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1988 |
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Front
Cover (1988) No author
information available
Summary: Not
available |
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Table of Contents (1988) No
author information available
Summary: Not
available |
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Conference Information
(1988) No author information
available
Summary: Not
available |
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High-speed
Josephson integrated circuit
technology S.
Hasuo
Summary: The author describes recent
progress in high-speed integrated circuits using niobium
junctions. He briefly describes the circuit fabrication
process and then introduces the modified variable threshold
logic (MVTL) gate family. The lowest experimentally obtained
MVTL OR-gate delay was only 2.5 ps with a power consumption of
17 mu W/gate. This gate family is used in various high-speed
logic circuits, such as 8-bit shift registers, 16-bit ALUs
(arithmetic logic units), and 4-bit microprocessors. The
author confirmed the high-speed operation of less than 10 ps
per gate on average for these circuits. A novel
high-sensitivity magnetic sensor using a SQUID
(superconducting quantum interference device) was also
developed. It is called a single-chip SQUID magnetometer
because the feedback circuit, which is operated at room
temperature is a conventional SQUID system, has been
integrated on the same chip as the SQUID sensor
itself. |
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Superconducting
magnetic energy storage W.
Hassenzahl
Summary: The author presents the
rationale for energy storage on utility systems, describes the
general technology of SMES (superconducting magnetic energy
storage), and explains the chronological development of
technology. The present ETM (Engineering Test Model) program
is outlined. The impact of high-T/sub c/ materials on SMES is
discussed. It is concluded that SMES is marginally competitive
with other storage technologies and is likely to remain
so. |
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The
status, recent progress and promise of superconducting
materials for practical
applications J.M.
Rowell
Summary: The author summarizes the
progress in materials science and engineering that created
today's superconducting technology. He reviews the state of
the technology with conventional materials by looking at two
particular applications: large-scale applications involving
conductors, for example, magnets; and electronics and
instrument applications. The state-of-the art is contrasted
with the present understanding of the high-T/sub c/ oxide
materials. |
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CEBAF,
a large scale application of superconducting
RF A.K.
Chargin
Summary: Summary form only given. The
Continuous Electron Beam Accelerator Facility (CEBAF) is a
4-GeV, multipass, CW accelerator. The machine is
racetrack-shaped with the linac split into two equal parts
that are located in the straight sections. The recirculating
magnets, which will guide the beam through four passes around
the accelerating structure, are in the arc sections connecting
the two linacs. At the end of the accelerator, the beam passes
into the switchyard where it is guided into any of three
experimental end stations. The linac consists of 418 Nb
superconducting RF cavities operating at 1500 MHz to produce a
high-quality beam having a 100% duty cycle with an energy
spread of 10/sup -4/ and emittance of 10/sup -9/ m. A pair of
high resolution spectrometers and a high-acceptance,
high-energy electron spectrometer will both utilize
superconducting dipoles and cos 2 theta
quadrupoles. |
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An
all-NbN time domain reflectometer chip functional above 8
K S.R. Whiteley, F. Kuo, M.
Radparvar and S.M. Faris
Summary: Niobium
nitride has a superconducting transition temperature nearly
twice that of niobium. A 5-ps time-domain reflectometer chip
based on NbN technology has been designed, fabricated, and
tested. The circuit is operable up to 9 K. The NbN process and
limitations are discussed, and present drawbacks in the
junction fabrication method are pointed out. Electrical
properties are discussed, the circuit operation is described,
and simulations are presented that are based on model
parameters extracted from device measurements. The actual
output of the circuit is presented as evidence of basic
functionality. This is the first demonstration of a functional
high-speed circuit based entirely on a compound superconductor
technology and operable at temperatures above 8
K. |
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Nb
multilayer planarization technology for a subnanosecond
Josephson 1K-bit RAM S. Nagasawa,
Y. Wada, H. Tsuge, M. Hidaka, I. Ishida and S.
Tahara
Summary: A fabrication process for a
Josephson 1-kb RAM (random access memory) has been developed
using a Nb multilayer planarization technology. The technology
consists of an etchback technique using 2000-molecular-weight
polystyrene and SiO/sub 2/ for the junction layer and wiring
layers, and a tapered edge etching technique for contact
between individual wiring layers. Excellent planarity, wherein
level differences in all step areas were reduced to less than
1/20th of their original values, was achieved. Appropriate RAM
operation with 570-ps minimum access time and 13-mW power
dissipation, were confirmed. |
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A
4K Josephson memory H. Suzuki, N.
Fujimaki, H. Tamura, T. Imamura and S.
Hasuo
Summary: The authors describe the
design and experimental performance of a 4 K*1-bit Josephson
(RAM). For high-speed memory operation, the authors have
developed a compact AND gate for the decoder, a high-voltage
driver gate, and a capacitively coupled single-flux quantum
memory cell. The 4 K memory was designed using these gates and
cell and was fabricated with Nb/AlO/sub x//Nb junctions. The
minimum access time was 590 ps, and the total power
dissipation was 19 mW. |
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Josephson
address control unit IC for a 4-bit microcomputer
prototype S. Kosaka, H. Nakagawa,
H. Kawamura, Y. Okada, Y. Hamazaki, M. Aoyagi, I. Kurosawa, A.
Shoji and S. Takada
Summary: The authors
describe the design and operation of a Josephson address
control unit IC (integrated circuit), which will be used for
controlling the instruction sequence of an experimental 4-bit
Josephson microcomputer prototype system. The IC is composed
of three sets of 7- to 10-bit-wide registers and combinational
logic circuits driven by a two-phase monopolar power supply.
593 four-function logic gates have been used in the circuit
and fabricated using 2.5- mu m NbN/oxide/NbN junction
technology with Mo resistors and SiO/sub 2/ insulation. The
operation of the circuit has been successfully tested for all
the instructions which control the program sequence of the
computer system. |
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Fabrication
of S-N-S Josephson junctions of Y-Ba-Cu-O/Au/Nb thin film
sandwiches H. Akoh, F. Shinoki,
M. Takahashi and S. Takada
Summary: The
authors fabricated S-N-S
(superconductor-normal-superconductor) Josephson junctions
which consist of Y-Ba-Cu-O/Au/Nb thin-film sandwiches with
various thicknesses of the Au barrier. Due to the AC-Josephson
effect, Shapiro steps are observed in the S-N-S junctions
under microwave radiation. The magnetic-field dependence of
the critical current shows that the junctions behave as
self-field-limited Josephson junctions. From the temperature
dependence of the critical current near the transition
temperature of the junctions, it is found that the thickness
of the Au barrier is shorter than the coherence length of
Au. |
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Y-Ba-Cu-O/Nb
Josephson tunnel junctions A.
Nakayama, A. Inoue, K. Takeuchi, H. Ito and Y.
Okabe
Summary: The authors fabricated
Y-Ba-Cu-O/Au/AlO/sub x//Nb and Y-Ba-Cu-O/AlO/sub x//Nb
Josephson tunnel junctions using electron-beam evaporation of
Al and Nb films and natural oxidation. Sintered Y-Ba-Cu-O was
used as the base electrode. Superconducting Josephson current
and hysteresis of the current-voltage characteristics, which
are typical features of Josephson tunnel junctions, have been
observed at 4.2 K. RF-induced voltage steps at a voltage
greater than 0.4 mV have been clearly observed, and RF-induced
subharmonic steps have also appeared. The superconducting
Josephson current was modulated by the magnetic
field. |
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Optimization
of YBCO surfaces of tunnel
junctions J.R. Gavaler, A.I.
Braginski, M.G. Forrester, J. Talvacchio and J.
Greggi
Summary: It is established that in
YBa/sub 2/Cu/sub 3/O/sub 7/ films prepared by annealing
amorphous oxide deposits, Ba segregation in the amorphous
phase and YBCO decomposition after recrystallization are the
major causes of surface degradation. The authors have grown
films by entirely in-situ processing in which these effects
are minimized. The films were epitaxially grown
on |
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Flux
noise and flux creep in YBCO thin
films M.J. Ferrari, M. Johnson,
F.C. Wellstood, J. Clarke, P.A. Rosenthal, R.H. Hammond and
M.R. Beasley
Summary: The authors used a DC
SQUID (superconducting quantum interference device) to measure
the low-frequency magnetic flux noise produced by thin-film
rings of YBa/sub 2/Cu/sub 3/O/sub 7- delta / (YBCO) with
various microstructures. Below the transition temperature
T/sub c/ of the YBCO, the spectral density of the noise scales
as 1/f (f is the frequency) from 1 Hz to 1 kHz. This noise
generally increases with temperature and vanishes abruptly at
T/sub c/. Improvements in crystalline microstructure greatly
reduce the magnitude of the noise, which was lowest for a
highly orientated sample with its c-axis perpendicular to the
substrate. Making a radial cut to interrupt current paths
around the sample ring does not significantly affect the
magnitude of the noise, demonstrating that the noise arises
from a local mechanism such as the thermally activated hopping
of flux bundles. Flux creep was observed in one sample cooled
in a magnetic field of 1 mT, and the creep rate exhibited a
sharp maximum near 80 K. It is concluded that SQUIDs and flux
transformers of YBCO must be fabricated from highly orientated
films to obtain low noise at low frequencies. |
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Surface
impedance studies of the high-T/sub c/ oxide
superconductors T.L. Hylton, M.R.
Beasley, A. Kapitulnik, J.P. Carini, L. Drabeck and G.
Grunner
Summary: The authors briefly review
results of measurements of the multimeter-wave surface
impedance of ceramic, thin-film and single-crystal samples of
the high-T/sub c/ oxide superconductors. The observed losses
and temperature dependences do not agree with BCS
(Bardeen-Cooper-Schriefer) theory. The authors discuss recent
measurements of single-crystal B/sub 2/CaSr/sub 2/Cu/sub
2/O/sub x/ and the effect of the application of a
perpendicular magnetic field. They consider the possibilities
of intrinsic and defect mechanisms to explain the
observations. In particular, they discuss the effects of
weakly coupled granular films on surface impedance. It is
concluded that better samples are necessary to resolve the
issue of the source of the large values of the surface
impedance observed thus far. |
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Ultrafast
response of superconducting transmission
lines J.M. Chwalek, D.R. Dykaar,
J.F. Whitaker, R. Sobolewski, S. Gupta, T.Y. Hsiang and G.A.
Mourou
Summary: The authors report
investigations of picosecond transient propagation on normal
and superconducting transmission lines and results of a
variety of lines that include YBa/sub 2/Cu/sub 3/O/sub 7-x/
(YBCO) coplanar lines, a superconducting coaxial cable, and a
dielectric-matched gold-line structure. A previously developed
algorithm for analyzing transient propagation was used to
identify the dominant mechanisms for signal distortion in most
of these cases, and the essential properties of all tested to
date are summarized for a direct comparison. |
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A
Josephson analog limiter
circuit D.A. Petersen, D. Herbert
and T. Van Duzer
Summary: The authors have
designed, fabricated and successfully tested a Josephson
integrated circuit that is used to limit the range of an
analog input signal. The circuit consists of a single
Josephson junction with a suppressed critical current. The
impedance of this suppressed junction consists of the
nonlinear quasiparticle resistance in parallel with the
capacitance of the junction. The junction capacitance is used
to advantage as one pole of a second-order low-pass filter,
the other element of which is a thin-film inductor. This
filter can perform the function of a slew-rate-limiting
filter, at the input to a high-speed comparator. The authors
fabricated individual junctions and measured their suppression
characteristics and have found that a 2- mu m*4.5 mu m
junction with a nominal critical current of 170 mu A can be
suppressed to less than 4 mu A of critical current with a
14-mA control current. Complete limiter circuits have been
fabricated and tested for both their DC and transient
characteristics. Measured DC response is in good agreement
with simulation, but parasitic capacitance present in the
fabricated devices limited the -3 dB bandwidth to about 3.6
GHz. |
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Speed-limiting
factors in flash-type Josephson A/D
converters E.S. Fang and T. Van
Duzer
Summary: The authors discuss the causes
of speed limitations in various A/D (analog/digital) converter
designs. The upper limit on bandwidth is extracted with the
help of Josephson SPICE simulations. In the Josephson A/D
converter circuits discussed, the dynamic properties of the
SQUIDs (superconducting quantum interference devices)
determine the aperture time and dictate the bandwidth. Designs
for 4-bit A/D converters that show potential for bandwidths on
the order of 10 GHz are described. Particular attention is
given to the bit-parallel A/D converter with self-gating AND
comparator and bit-parallel A/D converters with CLAM (current
latching analog microcomparator) and variable-pulse peak
comparators. |
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A
flash Josephson A/D converter constructed with one-junction
SQUIDs H. Ko
Summary:
A novel approach to the construction of a flash-type
Josephson A/D (analog/digital) converter is presented.
Simulations show that one-junction SQUID (superconducting
quantum interference device) comparators can have a greater
than fivefold advantage in bandwidth over the two- or
three-junction SQUIDs in an A/D circuit. Assuming a Nb
junction technology, the simulations show that a 6-bit A/D
converter using one-junction SQUID comparators could have a
sampling rate of approximately 20 GHz with approximately 5
bits of resolution for a 5-GHz input signal. Detailed analysis
and simulations of an A/D converter constructed with
one-junction SQUIDs are presented. Further improvement can be
made by using a coding algorithm which requires 2N-1
comparators, instead of N, for an N-bit A/D
converter. |
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A
variable hysteresis aperturing method for superconducting
counting A/D conversion G.S.
Lee
Summary: The authors present an
aperturing method for superconducting counting A/D
(analog/digital) converters. The scheme, called TRAP (time
release after aperturing of pulses), utilizes a large beta
/sub L/ two-junction SQUID (superconducting quantum
interference device) quantizer and plays on the ability to
control the amount of hysteresis available in such a SQUID.
Incoming information from the analog signal can be either
immediately processed by the quantizer or trapped and
outputted at a later time. The author explains how this
control can be used to form a picosecond aperture for counting
A/D converters, eliminating pulse-skipping problems. SPICE
simulations confirm the ideal behaviour of the
TRAP. |
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An
on-chip superconducting clock with two
modes G.S. Lee, A.H. Silver and
R.D. Sandell
Summary: A superconducting clock
based on a 2-junction SQUID (superconducting quantum
interference device) flip-flop with feedback is presented. The
feedback consists of transmission lines that emanate from and
return to the SQUID; the entire clock is built as a compact
integrated circuit. The period of the clock is mainly
determined by the length of transmission line, and there are
two modes of operation that can be separately excited whose
periods are in a ratio of 2:1. I-V data for a pair of clocks
with different designed periods confirm the presence of the
two modes, show how period depends on length, and give
information on the switching time of the SQUID flip-flops.
Specifically, the I-V data show that there exist both a
fundamental model for flux bias at odd multiples of Phi /sub
0//2 (half-flux quantum) and a doubled mode with precisely
half the period flux bias at even multiples. Clocks with
longer transmission lines have longer periods, but simple
scaling does not occur due to other sources of time
delay. |
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Josephson
shift register design and
layout J.X. Przybysz, R.D.
Blaugher and J. Buttyan
Summary: Integrated
circuit chips were designed and fabricated, based on a
Josephson shift register circuit that simulated operation at
25 GHz using the SPICE program. The 6.25-mm/sup 2/ chip
featured a twelve-gate, four-stage shift register fabricated
with Nb/AlO/sub x//Nb Josephson junctions with a design value
of 2000 A/cm/sup 2/ critical current density. SUPERCOMPACT, a
general program for the design of monolithic microwave
integrated circuits, was used to model the effects of layout
geometry on the uniformity and phase coherence of logic gate
bias currents. A layout geometry for the superconductive
transmission lines and thin-film bias resistors was developed.
The original SPICE-designed circuit was modified as a result
of these calculations. Modeling indicated that bias current
variations could be limited to 3% for all possible logic
states of the shift register, and phase coherence of the gates
could be maintained to within 2 degrees at 10 GHz. The
fundamental soundness of the circuit design was demonstrated
by the proper operation of fabricated shift
registers. |
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A
fast Josephson SFQ shift
register F. Kuo, S.R. Whiteley
and S.M. Faris
Summary: A Josephson SFQ
(single flux quantum) shift register circuit operating under a
two-phase power was designed and tested. The test cell was
fabricated using the Nb/AlO/sub x//Nb junction SNEAP process.
The main data latch is DC biased; the data storage is
accomplished by the transfer of a single flux quantum in or
out of a SQUID (superconducting quantum interference device)
superconducting loop. A two-phase sinusoidal clock with offset
is used to operate the circuit. Simulations verify that the
operating frequency can go beyond 70 GHz, with operating
margins exceeding 20%. Preliminary measurements indicate that
the circuit operates as intended. |
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A
novel ternary logic circuit using Josephson
junction M. Morisue, K. Oochi and
M. Nishizawa
Summary: A novel Josephson
complementary ternary logic (JCTL) circuit is described. This
fundamental circuit is based on the combination of two SQUIDs
(superconducting quantum interference devices), one of which
is switched in the positive direction and the other in the
negative direction. The JCTL can perform the fundamental
operations of AND, OR, NOT, and Double NOT in ternary form.
The principle of the operation and design criteria are
described in detail. Simulation results show that reliable
operation of these circuits can be achieved with a high
performance. |
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A
Josephson ternary associative memory
cell M. Morisue and K.
Suzuki
Summary: The authors describe a
three-valued content-addressable memory cell using a Josephson
complementary ternary logic (JCTL) circuit. The memory cell
can perform the operations of searching, writing and reading
in the ternary logic system. The principle of the memory
circuit is illustrated in detail by using the threshold
characteristics of the JCTL. Computer simulations were
performed to investigate how high-performance operation can be
achieved. Simulation results show that the cycle time of
memory operation is 120 ps, power consumption is about 0.5 mu
W/cell, and tolerances of writing and reading operation are
+or-15% and +or-24% respectively. |
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Quantum
flux type logic circuits utilizing Josephson junction
transmission lines M. Morisue, M.
Satoh and K. Araki
Summary: A
quantum-flux-type logic circuit is proposed which is composed
of Josephson junction transmission lines, along which
localized magnetic flux can propagate. By choosing bias
current properly, the duplication of magnetic flux and a
variety of logical functions can be obtained without changing
the circuit topology. Computer simulation results are
presented on AND and OR operations with two and three inputs
for the same circuit topology, confirming that these circuits
can be used as logic circuits. The simulations demonstrate the
high-speed operation and low power consumption of this
circuit. |
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RSFQ
logic arithmetic O.A. Mukhanov,
S.V. Rylov, V.K. Semonov and S.V.
Vyshenskii
Summary: Several ways to achieve
local timing of Josephson-junction RSFQ (rapid single flux
quantum) logic elements are proposed. Several examples of
serial and parallel pipelined arithmetic blocks using various
types of timing are suggested and their possible performance
is discussed. Serial devices enable one to perform n-bit
functions relatively slowly but using integrated circuits of a
moderate integration scale, while parallel pipelined devices
are more hardware-wasteful but promise extremely high
productivity. The possible local and self-timing of RSFQ logic
elements has been demonstrated, making it possible to
construct digital blocks and complex devices operating at
extremely high clock frequencies, limited only by logic delays
of the RSFQ elements ( approximately 100 GHz for the
present-day Nb technologies). |
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Experimental
study of the RSFQ logic
elements V.K. Kaplunenko, M.I.
Khabipov, V.P. Koshelets, K.K. Likharev, O.A. Mukhanov, V.K.
Semenov, I.L. Serpuchenko and A.N.
Vystavkin
Summary: New elements of the rapid
single flux quantum (RSFQ) logic family have been designed,
fabricated, and tested. All-Nb 14-layer 5 mu m technology
using externally shunted tunnel junctions with j/sub c/=500
A/cm/sup 2/, I/sub c/R/sub s/=300 mu V, and B/sub c/ |
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A
new A/D converter with quantum flux
parametron N. Shimizu, Y. Harada,
N. Miyamoto and E. Goto
Summary: A flash-type
superconducting A/D (analog/digital) converter is proposed.
The basis low-frequency operation of this converter, which
compromises a quantum flux parametron (QFP) and an RF SQUID
(superconducting quantum interference device), has been
observed. The RF SQUID consists of a Josephson junction and a
load inductor which is part of the QFP. It converts an analog
signal to period digital signals of one quantum flux
(2.07*10/sup -5/ W-b) per cycle. The QFP works as both an
amplifier and a comparator of the periodic digital signals, by
adding an exciting signal. This A/D converter is expected to
achieve high-speed operation of 20 GHz. Several 4-bit A/D
converter circuits were fabricated, using the conventional
Nb-Pb alloy process, with a junction size of 5 mu m*5 mu m and
supercurrent density of 200 A/cm/sup 2/. Basic operation at
100 KHz was confirmed experimentally. |
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Preparation
and performance of toroidal Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub
7-x/-RF SQUIDs Y. Zhang, M.
Diegel and C. Heiden
Summary: Starting from
disk-shaped bulk specimens of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub
7-x/ with T/sub c/ near 90 K, RF SQUIDs (superconducting
quantum interference devices) to toroidal topology with a
permanently adjusted mill-bridge contact were prepared.
Contact trimming was done by reducing the cross section so
that a good SQUID signal was obtained at 77 K. The SQUIDs were
sealed after the adjustment in order to prevent degradation by
moisture. Using conventional 20-MHz readout electronics the
SQUIDs were operated in the flux-locked loop mode. Flux noise
of the order of 10/sup -4/ Phi /sub 0// square root Hz was
obtained at 77 K in the white region of the spectrum, with
excess noise below about 100 Hz. The devices are quite robust
due to their monolithic structure and demonstrate good
stability against thermal cycling. |
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The
properties and potential applications of bulk RF SQUIDs made
from YBaCuO, BiSrCaCuO and
TlCaBaCuO C.M. Pegrum, J.R.
Buckley and M. Odehnal
Summary: The authors
have developed methods for making RF SQUIDs (superconducting
quantum interference devices) from bulk samples of
high-temperature ceramic superconductors. The technique relies
on intrinsic closed superconducting pathways within the
material to form the SQUID loop, and intergrain contact to
provide the necessary Josephson elements. The material does
not need to be multiply connected (for example, ring-shaped)
or to have a region which is deliberately weakened or
constricted. In this sense this type is quite unlike other RF
SQUIDs that have been developed recently, which use break
junctions or have machined constrictions. It is also much
simpler to make and reveals some interesting properties
related to the nature of the contact between grains. The
authors conclude that a very simple system can make a
magnetometer with a field sensitivity at 77 K of 1.5*10/sup
-10/ T-Hz/sup -1/2/, comparable to that of a flux gate
magnetometer. The technique also provides a good educational
demonstration of quantum interference and can be used for
detecting the superconducting state in small bulk
samples. |
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Noise
measurements in a 2-hole radio frequency SQUID at liquid
nitrogen temperature S.P. Harrop,
M.S. Colclough, C.E. Gough, M.N. Keene and C.M.
Muirhead
Summary: The authors report
measurements of the noise in a two-hole RF SQUID
(superconducting quantum interference device) fabricated from
bulk YBCO by an erosion technique in the frequency range 30
mHz to 1 kHz. Above 100 Hz the noise is essentially white with
an RMS (root mean square) value of 5*10/sup -4/ phi /sub 0//
square root Hz and is dominated by noise from the electronics.
Below 10-Hz the noise power rises as 1/f/sup
0/9/. |
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High
T/sub c/ RF-biased SQUID C.H.
Harmston, O.G. Symko, W.J. Yeh, D.J. Zheng and S.K.
Kulkarni
Summary: Results are presented on
the behaviour of a simple RF-biased SQUID (superconducting
quantum interference device) made out of YBa/sub 2/Cu/sub
3/O/sub 7/. It consists of a fractured ceramic pellet stuck
together at room temperature. This simple device is shielded
by a high-T/sub c/ tube and shows the characteristic flux
quantization behaviour up to 77 K. At 4.2 K its magnetic flux
resolution is less than 2*10/sup -4/ phi /sub 0// square root
Hz, and it shows sensitivity degradation as the temperature is
raised. |
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Properties
of the microwave SQUID with an YBaCuO point contact
junction T. Ryhanen and H.
Seppa
Summary: A high-sensitivity microwave
SQUID (superconducting quantum interference device), suitable
for RF attenuation measurements, is constructed taking
advantage of the low density of states of current carriers in
high-T/sub c/ materials. The dynamics, the noise properties,
and the sensitivity of the microwave SQUID are theoretically
analyzed. The ultimate energy resolution of the waveguide
SQUID is inversely proportional to the characteristic
frequency omega /sub c/=R/sub q//L of the loop. omega /sub c/
is high for YBaCuO junctions at 4.2 K because of small
leakage. A flux sensitivity of 4*10/sup 4-6/ phi /sub 0//
square root Hz has been achieved, which is in good agreement
with the theoretical predictions. |
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Josephson
junction and DC SQUID made from superconducting Y-Ba-Cu-O
thick film A.Z. Lin, H.Q. Li, L.
Tang and F.W. Liu
Summary: YBa/sub 2/Cu/sub
3/O/sub 7-x/ thick films were fabricated by the method of
screen printing. A zero-resistance state was achieved at 90 K.
The highest critical current density was about 150 A/cm/sup 2/
at 77 K. A bridge-type Josephson junction and DC SQUID
(superconducting quantum interference device) were fabricated
from thick films and operated at 77 K. The induced steps
produced by the AC Josephson effect were observed on the I-V
(current voltage) curve when microwave radiation was applied
to the junction. The clear periodic patterns of a DC SQUID are
observable at 77 K. Environmental protection and the stability
of thick films and devices are discussed. |
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DC
SQUID in YBaCuO ceramics operating at 77
K M.L.C. Sing, D. Robbes, Y.
Monfort, D. Bloyet, J. Provost and B.
Raveau
Summary: Constrictions with typical
cross-sectional areas of 10/sup -8/-10/sup -7/ m/sup 2/
engraved in bulk YBa/sub 2/Cu/sub 3/O/sub 7-d/ ceramics
operate as DC SQUIDs (superconducting quantum interference
devices) at 77 K. These sensors have a magnetic field
sensitivity of 1 to 20 pT/ square root Hz, a flux sensitivity
of 6*10/sup -6/ to 3*10/sup -5/ phi /sub 0// square root Hz,
and a corresponding energy resolution in the 10/sup
-30/-10/sup -29/ J/Hz range around 20 kHz. The noise spectral
density of the device clearly exhibits low-frequency 1/f
noise, which scales throughout the whole investigated range of
frequencies. The noise spectra at 1 Hz varies between 5*10/sup
-27/ and 2*10/sup -26/ J/Hz. |
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DC
SQUIDs fabricated with YBaCuO thin
films S. Wang, G. Cui, Y. Dai, H.
Jiang, X. Zeng, J. Li, Z. Bao, C. Li, D. Yin, K. Shao and C.
Cao
Summary: DC superconducting quantum
interference devices (SQUIDs) have been fabricated from thin
films of YBaCuO. The devices were made by the
photolithographic method with wet etching. The oscillatory
output voltage of the devices in a sweeping magnetic field is
shown to have a good periodicity. Triangular wave patterns of
the DC SQUID output were observed. |
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A
novel HTS magnetometer, exploiting the low jc of bulk
YBCO J.C. Gallop, S. Lilleyman,
C.D. Langham, W.J. Radcliffe and M. Stewart
Summary:
The authors report a novel form of magnetometer which
is based on the low critical magnetic field H/sub c1/ of
sintered samples of the high-temperature ceramic
superconductor YBa/sub 2/Cu/sub 3/O/sub y/. By driving a
sample of the superconductor around a magnetization hysteresis
loop at a frequency of approximately 100 kHz and detecting the
induced voltage in a coil coupled to the sample at the second
harmonic of the drive frequency, they find that this voltage
is linearly dependent on the applied DC magnetic field in
which the sample is situated. They present a model which
explains the operation of this magnetometer. The device, while
not sensitive as a SQUID (superconducting quantum interference
device), has the advantage of a wider dynamic range and direct
measurement of flux density unlike a SQUID, which is only
capable of sensing flux density changes. When operated at 77 K
the prototype magnetometer has already demonstrated a
sensitivity at least 10 times better than that of a commercial
flux-gate magnetometer. The system also appears to provide a
simple method for investigation of flux flow in these
materials. |
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Memory
characteristics of ring-shaped ceramic
superconductors A. Takeoka, M.
Hasunuma, S. Sakaiya, T. Hirano and Y.
Kuwano
Summary: The authors investigated the
residual magnetic field characteristics of ring-shaped ceramic
superconductors in a Y-Ba-Cu-O system with high T/sub c/. The
residual magnetic field of a ring with asymmetric current
paths, supplied by external currents, appeared when one of the
branch currents was above the critical current. The residual
magnetic field saturated when both branch currents exceeded
the critical current of the ring and showed hysteresis-like
characteristics. The saturated magnetic field is subject to
the critical current of the ring. The time constant of the
persistent current in a ring, evaluated from the damping
coefficient, was 5000 hours at most. A superconducting ring
with asymmetric current paths suggests a simple, novel
persistent-current-type memory device. |
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Three-dimensional
flux-sensor composed of high-T/sub c/
superconductor S. Nishijima, K.
Takahata, T. Okada, H. Okushiba, Y. Nagamori, S. Okamoto and
K. Hayashi
Summary: A three-dimensional flux
sensor has been studied with the aim of realizing a practical
application of high-T/sub c/ oxide superconductor Y-Ba-Cu-O.
Thick-film and cylindrical sensors were made and examined. The
current-voltage characteristics were investigated as a
function of magnetic field at liquid nitrogen temperature. The
flux flow was observed in a wide range of magnetic fields from
0.001 to 1 Tesla, and the magnetic field could be evaluated by
the output voltage. The dependence of the voltage on the angle
between the current and magnetic field direction was also
examined. The flux flow was affected by the direction of the
external magnetic field, resulting in a change of the output
voltage. The output voltage was found to show hysteresis with
respect to the external field. On the basis of these
phenomena, a three-dimensional flux sensor was designed and
fabricated. |
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Millimeter-wave
detection by GBJJ using high-T/sub c/ superconducting YBaCuO
films S. Kita, H. Tanabe and T.
Kobayashi
Summary: Millimeter-wave detection
in the heterodyne mixing mode has been carried out using the
high-T/sub c/ superconducting GBJJ (grain boundary Josephson
junction). The GBJJ was fabricated from high T/sub c/
superconducting Y-Ba-Cu-O films, prepared on YSZ and SrTiO/sub
3/ substrates using an RF sputtering technique. The junction
was patterned by chemical wet etching with phosphoric acid
solution. The authors carried out experiments in the
heterodyne mixing mode for a signal of 35 GHz with local
oscillation at 34.97 GHz. The detected IF (intermediate
frequency) output power was obtained at a bias voltage lying
midway between the zeroth and first Shapiro steps. A mixer
conversion efficiency of -15 dB was obtained with a dynamic
range of about 20 dB at 35 GHz. This result strongly implies
that high-T/sub c/ superconducting GBJJ works well as a
detector at millimeter-wave frequencies. |
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Josephson
harmonic mixing and internal oscillation-mixing in YBCO
superconducting weak link at liquid nitrogen
temperatures J. Song, P.H. Wu,
Q.H. Cheng, S.Z. Yang, J. Chen, D. Jin, H.X. Luo and J.S.
Kui
Summary: Using a superconducting weak
link (bridge) made of YBCO, the authors have successfully
observed Josephson harmonic mixing between a local oscillation
(LO) at X band and a signal at Ka band. Specifically,
fourth-order harmonic mixing between 35.47 GHz and 8.843 GHz
was observed. For a given signal level, the IF (intermediate
frequency) responses depended strongly on both LO level and DC
voltage bias across the junction. For zero bias, there is
still IF output which is characteristic of even-order harmonic
mixing. When the LO power level is such that the zero voltage
current is suppressed to the second zero, the IF responses are
maximum if the junction is biased between the zeroth and first
steps induced by the LO. Also studied is the internal
oscillation-mixing. An incoming signal at either X or Ka band
gives rise to IF signal peaks if the junction is biased at
voltages such that the corresponding internal Josephson
frequencies are equal to, or harmonics of, the incoming signal
frequency. |
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Heights
of microwave induced steps, temperature dependent supercurrent
and other experimental observations in YBCO weak
link N.X. Shen, P.H. Wu, S.Z.
Yang, Q.H. Cheng, Y.B. Sheng, H.X. Luo and J.S.
Kui
Summary: For YBCO bridges made of bulk
samples, the authors have carefully measured the height of the
microwave-induced steps as a function of applied microwave
power and found, for Omega =hf/2eI/sub c/R=0.25, approximate
agreement with the Bessel function dependence. The
supercurrent versus temperature dependence follows the I/sub
c/ varies as (1-T/T/sub c/) relationship quite closely at
temperatures far from T/sub c/, while in the vicinity of T/sub
c/ the deviation from it can be pronounced. For bridges made
of YBCO thin films, the temperate-dependent supercurrent can
be expressed as I/sub c/ varies as (1-T/T/sub c/)/sup 3/2/.
Also discussed are some possible explanations of
experimentally observed I-V curves which might be attributed
to mechanisms such as self-heating and junctions in
series. |
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Properties
of lift-off structured high T/sub c/
microbridges B. Hauser, B.
Klopman, D. Blank and H. Rogalla
Summary:
Microbridges and DC SQUIDs (superconducting quantum
interference devices) were fabricated, using a lift-off
technique, from RF sputtered YBaCuO films on MgO
single-crystal substrates. Microwave measurements at 9 GHz on
microbridges and the magnetic-field dependence of their
critical current reveal wide bridge behaviour up to
temperatures near the maximum operating temperature of the
bridge. Mostly, a linear dependence of the critical current on
the temperature is found, which is connected with high
intrinsic 1/f noise if the bridge is constant-current-biased
slightly above the critical current. In some bridges and DC
SQUIDs, regimes with a temperature dependence proportional to
(1-T/T/sub c/)/sup 1.5/ are found. In this case the 1/f noise
level is much smaller and SQUID modulation can be followed to
about 65 K. |
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Grain
boundary Josephson devices by YBaCuO films and 77 K
operations T. Yamashita, A.
Kawakami, S. Noge, W. Xu, M. Takata, T. Komatsu and K.
Matusita
Summary: Magnetron sputtering and
screen printing methods were used to fabricate YBaCuO films
with thicknesses of 5 to 30 mu m. The annealing of the films
at 1000 degrees C gave abnormal grain growth. The grain size
of the films was about 2 to 70 mu m. With photolithography and
razor cutting techniques, the films were formed into
bridge-type Josephson junctions having a few grain boundaries
in the bridge regions. In the devices, clear Shapiro steps and
SQUID (superconducting quantum interference devices) patterns
were observed at 77 K. The experiments showed that all
currents flowing through grain boundaries are Josephson
currents in YBaCuO polycrystalline films. Clear Josephson
effects were observed in about 30% of the fabricated devices.
Such devices may have high potential for high-frequency
detectors and SQUID flux sensors operative at 77
K. |
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Three
terminal YBaCuO Josephson device with quasi-particle injection
gate T. Kobayashi, K.-I.
Hashimoto, U. Kabasawa and M. Tonouchi
Summary:
A novel type of three-terminal Josephson device called
the SCST (superconducting current switching transistor), was
fabricated using high-T/sub c/ LnBaCuO (Ln=Y, Er) thin
epitaxial and/or polycrystalline films. The hot quasi-particle
injection effect on the Josephson (or superconducting) current
in the films was closely examined. The zero bias drain current
was efficiently suppressed by the injection of the hot
quasi-particles through the gate electrode. A comparison of
the experimental results and analyses based on the BCS theory
suggests that the main mechanism of the current modulation is
the nonequilibrium superconductivity due to the accumulation
of the excess quasi-particles. The maximum current modulation
gain was 5 approximately 7 and the modulation current ranged
as low as 3 approximately 10 A/cm/sup 2/. |
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Growth,
patterning, and weak link behaviour in high T/sub c/ thin
films G.C. Hilton, R.A.
Schweinfurth and D.J. Van Harlington
Summary:
The authors report techniques for making Josephson weak
links in thin films of the high-temperature ceramic
superconductors. They have fabricated superconducting thin
films of YBaCuO and BiSrCaCuO by several methods. These films
have been patterned into fine lines and small loops, and a
variety of measurements have been performed on these
structures, Intrinsic weak link behaviour is observed between
grains in patterned films and can be modified by an in-situ
ion beam milling and measurement procedure. |
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Laser
patterning of YBaCuO weak link
bridges E. Wiener-Avnear, J.E.
Cooper, G.L. Kerber, J.W. Spargo, A.G. Toth, J.Y. Josefowicz,
D.B. Rensch, B.M. Clemens and A.T. Hunter
Summary:
A laser ablation process was developed to pattern
thin-film superconducting devices with 1- mu m resolution. The
process was utilized to fabricate bridge structures in high
T/sub c/(>90 K) YBaCuO superconducting thin films sputtered
on various substrates. For bridges patterned on thin films
with a high degree of polycrystallinity, the authors observed
weak-link Josephson behaviour, from 4.2 K up to T/sub c/. The
weak-link behaviour was attributed to grain-boundary
superconducting tunneling. The results suggest that direct
laser patterning can be a powerful technique for evaluation
and optimization of prototype superconducting
devices. |
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Josephson
junctions with bulk YBa/sub 2/Cu/sub 3/O/sub 7-x/, Bi/sub
2/Sr/sub 2/CaCu/sub 2/O/sub x/ and Tl/sub 2/Ca/sub 2/Ba/sub
2/Cu/sub 3/O/sub 10+x/ W.
Eidelloth, F.S. Barnes, S. Geller, K.Y. Wu, Z.Z. Sheng and
A.M. Hermann
Summary: The characteristics of
point contacts with combinations of Pb, YBa/sub 2/Cu/sub
3/O/sub 7-x/, Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub x/, (YBCO),
(BSCCO), and Tl/sub 2/Ca/sub 2/Ba/sub 2/Cu/sub 3/O/sub 10+x/
(TCBCO) have been measured in a shielded liquid helium dewar.
Zero-voltage currents were found in all arrangements;
diffraction patterns in the voltage-flux curves were observed
with all materials. Electron pairs were confirmed as carriers
in all investigated high-T/sub c/ materials. All junctions
resembled weak-link bridge structures rather than tunnel
junctions. Despite strong shunting, hysteresis was found in
the I-V curves of some point contacts. A structure of unknown
origin was found in the I-V curves of YBCO-BSCCO junctions.
Both YBCO-YBCO and BSCCO-BSCCO junctions had much lower normal
resistances and considerably higher critical currents than
YBCO-BSCCO point contacts. The reason for this phenomenon is
not yet understood. |
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Josephson
effect and macroscopic quantum interference in high-T/sub c/
superconducting thin-film weak links at 77
K A.I. Golovashkin, A.L. Gudkov,
S.I. Krasnosvobodtsev, L.S. Kuzmin, K.K. Likharev, Y.V.
Maslennikov, Y.A. Pashkin, E.V. Pechen and O.V.
Snigirev
Summary: The authors studied the
properties of single and double superconducting weak links
(Dayem microbridges) fabricated by direct photolithography
from thin films of 1-2-3 copper oxides, deposited on SrTiO/sub
3/ and LiNbO/sub 3/ substrates by laser sputtering of ceramic
targets. Values of critical current I/sub c/ and normal
resistance R/sub n/ of the weak links could be altered by
plasma etching of the structures. I/sub c/R/sub n/ products up
to 100 mu V were achieved, and well-pronounced Shapiro steps
under 8-GHz microwave radiation were observed at 77 K.
Macroscopic quantum interference was clearly registered in
double microbridge structures at the same
temperature. |
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Metal-oxide
ceramic RF SQUID operating at 77
K V.M. Zakosarenko, E.V.
Ilyichyov and V.A. Tulin
Summary: The authors
fabricated several versions of Zimmerman-type RF SQUIDs
(superconducting quantum interference devices) from bulk
ceramic YBa/sub 2/Cu/sub 3/O/sub x/ samples and studied their
characteristics. The SQUIDs operated reliably at liquid
nitrogen temperature and proved to be stable over time. Some
of their properties were similar to those of RF SQUIDs based
on conventional superconductors, but there were also some
major distinctions. The latter may be due to the fact that
because of the macroscopic size of the weak link, the magnetic
flux vortex enters reversibly (without respect to the RF
current) the body of the weak link without crossing it. One of
the SQUIDs operated as a conventional nonhysteretic SQUID, but
the small value of the critical current of the weak link led
to a lower operating temperature (T |
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Microlithography
of high-temperature superconducting films: laser ablation vs.
wet etching P.H. Ballentine, A.M.
Kadin, M.A. Fisher, D.S. Mallory and W.R.
Donaldson
Summary: Narrow lines and
microbridge structures have been etched in sputtered
superconducting films of Y-Ba-Cu-O by variations of two
methods. The first uses standard photolithography followed by
wet etching in weak acid. The second uses a maskless process
involving focused pulsed YAG (yttrium-aluminium-garnet) laser
together with a computer-controlled x-y stage to produce local
ablation of the superconducting film. Issues relating to
limits of resolution, annealing of films, and degradation of
superconducting properties are critically discussed for the
two approaches. |
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Single
superconducting thin film devices for applications in high
T/sub c/ materials
circuits G.K.G. Hohenwarter, J.S.
Martens, D.P. McGinnis, J.B. Beyer, J.E. Nordman and D.S.
Ginley
Summary: The authors investigated
several different devices based on regions of weak
superconductivity and multiple parallel links in thin films.
Devices were fabricated with Nb and YBa/sub 2/Cu/sub 3/O/sub
7-x/ films. Hysteretic symmetric and asymmetric I-V
(current-voltage) curves have been observed. Flux flow was
indicated. Device switching properties and the dependence of
the flux-flow signature in the I-V curve on applied magnetic
field were explored. Contrary to vortex flow devices based on
Josephson junctions, the devices described here do not possess
a tunnelling barrier and are made of only a single
superconducting layer. Hence they should be applicable to
electronic circuits based on high-T/sub c/ superconducting
materials without the need for tunnel junctions. |
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Germanium
thin film growth onto high T/sub c/ superconducting
films M. Tonouchi, Y. Sakaguchi,
K. Hashimoto, Y. Yoshizako and T. Kobayashi
Summary:
Germanium thin film growth on high-T/sub c/
superconducting films has been studied. The crystallized Ge
films were prepared on YBaCuO films at a substrate temperature
above 200 degrees C. An ECSA (electron spectroscopy for
chemical analysis) study revealed that the Ge films were
formed without intense reaction between Fe and
YBaCuO. |
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Characterization
of YBaCuO and ErBaCuO thin films deposited on silicon and
gallium arsenide substrates D.K.
Chin, T. Van Duzer, W.L. Hansen, K.M. Yu, W.F. Stickle, S.Y.
Lee and B. Murdock
Summary: YBaCuO and
ErBaCuO films have been deposited on Si substrates with and
without a ZrO/sub 2/ buffer layer and on GaAs substrates by RF
diode sputtering from stoichiometric oxide targets. The films
and interface between the films and semiconductor substrates
are analyzed by Rutherford backscattering spectrometry (RBS),
X-ray fluorescence spectroscopy (XRF), Auger electron
spectroscopy (AES), energy dispersive X-ray spectrometry
(EDAX), and scanning electron microscopy (SEM). The films
grown on Si substrates with a ZrO/sub 2/ buffer layer show
superconductivity above 65 K, and no significant interaction
at the interface is observed. High-T/sub c/ films can be
obtained either by slow-furnace annealing or by rapid
heat-pulse annealing. No significant interaction is observed
between YBaCuO (ErBaCuO) and GaAs after rapid thermal
annealing at temperatures below 750 degrees C. |
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High
T/sub c/ thin film and device
development K. Betts, M.B.
Burbank, A. Cragg, A.A. Fife, P.R. Kubik, S. Lee, J. McCubbin,
D. McKenzie, M. Tillotson, B. Taylor, H. Tran, J. Vrba, A.C.D.
Chaklader, G. Roemer, B. Heinrich, J. Chrzanowski and J.C.
Irwin
Summary: Thin films of the high-T/sub
c/ superconductor YBa/sub 2/Cu/sub 3/O/sub y/ have been
deposited on various substrates by diode and magnetron
sputtering using bulk sintered targets. These films have been
analyzed by a variety of methods: scanning electron
microscopy, X-rays, electron beam microprobe, mass
spectroscopy and Raman spectroscopy. The stoichiometries of
the films have been measured as a function of the radial
position from the center of the sputtered beam at a fixed
target-substrate distance. Patterning of the films has been
carried out to form planar structures such as strip lines,
microbridges and RF SQUIDs (superconducting quantum
interference devices). The DC current-voltage characteristics
of the microbridges were measured as a function of
temperature. RF SQUID behavior has been observed for
single-loop devices and their properties established at 4.2 K
and higher temperatures. Flux-locked noise spectra with a 1/f
noise power response were recorded in the frequency range from
0.01 to approximately=100 Hz. RF SQUID signals have been
observed for temperatures up to 55 K. |
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Interaction
among superconducting weak links in a bulk sample of YBaCuO at
77 K T. Yang, L. Dong, Z.Q. Sun,
X. Chen, C. Liu, J. Li, X. Wu and L. Zhou
Summary:
The DC and AC Josephson effects have been directly
observed for a piece of superconducting material about 5 mm*1
mm*0.2 mm which was cut from a bulk sample of Y/sub 1/Ba/sub
2/Cu/sub 3/. the I-V (current voltage) characteristics were
similar to those of a microbridge. When the sample was
irradiated by 35.6-GHz microwaves, about 30 Shapiro steps on
the I-V curves were observed at 77 K. Microwave-induced steps
at both n(hf/2e) and 2n(hf/2e) were coexistent in the sample.
It was found that the step width is not always consistent with
the Josephson I-V relation. This phenomenon might result from
complex interaction among a variety of superconducting links
in the bulk sample of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub
y/. |
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Low
frequency resistance fluctuations in films of high temperature
superconductors P. Rosenthal,
R.H. Hammond, M.R. Beasley, R. Leoni, P. Lerch and J.
Clarke
Summary: Low-frequency voltage
fluctuations in thin films of YBa/sub 2/Cu/sub 3/O/sub 7-x/ at
and above the superconducting transition temperature have a
spectral density proportional to the ratio of the average
voltage across the film to the frequency. The ratio of the
spectral density to the average voltage decreases markedly as
the microstructure of the films is improved. In contrast to
classic superconductors, the noise at the resistive transition
does not arise from equilibrium temperature
fluctuations. |
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Switching
noise in YBa/sub 2/Cu/sub 3/O/sub x/
'macrobridges' R.H. Ono, J.A.
Beall, M.W. Cromar, P.M. Mankiewich, R.E. Howard and W.
Skocpol
Summary: Intermittent switching was
observed in the voltage current characteristics of thin-film
bridges of YBa/sub 2/Cu/sub 3/O/sub x/. At a fixed bias point
there are multiple metastable voltage states with lifetimes
which depend on the bias current and applied magnetic field.
The microbridges are made of thin (<500 nm) polycrystalline
films of YBa/sub 2/Cu/sub 3/O/sub x/ which are patterned by
liftoff into structures with dimensions ranging from less than
1 mu m. to 100 mu m. Details of the fabrication process and
the measurements are presented. The results are discussed in
the context of fluctuations in the effective resistance of the
bridge due to motion of trapped flux. |
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Ultra
broad band measurements on high T/sub c/ ceramic
superconducting transmission
lines K. Araki, I. Iwasa, Y.
Kobayashi, S. Nagata and M. Morisue
Summary:
Transmission lines were fabricated on high-T/sub c/
ceramic conductor thin films by means of a lithographic
technique. A scalar network analyzer was used to perform
measurements from 10 MHz to 26.5 GHz of the transmission and
reflection characteristics of superconducting transmission
lines at 4.2 K, 77 K, and room temperature. The lines were
fabricated from 0.8- mu m-thick YBa/sub 2/Cu/sub 3/O/sub 7/ on
1.0-mm-thick SrTiO/sub 3/ substrates. Characteristic
impedance, attenuation constant, phase constant, group time
delay and group velocity were calculated by a novel algorithm
which is processed in real time. Time domain responses, e.g.,
pulse delay and waveform distortion, were also easily obtained
by a computer technique. The degradation due to the etching
process is investigated, and a comparison is made of copper
and ceramic transmission lines. |
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A
film transmission line resonator to measure the microwave
surface resistance of YBa/sub 2/Cu/sub 3/O/sub
7-x/ J.S. Martens, G.K.G.
