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| 1970 |
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Front Cover (1970)
No
author information available
Summary: Not
available |
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Table of Contents (1970)
No
author information available
Summary: Not
available |
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Special Section on Applied Superconductivity -
Introduction (1970)
No author information
available
Summary: Not
available |
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Report of Panel Discussion Applied
Superconductivity Conference (1970)
No
author information available
Summary: Not
available |
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Astronomical Observations with Josephson
Junction Detectors (Title Only)
B.
Ulrich
Summary: Not available |
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Superconducting Magnetic Suspension and
Propulsion Systems for High-speed Ground Transport (Title
Only)
J.R. Powell
Summary:
Not available |
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University of Virginia Superconducting
Wind-Tunnel Balance
R.N. Zapata, H.M.
Parker, F.E. Moss, L.L. Hamlet and R.S.
Kilgore
Summary: The design of an
electromagnetic balance using superconducting coils is
reported. Both dc and ac coils are used to support aerodynamic
models in a supersonic (Mach 3) wind tunnel and to
simultaneously measure the forces acting on them along 3
orthogonal axes. Major design characteristics include:
adoption of symmetrical coil arrangement to provide maximum
space for the wind tunnel; 3 gradient-coil pairs capable of
being driven between 0 and 350 A at a frequency of 30 Hz by
specially designed power supplies; a vertical wind tunnel with
a 6-in. test section located in the axial room-temperature
access of a 2.50.liter liquid-helium Dewar. Results on ac
losses for prototype gradient coils wound of three different
superconducting materials are reported. |
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Forces on Superconducting Coils by Induced Body
Currents
R.H. Borcherts and J.R.
Reitz
Summary: The lift and drag forces on a
persistent-current superconducting coil suspended above the
rim of a rotating aluminum wheel have been measured.
Calculations have been made for various coil geometries above
conducting roadbeds that are infinrte in extent as well as of
finite width. Experimental measurements carried out up to a
speed of 300 mph over a solid as well as a "hoop-like"
aluminum wheel give support to the existence of a magnetic
Reynolds number, /spl lambda/=/spl mu//spl sigma//spl
upsilon/h; for the infinite roadbed case the theoretical
lift-to-drag ratio is F/sub L/F/sub D/ /spl qp/2(/spl
lambda////spl pi/)/sup 1/2 /. Here /spl sigmn/ is the
conductivity of the roadbed, /spl upsilon/ the relative
velocity of the coil and roadbed, and h the height of the coil
above the roadbed. Above 70-80 mph we measure a 0.55
dependence for the lift-to-drag ratio. This ratio reaches a
value of 8 at 300 mph-approximately one-half that predicted by
the above expression for the infinite geometry. The
"hoop-like" wheel shows lift and drag forces to fall off more
rapidly at low speeds than for the solid wheel-as might be
expected from skin depth considerations. |
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Superconducting Elements of a Magnetic Monopole
Detector (Title Only)
J.D. Taylor, R.R.
Ross, P.H. Eberhard and R.A. Byrns
Summary:
Not available |
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Operation of a Levitated Superconducting Ring in
a Plasma Physics Experimental Device
J.
File, G.D. Martin, R.G. Mills and K.E.
Wakefield
Summary: An isolated, isochoric
Dewar and superconducting ring, operable up to 130,000 ampere
turns, has been installed in the Princeton Spherator, and
plasma physics experiments have been performed with the ring
levitated and stabilized by means of a system similar to that
discussed previously. The evolution of this device has
progressed through three stages. Initially the poloidal field
coil was a conventional copper conductor ring within a vacuum
jacket supported within the reaction chamber from top and
bottom by four thin bars. In its second stage of development,
the conventional coil was replaced by a superconducting coil
and Dewar, supported by thin wire-like hangars. The present
mode eliminates the need for mechanical supports of any
sort. |
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Alternating Field Losses in Superconducting
Strip Conductors
R.M.F. Linford and R.G.
Rhodes
Summary: A wattmeter technique is
described which advances the measurement of alternating field
losses in superconducting materials. In particular, it allows
realistic experiments on strip conductors, the geometric form
most promising for application to electrical power
engineering. Uniform current distribution is achieved, contact
problems are eliminated, and sample preparation is minimal. It
is shown how the alternating field measurements can be
interpreted to yield values for several properties of the
material studied. Critical fields, the surface shielding
currents, and the bulk critical currents can all be
determined. When these quantities are inserted into
expressions derived from a modified critical state model, a
good match to the experimental results is obtained. Comparison
of the measured alternating field dissipation with the area
enclosed by magnetization curves, derived in slowly varying
fields, confirms the validity of the technique and the
applicability of critical state models to results obtained at
audiofrequencies. |
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Low-Frequency Electromagnetic Losses at 4.2/spl
deg/K of Neutron-Irradiated Nb
S.T.