Hohenwarter, D.P. McGinnis, J.B. Beyer and D.S.
Ginley
Summary: The authors constructed
microstrip transmission line resonators with YBa/sub 2/Cu/sub
3/O/sub 6.9/ ground planes, SiO dielectric, and Nb top
conductors. The film resonator is a low-impedance section n
lambda /2 long, and the wave is launched from a coaxial line.
Loose coupling is provided by the discontinuity between 50-
Omega feed lines and the 0.05- Omega resonator section. After
de-embedding the transmittance data of the structure, the
surface resistance of the ceramic superconductor can be
computed. The surface resistance of YBa/sub 2/Cu/sub 3/O/sub
6.9/ at 4.2 K was about a factor of 60 higher than the
theoretical value for Nb at 4.2 K over the range 2.9-6 GHz.
The power law behavior versus frequency of the surface
resistance is roughly quadratic, which is consistent with
two-fluid analysis. |
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RF
electromagnetic investigation of an YBa/sub 2/Cu/sub 3/O/sub
7/ thin film by proximity
coupling H.P. Baum, A.
Schenstrom, Y. Zheng, B.K. Sarma, M. Levy, J.H. Kang and R.T.
Kampwirth
Summary: Reports an investigation
of the RF electromagnetic (EM) attenuation by a YBa/sub
2/Cu/sub 3/O/sub 7/ thin film at 1 GHz as a function of
temperature. An increase in the attenuation was observed as
the temperature was decreased below the superconducting
transition temperature T/sub c/ approximately=84 K, followed
by a rapid decrease at lower temperature ( approximately 25
K). A theoretical analysis of EM absorption due to M. Fibich
(1965) is evoked to explain the experimental results. It is
concluded that the absorption of 1-GHz electromagnetic waves
in the present experimental arrangement is determined by the
real part of the conductivity in the superconducting state.
This implies that even for the high-temperature
superconductors considered, the Em absorption is determined by
the BCS (Bardeen-Cooper-Schrieffer) coherence factor II,
appropriate for electromagnetic absorption and nuclear spin
relaxation, as contrasted with the factor I with opposite
sign, which is appropriate for sound absorption. |
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Microwave
cavity made from YBaCuO W.J.
Radcliffe, J.C. Gallop, C.D. Langham, M. Gee and M.
Stewart
Summary: The authors report surface
impedance measurements made on a cylindrical cavity resonator
constructed entirely from the sintered high-temperature
superconductor YBa/sub 2/Cu/sub 3/O/sub 7/. The temperature
dependence of the surface impedance has been measured for five
different modes in the frequency range 10-18 GHz and over the
temperature range 4-300 K. Values for the penetration depth,
linear coefficient of expansion, and normal state resistivity
are derived from the data. The ultimate aim of this work is to
produce very high Q cavities, operating far below their
transition temperatures, where the temperature dependences of
superconducting parameters become negligible. Such
superconducting cavity resonators show promise as secondary
frequency standards of the highest stability. The preliminary
results with the first cavity are disappointing from this
point of view. |
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Tunnel
spectroscopy in Nb/BiSCCO point contact
junctions A. Barone, A. Di
Chiara, F. Fontana, G. Paterno, L. Maritato, G. Peluso, G.
Pepe and U. Scotti di Uccio
Summary: Results
concerning Nb/BiSCCO point contact junctions (PCJs) are
presented. I-V (current voltage) and (dV/dI)-V measurements
were performed at various temperatures and contact pressures.
Differential resistance measurements performed by a standard
modulation technique exhibits structures which can be related
to the energy gap of BiSCCO. Features of the derivative plots
are discussed in connection with models proposed previously
for similar junctions. Nb-Nb PCJs based on sintered pellets of
granular Nb have been investigated for
comparison. |
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Microwave
surface resistance of YBa/sub 2/Cu/sub 3/O/sub 7- delta
/ M. Kobayashi, K. Okabe, S.
Tanuma, I. Sankawa, M. Sato, T. Konaka and K.
Ishihara
Summary: The magnetic-field
dependence of the microwave surface resistance of both single
crystals and sintered crystals of YBa/sub 2/Cu/sub 3/O/sub 7-
delta / (YBCO) has been measured at 24 GHz. The resistance
decreases below the superconducting transition temperature and
reaches a constant residual resistance. The value is much
lower for single crystals than for polycrystals. Furthermore,
polycrystals showed magnetoresistance at low field and
magnetoresistance with hysteresis at high field, which
resembles the magnetization behavior of typical type-II
superconductors. The observed magnetoresistance is shown to be
inherent to a sintered polycrystal which is composed of grains
and contains many voids. The single crystal showed lower
magnetoresistance. |
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Hot
electron effect in the DC
SQUID F.C. Wellstood, C. Urbina
and J. Clarke
Summary: The authors
investigated the temperature dependence of the noise in
thin-film DC superconducting quantum interference devices
(SQUIDS) down to 20 mK. The white noise measured in the early
versions of the SQUIDS did not decrease as the bath
temperature was lowered below 150 mK. The authors have
attributed this saturation to a hot electron effect in the
thin-film AuCu resistors shunting the Josephson junctions. A
theoretical investigation showed that the temperature of the
electrons in the shunts should be given by T/sub e/=(P/ Sigma
Omega )/sup 1/5/, where P is the power dissipated in the
shunts, Omega is the shunt volume, and Sigma is a
proportionality constant. Experimentally, the authors found
epsilon =(2.4+or-0.6)*10/sup 9/ WK/sup -5/ m/sup -3/. The
shunts were redesigned, adding large thin-film cooling fins,
to increase their volume substantially. This technique has
reduced T/sub e/ to about 50 mK, with a corresponding
improvement in the sensitivity of the SQUIDS. |
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Noise
in DC SQUIDS with Nb/Al-oxide/Nb Josephson
junctions M.W. Cromar, J.A.
Beall, D. Go, K.A. Masarie, R.H. Ono and R.W.
Simon
Summary: The authors have developed a
process which incorporates very-high-quality Nb/Al-oxide/Nb
Josephson junctions. The junctions had low subgap conductance,
yielding V/sub m/ greater than 50 mV for critical current
densities of 1000 A/cm/sup 2/. Low-inductance SQUIDS
(superconducting quantum interference devices) made with these
junctions were apparently free from junction conductance
fluctuations, at least for frequencies above 1 Hz. A
correlation between low leakage junctions and low noise from
critical current fluctuations was observed. The SQUIDS
exhibited flux noise with an unusual frequency dependence of
currently unknown origin. |
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Low
frequency noise in flux-locked DC
SQUIDS Y. Miki, B. Muhlfelder,
J.M. Lockhart and C.D. Tesche
Summary: The
authors present the results of measurements of the noise of
open-loop and flux-locked DC SQUIDs (superconducting quantum
interference devices) at frequencies from 100 kHz to 10/sup
-4/ Hz. The SQUIDs studied in this work were low-1/f-noise
devices. Measurements were made in DC magnetic fields of
2*10/sup -3/ G using conventional shielding and also in an
ultralow magnetic field environment of 10/sup -7/ G using
expanded superconducting foil shields and a
feedback-controlled slow cooling technique. The open-loop
results are consistent with the earlier work of C.D. Tesche
(1985). Some of the runs in the moderate-background-field
environment (those with reduced shielding) showed a metastable
structure in which switching between the metastable states was
stimulated by electromagnetic interference. A description of
the apparatus used for obtaining the various noise
measurements is provided. |
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Investigation
of noise sources in SQUID
electronics T.R. Clem, M.J.
Goldstein, J.W. Purpura, L.H. Allen, J.H. Claassen, D.U.
Gubser and S.A. Wolf
Summary: The performance
of SQUID (superconducting quantum interference device)-based
electronics may be degraded from that found in laboratory
operation. Investigations on superconducting tubes, wires, and
sheets have been conducted to identify contributions to such
noise. Results have been obtained for bulk and thin-film
samples utilizing both the conventional low-temperature
materials and the new high temperature oxide materials.
Experiments have been conducted to quantify flux
redistribution and flux motion in superconducting samples
subjected to temperature changes, temperature gradients, and
magnetic field gradients. These investigations have been
conducted for magnetic fields typical of many SQUID
applications, with field intensities much smaller than the
critical values H/sub c1/. Penetration-depth, flux-pinning,
and flux-motion effects have been observed. The various types
of experiments conducted along with specific results are
described. |
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Seven-channel
RF SQUID with 1/f noises only at very low
frequencies H. Ohta, M. Takahata,
Y. Takahashi, K. Shinada, Y. Yamada, T. Hanasaka, Y. Uchikawa,
M. Kotani, T. Matsui and B. Komiyama
Summary:
A seven-channel RF SQUID (superconducting quantum
interference device) with anodized short weak links has been
fabricated. The devices exhibit a sensitivity of 23.6 fT/
square root Hz and a 1/f noise onset of 0.05 Hz at a white
noise level of 1.8*10/sup -4/ phi /sub 0// square root Hz. The
intrinsic flux noise is estimated from an observed step
pattern to be 4.9*10/sup -5/ phi /sub 0// square root Hz. It
is shown that Arnold theory explains very well the behavior of
Josephson junctions with a critical current density of around
10/sup 4/ A/cm/sup 2/. |
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Low
noise switching of a superconducting circuit by a laser
induced weak link C.E.
Cunningham, B. Cabrera, D.P. Saroff, J. Price and T.
Stevenson
Summary: The authors have designed
and begun testing a high-speed superconducting switch. The
switches are lines of Nb about 400-A thick, 2- mu m wide, and
2-mm long on a sapphire substrate. The switching is done by
illuminating the line with a laser through an optical fiber.
The photons break Cooper pairs mostly by scattering, since the
film thickness is on the order of the superconductor's
coherence length and the optical penetration depth. The order
parameter and critical current are depressed to zero through
nonequilibrium dynamics, with the film remaining below its
thermodynamic transition temperature. Thus, in principle, the
switching rates are limited by the quasiparticle recombination
time, rather than by the slower time of phonon escape from the
film. The noise seen in the normal state is comparable to that
in the superconducting state, and switching rates up to 300
kHz were used. |
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Reliable
low noise DC-SQUID P. Carelli, V.
Foglietti, R. Leoni and M. Pullano
Summary: A
novel type of multiloop DC SQUID (superconducting quantum
interference device) has been fabricated. The device is made
of six different layers deposited on a 2-in silicon wafer; the
layers are patterned by means of optical lithography. Every
chip is a square 6.35 mm on a side and contains two SQUIDs and
their input coils; the typical coil inductance is about 1 mu
H. The junctions are 2 mu m/sup 2/ planar windows on SiO made
of Nb-NbO/sub x/-PbAuIn with a current density of 500 A/cm/sup
2/ and V/sub m/ approximately=20 mV. The device was repeatedly
cooled and stored in air for six months; after these thermal
cycles no variations in critical current were observed. The
measured mutual inductance between the input coil and SQUID is
2.1 nH, and the SQUID inductance is about 28 pH. The energy
sensitivity referred to the input and in the flux-locked loop
is approximately 1400 h in the white region with a 1/f corner
frequency at 10 Hz. These SQUIDS are already operating in the
gravitational wave experiment of the Rome group in
Geneva. |
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A
DC SQUID amplifier with a novel tuning
circuit T. Takami, T. Noguchi and
K. Hamanaka
Summary: The authors report a
SQUID (superconducting quantum interference device) amplifier
with a novel tuning circuit that can be easily coupled to a
finite-impedance source. The SQUID amplifier is designed for a
source impedance of 50 Omega and a signal frequency of nearly
150 MHz. The performance of the SQUID amplifier has been
measured at 4.2 K. The measured gain and noise temperature of
the SQUID amplifier are 20 dB and 0.7 K, respectively. A
saturation temperature as high as 200 K at the input of the
SQUID amplifier is achieved. It is concluded that this type of
tuned SQUID amplifier is applicable to the IF (intermediate
frequency) amplifier of the SIS
(superconductor-insulator-semiconductor)
receiver. |
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Measured
performance parameters of gradiometers with digital
output D. Drung, E. Crocoll, R.
Herwig, M. Neuhaus and W. Jutzi
Summary: A
planar first-order gradiometer and a DC SQUID (superconducting
quantum interference device) comparator have been integrated
on the same 6*6 mm/sup 2/ chip in Nb-Nb/sub 2/O/sub 5/-PbInAu
technology and operated in a flux-locked loop. Owing to the
digital gradiometer output, a very low flux noise of 7*10/sup
-7/ phi /sub 0// square root Hz, a coupled noise energy of 34
times Planck's constant h, and a gradient noise of 3.8 pT/m
square root Hz have been achieved in the white noise region
above about 60 Hz. In spite of the very low flux noise, a
maximum slew rate of 1.5*10/sup 5/ phi /sub 0//s at a signal
frequency of 4 kHz has been measured. A simple modulation
scheme has been developed to cancel Josephson junction
critical-current fluctuations, yielding 4.5*10/sup -6/ phi
/sub 0// square root Hz at 0.1 Hz. |
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Phase
sensitive heterodyne receivers with SIS quasiparticle
mixers M.F. Bocko, M.J. Wengler
and Z.N. Zhang
Summary: It is noted that it
is possible to operate a mixer as a phase-sensitive linear
amplifier by pumping with two local oscillators (LOs)
separated in frequency by twice the IF (intermediate
frequency) and symmetrically located about the signal
frequency. In principle a phase-sensitive amplifier can detect
a signal without adding any noise. The authors present a
formalism developed to describe two-LO mixers, and extend the
quantum mixer theory to calculate expressions for the gain and
noise of a two-LO mixer. The full quantum mechanical
formulation used here enables an analysis of the mixer noise
for general input states, for example, a squeezed state. It is
demonstrated that a two-LO mixer is phase-sensitive. The
results are pertinent to the design of phase-sensitive
heterodyne receivers with SIS
(superconductor-insulator-superconductor) quasiparticle
mixers. |
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Numerical
computation of two-LO SIS mixer gain and
noise Z.N. Zhang, M.F. Bocko and
M.J. Wengler
Summary: The authors present
numerical calculations of the gain and noise of a two-LO
(local oscillator) mixer based on an ideal SIS
(superconductor-insulator-superconductor) IV curve. They
demonstrate the phase-sensitive response of the mixer with
two-LO pumping and present calculations of the noise for
various DC bias conditions and embedding impedances.
Preliminary results indicate that for certain DC bias
conditions and signal terminations, the noise added to a
signal by the two-LO mixer operated in the single-sideband
mode can be a factor of 25 less than that added by a
conventional single-sideband single-LO mixer. |
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Quasi-particle
and pair current steps of Josephson junctions with radiation
noise in a DC magnetic field W.
Jutzi, E. Crocoll, D. Drung and G. Kramer
Summary:
The current-voltage characteristics of a Josephson
junction approximated by a long interferometer with many point
junctions are simulated under the influence of a local
oscillator at about half the gap frequency for different
oscillator resistances. Without a DC magnetic field, pair
current steps with hysteresis and quasi-particle steps appear
in the same I-V characteristic. With appropriate magnetic
fields, Shapiro steps can be reduced. However, even at the
first and second zeros of the Josephson current, Shapiro steps
of finite size are simulated. Modifications of current steps
are described as a function of local oscillator noise. The
simulated results are in qualitative agreement with
measurements reported in the literature. |
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Numerical
simulation of experimental data from planar SIS mixers with
integrated tuning elements C.A.
Mears, Q. Hu and P.L. Richards
Summary: The
authors used the full Tucker theory including the quantum
susceptance to fit data from planar lithographed
millimeter-wave mixers with bow-tie antennas and integrated RF
coupling elements. Essentially perfect fits to pumped I-V
(current-voltage) curves have been obtained. The deduced
imbedding admittances agree well with those dependently
calculated from the geometry of the antenna and matching
structures. It is found that the quantum susceptance is
essential to the fit and thus to predictions of the mixer
performance. For junctions with moderately sharp gap
structures, the quantum susceptance is especially important in
the production of steps with low and/or negative dynamic
conductance. |
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NbN/MgO/NbN
SIS tunnel junctions for submm wave
mixers J.A. Stern, B.D. Hunt,
H.G. LeDuc, A. Judas, W.R. McGrath, S.R. Cypher and S.K.
Khanna
Summary: The authors report on the
fabrication and testing of all-refractory NbN/MgO/NbN SIS
(superconductor-insulator-superconductor) tunnel junctions for
use as high-frequency mixers. Progress in the development of
techniques for the fabrication of submicron-area tunnel
junctions is described. Junction structures which have been
investigated include mesa, crossline, and edge geometries.
Using reactive sputtering techniques, NbN tunnel junctions
with critical currents in excess of 10/sup 4/ A/cm/sup 2/ have
been fabricated with V/sub m/ values as high as 65 mV and
areas down to 0.1 mu m/sup 2/. Specific capacitance
measurements on NbN/MgO/NbN mesa-type tunnel junctions give
values in the range 60-90 fF/ mu m/sup 2/. These SIS tunnel
junctions have been integrated with antennas and coupling
structures for mixer tests in a waveguide receiver at 207 GHz.
Preliminary mixer results are reported. |
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SNAP
structures with Nb-AlO/sub x/-Nb junctions for MM-wave
receivers A.B. Ermakov, V.P.
Koshelets, I.L. Serpuchenko, L.V. Filippenko, S.V. Shitov and
A.N. Vystavkin
Summary: High-quality
small-area Nb-AlO/sub x/-Nb junctions have been fabricated by
the selective niobium anodization process (SNAP). The
influence of the preparation conditions on the junction
properties were investigated. Numerical calculations based on
I-V (current-voltage) characteristics measured by a data
acquisition system were used to simulate the mixer
performance. It has been shown that the knee-shaped structure
on the I-V curve of refractory material junctions
significantly affects the signal properties of the mixer. The
signal and noise properties for different types of integrated
SIS (superconductor-insulator-superconductor) mixing elements
have been investigated experimentally in the frequency range
of 37-53 GHz; the mixer conversion loss was as low as 4 dB at
45 GHz. The mixing elements consist of an odd number of SIS
junctions connected in series for RF and in parallel for DC
biasing. The DC bias wiring and IF (intermediate frequency)
filters of these structures were designed to provide tuning
out of junction capacitance and thin-film-electrode
inductance. |
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Nb
edge junction process for submillimeter wave SIS
mixers W.C. Danchi, E.C. Sutton,
P.A. Jaminet and R.H. Ono
Summary: The
authors describe a junction fabrication process that produces
high-quality Nb edge junctions of areas less than 0.2 mu m/sup
2/ on thin quartz (0.10 nm) substrates. This process utilizes
a 10:1 projection wafer stepper for high-resolution and
high-accuracy layer-to-layer registration. Large numbers of
junctions were fabricated reliably with high-quality I-V
(current-voltage) characteristics and with impedances suitable
for use in SIS (superconductor-insulator-superconductor)
mixers for submillimeter astronomy. Junctions produced by this
process can be stored on the shelf for more than two years
with no special precautions taken, and with an impedance
change of less than 10%. No failures have been observed after
thermal cycling. Currently these junctions are being used in
an astronomical receiver that demonstrates state-of-the-art
performance in the atmospheric window centered at 345 GHz.
With some improvements to the process, it is possible to make
junctions with areas sufficiently small for omega RC
approximately=3 at 800 GHz. |
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All-NbN
nanobridges as Josephson
mixers T. Matsui, Z. Wang, K.
Hamasaki, T. Yamashita and M. Endo
Summary:
Using RIE (reactive in etching) and lift-off
techniques, all-NbN nanobridges with gap structure in the I-V
(current-voltage) curves have been reproducibly constructed
for testing as millimeter-wave mixers. The nanobridges were
constructed on NbN/MgO/NbN edge junctions. They were
characterized by measurements of their I-V curve and dI/dV
versus V and by mixing experiments at 105.9 GHz. These devices
had high normal-state resistance, and showed almost ideal
Josephson response to external magnetic flux and to
millimeter-wave irradiation. The I-V curve remained
nonhysteretic from T/sub c/ to 4.2 K, and
millimeter-wave-induced steps in the I-V curve were observed
up to approximately 3 mV. The cooling of the NbN nanobridges
was quite good due to their small size and VTB geometry and
the high thermal conductivity of the MgO films. Hence the
Josephson response of these devices to millimeter waves was
comparatively less limited by self-heating. |
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Harmonic
mixing in a series array of short superconducting
weak-links T. Matsui, B. Komiyama
and H. Ohta
Summary: A new type of harmonic
mixer using a series array of short superconducting weak-links
is proposed for mixer applications at far-infrared and
submillimeter-wave frequencies. The problem of limited mixer
dynamic range can be eliminated by the use of a Josephson
junction series array structure. Using a eleven-element series
array, a preliminary experiment on harmonic mixing was carried
out for two signals at 105.9 GHz and 5.4-12.6 GHz. IF
(intermediate frequency) output was measured for each harmonic
order from 9th to 19th by optimizing local power and bias
voltage. In this experiment, 5-dB heightening of the
signal-to-noise ratio was obtained at the IF output when the
harmonic mixing order n was equal to the series element
number, N=11. |
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Submillimeter
wave generation using Josephson junction
arrays K.-L. Wan, A.K. Jain and
J.E. Lukens
Summary: A submillimeter-wave
oscillator using a phase-locked array of Josephson junctions
is demonstrated. With an array of 40 shunted tunnel junctions,
the oscillator delivers 1 mu W of power at 350 GHz into a 60-
Omega load on the chip. The operating frequency of the
oscillator is tunable over a range of about 10%. Experiments
on two coupled lead alloy tunnel junctions show that Josephson
oscillations exist to about 1 THz. Using a similar design and
a higher gap superconductor such as NbN for the transmission
lines, arrays operating at 1 THz with power outputs of 1 mW
should be feasible. |
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Microwave
oscillator using arrays of long Josephson
junctions S. Pagano, R. Monaco
and G. Costabile
Summary: The authors report
on measurements performed on integrated superconducting
devices based on arrays of long Josephson tunnel junctions
operating in the resonant fluxon oscillation regime (i.e.
biased on the zero field steps). The electromagnetic coupling
among the junctions causes a mutual phase-locking of the
fluxon oscillations with a corresponding increase of the
emitted power and a decrease of the signal linewidth. This
phase-locked state can be controlled by means of an external
DC bias current and magnetic field. The effect of the
generated microwave signal has been observed on a small
Josephson tunnel junction coupled to the array via a
microstrip transmission line. The feasibility of the reported
devices as local oscillators in an integrated microwave
Josephson receiver is discussed. |
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Inductive
coupling of a flux-flow type Josephson oscillator to a
stripline K. Yoshida, J. Qin and
K. Enpuku
Summary: A theoretical and
experimental study of the coupling of the flux-flow-type
Josephson oscillator (FFO) to an external stripline has been
carried out to overcome the large impedance mismatch between
them. For efficient coupling and impedance matching at
millimeter and submillimeter wavelengths, the authors have
proposed a scheme where the Josephson oscillator is
inductively coupled to the stripline by a thin superconducting
electrode. Theoretical modeling of the coupling section and
the design of the stripline impedance-transformer have been
carried out. In a preliminary experiment, the authors obtained
a power of about 10/sup -7/ W emitted from the FFO into the
stripline in the frequency range from 184 GHz to 193
GHz. |
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Noise
driven fluctuations of Josephson junction series
arrays P. Hadley, M.R. Beasley
and K. Wiesenfeld
Summary: The shunted
junction model is used to analyze the noise-driven
fluctuations of series arrays of Josephson junctions. The
total voltage across an array of coherently oscillating
Josephson junctions is shown to exhibit two fundamentally
different types of fluctuations, each of which makes its own
characteristic type of contribution to the power spectrum.
Phase fluctuations broaden the peaks in the power spectrum
that correspond to the basic oscillations of the junctions and
are primarily responsible for the linewidth of these
oscillations. Transverse fluctuations contribute
Lorentzian-shaped noise bumps to the power spectrum at the
fundamental and harmonics of the basic Josephson oscillations.
These noise bumps become larger and narrower at T/ beta /sub
c/ increases, making a contribution to the linewidth for large
beta /sub c/ and large bias currents. The form of these
fluctuations is calculated in the limit of small noise, and it
is shown that the fluctuations increase as a dynamical
instability is approached. |
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Anomalous
RF induced steps in resonant Josephson tunnel junctions for
voltage standard maintenance U.
Klein, J.H. Hinken, T. Gunther and D.
Teklenburg
Summary: The authors theoretically
investigated the current steps in the I-V (current-voltage)
characteristics of a long Josephson tunnel junction driven by
microwaves at frequencies near its geometrical resonance. They
used a numerical algorithm based on a lossy transmission line
model of the Josephson tunnel junction to calculate the DC
current density distribution. In addition to the fundamental
microwave frequency, harmonic contents of the tunneling
current are also considered. The calculations show a strong
asymmetric behavior of the current steps with respect to the
quasi-particle curve. It is suggested that the excitation of
higher harmonic waves gives rise to these anomalous
microwave-induced steps. Due to the nonuniform
DC-tunneling-current density distribution, the amplitudes of
these current steps are not usually larger than 10% of the
critical current. Since the critical current in long junctions
is of the order of milliamperes, these amplitudes are
considered relatively large. |
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Modulation
of the penetration depth of Nb and NbN films by quasiparticle
injection E.K. Track, M.
Radparvar and S.M. Faris
Summary: A novel
approach to modulating the inductance of a superconducting
microstrip is described. This approach could be the basis for
numerous practical applications, such as phase-shifters and
high-frequency tuning elements. The physical mechanisms
involved are quasiparticle injection, gap suppression, and
penetration-depth modulation. The authors have investigated
the modulation of the penetration depth of niobium and niobium
nitride films by excess quasiparticle injection. To this
effect, all niobium and all-niobium-nitride SQUID
(superconducting quantum interference device) circuits were
designed and fabricated. These circuits allow quasiparticle
injection into the inductive element of the SQUID. This
injection is achieved by optical irradiation through an
opening in a Nb reflective layer which partially masks the
rest of the circuit or electronic current injection through a
tunnel junction overlaid on the microstrip inductance.
Penetration-depth modulation is achieved with both methods.
The magnitude of the effect varies from 10% to over 200%
change in inductance. |
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Design
of variable phase velocity kinetic inductance delay lines and
their measured characteristics when fabricated by a simple Nb
based process G.K.G. Hohenwarter,
J.S. Martens, J.B. Beyer, J.E. Nordman and D.P.
McGinnis
Summary: A simple design for
superconducting kinetic inductance delay lines is presented.
Delay lines were fabricated with a thin-film process involving
only DC magnetron deposition and anodization of Nb.
Phase-velocity measurements demonstrated wave slowing to less
than 1/150 of the velocity of light in free space with
associated impedances in the ohm range. Phase velocity changes
on lines which were in physical proximity to long Josephson
junctions and normal conducting lines were investigated.
Variations larger than 10% of the equilibrium value were
observed with junction-coupled lines under bias application.
Locally heated lines exhibited changes exceeding a factor of
two. Loss and phase velocity were determined as a function of
bias conditions in both cases. Measured line loss was within
expectations. |
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Superconducting
stripline resonator
performance B.R. McAvoy, G.R.
Wagner, J.D. Adam, J. Talvacchio and M.
Driscoll
Summary: Reliable techniques for
evaluating the microwave properties of superconductors are
essential in providing calibrated data for exchange between
laboratories and for developing practical device designs. The
authors are examining the techniques which utilize microwave
stripline resonators. These resonators provide for the rapid
measurement of microwave parameters in a repeatable fashion
with minimal constraints on processing. Sandwiched microstrip
line resonators are used to compare the performance at 4.2 K
of OFHC copper and superconducting films of Pb, Nb, and
YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) at C-band and X-band.
Typical results for the Nb resonators show a loaded Q/sub L/
of about 8*10/sup 4/ with a transmission insertion loss of 5
dB at 3 GHz. Initial results on a YBCO a-axis film used as a
ground plane in the Nb resonator yield a surface resistance
value of about 10/sup -3/ Omega at 2.8 GHz. Preliminary
results on the phase noise performance of a Nb resonator at
2.9 GHz are presented. |
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Thermal
study of semiconductor-superconductor
hybrids E. Raicu and T. Van
Duzer
Summary: The authors have conducted an
experiment to evaluate the thermal problems that would result
if a GaAs amplifier chip were flip mounted via solder-ball
connections to an otherwise superconducting receiver on a
larger Si chip. The GaAs chip is modeled by a chip carrying a
thin-film resistor, which is used to dissipate the same amount
of heat as the GaAs IF (intermediate frequency) amplifier. The
energy gaps exhibited by Nb tunnel junctions are used to
monitor the temperature in a number of places surrounding the
heat source. The results indicate that thermal effects do not
preclude the use of semiconductor-superconductor hybrid
receivers. The data indicate that for applications involving
power dissipation less than 150 mW (gain of up to 45 dB) on a
chip of area of approximately 0.1 cm/sup 2/, neither switching
of lines nor thermal fluctuations of devices is likely to be a
problem when the devices are at least 1 mm from the edge of
the chip and away from the path of the helium bubbles. The
limit on power dissipation is likely to be temperature
fluctuations rather than excessive temperature. |
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Inductance
calculation system for superconducting
circuits M. Hosoya, E. Goto, N.
Shimizu, N. Miyamoto and Y. Harada
Summary: A
method for calculating the inductance of complicated
three-dimensional superconducting circuits is described. The
current distribution is obtained by assuming that the edge
currents satisfy both Maxwell's and London's equations.
Inductance is calculated from the magnetic energy resulting
from the current. Extrapolations are used to reduce the
computational requirements and to increase the accuracy of the
results. Using the method, a CAD (computer-aided design)
system was developed for superconducting circuits. The
inductance of 3-D superconducting circuits, which was
unattainable except by experiments, can be easily calculated
by this system. The coupling inductance of a DC SQUID
(superconducting quantum interference device) is calculated
using the CAD system. Agreement between experiment and the
computation is good. |
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Dielectric
constant of evaporated SiO at frequencies between 13 and 103
GHz H.K.
Olsson
Summary: An integrated Josephson
tunnel junction and microstrip resonator have been used to
determine the dielectric constant of evaporated SiO. The
method is straightforward in that it uses conventional
microwave techniques to calculate the impedances for different
frequencies and parasitic effects are negligible. A
frequency-independent value of 5.5+or-0.4 was calculated for
the 13- to 103-GHz range. At each resonant frequency a step
appeared at the corresponding voltage in the current-voltage
curve. For each resonant frequency, a dielectric constant was
calculated. The constant does not change appreciably from the
average value of 5.5 throughout the whole frequency range, in
agreement with previous measurements. |
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Effects
of intrinsic stress on submicrometer Nb/AlO/sub x//Nb
Josephson junctions T. Imamura
and S. Hasuo
Summary: The intrinsic stress
and its relaxation process are discussed for sputtered Nb
films used for the electrodes in Nb/AlO/sub x//Nb Josephson
junctions. The stress, optically measured for Nb films,
depends on the Ar pressure during sputtering, and it changes
from compressive to tensile when the Ar pressure is increased.
From X-ray diffraction, the shift in the lattice constant was
observed to be proportional to the film stress. Changes in the
lattice constant were also clearly observed when Nb films were
etched to fine patterns. This suggests that the intrinsic
stress in Nb films is relaxed at peripheral areas after the
patterning process, and that such relaxation is one cause of
deterioration in current-voltage characteristics frequently
observed in small Josephson junctions. This indicates that
stress-free Nb should be used for submicrometer
junctions. |
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A
new self-aligning process for whole-wafer tunnel junction
fabrication M.G. Blamire, J.E.
Evetts and D.G. Hasko
Summary: The authors
have developed a processing method for whole-wafer tunnel
junctions which allows the preparation of planar tunnel
junctions with just two lithographic steps and largely
eliminates the inherent capacitance and potential failure
problems associated with overlap between the base electrode
and the counterelectrode metallization common to all existing
methods. The basic feature of this self-aligning whole-wafer
(SAWW) process is that the pattern used to create the
counterelectrode metallization also defines the junction area.
Results of preliminary trials of this method are presented and
possible future developments discussed. |
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Processing
techniques for refractory integrated circuits
(superconducting) J.X. Przybysz,
R.D. Blaugher and J. Buttyan
Summary:
Processing techniques have been developed to increase
yields and uniformity in superconductor integrated circuits
fabricated with refractory materials. An eight-level process
was used to define a ground plane, ground plane insulator,
Josephson junction base and counterelectrodes, a second
insulator layer, superconductor interconnections, resistors,
and gold contact pads. Every layer, except the gold, was
patterned by reactive ion etching (RIE). A resistor structure
was developed that included an etch stop layer. The formation
of polymers, which occurs with etch gases containing carbon,
was inhibited by the addition of oxygen to the plasma. RIE of
insulator vias was accomplished with a mixture of NF/sub 3/
and Ar that gave good selectivity for silicon dioxide over
niobium. Stress-free films of niobium, molybdenum, and silicon
dioxide were obtained by adjusting the sputtering gas
pressure. Molybdenum resistors, deposited as a top layer, were
trimmed by RIE as a post-testing step to improve circuit
performance. |
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Characterization
of Nb/AlO/sub x/-Al/Nb junction structures by anodization
spectroscopy T. Imamura and S.
Hasuo
Summary: The thin tunneling barrier in
a Nb/AlO/sub x/-Al/Nb Josephson junction was characterized by
anodization spectroscopy. Nb/AlO/sub x/ and Al/Nb interfaces
made by varying certain process parameters were examined. The
interface quality is greatly affected by film thickness, layer
sequence, annealing, and existence of a thin oxide. It is
concluded that anodization spectroscopy is a useful technique
to diagnose the tunneling barrier in the Nb/AlO/sub x/-Al/Nb
Josephson junctions during fabrication
processes. |
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Characteristics
of vertically-stacked planar tunnel junction
structures M.G. Blamire, R.E.
Somekh, G.W. Morris and J.E. Evetts
Summary:
Whole-wafer multilayer tunnel structures consisting of
vertically stacked Al/sub 2/O/sub 3/ tunnel barriers separated
by thin Nb layers have been deposited under UHV (ultrahigh
vacuum) conditions, with repeat distances ranging from 10-50
nm. Using a modification of the conventional SNEP process,
these structures have been fabricated into vertical series
arrays. Using this technique it is possible to select
different numbers of junctions on the same substrate and so
determine the properties of each barrier. Information gained
from such structures provides a considerable insight into the
factors determining junction characteristics such as critical
current density and quality as well as serving as the basis
for the study of novel types of structures and
devices. |
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Niobium
trilayer process for superconducting
circuits J.M. Murduck, J. Porter,
W. Dozier, R. Sandell, J. Burch, J. Bulman, C. Dang, L. Lee,
H. Chan, R.W. Simon and A.H. Silver
Summary:
A Nb-AlO/sub x/-Nb trilayer process was developed that
consistently produces junctions with V/sub m/>40 mV.
Critical current density was controlled reproducibly from 200
to 7000 A/cm/sup 2/ by controlling the oxygen pressure in the
process chamber during thermal oxidation of the aluminum. A
multilayer AlO/sub x/ barrier was used to produce junctions
for Josephson circuit applications requiring low critical
currents (<50 A/cm/sup 2/). A second Al deposition and
oxidation was able to reproducibly control the critical
current density from 6 to 200 A/cm/sup 2/. High quality (V/sub
m/>40 mV) Josephson junctions have been fabricated with low
stress (<1 Gdyne/cm/sup 2/) by an appropriate choice of
niobium deposition rate (8 to 9 AA/s) and argon sputtering
pressure (4 mtorr). |
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Low-loss
lumped-element capacitors for superconductive integrated
circuits M. Bhushan, J.B. Green
and A.C. Anderson
Summary: Low-loss
lumped-element capacitors for superconductive circuits were
fabricated using sputter-deposited Nb electrodes. The
dielectric layer was formed by partially anodizing the Nb base
electrode. The deposition technique for the counterelectrode
strongly affected the parasitic shunt conductance of the
capacitors. It was found that this conductance could be
reduced by depositing the Nb counterelectrode films by DC
magnetron sputtering at a low rate and at a high Ar pressure.
By optimizing these process parameters, capacitors with
breakdown voltages greater than 85% of the anodic oxide
formation voltage and loss tangents less than 0.003 at 10 MHz
were fabricated. These capacitors were integrated with Nb
thin-film inductors to produce L-C resonators with quality
factors greater than 400. |
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A
DC SQUID with intrinsically shunted submicron junctions near
the hysteretic limit exhibiting an extremely large dV/d phi
transfer function E.P. Houwman,
R. Cantor, M. Peters, H.J. Scheer and H.
Koch
Summary: A fabrication process yielding
submicron-scale Josephson junctions has been developed. The
junction consists of two metal striplines lying in line and
separated by a vertical barrier. Thus the contact area is
determined by the width and thickness of the striplines. The
capacitive coupling is only due to the small contact area,
because there is no layer overlap. The process was applied to
all-Nb thin-film junction technology with nitrided Si
barriers. The I-V (current-voltage) curves of these junctions
show the characteristic features of
superconductor-normal-superconductor contacts. DC SQUIDs
(superconducting quantum interference devices) made of these
junctions exhibit characteristics competitive with those of
high-quality tunnel-junction DC SQUIDs. |
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Simple
DC-SQUID system based on a frequency modulated relaxation
oscillator M. Muck and C.
Heiden
Summary: Nanobridges with hysteretic
I-V (current-voltage) characteristics can be operated as
relaxation oscillators when connected to an R-L shunt. A DC
SQUID (superconducting quantum interference device)
configuration results if two such oscillators are incorporated
in a superconducting ring. Operating such a self-oscillating
SQUID (SOS) with a suitable bias current leads to an output
signal of the order of 10 mV, whose frequency depends on the
magnetic flux through the SQUID ring modulo Phi /sub 0/. Due
to this large output voltage, no impedance matching or special
low-noise preamplifier is needed for the readout scheme.
Standard integrated circuits for FM (frequency modulated)
signal processing can be used, leading to matchbox-size
electronics. Such DC SQUIDs have been made of thin Nb, NbN,
and Nb/sub 3/Ge films. The highest sensitivity achieved so far
is Nb devices with a white flux noise below 10/sup -5/ Phi
/sub 0// square root Hz at 4.2 K. |
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Low
noise operation of a DC SQUID in a large beta regime and its
application to the design of a high T/sub c/
SQUID K. Enpuku, S. Kohjiro and
K. Yoshida
Summary: The noise characteristics
of a large- beta DC SQUID (superconducting quantum
interference device) have been studied experimentally in the
case where the SQUID inductance is resistively shunted. Energy
resolutions of the SQUIDs, epsilon , have been measured for
various beta values up to beta =19. It is shown experimentally
that the energy resolution of these large- beta SQUIDs is as
small as that of the conventional SQUID with beta =1. For
example, an energy resolution of 25h was obtained for beta =12
(L= 340 pH and I/sub 0/=37 mu A), where h is Planck's
constant. The experimental results agree quantitatively with
theoretical ones. It is also shown that the present
resistivity shunted inductance scheme is very useful in
developing SQUIDs operating at T=77 K with high-T/sub c/
superconductors, where the value of beta will necessarily be
large in order to avoid noise effects. Numerical simulations
of the performance of the high-T/sub c/ SQUID are
presented. |
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Current
amplifier and flux-buffer designs using an exponential flux
shuttle with a Josephson junction synthetic
inductor M.
Gershenson
Summary: A current amplifier
design based on the principle of fluxon propagation in a
multijunction exponential flux shuttle has been investigated.
In this design, the critical current of the junctions is
increased exponentially and the SQUID (superconducting quantum
interference device) inductance is a Josephson-junction
equivalent inductance. Current gain can be achieved by
generating fluxons at the low end and dissipating them at the
high end where the load is located. Advantages over other
types of linear devices are discussed. Two parallel
exponential flux shuttles can be used to duplicate flux from a
high-inductance input coil to a low-inductance output. The
device performance of the two circuits is evaluated by
computer simulation, and noise performance is
discussed. |
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DC-SQUID
magnetometer system for detecting vector value of magnetic
field M. Nakanishi, N. Kasai, H.
Kado and M. Koyanagi
Summary: A vector
magnetometer system for biomagnetic applications is described.
The system consists of two vector magnetometers, each
comprising three integrated SQUID (superconducting quantum
interference device) magnetometers located perpendicularly to
each other to detect three orthogonal components of the
magnetic field simultaneously. The resolution of each
orthogonal component is 11 fT/ square root Hz in the write
noise region. This system has been used in
magneto-cardiography and for measuring the spontaneous
magnetic field from the human brain. |
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An
integrated DC SQUID gradiometer for biomagnetic
application M. Koyanagi, N.
Kasai, K. Chinone, M. Nakanishi, S. Kosaka, M. Higuchi and H.
Kado
Summary: A first-order off-diagonal
gradiometer was fabricated and tested. The gradiometer
consisted of two field pickup coils and a planar DC SQUID
(superconducting quantum interference device) with two
superconducting loops connected in parallel, two multiturn
input coils, and a modulation-feedback coil. The size of the
pickup coils was 6*6 mm/sup 2/ with a base line of 8 mm. The
overall size of the gradiometer was 15*7.5 mm/sup 2/. The
resolution of the magnetic field gradient of the gradiometer
increased from 11 to 1.8 pT/m square root Hz (the noise
limits), corresponding to the frequencies from 1 Hz to 600 Hz.
The resolution became nearly white in a frequency range above
600 Hz. The intrinsic balance of the gradiometer was better
than 1000 p.p.m. for the field perpendicular to its
plane. |
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Experimental
localization ability of planar gradiometer systems for
biomagnetic measurements A.C.
Bruno, V. Pizzella, G. Torrioli and G.L.
Romani
Summary: The localization ability of
SQUID (superconducting quantum interference device) planar
gradiometers for biomagnetic applications has been
investigated. A linear planar superconducting-wire gradiometer
was constructed and tested. Localization tests were performed
both on a current dipole model and on human subjects. It is
shown that the accuracy of localization results was not
significantly affected by the use of the planar
configuration. |
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Thermal
activation and macroscopic quantum tunneling in a DC
SQUID F. Sharifi, J.L. Gavilano
and D.J. Van Harlingen
Summary: The authors
report measurements of the transition rate from zero-voltage
metastable minima in the two-dimensional potential of a DC
SQUID (superconducting quantum interference device) as a
function of applied flux and temperature. A crossover is
observed from thermally activated escape to macroscopic
quantum tunneling at a critical temperature that is a strong
function of the bias flux. The macroscopic quantum tunneling
rate is substantially reduced by damping, which also broadens
the crossover region. The authors unexpectedly observed
thermal rates that are significantly suppressed from those
predicted by the classical two-dimensional thermal activation
model, as if the potential barrier for activation were
effectively enhanced. They discuss possible explanations for
this result, based on the interaction of the macroscopic
degrees of freedom in the device and energy level quantization
effects. |
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Relaxation-oscillation-driven
DC SQUIDs S.A. Gudoshnikov, Y.V.
Maslennikov, V.K. Semenov, O.V. Snigirev and A.V.
Vasiliev
Summary: Simple and reliable
relaxation-oscillation (RO)-driven DC SQUIDs (superconducting
quantum interference devices) have been fabricated with a
flux-to-voltage factor H up to 1 mV/ Phi /sub 0/ and internal
energy sensitivity E/sub v/ of (4+or-1) * 10/sup -31/ J/Hz at
frequencies above 10 Hz and 1*10/sup -29/ (1 Hz/f) J/Hz at
lower frequencies. Supplied with a 50- mu m-wire 20-turn
spiral input coil and a 6-cm/sup 2/ pickup loop, such SQUIDs
provided magnetic field sensitivity close to (6+or-3)*10/sup
-15/ T/Hz/sup 1/2/ above 10 Hz. The dynamic range of the
magnetometers was close to 1.5*10/sup 7/ Hz/sup 1/2/ above 10
Hz. The maximum slew rate was about 10 mu T/s at 0.1 kHz.
Successful records of Earth-ionosphere magnetic resonances
have been obtained with these magnetometers housed in a
stainless steel transport dewar. A balanced version of the
RO-driven DC SQUID is suggested with improved H and E/sub
v/. |
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DC
SQUID preamplifier for DC-SQUID
magnetometer V.P. Koshelets, A.N.
Matlashov, I.L. Serpuchenko, L.V. Filippenko and Y.E.
Zhuravlev
Summary: A DC SQUID
(superconducting quantum interference device) magnetometer
with DC SQUID preamplifier has been designed and
experimentally tested. The total magnetometer noise was
reduced to the intrinsic noise level of the input DC SQUID,
and magnetometer cryogenic containment was minimized by using
the DC SQUID preamplifier. Two identical thin-film DC SQUIDs
with 15- mu m/sup 2/-area resistively shunted Nb-AlO/sub x/-Nb
junctions were utilized. The resulting magnetometer had a
coupled energy sensitivity equivalent to 3*10/sup -31/ J/Hz in
the range from 1 Hz to 30 kHz and about 100-dB dynamic
range. |
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Picosecond
magnetic spectroscopy with integrated DC
SQUIDs D.D. Awschalom and J.
Warnock
Summary: Advanced VLSI (very large
scale integration) technology was used to fabricate
ultraminiature integrated SQUID (superconducting quantum
interference device) susceptometers. With the appropriate
design parameters, the sensitivity of the devices approaches
the quantum limit. The use of integrated circuits in
conjunction with pulsed optical techniques allows magnetic
systems to be probed with a picosecond time resolution. The
response can be mapped out as a function of the energy of the
optical excitation, providing detailed spectroscopic
information. Applying these techniques to the study of II-VI
dilute magnetic semiconductors has yielded new insight into
the mechanics of magnetic polaron formation and the dynamics
of the magnetic spins. First experiments were carried out on a
small approximately 10*10*1- mu m/sup 3/ single crystal
platelet of Cd/sub 0.8/Mn/sub 0.2/Te. The results of the
time-averaged magnetic spectroscopy at two different
temperatures are presented, displaying the magnetic response
to optical excitation at constant intensity from a photon
energy of 1.83 to 2.0 eV. |
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Nuclear
magnetic resonance with DC SQUID
preamplifiers N.Q. Fan, M.B.
Heaney, J. Clarke, D. Newitt, L.L. Wald, E.L. Hahn, A.
Bielecki and A. Pines
Summary: Describes five
experiments illustrating the application of DC SQUID
(superconducting quantum interference device) amplifiers to
magnetic resonance experiments. The first experiment involved
the observation of nuclear spin noise, that is the spontaneous
emission of photons from an ensemble of /sup 35/Cl nuclei in
the zero polarization state. The second experiment involved
the use of the Q-spoiler in conventional NQR (nuclear
quadrupole resonance) and NMR measurements in which one
applies a large RF pulse to the nuclei to make them precess.
The Q-spoiler was then used in an experiment to detect the
oscillating electric polarization induced by /sup 35/Cl
nuclear quadrupole moments. The fourth experiment involved the
extension of the use of the Q-spoiler and SQUID amplifier to
NMR, detecting the signal from /sup 119/Sn nuclei at 30 MHz.
Finally, a SQUID amplifier was used with an untuned input
circuit to detect the low-frequency NMR signal at 55 kHz from
/sup 195/Pt nuclei in an applied field of 60
Gauss. |
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A
comparison of the performance of planar and conventional
second-order gradiometers coupled to a SQUID for the NDT of
steel plates S. Evanson, R.J.P.
Bain, G.B. Donaldson, G. Stirling and G.
Hayward
Summary: The authors have developed a
DC magnetic NDT (nondestructive testing) technique using a
SQUID (superconducting quantum interference device) coupled to
a planar gradiometer for the inspection of structures such as
flat steel plates and steel pipes. Design criteria for the
gradiometer are presented and used to demonstrate the superior
performance of the planar approach compared to that of a
second-order axial gradiometer of conventional design. The
authors show an example of the use of system for mapping the
magnetic field above a steel plate containing ideal defects.
The proposed SQUID system is shown to operate normally in an
unscreened laboratory close to a steel specimen in the
presence of a DC magnetizing field in excess of 20
mT. |
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Use
of a superconductive gradiometer in an ultrasensitive
electromagnetic metal
detector P.V. Czipott and W.N.
Podney
Summary: The authors present an
electromagnetic gradiometer that uses a SQUID (superconductor
quantum interference device) sensor as the receiver in an
active, electromagnetic detector of nonferrous as well as
ferrous objects. The gradiometer pickup loops sit in the
center of magnetic coils that generate a time-varying magnetic
field inducing eddy currents in conductive bodies. The
gradiometer measures the secondary magnetic field of the eddy
currents. The SQUID's sensitivity at frequencies below 1 kHz
makes electromagnetic metal detectors practical in the marine
environment, where the electrical conductivity of seawater
precludes the use of conventional systems. The authors
describe a prototype system that attains a detection range of
10 m in seawater for targets 50 cm in diameter. It operates at
frequencies from 1 Hz to a few hundred Hz. Uses of the
electromagnetic gradiometer include locating naval mines
buried in sea bottom sediments and hunting undersea treasure.
The system's response to the seawater itself enables
application to airborne electromagnetic bathymetry. Over land,
its sensitivity to crustal conductivity contrasts makes it
suitable for airborne mineral exploration. |
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Status
of Stanford magnetic monopole search with eight-loop
superconducting detector M.E.
Huber, B. Cabrera, M.A. Taber and R.D.
Gardner
Summary: A model has been developed
which includes all significant inductive coupling of the
Stanford monopole detector pickup loops and allows variation
of the individual inductances to optimize the fit to the data.
With this model, one can calculate the response of the
detector to monopoles, with allowances for the inoperative
panels and discrepancies in the inductances. The detector has
been in operation for approximately 450 days at this time. Of
this, there are 6600 hours of computer data, and 6000 of these
hours meet the criteria for active sensing time. No candidate
monopole events have been observed. The exposure to date
represents a limit on the flux of cosmic ray magnetic poles of
7.8*10/sup -13/ cm/sup -2/ s/sup -1/ sr/sup -1/ at 90%
confidence level. |
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Design,
fabrication, and performance of integrated miniature SQUID
susceptometers M.B. Ketchen, D.D.
Awschalom, W.J. Gallagher, A.W. Kleinsasser, R.L. Sandstrom,
J.R. Rozen and B. Bumble
Summary: The design,
construction, and performance of miniature SQUID
(superconducting quantum interference device) susceptometers
is discussed. Spins (in units of mu /sub beta /) per square
root Hz has been identified as an important figure of merit.
Simple expressions for S/sub n/ (spin sensitivity) in
miniature SQUID susceptometers are developed and the
implications of dimensional scaling explored. The details of
several existing and proposed designs are reviewed, including
versions that utilize commercial SQUIDs. With thin-film DC
SQUIDs, S/sub n/ values of a few thousand spins/ square root
Hz have already been obtained, and it is projected that values
of a few hundred will be achieved soon. |
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SQUID
detection of electronic
circuits R.L.
Fagaly
Summary: The author reports on the use
of a SQUID (superconducting quantum interference device)
magnetometer to detect audio-frequency timing signals from
electronic and electromechanical clock circuits. Narrowband
spectrum analysis was used to determine the quality factor (Q)
in one case. Magnetic field maps indicated that the source of
the magnetic fields could be localized to better than 1 mm.
X-rays showed the presence of circuit elements in the region
of the magnetic field maxima. Interpretation of the field
sources as magnetic dipoles yielded m/sub 2/ values ranging
from 10/sup -6/ Am/sup 2/ to 10/sup -8/ Am/sup 2/. The
implication of this work is that SQUID magnetometers have
potential use in noncontact detection of timing signals of
timing circuits and microprocessors. |
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Spatial
deconvolution algorithm for superconducting planar gradiometer
arrays A.C. Bruno and P.C.
Ribeiro
Summary: A digital filter model used
previously to describe axial gradiometers is used to study
arrays of planar first-order gradiometers. As an application
of this technique, a spatial deconvolution algorithm is
developed to recover the input field from the output
measurements. The influence of parameters such as source depth
and the gradiometer array density is investigated by means of
computer simulations. With properly designed array the input
flux was recovered with errors between 3% and 12%. This
provided a contour plot quite similar to the one that would be
obtained if the source were detected with a single-loop coil
array. |
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Submicron
NbN Josephson tunnel junctions for digital
applications M. Aoyagi, A. Shoji,
S. Kosaka, H. Nakagawa and S. Takada
Summary:
Submicron NbN/MgO/NbN Josephson tunnel junctions for
Josephson integrated circuits were investigated. The junctions
have been fabricated by the cross-line patterning (CLIP)
method with an electron-beam (EB) direct-writing technique.
The all-refractory fabrication process for logic circuits
using the CLIP method is presented. This process is applied to
the fabrication of a logic gate of 4JL containing 0.8- mu
m-square junctions as an example of digital application. A
logic gate has been fabricated by this process, and its
characteristics are discussed. |
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RF-sputter-deposited
magnesium oxide films as high-quality adjustable tunnel
barriers J.C. Villegier, M.
Radparvar, L.S. Yu and S.M. Faris
Summary:
High-quality RF-sputtered MgO films as tunnel barriers
to fabricate small-area nitride tunnel junctions. A magnetism
oxide barrier deposited as a single layer or as a multilayer
film results in devices with similar characteristics.
Annealing trilayers at temperatures in excess of 250 degrees C
for several hours decreases junction current density and
improves device quality, presumably by increasing barrier
heights through reducing resonant tunneling states. A
self-aligned process utilizing only two mask levels is used to
produce junctions as small as 0.5 mu m/sup 2/ with excellent
critical current uniformity. These junctions exhibit energy
gaps of 5.1 mV and low subgap currents at current densities in
excess of 1000 A/cm/sup 2/, which make them suitable for a
variety of applications such as SIS
(superconductor-insulator-superconductor) mixers and logic
circuits. |
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Properties
of stacked NbN tunnel
junctions H. Hedbabny and H.
Rogalla
Summary: Stacks of up to three tunnel
junctions were fabricated using the NbN-MgO technique.
Different preparation methods were tested, and two gave
favorable results. In the first one, the whole stack is
prepared in situ and structured afterwards by lift-off and
reactive and argon ion etching. The authors investigated the
resulting I-V (current-voltage) characteristics of stacks of
8.3 mV. Since it was not possible to establish electrical
connections to the intermediate electrodes by this method, a
second one was applied in which each NbN/MgO layer is prepared
in a separate step and structured by lift-off. Here the I-V
characteristics, the interaction between the tunnel junctions,
and their RF properties were investigated. Shapiro steps and
photon-assisted tunneling were observed in the I-V
characteristics of a receiver junction, while the bottom
tunnel junctions were used as microwave
generators. |
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NbN
based Josephson junctions with silicon barriers deposited at
700/spl deg/C E.J. Cukauskas and
W.J. Carter
Summary: The authors have
fabricated NbN/Si/Nb tunnel junctions where the barriers were
deposited at substrate temperatures to 700 degrees C. The
device characteristics for those tunnel junctions with V/sub
m/ (figure of merit) values exceeding 15 mV were studied as a
function of the barrier deposition temperature. A reduced NbN
energy gap was observed for barriers deposited at high
substrate temperatures, possibly due to an interaction of the
silicon with the NbN or an off-stoichiometric surface
resulting from the high temperature sputter etch of the base
electrode. The zero-bias conductivity as a function of
temperature was measured for thick barriers and found to
follow an activated conduction process at the high
temperatures and Mott hopping conduction at the lower
temperature. Some initial measurements on germanium barriers
deposited at 250 degrees C were made and found to follow a
similar behavior except at a much lower temperature, possibly
due to a lower activation energy. |
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NbN
edge junction fabrication: edge profile control by reactive
ion etching X.F. Meng, R.S. Amos,
A.W. Lichtenberger, R.J. Mattauch and M.J.
Feldman
Summary: In order to fabricate NbN
edge junctions with low parasitic capacitance, an insulator
with low dielectric constant, such as SiO/sub 2/, must isolate
the base and counterelectrode except on the edge. Reactive ion
etching was used to cut an edge in SiO/sub 2//NbN bilayer
films. For this process it is necessary that SiO/sub 2/ be
etched more rapidly than NbN to form a suitably sloped edged
profile. The authors investigated the influence of etching gas
composition and other parameters on etching rates and edge
profiles, using CF/sub 4/, CF/sub 4//O/sub 2/, CF/sub
4//CH/sub 4/, CF/sub 4//CHF/sub 3/, and CHF/sub 3/. It was
found that CF/sub 4/ and CF/sub 4//O/sub 2/ plasma etching
generally yields poor, undercut edge profiles. However,
satisfactory edge profiles were obtained with the other three
gas combinations. The edge angle can be controlled by changing
the proportions of the gases. Using this process, the authors
have successfully fabricated NbN/oxide/PbBi edge junctions
with <1 mu m/sup 2-/ area by standard optical
photolithography. |
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The
effects of ion gun beam voltage on the electrical
characteristics of NbCN/PbBi edge
junctions A.W. Lichtenberger,
M.J. Feldman, R.J. Mattauch and E.J.
Cukauskas
Summary: The authors have succeeded
in fabricating high-quality submicron NbCN edge junctions
using a technique which is commonly used to make Nb edge
junctions. A modified commercial ion gun was used to cut an
edge in SiO/sub 2//NbCN films partially covered with
photoresist. An insulating barrier was then formed on the
exposed edge by reactive ion beam oxidation, and a
counterelectrode of PbBi was deposited. The electrical quality
of the resulting junctions was found to be strongly influenced
by the ion beam acceleration voltages used to cut the edge and
to oxidize it. For low ion beam voltages, the junction quality
parameter was as high as V/sub m/=55 mV (measured at 3 mV),
but higher ion beam voltages yielded strikingly poorer quality
junctions. In light of the small coherence length of NbN ( xi
approximately=3 nm), the dependence of the electrical
characteristics on ion beam voltage is presumably due to
mechanical damage of the NbCN surface. In contrast, for
similar ion beam voltages, no such dependence was found for
Nb( xi approximately=30 nm) edge junctions. |
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Fabrication
of Nb/Al-Al/sub 2/O/sub 3//Nb junctions with extremely low
leakage currents A.W.
Lichtenberger, C.P. McClay, R.J. Mattauch, M.J. Feldman, S.
Pan and A.R. Kerr
Summary: Nb/Al-Al/sub
2/O/sub 3//Nb trilayer films were deposited using DC magnetron
sputtering guns in a UHV (ultrahigh vacuum) system which is
capable of 5*10/sup -10/ Torr. SIS
(superconductor-insulator-superconductor) junctions as small
as 3.2*3.2 mu m/sup 2/ were isolated from the trilayer by
standard photolithography. The junctions typically have V/sub
m/=70-90 mV at 4.2 K, while at 2.0 K, V/sub m/ is as large as
1 V. This corresponds to a subgap current of 0.15% of the
quasiparticle current rise. The subgap leakage current is
compared to the predictions of the BCS
(Bardeen-Cooper-Schrieffer) theory. The specific capacitance
is preliminarily measured to be 45+or-5 fF/ mu m/sup
2/. |
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Supercurrent
nonuniformities by ion implantation in niobium based Josephson
junctions C. Camerlingo, B.
Ruggiero, M. Russo and E. Sarnelli
Summary:
Nitrogen ion implantation was used to induce local
modifications of the superconductive properties of the niobium
base layer in Nb-NbO/sub x/-Pb Josephson tunnel junctions. A
local depression of the niobium critical temperature and
energy gap turns into a nonuniform critical current density
profile J/sub c/ in the sample. Different profiles have been
obtained by modifying the implanted region geometry and the
nitrogen ion concentration. The external magnetic field
dependence of the maximum DC Josephson current has been used
as a diagnostic tool to obtain a preliminary characterization
of the shape of J/sub c/ in a small sample. Preliminary
investigations on long junctions are also
reported. |
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Processing
of thick-film dielectrics compatible with thin-film
superconductors for analog signal processing
devices S.C. Wong, A.C. Anderson
and D.A. Rudman
Summary: The authors
demonstrate the feasibility of integrating thick-film
dielectrics with thin-film superconductors in the fabrication
of signal processing devices. Thin films of Nb and NbN are
deposited by RF sputtering and patterned
photolithographically. Thick film dielectrics can be spun on
to a controlled thickness of 10 to 25 mu m, which is an
optimum range for high-density, low-loss microwave delay
lines. After an organic-binder burnout step at 450 degrees C
in air, the ceramic dielectrics are sintered onto the NbN thin
films at 850 degrees C in vacuum. The transition temperature
of the NbN changes from 15 K as sputtered to 9 K after
sintering, but superconductivity is preserved by the presence
of thin-film MgO and SiO/sub 2/ barrier layers. Lumped-element
resonators, in which the inductor and the capacitor electrodes
are made of NbN and the dielectric of thick-film ceramic, have
been successfully fabricated for measuring the dielectric
properties of the ceramics at 4.2 K and 10 MHz. Preliminary
results indicate a dielectric constant of 6-8 and tan delta of
10/sup -2/. |
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Experimental
results of a vortex flow transistor
amplifier D.P. McGinnis, G.K.G.
Hohenwarter, M. Ketkar, J.B. Beyer and J.E.
Nordman
Summary: A niobium-based
superconducting amplifier consisting of a series array of 60
vortex flow transistors (VFT) was fabricated and tested. Each
VFT consisted of a long Josephson junction biased in the flux
flow state, magnetically coupled to a current based on a
distributed amplifier configuration. The transresistance of
the amplifier in a 50- Omega environment for frequencies up to
1 MHz was measured. The transresistance of the array was less
than the expected value calculated from measurements obtained
on a single VFT. The difference was probably due to
nonuniformities in the fabrication process, which caused some
of the junctions not to be biased in the flux flow state or
receive the same amount of magnetic field from the control
line. However, measurements of the array showed that a large
number of VFTs could be connected in series and simultaneously
be biased into the flux flow state. |
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A
modified superconducting current injection transistor and
distributed amplifier design D.P.
McGinnis, J.B. Beyer and J.E. Nordman
Summary:
The authors have modified the design of the
superconducting current injection transistor (Super-CIT) for
use in a distributed amplifier. The super-CIT consists of a
resistively damped long Josephson junction in which the
maximum Josephson current is modulated by the magnetic field
produced by current in a control line. The device is modified
by DC-coupling the control line from the Josephson junction so
that the Super-CIT can be embedded in a distributed amplifier
configuration. The distributed amplifier design is used to
raise the input impedance and the power gain of the amplifier
over a wide bandwidth. The small signal current gain of the
device can be increased by biasing the Josephson junction in
an asymmetric configuration. A simulation using practical
values results in an amplifier with an input impedance of 6.5
Omega using only four devices, which produces 15 dB of gain
into a 2.5- Omega load over a bandwidth of 90 GHz with very
little reverse gain. |
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Performances
of a flux-flow-type Josephson
amplifier T. Hashimoto, K. Enpuku
and K. Yoshida
Summary: The authors have been
developing an analog amplifier utilizing a flux flow phenomena
in a Josephson line, i.e. the flux-flow-type Josephson
amplifier (FFA). The FFA converts a signal current I/sub s/ to
a voltage V/sub dc/ across the Josephson line, and the I/sub
s/-V/sub dc/ characteristic determines the performance of the
FFA, e.g. the gain and saturation level. Studies have been
made to improve the I/sub s/-V/sub dc/ relation. It is shown
that the transresistance can be improved significantly by
reducing the width of the Josephson line and increasing the
kinetic inductance of the line. The kinetic inductance can be
increased by thinning the line electrode and by using an
electrode with large London penetration depth, such as NbN.
The transresistance obtained is as large as 6.8 Omega for
NbN/Pb-alloy Josephson lines with a thickness of 5 nm and a
width of 4 mu m. It is also shown that the linear operating
range of the amplifier is as large as 1 mV. |
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Experiment
of filed-effect transistor with a channel made of NbN granular
thin film M. Sugahara, N.
Yoshikawa and T. Murakami
Summary: The
authors describe an experimental study of the effect of an
electric field on a granular superconducting thin film using a
three-terminal device which has the structure of a
field-effect transistor, with the channel made of NbN granular
thin film. When the gate voltage is varied at low temperature,
the channel resistance shows a cyclical variation of more than
one cycle. The maximum resistance variation observed at 4.2 K
is about 40% . The voltage period for the resistance variation
seems to correspond to the induction of the electric pair
charge, 2e, on a grain. It is concluded that this result is
due to a new microscopic quantum effect dual to the Josephson
effect and could lead to a novel three-terminal superconductor
device. |
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Prospects
for proximity effect superconducting
FETs A.W. Kleinsasser and T.N.
Jackson
Summary: The authors address the
question of how a JOFET (Josephson field effect transistor)
works and whether or not a useful device of this type is
feasible. They show that JOFETs represent a limiting case of
normal FETs in which superconducting source and drain contacts
result in zero channel resistance. This results in improved
device characteristics, but does not have a major effect on
speed or power dissipation. With conventional superconductors,
the input and output voltage scales are incompatible, but the
authors describe the extent to which this can be overcome by
using high-temperature superconductors, making possible
voltage gain exceeding unity. Device length and operating
temperature are constrained by the natural length scale for
the penetration of superconductivity into normal materials, so
that submicrometer device dimensions and operating
temperatures less than approximately 77 K are required.
Silicon is effectively ruled out in favor of III-V materials
for device operation above approximately 20 K. |
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Noise
model for the superconducting-base semiconductor-isolated
transistor A. Davidson and D.J.
Frank
Summary: A simple noise model for
bipolar transistors is adapted to the superconducting base
semiconductor isolated transistor (SUBSIT). The model includes
two statistically independent shot noise current sources in a
lumped element model. For the case where the emitter of the
SUBSIT is a normal metal, it is shown that the bipolar model
carries over intact. For a SUBSIT with a superconducting
emitter as well as base, the noise performance improves
tremendously, due to the sharper turn-on characteristic. Noise
temperatures in the milli-Kelvin regime are possible, and the
energy resolution is likely to be
quantum-limited. |
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Gap
measurement in Nb/NbO/sub x//Al injection controlled planar
three terminal devices K.
Takeuchi and Y. Okabe
Summary: The
characteristics of Nb/NbO/sub x//Al injection-controlled
planar three-terminal superconducting devices
(injection-controlled links) have been studied experimentally.
A built-in Nb/AlO/sub x//Nb Josephson junction is used to
perform direct tunneling measurement, whose results show that
the gap in the Nb strip decreases as its critical current is
suppressed. It is also shown that the static characteristics
of the devices with high and low injector resistance can be
fitted to a simple heating model and a modified heating model,
respectively. |
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The
circuits of phase-quantum-tunneling
device N. Yoshikawa, M. Sugahara
and T. Murakami
Summary: The authors analyze
the dynamics of logic gates and a discrete transmission line
composed of PQT (phase-quantum-tunneling) junctions, each of
which has characteristics completely dual to those of the
Josephson junction. A PQT transistor which is represented as a
series connection of two PQT junctions has a periodic
threshold-voltage-charge relation with a period 2e. It is
shown that it is possible to construct a PQT inverter and
other PQT logic gates similar to the FET logic gates. The PQT
logic gates have high performance (high switching speed and
low power dissipation) even in high-temperature operation.
Charge-soliton logic circuits can be formed using discrete PQT
transmission lines. It is concluded that PQT devices are
highly suitable for high speed electronics using high-T/sub c/
superconductors. |
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Superconductor
electronics: new prospects K.K.
Likharev, V.K. Semenov and A.B. Zorin
Summary:
The authors review research on superconductance
electronics carried out at Moscow State University. It is
shown that the utilization of high-T/sub c/ superconductors
allows cheaper but hardly more perfect electronic components
and devices, so that the discovery of these superconductors is
expected to have the largest impact on the practical use of
the simplest components, such as EMF (electromagnetic field)
shields, microwave modulators, SQUIDs (superconducting quantum
interference devices), and samplers. Superconducting LSI
(large scale integrated) circuits, including multijunction
voltage standards, A/D (analog/digital) and D/AS
(digital/analog) converters, and digital signal processors,
although less affected by the new materials, nevertheless are
expected to benefit from other recent ideas and results, in
particular, the invention of the RSFQ (resistive or rapid
single flux quantum) logic circuit family and the discovery of
correlated single-electron tunneling. |
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NbN/MgO/NbN
Josephson tunnel junctions fabricated on thin underlayers of
MgO G.L. Kerber, J.E. Cooper,
R.S. Morris, J.W. Spargo and A.G. Toth
Summary:
Underlayers of MgO as thin as 8.0 nm have been used in
the fabrication of NbN/MgO/NbN Josephson tunnel junctions.
NbN/MgO/NbN trilayers with and without MgO underlayers were
deposited on thermally oxidized Si substrates at 100 degrees C
using RF magnetron sputtering in a semi-UHV (ultrahigh vacuum)
load lock vacuum system. Sputtering parameters were first
optimized to produce NbN with T/sub c/=14.7 K on SiO/sub 2//Si
substrates, and then thin MgO underlayers were used to enhance
the T/sub c/ of the trilayers to 15.7 K. X-ray diffraction of
NbN films indicates that thin MgO underlayers of 8.0 nm are
capable of almost completely removing the NbN [111]
diffraction peak found in lower T/sub c/ films and enhancing
the NbN [200] peak. MgO underlayers were found to be oriented
in the [100] direction when sputtered in an atmosphere of Ar
and N/sub 2/ and randomly oriented when sputtered in Ar alone.
The authors present details for the preparation and analysis
of NbN and MgO films as well as the fabrication and electrical
performance of tunnel junctions with and without MgO
underlayers. |
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The
observation of the AC Josephson effect in a YBa/sub 2/Cu/sub
3/O/sub 7//Au/YBa/sub 2/Cu/sub 3/O/sub 7/
junction B. Schwartz, P.M.
Mankiewich, R.E. Howard, L.D. Jackel, B.L. Straughn, E.G.
Burkhart and A.H. Dayem
Summary: The authors
have demonstrated proximity effect coupling between a
high-transition-temperature superconductor and a normal metal.
In a device with a 1- mu m long gap in a YBa/sub 2/Cu/sub
3/O/sub 7/ film spanned by an Au shunt, the authors observed a
DC supercurrent and the AC Josephson effect under microwave
irradiation from 2 GHz to 15 GHz. Preliminary work has also
begun with Ag shunts. It is concluded that these high quality
S-N (superconductor-normal) interfaces should be applicable
both to probing the superconducting state in oxide
superconductors and to building high-T/sub c/ electronic
devices. |
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Low
temperature synthesis of Y-Ba-Cu oxide films and preparation
of DC-SQUIDs M. Matsuda, A.
Matachi and S. Kuriki
Summary: The authors
have synthesized superconducting Y-Ba-Cu oxide thin films by
RF sputtering at a relatively low substrate temperature (about
650 degrees C). Films of 300-700 nm thickness have Tc of 60-72
K without annealing after the deposition. A preferential
orientation with a c-axis perpendicular to the surface was
observed. DC SQUIDs (superconducting quantum interference
devices) having two parallel microbridges with constriction of
submicrometer to 40- mu m width were fabricated from the oxide
films by chemical etching. The microbridges with greater than
2- mu m-width did not operate as Josephson devices. The
characteristics of these SQUIDs suggested vortex flow.
Periodic modulation of the voltage with magnetic flux was
observed in a SQUID having submicrometer microbridges.
Telegraph-like noise of the voltage which exhibits switching
between two or more discrete levels was observed in all of the
SQUIDs irrespective of the width of the microbridges. This
noise was also observed in a single microbridge, and its
effective switching rate increased exponentially with the
increase of temperature above 15 K. |
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Characterization
of a high temperature superconducting oxide thin-film RF
SQUID K.P. Daly, A.H. Silver,
R.W. Simon, C.E. Platt, A.E. Lee, M.S. Wire and J.E.
Zimmerman
Summary: The authors have
successfully fabricated and operated RF SQUIDs
(superconducting quantum interference devices) made from thin
films of the high-temperature superconducting oxide ErBa/sub
2/Cu/sub 3/O/sub 7-x/. The ideal triangular quantum
interference pattern characteristics of RF SQUIDs are
observed. The performance of a particular device, operating
between 60 and 65 K, is described in detail. Except for a
possible increase of noise due to operation close to T/sub c/,
the limited temperature range is shown to have no decisive
practical disadvantage. The precise nature of the Josephson
coupling in the device is unclear. It seems likely that
Josephson coupling occurs between individual grains and
probably within the microbridge. |
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Fabrication
of submicrometer features in Y-Ba-Cu-O superconducting thin
films C.T. Rogers, T.L. Cheeks,
P. England, S.-W. Chan, H.G. Craighead and T.
Venkatesan
Summary: The authors explored some
of the processes required to fabricate superconducting devices
in Y-Ba-Cu-O thin films. Photolithography was used to define
superconducting wires with dimensions from several micrometers
to less than one micrometer in films grown both by laser and
electron-beam deposition. Patterns were transferred with a
variety of techniques including lift-off, chemical etching,
and argon ion milling. Low-resistance contacts were reliably
made by in situ Ar ion beam presputter and Au or Ag
deposition. Submicrometer wires produced with these processes
behaved as scaled-down bulk superconducting films; wire
properties were determined largely by initial film morphology
and wire geometry. No naturally occurring weak link behavior
was observed in wires down to 0.5 mu m in width. The
fabrication process and the electrical behavior of the
resulting wires are discussed. |
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Critical
currents in a high-T/sub c/ superconducting short dipole
antenna C.E. Gough, S.K. Khamas,
T.S.M. Maclean, M.J. Mehler, N.M. Alford and M.M.
Harmer
Summary: Measurements on a
liquid-nitrogen-cooled, superconducting short-dipole aerial at
550 MHz are presented. The measured relative field strength as
a function of frequency is shown for an excitation current
well below the critical value. The field strength is shown as
a function of frequency for input powers of 25 dBm, 28 dBm,
and 31 dBm. An estimate of the RF critical current is made
from deviations in linearity with input power. |
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Photoconductive
response of granular superconducting
films U. Strom, E.S. Snow, R.L.
Henry, P.R. Broussard, J.H. Claassen, S.A. Wolf, M. Leung and
R.W. Simon
Summary: The application of a DC
magnetic field to disordered or granular films of Y-Ba-Cu-O is
shown to lead an enhancement of a nonbolometric photoresponse
at temperatures near and below T/sub c/. The disorder is
evidenced by the broadened resistive transition to
superconductivity and by the higher normal state resistance of
the films. This observation is consistent with a
nonequilibrium effect, which is described by the flux-flow
model of the low-temperature photoresponse of granular
superconductors. |
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Optical
detection by suppression of the gap voltage in niobium
junctions D.P. Osterman, M.
Radparvar and S.M. Faris
Summary: The
application of niobium Josephson junctions as detectors in
infrared focal plane arrays is being investigated. The
dependence of the optical response of these junctions on
several variables including temperature, frequency of light
chopping, and light intensity has been measured. In addition
the authors have investigated the effect of different
substrate materials, namely, single-crystal silicon and
amorphous fused quartz, and the effect of two different
cryogenic environments, namely, vacuum and superfluid helium.
For measurements in vacuum, cooling is provided by contact
between the substrate and a cold surface. Immersion in
superfluid helium is expected to reduce heating effects
considerably owing to its extremely high thermal conductivity,
thus isolating nonequilibrium effects. The NEP (noise
equivalent powers) and D* (detectivity) of the detectors is
limited by noise in the room-temperature electronics. For a
typical niobium junction of area 100 mu m/sup 2/ on a fused
quartz (SiO/sub 2/) substrate in vacuum at 4.2 K the
responsivity is 3000 V/W, NEP is 6*10/sup -13/ W, and D* is
2*10/sup 9/ cm-Hz/sup 1/2//W for a 1-Hz bandwidth at a
wavelength of 1 mu m. |
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Optical
response of YBCO thin films and
weak-links D.P. Osterman, R.
Drake, R. Patt, E.K. Track, M. Radparvar and S.M.
Faris
Summary: The authors have fabricated
films of the high-temperature superconductor YBCO and measured
their response to optical and infrared radiation. The response
is manifested by a change in the current-voltage
characteristics of YBCO weak links. The change is found to be
dependent on film quality, operating point, light chopping
frequency, and temperature. Depending on the type of anneal,
the superconducting films exhibit metallic or semiconducting
resistivity behavior above T/sub c/. The optical responsivity
of semiconducting films is larger than that of metallic films.
By further annealing, semiconducting films could be converted
into metallic films with a concurrent decrease in their
optical responsivity. Some of the measurements have been
performed with the films immersed in superfluid helium to
allow the separation of nonequilibrium effects from the
equilibrium bolometric response. The results of this
preliminary study indicate that patterned YBCO films in the
superconducting state can be used as sensitive detectors of
light. |
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Optical
response of epitaxial and granular films of YBa/sub 2/Cu/sub
3/O/sub 7- delta/ at temperatures from 25 K to 100
K M.G. Forrester, M. Gottlieb,
J.R. Gavaler and A.I. Braginski
Summary: The
authors present the results of optical detection in epitaxial
and granular films of YBa/sub 2/Cu/sub 3/O/sub 7- delta / at
wavelengths of 0.63, 3.39, and 10.6 mu m and at temperatures
from 25 K to 100 K. Both types of film exhibit only bolometric
detection, with no evidence for nonequilibrium effects in this
temperature range. For 0.63- mu m radiation, mechanically
chopped at 725 Hz, measurements of a 10*90 mu m/sup 2/
epitaxial bridge yield a bolometric responsivity of
approximately 4*10/sup 3/ V/W, and detectivity D* of more than
10/sup 8/ cm square root Hz/W. Granular films biased above
their critical current are found to exhibit two-level
switching noise, resulting in Lorentzian deviations from an
otherwise 1/f noise spectrum. |
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Superconducting
kinetic inductance bolometer J.E.
Sauvageau and D.G. McDonald
Summary: The
authors are developing a bolometer with a temperature sensor
based on the temperature dependence of the inductance of a
superconducting microstrip line. As a first step in exploring
this idea experimentally, they have designed experiments to
test only the temperature sensor. The experimental devices are
all-niobium inductance thermometers fabricated on silicon
substrates which have been deeply etched to provide areas of
relative thermal isolation. The ground plane superconductor is
thin enough so that its kinetic inductance dominates the audio
frequency impedance of the stripline near its critical
temperature, at 0.9 T/sub c/. This differential thermometer
uses a commercial SQUID (superconducting quantum interference
device) as the preamplifier. Preliminary results demonstrate a
proof-of-principle for the thermometer design. An important
quality factor for this type of measurement system is the
depth of the null. The authors achieved a high value of
0.4*10/sup -6/, which is satisfying for a first
try. |
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The
high T/sub c/ superconducting
bolometer P.L. Richards, S.
Verghese, T.H. Geballe and S.R. Spielman
Summary:
A description is given of the optimization of a
bolometric infrared detector which uses the resistive
transition of a high-T/sub c/ film as the thermometer. The
performance of an LN (Liquid nitrogen)-cooled far-infrared
bolometer operated with a cooled low-pass filter is computed
for the ideal case of a noise-free readout. The theory is then
extended to include various contributions to the readout
noise. Measurements are presented of the low frequency noise
near T/sub c/ in current-biased films of ErBa/sub 2/Cu/sub
3/O/sub 7/, which show that useful performance can be
achieved. Comparisons are made with other infrared detection
technologies which show that practical high T/sub c/
bolometers will be especially useful for wavelengths longer
than the -20 mu m cutoff of LN-cooled photovoltaic detectors.
Potential applications include far-infrared laboratory
spectroscopy and passively cooled space observations of bright
sources such as the earth. |
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Josephson
junctions as ultrafast nuclear particle
detector R. Cristiano and P.
Silvestrini
Summary: The problem of
optimizing Josephson junction parameters to realize a fast
particle (radiation) discriminator is discussed. The switching
operation is investigated, focusing on problems of intrinsic
noise and nonequilibrium processes induced by the radiation.
The key concept is the effective radiation flux, namely, the
flux which produces a number of switchings equal to that
produced by intrinsic noise. This quantity gives a direct
measure of the device performance. The effects of some
relevant junction parameters, such as the junction area, the
Josephson current density, and the bath temperature, on the
behavior of the effective flux as a function of the response
time of the device are presented. |
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Characteristics
of high-transmission-probability tunnel junctions for use as
particle detectors D.A. Stricker,
G.P. Alba, C.C. Anderson, D.D. Bing, R.W. Bland, S.C. Dickson,
T.G. Dignan, P. Gagnon, R.T. Johnson and C.M.
Seneclauze
Summary: Interest in the problem
of the galactic dark matter has stimulated development of
particle detectors sensitive to very low energies. The authors
describe superconducting tunnel junctions with thin barriers
which may be suitable for this purpose. They present I-V
(current-voltage) characteristics and data on the temperature
dependence of the subgap tunneling current. They examine the
barrier transmission probability and the resistance of the
null junction. They also present some
scanning-electron-microscope observations of the thin films of
the tunnel junctions, which show that faster deposition
produces smaller grains and increasing film thickness results
in larger grains from an apparent fusing of smaller
grains. |
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Phonon-mediated
detection of X-rays in silicon crystals using superconducting
transition edge phonon
sensors B.A. Young, B. Cabrera,
A.T. Lee, C.J. Martoff, B. Neuhauser and J.P.
McVittie
Summary: The authors present data on
the operation of thin-film superconducting strips of titanium
as phonon sensors on the surface of silicon crystals. The
superconducting films are biased at the foot of the resistive
transition in temperature and below the critical latching
current (the current above which a normal region in the film
grows from self-heating). The interaction of an incident X-ray
in the Si crystal generates a phonon source which propagates
to the surface at the speed of sound. Such an event produces a
several-microsecond-long self-terminating voltage pulse which
is proportional to the amount of the sensor area driven
normal. It is shown that these Ti superconducting transition
edge sensors operated at 0.3 K have sufficient resolution for
detecting particles with energy deposition above several keV,
which makes them good candidates for use in neutrino (and
other) experiments. |
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The
detection of 6 keV X-rays with Nb
junctions P. Gare, R. Englehardt,
A. Peacock, D. Twerenbold, J. Lumley and R.E.
Somekh
Summary: Refractory metal
Nb/Al/Al-oxide/Al/Nb junctions are shown to be sensitive to
6-keV X-rays over the temperature range from 2.8 to 1.4 K. For
such junctions, which have an observed minimum ionizing energy
of 12 MeV, a limiting energy resolution of 8 eV is predicted.
Currently an energy resolution of 250 eV is observed at 1.4 K
which is primarily dominated by system electronic noise. The
Nb-based junctions are shown to be very stable with respect to
thermal cycling while the nonequilibrium physics can be simply
scaled from the theory of Sn junctions. It is concluded that
on-chip arrays having a broad band pass and good energy
resolution should be feasible to construct. |
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Characteristics
of Nb-based Josephson junctions at a temperature below 1
K K. Ishibashi, K. Takeno, Y.
Oae, T. Sakae, Y. Matsumoto, A. Katase, S. Takada, H. Akoh and
H. Nakagawa
Summary: A lithographic technique
for Josephson LSI (large scale integrated) circuits was
applied to the production of a superconducting tunnel junction
for radiation detection. Nb-Al/aluminum oxide/Nb tunnel
junctions with different junction sizes and barrier oxidation
times have been fabricated. The current-voltage curves were
measured in the temperature range from 4.2 to 0.45 K. The
junctions showed some amounts of leakage current, which
remained below 2 K. The behavior of the leakage current was
successfully understood from its experimental dependence on
the junction size. |
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A
RF superconducting electromechanical transducer for
gravitational wave antennae M.F.
Bocko, W.W. Johnson and V. Iafolla
Summary:
An electromechanical transducer based on a
superconducting radiofrequency bridge circuit has been
developed for use on a gravitational radiation detector. The
low electrical loss of superconductors has made it possible to
achieve electrical quality factors of several thousand in a
lumped-element circuit which operates at 4 MHz. The bridge
could be remotely balanced to one part in 50000, which led to
a displacement noise level of 10/sup -15/ m/ square root Hz.
It should be useful in measuring any physical quantity which
can be made to change a capacitance. At the present stage of
development, capacitance changes of 10/sup -20/ F could be
detected in a 1-s integration time. One straightforward
improvement, namely, the use of a low-phase-noise quartz
crystal oscillator as the bridge excitation source will reduce
the noise to 10/sup -17/ m/ square root Hz. |
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Superconducting
microwave resonators for physics
experiments N. Klein, G. Muller,
H. Piel and J. Schurr
Summary:
Superconducting resonators at K-band frequencies have
been developed for different applications in general physics.
Niobium pillbox cavities have been built for the one-atom
maser experiment by which the interaction of Rydberg atoms
with single microwave photons has been investigated. At 21.5
GHz and 1.3 K, quality factors of up to 10/sup 11/ were
obtained. Coating of the cavity with Nb/sub 3/Sn resulted in
quality factors of 6*10/sup 8/ at 4.2 K and 6*10/sup 9/ at 2
K. A superconducting Fabry-Perot resonator consisting of two
spherically curved niobium mirrors was also investigated. The
quality factor of 1.8*10/sup 7/ measured at 25 GHz and 4.2 K
was found to be two orders of magnitude higher than for a
corresponding copper resonator. Fabry-Perot resonators can be
used for detecting small position changes for one mirror with
respect to another caused by gravitational forces. First
experiments with copper Fabry-Perot mirrors suspended in a
vacuum chamber provided a maximum sensitivity for a
gravitational acceleration of one mirror of 4*10/sup -11/
m/s/sup 2/. These results are promising for a possible fifth
force detector based on a superconducting Fabry-Perot
resonator. |
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Dual
channel 115 and 230 GHz SIS receivers in operation at the
Owens Valley Radio
Observatory D.P. Woody, C.J.
Giovanine and R.E. Miller
Summary: The Owens
Valley Radio Observatory millimeter-wave interferometer array
is presently operating with dual-channel SIS
(superconductor-insulator-superconductor) tunnel junction
receivers. The first channel covers the frequency range from
85 to 120 GHz and the second channel covers the frequency
range from 200 to 300 GHz. The mixers consist of a corrugated
feed horn, a single-stage circular-to-rectangular waveguide
transition a reduced-height waveguide with an SIS junction
mounted across the E-plane, and a non-contacting backshort.
The mixer block has a built-in RF choke for the IF
(intermediate frequency) signal path which is designed to
present a short circuit to the junction at frequencies above
the 2-GHz IF frequency. The small-area (<1 mu m/sup 2/)
PbInAu-native oxide-PbAu SIS tunnel junctions are fabricated
using a bridge lift-off technique. The receivers in the 85- to
120-GHz band have noise temperatures of <100 K, while the
receivers in the 100- to 300-GHz band have noise temperatures
in the range from 200 to 300 K. These dual-channel receivers
are mounted in 4.5 K closed-cycle refrigerators and are in
continuous use on the array. |
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Observation
of 4.2 equilibrium noise squeezing via a Josephson-parametric
amplifier B. Yurke, P.G.
Kaminsky, R.E. Miller, E.A. Whittaker, A.D. Smith, A.H. Silver
and R.W. Simon
Summary: The authors observed
the squeezing of 4.2 K thermal noise using a Josephson
parametric amplifier operated in the degenerate mode at 19.4
GHz. A 42% reduction in the equilibrium noise was observed.
The amplifier has also been operated at 0.1 K with an excess
noise of 0.28 K referred to the amplifier's input port. This
is less than the vacuum fluctuation noise of 0.47 K at the
amplifier's input. Recently the amplifier has been operated in
a nonchaotic mode with a signal gain in excess of 16
dB. |
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Beating
the quantum limit in SIS
mixers M.J. Wengler and M.F.
Bocko
Summary: The 2LO (double local
oscillator) mixer is analyzed in the low-LO-power limit for a
perfect SIS (superconductor-insulator-superconductor) diode.
It is shown that a 2LO SIS mixer can beat the noise
performance of a one-LO mixer by a factor of two. It is
suggested that even lower noise is possible with an SIS when
operated in some other way. |
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MM
wave quasioptical SIS mixers Q.
Hu, C.A. Mears, P.L. Richards and F.L.
Lloyd
Summary: The authors have tested the
performance of planar SIS
(superconductor-insulator-superconductor) mixers with
log-periodic antennas at near millimeter and submillimeter
wave frequencies from 90 to 360 GHz. The large omega R/sub N/C
product ( approximately 10 at 90 GHz) of the Nb/NbO/sub
x//Pb-In-Au junctions requires an integrated inductive tuning
element to resonate the junction capacitance at the operating
frequencies. Two types of integrated tuning element were used
which were designed with the aid of measurements using a
Fourier transform spectrometer. Preliminary results indicate
that the tuning elements can give very good mixer performance
up to at least 200 GHz. An inductive wire in parallel with a
5-junction array gives a minimum mixer noise temperature of
115 K (DSB) at 90 GHz with a FWHM (full width at half maximum)
bandwidth of 8 GHz. An open-ended microstrip stub in parallel
with a single junction gives minimum mixer noise temperatures
at 150 and 200 K (DSB) near 90 and 180 GHz with FWHM
bandwidths of 4 and 3 GHz, respectively. The relatively high
mixer noise temperatures compared to those of waveguide SIS
mixers in a similar frequency range are attributed mainly to
the losses in the optical system. |
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Josephson
terahertz local oscillator R.P.
Robertazzi and R.A. Buhrman
Summary:
Voltage-tunable Josephson junction terahertz
oscillators have been fabricated using rugged,
high-current-density NbN/sub 1-x/C/sub x/ tunnel junctions
with M O barriers. The radiation emitted from such junctions
is detected on chip by a second Josephson junction which is
capacitively coupled to the first. For oscillator junctions
with a critical current density of J/sub c/ approximately
3.5*10/sup 4/ A/cm/sup 2/ the junction oscillates with a
voltage amplitude of approximately=1.5 mV. The detected RF
voltage level remain essentially constant from 300 GHz to
above 1 THz, the upper limit of the detector. From
measurements of the Josephson step height in the detector
current-voltage characteristics, it is determined that the
oscillator junction produces 0.5 mu W of terahertz radiation
of which, due to impedance mismatch, 10 nW is coupled into the
detector junction. |
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A
current controlled variable delay superconducting transmission
line S.M. Anlage, H.J. Snortland
and M.R. Beasley
Summary: The authors present
a device concept for a superconductive current-biased
variable-delay transmission line structure which is capable,
in principle, of operating up to the terahertz regime. The
device makes use of the change in kinetic inductance of
superconductors with transport current. The relevant material
figures of merit for optimum performance of such a device are
defined, and suitable candidate materials are identified. The
device concept has been tested in niobium technology, where
temperature-dependent changes in the inductance are easily
achieved. Preliminary measurements on the temperature and
current dependence of niobium transmission line resonators
operating in the 1-20 GHz range are presented. The expected DC
bias current variable delay has not yet been observed, but
niobium is not expected to be the optimum material for such an
effect. Suggested improvements include the use of more
favorable materials, such as amorphous alloys and oxide
superconducting films, and the use of modified microstrip
geometries where a closer approach to the depairing critical
current density should be possible. |
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Picosecond
switching dynamics of a Josephson tunnel
junction D.R. Dykaar, R.
Sobolewski and T.Y. Hsiang
Summary: The
authors previously reported (D.R. Dykaar et al., 1987)
switching measurements of a tunnel junction excited by
photogenerated picosecond current pulses. The junction was
found to switch from the zero- to the finite-voltage regime
for subcritical current pulses, and the results were modeled
with a critical charge concept. It is shown that in addition
to pulse charge, pulse risetime is a critical parameter in
determining the junction response. Simulations have been
performed using exponentially rising and failing pulses to
simulate the experimentally measured waveform. Significant
deviations from DC behavior have been found. Pulse risetime
has been found be the parameter which causes the onset of the
observed chaos in these devices. Pulse falltime has no effect,
which is consistent with the critical risetime
interpretation. |
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Noise
characteristics and instabilities of long Josephson
junctions B.S. Han, B. Lee, O.G.
Symko, W.J. Yeh and D.J. Zheng
Summary: The
noise characteristics and chaotic behavior of long Josephson
junctions in a magnetic field are investigated. In a magnetic
field, current-biased junctions exhibit the dynamics of fluxon
motion affected by fluctuations. These consist of telegraph
noise at voltage steps and instabilities due to chaotic
behavior. Results on long junctions with McCumber number
ranging from 10 to 100 show such behavior. The telegraph noise
is driven by thermal fluctuations. Modeling of the junctions
using a perturbed sine-Gordon equation shows chaotic and
periodic regions. Some of the chaotic regions are between
Fiske steps, while others are at the edge of the Fiske steps.
These observations are confirmed by numerical
calculations. |
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Chaos
and catastrophe near the plasma frequency in the RF-biased
Josephson junction R.L. Kautz and
R. Monaco
Summary: The authors examine two
forms of complex behavior in the RF-biased junction that occur
at bias frequencies near the plasma frequency: the chaotic
instabilities that influence the design of voltage standards;
and a cusp catastrophe. Using digital simulation the authors
determine how chaos and catastrophe affect the zero-voltage
part of the I-V (current voltage) curve as a function of the
RF amplitude and frequency. In particular, they show that
chaotic instabilities result in discontinuities in the DC-bias
range of the zero-voltage state and the cusp catastrophes
result in two dynamically different zero-voltage states that
exist over the same DC-bias range. They also report the
experimental verification of the former effect. |
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Studies
of chaos and thermal noise in a driven Josephson junction
using an electronic analog C.M.
Pegrum, W.S.C. Gurney and R.M. Nisbet
Summary:
Using an electronic analog of a resistively shunted
driven Josephson junction, the authors have demonstrated a
number of effects, including the appearance of a devil's
staircase in the current-voltage characteristic, the onset of
chaos, and the effect of noise on these phenomena. It is
stressed that the analog is simple, but models the junction
behavior with a high degree of accuracy and
detail. |
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Dynamic
stabilization of a microwave-driven Josephson tunnel junction
against a period-doubling
bifurcation J.B. Hansen, G.F.
Eriksen, M.R. Samuelsen and H. Svensmark
Summary:
The authors present theoretical and experimental
results on the effect of periodic perturbations on
microwave-driven Josephson tunnel junctions close to a
period-doubling bifurcation. The theory, which is quite
general, describes how a periodic near-resonant perturbation
changes the stability of the system. The phenomenon was
investigated experimentally in small Nb-NbO/sub x/-Pb tunnel
junctions, and experimental findings were found to be in
agreement with the theory. The effect has importance for the
performance of parametric amplifiers based on Josephson tunnel
junctions. |
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Mutual
locking, chaos and devils staircase in two Josephson tunnel
junctions with a common resistive
shunt H.D. Jensen, A. Larsen, J.