Sekula
Summary: Low-frequency (3.50 Hz)
permeability measurements at 4.2°K using phase-sensitive
techniques were carried out on a Nb sample irradiated in the
Oak Ridge Research Reactor at 40°C to several fast neutron
doses up to 6X10/sup 19/ neutrons/cm/sup 2/ (E>1 MeV). In
the mixed-state region a signal proportional to the
electromagnetic loss in the sample is observed which decreases
with increasing fast neutron dose. Quantitative estimates of
the power dissipation in irradiated Nb are given which are
then related to the magnetic field dependence of the critical
current density J/sub c/(H). |
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Alternating-Current Loss Measurements in
Thin-Film Type II Superconductors
D.W.
Deis, J.R. Gavaler, C.K. Jones and A.
Patterson
Summary: An electrical technique is
described for measuring the audio frequency power dissipation
in superconducting thin films as a function of the ac current
density, frequency, and applied dc magnetic field. This system
has been used to study the ac losses in thin films of Nb and
NbN, and also the ac and dc critical current densities in
these materials. The results can be explained by assuming that
both hysteresis and eddy current losses
contribute. |
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Alternating-Current Losses in Pancake Coils of
Nb/sub 3/Sn Type (Title Only)
S.L. Wipf
and C.A. Guderjahn
Summary: Not
available |
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Versatile Superconducting Femtovolt Amplifier
and Multimeter
J.E. Lukens, R.J.
Warburton and W.W. Webb
Summary: We describe
the adaption and use of superconducting quantum interference
magnetometers for the measurement of voltage, resistance, and
inductance. The instrument which we discuss achieves the
Nyquist noise limit, about 10/sup -22/ W of noise power in a
1-Hz bandwidth, over a wide range of conditions. This permits
resistance measurements down to this power level and a voltage
sensitivity of a few femtovolts for low impedance sources.
Using a 10-/spl mu/A measuring current, inductance changes of
0.1 pH have been measured. We discuss in detail the noise
limitations of the device and the design criteria for optimum
performance. Circuit and construction designs are presented
which allow the device to be used, with minimal changes, for
measurements over a wide range of parameters. |
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Recent Developments in Superconducting
Devices
J.E. Zimmerman
Summary:
A simple model of the electrical characteristics of
thin-film-bridge, point-contact, and tunnel junction Josephson
devices is given, along with some comments on their relative
performance at very low and at very high frequencies. A
particular example is the dc IV characteristic of a point
contact at the center of a parallel-disk microwave cavity.
Some recent developments in devices and the application of a
pointcontact loop (SQUID) device to magnetocardiography is
described. |
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Analog Computer Simulation of Weakly Connected
Superconducting Rings
M.B. Simmonds and
W.H. Parker
Summary: A system comprising a
weakly connected superconducting ring inductively coupled to a
resonant L-C circuit has been studied using an analog
computer. The weak link was characterized either by a curren
tphase relation of the form I=I/sub c/ sin/spl theta/ or by
"supercurrent breakdown," and the results were compared. The
effects of a normal resistance shunting the junction were
investigated, as well as the effects of thermal current noise
in the ring. Some criteria for achieving optimum coupling to
the superconducting ring are discussed. |
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Interpretation of Quantum Interference
Properties of Double Josephson
Junctions
T.A. Fulton and R.C.
Dynes
Summary: Experiments and analysis of
the quantum interference of the critical current I/sub c/ in a
magnetic field B for double Josephson (tunneling barrier)
junctions are described. A recent treatment of this problem is
employed which takes account of geometric asymmetries, loop
self-inductance L, and possible nonsinusoidal
supercurrent-phase relations I/sub c/(/spl theta/). The
observed I/sub c/(B) curves are found to agree with the
predictions of the model. The I/sub c/(/spl theta/) are
directly extracted from the I/sub c/(B) and accurately conform
to Josephson’s original prediction of I/suc c/(/spl theta/)
=I/sub c/sin/spl theta/. For larger values of L, I/sub c/ is
often observed to be multivalued for some or all B, or to
display discrete, periodic jumps. This behavior is shown to be
the combined result of multiple local maxima in an effective
current-phase relation for the double junction and the
particular dynamics of /spl theta/ encountered in tunnel
junction geometries, |
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Weak Link Point Contact
Devices
R.A. Buhrman, J.E. Lukens, S.F.
Strait and W.W. Webb
Summary: Recent progress
in our laboratory on permanently adjusted niobium
point-contact superconducting devices is reported. We describe
an rf noise-biased magnetometer obtained by appropriate tuning
and adjustment of the point contact in a superconducting ring
that is coupled to an LC tank circuit which provides an rf
noise-bias excitation at its resonant frequency. Thus, the rf
oscillator normally used is eliminated although the
magnetometer noise increases somewhat. In addition,
mechanically stable,
room-temperature-helium-temperature-cyclable insulated double
and single point contacts have been obtained by use of an
insulating glass with thermal expansion matching that of
niobium. When two of these contacts are connected in parallel
the resultant device works quite well as a cyclable dc-biased
magnetometer. Study of the temperature dependence of the
critical current is facilitated by the stable configuration.
Preliminary measurements showed sharp rises of the critical
current with decreasing temperature similar to those observed
with S-AT-S sandwiches. Properties of these devices are
discussed. |
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High-Field Superconductivity of Alloys in Porous
Glass
J.H.P. Watson
Summary:
Superconductors have been prepared by impregnating
porous glass with low-melting-point alloys such as those in
the Pb-Bi-Sb system. The critical fields of many alloys in
porous glass with a pore diameter of 32 /spl Aring/ are in
excess of 100 kOe at 4.2/spl deg/K. SampIes have been prepared
having critical current densities near 10/sup 5/ A/cm/sup 2/
in low (< 10 kOe) magnetic fields. |
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Properties of Very Thin Aluminum
Films
R. Meservey and P.M.
Tedrow
Summary: Thin films of aluminum have
been produced in the thickness range of 1000-30 /spl Aring/ in
small area samples with photoetched edges. The superconducting
transition temperature T/sub c/, the critical magnetic field
H/sub c/, the room-temperature conductivity have been
measured. as a function of thickness d. The results indicate
that films as thin as 30 /spl Aring/ act essentially as
uniform layers in which the crystal size is approximately
equal to the film thickness. The transition temperature was
found to vary linearly with d/sup -1/. H/sub c/(T) was
measured from T/sub c/ to 0.4/spl deg/K. For thickness from
1000 to 200 /spl Aring/, H/sub c//spl sim//sup -3/2 /, as
expected from the Ginzburg Landau theory. For d<200 /spl
Aring/, H/sub c/ is paramagnetically limited to about 49 kOe=
19.6T/sub c/, slightly above the Clogston limit. |
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Very High Critical Current and Field
Characteristics of Niobium Nitride Thin
Films
J.R. Gavaler, M.A. Janocko, A.
Patterson and C.K. Jones
Summary: Previous
results indicated that NbN thin films possess critical current
and field characteristics significantly superior to that of
bulk NbN having a similar T/sub c/. Further measurements made
on NbN films, with thicknesses between 50 /spl Aring/ and 8
/spl mu/ now show that, at 4.2°K, certain of these films
exhibit the highest current densities of any presently known
superconductor in all fields from zero up to the limit of our
measurement capability (210 kOe) . In addition, anomalously
high current and field values have been measured in very thin
(<300 /spl Aring/) films. These thinner films show no
depressions in J/sub c/ (measured at 4.2°K) or in H/sub c2/(0)
values despite decreases in T/sub c/ from almost 16°K (in the
thicker films) down to 11°K. |
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Preparation and Superconducting Properties of
Ta-Sn and Nb-Ge Diffusion Layers
G.
Otto
Summary: Intermetallic compounds of the
binary systems Ta-Sn and Nb-Ge were formed on Ta or Nb wires
by the method of vapor phase diffusion at elevated
temperatures. Critical temperature T/sub c/, critical magnetic
field H/sub c2/ at 4.2°K, and composition of these layers were
measured as a function of heating conditions. Ta-Sn diffusion
layers prepared at temperatures between 800° and 1200°C
consisted predominantly of the A15 compound Ta/sub 3/Sn with a
lattice parameter of a=5.278 /spl Aring/. The observed layer
thickness was always less than 1/spl mu/. Simultaneous maxima
in T/sub c/ (7.0°K) and H/sub c2/ (50 kOe) were measured on
wires heated at 950°C for 3 days. On the other hand, in the
Nb-Ge system the A15 compound NbaGe did not form in the same
temperature range. The layers, of thickness from 1 to 20 /spl
mu/, consisted only of NbGe/sub 2/ for a diffusion temperature
of 800°C, Nb/sub 2/Ge/sub 2/ for 1200°C, and a mixture of the
two at 1000°C. Consequently H/sub c2/ at 4.2°K for these
samples did not exceed 12 kOe. |
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Multifilament Nb/sub 3/Sn Superconducting
Wire
A.R. Kaufman and J.J.
Pickett
Summary: A superconducting copper
matrix wire containing many filaments of 0.4mil-diam Nb/sub
3/Sn has been prepared and tested. The critical current in
transverse magnetic fields in the range of 50-100 kG is 33-5
times greater than that of the Nb-Ti alloy but not as high as
that of the best Nb,Sn. The critical temperature of this
material is about 17°K. |
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H/sub c2/(4.2/spl deg/K) of High-Temperature
Superconducting Alloys
S. Foner, E.J.
McNiff Jr., B.T. Matthias, T.H. Geballe, R.H. Willens and E.