Mygind and M.T. Levinsen
Summary: The authors
studied the phase-locking in a pair of individually biased,
resistively and capacitively shunted Josephson junctions,
coupled by a common resistive or capacitive shunt, on an
analog computer. Under certain conditions, locking is found to
occur at all rational frequency ratios. A critical line is
found in parameter space along which the steps form a complete
devil's staircase having a fractal dimension of 0.87. Beyond
the critical line bifurcations occur on all steps following
the Feigenbaum sequence to chaos. The Feigenbaum constants are
recovered to a fair accuracy. Preliminary experiments with two
Josephson tunnel junctions shunted on the chip by a
gold-indium film are discussed. |
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Experimental
results on the metastability of the resistive state in
Josephson junctions R. Cristiano,
S. Pagano and P. Silvestrini
Summary: The
authors report experimental results on the metastability of
both the V=0 and the V not=0 states in Josephson junctions. A
comparative analysis of both the decay mechanisms at various
values of the Josephson coupling energy makes it possible to
obtain useful information on the relevant parameters in the
lifetime expression of the V not=0 state. In particular, cases
were investigated where the lifetime of the V=0 state is
comparable with the lifetime of the V not=0 state. This
situation is experimentally realized by applying an
appropriate magnetic field. Experimental data show good
agreement with theory for junctions with a low level of
damping. As the hysteresis parameter increases, discrepancies
appear. |
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Structure
of resistive networks for dispersive tunnel
processes G. Brunk, C. Zurbrugg
and H. Lubbig
Summary: Fundamental
construction principles for equivalent circuits for
superconducting pair and quasiparticle tunnel junctions are
discussed. To the lowest order of microscopic tunneling, the
two basic features of the corresponding macroscopic dynamics,
i.e the energy-neutral nonlinear self-coupling of the quantum
phase shift and the energy-effective tunnel admittance, are
discussed in the framework of linear response theory
(equivalent to the KUBO formalism) and are represented by
means of the corresponding equivalent two-subcircuit network.
Approximation formulas describing a singularity-equivalent
model are discussed in detail, and consequences for secondary
quantum processes are indicated. |
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Anomalously
large Josephson switching currents in low dissipation
Josephson junctions C.D. Tesche,
J.R. Kirtley, W.J. Gallagher, A.W. Kleinsasser, R.L. Sandstrom
and S.I. Raider
Summary: The authors observed
marked effects on the switching characteristics of a
high-quality junction due to the frequency dependence of the
external circuitry. This involved anomalously high values of
the maximum switching current of Josephson junctions at
temperatures where thermally activated switching was expected
to greatly lower it. This effect was modeled by evaluating
junction parameters from the measured characteristics. The
junction resistance determined from the return switching
current is that of a nearly ideal quasiparticle tunneling
resistance, increasing exponentially fast at low temperatures.
As a result, the load resistance becomes dominant over a wide
range of temperatures. The frequency dependence of this load
resistance then produces the anomalously large Josephson
switching currents, which have not been observed previously in
junctions with significant normal conductance. |
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Dynamical
study of the superconducting phase transition of
two-dimensional networks B.
Jeanneret, P. Fluckiger, C. Leemann and P.
Martinoli
Summary: Granular aluminum wire
networks forming square arrays of N*N approximately=10/sup 6/
nodes connected by strips of 8- mu m long were fabricated with
photolithographic techniques. For strip resistances of the
order of 1 k Omega the superconducting transition of the
network, as evidenced by AC conductance measurements is shown
to be of the Kosterlitz-Thouless type. In a perpendicular
magnetic field, flux quantization in the loops of the network
leads to periodic oscillations of the
magnetoconductance. |
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DC
voltage multipliers: a novel application of synchronization in
Josephson junction arrays V.K.
Semenov and M.A. Voronova
Summary: New types
of followers and follower/doublers of DC voltage are proposed
and analyzed. Operation of the devices is based on
synchronization of the single-flux quantum (SFQ) pulses
generated by overdamped Josephson junctions coupled by means
of passive capacitive or inductive circuits. Two ways of
combining the followers and doublers to get DC voltage
multipliers and standards are described. Possible applications
of these devices in A/D and D/A converters and RSFQ (rapid
SFQ) logic circuits are also discussed. It is believed that
the proposed multipliers and standards could produce stable
output voltages in the 1-10 V range with output currents in
the 0.1-1 mA range. |
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Single-electron
tunnel junction array: an electrostatic analog of the
Josephson transmission line K.K.
Likharev, N.S. Bakhvalov, G.S. Kazacha and S.I.
Serdyukova
Summary: The authors carried out
static and dynamic analyses of uniform one-dimensional arrays
of ultrasmall tunnel junctions. The correlated single-electron
tunneling in the junctions of the array results in a behavior
qualitatively similar to that of the Josephson transmission
line. In particular, external electric fields applied to the
array edges can inject single-electron-charged solitons into
the array interior. The shape of such a soliton and the
character of its interactions with other solitons and the
array edge are very similar to those of Josephson vortices
(sine-Gordon solitons) in the Josephson transmission line.
Under certain conditions, a coherent motion of the soliton
train along the array is possible, resulting in generation of
narrowband SET (single-electron tunneling) oscillations with
frequency f=/e, where is the DC current flowing along
the array. |
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Multi-fluxon
dynamics in inhomogeneous Josephson junctions:
experiment A.V.
Ustinov
Summary: Long Nb-NbO-Pb Josephson
junctions with artificially formed SiO inhomogeneities in the
insulator layer have been studied. A fine structure has been
observed on the current-voltage characteristics of the
junctions. Such structure is considered to be due to the
resonant interaction between the fluxon and its radiation. The
single-fluxon effect in the zero magnetic field and the
multifluxon effect in a large field have been studied
experimentally. The results are compared with those of the
perturbation theory and numerical simulation. A
multifluxon-chain pinning effect observed in the commensurable
lattice of inhomogeneities is reported and
discussed. |
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Status
of the SSC superconducting magnet
program J.
Peoples
Summary: The Superconducting Super
Collider (SSC) is a proposed proton-proton collider designated
to achieve collisions with 20 TeV per beam. In the SSC,
protons are accelerated and stored in two storage rings that
are stacked one on top of the other in an underground tunnel
83 km in circumference. In each collider ring the protons are
kept in a roughly elliptical orbit by two types of magnets:
dipoles, which bend the proton orbit into a closed loop, and
quadrupoles, which deflect the protons back toward the central
orbit when they diverge from it. Both types of magnets use
high-current superconducting coils to meet the SSC design
requirements. A dipole field of 6.6 T was chosen as a
compromise between the need for the highest practical field
and the limits of superconducting technology. The author
reviews the work done on the SSC dipole. Topics discussed are
cable development and production, cryostat design, cold mass
design, and model magnet testing. |
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Test
results from recent 1.8-M SSC model
dipoles P. Wanderer, J.G.
Cottingham, P. Dahl, M. Garber, A. Ghosh, C. Goodzeit, A.
Greene, J. Herrera, S. Kahn, E. Kelly, G. Morgan, A. Prodell,
W. Sampson, W. Schneider, R. Shutt, P. Thomspon and E.
Willen
Summary: The authors report results
for four 1.8-m-long dipoles built as part of the
Superconducting Super Collider (SSC) research and development
program. Except for length, these models have the features of
the SSC design, which is based on a two-layer cosine theta
coil with a 4-cm aperture. Compared to the 17-m design-length
SSC dipoles, the 1.8-m magnets are faster and more economical
way of testing changes in field shape design, conductor
support in the coil straight section and ends, etc. The four
magnets reported all reach fields in excess of 7.5 T with
little training and have excellent field shape. |
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Tests
of full scale SSC R&D dipole
magnets J. Strait, B.C. Brown, R.
Hanft, M. Kuchnir, M. Lamm, R. Lundy, P. Mantsch, P.O. Mazur,
A. McInturff, J.R. Orr, J.G. Cottingham, P. Dahl, G. Ganetis,
M. Gerber, A. Ghosh, C. Goodzeit, A. Greene, J. Herrera, S.
Kahn, E. Kelly, G. Morgan, A. Prodell, W. Sampson, W.
Schneider, R. Shutt, P. Thompson, P. Wanderer, E. Willen, S.
Caspi, W. Gilbert, R. Meuser, C. Peters, J. Rechen, R. Royer,
R. Scanlan, C. Taylor, J. Zbasnik, M. Chapman, A. Devred, J.
Kaugerts, J. Peoples, J. Tompkins and R.
Schermer
Summary: Four full-scale SSC
(Superconducting Super Collider) research and development
dipole magnets, incorporating successive mechanical design
improvements, have been quench-tested. Three of the magnets
are heavily instrumented with sensors to measure their
mechanical behavior and verify the effectiveness of the
mechanical improvements and with multiple voltage taps to
locate the origin of quenches. The last two magnets of this
series reach the SSC design operating field of 6.6 T in two or
fewer quenches. Load cells and motion sensors show that in
these two magnets the azimuthal clamping stress is higher at
zero current and drops more slowly with excitation that in
previous long magnets, and that the axial motion of the coil
upon excitation has been greatly reduced. Quenches are found
to originate preferentially in several locations, suggesting
other design improvements. |
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Magnetic
field decay in model SSC
dipoles W.S. Gilbert, R.F.
Althaus, P.J. Barale, R.W. Benjegerdes, M.A. Green, M.I. Green
and R.M. Scanlan
Summary: The authors have
observed that some of the model SSC dipoles have
long-time-constant decays of the magnetic field harmonics with
amplitudes large enough to result in significant beam loss, if
they are not corrected. The magnets were run at constant
current at the SSC injection field level of 0.3 Tesla for one
to three hours, and changes in the magnetic field were
observed. One explanation for the observed field decay is
time-dependent superconductor magnetization. Another
explanation involves flux creep or flux flow. Data are
presented on how the decay changes with previous flux history.
Similar magnets with different Nb-Ti filament spacings and
matrix materials have different long time field decay. A
theoretical model using proximity coupling and flux creep as
mechanisms for the observed field decay is
discussed. |
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Measurement
of internal forces in superconducting accelerator magnets with
strain gauge transducers C.L.
Goodzeit, M.D. Anerella and G.L. Ganetis
Summary:
An improved method has been developed for the
measurement of internal forces in superconducting accelerator
magnets, in particular the comprehensive stresses in coils and
the end restraint forces on the coils. The transducers have
been designed to provide improved sensitivity to purely
mechanical strain by using bending mode deflections for
sensing the applied loads. Strain gauge resistance
measurements are made with a new system that eliminates
sources of errors due to spurious resistance changes in
interconnecting wiring and solder joints. The design of the
transducers and their measurement system is presented along
with a discussion of the method of compensation for thermal
and magnetic effects, methods of calibration with typical
calibration data, and measured effects in actual magnets of
the thermal stress changes from cool-down and the Lorentz
forces during magnet excitation. |
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Magnet
systems for the International Thermonuclear Experimental
Reactor C.D. Henning and J.R.
Miller
Summary: Preliminary parameters for
the superconducting magnet system for the International
Thermonuclear Experimental Reactor has been established to
guide more detailed design work. Radiation tolerance of the
superconductors and insulators has been important because it
sets requirements for the neutron-shield dimension and
sensitively influences reactor size. Major levels of
mechanical stress appear in the structural cases of the
inboard legs of toroidal-field (TF) coils. The winding packs
of the TF coils include significant fractions of steel that
provide support against in-plane separating loads, but they
offer little support against out-of-plane separating loads
unless shear-bonding of the conductors can be maintained.
Removal of heat from nuclear and AC loads has not limited the
fundamental design, but it has nonnegligible economic
consequences. |
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TORE
SUPRA: a tokamak with superconducting toroidal field
coils-status report after the first
plasmas B.
Turck
Summary: The technology for TORE SUPRA
involves all the problems related to the fabrication and the
operation of large cryogenic superconducting coils and of the
associated cryogenic system, so is expected to provide
significant experience for developing the next generation of
machines for controlled fusion. TORE SUPRA has been designed
to operate with plasma pulses in excess of 30 s. Operation
started in April 1988 with a toroidal field of 2 Teslas.
Discharges of 500 kA with a 3-s current flat top were obtained
after a few days. Machine parameters where then raised to
their normal values (B approximately 4.5 and I/sub p/
approximately 1.7 MA). the discussion covers the goals of the
SUPRA, an overview of the basic machine, the conductor the
toroidal field coil system, and the cryogenic
system. |
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The
design of a high field ohmic heating coil for a
superconducting tokamak based on the US-DPC test
coil M.O. Hoenig and M.M.
Steeves
Summary: The fabrication of the
US-DPC (demonstration poloidal coil) test coil in the US is
using industrial processes capable of manufacturing advanced
30- to 40-kA ternary Nb/sub 3/Sn cable-in-conduit conductors
(CICCs) and coils for compact, high-field tokamaks. The
authors examine the feasibility of 1- to 1-m-bore 15-T ohmic
heating (OH) coils using state-of-the-art CICC conductors. The
proof of concept of this technology is expected to be
confirmed in the 1990 US-DPC tests at the Japanese
Demonstration Poloidal Coil Test Facility. The OH coil design
concepts presented appear to satisfy all known
superconducting, magnetic, pulsed loss, structural, hydraulic,
thermodynamic, and economic requirements of an ohmic heating
central solenoid for a compact steady-state
tokamak. |
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Recent
progress in the demo poloidal coil
program H. Tsuji, K. Okuno, H.
Nakajima, T. Ando, Y. Takahashi, M. Nishi, K. Yoshida, E.
Tada, K. Koizumi, T. Kato, T. Isono, M. Oshikiri, T. Hiyama,
K. Kawano, H. Yamamura, M. Sato, J. Yoshida, N. Itoh, S.
Shimamoto, T. Satou and T. Ichihara
Summary:
The fabrication of two 30-kA NbTi pulsed coils and one
10-kA Nb/sub 3/Sn coil with a total stored energy of 40 MJ is
in progress as a part of the DPC (Demonstration Poloidal Coil)
Program. All the Nb-Ti superconducting strands have been
fabricated, and their loss time-constant has been measured at
0.32 ms at 7 T, which is well below the initial target of less
than 1 ms. A novel winding technique is described for the
large current conductor which provides good mechanical
contact. |
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Design
of the prototype conductors for the Fusion Experimental
Reactor K. Yoshida, M.F. Nishi,
Y. Takahashi, H. Tsuji, K. Koizumi, K. Okuno and T.
Ando
Summary: The prototype conductors of the
toroidal coil for the Fusion Experimental Reactor have been
investigated. Specifications of the conductor are as follows:
forced flow cooling; rated current of 30 kA at 12 T and 4.2 K;
margin of critical current of 2; current density in winding of
30-40 A/mm/sup 2/; inlet helium at 4.2 K and 4-10 bar; and
minimum bending radius of 1.5 m. Three types of conductors
were considered as candidates: a test module coil of FF type
(hollow cooling type using test module coil Nb/sub 3/Sn
conductor techniques); preformed armor type; and advanced disk
type. Detailed design parameters and analysis results are
given for the three candidate conductors. |
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Calculation
of normal zone propagating velocities and transient thermal
effects dependence on the superconducting wire
parameters J. Casas and L.
Rinderer
Summary: Normal phase propagation
velocities along superconducting wires immersed in a cooling
normal helium bath were investigated experimentally and
theoretically. The measurements have been done for bare tin
wires with different radii and resistivities. The theoretical
calculations of the normal zone propagation velocity versus
the circulating current assume a transient mechanism for heat
transfer between the wire wall and the normal helium bath. The
transient-heat-transfer term is of the form alpha theta /sup
2/ delta theta / delta t, where theta is the temperature
relative to the normal helium bath temperature. The
transient-heat-transfer coefficient, alpha , is then defined,
and good agreement between the theoretical and experimental
data is obtained. The coefficient depends on the bath
temperature and, nonrandomly, on the maximum power per unit
surface released to the normal helium bath, which in turn
depends on the wire radius, critical current, and
resistivity. |
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Thermo-electromagnetic
stability of ultrafine multifilamentary superconducting wires
for 50-60 Hertz use A. Fevrier,
A. Gueraud, J.P. Tavergnier, Y. Laumond and A.
Lacaze
Summary: The authors present
calculations of the theoretical thermoelectromagnetic
stability of ultrafine multifilamentary superconducting wires.
These calculations take into account the self-field effect of
the transport current. The authors then compare theoretical
and experimental results for conductors comprising typically
several hundred thousand 0.17- mu m diameter filaments. These
first measurements of the electromagnetic stability of
multifilamentary wires under AC conditions have given
encouraging results and have shown good agreement with theory.
The results are pertinent to the development of Nb-Ti
ultrafine filamentary wires for 50-60-Hz
applications. |
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Investigation
of the pressure rise during the quench of a force cooled
superconducting coil Y. Wachi, M.
Ono, S. Kimura, A. Tanaka, Y. Sanada, A. Miura and T.
Fujioka
Summary: Pressure rise during the
quench of NbTi/Cu cable-in-conduit conductor has been
experimentally and analytically investigated. The pressure
rise under full-length quenching of the conductor is shown to
be in good agreement with the calculated pressure rise based
on Miller's equation, independently of joule power density.
The time elapsed to maximum pressure rise depends on the
transport current, which is determined by the joule power
density. Pressure rise during quench with a local normal zone
strongly depends on the time from the quench to the dump. The
measured pressure rise under local quench condition is in good
agreement with the calculated value based on L. Dresner's
(1986) analytical equation. It is concluded that the maximum
pressure rise of a conduit conductor can be restricted by
quick discharge in actual fusion magnets. |
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Analysis
of transient thermally-induced convection of supercritical
helium in a conduit P.E. Phelan,
S.M. Kuo and C.L. Tien
Summary: The author
investigates the thermally induced transient flow and heat
transfer in cable-in-conduit conductors (CICCs) cooled with
initially stagnant supercritical helium. Prior studies have
demonstrated that thermally induced flow results in a high
heat-transfer coefficient and a large thermal stability
margin. Accurate prediction of the stability margin requires a
quantitative description of the variation of heat transfer
with heat flux. This study focuses on the
heat-transfer/heat-flux relationship and on the basic
mechanisms governing the early stages of transient heating.
For modeling the problem is divided into two parts: a
low-heat-flux regime and a high-heat-flux regime. Results from
the models indicate that the heat-transfer coefficient
exhibits a minimum, as observed experimentally in previous
study, and that, in general, heat transfer can be considerably
enhanced by using a high compressible fluid like supercritical
helium. For low heat fluxes, heat transfer decreases with
increasing heat flux because the thermal boundary layer is
thickened by the expansion-driven velocity directed away from
the heat transfer surface. For high heat fluxes, heat transfer
increases with increasing heat flux because the thermal
boundary layer thickness is reduced by the action of the
expansion-driven axial velocity. |
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Two
dimensional transient heat transfer in He
II J.M. Pfotenhauer and X.
Huang
Summary: A comparison is presented of
transient heat flow in He II as measured experimentally and as
predicted by analysis based on the Gorter-Mellink equation.
The geometry is that envisioned for the He II cooling of a
SMES (superconducting magnetic energy storage) system, namely,
an annular layer of He II in direct contact with one layer of
a solenoid and extending the full height of the coil. A normal
zone over a fraction of a turn provides for two-dimensional
heat flow in the annular layer of the He II. The comparison is
given for both adiabatic and isothermal boundary conditions at
the end of the channel. Good agreement between the analysis
and the experimental data is found, verifying the usefulness
of the analysis for large-scale systems. In addition,
discrepancies between the analysis and data provide insight
into the stability process of the He II cooled
superconductor. |
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Pressure
drop measurements on supercritical helium cooled cable in
conduit conductors M.A.
Daugherty, Y. Huang and S.W. Van Sciver
Summary:
For extensive projects such as the International
Thermonuclear Experimental Reactor design cooperation it is
essential to know the pressure drops to be expected from
different conductor geometries and operating conditions. To
measure these drops a flow loop was constructed to circulate
supercritical helium through different conductors. The loop is
surrounded by a 5-K radiation shield to allow for stable
operation at the required temperatures. A coil heat exchanger
immersed in a helium bath is used to remove the heat generated
by the pump. Pressure drops are measured across 1-m lengths of
the conductors for various mass flow rates. Plots of friction
factor versus Reynolds number are used to correlate the data.
Supercritical helium flow through the two cable-in-conduit
conductors tested in the 3 K to 7 K temperature range appears
to undergo laminar to turbulent transition at Reynolds numbers
varying from 1000 to 7000. It is concluded that flow area and
cooled perimeter alone do not adequately correlate the
observed pressure drop along the conductors. |
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Stability
projections for high temperature
superconductors H.L. Laquer, F.J.
Edeskuty, W.V. Hassenzahl and S.L. Wipf
Summary:
The stability of the high-temperature superconducting
oxides has been analyzed, using the methodology developed for
conventional type II superconductors. The results are
presented in graphical form for the temperature range from 4
to 100 K. For a 90-K superconductor the first flux jump field
peaks above 7 T at 60 K, and for a 120-K superconductor it
peaks above 12 T at 75 K. The maximum adiabatically stable
thickness increases greatly. The linear dimension of the
minimum propagating zone increases by a factor of 3 to 5, and
the quench propagation velocity drops by four orders of
magnitude. It is concluded that the high-temperature
superconducting materials will have much higher stability than
conventional type II superconductors and that their high flux
jump fields will make ultrafine multifilamentary conductors
unnecessary and improve the outlook for tape conductors. The
energy required to create a propagating zone is increased.
However, methods of coil protection will have to be
modified. |
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Quench
detection of superconducting magnets using ultrasonic
wave A. Ninomiya, K. Sakaniwa, H.
Kado, T. Ishigohka and Y. Higo
Summary: An
ultrasonic quench-detection method for superconducting magnets
is presented. This nondestructive method monitors the change
of the acoustic transfer function of a superconducting magnet
induced by a local temperature rise or an epoxy crack.
Experiments were carried out on a small epoxy-impregnated
magnet. The experimental results show that a local temperature
rise of about 2-3 K can be detected by this method. It is
concluded that the technique can provide early quench warning
by monitoring continuously the change of the acoustic transfer
function of the magnet. |
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Instabilities
of impregnated windings induced by epoxy
cracking T. Yamashita, S.
Nishijima, K. Takahata, T. Okada, T. Fukutsuka, K. Matsumoto
and M. Hamada
Summary: Changes in the
mechanical behaviour and the instabilities of impregnated
windings induced by epoxy cracking have been studied in order
to establish a methodology for constructing a superconducting
magnet. Epoxy cracks are inevitably induced in the process of
training. The cracks degrade the rigidity of the
superconducting windings, hence the evaluation of epoxy cracks
in impregnated windings is important in analyzing the
stability of the windings. Test windings in which epoxy cracks
were purposely introduced were used, and the degradation and
the training behavior were measured before and after crack
introduction. Before crack introduction the winding showed
training, that is, the quench current increased with quench
number. After crack introduction the quench current did not
increase with quench number. |
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Stabilization
of superconducting dry
solenoids M. Urata and H.
Maeda
Summary: Premature quenches in
superconducting solenoids wound with Formvar-coated NbTi
conductors have been studied. Some model coils wound with
various tensions were tested. The experimental results are
discussed with attention given to the stress distribution for
coil winding, cool-down to liquid helium temperature, and
energization at 4.2 K. Some mechanisms of premature quench are
classified according to the winding tension, and some
stabilization methods are presented on the basis of these
quench mechanisms. It is found that if a solenoid is wound
loosely, macroscopic slips occur in inner layers due to low
frictional force. Although such motions are removed if the
coil is wound tightly, shear stress concentration at the
interface between the winding and the bore tube then causes
quenches by microslips. Inserted polymer films affect the
sliding behavior of the conductor, which can improve the coil
performance. |
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High
current density aluminium stabilized conductor concepts for
space applications X. Huang, Y.M.
Eyssa and M.A. Hilal
Summary: The authors
report on a high-current-density aluminium-stabilized
conductor concept for large spaceborne energy storage
inductors. High-purity-aluminium-stabilized NbTi composite
conductors cooled by 1.8-K helium can provide a winding
current density up to 15 kA/cm/sup 2/ at fields up to 10 T.
The conductors are edge-cooled with enough surface area to
provide recovery following a normalizing disturbance. The
conductors are designed so that current diffusion time in the
high-purity aluminium is smaller than the thermal diffusion
time in helium. Conductor design, stability, and current
diffusion are considered. The numerical analysis of transient
stability shows that aluminium-stabilized conductors with
final resistivity ratio greater than 800 can be stable in a
1.8-K pressurized helium II bath up to 50 kA (J=15 kA/cm/sup
2/) at fields up to 10 T. single-layer toroids are preferred
over multilayer ones because of their simplicity of
construction, large current requirement, and better
magnetoresistance. |
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Effects
of insulation on potted superconducting
coils A.F. Zeller, J.C. DeKamp,
C.T. Magsig, J.A. Nolen and A.D. McInturff
Summary:
The authors studied the effects of the insulation on
potted superconducting coils wet-wound with Stycast 2850 FT
epoxy. The wire was insulated with one or two insulating
varnishes: Formvar (a polyvinyl formal resin) or Polyesterimid
(a phenolic resin). Although differences in maximum currents
were observed, it was found that the type of insulation did
not affect the training behavior. |
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Quenching
of multisection superconducting magnets and internal and
external shunt resistors A.A.
Konjukhov, V.A. Malginov, V.V. Matokhin and V.R.
Karasik
Summary: Self-protection of
superconducting magnets by subdivision using internal and
external shunt resistors was investigated. Temperature rise at
various points of the winding was measured by means of
thermocouples soldered to the conductor. The velocity of the
normal-zone propagation along radial and axial directions was
obtained. Initial current was varied from 0.3 to 0.9I/sub
q/=1000 A. The effectiveness of protection by subdivision with
external shunts depended on the number and dimensions of
sections. It increased with decreasing single section size.
When placed in the magnet, shunts work as a heater and
diminish the quenching time and maximum winding
temperature. |
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Protection
of superconducting magnets with high current
density V.R. Karasik, A.I.
Rusinov, V.S. Vysotsky and A.A. Konujkhov
Summary:
The effectiveness of superconducting-magnet protection
by subdivision is demonstrated. An analytical model of
quenching is proposed which is consistent with experimental
results and makes it possible to estimate maximum temperature
and voltage, the correct volume of a single section, and other
parameters of protected winding. It is shown that subdivision
decreases overheating and transient voltage in the
winding. |
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Study
of quench propagation velocity in superconducting magnets for
UNK I.V. Bogdanov, P.A.
Shcherbakov, V.P. Snitko, N.P. Tkachenko, L.M. Vasiliev, M.G.
Vybornov and A.V. Zlobin
Summary: Two
superconducting magnet models, warm-iron and cold-iron
designs, are studied. The authors describe a method for
measuring quench propagation velocity in superconducting
cables with a transport current in an external field under
cooling conditions typical for those of the magnet winding.
Results on measuring quench propagation velocities in
warm-iron and cold-iron designs are presented, and the results
obtained for short samples and model coils are compared. They
make it possible to simulate numerically a quench process in
the coil. |
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Magnets
for 150 GHz gyrotrons W. Maurer,
B. Piosczyk, M. Forster, D. Krischel and H.
Peschel
Summary: The development of three
magnet sets for a 150-GHz gyrotron is described. This
development has the ultimate goal of producing a 150-GHz
gyrotron unit with 1-MW power in steady-state operation for
plasma heating. The design criteria and the problems and
difficulties during development phase are discussed. The first
set, consisting of normal-conducting and superconducting
solenoids and normal-conducting steering coils, showed
problems during operation. The second magnet set, with only
two normal-conducting magnets in the emitter zone, is working
without any problems. The third set, (in the construction
phase), is all superconducting. Using the second set an output
RF power of 200 kW was achieved for a few
milliseconds. |
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Impact
of high temperature superconductors on the possibility of
radio-frequency confinement S.O.
Dean
Summary: It is suggested that recent
high-temperature superconductors may impact the feasibility of
RF confinement in two important ways: higher-temperature
superconductors should have higher critical B fields and
consequently may allow higher critical B fields to be
sustained in the cavity, thus allowing the necessary confining
pressure to be achieved; and the higher-temperature
superconductors lower the refrigeration power necessary to
maintain the superconducting cavity, thus allowing a favorable
energy balance. Issues discussed include the frequency
condition and the pressure balance, the maintenance of a
superconductor near a thermonuclear plasma, and the field
configuration. It is concluded that the old bases for
rejecting the possibility of RF confinement of thermonuclear
plasma are no longer valid. |
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Standard-II
and extended tests of the Swiss
LCT-coil J.A. Zichy, B. Jakob, K.
Kwasnitza, C. Sborchia, G. Vecsey, M.S. Lubell and S.S.
Shen
Summary: The Swiss contribution to the
international Large Coil Task was a D-shaped 2.5-m*3.5-m-bore,
superconducting toroidal field coil of 8 T. The conductor was
made from copper-stabilized, fully transposed NbTi filaments,
cabled in three stages and cooled with pressurized
supercritical helium. The testing of the six coils was
successfully finished last year at Oak Ridge National
Laboratory. The AC loss measurement data taken during the
standard-II tests are presented. The results are compared with
predictions made while designing the coil. The results of the
extended-condition tests are discussed. Recently analyzed
quench data, taken during the entire test period, are
presented. The data showed that a fully transposed filamentary
conductor with short twist pitch and well-designed resistive
barriers reduces substantially the AC losses in the windings.
The overall current density reached 33 A/mm/sup 2/ at 8.96 T
and 29.6 T/mm/sup 2/ at 9.1 T in the high- and maximum-field
torus tests, respectively. This demonstrated that the
NbTi-based CH conductor had a sufficient current
margin. |
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Advanced
superconducting MHD magnet design for a retrofit power
plant P.G. Marston, J.R. Hale and
A.M. Dawson
Summary: A magnet has been
designed for an MHD (magnetohydrodynamic) topping cycle
retrofit of a conventional power plant. The channel power
output will be approximately 35 MWe. The magnet which will
have 4.5-T peak on-axis field, will be constructed of an
unusual NbTi superconductor wound into four subunits per
dipole half. These will consist of three 45 degrees saddle
coils with circular or ellipsoidal end turns and a single
planar coil with a modified racetrack shape that will serve
principally as a field-shaping coil. This planar coil also
allows a substantial reduction in the ratio of peak to central
field strength. Among the unique features of this design will
be the use of flexible bands in tension as the primary element
of the transverse force containment structure. The conductor
will be of the cable-in-conduit type with a cable having a low
copper-to-superconductor ratio and a thick-walled aluminum
conduit sheath. The sheath will support the axial loads on the
saddles and will also provide thermal mass for protection
against overheating in the event of an energy dump. The
analysis and design of this magnet system and its projected
advantages in both performance and economics are
discussed. |
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Comparison
of two ICCS conductors for MHD
application J.R. Hale, P.G.
Marston and A.M. Dawson
Summary: Two
subscale, internally cooled, cabled superconductors (ICCSs)
have been examined as candidates for use in a retrofit MHD
(magnetohydrodynamic) topping cycle magnet. One of these was a
3*3*3 cable in which all the strands were multifilamentary
NbTi stabilized with copper. The other was a 3*3*3 cable in
which two strands in each of the nine triplets wa OFHC copper
and one was multifilamentary NbTi. The overall
copper-to-superconductor ratio for each of the two 27-strand
cables was approximately the same. The two conductors were
cowound onto a grooved mandrel in such a way that they could
be tested alternately. Each sample was instrumented with a
heater at the center of the conductor length and with a
pressure transducer, four pairs of voltage taps, and one
iron-doped gold/constantan thermocouple. Performance tests of
the conductors were made at 6-, 7-, and 7.8-T background
magnetic fields and at heater input energies ranging from 60
mJ/cm/sup 3/ to 1758 mJ/cm/sup 3/ of conductor. The results of
these tests and their significance for MHD magnet design and
economics are discussed. |
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An
overview of the SMES ETM program: the Bechtel team's
perspective R.J. Loyd, T.E.
Walsh, E.R. Kimmy and B.E. Dick
Summary:
Phase I of the conceptual design and component
development for the SMES/ETM (Superconducting Magnetic Energy
Storage/Engineering Test Model) program is discussed. The
approach chosen is to develop and test all key components at
full scale; in particular, the selected operating current for
the conductor is 200 kA. The ETM is supposed to deliver from
400 to 1000 MW for 100 s and from 10 to 20 MW for 2 to 3 h.
The lower end of each range was chosen for the design
described. The result is a 21-MWh coil discharging 56% of its
energy at 400 MW in 100 s or discharging 95% of its energy at
10 MW over 2 h. The design philosophy is
discussed. |
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Current
diffusion effects on the performance of large monolithic
conductors C.A. Luongo, R.J. Loyd
and C.L. Chang
Summary: Describes a
mathematical model developed to study the effects of slow
current diffusion on the performance of large monolithic
conductors. The model is based on the numerical solution of
the coupled heat and magnetic flux (current) diffusion
equations in slab geometry. It is used to calculate stability
margins for large monoliths; relatively low margins are
predicted. Normal-zone propagation velocities are also
calculated. It is shown that for large monolithic conductors
with long current diffusion times a propagating normal zone
can be established even in a conductor designed under the
traditional cryostability criterion. Examples are given to
illustrate the fact that for large monoliths cryostability
must be assigned a new meaning (or lack of). The results are
pertinent to the development of full-scale superconducting
magnetic energy storage. |
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AC
losses in the SMES conductor and coil
structure H. Gurol, L. Motowidlo
and C. Luongo
Summary: The authors present a
methodology for calculating AC losses in the SMES/ETM
(Superconducting Magnetic Energy Storage/Engineering Test
Model) conductor and coil structure. Coupling and eddy current
losses are discussed for a 100-s discharge of a 143-m-diameter
ETM with a total stored energy of 21 MWh. The chosen pitch
length, filament size, and winding configuration are shown to
result in coupling losses that are small during this 400-MW.
The total coupling losses are about 0.0048 J/cm/sup 3/. The
structural current losses result in energy deposition of about
0.05 J/cm/sup 3/ during the same discharge. The energy is
being deposited in the structure (away from the conductor) and
over a very long time as far as stability is concerned
(100/s). In addition, the highest power deposition occurs
toward the end of the discharge, when the conductor current is
lowest. The AC losses during slow utility discharge,
calculated using the proposed methodology, represent part of
the steady-state heat load to be removed by the refrigeration
system. |
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SMES
conductor test program J. Colvin,
F.R. Huson, P. Pissanetzky, R. Rocha, W. Schmidt, G. Shotzman,
D. Stanko, J. Zeigler, P. VanderArend and J.
Purcell
Summary: Phase I of the
Superconducting Magnetic Energy Storage/Engineering Test Model
(SMES/ETM) project includes a test program to evaluate
superconductors under development. Key aspects of this program
are presented including the operation of the high test
facility at 1.8 K. Descriptions are given of the 1.8-K
refrigeration, the background field, the current supply, and
the test setup. |
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Thermal-hydraulic
simulation of helium expulsion from a cable-in-conduit
conductor C.A. Luongo, R.J. Loyd,
F.K. Chen and S.D. Peck
Summary: The authors
present simulation results for a quenching cable-in-conduit
conductor (CICC) for application in superconducting magnetic
energy storage (SMES). The details of the model and its
computational implementation (the code HEDUMP) are discussed.
An extensive verification process shows that HEDUMP can
accurately model a quenching CICC. Preliminary results of the
simulation are given and discussed. In particular, the normal
zone propagation is studied. It is shown that CICCs exhibit a
thermohydraulic quench-back behavior whereby superconducting
regions ahead of the normals front are suddenly driven normal
by frictional dissipation and/or compression heating of the
fluid. |
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SMES
conductor design D.L. Walker,
F.M. Kimball, E.R. Kimmy, S.D. Peck, H.H. van den Bergh, R.J.
Loyd and C.A. Luongo
Summary: The authors
report the current status of the 200-kA SMES (superconducting
magnetic energy storage) conductor design and the future test
and development plans. The basis is given for selecting the
design criteria for each requirement used in the conductor
tradeoff study. A CICC (cable-in-conduit conductor) concept
has been selected for the SMES system because it represents
the most appropriate route for current scale-up to 200 kA (or
more) as required by SMES. A CICC has a high stability margin,
utilizes proven manufacturing processes, eliminates the need
for a high-risk helium vessel and complex helium dump system,
and is cost-effective. The results of thermal and stress
analyses are given for stability and thermal and magnetic
loadings. Finally, a preliminary manufacturing plan is
described. |
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Design
for a superconductive power cable for
SMES Q. Xiuying, R.L. Kustom and
J.J. Skiles
Summary: Superconductive power
cables are considered for use as the busbar between a 5500-MWh
superconductive magnetic energy storage (SMES) coil and the
power-conditioning circuits. The busbar is configured to
minimize forces due to the magnetic field. Three design
options for the superconducting cable are presented. The peak
operating current is 230 kA. Room-temperature copper cable is
connected in parallel to the superconductive cable for
redundant operation in the event of loss of cooling of the
superconductor. Calculations of temperature rise versus time
without cooling are presented. |
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Self
protection of high current density superconducting
magnets M.A. Hilal and Y.M.
Eyssa
Summary: Two schemes are proposed for
superconducting magnet protection. The first is a
self-protection scheme for high-current-density conductors
using a metallic structure such as beryllium to act as a
transformer secondary, with the conductor winding acting as
the primary. A dump resistor is proposed as the second
protection scheme for comparison. It requires the use of a
lightweight high-temperature dump resistor with the current
density limited to 15 kA cm/sup 2/. The self-protection scheme
is being developed for spaceborne and mobile magnet
systems. |
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A
unique cabling machine designed to produce Rutherford-type
superconducting cable for the SSC
project J. Grisel, J.M. Royet,
R.M. Scanlan and R. Armer
Summary: Up to
25000 km of keystoned flat cable must be produced for the SCC
(Superconducting Super Collider) project. Starting from a
specification developed by Lawrence Berkeley Laboratory, a
special cabling machine has been designed to run at a speed
corresponding to a maximum production rate of 10 m/min. This
cabling machine is the key part of the production line, which
consists of a precision Turkshead equipped with a variable
power drive, a caterpillar, a dimensional control bench, a
data acquisition system, and a take-up unit. The main features
of the cabling unit described are a design with nearly equal
path between spool and assembling point for all the wires and
the possibility of running the machine with several over- or
under-twisting ratios between cable and wires. These
requirements led to the choice of an unconventional mechanical
concept. With this equipment, the first trial run using 30
strands of superconducting wire produced cable within the
dimensional tolerances at speeds up to 10 m/min without any
crossovers. |
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An
alternate method for designing dipole magnet
ends W.L. Pope, M.A. Green, C.
Peters, S. Caspi and C.E. Taylor
Summary: The
authors present a novel procedure for designing dipole magnet
ends which can be applied to magnets with either cylindrical
or conical bulged ends to produce integrated field multipoles
which meet the constraints imposed by the SSC (Superconducting
Super Collider) lattice. The method makes it possible to
couple existing multiparameters optimization routines (i.e.
MINUIT with suitable independent parameter constraints) with a
computer code DIPEND, which describes the multiples, so that
any reasonable objective (i.e. minimizing integrated sextupole
and decapole) can be met. The authors describe how the
computer method was used to analyze the bulged conical ends
for an SSC dipole. |
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Superconducting
Super Collider second generation dipole magnet cryostat
design R.C. Niemann, R.C.
Bossert, J.A. Carson, N.H. Engler, J.D. Gonczy, E.T. Larson,
T.H. Nicol and T. Ohmori
Summary: The
second-generation cryostat design for the Superconducting
Super Collider has been developed utilizing the experiences
gained during the construction, installation and operation of
several full-length first-generation dipole magnet models. The
nature of the cryostat improvements is described. Considered
are the connections between the magnet cold mass and its
supports, cryogenic supports, cold mass axial anchor, thermal
shields, insulation, vacuum vessel, and interconnections. The
details of the improvements are enumerated and the results of
available component and system evaluations are
presented. |
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SSC
detector solenoid R.W. Fast, J.H.
Grimson, R.D. Kephart, H.J. Krebs, M.E. Stone, E.D. Theriot
and R.H. Wands
Summary: A detector utilizing
a superconducting solenoid is being discussed for the
Superconducting Super Collider (SSC). A useful field volume of
8-m diameter*16-m length at 1.5-2 T ( approximately 1 GJ at 2
T) is required. It has been decided that all of the particle
physics calorimetry will be inside the bore of the solenoid
and that there is no need for the coil and cryostat to be thin
in radiation lengths. An iron yoke will reduce the excitation
required and will provide muon identification and a redundant
momentum measurement of the muons. A conceptual design was
developed to meet these requirements. The magnet will use a
copper-stabilized Nb-Ti conductor sized for a cryostable pool
boiling heat flux of approximately 0.025 W/cm/sup 2/. The
operating current, current density, coil subdivision, and dump
resistor have been chosen to guarantee that the coil will be
undamaged should a quench occur. The 5000 metric tons of
calorimetry will be supported from the iron yoke through a
trussed cylindrical shell structure separate from the
cryostat. The coil and case, radiation shield, and stainless
vacuum vessel would be fabricated and cryogenically tested as
two 8-m sections. |
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Integrated
thermal analysis of the production dipole magnets for the
superconducting super
collider G.E. Hardy and S.D.
Peck
Summary: A thermal model of a
16.6-m-long Superconducting Super Collider dipole cryostat was
developed to evaluate the system thermal behavior of the
dipoles. The model incorporates in detail all the heat flow
paths in the cryostat with the capacity to model all
steady-state and transient thermal boundary conditions to
which the cryostat is subjected during normal and upset
conditions. The model predicts heat leaks to the cryogenics of
25.4 W per dipole to 80 K, 2.64 W per dipole to 20 K, and
0.174 W per dipole to 4.35 K during normal operation, which
compare favorably to the established heat leak budgets. The
model shows that the synchrotron radiation does not raise the
temperature of the winding more than 0.2 K relative to the
inlet temperature of the helium. Cooldown from room
temperature to 55 K is shown to take from 1.5 to 3 h with flow
rates of the order of 100-200 g/s. Warmup to room temperature
from steady-state operating conditions takes approximately
17.5 h using 5-kW heaters. Recooling of the winding following
a quench takes about 3.5 min from 70 K and 9 min from 300
K. |
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Design
study on future accelerator magnets using largely keystoned
cables K. Ishibashi, K. Kamezawa,
A. Katase, A. Terashima, Y. Sakakibara, Y. Funahashi and H.
Hirabayashi
Summary: In the near future,
superconducting magnets with a small beam aperture will be
required for high-energy proton colliders. When coils are
designed to be self-arched over a small beam pipe, the magnet
construction is simplified both in the winding procedure and
in the field quality control. Fabrication of these coils
requires cables with large keystone angles. A braid-in-strand
structure was devised to produce these cables. Two types of
self-arched coils were found to produce good field
homogeneity. Prior to construction of a whole magnet, a
straight section of a circular-aperture magnet was fabricated
to obtain detailed information on the cable position. Strand
positions have been measured on the cross section of the coil.
Magnetic field analysis is made on the basis of the position
of the strands. |
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The
first, industry made, model magnet for the CERN Large Hadron
Collider R. Perin, D. Leroy and
G. Spigo
Summary: The first magnet model for
the LHC (Large Hadron Collider) is a single-aperture,
1-m-long, dipole magnet based on NbTi cable technology and
designed for an 8-T nominal field at 2 K. It reached and
passed its nominal field without any quench and attained 9.1
T, where it operated without quenching before the first
campaign of tests was voluntarily stopped. In a second test
campaign a 9.3 T central field was attained. After a
description of the magnet, the authors discuss the cable
characteristics, manufacturing tools, and magnet
performance. |
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Towards
a 1 m high field Nb/sub 3/Sn dipole magnet of the ELIN-CERN
collaboration for the LHC-project-development and
technological aspects S. Wenger,
F. Zerobin and A. Asner
Summary: The main
design features of the 1-m-long Nb/sub 3/Sn superconducting
dipole model for the Large Hadron Collider (LHC) are
presented. The magnets are designed to operate at 4.2 K with a
nominal field of about 9-10 T. After describing the general
design of the model magnet, the authors discuss electrical and
mechanical computations, short-model tests, cable tests, and
coil-winding experience. |
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Optimization
of multiwire coil ends having 45 degree
bends G.H.
Morgan
Summary: Multiwire is the name of a
proprietary process for affixing small-diameter wires to a
flat substrate using digitally controlled machinery. It is
currently being used to wind trim coils for the SSC
(Superconducting Super Collider) dipoles on a flexible
substrate which is wrapped around the beam tube. The current
multiwire process does not permit a change in direction of the
wire other than 45 degrees . The author investigates the
location of 45 degrees bends in the flattened coil along
straight lines in such a way as to eliminate or reduce higher
harmonics in the ends. |
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Time
variations of fields in superconducting magnets and their
effects on accelerators D.A.
Herrup, M.J. Syphers, D.E. Johnson, R.P. Johnson, A.V.
Tollestrup, R.W. Hanft, B.C. Brown, M.J. Lamm, M. Kuchnir and
A.D. McInturff
Summary: The time dependence
of magnetic fields in the superconducting magnets of the
Fermilab Tevatron is discussed. A field variation of order 1 G
at the aperture radius is observed. Studies on both full-sized
Tevatron dipoles and prototype magnets have been used to
elucidate these effects. Explanations based on eddy currents
in the coil matrix or on flux creep in the superconducting
filaments are explored with these tests. Measurement results
and techniques for controlling the effect based on novel
laboratory tests and the latest accelerator operation are
presented. It is shown that small but important changes in the
fields occur with (ln t) time dependencies. For accelerator
operation, in both laboratory and accelerator measurements,
some events which might be associated with nonsmooth behavior
(sudden changes on the relevant time scale) and with
deviations at long times from observed (ln t) behavior have
been seen. |
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Studies
of time dependence of fields in Tevatron superconducting
dipole magnets R.W. Hanft, B.C.
Brown, D.A. Herrup, M.J. Lamm, A.D. McInturff and M.J.
Syphers
Summary: The time variation in the
magnetic field of a model Tevatron dipole magnet at constant
excitation current has been studied. Variations in symmetry
allowed harmonic components over long time ranges show a log t
behavior indicative of 'flux creep'. Both short-time-range and
long-time-range behavior depend on the excitation history.
Similar effects are seen in the remnant fields present in
full-scale Tevatron dipoles following current ramping. Both
magnitudes and time dependences are observed to depend on
details such as ramp rate, flattop duration, and number of
ramps. In a few magnets, variations are also seen in
symmetry-disallowed harmonics. |
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Experience
during series production of the superconducting HERA dipole
magnets C.-H. Dustmann, M.
Forster and D. Bonmann
Summary: A production
flow chart for the series production of the superconducting
HERA dipoles at a rate of one magnet per day is presented.
Some of the major problems which occurred during the series
production of the HERA magnets and their solution are
discussed. Experience obtained with collared coil production,
coil sizes, and quench heaters is outlined. |
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Manufacture
and testing of 465 km superconducting cable for the HERA
dipole magnets R.K. Maix, D.
Salathe, S.L. Wipf and M. Garber
Summary:
Experience with strand and cable manufacturing are
presented together with mechanical and electrical data. A
comparison of the critical currents of the cables with the sum
of the critical currents of the 24 individual strands before
cabling shows that with the usual definition the cables have
practically no degradation. Finally, it is shown how different
cable compactions and an additional calibration run through
the cabling machine influence the critical
current. |
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First
industry made superconducting quadrupoles for
HERA R. Auzolle, J. Perot, J.M.
Rifflet, A. Fokken, O. Peters and S. Wolff
Summary:
246 superconducting quadrupoles of ten different kinds
have to be built for the HERA proton machine at DESY, Hamburg,
West Germany. After several prototypes were made and tested in
the SACLAY laboratory the mass fabrication was committed to
industry. Three different European firms have been selected to
build both the magnets and their cryostats. The technology
transfer from laboratory to industry, the fabrication control,
and the results on the first magnets made are
described. |
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Prototype
thin superconducting solenoid for particle astrophysics in
space T. Mito, A. Yamamoto, Y.
Makida, T. Haruyama, H. Inoue, N. Kimura, N. Minowa, H.
Yamaoka and Y. Doi
Summary: A description is
given of a prototype magnet which is a very thin and
lightweight superconducting solenoid 1 m in diameter, 1.3 m in
length and with a 1.27 T central field. The authors plan to
use this device in balloon experiments scheduled for 1990 to
measure high-energy cosmic rays as preliminaries to the
experiment in space. They have constructed the model magnet,
which is the same size as the prototype magnet and has 1/4
superconducting wire and 3/4 dummy wire. The cooling,
exciting, and quench properties of this magnet were measured
and compared with design values to verify the coil winding
technique and the static indirect cooling method using pure
aluminum. |
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Design
of a large superconducting spectrometer
magnet T. Shintomi, O. Hashimoto,
Y. Makida, T. Mito, T. Nagae and Y. Yamanoi
Summary:
A superconducting spectrometer magnet for nuclear
physics experiments is currently under construction. The
magnet has a sector-type coil. The magnetic field is 3 T with
a magnet gap of 50 cm, and the stored energy is 11.8 MJ. Ease
of operation and maintenance are taken into consideration in
the design. Three-dimensional magnetic field calculations and
the stress analysis have been performed. The code QUENCH was
applied to determine the operating current and check coil
safety. A current of 500 A was selected. Heat leaks were
checked and estimated to be <2 W at 4 K. A small
refrigerator is to be used for thermal insulations at 80 and
20 K. |
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Magnetic
structure for a superconducting variable frequency electron
cyclotron resonance ion
source T.A. Antaya, A.F. Zeller,
J.M. Moskalik, H.G. Blosser, J.A. Nolen and K.A.
Harrison
Summary: Electron cyclotron
resonance (ECR) ion source are, at present, the leading source
of highly charged DC beams of positive ions. Fully stripped
light ions through argon have been obtained, and ionization
levels of 25-35 electrons removed have been obtained for heavy
mass species. Major applications of ECR ion sources for highly
charged ions now include approximately 30 nuclear, atomic, and
high-energy physics laboratories worldwide. |
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A
17 T NbTi-Nb/sub 3/Sn superconducting magnet with low-purity
holmium core L.Z. Lin, K.W. Li,
C.Y. Lin, S. Han, X.S. Li and L.G. Yan
Summary:
A 14-T NbTi-Nb/sub 3/Sn superconducting magnet system
with 31-mm bore has been constructed and tested. The magnet
system consists of three coils one of NbTi and two of Nb/sub
3/Sn. Two different cross sections of Nb/sub 3/Sn wire
constructed using the bronze method were chosen for the outer
Nb/sub 3/Sn coil of 80-mm bore, and an (NbTi)/sub 3/Sn wire
made by the high-tin-content Nb tube method was used to build
the inner Nb/sub 3/Sn coil. The NbTi-Nb/sub 3/ magnet system
has successfully achieved a 14.42 T central field at 4.2 K. To
obtain a higher magnetic field, a pair of low-purity holmium
cores of 25-mm diameter were used. The test of the NbTi-Nb/sub
3/Sn magnet with the holmium cores shows that the central
field increases from 14 T to 17 T at 4.2 K. |
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The
optimization of NbTi-Nb/sub 3/Sn high field superconducting
magnet used for physics
experiments B. Han, S. Han and
Z.X. Feng
Summary: An approach to
optimum-cost design of a multigraded NbTi-Nb/sub 3/Sn
high-field superconducting magnet is proposed. Investigation
shows that by reasonable choice of the contribution of NbTi
and Nb/sub 3/Sn coils to the central field and by properly
increasing the beta of both, an optimum-cost design of the
NbTi-Nb/sub 3/Sn solenoid magnet can be obtained. As an
example, a calculation of a 14-T three-graded NbTi-Nb/sub 3/Sn
superconducting magnet with a bore of 31-mm diameter is
given. |
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Design
and model tests of a 7.5 T wiggler for the Max-Lab synchrotron
radiation facility J.-T.
Eriksson, L. Kettunen, R. Mikkonen and L.
Soderlund
Summary: A three-pole
superconducting wiggler magnet has been designed to provide a
peak flux density of 7.5 T. Two-dimensional software was used
for narrowing down size parameters and a 3-D program was used
for calculating the end fields. The design criteria were
maximum peak flux density at minimum overall length and
minimum helium boil-off. A 2/3-scale model magnet has been
built and tested in order to gain information about the
three-pole concept. Experimental verification of the
calculated flux densities along the beam track has been
obtained. |
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Construction
of optimized superconducting spin precession magnets for
neutron spectroscopy H.H.J. ten
Kate, R.A. Hartmann, C.M.E. Zeyen, B. ten Haken and L.J.M. van
de Klundert
Summary: The design and
construction of a superconducting magnet system for a
high-resolution neutron spin echo spectrometer. The principal
solution for the field shape of optimal precession magnets is
B/sub 0/cos/sup 2/( pi z/L). In practical precession magnets,
this field shape is approximated by 30 superimposed concentric
solenoids with a bore of 80 mm. The required field integral of
1 Tm, providing 10/sup 4/ precession turns, is achieved in a
magnet with a length of about 1.5 m. The field in the center
is 1.5 T maximum. The relative line integral inhomogeneity of
about 10/sup -3/ obtained with such a coil is improved to less
than 10/sup -6/ by two in-beam correction coils. The advanced
homogeneity level means that after 10000 precession turns the
precession angle remains still well defined without using
tedious correction procedures. |
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Some
aspects of modern theory of applied
superconductivity N.N.
Martovetsky
Summary: Insight into applied
superconductivity (SC) based on C.P. Bean's model (1964) of
critical state concerning processes which take place in the SC
wires is used to develop a series of recommendations for
superconductor advances. There are many principal facts from
experiments and practice which can not be explained within the
framework of this understand, especially for nonstabilized
conductors with high currents. This contradiction originates
from the fact that a transition characteristic of a
superconductor is smoothed in a region of electrical fields
where modern superconductors work. This fact required revision
of many aspects of the theory and practice of applied
superconductivity. A review of the main results which have
been obtained is presented. The possibility of stable
performance of SC composites with high currents and poor
cooling with various types of disturbances is
given. |
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General
formulas for the adiabatic propagation velocity of the normal
zone A. Devred
Summary:
The influence on the propagation velocity of the normal
zone of four phenomena was investigated: (1) the temperature
dependence of the specific heat and the thermal conductivity;
(2) the current-sharing zone; (3) the electromagnetic
diffusion of current through a possible superstabilizer; and
(4) the thermal diffusion through a possible insulator. At the
beginning, these influences were studied independently of each
other. In all cases, after a model of the particular
phenomenon was created, the equations of thermal and
electromagnetic behavior were solved analytically and the
expressions for the propagation velocity obtained. These
expressions were then put into nondimensional forms, which
have allowed four correction factors to be defined that take
into account the influences studied, and depend only on one or
two nondimensional parameters. Subsequent studies were made to
investigate how to combine these correction factors to obtain
general formulas for the velocity, taking into account a part
or all of these influences. A review is presented of these
formulas, which are of interest for the superconducting
windings where the heat transfer to helium can be neglected on
the time scale of the quench process. |
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Designing
for superconducting magnet stability using minimum propagating
zone theory P.W.
Eckels
Summary: Minimum propagating zone
(MPZ) theory has been applied to the design of several
superconducting coils which were, at the time of their
conceptualization, novel and unique concepts. Each of the
coils, the 300-kJ pulsed discharge coil, the Nb/sub 3/Sn
forced flow-cooled Large Coil, and an epoxy potted coil,
utilized a form of the MPZ theory to relate a different type
of cooling and structural support system to the chosen magnet
stability criterion. The design philosophy, analysis, and
performance of the magnets are reviewed, and correlation of
the quench conditions with conductor diameter is clearly
indicated. The MPZ concept is shown to be a powerful tool, and
it has had a major impact on the design parameters and
operating modes of these coils. |
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Quench
pressure, thermal expulsion, and normal zone propagation in
internally cooled
superconductors L.
Dresner
Summary: When a nonrecovering normal
zone appears in an internally cooled superconductor, the
pressure in the conductor rises, helium is expelled from its
ends, and the normal zone grows in size. A model of these
processes is presented that allows calculation of the
pressure, the expulsion velocity, and the propagation velocity
with simple formulas. The model is intended to apply to
conductors such as the cable-in-conduit conductor of the
Westinghouse LCT (Large Coil Task) coil, the helium volumes of
which have very large length-to-diameter ratios (3*10/sup 5/).
The predictions of the model agree with the rather limited
data available from propagation experiments carried out on the
coil. |
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Quantification
of disturbance energy due to conductor motion and stability
analysis of superconducting composite
wire O. Tsukamoto, T. Takao and
S. Honjo
Summary: The authors propose a model
to quantify the disturbance energy due to conductor motion.
The model relates the disturbance energy to mechanical
properties of the conductor, electromagnetic force, and
winding structure. The stability of a superconducting
composite conductor subject to an abrupt conductor motion is
analyzed on the basis of the proposed model. It is pointed out
that the stability of the conductor is improved by decreasing
the distance between spacers placed between the conductors.
However, it is pointed out that there is a limit to the
improvement, because of limited accuracies of the conductor
and spacer dimensions. |
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Influence
of dump voltage and allowable temperature rise on stabilizer
requirements in superconducting
coils S.W.
Schwenterly
Summary: The authors present some
simple relations between the dump voltage and the stored
energy, temperature rise, and coil geometry that are useful in
scoping the required amount of stabilizer. Comparison with
some recently proposed fusion magnet system designs indicate
that excessive dump voltages could result in some cases.
High-temperature superconductors may require more stabilizer
than the conventional alloys. Calculations with simple model
coil systems indicate how tradeoffs between between various
coil parameters affect the dump voltage. |
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Development
of a forced-cooled superconducting coil with high average
current density (DPC-TJ) T.
Hamajima, A. Tanaka, H. Shiraki, M. Shibui, Y. Sanada, M.
Naganuma, M. Shimada, T. Fujioka, M. Nishi, H. Nakajima, K.
Koizumi, Y. Takahashi, T. Ando, H. Tsuji and S.
Shimamoto
Summary: A forced-cooled
superconducting coil (DPC-TJ) using a double-walled
cable-in-conduit, the so-called Performed Armor CICC, has been
developed. The DPC-TJ is an advanced coil with an average
current density of 40 A/mm/sup 2/, operating current of 24 kA,
and field of 12 T. The DPC-TJ coil has several advantages such
as mechanical rigidity, negligible degradation of critical
current, sound electrical insulation, and excellent winding
tolerance compared with a conventional forced-cooled coil. A
superconducting strand was developed with the aim of improving
the critical current density and resistivity. The resulting
Nb-tube-processed (NbTi)/sub 3/Sn strand fulfils the
requirement that the critical current density be more than 600
A/mm/sup 2/ at 12 T and the RRR (residual resistivity ratio)
be higher than 50. |
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Structural
characteristics of proposed ITER TF coil
conductor C.R. Gibson and J.R.
Miller
Summary: An analysis has been
conducted of the effect of transverse loading on a
cable-in-conduit conductor which has been proposed for the
toroidal field coils of the International Thermonuclear
Experiment Reactor. The primary components of this conductor
are a loose cable of superconducting wires, a thin-wall tube
for helium containment, and a U-shaped structural channel. A
method is given where the geometry of this conductor can be
optimized for a given set of operating conditions. It is
shown, using finite-element modeling, that the structural
channel is effective in supporting loads due to transverse
forces and internal pressure. In addition, it is shown that
the superconducting cable is effectively shielded from
external transverse loads that might otherwise degrade its
current-carrying capacity. |
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Detailed
distribution of AC loss in the NET TF
coils L. Bottura and J.
Minervini
Summary: The AC loss in the NET
(Next European Torus) TF (toroidal field) coil has been
computed as a function of space and time using a field map of
the DC toroidal field components and AC poloidal field
components for three proposed conductor designs. Resolving the
field B, field change Delta B, and field derivative dB/dt into
components transversal and parallel to the conductors, the
value of the coupling, eddy currents, and hysteresis losses
was computed using a set of simplified expressions. The
calculation performed on the whole coil is presented, using
the distributions along a developed pancake of the
instantaneous and average values of AC loss. |
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Large
Coil Task instrumentation and diagnostics-a
review J.W. Lue, J.F. Ellis, W.A.
Fietz, M.S. Lubell, J.N. Luton, S.W. Schwenterly, C.T. Wilson
and R.E. Winterberg
Summary: The goal of the
Large Coil Task (LCT) was to develop large toroidal
superconducting magnets for fusion reactors. Each of the six
coils built for this task was heavily instrumented with some
200 to 400 sensors and diagnostic voltage taps to test its
performance and characteristics. In addition, more than a
thousand sensors were installed in the facility and test-stand
components to ensure safe and controlled operation. A review
is presented of the various types of thermometers, pressure
transducers, flowmeters, strain gages, displacement
transducers, acoustic emission sensors, field probes, and
other diagnostic instrumentation used in the LCT. The
usefulness of the sensors, the difficulties with some of them,
and the reliability of different groups of sensors in this
task are described. |
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Progress
in the manufacture of the US-DPC test
coil M.O. Hoenig, M. Takayasu,
R.N. Randall, J.E. Tracey, J.R. Hale, M.M. Morra, I. Hwang and
P. Marti
Summary: A superconducting ohmic
heating coil is being built by MIT to be tested in early 1990
at the Japan Atomic Energy Research Institute (JAERI). This
2-m-diameter coil will be wound from Nb/sub 3/Sn
cable-in-conduit conductor. The coil and conductor are briefly
described. Manufacturing procedures and problems encountered
in the fabrication of the conductor up to August 1988 are
considered. Included are descriptions of wire chrome plating,
cable manufacture, and conduit fabrication. |
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Current
leads with high T/sub c/ superconducting bus
bars A. Matrone, G. Rosatelli and
R. Vaccarone
Summary: The feasibility of
combined current leads with a superconducting extension using
high-T/sub c/ superconductors is demonstrated. The performance
of systems with heatsink at room temperature or at LN/sub 2/
temperature are compared. In both cases the leads and the
busbars are cooled by counterflowing helium gas and the
superconducting part can be, depending on T and I, in the
superconducting state, in the normal state, or in the
current-sharing regime. The improvement obtained by such a
system is important notwithstanding the limited current
capacity of today high-T/sub c/ superconductors, at least for
the LN/sub 2/-cooled system. |
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Superconducting
cyclotron for medical
application H. Blosser, J.
Bailey, R. Burleigh, D. Johnson, E. Kashy, T. Kuo, F. Marti,
J. Vincent, A. Zeller, E. Blosser, G. Blosser, R. Maughan, W.
Powers and J. Wagner
Summary: A medical
superconducting cyclotron, the first such project to be
undertaken, is under construction. The cyclotron will be used
to produce neutron beams for use in radiation treatment of
cancer. The purpose of such treatment is to administer lethal
doses of radiation to tumor cells while minimizing dose to
normal tissue; rather intense neutron beams are required for
the treatments to occur in a reasonably expeditious time and
the device needs to be able to fire the neutrons at the
patient from any angle. The authors briefly review the medical
rationale for such a device and then describe the cyclotron
design with special emphasis on details of the superconducting
coil and cryostat. |
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Superconducting
magnets for whole body magnetic resonance
imaging M.F.
Murphy
Summary: Superconducting magnets have
achieved preeminence in the magnetic resonance imaging (MRI)
industry. Further growth in this market will depend on
reducing system costs, extending medical applications, and
easing the present siting problem. Novel magnet designs are
described that address these issues. Compact magnets are
economical to build and operate. Two 4-T whole-body magnets
for research in magnetic resonance spectroscopy (MRS) are now
in operation. Active-shield magnets, by greatly reducing the
magnetic fringe fields, will allow MRI systems with
superconducting magnets to be located in previously
inaccessible sites. |
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A
5 Telsa imaging magnet for imaging laboratory
animals J.L. Carolan, W.A. Burns
and M.A. Green
Summary: A report is presented
on the construction of the first of a series of magnetic
resonance imaging (MRI) imaging magnets for laboratory
animals. The magnet has a 33-cm warm bore with a design
central induction of 5.5 T without active shielding and 5.0 T
with active shielding. The magnet will be used for both
imaging and spectroscopy of living animals. The active shield
system is designed so that the 5-G line is less than 3 m from
the magnet center when the magnet operates at design field.
This permits the magnet to be used within an experimental
space commonly available within a university
building. |
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A
4.7-T magnet with a 0.3 M bore for P-31 magnetic resonance
spectroscopic imaging H. Maeda,
M. Urata, T. Wada, T. Yazawa and A. Sato
Summary:
A 0.3-m-bore, 4.7-T, high-homogeneity magnet, used for
P-31 magnetic resonance imaging, has been constructed and
installed. The magnets are comprised of a set of solenoids,
which attained designed 94.8-A current (1.8 MJ) without
quenching. Diodes in the cryostat resulted in a fast ramp rate
of 3 T in 10 min. The field inhomogeneity for the bare magnet
was -250 p.p.m./0.1 m dsv, which was finally reduced to 6
p.p.m./0.1 m dsv by shim coils. The field decay was less than
0.05 p.p.m./h. The magnet system is currently used for P-31
magnetic resonance spectroscopic imaging experiments on living
animals in the laboratory. |
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600
MHz spectrometer magnet J.E.C.
Williams, S. Pourrahimi, Y. Iwasa, L.J. Neuringer and L.
Motowidlo
Summary: An NMR-grade
superconducting magnet has been constructed which operates in
persistent mode at 14.25 T at reduced temperature. The stored
energy is 840 kJ. The magnet incorporates niobium-titanium and
niobium-tin multifilamentary conductors. In the persistent
mode all windings are in series with superconducting joints
between sections. Six of the joints are hybrids, niobium-tin
joined to niobium-titanium. The niobium-titanium windings
reached short sample without training. The niobium-tin
sections suffered two training quenches before reaching 14.25
T. |
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A
full scale superconducting rectifier for powering an
MRI-magnet J. Sikkenga and H.H.J.
ten Kate
Summary: The design, construction,
and experimental investigation of a full-scale superconducting
rectifier is described. A short description of the principles
of superconducting rectifiers is given. The results of the
experiments are very satisfactory. The main goal, to establish
the reliability of the superconducting rectifier as power
supply for an MRI magnet system, was fully achieved. The
demonstrated behavior of the rectifying system was as
theoretically expected. |
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Transient
characteristics of parallel running of the 20 kVA
superconducting synchronous generator and a conventional
one T. Nitta and T.
Okada
Summary: A description is given of
electrical transient characteristics of the parallel operation
of a 20-kVA superconducting synchronous generator and a
conventional one. In the experimental power system, the
superconducting generator is connected through reactors
(artificial transmission lines) to a regional power system
(infinite bus), and the conventional generator (20 kVA) is
connected to the terminal of the superconducting generator.
Synchronous closing tests, loss of synchronism tests, and
disconnecting and reclosing tests were performed. It was found
that by installing a superconducting generator (SCG) in power
systems, voltage stability and power system stability can be
improved in transient states as well as in steady states, and
the variation of armature current of SCG during a transient
period is much larger than that of the conventional one.
Simulation results in good agreement with the experimental
results were also obtained. |
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Using
a small hybrid pulse power transformer unit as component of a
high-current opening switch for a
railgun E.M.W. Leung, R.E. Bailey
and P.H. Michels
Summary: The hybrid pulse
power transformer (HPPT) is a unique concept utilizing the
ultrafast superconducting-to-normal transition process of a
superconductor. When used in the form of a hybrid transformer
current-zero switch (HTCS), this creates an approach in which
the large, high-power, high-current opening switch in a
conventional railgun system can be eliminated. This represents
an innovative application of superconductivity to pulsed power
conditioning required for the Strategic Defense Initiative
(SDI). The authors explain the working principles of a 100-kJ
unit capable of switching up to 500 kA at a frequency of 0.5
Hz and with a system efficiency of greater than 90%. Circuit
analysis using a computer code called SPICE PLUS was used to
verify the HTCS concept. This concept can be scaled up to
applications in the several mega-ampere levels. |
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Future
development of large superconducting
generators S.K. Singh and C.J.
Mole
Summary: Scientific and technological
progress in superconductivity and cryogenics that has brought
the use of superconductors to reduce the electrical power
dissipation in power equipment closer to reality are briefly
reviewed. Recently completed US programs and current foreign
and US programs are described. A US development program to
maintain leadership in the field is proposed. |
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Impact
of high temperature superconductors on pulsed power
systems S.K. Singh, J.H. Murphy,
P.W. Eckels, C.J. Mole, A.T. Male and C.K.
Jones
Summary: Some recent parametric
analyses conducted on a series of alternative baseline designs
of 5-50-MVA pulsed AC generators with
low-temperature-superconductor, high-temperature
superconductor, and cryocooled windings are discussed with
emphasis of the influence of operating temperature from liquid
helium to liquid nitrogen coolant system design, conductor
configuration, and consequent energy economics of the machine.
These are related to the projected performance of a novel
conductor concept presently under development. |
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Practical
experience on the operation of a 320 kVA synchronous generator
with a superconducting field
winding H. Bischof, W. Engl,
H.-P. Groter, H.W. Lorenzen and J. Schaller
Summary:
Eight years of experiments carried out on a 320-kVA
two-pole, external-pole-synchronous generator with a
superconducting field winding. A summary is presented of
practical experience and empirical results, which were
obtained in 39 test cycles. On back-strokes, difficulties and
problems concerning the generator and the whole test setup are
reported as well as successes. |
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Controller
action and control of a 330 kVA synchronous generator with a
superconducting excitation
winding H. Bischof, H.-P. Groter
and H.W. Lorenzen
Summary: Theoretical and
experimental investigations were conducted during the last two
years on the control of a 330-kVA synchronous generator with a
superconducting field winding. The generator is connected
either to the mains or to an island network. The
superconducting field winding of the two-pole external pole
synchronous generator is cooled indirectly by supercritical
helium in a closed cooling circuit. The machine is controlled
digitally by microcomputers at grid or island operation. A
summary is given of test results on controlled generator
operation (load decrease, variation of active and reactive
power). Measured values are compared to the values gained from
simulation are discussed. |
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A
comparison of the conductor requirements for energy storage
devices made with ideal coil
geometries W.
Hassenzahl
Summary: Superconducting magnetic
energy storage (SMES) plants have previously been proposed in
both solenoidal and toroidal geometries. The former is
efficient in terms of the quantity of superconductor required
per unit of stored energy. For applications where a fringe
field could be a problem, the toroidal geometry, which
requires at least a factor of two more material, has been
proposed. In addition to the solenoid and toroid, other
geometries are possible, such as linear multipoles and
spherical coils. These geometries have been considered for use
in applications other than energy storage. Here the
effectiveness (quantity of superconductor/stored energy) is
calculated for various coil geometries. |
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Lightweight
power bus for a baseload nuclear reactor in
space C.E. Oberly, L.D. Massie
and D.J. Hoffmann
Summary: Space
environmental interactions with the power distribution/power
processing subsystem can become a serious problem for power
systems rated at tens to hundreds of kilowatts. Utilization of
ceramic superconductors at 1000 A/cm/sup 2/, which has already
been demonstrated at 77 K, in a conductor configuration may
eliminate both bus mass and distribution voltage problems in a
high-power satellite. The analytical results presented
demonstrate that a superconducting coaxial power transmission
bus offers significant benefits in reduced distribution
voltage and mass. |
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Economic
potential of applying HiT/sub c/ superconductors to magnetic
energy storage J.-T. Eriksson and
J. Korpijarvi
Summary: The authors verify the
technical and economic specifications of SMES (superconducting
magnetic energy storage) units of arbitrary sizes, i.e. to
provide models for further, energy-management-related
simulations. The weekly energy efficiency is based on a
five/seven days discharge duty cycle (51 out of 52 weeks a
year). The efficiency includes thermal heat leak through walls
and reinforcement, current lead losses, power equipment
losses, and nitrogen production losses. Superconductor AC
losses and losses due to eddy currents in massive aluminum
parts are the only major sources that the authors have not
tried to estimate so far. The cost distribution of the 500-MWh
facility is presented. |
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Experimental
results of an experimental three-phase AC superconducting
armature Y. Brunet, P. Tixador
and P. Vedrine
Summary: Owing to the recent
development of low-loss AC superconducting conductors,
50/60-Hz applications of superconductivity in electrical
engineering can be developed, and power transformers and
limiters, AC cables, and full-superconducting machines can be
investigated. As a first step in the realization of a
full-superconducting synchronous generator, a three-phase
superconducting armature has been manufactured and tested with
a normal rotor inside. Cryogenic and electrical tests have
been made during more than 100 h, and include transients,
connection to the power grid and asynchronous start-up and
operation. The results are relevant for the next step of the
experiment which is to complete a 20-kVA, 3000-r.p.m. machine
with a superconducting rotating field winding, which is now
under construction. |
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A
two-phase full-wave superconducting
rectifier T. Ariga and A.
Ishiyama
Summary: A two-phase full-wave
superconducting rectifier has been developed as a small
cryogenic power supply of superconducting magnets for
magnetically levitation trains. Those magnets will be operated
in the persistent current mode. However, small ohmic loss
caused at resistive joints, and AC loss induced by the
vibration of the train cannot be avoided. Therefore, the
low-power cryogenic power supply will be required to
compensate for the reduction in magnet current. The
superconducting rectifiers consists of two identical full-wave
rectifiers connected in series. The main components of each
rectifier are a toroidal superconducting setup transformer and
two thermally controlled switches. The test results using a
47.5-mH-load magnet at 0.2 Hz and 0.5 Hz operations are
described. To estimate the characteristics of the
superconducting rectifier, the authors have developed a
simulation code. From the experiments and the simulations, the
transfer efficiency is determined. |
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Thermally
and magnetically controlled superconducting
rectifiers G.B.J. Mulder, H.H.J.
ten Kate, H.J.G. Krooshoop and L.J.M. van de
Klundert
Summary: The switches of a
superconducting rectifier can be controlled either
magnetically or thermally. The authors point out the
differences between these methods of switching and discuss the
consequences for the operation of the rectifier. The
discussion is illustrated by the experimental results of a
rectifier which was tested with magnetically as well as
thermally controlled switches. It has an input current of 30
A, an output current of more than 1 kA and an operating
frequency of a few Hz. A superconducting magnet connected to
this rectifier can be energized at a rate exceeding 1 MJ/h and
an efficiency of about 97%. |
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Method
for critical current testing: software
corrections M.B. Siddall and D.B.
Smathers
Summary: A digital method for
accumulating critical current date allows the test to be
automated. Digital techniques provide the opportunity to make
corrections for self field, protection shunt, and transient
voltages which are not feasible with an analog system.
Automation also allows rapid data analysis to inspect the test
for bad data. The digital current incrementing method deviates
from the ASTM test method, but the authors' measurements on
the NbTi SRM and the Japan-US B sample from round robin
testing show it to be accurate to within the limits set by
ASTM. |
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The
aluminum stabilized Nb-Ti conductor for the ZEUS thin
solenoid R. Bruzzese, S.
Ceresara, G. Donati, S. Rossi, N. Sacchetti and M.
Spadoni
Summary: The ZEUS experiment, an
experimental apparatus for high-energy physics that is to be
installed on the hadron electron ring accelerator HERA at
DESY-Hamburg, West Germany, will use a superconducting thin
solenoid surrounding a number of particle detectors. The
essential features of this magnet are 1.8-T magnetic field on
the axis, 2.3-T maximum field on the conductor, inner diameter
of 2 m, and solenoid length of 2.5 m. As physics requirements
call for a total solenoid thickness less than 0.9 radiation
length, a conductor consisting of a Rutherford-type Nb-Ti
cable clad with a high purity aluminum has been developed. A
report is presented on measurements of the main parameters
needed for a complete characterization of a conductor to be
used in this particular kind of magnet. Data on critical
current, transfer length, and residual resistivity ratio are
given and discussed, including the influence that the various
fabrication steps can have on them. |
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Training
in test samples of superconducting cables for accelerator
magnets A.K. Ghosh, M. Garber,
K.E. Robins and W.B. Sampson
Summary: In the
critical current measurement of some high-current NbTi cables,
the samples have to be 'trained' by repeated quenching to
obtain a usable voltage-current curve for I/sub c/
determination. This training behavior is most pronounced when
the applied field is perpendicular to the wide face of the
conductor and is strongly dependent on the
copper-to-superconductor ratio and the clamping pressure. Data
are given for SSC (Superconducting Super Collider) prototype
cables as well as for HERA production conductors. Although a
quantitative understanding of the experimental data is still
lacking, some speculations regarding stability are
presented. |
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The
effect of transverse stress on the critical current of Nb/sub
3/Sn cable-in-conduit
superconductors L.T. Summers and
J.R. Miller
Summary: An investigation has
been conducted of the effects of transverse stress on the
critical current of Nb/sub 3/Sn cable-in-conduit conductors
(CICC). The sensitivity of the critical current (I/sub c/) to
applied stress at 12 T has been determined for CICCs with
helium void fractions (f/sub He/) in the range of 0.25-0.4.
I/sub c/ has been found to be a function of transverse stress
with good correlation with single-wire data up to stress
levels of 50 MPa. At higher stress levels with the CICCs show
significantly higher degradation with transverse stress. No
clear correlation has been found between the stress
sensitivity of I/sub c/ and f/sub He/. The results fall into a
broadband, suggesting that geometric factors, such as wire
position within the cable bundle, exert a strong influence on
the I/sub c/ vs. stress dependence. |
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Flux
creep in a Tevatron cable M.
Kuchnir and A.V. Tollestrup
Summary: The
authors measured the slow magnetization decay of short sample
(2.3 cm) of Tevatron cable in fields up to 0.3 T. The special
susceptometer in development for these measurements is based
on a commercial SQUID (superconducting quantum interference
device) and is described in detail. The observed decay is
logarithmic in time as expected from flux creep in the NbTi
filaments. A strong correlation was found between the decay
and the magnetization status of the sample. It is too early
yet to present a quantitative correlation between what the
authors observed and the decay observed in the sextupole
component of Tevatron dipoles. The detailed understanding of
this phenomenon may be instrumental in guiding the search for
efficient superconducting synchrotron operational
procedures. |
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Self
field effects on the superconducting to normal transition of
S/C cables P. Fabbricatore, P.
Fernandes, R. Parodi and R. Vaccarone
Summary:
The onset of the transition of a superconducting (S/C)
cable to the normal state is described by the relation V=V/sub
c/ (the critical voltage, i/sup n+1/) where V is the voltage
drop per unit length, V/sub c/ the critical voltage, i the
reduced current I/I/sub c/ (I/sub c/ is the critical current),
and n an integer ranging from 10 to 40 depending on the cable
quality. A correct evaluation of the critical current and n
value must take into account the effects due to the self-field
at the cable. For high-current cables the self-field can
affect the measurement by about 15%. A theoretical model is
developed and it is applied to the results obtained in the
critical current measurements of the S/C cables for HERA
dipoles. It is found that the critical current and n are
strongly dependent on the self-field. |
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Application
of current simulation method for calculating magnetic field of
superconducting AC
generator Qingfu Li and Fei
Wang
Summary: A current simulation method is
presented for calculating the three-dimensional magnetic field
of a superconducting AC generator. The method represents the
influence of the boundaries by a large number of current loops
appropriately positioned at a distance behind the boundary
surface so that the boundary conditions on the actual surface
of the boundaries are satisfied. The calculated results
obtained for the model of the superconducting AC generator are
close to the measured values. |
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Application
of image method to calculate 3-D magnetic field and parameters
of SC alternator Qingfu Li and
Fei Wang
Summary: The image method has been
used for calculating the three-dimensional magnetic field and
parameters of a superconducting AC generator. The method can
represent the actual shape of the coils and take account of
the conductor cross-section. The influence of the magnetic and
conducting boundaries are represented approximately by their
images in the boundaries. The influence of either conical or
stepped boundaries in the end region is represented by forming
images on an adjustable conical surface in association with
the boundary condition, improving the accuracy of the
calculation. |
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Considerations
against a force compensated
coil W.
Hassenzahl
Summary: The cost of structural
components in a large superconducting coil such as SMES may
well exceed the coil and cryostat cost. As a result, the idea
of constructing a system composed of two different coil types
assembled in such a way that the forces balance and reduce the
total structural requirement is often proposed. A suitable
geometry has never been found for the fundamental reason that
there can be no force-compensated solution. The general
problem is presented, and an analysis of the energy stored and
stresses produced in the structure is described in a
fundamental way. Finally, relation between structural mass M
and stored energy E that is valid for all magnetic systems is
developed. |
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Structure
optimization of space borne toroidal
magnets X. Huang, Y.M. Eyssa,
M.K. Abdelsalam, L. El-Marazki, M.H. Abdelmohsen, M.A. Hilal
and G.E. McIntosh
Summary: An optimization
study aimed at minimum weight under certain constraints on
module size and weight has been carried out. This study
includes a structural material survey of materials with high
specific strength and modulus, and an analysis of the
effectiveness of coil configuration parameters on the value of
E/M, energy stored per unit mass. Some typical structure
problems, such as buckling in the bucking cylinder, are also
discussed. |
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Nb/sub
3/Sn coating of high purity Nb
cavities D. Dasbach, G. Muller,
M. Peiniger, H. Piel and R.W. Roth
Summary:
The authors have developed a modified vapor diffusion
technique using specially constructed UHV furnaces to produce
uniform Nb/sub 3/Sn layers on 1- and 3-GHz accelerator
cavities without losing the high thermal conductivity of the
Nb wall. In a first experiment with a single-cell 3-GHz cavity
of medium Nb purity (residual resistivity ratio, RRR=156) they
have achieved E/sub acc/=10 MV/m with an 0.6- mu m-thick
Nb/sub 3/Sn layer. This is so far the highest accelerating
field for their Nb/sub 3/Sn cavities, but it is low compared
to the 25 MV/m achieved in the same cavity before coating.
Possible reasons for the reduced breakdown field for Nb/sub
3/Sn are discussed on the basis of the local thermal breakdown
model. |
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Superconducting
properties of B1 nitrides film obtained by gas-metal reaction
for RF application P.
Fabbricatore, P. Fernandes, G.C. Gualco, R. Musenich and R.
Parodi
Summary: The authors investigated the
RF behavior of the B1-type superconductors NbN obtained by
reacting reactor-grade niobium samples in high-purity (10
p.p.m. of total impurities) nitrogen atmosphere after a baking
at high temperature (800 degrees C) in an UHV furnace. They
obtained a critical temperature of 14.6 K for a tetragonally
distorted cubic lattice with cell parameters a=4.381 AA and
c=4.324 AA, as determined by X-ray diffraction. The RF test
showed for the resonator, operating at 4.5 GHz, a surface
resistance at 4.2 K of 1 mu Omega in good agreement with the
value predicted by the BCS theory. |
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Influence
of condensed gases on field emission and the performance of
superconducting RF cavities Q.S.
Shu, K. Gendreau, W. Hartung, J. Kirchgessner, D. Moffat, R.
Noer, H. Padamsee, D.L. Rubin and J. Sears
Summary:
In a program to study the field emission (FE) and to
improve the performance of one-cell 1500-MHz superconducting
Nb microwave particle accelerator cavities, the authors
recently achieved peak surface fields as high as 51 MV/m
through the use of 1200 degrees C UHV annealing, methanol
rinsing, and high-power He processing. Performance is limited
by excess FE from localized points on the cavity walls.
Cycling of these cavities to room temperature and admission of
He processing gas frequently produce large changes in Q
correlating with the appearance or disappearance of the
dominant field emitter, suggesting that condensed residual and
impurity gases play a significant role in enhancing FE. By
intentionally condensing O/sub 2/ into a cold cavity, the
authors have produced similar effects, increasing the
dissipated power and reducing Q, each by an order of magnitude
at the same field level. Preliminary tests have also been
carried out with H/sub 2/ and water vapor. These results
suggest that improvements in the outgassing and vacuum
environment of these cavities may be important. |
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A
laboratory superconducting high gradient magnetic
separator L.G. Yan, Y.J. Yu, Z.K.
Wang, Z.Y. Kao, Z.X. Ye, C.L. Xue, P. Ye, Y.L. Cheng, X.M. Li,
Q.W. Kong, S.S. Song, H.L. Nan, Y.M. Dai and H.T.
Tang
Summary: To determine the effectiveness
of high-gradient magnetic separation (HGMS) for kaolin clay
purification and coal desulfurization in China and to develop
suitable technology, a superconducting HGMS facility has been
constructed and put into operation. The working separation
chamber is 80 mm in diameter and 400 mm in length. The magnet
is wound with 0.75-mm and 0.5-mm diameter NbTi superconducting
composite. The winding is compact and wax-filled. Tests prove
that the magnet can operate at 5 T. Special attention has been
paid to design and construction of the magnet cryostat. In the
wet benefication mode, there are two separation systems
available; one is an upward pumping feeding system and another
is downward gravity feeding. The rate of flow and the linear
velocity are 0-0.5 L/s and 0-100 cm/s, respectively.
Preliminary sample test results for kaolin clay purification
and coal desulfurization show the feasibility of magnetic
separation. |
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Superconducting
permanent magnets S.L. Wipf and
H.L. Laquer
Summary: The authors discuss
means to verify the analysis and the limitations imposed by
the low critical current densities in presently available
high-temperature superconductors. Based on experimental data,
it is concluded that : (1) high-temperature superconductors
can be used as permanent magnets with trapped fields of at
least 3 T at 77 K; (2) their energy product should be as high
as 1.8 MJ/m/sup 3/ or 255 MG-Oe; (3) the magnetizing field has
to be four times as large as the trapped field, or 12 T; (4)
permanent-magnet use of high-temperature superconductors
bypasses the problems of wire or tape manufacture; and (5)
widespread use of this concept will depend on improved
critical current densities. |
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The
investigation of the superconducting NMR-imaging main
magnets Y. Zhang, S. Han and Z.X.
Feng
Summary: The design principles of MRI
(magnetic resonance imaging) main magnets and the problems in
the design process are analyzed. A computer program in which
the critical characteristics of superconductor, the uniformity
of the magnetic field, the economization of the magnet, and
the selection of magnet constructions are considered has been
established. The program can also be used to design
high-uniformity superconducting magnets in some other uses.
Five different magnet constructions are treated. By analyzing
the computer results some conclusions useful for the practical
design of the magnets are obtained. |
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Optimal
design of MRI magnets with magnetic
shielding A. Ishiyama, M. Hondoh,
N. Ishida and T. Onuki
Summary: A novel
optimal design method has been developed for a highly
homogeneous superconducting coil system with magnetic
shielding for magnetic resonance imaging (MRI). The design
method is based on a combination of the hybrid
finite-element-boundary-element method for analysis of an
axially symmetric nonlinear open-boundary magnetic-field
problem and a mathematical programming method for solving the
corresponding optimization problem. The goal programming and
the nonlinear least-squares methods have been adopted. The
relative merits of a combination of an active shielding and a
room shielding have been investigated. The proposed design
method can optimize both an MRI magnet and its magnetic
shielding system simultaneously, taking into account the
maximum permissible intensity of stray field at a specified
distance from the magnet center. Several design procedures and
results of a 1-T magnet system are shown. |
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Magnetic
shielding by a tubular superconducting winding in parallel and
transverse fields K. Takahata, S.
Nishijima, M. Ohgami, T. Okada, S. Nakagawa and M.
Yoshiwa
Summary: The shielding efficiency for
parallel and transverse fields was examined using three types
of shield, and the possibility of magnetic shielding with
superconducting multifilament wire was investigated. It was
concluded that the magnetic shields with superconducting
windings could shield not only the parallel but also the
transverse fields to the axis. The stability of the
superconducting shield is discussed. |
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Development
of superconducting networks for a large scale magnetic
shield K. Takahata, S. Nishijima,
T. Okada, S. Nakagawa and M. Yoshiwa
Summary:
The shielding capabilities of a multiringed disk were
analyzed to design the shielding networks. The applicability
to a large-scale shield was examined experimentally and four
conclusions are drawn: (1) the multiringed shields show the
highest performance when the diameter of the rings is half of
that of the disks; (2) if the diameters of the rings are
adequate, a multiringed shield can control the field
distribution; (3) the shielding efficiency is almost
independent of the external field and the field sweep rate
when the shielding currents in the rings does not exceed the
effective critical current; when the shielding current reaches
the effective critical current, the efficiency decreases with
increasing external field, and in this case, flux jumps are
apt to occur; and (4) for a large-scale shielding network, the
effective critical current of rings must be
enlarged. |
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CEBAF
superconducting spectrometer
design P. Brindza, J. Alcorn, R.
Carlini, L. Harwood, J. LeRose, B. Mecking, J. Mougey and J.
O'Meara
Summary: The CEBAF (Continuous
Electron Beam Accelerator Facility) Experimental Nuclear
Physics Program is based on three independent facilities, each
with unique capabilities. Hall A consists of a pair of
high-resolution spectrometers (HRSs), hall B features a large
acceptance spectrometer (LAS), and hall C will have a
high-acceptance, high-energy electron spectrometer and
matching special-purpose hadron spectrometer. The authors
demonstrate that the performance and operational requirements
of these devices can be met effectively through the use of
superconducting devices. The physics requirements and the
rational basis for design selection are discussed. An analysis
that supports the cost-effectiveness of the selected design is
presented. The cryogenic system is covered as well as the
integration of the experimental hall cryogenic system and main
CEBAF central helium refrigerator. |
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A
superconducting toroidal magnet for the CEBAF large acceptance
spectrometer J. O'Meara, J.
Alcorn, P. Brindza, M.-S. Chew, G. Doolittle, M. Fowler, B.
Mecking, C. Riggs, D. Tilles and W. Tuzel
Summary:
The CEBAF (Continuous Electron Beam Accelerator
Facility) physics program requires a large acceptance
spectrometer covering nearly all of the 4 pi solid angle for
studying complex electronuclear interactions. This device
relies on a six-coil toroidal magnetic field to provide
momentum analysis. The desire to maximize solid angle coverage
led to a thin superconducting toroid design. The main device
has an outside diameter of 4.6 m, an active length of 5 m, a
peak field in the toroidal volume of 1.2 T, and a stored
energy of 18 MJ. An intrinsically stable design was selected
to minimize the overall space requirements of the toroid. The
individual coils of the toroid are approximately
kidney-shaped; the negative curvature provides a field-free
region for targets coming from the requirement to have good
particle focusing properties. The overall design is
presented. |
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A
superconducting dipole concept for the CEBAF high resolution
spectrometers J. Alcorn, P.
Brindza, M. Fowler, L. Harwood, T. Hunter and W.
Tuzel
Summary: CEBAF (Continuous Electron
Beam Accelerator Facility) requires a pair of high-resolution
spectrometers (HRS) for performance of electron/hadron
coincidence experiments at up to 4 GeV/c. Momentum analysis
for each arm is provided by a 45 degrees vertical bending
dipole having a useful field volume 6.0 m long * 30-40 cm wide
(pole gap) * 80 cm high. A resistive 30-cm-gap dipole has been
selected for the reference 4-GeV/c HRS, utilizing a qQQDQQ
magnet configuration. However, recent studies aimed at
equipment cost reduction indicate that a 40-cm-gap
superconducting dipole based on less stringent field
uniformity requirements may be attractive, either with the
reference HRS or with a revised design, using fewer magnet
elements. The concept described is an H-frame superconducting
magnet, with contoured poles and resistive interpole field
shaping coils, to achieve a field quality of 1 G/cm across the
aperture at all useful excitation levels. This approach yields
minimal pole width and requires only conventional fabrication
tolerances. |
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A
superconducting iron-dominated quadrupole for
CEBAF L.H. Harwood, S. Lassiter
and W. Tuzel
Summary: The current design for
the high-resolution spectrometers at CEBAF (Continuous
Electron Beam Accelerator Facility) requires a front
quadrupole that has a gradient * length of 6.8 T with a 'good
field' aperture (1*10/sup -3/ uniformity in gradient) of 16 cm
radius. A room-temperature design was found too power hungry
and interferred with the beam. Engineering and construction of
a small cos 2 theta magnet was considered to be quite
expensive. A Panofsky design was not considered due to the
extreme sensitivity of the field quality to errors in
conductor placement. A conformal mapping of a window-frame
dipole into quadrupole geometry worked well. A conceptual
design has been developed with the following characteristics:
physical length (total)=1.2 ; iron length=1.1 m; iron outer
dimensions=54 cm*80 cm; peak gradient=6.2 T/m; pole radius=20
cm; 'good field' radius=16 cm; coil peak field=1.5 T;
conductors=1 mm diameter; Cu/NbTi=7:1; current=400 A;
turns=250/quadrant; and stored energy=50 kJ. |
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A
superconducting cos 2/spl theta/ quadrupole design for the
CEBAF high resolution
spectrometer W. Tuzel, J. Alcorn,
P. Brindza, A. Gavalya, J. Mougey and S.
Nanda
Summary: The pair of high-resolution
spectrometers at CEBAF (Continuous Electron Beam Accelerator
Facility) require large-aperture, high-gradient quadrupoles. A
superconducting cos 2 theta design is under consideration to
meet these requirements. The quadrupole has a coil diameter of
1 m and a peak gradient of 5 T/m with an effective field
length of 1.8 m. The stored energy of this quad is 3.0 MJ. The
details of the design of the coil, shrink fit support collar,
and cryostat are presented. The analysis of the
superconductor, quench behavior, and coil protection system is
covered. |
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Microstructure
control in high Ti NbTi
alloys P.J. Lee, J.C. McKinnell
and D.C. Larbalestier
Summary: The
microstructures of five high-Ti NbTi alloys (Nb49-62-wt.%Ti)
and a ternary Nb-54-wt.%Ti, 4-wt.%Zr alloy have been observed
following heat treatments of 80 h at 420 degrees C at
prestrains of between 8.3 and 12.6. It was found that
increasing prestrain suppressed the formation of deleterious
Widmanstatten alpha -Ti and omega phases and that the higher
the Ti content, the higher the prestrain required for
elimination of these forms of precipitation. The Zr addition
increased the precipitation rate and reduced the precipitate
size without promoting the formation of Widmanstatten alpha
-Ti or omega . |
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Fine
filament NbTi conductors for the
SSC H.C. Kanithi, C.G. King, B.A.
Zeitlin and R.M. Scanlan
Summary: Inner and
outer grades of wire have been processed with NbTi filament
diameters of 5 to 9 mu m, depending on the magnet design
requirements. An increase in billet diameter from 10-in to
12-in resulted in an improved critical current density
approaching 3000 A/mm/sup 2/ at 5 T. The authors report on the
electrical properties (J/sub c/ and n value) and mechanical
properties (tensile, springback, and bend characteristics) of
the strand. Piece length distributions obtained for various
production billets are presented. The performance of cables
fabricated from the strand is discussed. Specifically, the
critical current degradation due to cabling and residual twist
in the cable are addressed. |
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A
conductor, with uncoupled 2.5 mu m diameter filaments,
designed for the outer cable of SSC dipole
magnets E. Gregory, T.S.
Kreilick, J. Wong, E.W. Collings, K.R. Marken Jr., R.M.
Scanlan and C.E. Taylor
Summary: A conductor
with a stabilizer-to-superconductor ratio of 1.8/1 and
containing 22900 2.5- mu m diameter filaments at a wire
diameter of 0.65 mm has been produced for the outer cable of
the Superconducting Super Collider (SSC). The fabrication
procedures used to make this material from a full
production-size billet 305 mm in diameter are described.
Metallographic, electrical, and magnetization data are
presented, and these illustrate the filaments are completely
uncoupled at the size used in the outer SSC cable. This work
indicates the feasibility of the commercial manufacture of
such a 2.5- mu m-diameter filamentary material. Preliminary
tests show relatively low J/sub c/ and n values. It is likely
that materials with smaller spacings than those used here will
be required before the J/sub c/ values specified for the SSC
can be obtained reproducibly. |
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The
effect of titanium content on the pinning force in Nb44wt.%Ti
to Nb62wt.%Ti J.C. McKinnell,
P.J. Lee and D.C. Larbalestier
Summary: The
authors have been studying high Ti alloys to establish whether
they offer higher low-field J/sub c/ values than Nb46.5wt.%Ti
and find that they do, with values, as high as 7500 A/mm/sup
2/ at 2 T, 4.2K. They have established that the pinning
centers are alpha -Ti precipitates for all alloys from
Nb44wt.%Ti to Nb62wt.%Ti, but find a steady shift in the
pinning curve peak to lower reduced fields at 4.2 K. Recently,
it was shown that both Delta H/sub c/ and Delta kappa pinning
can be recognized in Nb47wt.%Ti. Since T/sub c/ and H/sub c/
vary rapidly in the high Ti alloys, Delta H/sub c/ pinning
should be proportionately stronger in the high Ti alloys,
leading to a displacement of the peak in the pinning force
curves to lower fields. The authors present pinning force data
at 1.8 K and 4.2 K across the alloy range and find that while
lower Ti alloys do exhibit b(1-b) behavior, the higher Ti
alloys never realize b/sup 1/2/(1-b)/sup 2/
behavior. |
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High
current density of NbTi
composite S. Hong, D.
Geschwindner, A. Mantone, W. Marancik, S. Zalek and R.
Zhou
Summary: Various NbTi composites that
can sustain very high current density are reported. J/sub
c/=3000 A/mm/sup 2/ in a 5-T background field has been
achieved with 0.85-mm-diameter wire for production and J/sub
c/=38000 A/mm/sup 2/ has been achieved in some experimental
composites. The development results on a fine filament
composite to be used in conjunction with SSC are
reported. |
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Refinements
in billet design for SSC
strand P. Valaris, T.S. Kreilick,
E. Gregory and J. Wong
Summary: Refinements
in the design of superconducting strand to be used in the
manufacture of magnets for the Superconducting Super Collider
(SSC) are described that have considerably improved
performance characteristics in key areas; first, by
maintaining geometric uniformity and second, by contributing
to piece length yield. With 2.5- mu m-diameter filaments and a
copper-to-superconductor ratio of 1.8:1, the total number of
filaments required is approximately 23000. Stacking hexagon
units and reducing the amount of void space has enabled the
production of largely uniform arrays and consequently uniform
spacing. A strand with approximately 23000 fine filaments has
been produced with a mostly uniform filament
array. |
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The
effect of self field on the critical current determination of
multifilamentary
superconductors M. Garber, A.K.
Ghosh and W.B. Sampson
Summary: In
determining the short sample critical current of conductors of
large cross section or high current density, the self-field
produced by the transport current must be taken into account
to obtain a true value for the critical current. A simple
model calculation for determining this effect is described.
Measurements on wires, cables, and monoliths show the validity
and self-consistency of the procedures. |
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Reduction
of AC losses in ultra-fine multifilamentary NbTi
wires J.R. Cave, A. Fevrier, T.
Verhaege, A. Lacaze and Y. Laumond
Summary:
The authors present AC loss measurements for a series
of wires in which the proximity effect has been greatly
reduced by increasing the distance between the filaments while
retaining high values of the overall critical current density,
typically 3000 A/mm/sup 2/ at 0.5 T. For the best performing
wires, 50-Hz AC losses in W/(Am) (=(losses in W/m/sup
3/)/(overall J/sub c/ in A/m/sup 2/)) for small test coils are
a factor of ten lower for 0.5-T peak field and a factor of
three lower at 1-T peak field. For wires with negligible
proximity coupling, losses at low fields are reduced compared
to that calculated by the Bean model, due to the small number
of flux lines that thread across the width of each
filament. |
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Current
capacity degradation in superconducting cable
strands L.F. Goodrich and S.L.
Bray
Summary: The electromagnetic properties
of NbTi strands extracted from Rutherford cables were studied
to clarify the effect of mechanical deformation, caused by the
cabling process, on the current capacity of the strands. Three
different cables were studied, all of which are prototypes for
the Superconducting Super Collider's dipole magnets. The
extracted cable strands were instrumented to allow measurement
of the voltage across several key regions of mechanical
deformation as a function of current and the orientation of
the applied magnetic field. The resulting data are presented
in terms of the strand's voltage profile as well as its
critical current to more thoroughly characterize the
conductor's electromagnetic properties. The cable strands show
very localized reductions in current capacity that are well
correlated with the regions of high mechanical
deformation. |
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Magnetic
evaluation of Cu-Mn matrix material for fine-filament Nb-Ti
superconductors R.B. Goldfarb,
D.L. Ried, T.S. Kreilick and E. Gregory
Summary:
The authors have examined some of the magnetic
properties of Cu-Mn alloys with Mn content in the range 0.16
to 3.75 wt.% to determine whether spin-glass effects could
impair the performance of fine-filament Nb-Ti conductors. It
appears that for Mn content in this range, the adverse
magnetic properties of the matrix are not significant.
Magnetization and susceptibility measurements show that
adverse magnetic effects arising from spin-glass properties
are minimal for concentrations of Mn up to at least
4%. |
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The
critical current of NbTi multifilamentary wire: a comparison
between critical current densities using AC magnetization
techniques (J/sub CM/) and DC transport measurement (J/sub
CT/) D.P. Hampshire and D.C.
Larbalestier
Summary: AC magnetization
measurements are presented for transverse field orientation on
a NbTi standard reference material supplied by the National
Bureau of Standards. These data, generated in DC background
fields for the range 0 |
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Calculations
of plastic strain and resistivity increase in a composite
conductor subjected to cyclic
strain K.T. Hartwig, J.T. Gehan
and G.L. Richards
Summary: Graphical
calculations are presented for plastic strain per cycle and
accumulated plastic strain in aluminum composite conductors
subjected to cyclic strain at 4.2 K. The graphical model is
based on a constant-strain-range experiment with monolithic
pure aluminum. Calculations are derived from synthesized
stress-strain hysteresis loops. The resistivity increase
expected from force controlled cyclic stress is shown to be
substantially less than that arising from constant strain
range cycling. Suggestions are given for decreasing
resistivity degradation in the composite conductors of cycled
magnets. |
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Intermediate
scale industrially processed Nb/sub 3/Sn(Ti) by powder
metallurgy S. Pourrahimi, C.L.H.
Thieme and S. Foner
Summary: Powder
metallurgy processing of Nb/sub 3/Sn(Ti) wire was scaled up to
75-mm-diameter billets, each containing 4 kg of pressed powder
composite. Prealloyed Nb-1.5-wt.% Ti powder was used for
optimum high-field properties. An initial hot extrusion step
provided complete consolidation and excellent integrity of the
powder composite. Using the extruded bars, various single and
multiple tin core billets were prepared and hydrostatically
extruded to 12-mm-diameter bars. Long lengths of internal tin
core wire were drawn without any breakage to less than 1 mm
diameter. To evaluate the performance of long-length wires,
small magnets were fabricated from these wires and tested in
background fields of water-cooled Bitter magnets up to 20 T.
The processing, performance, and microstructure of these small
magnets are discussed. This intermediate-scale process used
conventional metal-forming techniques that can be scaled to
larger scale industrial processing. |
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Hysteresis
losses in hollow
superconductors J.A. Eikelboom,
R.A. Hartmann and L.J.M. van de Klundert
Summary:
Flux penetration into a hollow superconducting filament
in a time-varying transverse magnetic field is determined
numerically. The magnetization of the filaments is calculated
for field variations below and above the penetration field of
the filament. The influence of the inner radius of the
superconducting filament on the magnetization and the
hysteresis losses in the filament is shown. The critical
current density is taken to be constant during the external
field cycle and depends within the superconducting filament on
the local magnetic field, which is the sum of the externally
applied field and the field induced by the local screening
currents. Calculations based on the theory presented here show
good agreement with experimental results. |
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Mo-Re
superconducting thin films by single target magnetron
sputtering A. Andreone, A.
Barone, A. Di Chiara, G. Mascolo, V. Palmieri, G. Peluso and
U. Scotti di Uccio
Summary: The authors
prepared Mo/sub 60/Re/sub 40/ thin films by single target
magnetron sputtering. All the films analyzed exhibited a sharp
superconductive transition and values of the critical
temperature T/sub c/ up to 13 K were achieved. Structural
analyses (EDS and X-rays) were performed on the samples. A15
phase was obtained even at low deposition temperatures. From
the measured low temperature resistivity O/sub c/ and critical
temperature T/sub c/ an estimate of the theoretical BCS
surface resistance is inferred and compared with that of
niobium. |
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The
influence of transverse, compressive stress on the critical
current of multifilamentary Nb/sub 3/Sn and NbTi
wires H. Boschman, P.P.E.
Fornerod and L.J.M. van de Klundert
Summary:
To investigate the critical current of multifilamentary
wires as a function of the applied field and transverse
compressive stress, a special experimental arrangement has
been developed. The repulsive Lorentz force generated by a set
of magnets is used to press a sample between two parallel
surfaces. From the first experimental results on a
multifilamentary NbTi wire, it is concluded that this approach
functions properly. |
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Mechanical
properties of composite conductors containing niobium
filaments in bronze matrix A.D.
Nikulin, A.K. Shikov, A.E. Vorob'eva, I.V. Golikov, I.I.
Davydov, V.N. Shishov and T.N. Izmailova
Summary:
A discussion is presented of an investigation into
mechanical properties of multifilamentary composites 0.3-1.5
mm in diameter and containing up to some tens of thousands of
Nb filaments in a bronze matrix. The influence of design
features, geometrical sizes, and heat treatment schedules on
the strength and plasticity of the materials was studied.
Using metallographic and electron microscopic analysis, data
were obtained on the structure and morphology of
multifilamentary conductors that give an explanation of the
observed differences in the mechanical properties of the
composites discussed. |
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Nb/sub
3/Al formation process in powder metallurgy processed wires
and sputtered multilayer films K.
Watanabe, K. Noto, H. Morita, H. Fujimori and Y.
Muto
Summary: The formation of Nb/sub 3/Al by
a solid-state reaction between Nb and Al is examined for both
powder-metallurgy-processed wires and sputtered multilayer
films. It is found that the diffusion of Al into the Nb layer
plays an important role in the formation of Nb/sub 3/Al.
However, an extremely thin Nb film is not necessarily needed
for the solid-state reaction, as indicated by the results
obtained with multilayered films. A discussion is presented of
the relationship of the Nb layer thickness and the areal
reduction ratio to Nb/sub 3/Al formation in the
powder-metallurgy process. |
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Comparison
of Al5-Nb/sub 3/Al superconductors produced by laser alloying
and infiltration W. Schaper, M.
Kehlenbeck, I. Zimmermann and H.C.
Freyhardt
Summary: Nb/sub 3/(Al, Si, Bi) and
Nb/sub 3/Al superconductors were successfully prepared by a
laser alloying and an infiltration process. Continuous CO/sub
2/ laser irradiation of Nb-sheathed Nb-25-at.% Al composite
tapes prepared by a conventional powder metallurgical method
was carried out. For a laser power of less than 3.5 kW and
specimen velocities up to 30 m/min, almost stoichiometric
Nb/sub 3/Al with critical temperatures of 16.6 K and critical
current densities of 10/sup 4/ A/cm/sup 2/ at 15 T was
obtained. For infiltration, sintered Nb powder was immersed in
a liquid (Al/sub 88.7/Si/sub 11.3/)/sub 99/Bi/sub 1/ alloy.
Subsequently the infiltrated rod was deformed and reacted at
temperatures up to 2000 degrees C. The microstructure and the
superconducting properties have been studied as a function of
the degree of deformation and the reaction temperature.
Critical temperatures between 14 and 18 K, upper critical
fields B/sub c2/ of 21 T and critical current densities of
10/sup 4/ A/cm/sup 2/ at 16 T were obtained. |
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Nb/sub
3/Al wire produced by powder metallurgy and rapid quenching
from high temperatures C.L.H.
Thieme, S. Pourrahimi and S. Foner
Summary:
Powder-metallurgy-processed Nb-25-at.% Al wires were
annealed at temperatures just below the melting temperature.
Depending on anneal conditions, the entire Nb-Al part of the
wire could be rapidly quenched as a metastable Al Nb(Al) solid
solution with an Al concentration exceeding 23 at.%. This A2
phase is sufficiently ductile to be bent without incurring
damage. A second anneal at 950 degrees C converted the A2 into
A15. J/sub c/ was 10/sup 4/ A/cm/sup 2/ at 22 T, and T/sub c/
was 17.8 K. The rapid quenching of Nb-Al powder metallurgy
processed wire is a possible alternative to obtain improved
high-field properties. |
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Further
studies of a laser quenched superconducting opening
switch T.L. Francavilla, H.H.
Nelson, A.P. Baronavski, S.A. Wolf, D.U. Gubser and R.A.
Hein
Summary: NbN films were prepared on
planar sapphire and on small-diameter cylindrical quartz and
sapphire substrates. Films prepared on cylindrical substrates
had a greater critical current density than those prepared on
planar substrates. The highest current densities of 1*10/sup
7/ A/cm/sup 2/ were achieved on cylindrical sapphire
substrates. Measurements of the laser power required to drive
the switch into the normal state were made as a function of
current being carried by the NbN-clad sapphire rod. These
experiments indicated that near J/sub c/ very small amounts of
laser energy were needed to drive the sample normal as
expected. At current values well below J/sub c/, much larger
energies were required. These large energies, which are
roughly 10/sup 3/ larger than the condensation energy, were
attributed to heating of the sapphire rod necessary to raise
the NbN above its transition temperature. The desirable
properties of a superconducting switch are the ability to
carry large currents in the superconducting state and have a
high resistance in the normal state. Considerations for using
the high-T/sub c/ oxides in a superconducting switch are
examined. |
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Superconducting
properties of TaN and VN
films V.M. Pan, V.G. Prokhorov,
V.A. Komashko, G.G. Kaminsky, M.A. Kousenetsov and C.G.
Tretiatchenko
Summary: Thin superconducting
TaN and VN films produced by reactive cathode sputtering have
been investigated. Superconducting-phase-transition smearing
in applied field seems to be due to spatial fluctuations of
the electron diffusion coefficient as well as dimensional
crossover of dynamical superconducting order parameter
fluctuations. Flux pinning occurs at grain boundaries by
electron scattering mechanism. Transition to a dissipative
state is induced by flux line separation from the pins but not
by flux line lattice plastic shear. |
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Effects
of alloying on the superconducting properties of ErBa/sub
2/Cu/sub 3/O/sub 7/ C.S. Pande,
H.A. Hoff, A.K. Singh, M.S. Osofsky, M.A. Imam, K. Sadananda
and L.E. Richards
Summary: In high-T/sub c/
superconductors, the low critical current densities in
polycrystalline materials have been attributed to a
combination of critical current anisotropy and poor
superconducting coupling across grain boundaries. Theoretical
calculations indicate that although the flux pinning should
vary roughly inversely as grain size, the polycrystalline
critical current behavior could possibly be understood in
terms of stresses due to the grain boundary. Experiments have
been conducted to increase the coupling between adjacent
grains by modifying grain boundary chemistry. These include
adding either a conducting layer or a superconducting layer at
the interfaces. The effect of additions such as Ag, B, Bi, Ga,
and In to produce a conducting layer and the alloying of
RBa/sub 2/Cu/sub 3/O/sub 7/ with another superconductor to
produce a superconducting layer were analyzed by measuring
T/sub c/ and J/sub c/ and observing changes to the
microstructure. Early results indicate some J/sub c/
enhancement with silver addition. However, the addition of a
different superconductor appears more promising. |
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Preparation
of dense bulk high T/sub c/ superconducting materials using
hot isostatic pressing M.A. Imam,
A.K. Singh, K. Sadananda and M. Osofsky
Summary:
Sintered superconducting materials were encapsulated in
evacuated stainless-steel or pyrex glass containers and
processed in a hot isostatic pressing (HIP) unit at high
temperatures and pressures. Optimum HIP conditions to obtain
bulk samples of nearly theoretical density were determined.
Results indicate that in addition to density, HIP also
improved T/sub c/. For the one-two-three system, the grain
size is significantly reduced during HIP. This is attributed
to fracturing of coarse particles in the sintered compact due
to high strain rates experienced during the pressurization.
HIP materials can be machined to any shape and size, and the
process being used for making specimens for J/sub c/
measurements and superconducting devices. |
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Properties
of high-T/sub c/ oxide fibers from laser heated pedestal
growth J.Z. Sun, R.S. Feigelson,
D. Gazit, D. Fork, T.H. Geballe and A.
Kapitulnik
Summary: Bi-containing oxide
superconductors have been made into fine fibers using the
laser-heated pedestal growth (LHPG) method. These fibers grow
with the c-axis of the grain normal to the growth direction.
Studies of critical currents obtained by pulsed-current
transport measurement and by magnetic shielding studies are
compared. The transport and magnetic properties are promising
for 60-Hz power applications and devices where the current is
maintained by an outside source and a small amount of
dissipation can be tolerated. Low pinning barriers and
consequently appreciable flux creep seem to be intrinsic
characteristics of the high-T/sub c/ oxide superconductors. To
utilize them in situations where persistent currents are
required, it will be necessary to incorporate more pinning
centers. |
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Bi-Sr-Ca-Cu-O
bulk superconductors with zero resistance above 110
K H.S. Huang, S.N. Mao, Z.T. Yao,
M.L. Liu, X.D. Ren, K.S. Zhang, Z.Y. Wu, X. Zhao and Y.-Z.
Liu
Summary: Optimized processing to obtain
bulk superconductors in Bi-Sr-Ca-Cu-O system with zero
resistance above 110 K has been studied. The superconducting
path of the high-T/sub c/ phase has been realized in
multiphase polycrystalline samples. The resistance of a
typical sample reaches zero at 111 K. The usual resistive tail
extended to 80 K has been successfully cut off. The importance
of the role CuO played in the sintering reactions for the
formation of the 110 K phase is discussed on the basis of the
results of DTA (differential thermal analysis) and TG
experiments. |
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Preparation
and characterization of magnetically textured Y and DY 123
compounds J. Seuntjens, X. Cai
and D.C. Larbalestier
Summary: The authors
prepared homogeneously (i.e through volume-textured material
in comparison to sinter-forged samples where the texture is
preferentially at the surface) c-axis-textured samples with
approximately 200 mu Omega -cm resistivity at 100 K and
approximately 88% density by aligning powders in a magnetic
field and then sintering. Light microscopy, X-ray diffraction,
and transport J/sub c/ measurements were used to characterize
the samples. Longer sintering times improved the texture.
Dilute suspensions in which particles did not touch could be
very well aligned. Such samples, encased in epoxy, had 50%
of |
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Zero
resistance above 100 K in Bi-based
superconductors S.M. Green, C.
Jiang, Y. Mei, H.L. Luo and C. Politis
Summary:
The authors have concentrated on the effect of Pb on
the superconducting properties of the Bi-Sr-Ca-Cu-O system.
Preliminary results are also presented on the effect of
varying the Sr:Ca and Bi:Ca ratios in Pb-substituted samples.
It is shown that the addition of Pb does not guarantee a
step-free resistive transition. The steps in the resistivity
become more pronounced as the Bi:Ca ratio is increased. This
gradual, systematic change suggests that the reduced
continuity is not the result of an abrupt reduction in the
c-spacing near grain boundaries as observed in Pb-free
samples. |
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Preparation
of high-T/sub c/ superconducting thick films and power
conducting tubes by a low-pressure plasma
spraying K. Tachikawa, M. Ono, Y.
Shinbo, T. Suzuki, M. Kabasawa and S.
Kosuge
Summary: Superconducting thick films
of YBaCuO and BiSrCaCuO systems have been prepared using a
low-pressure plasma spraying technique. The chemical
composition of the deposits was very close to that of the
spraying powders. After postannealing the films showed zero
resistance temperatures, T/sub c/ of approximately 90 K for
YBaCuO and approximately 60 K for BiSrCaCuO. Superconducting
power tubes were successfully produced by a low-pressure
plasma spraying. 100-200- mu m-thick YBaCuO superconducting
films were deposited on the outer surface of the Ni-plated Cu
tubes. After postannealing at 930 degrees C for 1 h the films
exhibited T/sub c/ of 85-90 K. An improved film preparation
process in which a flux layer is coated on the Y/sub 1/Ba/sub
2/Cu/sub 3/O/sub x/ matrix layer by a low-pressure plasma
spraying has been developed. Enhanced critical currents have
been achieved by this process. |
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Round
robin tests of T/sub c/ and I/sub c/ on YBa/sub 2/Cu/sub
3/O/sub x/ Y. Kimura, N. Higuchi,
S. Meguro, K. Takahashi, K. Uyeda, T. Ishihara, E. Inukai and
M. Umeda
Summary: Methods for measuring the
critical properties such as critical temperature and critical
current were investigated and studied by round robin tests.
The investigation was carried out in five steps: (1) decision
on the specifications of common samples and measuring methods;
(2) preparation and characterization of common samples; (3)
distribution of samples to ten member research organizations;
(4) experimental measurement at the respective research
organizations; and (5) analysis of measurement results. The
measurement of critical temperature and critical current were
made mainly by the four-probe methods and partially by the
magnetization method. The results were found to disperse,
depending on the measurement methods and definitions of T/sub
c/ and I/sub c/. |
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Relation
of the N-value of the resistive transition to microstructure
and inhomogeneity for YBa/sub 2/Cu/sub 3/O/sub 7/
wires J.E. Evetts, B.A. Glowacki,
P.L. Sampson, M.G. Blamire, N.M. Alford and M.A.
Harmer
Summary: Measurements are reported of
resistive transitions at 77 K for a series of YBa/sub 2/Cu/sub
3/O/sub 7/ wires as a function of applied magnetic field. The
transitions are very broad and for electric fields in the
range 10/sup -6/ to 10/sup -3/ V m/sup -1/ can be fitted
approximately by a power law with n-values in the range 4-60.
The n-value is observed to fall with increasing applied
magnetic field and shows strong hysteresis related to trapped
flux when the field is cycled. The variation of both critical
current and n-value with applied field is related to the
microstructure of wires with different additives, and on a
series of wires of different bulk density prepared using
viscous processing techniques. Extensions to the analysis for
conventional composites makes it possible to relate the
n-value to the statistical critical current
distribution. |
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Deterioration
mechanism of oxide
superconductor K. Imai and H.
Matsuba
Summary: Deterioration of oxide
superconductors has been investigated by evaluating the
critical current densities, J/sub c/, at 77 K and the
conductivity at room temperature. The oxide superconductors
were exposed to both moisture and heat cycles. The authors
observed slow decreases in J/sub c/ often followed by fast
decreases. The experimental results indicate that an external
substance penetrates into the superconductors and brings about
chemical reactions to form nonsuperconducting products so that
volume expansion occurs at the site causing
cracks. |
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Low
resistance ohmic contacts to high-T/sub c/ superconducting
thin films Y. Tazoh, K. Aihara,
K. Miyabara, K. Hohkawa and M. Oshima
Summary:
Metal contacts to high-T/sub c/ superconducting thin
films have been studied. By analyzing the I-V characteristics
at contact junctions and by synchrotron radiation
photoemission spectroscopy, the effect of contact metal and
surface treatment before depositing contact metal on the
metal-superconductor interface characteristics has been
investigated. Low-resistance ohmic contacts are realized by
the following sequence of processes: (1) oxygen ion sputter
etching; (2) high-pressure oxygen plasma treatment, and (3) in
situ deposition of Au with no interfacial reaction with the
superconductor. A low-contact resistivity equal to 5.5*10/sup
-7/ Omega -cm/sup 2/ at 77 K is realized
experimentally. |
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Low
contact-resistivity junctions to ceramic
superconductors M.P. Maley, J.O.
Willis, J.D. Katz, R.G. Castro and R.M.
Aikin
Summary: A report is presented on the
development of a plasma-arc spray process for the application
of low-resistivity silver contact junctions onto YBa/sub
2/Cu/sub 3/O/sub 7/ sintered ceramic materials. The contact
resistances decrease montonically with decreasing temperature
and are typically approximately 10/sup -8/ Omega -cm/sup 2/ at
75 K. I-V characteristics are linear up to a critical current
density that agrees with that of the underlying
superconducting material. Advantages over other contact
methods include ease of application, absence of any
postprocessing treatment, and applicability to large areas and
arbitrary shapes. |
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Ag
screen contacts to sintered YBa/sub 2/Cu/sub 3/O/sub x/ powder
for rapid superconductor
characterization J. Moreland and
L.F. Goodrich
Summary: The authors have
developed a method for making current contacts and voltage
taps to YBa/sub 2/Cu/sub 3/O/sub x/ sintered pellets for rapid
superconductor characterization. Ag wire screens are
interleaved between calcined powder sections and then fired at
930 degrees C to form a composite pellet for resistivity and
critical current measurements. The Ag diffuses into the powder
during the sintering process forming a proximity contact that
is permeable to O/sub 2/. Contact surface resistivities
(area-resistance product) range from 1 to 10 mu Omega -cm/sup
2/ at 77 K for the Ag-powder interface. In this configuration,
current can be uniformly injected into the ends of the pellet
through the bonded Ag screen electrodes. Ag screen voltage
contacts, which span a cross section of the pellet, may
provide an ideal geometry for detecting voltage drops along
the pellet, minimizing current transfer effects. |
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Experimental
study of flux pinning in NbN films and multilayers: ultimate
limits on critical currents in
superconductors K.E. Gray, R.T.
Kampwirth, D.W. Capone II and J.M. Murdock
Summary:
A flux-pinning model is presented which predicts the
maximum critical current density attainable in
superconductors. That such a limit must exist comes from the
realization that flux pinning is strongest in regions of weak
superconductivity, but these regions cannot carry a large
supercurrent. Since the same regions within the superconductor
cannot be used for both pinning and supercurrent conduction,
there must be an optimum mix, leading to a maximum J/sub c/.
Measurements on films and multilayers of NbN have verified
many details of the model including anisotropy effects and a
strong reduction in J/sub c/ for defect spacings smaller than
the flux core diameter. In an optimized multilayer the pinning
force reached approximately 22% of the theoretical maximum.
The implications of these results on the practical
applications of NbN films and on the maximum critical current
density in the high-temperature oxide superconductors are also
discussed. |
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Effects
of additive elements on continuous ultra-fine Nb/sub 3/Al MF
superconductor T. Takeuchi, Y.
Iijima, M. Kosuge, T. Kuroda, M. Yuyama and K.
Inoue
Summary: Nb/sub 3/Al multifilamentary
(MF) wire can be fabricated by the composite process using
various Al-based alloy cores and pure Nb matrix. Additive
elements of Mg, Ag, Cu and Zn harden the Al core
preferentially and then improve the workability of the Nb/Al
composite, permitting it to be cold-drawn into a wire with
ultrafine Al-based alloy filaments (filament number: 1.8
million, diameter: about 0.1 mu m). Reacted wires at 700-900
degrees C show T/sub c/ of 15-16 K, mu /sub 0/H/sub c2/ (4.2
K) of 21-22 T and J/sub c/ (4.2 K, 10 T) of 1-1.5*10/sup 9/
A/m/sup 2/. A two-stage reaction consisting of a reaction
above 950 degrees C and a subsequent reaction around 700
degrees C is carried out to improve critical values; T/sub c/
and mu /sub 0/H/sub c2/ are increased to 17.4 K and 25.4 T,
respectively. Excellent superconducting properties involving
good strain tolerance ( epsilon /sub irrev/=1.3%) indicate
that this Nb/sub 3/Al is very promising for practical
high-field superconducting cables. |
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Local
pinning-force distribution in NbN
films A. Pruymboom, W.H.B.
Hoondert and P.H. Kes
Summary: The authors
performed measurements of the flux-flow resistivity on a thin
film of NbN. By fitting the resistivity, measured as a
function of the current, to a theoretical expression, they
were able to obtain the distribution function of local
depinning forces in the film. It appears that the distribution
is very broad and that only a minor fraction of flux lines is
flowing at what is usually defined as J/sub c/. These
observations are in agreement with the results of the
pinning-force measurements. The measurements reveal that at
high fields pinning is limited by flux-line shear and that the
effective channel width for shear is equal to the flux-line
spacing. They suggest that variations of the local depinning
force on various length scales always allow the flow of single
rows of flux lines. |
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Neutron
irradiation of sputtered NbN
films H.W. Weber, P. Gregshammer,
K.E. Gray and R.T. Kampwirth
Summary: The
authors report neutron irradiation experiments on sputtered
NbN films, up to a fluence of 10/sup 23/ m/sup -2/ (E> 0.1
MeV), which prove that NbN films are also extremely
radiation-hard high-field superconductors. Both the transition
temperatures, T/sub c/, and the normal state resistivities
show only small changes with neutron fluence. Concerning the
critical current densities, j/sub c/, degradations by as much
as 30% are observed at low fields, whereas in an intermediate
field range (11-15 T), virtually no change of j/sub c/ is
seen, and at high fields (near 20 T), even an increase of
j/sub c/ is found. The latter observation is ascribed to a
radiation-induced increase of the upper critical field, B/sub
c2/, and to the occurrence of peak effects near B/sub
c2/. |
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Process
control for the low temperature deposition of niobium-nitride
thin films A.C. Anderson, D.J.
Lichtenwalner and W.T. Brogan
Summary: The
authors investigated the process of DC and RF magnetron
reactive sputtering of NbN with the purpose of clarifying the
issues which are important for process control. Based on a
simple physical model of the deposition process, parameters
(plasma impedance and plasma concentration of reactive
species) have been selected for study and their effects on the
properties of the films (transition temperature, room
temperature resistance, and RF surface impedance) have been
evaluated experimentally. Based on these experiments, a simple
feedback scheme using the plasma impedance as a control
parameter was implemented, allowing for the consistent
deposition of high-quality films. The same scheme can be used
for the control of the stoichiometry of any reactive
sputtering process, including the deposition of the
high-transition-temperature oxide films. |
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Fabrication
of multifilamentary Nb/sub 3/(Al,Ge) wires through a modified
jelly roll process K. Tachikawa,
Y. Kamisada and E. Suzuki
Summary: Nb/Al-Ge
alloy composites were fabricated into multifilamentary wires
through a modified jelly roll (MJR) process. A Nb mesh sheet
was used as the Nb component. An Al-Ge alloy prepared by a
conventional casting process was forged and rolled into a
sheet of 0.2 mm in thickness. The Nb/Al-Ge composite was
prepared by wrapping the Nb mesh sheet together with the Al-Ge
alloy sheet around a Nb core into a jelly-roll form. The MJR
composite was encased in a Cu-Ni alloy tube of which the outer
diameter was 43 mm. The resulting composite was
hydrostatically pressed and extruded into a rod of 18 mm in
diameter. A Nb barrier was then inserted between the MJR and
the Cu-Ni jacket. The composite rod was swaged and drawn into
a wire without any intermediate annealing. The wire was able
to be drawn down to a very small diameter of 0.1 mm. The
cross-sectional configuration of the MJR composite was not
much disturbed by the fabrication. An onset T/sub c/ of 19.3 K
has been achieved. |
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Reaction
kinetics of phase formation in Nb-Al powder metallurgy
processed wire K.R. Coffey, K.
Barmak, D.A. Rudman, C.L.H. Thieme and S.
Foner
Summary: The sequence of phases formed
in the low-temperature reaction of niobium and aluminium to
form the A15 superconductor, Nb/sub 3/Al, was studied in high
areal reduction powder metallurgy processed wire samples. The
reaction path observed was the same as that reported
previously for multilayer Nb/Al thin films. The reaction
proceeds without the formation of the sigma phase, Nb/sub 2/Al
resulting in a metastable A15 of higher Al concentration. The
extent of this metastable extension is estimated to be 22-24
at.% Al, based on the resistive T/sub c/ of 17.0 K observed.
The reaction kinetics of the wire samples were examined by
scanning heat flux calorimetry, and the activation energy of
A15 formation was found to be 3.1 eV. The formation of
NbAl/sub 3/ and dissolution of Al into the Nb lattice were
both observed as precursor reactions with much lower
activation energies. |
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Normal
state resistance and low temperature magnetoresistance of
superconducting cables for accelerator
magnets W.B. Sampson, M. Garber
and A.K. Ghosh
Summary: Resistance
measurements which are routinely made on superconducting wires
and cables for accelerator magnets can provide an accurate
measure of the copper-to-superconductor ratio. Compared to the
etch and weight technique, a resistance specification for
Cu/Sc is preferable for monitoring of this parameter during
wire and cable production. Results shown here indicate that by
using the 4 K magnetoresistance data of bulk copper and the
size effect formula for the resistivity enhancement of copper,
the measured residual resistance ratio can be used to estimate
the resistance at high fields. |
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Losses
in superconducting wires with inhomogeneous and anisotropic
matrix conductivity over cross
section E.N.
Aksenova
Summary: Methods of calculating
energy losses for multifilamentary wires with anisotropic and
inhomogeneous conductivity of a matrix over the cross section
are developed, in particular, for wires with a cluster
assembly. Analysis of diffusion of the magnetic flux and
dissipative characteristics for the multifilamentary wire in
the transverse AC magnetic field shows that: (1) in the case
of the matrix anisotropic conductivity the processes of
current coupling are antiparallel, making a contribution to
losses, which is proportional to one of the matrix components,
sigma /sub 1/; (2) the presence of the matrix conductivity
inhomogeneity causes peculiarities in the saturation process,
namely, that the longitudinal-to-the-filaments electric field
has local maxima at the outer and the inner radii; (3) this
circumstance explains the process of parallel saturation of
groups of filaments and the wire as a whole, taking place in
the wire with cluster assembly; and (4) the rise of the ratio
of hysteresis losses to the filament diameter with decrease of
the latter for diameter approximately 1 mu m can be accounted
for by the cluster saturation and increase of filament
inhomogeneity in their thinning without recourse to the
proximity effect. |
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AC
losses of NbTi superconducting wires with fine
filament K. Ohmatsu, M. Nagata,
M. Kawashima, H. Tateishi and T. Onishi
Summary:
An investigation was conducted of the AC losses in
coils with NbTi superconducting wires. The authors used the
multiple-stacking procedure to prepare the wires, which varied
in filament size from 10 mu m to 0.07 mu m. Three kinds of
wires with 0.07- mu m filament were specially fabricated to
estimate the proximity effect on AC losses. The distances
between the filaments of these wires were 21 nm, 42 nm, and 70
nm, respectively. AC losses were measured for small coils at
60 Hz. The proximity effect was observed for the wires with
0.07- mu m filament when the distance was 21 nm. Conversely,
when the distance was 42 nm, the loss decreased to the small
value of 11 kW/m/sup 3/ at 0.5 T and 4.2 K. |
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Initiation
of flux jump in SC composite by heat
pulse E.Y. Klimenko and N.N.
Martovetsky
Summary: Nonisothermal diffusion
of magnetic flux after a heat pulse shot on the surface of SC
(superconductor) composite has been studied numerically taking
into account the smoothed transition characteristic of the SC.
It is shown that for an SC composite with poor stabilization,
the current and heat redistribution change significantly the
estimations of stability based on steady-state functions of
heat generation and heat transfer. The critical pulsed energy
strongly depends on the initial current distribution over the
conductor cross section, and the energy may be much less for a
conductor with growing current than for a conductor with the
same current in steady state. It has been found that
undercritical heat pulses only slightly affect current density
profile, but stability increases as time delay increases
between the current input halt and the pulse shot. It has also
been found that for a SC composite with poor stabilization,
the transverse thermal conductivity is more important than the
electrical resistivity of the matrix from the stability
standpoint. |
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Simplified
loss expressions for superconductors carrying DC transport
current in an AC field J.V.
Minervini
Summary: Simplified expressions are
reported for the shielding and transport loss components as a
function of the applied transverse field and the transport
current. These expressions are based on a numerical solution
but are approximated by analytic expressions in such a way
that they are as simple to use as the equivalent expressions
derived from the one-dimensional slab model. A comparison of
the two models is presented. |
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Study
of serviceability of superconducting wires at 50-Hz
frequency E.N. Aksenova, P.V.
Aksenov, V.S. Kruglov, G.K. Zelensky and E.V.
Nikulenkov
Summary: A stationary equation for
thermal conductivity is solved for a multifilamentary
superconducting wire in the AC external magnetic field with an
in-phase varying transport current of the amplitude j/sub
m/< |
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Calculating
the J/sub c/, B, T surface for niobium titanium using a
reduced-state model M.A.
Green
Summary: A method is presented for
calculating the J/sub c/, B, T surface for commercial niobium
titanium superconductor given T/sub c/, B/sub c2/ and J/sub c/
over a range B at a given temperature. The J/sub c/ calculated
as a function of B and T agrees with measured values of J/sub
c/ quite well over a range of T from 1.5 K to 7 K and a range
of B from 0.1 T to 10 T. The values of J/sub c/ given by this
theory are good enough to be used to calculate the performance
of superconducting magnets at temperatures other than 4.2 K,
and the theory can be used to predict the magnetization
effects of superconductors over a wide range of temperatures
and inductions. |
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Temperature
dependence of penetration depth in thin film
niobium N. More, B. Muhlfelder
and J. Lockhart
Summary: A novel technique is
presented which should allow precise determination of the
temperature dependence of the inductance, and hence of the
penetration depth, of superconducting niobium thin-film
structures. Four niobium thin-film stripline inductors are
arranged in a bridge configuration, and inductance differences
are measured using a potentiometric technique with a SQUID
(superconducting quantum interference device) as the null
detector. Numerical simulations of the stripline inductances
are presented which allow the performance of the measurement
technique to be evaluated. The prediction of the two-fluid
model for the penetration-depth temperature dependence is
given for reduced temperatures of 0.3 to 0.9. The experimental
apparatus and its resolution and accuracy are
discussed. |
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Current
distribution and AC losses in twisted multifilamentary AC
superconductors A.J.M. Roovers
and L.J.M. van de Klundert
Summary: The
development of AC superconductors has led to multifilamentary
wires having a highly resistive matrix material and a small
twist pitch and filament diameter. A numerical one-dimensional
model is presented which enables the authors to calculate the
current distribution as well as the magnetization and
transport current loss in a current-carrying AC superconductor
subjected to a transverse alternating magnetic field. The
impact on wire design is also discussed. |
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Numerical
solution of the transverse resistivity of superconducting
cables under AC conditions R.A.
Hartmann, D. Dijkstra, F.P.H. van Beckum and L.J.M. van de
Klundert
Summary: The authors develop a
numerical method for calculating the transverse resistivity of
superconducting cables. A superconducting cable consists of a
twisted bundle of strands with a nonconducting inner region.
If such a cable is placed in an external magnetic field, the
induced currents will not merely flow in the axial direction,
but also around the center, in the plane of the cross section.
It is shown that the transverse transport current, which is
induced by external fields acting on the cable, can saturate
most of the filaments of the superconducting layer. This
results in a smaller maximal value of a longitudinal transport
current and small coupling losses. |
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Formation
of metal/superconducting oxide composites by oxidation of melt
spun metallic precursors H. Hsu,
L. Masur, C. Joshi, K. Sandhage, W. Carter and G.J.
Yurek
Summary: Melt spinning was used to
produce homogeneous, melt-spun ribbons of Yb/sub 1/Ba/sub
2/Cu/sub 3/ and Yb/sub 1/Ba/sub 2/Cu/sub 3/ plus various
amounts of Ag. While none of these alloys was completely
glassy, they did contain glassy alloy and they were
homogeneous. Metallic precursors as long as 12 m were
produced; the widths were typically 1 to 2 mm, and the
thicknesses ranged from 25 to 170 mu m. The dimensions of the
specimens varied with the alloy composition and the processing
conditions. |
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Critical
current densities for the high temperature ceramic
superconductors YBa/sub 2/Cu/sub 3/O/sub 7/ and Bi/sub
2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10+ delta
/ W.C. McGinnis, T.E. Jones, E.W.
Jacobs, R.D. Boss and J.W. Schindler
Summary:
A pulsed transport technique has been used to measure
the zero-field critical current densities of YBa/sub 2/Cu/sub
3/O/sub 7/ ceramic superconductors prepared under different
sintering conditions. This low duty-cycle, pulsed technique,
used on specially prepared samples with low-resistance current
contacts, allows one to determine the entire
superconducting/normal phase boundary without the problems
associated with sample heating. The results can be interpreted
in terms of two different critical-current densities. The
intergrain critical current is low and is limited by the weak
links between the grains; the intrinsic intragrain critical
current is greater than 10/sup 4/ A/cm/sup 2/ at 77 K.
Critical-current values inferred from magnetic hysteresis
measurements made on the same samples agree with the intrinsic
intragrain critical currents obtained using the pulsed
transport technique. In addition, the magnetization-derived
critical-current density has been determined for the high
T/sub c/ phase Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10+
delta / at 77 K. |
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Size
effects in the magnetisation of high T/sub c/
powders A.D. Hibbs and A.M.
Campbell
Summary: The critical-current and
flux profile within a series of different size powders of
YBa/sub 2/Cu/sub 3/O/sub 7/ has been measured. At zero field,
supercurrents flow unimpeded within the grains. However, for
fields above 100 mT, it appears that the grains are subdivided
on a scale smaller than 1 mu m. Using this interpretation it
is found that the interaction of the fluxoids with the pinning
centers is very similar to that taking place in V/sub 3/Si and
commercial NbTi. |
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Flux
trapping and magnetisation of hollow superconducting
cylinders F.J. Eberhardt, A.D.
Hibbs and A.M. Campbell
Summary: The
magnetisation of hollow cylinders of high T/sub c/ materials
and the field trapped inside them have been measured by
integrating the signal from coils outside and inside the
cylinders. The two coils allow the field trapped in the grains
themselves to be separated from the field due to the
circulating currents in the bulk material. The trapped field
indicates the maximum field that can be expected in a magnet.
For practical purposes the most important parameter is the
field at which J/sub c/ drops to half its zero-field value.
Cylinders have been made from yttrium-barium-copper oxide and
from bismuth-strontium-calcium-copper oxide. Various
preparation methods have been used. |
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High-T/sub
c/ superconducting glass ceramics based on the Bi-Ca-Sr-Cu-O
system T. Komatsu, R. Sato, K.
Imai, K. Matusita and T. Yamashita
Summary:
Various high-T/sub c/ superconducting glass ceramics
based on the Bi-Ca-Sr-Cu-O system were prepared by the
melt-quenching method. The melt-quenched sample of Bi/sub
1.2/CaSrCu/sub 2/O/sub x/ was found to be a glass that became
a high-T/sub c/ superconductor with T/sub c/ (zero)=85 K by
annealing at 820 degrees C. The Bi/sub 1-x/Pb/sub x/CaSrCu/sub
2/O/sub y/ glasses also became high-T/sub c/ superconductors
upon annealing at temperatures above 800 degrees C. The glass
ceramics of Bi/sub 0.9/Pb/sub 0.1/CaSrCu/sub 2/O/sub x/
exhibited superconductivity with T/sub c/ (onset)=116 and
T/sub c/ (zero)=100 K. |
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Superconductivity
and crystal structure in the Bi-Pb-Sr-Ca-Cu-O
system Xing Zhu, Sungi Feng, Jian
Zhang and Zizhao Gan
Summary: BiPbSrCaCuO
superconductors with various nominal compositions have been
studied. T/sub c,zer0/=111 K and an AC Meissner effect of
93.1% have been obtained in a (Bi/sub 0.8/Pb/sub 0.2/)/sub
2/Sr/sub 2/Ca/sub 2/-Cu/sub 3/O/sub x/ sample. The authors
have also synthesized a single-phase (Bi/sub 0.8/Pb/sub
0.2/)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ sample. The
crystal structure of the superconducting phase has been
determined by transmission electron microscopy, convergent
beam electron diffraction, and powder X-ray diffraction. The
crystal structure of the Pb-doped system has a tetragonal unit
cell (a=b=3.823 AA, c=37.074 AA), space group I4/MMM. The
atomic coordinates have been determined. Unlike the Bi-based
system without Pb, the Pb-doped 2223 phase has very weak or no
modulation structure, which in turn changes the symmetry from
pseudo-tetragonal to tetragonal. The relationship between
synthesis conditions and superconductivity is
discussed. |
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Effect
of synthesis condition on superconducting transmission
temperature in Tl-Ba-Ca-Cu-O
system Liu Chunfang, Zhou Lian,
Yang Zhaoling, Wang Shugian, Du Shejun, Xiu Jingren and Ye
Yongcai
Summary: The formation of the
high-T/sub c/ superconducting phases in the Tl-Ba-Ca-Cu-O
system has been investigated. Stable superconductivity up to
119 K was observed. One of the superconducting phases displays
an onset temperature of 122 K. The resistance of the
TlBaCaCu/sub 2/O/sub y/ sample, which is almost single-phase,
starts to drop sharply above 113 K and reaches zero at 108 K.
The bulk samples with different compositions in the
Tl-Ba-Ca-Cu-O system were synthesized. The electrical
resistivity and AC magnetic susceptibility dependence on the
temperature show that the zero-resistance transition
temperature in most of the samples is above 96
K. |
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Promoting
effects of steam on the sintering process of YBCO ceramic
superconductors S.T. Chen, X.S.
Li, F.C. Yin and X. Yao
Summary: The effects
of steam on the sintering of Y-Ba-Cu-O (YBCO) ceramic
superconductors are reported. It has been found that steam is
favourable for promoting the sintering of YBCO ceramics and
increasing the bulk density of the material as well as the
critical-current density. The linear contraction coefficient
increases with increasing steam pressure. The samples of YBCO
made by this method have shown good resistance against H/sub
2/O and CO/sub 2/. The reason why steam promotes the sintering
process is explained. |
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Studies
on the non rare earth oxide superconductors fabricated by
sintering H. Sekine, K. Inoue, H.
Maeda, K. Numata, K. Mori and H. Yamamoto
Summary:
Wire, tape, and bulk specimens of Bi-(Pb)-Sr-Ca-Cu-O
material have been prepared. Studies on the superconducting
properties and metallurgical studies have been conducted. The
results of thermogravimetric analysis indicate that the phase
transformation which accompanies absorption of oxygen does not
occur in this material from 860 degrees C down to 400 degrees
C. Tape specimens prepared by a combination of cold work and
sintering showed J/sub c/ of 1100 A/cm/sup 2/ at 77 K with a
good reproducibility. In these tape specimens, the c-axis of
the grains tends to align perpendicularly to the tape surface.
A 1330-filament Bi-Pb-Sr-Ca-Cu-O wire with a Ag matrix has
been fabricated, showing high-T/sub c/ transitions when
sintered at relatively low temperature. The concentration of
Pb in the oxide must be controlled (reduced) before packing
into an Ag sheath for the superconducting phase to be formed
in a long wire. The results of this study indicate that the
Bi-Sr-Ca-Cu-O system material has several favorable features
which could make practical use of this material
possible. |
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The
manufacturing process and properties of the high T/sub c/
oxide superconducting wire M.
Kawashima, M. Nagata, Y. Hosoda, S. Takano, N. Shibuta, H.
Mukai and T. Hikata
Summary: The
critical-current densities (J/sub c/) of Ag-sheathed tapes of
oxide superconductors were investigated using both the yttrium
and bismuth systems. Using a press process for Y/sub 1/Ba/sub
2/Cu/sub 3/O/sub 7-x/, BiSrCaCu/sub 2/O/sub x/, and Bi/sub
0.8/Pb/sub 0.2/SrCaCu/sub 2/O/sub x/, increases in J/sub c/
were inconsistently observed. J/sub c/ showed an increase for
Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ but not for BiSrCaCu/sub
2/O/sub x/. The Bi/sub 0.8/Pb/sub 0.2/SrCaCu/sub 2/O/sub x/
superconductor also showed increased J/sub c/. The press
processing improved the superconducting properties of
Ag-sheathed wire not only for J/sub c/ but also for T/sub c/.
The authors obtained high T/sub c/ phase T/sub c(R=zero)/=104
K and J/sub c/=1850 A/cm/sup 2/ for Bi/sub 0.8/Pb/sub
0.2/SrCaCu/sub 2/O/sub x/ superconductor. |
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Oxygen
kinetics and superconductivity in the high-T/sub c/ oxide
YBa/sub 2/Cu/sub 3/O/sub x/ O.F.
de Lima, R.F. Jardim, S. Gama and I.
Torriani
Summary: Disk-shaped samples of the
nominal compositions YBa/sub 2/Cu/sub 3/O/sub 7/, Y/sub
2/BaCuO/sub 5/ and BaCuO/sub 2/ were initially produced
through the solid diffusion method. These samples were then
cut into several small pieces, which were used for oxygen
intercalation and desorption studies for temperatures between
330 degrees C and 700 degrees C. The compound YBa/sub 2/Cu/sub
3/O/sub x/ went through a gradual transformation, going from
the pure superconducting orthorhombic phase (x=7.0) to the
pure semiconducting tetragonal phase (x=6.3). The compound
BaCuO/sub y/ also varied its oxygen content, in the range
2.00 |
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Compacted
1-2-3 ceramic powders treated as superconducting
composites E.W. Collings, A.J.
Markworth and K.R. Marken Jr.
Summary:
Magnetization studies of closely spaced
fine-filamentary Cu/NbTi composites have shown that, at very
low applied field strengths, proximity-effect coupling (across
the Cu matrix) between the filaments causes the assembly to
exclude flux in the manner of a solid block of superconductor
of the same outside diameter as the filamentary bundle. Then
at somewhat higher applied field strengths (but still with H |
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Critical
current density of Y-Ba-Cu oxide superconductor prepared by a
diffusion process N. Sadakata, M.
Sugimoto, O. Kohno and K. Tachikawa
Summary:
The formation of the YBa/sub 2/Cu/sub 3/O/sub 7-y/
layer has been investigated by a diffusion process as basic
research for high-T/sub c/ superconductor fabrication. The
starting materials were Y/sub 2/BaCuO/sub 5/ and barium
cuprate. A dense layer of YBa/sub 2/Cu/sub 3/O/sub 7-y/ was
formed on a Y/sub 2/BaCuO/sub 5/ substrate, and the starting
barium cuprate layer was fully converted to the
superconducting oxide. X-ray diffractometry revealed that the
layer consisted of an orthorhombic crystal structure. The
reacted specimens showed good superconductivity. A
zero-resistance temperature of 91.5 K and a critical-current
density of 1900 A/cm/sup 2/ (77 K, 0 T) were achieved in the
layer. Preparation conditions and superconducting properties,
particularly J/sub c/ characteristics, are investigated. The
diffusion phenomena that result in the formation of the
YBa/sub 2/Cu/sub 3/O/sub 7-y/ phase by this process are also
discussed. |
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Grain
size control in powder processed Y/sub 1/Ba/sub 2/Cu/sub
3/O/sub x/ K. No, J.D. Verhoeven,
R.W. McCallum and E.D. Gison
Summary: It is
demonstrated that small grained YBa2Cu30x (123) material can
be made from powders of Y2O3, BaC03 and CuO by avoiding liquid
phase formation in the high temperature treatments. Techniques
for avoiding liquid phase formation are demonstrated and
transport Jc measurements versus grain size are presented
. |
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Synthesis
of a high T/sub c/ superconducting phase in Bi-Sr-Ca-Cu-O
system S. Miyashita, M. Wakata,
A. Nozaki, K. Egawa, T. Ogama and K.
Yoshizaki
Summary: The synthesis of a
high-T/sub c/ phase in the Bi-Sr-Ca-Cu-O system by the
solid-state reaction and the J/sub c/ characteristics are
reported. The partial pressure of oxygen near 0.1 atm is
preferable as a sintering atmosphere, and Pb addition is
effective to produce a large amount of high-T/sub c/ phase.
Two classes of amorphouslike metastable superconducting phases
with T/sub c/ of approximately 90 K and approximately 40 K,
respectively, are found. It is thought that they have a
randomly stacked structure. J/sub c/ of 93 A/cm/sup 2/ at zero
field, 77 K is obtained for the sample with T/sub c/( rho
=0)=107 K, which includes a high-T/sub c/ phase and a small
amount of a low-T/sub c/ phase covered with the former phase.
J/sub c/ is exponentially decreased by increasing the field.
It is found that the transport J/sub c/ is limited by the
weak-link regions in this system, as evidenced by the
observation of a low resistive state above the J/sub c/ and of
superconductivity in that state. |
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Composite
core Nb/sub 3/Sn wires: preparation and
characterization R. Flukiger, W.
Specking, M. Klemm and S. Gauss
Summary:
Powder metallurgically processed composite-core Nb/sub
3/Sn wires, a novel type with filaments containing controlled
amounts of normal conducting inclusions elongated in the wire
axis, e.g., Ta or NbTi, have recently been developed.
Composite cores replace the Nb cores, which are inserted
either in a CuSn or in a Cu matrix for producing wires by the
bronze route or by the internal Sn diffusion process,
respectively. The ductile inclusions are deformed during the
wire drawing process to long ribbons being <100 nm apart.
If the reaction temperature is sufficiently low, the Ta or
NbTi shows little reaction with the Nb/sub 3/Sn phase but
reduces the A15 grain growth in the radial direction. The
resulting enhanced pinning at the grain boundaries (artificial
pinning) leads to higher critical-current densities,
particularly in the field range B<14 T. A difference
between the observed field dependence for Ta and NbTi
inclusions in the present configuration is explained by
different degrees of prestress. The achievement of higher
J/sub c/ values at 10-12 T may be advantageous for their
application in fusion magnets. |
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Influence
of cyclic pumped diffusion on the morphology and
microstructure of A15 layers in bronze processed
multifilamentary wire B.A.
Glowacki and J.E. Evetts
Summary: The authors
have used a novel reaction method called cyclic-pumped
diffusion (CPD) to achieve near uniform A15 reaction layers on
filaments throughout the cross-section of a multifilamentary
composite. A detailed investigation of the spatial variation
of the lattice parameter during reaction in filaments and in
the bronze matrix has enabled them to optimize the time and
temperature for successive reaction cycles. A 61*61 filament
Nb-7.5% wt Ta-bronze composite reacted using CPD in the range
600-800 degrees C shows a compound thickness variation between
the inner and outer filaments of approximately 30%, while the
same composite given a conventional isothermal heat treatment
at 700 degrees C shows a corresponding layer thickness
variation of approximately 200%. The consequences of the CPD
reaction method for the design of composites containing tin
reservoirs are discussed. |
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Low
loss and high current Nb/sub 3/Sn conductors made by the
internal-tin method H.C. Kanithi,
L.R. Motowidlo, G.M. Ozeryansky, D.W. Hazelton and B.A.
Zeitlin
Summary: Upon reaction heat treatment
of Nb/sub 3/Sn-type conductors, niobium filaments grow in size
due to a volume expansion. In conductors made by the
internal-tin method, the filaments are generally closer to
each other than in the bronze-route conductors. This can lead
to filament bridging after the reaction heat treatment. For
magnet applications such as in fusion and high-energy physics
where AC losses have to be limited, the authors have designed
and fabricated conductors with sufficiently large filament
separation. The conductors incorporate tantalum as a diffusion
barrier. An outline is given of the manufacturing process,
with details of significant design parameters. Required
heat-treatment conditions are addressed. Critical-current
performance and AC loss characteristics are presented and
compared to those of other internal-tin processed
conductors. |
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Ductility
enhancement in unreacted internal-Sn Nb/sub 3/Sn through
low-temperature anneals M.J.
Strum, L.T. Summers and J.R. Miller
Summary:
It is shown that a lack of tensile ductility in
as-drawn superconducting wires at room temperature can lead to
damage during magnet fabrication steps such as cabling and
coil winding. The tensile elongation of 0.42 mm internal-Sn
Nb/sub 3/Sn conductor in the as-drawn unreacted condition is
limited to less than 3% at room temperature. The tensile
elongation is substantially enhanced through low-temperature
annealing treatments at 200 degrees C, increasing to as high
as 10% after 24 h at 200 degrees C. This effect is attributed
to increased work hardening capacity from softening, which
reduces yield strengths up to 50% while maintaining 90% of the
tensile strength. Fracture surfaces of annealed wires reveal
increased adhesion between the Sn cores and the surrounding Cu
as well as an increase in ductile void coalescence as the
failure mechanism within the Cu. Preliminary results on the
prereaction strain sensitivity of reacted wire critical
currents confirm the increased strain tolerance in annealed
wires. |
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Metallurgy
and superconductivity in bronze-processed Nb/sub 3/Sn doped
cerium and cerium-titanium Liu
Chunfang, Zhou Lian and Yang Zhaoling
Summary:
Metallurgy and superconductivity in Ce-doped and
Ce-Ti-doped Nb/sub 3/Sn samples were investigated. The wires
containing Ce-bronze and Ce-Te-bronze were not noticeably
difficult to draw. The addition of Ce to the bronze not only
enhances the growth rate of the Nb/sub 3/Sn layer but also
improves the composition distribution within the wire. The
critical temperature T/sub c/ of Ce-doped samples treated at
700 degrees C is obviously higher than that of pure Nb/sub
3/Sn. The J/sub c/ difference between samples with small
amounts of Ce and pure Nb/sub 3/Sn is not obvious. J/sub c/
decreases slightly in low field with increasing Ce
content. |
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Internal
tin-Nb/sub 3/Sn composite S.
Hong, D. Geschwindner, A. Mantone, W. Marancik and R.
Zhou
Summary: Investigations are conducted of
Nb/sub 3/Sn multifilamentry composite fabrication by the
internal-tin method. One method is to distribute tin around
each filament uniformly, and the other approach is to
distribute tin semiuniformly. The authors report the
preliminary performance results for these two
composites. |
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YBa/sub
2/Cu/sub 3/O/sub 7/ thin films grown by high pressure reactive
evaporation and high pressure reactive
sputtering D.K. Lathrop, S.E.
Russek, K. Tanabe and R.A. Buhrman
Summary: A
high-pressure reactive evaporation process and a high-pressure
reactive sputtering process have been developed for the growth
of high-quality thin films of YBa/sub 2/Cu/sub 3/O/sub 7/.
Both techniques, when used with heated substrates, are
effective in the formation of the 123 phase in situ during the
film growth. With reactive evaporation only a cooldown anneal
in a higher-pressure oxygen ambient is necessary to obtain
good superconducting properties. For the reactive sputtering
process, a brief, postgrowth, rapid thermal anneal step is
required for best results. Fully epitaxial growth has been
achieved with single-crystal MgO substrates. The resultant
films, which can be quite smooth and uniform, have been
patterned to micron and submicron dimensions, and the
transport properties of these microstructures have been
examined. |
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Superconducting
properties of magnetron sputtered high T/sub c/ thin films
containing oxide compounds of yttrium, bismuth, or
thallium J.H. Kang, R.T.
Kampwirth and K.E. Gray
Summary: The authors
have used multiple-source magnetron sputtering to prepare thin
films of Y-Ba-Cu-O, Bi-Ca-Sr-Cu-O, and Tl-Ca-Ba-Cu-O
on |
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Electronic
noise in Ba/sub 2/YCu/sub 3/O/sub 7/ films at high
temperatures: a possible connection to stress
relaxation A. Davidson, N.F.
Pedersen, A. Palevski and M.R. Scheuermann
Summary:
Two types of electronic noise have been observed in
Ba/sub 2/YCu/sub 3/O/sub 7/ at high temperature. One type is
probably due to temperature and pressure fluctuations in the
environment of the sample. Because of the known sensitivity of
Ba/sub 2/YCu/sub 3/O/sub 7/ resistance to both temperature and
pressure, this mechanism translates into 10 to 100 nV per root
Hz of noise at temperatures above 750 K. The second type is
more intrinsic, and may be related to stress relaxation in the
film induced by the structural changes associated with the
orthorhombic-tetragonal transition near 950 K. |
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Inductive
measurements of critical current density in superconducting
thin films J.H.
Claassen
Summary: A noncontacting method of
probing the current-induced breakdown of superconductivity
(i.e., J/sub c/) in thin films is described which makes use of
a single pancake coil pressed against the film surface. The
technique has a sensitivity that is approximately 100 times
greater than direct transport measurements using
room-temperature electronics, and it eliminates many of the
attendant difficulties of the latter. Preliminary results on
Nb and Y-Ba-Cu-O films at 4.2 K reveal an exponential
voltage-current dependence, as expected from the activated
flux creep model. It is noted that, this being the case, no
unique critical-current density can be defined. In the case of
the oxide superconductors the flux-pinning parameters are such
that even a practical J/sub c/ definition is probably not
useful. |
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Dissipation
in high T/sub c/ thin films P.
England, T. Venkatesan, T.L. Cheeks, H.G. Craighead, C.T.
Rogers, S.-W. Chan, X.D. Wu and A. Inam
Summary:
Current methods of fabricating Y/sub 1/Ba/sub 2/Cu/sub
3/O/sub 7- delta / thin films by pulsed laser deposition
result in films that are multicrystalline, with grain sizes of
typically 1 mu m, or are quasi-monocrystalline, with no
discernable grain boundaries, according to whether the
deposition is carried out at high or low temperatures,
respectively. The authors studied the electrical
characteristics of both these types of material. It was found
that the I-V characteristics of the granular material follow
the rule V alpha (I-I/sub c/)/sup x(T)/ over at least six
orders of magnitude in voltage, at all temperatures and
magnetic fields. In addition, x(T) changes sharply from x=1 to
x=2 at the temperature at which I/sub c/ becomes finite. This
behavior can be explained quantitatively by a phase-ordering
transition. The nongranular material has a much sharper
resistive transition and larger critical current. The authors
examined the variation of critical current with magnetic field
and temperature in this material and observed an exponential
suppression of J/sub c/ with magnetic field. |
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Effects
of irradiation induced lattice disorder in superconducting
YBa/sub 2/Cu/sub 3/O/sub 7/
films P. Schmitt, H. Adrian, W.
Gerhauser, G. Adrian and R. Stumpf
Summary: A
report is presented on the preparation of thin films of the
high-T/sub c/ superconductor YBa/sub 2/Cu/sub 3/O/sub 7/ by
DC-magnetron sputtering and the effects of structural disorder
introduced by irradiation with 25 MeV /sup 16/O-ions on
superconductivity and normal state electrical resistivity. By
suitable heat treatments polycrystalline films of different
quality could be prepared on SrTiO/sub 3/ and Al/sub 2/O/sub
3/ substrates. The upper critical field B/sub c2/(T) of
high-T/sub c/ films measured up to 6 T is linear with dB/sub
c2//dT=-2 T/K. Irradiation with 25 Me oxygen ions leads to a
linear reduction of T/sub c/ and an increase of rho (100 K)
with fluence. The sensitivity of T/sub c/ to irradiation
depends significantly on the quality of the films. For
high-T/sub c/ films B/sub c2/(T) remains linear with
increasing slope for defect-induced T/sub c/-depressions of
about 30 K. As possible mechanisms for the observed T/sub
c/-degradation, a defect-induced metal-to-semiconductor
transition, which may be caused by a change of the local
stoichiometry, and electron localization are
discussed. |
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Microstructural
characterization of YBa/sub 2/Cu/sub 3/O/sub 7- delta / thin
films on SrTiO/sub 3/ using four-axis X-ray
diffraction J. Sizemore, R.
Barton, A. Marshall, J.C. Bravman, M. Naito and K.
Char
Summary: Microstructural characteristics
of YBa/sub 2/Cu/sub 3/O/sub 7- delta / thin films on SrTiO/sub
3/ have an important influence on their physical properties.
Four-axis X-ray diffraction methods have been used to
characterize textured and nontextured phases in several
YBa/sub 2/Cu/sub 3/O/sub 7- delta / thin films deposited on
single-crystal SrTiO/sub 3/. The type of epitaxial orientation
of a- and c-axis textured films has been analyzed by tilting
the specimen and observing the distribution of YBa/sub
2/Cu/sub 3/O/sub 7- delta / (102) plane X-ray intensities over
95% of the reciprocal lattice hemisphere. The ratio of a- to
c-axis material was determined. Several films were found to
contain nonepitaxially oriented YBa/sub 2/Cu/sub 3/O/sub 7-
delta /, Y/sub 2/BaCuO/sub 5/, and BaCuO/sub 2/. Methods for
quantifying the amounts of these random phases are discussed.
The existence of stacking faults, such as those caused by the
presence of extra Cu-O planes in YBa/sub 2/Cu/sub 3/O/sub 7-
delta /, can sometimes manifest itself as a broadening and
shifting of diffraction peaks. Consistent with the X-ray data,
the stacking faults are readily seen in high-resolution
electron microscopy images. |
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Spatial
resolution limit for the investigation of high-T/sub c/ films
by low temperature scanning electron
microscopy R. Gross, M. Hartmann,
K. Hipler, R.P. Huebener, F. Kober and D.
Koelle
Summary: Low-temperature scanning
electron microscopy represents a promising technique for
studying the local superconducting properties of high-T/sub c/
films. The spatial and temporal structure of the
electron-beam-induced thermal perturbation of high-T/sub c/
films and the resulting spatial resolution limit are
discussed. Typical examples illustrating the imaging technique
are presented. |
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Oxidation
kinetics in oxygen deficient YBa/sub 2/Cu/sub 3/O/sub 7-x/
studied by neutron powder
diffraction J. Als-Nielsen, N.H.
Andersen, C. Broholm, K.N. Clausen, B. Lebech, M. Nielsen and
H.F. Poulsen
Summary: A high-resolution,
multidetector neutron powder diffractometer has been
constructed and used for online studies of the oxidation
kinetics in ceramic powders of oxygen-deficient YBa/sub
2/Cu/sub 3/O/sub 7-x/ (YBACUO). The structural phase
transition between ordered orthorhombic and disordered
tetragonal phase has been studied in pure oxygen and under
vacuum. A continuous transition at 655 degrees C is observed
in a pure oxygen atmosphere under equilibrium conditions,
whereas an irreversible transition is observed at 470 degrees
C under vacuum. Reduced YBACUO corresponding to x
approximately=1.0 has been obtained by evacuation at 705
degrees C and has been shown to be stable in air. The
oxidation processes of reduced YBACUO in a pure oxygen
atmosphere have been studied using a semidynamical stepwise
increase in temperature and under isothermal conditions. The
results show that the in-diffusion under oxidation of reduced
YBACUO is more easily accomplished than the corresponding
out-diffusion process under reduction. From the structural
data it is concluded that oxidation takes place in a
mixed-phase medium of coexisting tetragonal and orthorhombic
phases. |
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Superconductivity
and crystal structure peculiarities of the oxygen deficient
YBa/sub 2/Cu/sub 3/O/sub 7- delta /
compounds V.G. Bar'yakhtar, I.G.
Mikhailov, A.E. Morozovsky, V.V. Nemoshkalenko, V.M. Pan, S.K.
Tolpygo, M.G. Vasilenko-Sheremetiev, V.S. Melnikov and N.P.
Pshentsova
Summary: Crystal structure
peculiarities, oxygen atom and vacancy ordering, and
superconducting properties of YBa/sub 2/Cu/sub 3/O/sub 7-
delta / compounds were investigated for the oxygen deficiency
range O< delta <0.8. It was shown that the
orthorhombic-to-tetragonal phase transition and
oxygen-atom-ordering process are quite different phenomena and
are observed at different temperature ranges. Oxygen
disordering in the z=o plane is thermally activated and occurs
mainly at low temperatures, when oxygen depletion by heating
above 400 degrees C leads to the significant local reduction
of the negative charge at the z=o plane. To restore a charge
balance, lattice negative and positive ions have to be driven
to shift along the z-axis in opposite directions. These
structure changes cause an increase in the distances between
layers. The superconducting transition temperature T/sub c/
appears to decrease along with delta . T/sub c/ versus delta
dependence has a two-step-like shape, for which an explanation
is offered. |
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Superconductivity,
critical current density, and magnetic properties of
Fe-substituted Y/sub 1/Ba/sub 2/(Cu/sub 1-x/Fe/sub x/)/sub
3/O/sub z/ materials S.T. Sekula,
J. Brynestad, D.K. Christen, J.R. Thompson and Y.C.
Kim
Summary: The superconductive properties
of a series of Fe-substituted Y/sub 1/Ba/sub 2/(Cu/sub
1-x/Fe/sub x/)/sub 3/O/sub z/ materials, with x=0 to 10
at.%Fe, have been studied. It was found that the T/sub c/
decreased linearly with Fe content x at the rate of 6.7
K/at.%. The intragranular critical-current density J/sub c/
decreased exponentially both with reduced temperature, T/T/sub
c/, and Fe concentration. Since the addition of Fe strongly
drives the structure toward tetragonality, it is concluded
that neither copper-oxygen chains nor macroscopic twinning is
essential for high-temperature
superconductivity. |
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Applicability
of oxide superconductor to magnetic
shielding T. Okada, K. Takahata,
S. Nishijima, S. Yoshida and T. Hanasaka
Summary:
Magnetic shielding capability has been investigated on
the Y-B-Cu-O oxide superconductor, aiming at a practical use
of oxide superconductors. Critical-transport-current density
and magnetization were measured using the four-terminal and
electromagnetic methods at temperatures of 77 K and 4.2 K. The
results indicated that the critical-current density by the
transport measurement was two orders of magnitude lower than
that by the magnetization. This means that the larger
shielding currents were found to be induced in the grain. The
magnetic field behind the disk-shaped specimen was measured
using a magnetic sensor. The shielding capability does not
depend upon the critical-current density derived from
magnetization but the critical-transport-current density. The
applicability of oxide superconductors to magnetic shielding
is discussed. |
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A
superconductive shielding can for high T/sub c/
SQUID G.J. Cui, S.G. Wang, H.M.
Jiang, J.Z. Li, C.Y. Li, C.D. Lin, R.Z. Liu, Q.L. Zheng, Y.S.
Fu, Z.L. Luo and W.C. Qiao
Summary: The
authors have fabricated yttrium-barium-copper-oxygen ceramic
superconductive shielding cans and measured the magnetic
shielding properties at liquid-nitrogen temperature by using a
Hall magnetic sensor and high-T/sub c/ SQUID (superconducting
quantum interference device). They report the fabrication
processing of the shielding cans and the measurement results.
The magnetic properties of the bulk samples are very different
from those of conventional superconductors. There are lots of
grain boundaries and defects inside the ceramic samples. The
magnetic field can penetrate through those defects into the
sample easily. The physical density of the samples is of great
importance for their superconductive magnetic shielding
ability. The isostatic pressing technique, which has been used
in this work to prepare a dense and uniform cylinder, is a key
processing method. |
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DC
critical currents in superconducting ceramic samples of Y/sub
1/Ba/sub 2/Cu/sub 3/O/sub 7/ G.
Paterno, C. Alvani, S. Casadio, U. Gambardella and L.
Maritato
Summary: DC transport critical
currents have been measured on Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub
7/ pellets, sintered under different conditions. Samples with
a density ranging from 3.8 to 5.42 g/cm/sup 3/ have been
investigated. The magnetic-field dependence of the critical
current has been measured at liquid-nitrogen temperature.
High-density samples, sintered at temperatures exceeding 950
degrees C, exhibit lower critical-current densities and a
stronger dependence on a weak magnetic field. |
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Voltage-current
characteristics of bulk YBa/sub 2/Cu/sub 3/O/sub 7-x/
superconductors Dingan Yu,
Junping Tang and Dunhou Zhang
Summary: V-I
characteristics of the sintered bulk YBa/sub 2/Cu/sub 3/O/sub
7-x/ superconducting samples with J/sub c/ ranging from 30 to
1000 A/cm/sup 2/ were measured in zero applied magnetic fields
and at liquid-nitrogen temperatures by using a
microcomputer-controlled I/sub c/ measuring system. The
results show that the V-I characteristics of these oxide
superconductors can be divided into four stages. In the first
stage, where voltage starts to go upwards, the voltage can be
expressed as an exponential function of the transport current,
which is different from the power relation often seen in the
cases of traditional superconductors such as NbTi alloys. In
the second stage, the voltage is a linear function of the
current, but its slope is much smaller than the normal-state
resistance of the sample. Finally, an unstable stage
characterized by a sharply rising voltage appears before the
V-I characteristic is turned into a normal-state one. These
features of the V-I characteristics are closely related to the
inhomogeneity existing in the sintered oxide
superconductors. |
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Superconductivity
and crystal structure of high T/sub c/ Bi-Ca-Sr-Cu-O
compounds V.M. Pan, V.P.
Dovgopol, A.G. Popov, V.B. Spivakovskij, V.S. Flis, A.V.
Zhalko-Titarenko, V.G. Bar'jakhtar, A.E. Pashitskij, V.S.
Melnikov, N.P. Pshencova, D.P. Demenko and S.M.
Rjabchenko
Summary: The superconducting
properties, phase chemical composition and crystal structure
of Bi-Ca-Sr-Cu-O system sintered samples were investigated.
Twostep resistive superconducting transitions with a 120 K
T/sub c/ onset were observed. The dominant phase composition
might be formulated as Bi/sub 2/CaSr/sub 2/Cu/sub 2+x/O/sub y/
where x=0.4-1.0, depending upon the heat-treatment
procedure. |
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Critical
current characteristics in high T/sub c/
superconductors T. Matsushita and
B. Ni
Summary: Critical-current densities of
superconducting oxides are theoretically estimated for
single-crystalline thin films and polycrystalline bulk
materials with oriented and random textures. The percolation
theory is used, and the effect of depression of the transport
current through grain boundaries is taken into account. The
results are compared in detail to previously published
experimental results. |
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Estimation
of stability and critical current density of oxide
superconducting wires S.
Nishijima, K. Takahata, T. Okada, Y. Sakai, K. Ikisawa, T.
Mori, N. Takasu, K. Ikeda and K. Ueminami
Summary:
The critical-current density of the high-T/sub c/ oxide
superconductor, YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO), has been
measured changing the temperature down to liquid-nitrogen
temperature (LNT) in order to estimate the adiabatic and
cryogenic stabilities at LNT. In estimating the
critical-current density, both four-terminal and magnetization
measurements were made. The specific heat was also measured.
The adiabatic stability was found to be much higher than that
of conventional superconductors at liquid-helium temperature.
The cryogenic stability, however, is found to be
poor. |
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Hall
effect and Hall resistivity in REBa/sub 2/Cu/sub 3/O/sub 7-
delta / N. Thier and K.
Winzer
Summary: Results of Hall effect and
Hall resistivity measurements for single-phase REBa/sub
2/Cu/sub 3/O/sub 7- delta / (RE=Y, Sm, Gd, Dy, Tm, Yb) and
YBa/sub 2/Cu/sub 3/O/sub 7- delta / single crystals around and
above T/sub c/ are reported. For all polycrystalline samples
the Hall numbers V/sub 0//R/sub H/e are positive and show a
linear temperature dependence in the range 120 K |
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Characterizing
the current carrying capacity of different high T/sub c/
superconductors with as much sensitivity as
versatility P. Metra, L. Gherardi
and R. Mele
Summary: To qualify quickly new
or optimized versions of high-T/sub c/ ceramic superconductors
that are not available in the shape of a wire, two simple and
accurate methods have been set up. The first measures the
transport critical-current density of small sintered specimens
or films with no electrical contacts, using a standard
magnetic-core transformer with a washer of superconducting
material fitted as a shorted, one-turn secondary winding. The
current is induced in the specimen by exciting the primary
coil with AC, and the resulting magnetic flux is very
accurately measured with a pick-up coil and a lock-in
amplifier. A similar approach, in which a small
superconducting disk is introduced to replace an air gap in
the magnetic circuit, measures the AC susceptibility
components. This method can be used to study the effect of
both temperature and magnetic field (up to 80 Oe) by clearly
discriminating between intra- and inter-grain conduction
transitions. Results of measurements on YBa/sub 2/Cu/sub
3/O/sub 7- delta / and bismuth compounds are
presented. |
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Influence
of fast neutron irradiation on inter- and intragrain
properties of ceramic YBa/sub 2/Cu/sub 3/O/sub
7/ H. Kupfer, U. Wiech, I.
Apfelstedt, R. Flukiger, R. Meier-Hirmer, T. Wolf and H.
Scheurer
Summary: Fast neutron irradiation of
YBa/sub 2/Cu/sub 3/O/sub 7/ leads to a decrease of T/sub c/,
the variation of the ratio T/sub c/( Phi t)/T/sub c/(O) being
comparable to that observed in conventional superconducting
compounds. Superconductivity vanishes above a fluence of
2*10/sup 19/ cm/sup 2/. The phase-locking temperature of the
intergrain junctions decreases faster with fluence than T/sub
c/. Lower AC losses due to a complete destruction of weak
links and lower intergrain critical-current densities with
increasing fluence reflect in addition a larger radiation
sensitivity of the intergrain junction properties. The
intragrain critical-current density j/sub cG/ increases with
increasing fluence, but its dependence on B and Phi t is not
at all in accordance with increasing pinning by
radiation-induced defects and decreasing T/sub c/. The
intragrain current drops remarkably at low fields, passing a
minimum and a subsequent maximum beyond which the common
decrease of j/sub cG/ with increasing field is observed. The
low-field behavior, together with the much lower increase of
j/sub cG/ with fluence than above the maximum, points to a
junction character of the current in this field
range. |
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Substitution
of Mn for Cu in the high T/sub c/ superconductor YBa/sub
2/Cu/sub 3/O/sub 7- delta / R.F.
Jardim, S. Gama, O.F. de Lima and I.
Torriani
Summary: Samples of YBa/sub
2/(Cu/sub 1-x/Mn/sub x/)/sub 3/O/sub 7- delta /, with x=0.0,
0.025, 0.05, 0.075, 0.10, 0.15, and 0.20, were prepared by the
solid diffusion method. Superconductivity with T/sub c/ in the
range of 84-92 K was observed through electrical resistivity
and magnetic susceptibility measurements. The substitution of
Mn for Cu practically did not affect the onset temperature in
the resistive transition, although a strong correlation
between the transition width broadening and the increase of Mn
content was observed. This fact is consistent with the
reduction of the superconducting volume fraction determined
through the normalized susceptibility data. X-ray diffraction
revealed the occurrence of additional phases increasing with
the Mn concentration. |
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High
magnetic field studies of YBa/sub 2/Cu/sub 3/O/sub 7/ and
NdBa/sub 2/Cu/sub 3/O/sub 7/ single
crystals J.S. Moodera, J.E.
Tkaczyk and P.M. Tedrow
Summary: The authors
have measured the upper critical magnetic field H/sub c2/ of
single crystals of the high-T/sub c/ superconductors YBa/sub
2/Cu/sub 3/O/sub 7/ and NdBa/sub 2/Cu/sub 3/O/sub 7/ as a
function of crystal orientation in DC fields up to 31.5 T. The
temperature dependence and anisotropy of H/sub c2/ and
inferences concerning spin-orbit scattering are discussed. The
lowest value of critical field is obtained for the crystal
orientation which makes the applied field perpendicular to the
Cu-O planes and is not much different from the values of H/sub
c2/ for some conventional superconductors. |
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Weak
link behavior in polycrystalline BaPb/sub 0.75/Bi/sub
0.25/O/sub 3/ L. Cooley, M.
Daeumling, T.C. Willis and D.C.
Larbalestier
Summary: A study has been made
of the critical-current densities of polycrystalline samples
of the perovskite oxide superconductor BaPb/sub 0.75/Bi/sub
0.25/O/sub 3/ (BPB). BPB does not have the complications of
twin boundaries and a highly anisotropic unit cell as is found
in the high-T/sub c/ oxide superconductors (HTSC), and the
coherence length ( xi ) is much longer than in HTSC materials
( approximately 7 nm as compared to approximately 0.5 to 2.5
nm). Thus some problems which have proved quite complex for
HTSC materials (e.g. twin pinning or weak links) should be
absent or have a different character in BPB. The authors have
produced several samples of BPB. X-ray diffraction showed that
the samples were single-phase, but an electron microprobe was
used to identify about 1 vol.% of other phases present. The
authors found that the transport critical-current density, as
high as 2 A/cm/sup 2/ in zero field, drops markedly in fields
of only 0.1 T. Flux penetration measurements place the
magnetization critical-current density at about 10/sup 4/
A/cm/sup 2/. This large discrepancy is attributed to weak
links. |
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Silver
encased high temperature superconductor ribbons produced by
rolling G.A. Whitlow, N.C. Iyer,
A.T. Male, J.C. Powell and G.R. Wagner
Summary:
Ribbons of silver/yttrium-barium-copper oxide/silver
(Ag/YBCO/Ag), nominally 0.01 in. (0.25 mm) thick, have been
fabricated by unidirectional rolling and their resulting
microstructures and superconducting properties evaluated. The
influence of a postfabrication sinter/oxygen replenishment
anneal on their properties has been determined. Grain
alignment/preferred orientation of the YBCO core was observed.
Typical transport critical-current densities measured in zero
field at 4.2 and 77 K were 600 and 125 A/cm/sup 2/,
respectively. Preliminary experiments on the
bismuth-containing high-temperature superconductors have
indicated that codeformation of a silver composite billet by
rolling to thin strip is easily accomplished. |
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Preliminary
studies for the development of superconducting composite
wires V. Provenzano, W.F.
Henshaw, A.S. Edelstein, A.K. Singh, M.A. Imam, M.S. Osofsky,
E.F. Skelton and S.B. Qadri
Summary: The
results reported are part of a larger effort, the basic aim of
which is to develop high-T/sub c/ superconducting composite
wires, films, and thin-gauge panels with large current
carrying capacity and engineered thermal and mechanical
properties. The results relate to the following aspects of
this ongoing research effort: the sputtering parameters used
to deposit Bi-Sr-Ca-Cu-O films on MgO and other substrates; a
dipping technique to produce Bi-Sr-Ca-Cu-O superconducting
layers and wires; the postannealing heat treatments; the
characterization of the microstructure and crystallographic
features of the deposited films; and the superconducting
properties of the films, layers, and wires. |
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Anisotropies
in the magnetic penetration depth and critical current density
of R/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ (R=Y or Ho), measured
on magnetically-aligned powder
composites D.K. Christen, Y.C.
Kim, S.T. Sekula, J.R. Thompson, J. Brynestad, B.C. Sales,
L.A. Boatner and J.D. Budai
Summary:
Dispersed, uniaxially-aligned powder composites of
R/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ (R=Y or Ho) were formed
by suspending the powder in a liquid organic matrix that was
then solidified in a magnetic field. From the observed
particle dimensions and magnetic susceptibility measurements
for fields applied either parallel or perpendicular to the
Cu-O planes, the authors infer anisotropies in the
superconducting magnetic penetration depth lambda . From the
magnetization hysteresis in the mixed state they derive
relative anisotropies in the critical-current densities J/sub
c/. The observed anisotropies are qualitatively similar for
both rare-earth-based materials but show some quantitative
distinctions. |
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ARXPS,
SEM, and SAM studies of highly textured YBa/sub 2/Cu/sub
3/O/sub x/ films J. Halbritter,
B. Hauser, E.G. Keim, H.-J. Mathes, P. Walk and H.
Rogalla
Summary: The electric conduction in
the new cuprate superconductors suffers from intragrain and
intergrain weak links. In transition metal oxides weak links
show up in a specific chemistry, which can be measured by
X-ray photoelectron spectroscopy and by Auger electron
spectroscopy. To measure a defective local chemistry, the
chemical signature of the reaction cinder has to be identified
and the chemical signature of the perfect crystal has to be
established. For highly textured cuprate films this has been
done successfully, yielding the signature of the reaction
cinder, e.g. BaO/sub 2/ or graphite, the signature of the
interface reactions, e.g. Ba-aluminate on Al/sub 2/O/sub 3/ or
Cu diffusion into MgO, and the signature of c-axis surfaces
being a BaO/sub x/ layer on YBa/sub 2/Cu/sub 3/O/sub x/, which
results in a 1-nm-thick insulating layer. |
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Properties
of the new high T/sub c/ materials: an analysis based on
fermiology V.Z. Kresin, G.
Deutscher and S.A. Wolf
Summary: A small
value of the Fermi energy, E/sub f/, in the new high T/sub c/
oxides and its consequences are the subject of this study. It
is shown that the small value of E/sub f/ allows separation of
the electronic contribution to the heat capacity in the
high-temperature region E/sub f/k/sub B/>T> theta /sub
D/ to determine the value of the electron-phonon coupling
constant lambda . The linear temperature dependence of the
normal resistance is mainly due to a large anisotropy of the
system. A small value of E/sub f/ allows the lattice
contribution to the thermal conductivity to play a dominant
role. A strong electron-phonon coupling is manifested in the
increase of the thermal conductivity in the region T |
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Measurement
and correlation of optical and TEM twins in Y/sub 1/Ba/sub
2/Cu/sub 3/O/sub 7- delta/ L.S.
Chumbley, J.D. Verhoeven, M.R. Kim, A.L. Cornelius and M.J.
Kramer
Summary: The spacing of the twins
present in Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7- delta / has been
investigated using optical and transmission electron
microscopy (TEM). Studies have reported that the value of the
average twin spacing obtained by optical measurements is much
larger than that measured in thin foils prepared for TEM
observation, and controversy exists as to the exact nature of
the optical twins. Experiments have been conducted which show
that the twins observed optically correspond to those seen
using TEM and are not produced by an optical interference
effect. Using electron channeling, equations have been
formulated which allow optical determination of the true
spacing of twins present in a selected grain as well as the
orientation of that grain. The average twin spacing measured
by TEM has been found to vary with the sample preparation
method, being smaller for crushed chip samples than for bulk
samples prepared by ion milling or jet-polishing. A variation
of the twin spacing with grain size has also been
noted. |
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Transport
and structural properties of Bi-Sr-Ca-Cu-oxide thin films
prepared by reactive magnetron
sputtering D.W. Face, J.T.
Kucera, J. Crain, M.M. Matthiesen, D. Steel, G. Somer, J.
Lewis, J.M. Graybeal, T.P. Orlando and D.A.
Rudman
Summary: Highly oriented
superconducting thin films of Bi-Sr-Ca-Cu-oxides have been
reproducibly prepared by reactive magnetically-enhanced triode
sputtering using three separate metal targets (Sr, Ca, and
Cu-Bi). The as-deposited films had composition ratios of
Bi:Sr:Ca:Cu that were very close to either 2:2:1:2 or 2:2:2:3.
The films were deposited on SrTiO/sub 3/, MgO, CaF/sub 2/,
Al/sub 2/O/sub 3/, and yttria stabilized zirconia (YSZ)
substrates and had thicknesses of about 250 nm. After
deposition, the films were annealed to 850-900 degrees C in
various partial pressures of oxygen (0.05, 0.2 and 1 atm).
Films deposited on |
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Critical
current density measurements of thin films of
YBaCuO L.H. Allen, J.H. Claasen
and P.R. Broussard
Summary: Thin films of the
YBaCuO superconductor have been deposited by evaporation on
SrTiO/sub 3/ substrates. After a post-anneal, the films were
patterned into a four-probe geometry, and critical-current
density measurements were made using a transport technique.
Compositions both on and off the stoichiometric Y/sub 1/Ba/sub
2/Cu/sub 3/O/sub 7-x/ were studied, and measurements were made
as a function of temperature and magnetic field. At 4.2 K and
in ambient field, a stoichiometric film has a J/sub c/ greater
than 10/sup 6/ A/cm/sup 2/ which falls off slowly out to
approximately=55 K and then follows a power law dependence
(1-t)/sup 3.3/, where t is the reduced temperature. The
magnetic-field dependence of J/sub c/ at 4.2 K is weak, and
for applied fields of 90 kOe, J/sub c/ has dropped only a
factor of 10 from the ambient field. Off-stoichiometric films
have lower J/sub c/'s which fall off faster and at lower
temperatures. |
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Torque
measurements of textured Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/
sintered pellets F. Celani, L.
Fruchter, C. Giovannella, R. Messi, S. Pace, A. Saggese and N.
Sparvieri
Summary: The authors present at 4.2
K magnetic torque balance study of a textured sintered YBCO
sample produced by pyrolysis using in the thermal treatments
an ozone-enriched oxygen atmosphere. The shape of the torque
signal as a function of the angle and of the measuring field
shows features that are intermediate between those of a
standard sintered pellet and those of a single crystal. The
anisotropic experimental values of the lower critical field
H/sub c1/ compare rather well to those previously found on
YBCO single crystals. The intensity of the remaining
magnetization, measured after a field cycling procedure,
suggests the existence of a strong pinning for H>or=H/sub
c1//sup (c)/ along the c-axis. |
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Thin
film growth of oxide superconductor
materials R.L. Henry, E.J.
Cukauskas, S.B. Qadri, A.H. Singer and G.G.
Campisi
Summary: Thin films of YBa/sub
2/Cu/sub 3/O/sub 7/, YBa/sub 2/Cu/sub 3/O/sub 7/:Ag and Bi/sub
2/CaSr/sub 2/Cu/sub 2/O/sub 8/ have been grown by spray
pyrolysis of aqueous nitrate solutions. Substrate temperatures
in the range of 300 degrees C to 875 degrees C have been
investigated. Following deposition, the films were annealed in
flowing oxygen at 960 degrees C for YBa/sub 2/Cu/sub 3/O/sub
7/ and 850 degrees C for Bi/sub 2/CaSr/sub 2/Cu/sub 2/O/sub
8/. The unit cell size of YBa/sub 2/Cu/sub 3/O/sub 7/:Ag did
not change from that of YBa/sub 2/Cu/sub 3/O/sub 7/ but the
X-ray diffraction patterns for the YBa/sub 2/Cu/sub 3/O/sub
7/:Ag films show that the |
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Characterization
of thin YBaCuO films grown by
coevaporation P.R. Broussard,
J.H. Claassen and S.A. Wolf
Summary: Thin
films of the Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub x/ superconductor
have been grown by coevaporation and postannealing in oxygen
on substrates of (100) SrTiO/sub 3/, (100) MgO, and (100)
Yttria-stabilized Zirconia. Films grown on SrTiO/sub 3/ show
evidence of both a- and c-axis growth normal to the substrate
and strong in-plane order. Films on MgO show c-axis growth
without any in-plane order, indicative of fiber texture.
Transition temperatures of the films on MgO are depressed
compared to those on SrTiO/sub 3/, but by using higher
post-anneal temperatures we are able to improve the
superconducting behavior of MgO samples. |
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Bi-Ca-Sr-Cu-O
superconductors: the connectivity
issue R. Ramesh, G. Thomas, S.M.
Green, M.L. Rudee, Y. Mei and H.L. Luo
Summary:
The connectivity problem in Bi-Ca-Sr-Cu-O
superconductors is addressed. The bulk samples always exhibit
a step in the resistivity plot, consequent to which zero
resistance is obtained around 75 K, although the onset is
above 110 K. Magnetic measurements indicate the presence of
almost equal fractions of two superconducting phases with
transition temperatures of 110 K and about 80 K. The
microstructure of these samples has been examined using
several transmission electron microscopy techniques. The
superconducting phases exhibit a polytypoid-type structure
with the general formula of Bi/sub 2/Sr/sub 2/Ca/sub
n-1/Cu/sub n/O/sub y/, where n takes on values of 1,2,3, and,
in some infrequent cases 4. The c-parameter and T/sub c/
increase with n. In almost all the superconducting grains, the
lower T/sub c/ (n=1 or 2) polytypoid is observed at the grain
boundaries. Thus, the 110 K polytypoid in each grain is not
'connected' to similar regions in other grains. The lower
T/sub c/ polytypoid is observed at the grain boundaries due to
a reduction of Ca and/or Cu. It is suggested that the
controlled addition of PbO allows the formation of a Pb-rich
liquid phase which makes the composition near the grain
boundaries uniform. Consequently, the step in the resistivity
plot disappears. |
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High
temperature superconductors in the radiofrequency
field J. Pirnat, J. Luznik and Z.
Trontelj
Summary: The characterization of
high T/sub c/ superconducting materials by a superregenerative
radio frequency oscillator is considered. The physical
principles of measurements performed on different YBa/sub
2/Cu/sub 3/O/sub 7- delta / samples are presented. A strong,
reproducible and measurable effect on the superregenerative
oscillator output was noticed when the sample in the tank
circuit coil subjected to the low magnetic modulation field
(about 4 mT, 25 Hz) passed through the superconductive
transition in either direction. The signal is roughly
independent of the spectrometer frequency from 2 MHz to 350
MHz. The results of measurements of the signal shape, the
frequency shift, the line splitting and the influence of
applied DC magnetic field are discussed. |
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VAMAS
intercomparison of critical current measurement in Nb/sub 3/Sn
wires K. Tachikawa, K. Itoh, H.
Wada, D. Gould, H. Jones, C.R. Walters, L.F. Goodrich, J.W.
Ekin and S.L. Bray
Summary: A technical
working party in the area of superconducting and cryogenic
structural materials has recently carried out the first
worldwide intercomparison of critical-current, I/sub c/,
measurement on multifilamentary Nb/sub 3/Sn wires. Three
sample wires were supplied from each of the European Community
countries, Japan, and the United States. There were few
restrictions for the I/sub c/ measurement at participant labs.
The standard deviations of the I/sub c/ values reported from
these labs varied among test samples, and were 6-21% of
averaged I/sub c/s at 12 T. |
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Nb/sub
3/Sn critical-current measurements using tubular
fiberglass-epoxy mandrels L.F.
Goodrich, S.L. Bray and T.C. Stauffer
Summary:
A systematic study of the effect of sample mounting
techniques on the superconducting critical-current measurement
was made. A seemingly small change in mandrel geometry can
result in a 40% change in the measured critical current of a
Nb/sub 3/Sn sample at 12 T. This is a result of a change in
the conductor prestrain at 4 K caused by variation in thermal
contraction between thick-and thin-walled fiberglass-epoxy
composite (G-10) tubes. An approximate measure of the
vibrations in thermal contraction (from room to
liquid-nitrogen temperature) indicate a 0.2% greater
contraction for the thick-walled tube. This difference,
combined with strain sensitivity measurements, is consistent
with the observed decrease in critical current. |
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Effect
of transverse compressive stress on the critical current of
cabled Nb/sub 3/Sn conductor B.
Jakob and G. Pasztor
Summary: A test
apparatus has been designed and built for the investigation of
cabled conductors subjected to transverse load. The apparatus
permits measurement of compressive stress on a loaded
multistage flat cable when a transport current is applied to
one of its constituent subcables. With the apparatus, the
critical-current behavior of a solder-filled Nb/sub 3/Sn cable
made by the external diffusion method exposed to transverse
loads up to 165 MPa, was investigated. A field-dependent
degradation of the critical current was found. The effect is
completely reversible up to about 100 MPa. Solder-filled
cables as used in large-current applications are less
sensitive to transverse compressive stress than single wires.
A 150 MPa compressive load on the SULTAN cable, for example,
degrades the critical current by 31% at 11 T. This is still a
high enough value in view of the large transverse compressive
stress acting on conductors required for fusion
magnets. |
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The
critical density throughout the superconducting phase of
multifilamentary Nb/sub 3/Sn wire for forced flow fusion
applications measured using the Bean-Campbell AC magnetization
technique D.P. Hampshire and D.C.
Larbalestier
Summary: AC magnetization
measurements for transverse orientation have been completed on
a multifilamentary Nb/sub 3/Sn composite wire, which is a
candidate for fusion applications. Using these data, it was
possible to determine the critical-current density using the
Bean-Campbell analysis in the ranges B |
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Measurements
of the stability margin of a Nb/sub 3/Sn cable-in-conduit
conductor T. Ando, M. Nishi and
S. Shimamoto
Summary: The stability margin of
a single quadruplex of Nb/sub 3/Sn/Cu composite strand
enclosed in a round stainless-steel tube was measured by using
an inductive heater whose duration time is around 0.1 ms. The
strand had a diameter of 1.04 mm and a copper fraction of 69%.
The fraction of helium in the cable space was 34.5%.
Measurements of the stability margin were carried out by
varying the transport current, external magnetic field, and
initial helium pressure. The helium flow rate was always zero.
The limiting transport current, which is the boundary value
between the upper and lower stable regions similar to NbTi
cable-in-conduit conductors, was observed. The
limiting-current density over the cable space inside the
conduit at 12 T was 100 A/mm/sup 2/, corresponding to 58% of
the critical current. This value does not depend on the
initial helium pressure in the range of 3-12 atm. Observed
upper and the lower stability margins were more than 200
mJ/cm/sup 3/ strand and less than 60 mJ/cm/sup 3/ strand,
respectively. The heat transfer coefficient in the
cable-in-conduit conductor is estimated to be around 0.1
W/cm/sup 2/K. |
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Microwave
studies of granular
superconductivity A.M. Portis and
K.W. Blazey
Summary: Microwave absorption in
low magnetic fields is a sensitive contactless indicator of
surface granular superconductivity, a significant source of
surface resistance. Granularity is also responsible for
microwave absorption in the ceramic high-temperature
superconductors and in ceramic Chevrel-phase compounds.
Modulated microwave adsorption signals from granular ceramics
show critical behavior with pinning and depinning of
intergranular flux over a modulation cycle. In single crystal
YBa/sub 2/Cu/sub 3/O/sub 7- delta / the adsorption arises from
the motion of flux within twin-plane domain boundaries.
Modulated adsorption signals from these domain boundaries show
additional adsorption from fluxon nucleation above critical
levels of microwave current. Such losses may contribute
significantly to microwave adsorption in zero magnetic
field. |
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Microwave
measurements on high T/sub c/ superconducting single crystals
and films W.W. Fuller, F.J.
Rachford, W.L. Lechter, P.R. Broussard, L.H. Allen and J.H.
Claassen
Summary: The authors have studied
the microwave properties of a single crystal of ErBa/sub
2/Cu/sub 3/O/sub 7-x/. The crystal was placed in the bottom of
a rectangular TE 103 copper cavity whose resonant frequency
was 9.2 GHz. The Q of the system, cavity and sample, as well
as the frequency shift, was monitored as the system was
cooled. Below about 50 K the measurements became limited by
the copper cavity. Close to the superconducting transition,
about 90 K, the real surface impedance is at least one order
of magnitude higher than that calculated from Mattis and
Bardeen's (1958) formula. The authors have also studied using
both resonant and nonresonant methods the microwave properties
of an oriented film of YBa/sub 2/Cu/sub 3/O/sub 7-x/ grown on
a MgO substrate. For both types of measurements they have
studied the effects of small magnetic fields, less than 100 G,
on the microwave losses at 9.2 and 10 GHz in a microwave
cavity and by a nonresonant technique. The results are
discussed in terms of flux entry and pinning at grain
boundaries. |
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Fabrication
and microwave properties of Y-Ba-Cu-O and Bi-Ca-Sr-Cu-O thin
films R. Sobolewski, J. Konopka,
W. Kula, P. Gierlowski, A. Konopka and S.J.
Lewandowski
Summary: Studies are reported of
the interaction of Y-Ba-Cu-O and Bi-Ca-Sr-Cu-O thin films with
microwaves. The films were prepared on cubic zirconia and
magnesium oxide substrates using a modified spray deposition
technique. The tested films were about 1-3 mu m thick and
exhibited good superconducting properties. Several test
structures have been designed by spraying a mixture of nitrate
precursors on heated substrates through very thin stencil
marks. The supercurrent dependence on millimeter wavelength
microwave illumination was measured, and microwave detection
and mixing experiments were performed. The films of both
materials exhibited properties characteristic of granular
superconductors, with Josephson-type coupling at the
intergrain connections. The results also indicate that these
materials can be successfully used as very sensitive
millimeter radiation detectors operational at liquid-nitrogen
temperatures. |
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Surface
impedance measurements on high T/sub c/
superconductors G. Muller, D.J.
Brauer, R. Eujen, M. Hein, N. Klein, H. Piel, L. Ponto, U.
Klein and M. Peiniger
Summary: An
investigation has been made of the RF properties of two oxide
superconductors, mainly Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ and
tentatively Bi/sub 2/CaSr/sub 2/Cu/sub 2/O/sub x/, between 4.2
and 300 K at S-, K-, and W-band frequencies. The surface
resistance R/sub s/ and the magnetic-field penetration depth
lambda of ceramic pellets and of polycrystalline layers on
silver substrates were measured in normal or superconducting
host cavities. Improvement factors for R/sub s/ of up to three
orders of magnitude at 3 GHz and one at 86 GHz have been
achieved, but the temperature dependence of R/sub s/ deviates
strongly from that of conventional superconductors. The main
features are a slow fall-off near T/sub c/ and a remaining
temperature dependence at 4.2 K, both depending on the
preparation and the resulting homogeneity of the samples. From
the frequency shift of the cavity near T/sub c/ the authors
extrapolate lambda (0) values between 200 and 1300 nm. Large
fractions of the samples remain superconducting up to surface
current densities on the order of 10/sup 5/ A/cm/sup
2/. |
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RF
properties of high-T/sub c/
superconductors C.L. Bohn, J.R.
Delayen, D.I. Dos Santos, M.T. Lanagan and K.W.
Shepard
Summary: An investigation was
conducted of the RF properties of high-T/sub c/
superconductors over a wide range of temperatures, frequencies
and RF field amplitudes. Both bulk polycrystalline samples and
thick films on silver substrates were tested. At 150 MHz and
4.2 K, surface resistances of 18 mu Omega at low RF field and
3.6 m Omega at an RF field of 270 G were measured. All samples
showed a strong dependence of the surface resistance on RF
field. However, no breakdown of the superconducting state was
observed up to the highest field achieved (320
G). |
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Relaxation
behavior of ultrasonic attenuation in YBa/sub 2/Cu/sub 3/O/sub
7/ K.J. Sun, W.P. Winfree, M.F.
Xu, B.K. Sarma, M. Levy, R. Caton and R.
Selim
Summary: It is shown that
temperature-dependence ultrasonic attenuation data of YBa/sub
2/Cu/sub 3/O/sub 7/ at various frequencies exhibit anomalies
at temperatures close to T/sub c/. These attenuation maxima
are found to be the result of a relaxation mechanism added on
top of an unusual attenuation background. It is proposed that
the grain boundary motions induced by the structural
distortion and the propagation of sound waves enhances the
energy dissipation around T/sub c/. Whether this structural
distortion is the consequence of the onset of a
superconducting state remains undetermined. It is also
possible that either a tunneling effect of the acoustoelectric
effect contributes to sound energy dissipation. The
temperature dependence of ultrasonic velocity shows a
softening around T/sub c/ which may be an intrinsic property
of high-temperature superconductors. |
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Ultrasonic
measurements in sinter-forged high T/sub c/ superconductor
YBa/sub 2/Cu/sub 3/O/sub 7-
delta/ M.F. Xu, A. Schenstrom, Y.
Hong, D. Bein, B.K. Sarma, M. Levy, Z. Zhao, S. Adenwalla, A.
Moreau, Q. Robinson, D.L. Johnson, S.J. Hwu, K.R. Poeppelmeier
and J.B. Ketterson
Summary: Both ultrasonic
attenuation and velocity as a function of temperature from 4.2
K to room temperature were measured on sinter-forged samples
of YBa/sub 2/Cu/sub 3/O/sub 7- delta /, which have more than
80% alignment of the c-axis of the grains parallel to the
forging axis. Strong anisotropy was found both in the
attenuation and the velocity when propagating sound waves in
different directions relative to the forging axis. Hysteresis
was observed in the velocity changes. |
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In
situ growth of superconducting YBaCuO using reactive
electron-beam coevaporation N.
Missert, R. Hammond, J.E. Mooij, V. Matijasevic, P. Rosenthal,
T.H. Geballe, A. Kapitulnik, M.R. Beasley, S.S. Laderman, C.
Lu, E. Garwin and R. Barton
Summary:
Conditions required for in situ growth of YBaCuO thin
films by reactive electron-beam evaporation have been
explored. Three sources of activated oxygen (atomic oxygen
from microwave discharge, plasma generated by electron beams,
and an ion beam) were compared. The best results so far were
obtained with atomic oxygen. Epitaxial films with high
critical currents were grown on SrTiO/sub 3/ [100] and [110],
Al/sub 2/O/sub 3/ (1102), and MgO [100] at 600 degrees C.
Evaporation rates were controlled with a rate monitor using
atomic absorption. |
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Growth
and properties of sputtered high-T/sub c/ oxide thin
films K. Char, M.R. Hahn, T.L.
Hylton, M.R. Beasley, T.H. Geballe and A.
Kapitulnik
Summary: Superconducting YBa/sub
2/Cu/sub 3/O/sub 7-x/ films of thicknesses ranging from 500 AA
to 5 mu m have been successfully made by the reactive
magnetron sputtering technique. The effects of composition,
annealing condition, and thickness of an epitaxial film on its
orientation are discussed. films show anisotropic
resistivities and critical-current densities that are
orientation-dependent. |
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Fabrication
and properties of Bi-Sr-Ca-Cu-O thin films made using pulsed
laser deposition D.K. Fork, T.H.
Geballe, J.B. Boyce, F.A. Ponce and R.I.
Johnson
Summary: Superconducting films of
Bi-Sr-Ca-Cu-O on (100) MgO were fabricated using the pulsed
laser deposition technique to deposit from sintered and
fluoride-containing disks. Fluoride-containing films were
observed to anneal more rapidly and at slightly reduced
temperatures and to contain an additional phase with 24.5 AA
c-axis spacing. The preferred orientation of all films is
c-axis normal to the substrate. On (100) SrTiO/sub 3/,
heteroepitaxial growth of the Bi/sub 2/Sr/sub 2/Ca/sub
1/Cu/sub 2/O/sub 8+ delta / phase was demonstrated using
transmission electron microscopy. |
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Superconducting
thin films of BiSrCaCuO made by sequential electron beam
evaporation J. Steinbeck, A.C.
Anderson, B.Y. Tsaur and A.J. Strauss
Summary:
Superconducting thin films of Bi/sub 2/Sr/sub 2/Ca/sub
1/Cu/sub 2/O/sub x/ have been made by sequential electron-beam
evaporation of multiple layers of Bi and Cu metals and
(Sr,Ca)F/sub 2/ on MgO substrates. The films were annealed at
high temperature, first in wet O/sub 2/ and then in dry O/sub
2/, and cooled to room temperature in dry O/sub 2/. The
resulting films, which are approximately 1 mu m thick, have
transition temperatures of approximately 85 K. X-ray
diffraction shows that the films are preferentially oriented
with their c-axis perpendicular to the MgO substrate. The best
film has a zero-resistance temperature of 90 K and
critical-current densities of 8*10/sup 4/ A/cm/sup 2/ at 77 K
and 2.5*10/sup 5/ A/cm/sup 2/ at 4.2 K. |
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Improvement
of average film quality in RBa/sub 2/Cu/sub 3/O/sub 7-x/
sputtered films R.W. Simon, C.E.
Platt, K.P. Daly, A.E. Lee and M.K. Wagner
Summary:
Thin films of RBa/sub 2/Cu/sub 3/O/sub 7-x/ (R includes
Y, Er, and Nd) have been deposited by four-gun DC magnetron
sputtering from metallic targets in a Xe-O/sub 2/ gas mixture
and postannealed in flowing O/sub 2/. Extensive optimization
of the film deposition process has been achieved for
erbium-based films which can be produced with good
characteristics on a variety of substrates. Fine-grained
polycrystalline films with sharp resistive transitions are
produced on single-crystal and polycrystalline YSZ, MgO,
ZrO/sub 2/-buffered sapphire, and silver-buffered sapphire.
Broader transitions are observed on buffered silicon wafers.
These erbium films often show grain sizes as small as 20-30 nm
and, with the addition of an RF oxygen glow discharge in the
deposition system, are quite smooth despite furnace
postannealing. Highly-oriented 1-2-3 film growth is observed
on SrTiO/sub 3/ and LaAlO/sub 3/ single-crystal substrates.
Four-point resistivity measurements show transitions as narrow
as 1 K, while magnetization and RF frequency-shift
measurements show more detail about film quality. Films are
characterized by XRD, SNMS, Auger, XPS, and cross-sectional
EDX profiling. The authors also report on the use of in-situ
deposition of silver as a passivation layer for the 1-2-3
surface. |
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High
temperature superconducting films by RF magnetron
sputtering A.M. Kadin, P.H.
Ballentine, J. Argana and R.C. Rath
Summary:
The authors have produced sputtered films of Y-Ba-Cu-O
and Bi-Sr-Ca-Cu-O by RF magnetron sputtering from an oxide
target consisting of loose reacted powder. The use of a large
8-in stoichiometric target in the magnetron mode permits films
located above the central region to be free of negative-ion
resputtering effects, and hence yields reproducible, uniform
stoichiometric compositions for a wide range of substrate
temperatures. Superconducting YBCO films have been obtained
either by sputtering at low temperatures followed by an 850
degrees C oxygen anneal, or alternatively by depositing onto
substrates heated to approximately=600-650 degrees C and
cooling in oxygen. Films prepared by the former method on
cubic zirconia substrates consist of randomly oriented
crystallites with zero resistance above 83 K. Those deposited
on zirconia at medium temperatures without the
high-temperature anneal contain smooth partially oriented
crystallites, with a slightly depressed T/sub c/
approximately=75 K. Finally, superconducting films have been
deposited on MgO using a BiSrCaCu/sub 2/O/sub x/ powder
target. |
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Laser
deposition of quality high T/sub c/ superconductor
films C.C. Chang, T. Venkatesan,
X.D. Wu, A. Inam, E.W. Chase, D.M. Hwang, J.M. Tarascon, P.
Barboux, P. England and B.J. Wilkens
Summary:
The laser deposition technique has been developed into
a versatile method for depositing films of YBa/sub 2/Cu/sub
3/O/sub x/ material. A brief description is given of the
method and important materials characteristics, such as
composition, surface smoothness, crystalline structure, and
interactions with substrates. T/sub c/ over 93 K, J/sub c/ in
the 10/sup 6/ A/cm/sup 2/ range at 77 K, tolerance to high
magnetic fields, low temperature deposition (near 650 degrees
C), as-deposited superconducting films, epitaxial growth, and
very smooth surfaces have been demonstrated. Relevance to
practical application is briefly discussed. |
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Epitaxial
feature of Y-Ba-Cu-O thin films prepared by multilayer
deposition S.Z. Wang, Z.L. Bao,
S.L. Wang, G. Li, Z.Y. Ye, C.Y. Li and D.L.
Yin
Summary: Y-Ba-Cu-O thin films have been
prepared on (100) SrTiO/sub 3/ substrates by a multilayer
deposition technique using a single electron beam source. The
epitaxial growth with a-axis normal to the film plane was
confirmed by X-ray diffraction. The oriented growth phenomenon
of the Y-Ba-Cu-O thin films is explained by theories of
epitaxy and texture. |
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Investigation
of the multilayer deposited BiSrCaCuO thin
films B.B. Jie, S.L. Wang, Z.L.
Bao, F.R. Wang, C.Y. Li, G. Li, S.Z. Wang and D.L.
Yin
Summary: Superconducting BiSrCaCuO thin
films with zero resistivity at 80 K have been prepared by a
multilayer deposition and diffusion method. The nominal
composition is BiSrCaCu/sub 2/O/sub y/. X-ray diffraction
shows that the same lattice structure as for the bulk material
but with strong preferred orientation in the film. The
superconductivity of this kind of film is sensitive to
treatment temperature but not sensitive to annealing
atmosphere. Critical parameters have been measured and
compared with the similar YBaCuO thin films. |
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Superconducting
characteristics of YBCO films M.
Muroi, T. Matsui, Y. Koinuma, K. Tsuda, M. Nagano and K.
Mukae
Summary: An investigation has been made
of superconducting YBa/sub 2/Cu/sub 3/O/sub y/ (YBCO) thin
films, which were prepared with two synthetic processes using
RF-magnetron sputtering: (a) deposition at low substrate
temperature with postannealing at around 900 degrees C, and
(b) deposition at high substrate temperature around 650
degrees C with or without postannealing. High-T/sub c/
superconducting YBCO thin films were obtained with either
process. The films prepared with process (a) had, however,
poor J/sub c/ below 10 A/cm/sup 2/ at 77 K and their surfaces
were rough. The films prepared with process (b) had higher
J/sub c/ and smoother surfaces. Further improvement in surface
morphology was made by raising substrate temperature to 700
degrees C at the start of the deposition. |
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Chemi-physical
diagnosis of BiSrCaCuO thin film
preparation T. Kobayashi, Y.
Fukumoto, H. Hidaka and M. Tonouchi
Summary:
Thin film and bulk formation of the new high T/sub c/
superconductive oxide BiSrCaCuO (BSCCO) and related crystal
structures has been investigated, emphasizing the beneficial
effect of Mg addition on the superconductivity. The onset
temperature of the high T/sub c/ phase was raised by a few
degrees, and a T/sub c/ (zero) higher than 100 K was easy to
obtain in the BiSrCaMgCuO system. The as-grown BSCCO thin film
formation is also considered. Sputter-deposited to 48 K
without using postanneal. After the film was subjected to the
furnace anneal, T/sub c/ (zero) rose to 90 K, though most of
the resistance dropped at even higher temperatures. The
structural change and chemical shift introduced during the
postanneal have been closely examined, showing that
sensitivity of the BSCCO crystallinity to the substrate
material, annealing time, and temperatures. These
characteristic features seem substantially different from
those of YBCO system. |
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Y-Ba-Cu-O
thin films by evaporating the three elements using resistive
heating A. Nakayama, N. Ochi, K.
Takeuchi, H. Ito and Y. Okabe
Summary: High
T/sub c/ superconducting Y-Ba-Cu-O thin films were fabricated
by evaporating the three metal elements Y, Ba, and Cu using
resistive heating. Subsequently, the films were annealed in
oxygen in order to obtain the superconducting phase. The
critical temperatures T/sub c/ (onset) and T/sub c/ (end) of
the obtained film were 88 K and 65 K,
respectively. |
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Fabrication
and interface reaction of Y-Ba-Cu-O superconducting thin films
by reactive ion beam coating C.X.
Ren, G.L. Chen, Y. Zheng, J.M. Chen, J. Yang, C.S. Zhang, L.M.
Xie and S.C. Zou
Summary: Y-Ba-Cu-O
superconducting thin films have been fabricated by the
reactive ion beam coating technique with good reproducibility.
The influence of atom stoichiometry on the critical
temperatures of the films is determined. Ba and Cu atoms
diffusing into substrates were detected by auger electron
spectroscopy and Rutherford backscattering. A thin silver film
deposited on ZrO/sub 2/ substrate as a buffer layer can
effectively retard the interface reaction. |
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High
T/sub c/ superconducting thin films prepared by flash
evaporation T. Hato, Y. Takai and
H. Hayakawa
Summary: Superconducting thin
films of Y-Ba-Cu-O and Bi-Ca-Sr-Cu-O were prepared by the
flash evaporation technique. The flash evaporation method
evaporates multi-elements instantly with different evaporation
temperatures. These films had critical temperatures T/sub c/
(zero resistance) of 78 K (unannealed Y-Ba-Cu-O) and 73 K
(annealed Bi-Ca-Sr-Cu-O). The thicknesses of these films were
from 200 to 600 nm. |
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High
T/sub c/ BiSrCaCuO superconductor grown by CVD
technique M. Ihara, T. Kimura, H.
Yamawaki and K. Ikeda
Summary: Single-crystal
high-T/sub c/ BiSrCaCuO films were grown on (100) MgO
substrates in an open-tube chemical-vapor deposition (CVD)
system using metal-halide sources and oxygen gas. The
morphology of the BiSrCaCuO superconducting layers and MgO
insulating layers was satisfactory. The resistivities of the
high-T/sub c/ layers at 300 K and critical temperatures were
rho =5-20 m Omega -cm and T/sub c/=70-80 K for Bi/sub 1/Sr/sub
0.8/Ca/sub 1/Cu/sub 3.7/O/sub x/ layers 0.1-to-0.3- mu m thick
on MgO substrates. The CVD system and characteristics of the
high-T/sub c/ layers and MgO heteroepitaxial layers as a
substrate are discussed. |
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Preparation
of superconducting Bi-Ca-Sr-Cu-O thin films by RF magnetron
sputtering Y. Yang, G.H.
Dieckmann and J.E. Nordman
Summary:
Superconducting thin films of Bi-Ca-Sr-Cu oxide, with
T/sub c/ onset of approximately 90 K were prepared by RF
magnetron sputtering on sapphire with and without a CaZrO/sub
3/ buffer layer, on ZrO-coated silicon (111), and on
single-crystal MgO (100). The as-deposited films were
amorphous insulators which were then annealed in various ways.
Zero resistance at 72 K for films on (100) MgO was achieved
after annealing in air at 810 degrees C for 2 h. However, the
X-ray diffraction pattern showed that two superconducting
phases, i.e., Bi/sub 2/CaSr/sub 2/Cu/sub 2/O/sub 8+x/ (2122)
and Bi/sub 2/Ca/sub x/Sr/sub 2-x/CuO/sub 6+y/ (2021), are
present, and that films have preferred orientation with c-axis
normal to the substrates. |
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Resistivity
behavior during anneal of YBa/sub 2/Cu/sub 3/P/sub 7-x/
films A. Hohler, H. Neeb and C.
Heiden
Summary: The resistivity behavior of
sputtered YBCO films has been investigated during anneal at
temperatures up to 1175 K. Characteristic variations in
resistivity under different experimental conditions are
related to microstructural changes and corresponding
superconducting properties. |
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A
simple method for preparing superconducting high T/sub c/ thin
films P. Romano, A. Nigro, R.
Vaglio, E. Signorelli and K.E. Gray
Summary:
High-quality superconducting films of the high-T/sub c/
compound YBa/sub 2/Cu/sub 3/O/sub 7/ were prepared on sapphire
using a simple method that combines DC magnetron sputtering
from a single target and evaporation from a heated source. The
method should be especially suitable for depositing thin films
on complex surfaces. YBa/sub 2/Cu/sub 3/O/sub 7- delta / films
with T/sub c/=92 K (onset) were prepared using a YCU/sub 3/
target and evaporating barium by a specially designed,
feedback-controlled, molybdenum heater using either pure
barium or barium-aluminum alloy. The films were characterized
by X-rays, electron-dispersion spectroscopy, and other
techniques. |
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YBCO
superconducting thin films prepared by vacuum coevaporation
without post treatment in
oxygen S. Chromik, V. Strbik, S.
Benacka, J. Levarsky, J. Sith, A. Plecenik, S. Gazi, V. Smatko
and J. Schilder
Summary: Results on
low-temperature preparation of superconducting thin films of
YBa/sub 2/Cu/sub 3/O/sub x/ are presented. YBCO thin films of
0.5-1.0- mu m thickness were prepared by vacuum codeposition
of Y, BaO, and Cu onto both polycrystalline and single-crystal
Al/sub 2/O/sub 3/, MgO, and SrTiO/sub 3/ and on Si+SrTiO/sub
3/ and Si+SiO/sub 2/ substrates. The temperature of the
substrates was 550-580 degrees C and the partial oxygen
pressure in the vacuum chamber was 10/sup -2/ Pa. The
preparation of in situ superconducting thin films was
successful with all substrates. Maximum critical temperature
at zero resistance was T/sub ce/=85 K, the onset critical
temperature T/sub con/ being from 90 to 95 K. The authors
believe that T/sub con/ represents the maximal critical
temperature of grains. Thus, the synthesis of superconducting
granules with high T/sub c/ is possible even at a relatively
low temperature during the deposition. On the other hand, the
zero-resistance critical temperature T/sub ce/ characterizes
the intercrystalline boundaries with electrical properties
sensitive to diffusion processes from substrate or surrounding
atmosphere. Additional oxidation of films (thermal or
plasmatic) does not much affect T/sub ce/ near 85
K. |
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Boundary
layers of oxide superconductors films pasted on Al/sub 2/O/sub
3/, MgO, SrTiO/sub 3/ and YSZ K.
Agatsuma, T. Ohara, K. Kaiho and H.
Tateishi
Summary: Energy-dispersive X-ray
analysis, wavelength-dispersive X-ray analysis, and scanning
microscopy examinations have been performed for annealed
Y-Ba-Cu-O(YBCO) and Bi-Sr-Ca-Cu-O(BSCCO) films which will be
adopted for future superconducting devices. Relatively thin
films pasted on various substrates and cross sections were
tested. The influence of the annealing on the substrate and
superconducting films has been investigated. The results show
the formation of boundary layers of intermediate products,
which consists of Ba compounds between the YBCO layer and
substrate (Al/sub 2/O/sub 3/, SrTiO/sub 3/, and ZrO/sub 2/)
and Sr and Bi compounds between the BSCCO layer and substrate
(SrTiO/sub 3/, MgO, and ZrO/sub 2/). |
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Possibility
of critical field enhancement due to field penetration in
high-T/sub c/ sponges and thin
films E.W. Collings, A.J.
Markworth and K.R. Marken Jr.
Summary:
Magnetic susceptibility measurements of a sample of
sintered high-T/sub c/ ceramic superconductor of nominal
composition Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-y/ were
conducted as a function of temperature from liquid-He
temperatures to T/sub c/. The fitted form of the
susceptibility temperature dependence yielded a
sample-particle size that was only a few times larger than the
field-penetration depth. The particle size was much less than
the grain size and commensurate with the thickness of the
optical twins. The results of the enhancement studies are also
discussed in the light of C.P. Bean's (1964) early experiments
on Pb sponges (in this case lambda >> particle size)
which exhibited spectacular enhancements of H/sub c/ in
association with flux trapping at or between the Pb filaments.
It is predicted that it should be possible, using presently
available film-deposition techniques, to produce high-T/sub c/
films possessing severalfold enhancements of H/sub c1/ beyond
the bulk value, and that, as with the Pb sponges, the
magnetization loops, even when taken within what passes for
the Meissner state in such materials, will be
hysteretic. |
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Magnetic
shielding effect on oxide superconducting
plates A. Yahara and H.
Matsuba
Summary: The authors have measured a
critical shielding flux density B/sub 01/ of YBa/sub 2/Cu/sub
3/O/sub y/ superconducting plates. Sintered specimens were
assumed to be bulk superconductors with interconnected
sub-structures and were considered to screen the magnetic
field completely for applied fields of less than B/sub 01/.
The critical shielding flux density was proportional to the
thickness d of the specimen, as expected from an equation
derived from the Bean-London critical state model. The authors
have obtained 16 G/mm as a maximum shielding capacity of B/sub
01//d. The measured flux showed hysteresis when the applied
field was raised above B/sub 01//. |
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Magnetic
shielding by superconducting Y-Ba-Cu-O hollow
cylinders J.O. Willis, M.E.
McHenry, M.P. Maley and H. Sheinberg
Summary:
The authors have measured the magnetic field H' at the
center of a hollow of sintered Y-Ba-Cu-O superconductor as a
function of applied field H, temperature, tube length, and
tube wall thickness. The maximum field that can be shielded
H/sub sh/ agrees with the value calculated from the critical
state model using the measured critical-current density J/sub
c/. The maximum trapped field H/sub tr/ in the tube, on
decreasing H to zero, exceeds H/sub sh/ by as much as a factor
of two, and large enhancements in J/sub c/ are observed in the
decreasing field. These phenomena are identified with
intragranular flux pinning present only after H has exceeded
H/sub c1/ of the grains. Finite tube length does not affect
the H/sub tr//H/sub sh/ ratio appreciably. H/sub sh/ depends
approximately on the square root of the wall
thickness. |
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The
fabrication and characterization of high temperature
superconducting magnetic
shields J.W. Purpura and T.R.
Clem
Summary: Tubes fabricated of
polycrystalline YBa/sub 2/Cu/sub 3/O/sub 7-x/ are
characterized, and details of the fabrication procedure are
discussed. The microstructure of the tubes determined by
scanning electron microscopy and X-ray diffractometry is
described. Resistive measurements of T/sub c/ and Delta T/sub
c/ have been made. The tubes have also been characterized by
means of SQUID (superconducting quantum interference device)
magnetometry. The temperature dependence of magnetic fields
trapped axially in the tubes has been measured and estimates
of penetration depth are given. Moreover, measurements of the
transverse shielding effectiveness of the tubes have been made
and are compared with theoretical predictions. Studies on flux
penetration into the tubes are described. Findings from the
microstructure studies are correlated with the observed
superconductivity properties. The results on the
high-temperature materials are compared to results obtained
previously on tubes made from conventional
superconductors. |
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Meissner
motor using high-T/sub c/ ceramic
superconductors A. Takeoka, A.
Ishikawa, M. Suzuki, K. Niki and Y. Kuwano
Summary:
The authors developed a superconducting motor using
high-T/sub c/ ceramic superconductors. This motor, which
utilizes the repulsive force caused by the Meissner effect
that appears below T/sub c/, is called the Meissner motor. The
motor rotated at a maximum speed of 40 rpm. Though the
repulsive force to drive the motor increased with the decrease
of temperature or the increase of the gradient magnetic field,
it was only about 1.1 gf/g at 77 K in 3500 G/cm. The motor has
a maximum torque of 5.0 gf-cm theoretically, but actually had
a torque below 0.66 gf-cm, because it took some time to be
cooled below T/sub c/. The rotating speed of the motor was
limited by heating ability, and its torque was limited by
cooling ability. |
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Synchronous
rotation of a floating magnet and flux penetration in
Y-Ba-Cu-O superconductor J.C.
Macfarlane, K.-H. Muller and R. Driver
Summary:
An experimental demonstration of induced rotation of a
rectangular magnet floating above a superconducting ring is
described. Synchronism between the rotating and the applied
alternating magnetic field was observed under certain
conditions. The stability of the levitated magnet and the
mechanism of the rotation are discussed on the basis of the
authors' studies of flux penetration into
superconductors. |
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Fabrication
of Y-Ba-Cu-O superconductor for magnetic
bearing M. Itoh, H. Ishigaki and
A. Hida
Summary: Although high-T/sub c/ oxide
superconductors, such as Y-Ba-Cu-O, should have great
potential for application to repulsive magnetic bearings,
various kinds of improvements on the mechanical and electrical
properties of the materials are needed. The authors tried to
improve these properties by changing compression pressure in
the process used in forming a pellet systematically and by
adding Ag powder to Y-Ba-Cu-O powder. All materials used here
were confirmed as Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ by X-ray
diffraction analysis. It was found that density, hardness,
temperature of zero resistance, and ratio of resistivity at a
temperature of 300 K and that at a temperature of 100 K
increase as the compression pressure becomes higher. On the
other hand, a temperature of resistance drop and a repulsive
force due to the Meissner effect were insensitive to the
pressure. The Ag content in the pellets makes the temperature
of zero resistance higher. It was also found that the ratio of
resistivity had possible use as a criterion for better
superconductive characteristics. |
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Ozone
processing of MBE grown of YBa/sub 2/Cu/sub 3/O/sub 7-x/
films D.D. Berkley, B.R. Johnson,
N. Anand, K.M. Beauchamp, L.E. Conroy, A.M. Goldman, J. Maps,
K. Mauersberger, M.L. Mecartney, J. Morton, M. Tuominen and
Y.-J. Zhang
Summary: Results of a new process
for producing high-quality thin films of the YBa/sub 2/Cu/sub
3/O/sub 7-x/ superconducting oxide are presented. This process
uses a reactive ozone vapor at a molar rate commensurate with
evaporated metal rates under true UHV conditions. A film
exhibiting a superconducting onset at 87 K and zero resistance
at 82 K was obtained. It was c-axis oriented and contained no
second phases as determined by X-ray diffraction. The failure
to produce good films by depositing BaF/sub 2/ rather than
barium in this configuration was noted. YBa/sub 2/Cu/sub
3/O/sub 7-x/ was formed when diatomic oxygen was used in place
of the ozone vapor under nearly identical conditions. The use
of pure ozone vapor is the key ingredient in this process,
which may be useful in producing thin films of the other high
transition temperature compounds. |
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Properties
of in-situ superconducting thin films of Y-Ba-Cu-O on Si,
Al/sub 2/O/sub 3/, and SrTiO/sub 3/
substrates R.M. Silver, A.B.
Berezin, E. Ogawa and A.L. de Lozanne
Summary:
The authors report on the as-deposited superconducting
properties and microstructure of YBa/sub 2/Cu/sub 3/O/sub 7-y/
thin films grown on Si, Al/sub 2/O/sub 3/ and SrTiO/sub 3/
(110) and (100). They deposited thin films of YBa/sub 2/Cu/sub
3/O/sub 7-y/, which have critical temperatures as high as 68
K, directly on Si. They also grew in situ superconducting
films on SrTiO/sub 3/ (110) with critical temperatures of 84.5
K and critical currents typically around 1*10/sup 6/ A/cm/sup
2/ at 4.2 K. Transmission electron microscopy and X-ray
diffraction studies showed that the films have a single
orientation and no grain boundaries. |
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Optimization
of thin-film YBa/sub 2/Cu/sub 3/O/sub 7/ deposition by DC
sputtering onto sapphire
substrates E.J. Tomlinson, Z.H.
Barber, G.W. Morris, R.E. Somekh and J.E.
Evetts
Summary: A report is presented on the
deposition of YBa/sub 2/Cu/sub 3/O/sub 7/ thin films onto
epitaxial magnesia-coated single-crystal sapphire substrates
at deposition temperatures in the range of 600 degrees C-850
degrees C. The dependence of film composition on sputtering
parameters was investigated using a UHV DC magnetron sputter
deposition system with both composite metal and ceramic oxide
targets. Films deposited onto epitaxial magnesia are compared
to those deposited directly onto sapphire and
yttria-stabilized zirconia. The degree of control required for
the sputter deposition of YBaCuO thin films at elevated
temperatures is shown. |
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Multiple
heteroepitaxy and superlattice formation of LnBaCuO/YBaCuO
system M. Tonouchi, Y. Yoshizako,
M. Iyori and T. Kobayashi
Summary: Multiple
heteroepitaxy and superlattice formation of high-T/sub c/
superconducting films were demonstrated. LnBaCuO/YBaCuO
structures (Ln=Er, Nd) were realized as a preliminary stage.
Superlattice formation made it clear that the crystal
structure of the system strongly depends on the substrate
materials. A (100)-oriented single-crystal MgO substrate
provided a superlattice with the sound perovskite structure,
whereas on (110) SrTiO/sub 3/ wafers a deformed crystal was
formed. The study of ultrathin YBaCuO film epitaxy revealed
that some kind of crystal, deformed from the perovskite
structure but not identified yet, was found to grow on
SrTiO/sub 3/ wafers regardless of their orientation at the
beginning of the deposition, whereas the ultrathin perovskite
crystal epitaxially grew on (100) MgO substrate. In addition,
MgO thin film epitaxy on NdBaCuO was examined. RHEED patterns
revealed that MgO epitaxially grew on both (110) oriented
YBaCuO and SrTiO/sub 3/ with the orientation of (100) and
(110), respectively. |
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Comparison
of YBa/sub 2/Cu/sub 3/O/sub 7/ films grown by solid-state and
vapor-phase epitaxy J.
Talvacchio, J.R. Gavaler, J. Greggi, M.G. Forrester and A.I.
Braginski
Summary: Epitaxial films of YBa/sub
2/Cu/sub 3/O/sub 7/ (YBCO) have been grown on single-crystal
SrTiO/sub 3/ and MgO substrates by two different routes using
RF and DC magnetron cosputtering. In the first case, amorphous
oxide films were deposited at a substrate temperature of 400
degrees C and the crystalline film grew by solid-state epitaxy
in an in situ postanneal at approximately 850 degrees C. In
the second case, a substrate temperature of 600-650 degrees C
was used which was sufficient to crystallize the film as it
was deposited from the vapor phase. Reaction with the
substrate was less for the films grown by vapor-phase
epitaxy-even when they were annealed at 850 degrees C-as shown
by the transition temperatures of very thin films, Auger depth
profile measurements, in situ XPS analysis, and transmission
electron microscopy. XPS was used to show that segregation of
Ba at the free surface can occur in either type of film. The
relative merits of each growth process are discussed for
microwave applications and tunnel junction
fabrication. |
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Single-electron
charging effects and implications for tunneling measurements
of the high-T/sub c/
superconductors J.B. Barner, K.
Mullen, M.J. Honkanen, S.T. Ruggiero, E. Ben-Jacob and A.R.
Pelton
Summary: A theory is presented for the
dynamics of two voltage-biased ultra-small capacitance tunnel
junctions connected in series when one or more electrodes are
superconducting, and experiments performed on parallel arrays
of such junctions are reported. Using the semiclassical model,
the authors find that the I-V characteristics display steps
and therefore multiple peaks in dI/dV, corresponding to the
time-average occupation of the interjunction region by
integral numbers of electrons. The voltage at which the first
step is located depends on the superconducting gap and the
capacitances of the junctions. The spacing between subsequent
steps depends solely on the capacitances. A discussion is
presented of electron tunneling results performed on
metal/Al/sub 2/O/sub 3//2-10 nm-diameter metal
particles/Al/sub 2/O/sub 3//metal junctions where this
multiple-peak structure is observed. The authors present
preliminary tunneling results in junctions using Pb-particles.
These results indicate that the multiple-peak structure
commonly observed in tunneling data of high-T/sub c/ oxide
superconductors can be explained in terms of charging effects
in a material with a single superconducting gap. |
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Magnetic
properties and microstructure of sintered high-T/sub c/
Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O
oxides H. Kumakura, K. Togano, M.
Uehara, H. Maeda, K. Takahashi and M. Nakao
Summary:
AC complex susceptibility chi '-i chi ", DC
magnetization, resistive transition curves in various magnetic
fields, and the critical-current density were measured for
sintered Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O samples whose major
phase was the high-T/sub c/ phase of each system. The single
peak observed in chi " vs. temperature curves was very sharp
for both samples of each system. The hysteresis in DC
magnetization curves at 77 K rapidly decreased with increasing
magnetic field for both systems. This may be related to the
layered morphology of the platelike grains. The resistive
transition curves of both samples were broadened by the
application of a magnetic field as in the case of Y/sub
1/Ba/sub 2/Cu/sub 3/O/sub 7/. In magnetic fields, rather long
tails to low temperatures were observed in resistive
transition curves for Bi-Sr-Ca-Cu-O. J/sub c/s at 77 K and
zero field were 250 A/cm/sup 2/ and 600 A/cm/sup 2/ for Bi-
and Tl-based samples, respectively. These J/sub c/s rapidly
decreased with increasing magnetic field. |
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Preparation
of YBa/sub 2/Cu/sub 3/O/sub 7/ powders by molten salt
route R. Millis, A. Celikkaya, M.
Akinc and W. McCallum
Summary:
Superconducting YBa/sub 2/Cu/sub 3/O/sub 7/ powders
were prepared by using a molten-salt process. Superconducting
powders were obtained by heating the molten salt to first
solidify, and then to decompose into oxide form. The precursor
from copper acetate salt underwent a pyrophoric reaction upon
heating while that from copper nitrate required external
heating for decomposition. Powders from copper acetate were
large ( approximately 10 mu m) and highly
porous. |
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RF
magnetron sputtering of Bi-Sr-Ca-Cu-O thin
films A. Kussmaul, G.M. Roesler
Jr., J.E. Tkaczyk, J.S. Moodera, X. Hao, G.A. Gibson, R.
Meservey and P.M. Tedrow
Summary: Thin films
of the high-T/sub c/ material Bi-Sr-Ca-Cu-O were prepared by
the RF-magnetron technique from a single reacted and sintered
target in pure argon atmosphere on sapphire substrates. The
samples were annealed for three minutes in pure oxygen at 810
degrees C. Their composition was studied by Rutherford
backscattering. The superconducting transition was very broad,
with R=O at 65 K for the best film. MgO substrates and
nonsintered targets were also used. The behavior of the
resistance in a magnetic field (up to 20 T) was
studied. |
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Evaluation
of grain-coupling strength of YBaCuO superconductors by their
magnetic-field profile
measurements K. Yoshida, T. Kisu,
N. Fuchigami and K. Enpuku
Summary: In order
to evaluate the strength of intergrain coupling, which
determines the transport critical current of polycrystalline
oxide superconductors, the relation between the transport
critical current and the spatial distribution of the magnetic
flux density for YBaCuO thin films as well as bulk
superconductors has been studied. The authors have measured
the profile of the magnetic flux trapped inside bulk YBaCuO
samples with a small Hall magnetic sensor and revealed the
correlation between critical-current densities and the slope
of the trapped-flux profile, as described by the
critical-state model. The measurements of the magnetic-field
penetration depth of YBaCuO thin films in a Meissner state is
also carried out by measuring the loop inductance of a DC
SQUID placed on the YBaCuO thin film to be measured. Observed
magnetic-field penetration depths of polycrystalline films
with thicknesses of 1-3 mu m deposited on MgO single-crystal
substrates amount to approximately 1 mu m for samples with
T/sub c/, (end) approximately 80 K and the critical-current
density J/sub c/ approximately 10/sup 4/ A/cm/sup 2/ at 4.2
K. |
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Resistance
measurements of high T/sub c/ superconductors using a novel
'bathysphere' cryostat J.
Moreland, Y. Li, R.M. Folsom and T.E.
Capobianco
Summary: A novel cryostat has been
developed for variable-temperature testing of high-temperature
superconductors. The cryostat is a bathysphere consisting of
an overturned stainless steel Dewar suspended in liquid
helium. A sample-heater-thermometer assembly is located at the
top of the encapsulated (and thermally insulated) vapor space
inside of the Dewar. The sample can be rapidly cycled from 300
K to 4 K at an average rate of 1 K/min with a thermal
hysteresis of less than 0.1 K. Helium vapor flows through a
plug in the bottom of the bathysphere so that pressure of the
vapor is roughly ambient. This provides ample heat transfer to
and from the sample to maintain thermal equilibrium in the
vapor space. Results for resistance versus temperature of some
high-temperature superconductors in a magnetic field are
presented. Various definitions for thermodynamic and practical
T/sub c/s derived from transport resistance measurements are
suggested and discussed. These definitions are based on T/sub
c/ midpoint, various relative resistance criteria, or absolute
resistivity criteria. |
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Characterization
of high T/sub c/ superconductor by luminescence
methods Y. Fujiwara and T.
Kobayashi
Summary: High-T/sub c/
superconductors have been investigated by photoluminescence
(PL) and X-ray excited thermally stimulated luminescence (TSL)
techniques. Characteristic PLs have been observed in the
infrared region in the Er-Ba-Cu-O and Nd-Ba-Cu-O systems,
which are tentatively assigned to electronic transitions
between weakly crystal-field-split spin-orbit levels of
trivalent Er and Nd ions. The spectral shapes and/or peak
positions are dependent on the crystal arrangement surrounding
the rare-earth ion. Such PLs have never been obtained in the
Eu-Ba-Cu-O and Yb-Ba-Cu-O systems. TSL measurements have been
performed with particular emphasis on the Gd-Ba-Cu-O system.
Characteristic TSLs have been observed in this system,
depending on the composition, preparation conditions and
cumulative X-ray irradiation time. The activation energy of
the related traps ranges from 0.1 eV to 0.3 eV. Characteristic
TSLs have also been observed in the Ho-Ba-Cu-O and new
high-T/sub c/ superconducting systems containing no rare-earth
elements. |
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Thermal
response of copper oxide superconducting thin
films M.E. Rizkalla and B.G.
Burridge
Summary: An analysis of the heat
transfer from a current carrying copper oxide superconducting
thin film to a copper substrate at 20 K was conducted for the
case of a superconducting thin film to be used for high-power
switching purposes. The calculations were extended to the
maximum operating frequency for a given conduction current. As
a result of these calculations, the thermal response time for
the thin film due to a pulse heat source has been
determined. |
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Response
of YBaCuO thin films to electromagnetic radiation and their
electrical characteristics A.S.
Afanasyev, A.F. Volkov, V.N. Gubankov, Y.Y. Divin and P.M.
Shadrin
Summary: An investigation was
conducted of the characteristic features of the interaction of
electromagnetic radiation with different YBaCuO thin films
varying from granular to epitaxial. The current-voltage
characteristics and the voltage response of these films to
millimeter-wave radiation were measured at temperatures from 4
to 100 K. The behavior of these characteristics for granular
and polycrystalline films can be accounted for by a
percolation mechanism of conductivity in a two-dimensional
system of weak links existing between granulas or grains of
the films and having different values of critical currents. It
is shown that with the improvement of film structure the
mechanism of film interaction with electromagnetic radiation
is changed from percolation in the system of weak links
between grains to heating in grains of the
films. |
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Complex
AC conductance of YBaCuO
films P.K. Srivastava, P. Debely,
H.E. Hintermann, C. Leemann, P. Fluckiger, O. Caccivio, J.-L.
Gavilano, J. Weber and P. Martinoli
Summary:
The AC linear response of YBaCuO films prepared by RF
magnetron sputtering to a small oscillating field was studied
as a function of temperature and transverse magnetic field.
The structures observed are quite similar to those found in
artificially prepared two-dimensional periodic junction arrays
where the transition from a phase-ordered to a
phase-disordered state at T/sub c/ is attributed to the vortex
unbinding mechanism predicted by the Kosterlitz-Thouless
theory for phase transitions in two dimensions. The magnetic
field data suggest that the typical cell size of the YBCO
array is a few mu m, on the order of the grain size. This
suggests that the ceramic films can be modeled as
two-dimensional networks of Josephson tunnel junctions, the
junctions being located at the grain boundaries. |
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Influence
of radiation defects on superconductivity in YBa/sub 2/Cu/sub
3/O/sub 7- delta / films A.A.
Nikonov, G.V. Sotnikov and A.S. Tokarev
Summary:
The influence of 3-3.7-MeV proton bombardment in vacuum
media on superconducting properties of Y-Ba-Cu-O films
obtained with the aid of pulsed laser deposition on SrTiO/sub
3/ substrates is investigated. It is shown that in the samples
investigated T/sub c/ drops linearly with increasing
irradiation dose, the rate of this drop is approximately five
times quicker than for Nb/sub 3/Sn and there also takes place
a broadening of the superconducting transition. |
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Tunneling
spectroscopy in thin films YBCO/Pb tunnel
structures S. Benacka, V.M.
Svistunov, A. Plecenik, S. Chromik, S. Gazi, J. Levarsky, V.
Strbik and S. Takacs
Summary: Experimental
results on electron tunneling into thin superconducting films
of YBa/sub 2/Cu/sub 3/O/sub x/ in YBCO/Pb tunnel junctions are
reported. Thin superconducting films of YBCO (thickness 0.7 mu
m) were prepared by codeposition of Y, BaO, and Cu onto
single-crystal substrates (sapphire, MgO, SrTiO/sub 3/). The
maximum critical temperature at zero resistivity was T/sub
ce/=85 K. The surface of films is smooth with pancake-shaped
grains. The tunnel structure was completed by vacuum
deposition of superconducting Pb film (thickness 0.3. mu m)
onto YBCO film with either an uncleaned surface or after
cleaning in Ar or O/sub 2/ plasma. Two mechanisms are
presented which can lead to resonances in the tunneling
current of high T/sub c/ superconductors. The first one is
caused by geometrical localization of quasiparticles in normal
regions in superconducting surroundings in the neighborhood of
the counter electrode. It is shown that normal regions of the
dimension about 10 nm lead to step distances obtained in
experiments. The resonances above the gap can therefore occur
not only in grainlike material but also in single crystals and
in uniaxially aligned microcrystals with lamellar structure.
The second mechanism is based on the model of localized
baglike excitations by M. Weinstein (1987). |
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New
Tl-Ba-Ca-Cu-O (1234 and 1245) superconductors with T/sub
c/>117 K H. Ihara, M.
Hirabayashi, N. Terada, M. Jo, K. Hayashi, M. Tokumoto, Y.
Kimura, R. Sugise, T. Shimomura and S.
Ohashi
Summary: TlBa/sub 2/Ca/sub 3/Cu/sub
4/O/sub 11/ (1234) and TlBa/sub 2/Ca/sub 4/Cu/sub 5/O/sub 13/
(1245) superconductors have been synthesized for the first
time. The compounds have T/sub c/ values of 116 approximately
122 K. The highest T/sub c/ value was obtained for the 1234
phase. These are single Tl-O layer compounds with less than
half the Tl content of the previous 2223 phase. The structure
of the compounds is a simple tetragonal phase with the lattice
constants of a=b=3.85 AA and c=19.1 AA for the 1234 phase and
a=b=3.85 AA and c=22.3 AA for the 1245 phase. The electron
diffraction patterns and the lattice images from a
high-resolution TEM have confirmed an oxygen-deficient
layered-perovskite structure with the P4/mmm space group. The
c-lattice constant follows the c-axis rule of a linear
relation of c=6.3+3.2 n (n=the number of Cu-O layers), The
T/sub c/ value is strongly related to the valence of Cu
ion. |
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Author Index (1988) No
author information available
Summary: Not
available |
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