Corenzwit
Summary: Previous measurements of
H/sub c2/ versus T in selected Nb/sub3/AL/sub1-x/Ge/sub x/,
and Nb/sub3/Al alloys for dc magnetic fields to 200 kG and for
14 K /spl les/ T/spl les/ T/sub c/ have now been extended to
4.2 K by means of pulsed magnetic fields and rf measurements.
Long pulse (10 msec) multipalyer coils producing fields to 450
kG were employed for the measurements. At 4.2 K, H/sub c2//spl
sime/410 kG and H/sub c2//spl sime/320 kG for the highest
T/sub c/, Nb/sub3/AI/sub 1-x/Ge/sub x/ and Nb/sub 3/Al alloys,
respectively. These values of H/sub c2/ are the highest
measured for any superconductor. Measurements of H/sub c2/in
several related alloys with somewhat lower values of To and
/sub c2/ will also be presented. The large values of /sub c2/
(4.2 K) for all of these alloys permit great latitude of
engineering design for practical wire materials. A summary of
the physical properties of these high-temperature
superconductors, measurement techniques, present limits of
high H/sub c2/ materials, and a current appraisal of technical
possibilities will be presented. A summary of some of the
measurements has appeared. |
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Comparison of NbTi and Nb/sub 3/Sn Material
Tests with the Actual Performance of
Coils
D.L. Coffey, W.F. Gauster and M.S.
Lubell
Summary: A 100-kG, 8.26-cm-bore, 40-kJ
NbaSn magnet and a 75.kG, 10.2-cm-bore, 95kJ twisted filament
NbTi magnet have been constructed and tested. A number of
other magnets of smaller size using both the Nb/sub 2/Sn and
twisted filament NbTi conductors will also be described.
Material evaluation tests leading to these magnet designs will
be reported. They include short sample tests with controlled
surface cooling conditions and cusp coil tests in which an
external field is applied perpendicular to the cusp coil axis.
Finally, two successful stabilization techniques for Nb/sub
3/Sn ribbon will be presented with coil performance data
before and after the stabilization has been
employed. |
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State of the Art of Superconducting
Magnets
Z.J.J. Stekly
Summary:
The superconducting magnet is a key component in
various applications including high-energy physics, fusion
research, magnetohydrodynamics, and electrical rotating
machinery. This paper presents the general characteristics of
state of the art of superconducting magnets as far as design
and performance are concerned. The properties of
superconductors are briefly discussed, and the basis for
design of stable conductors set forth. Examples of conductors
and magnet systems are given and discussed. The current state
of affairs is summarized in three plots showing size versus
field achieved, current density in coils as a function of
magnetic field, and specific weight versus stored magnetic
energy. |
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Superconducting Quadrupole Doublet for the Los
Alamos Meson Physics Facility
J.D. Roger,
W.V. Hassenzahl, H.L. Laquer, J.K. Novack and R.W.
Stokes
Summary: A superconducting quadrupole
doublet for beam focusing has been operated with a 30-kG field
at the center of the 30-cm-long straight section of the
magnets and a 3-kG/cm field gradient. The magnets used twisted
multifilament Nb-Ti superconductor imbedded in a Cu matrix of
0.050-in. diameter and operate at currents up to 500 A. Each
magnet is equipped with a persistent mode superconducting
switch. The roomtemperature beam aperture of the magnet-Dewar
system is 15 cm. The focal length of the magnet pair is about
1 m for 500-MeV pions. The magnets are mounted in a Dewar with
thermally compensated supports to minimize motion upon cooling
and a soft iron shield reduces external fields to less than 60
G. |
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Computation and Construction of a High-Field
Superconductive Dipole
W.F.
Westendrop
Summary: Superconductive dipoles
show promise as bending magnets for the output beams of
particle accelerators. For a specific case considered criteria
are fields of 40 kG or higher, lengths of 20 ft or more,
uniformity of field of 0.2%, within useful diameters of 4 in.,
and magnet currents below 500 A. Multilayer construction
employing tape-type superconductors will be discussed. The
0.2% uniformity criterion can be met if careful attention is
paid to the actual integral number of tapes per slot in the
computer program. The curved ends of the winding also require
support and some experimental work on this will be described.
The transfer of the forces on the conductors outwards to the
retaining rings will be discussed. The resulting bending
moments are computed. |
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Prototype Superconducting Magnets for NAL Beam
Lines
R.W. Fast, B.P. Strauss, C.M. Cohn,
J.R. Heim and E.H. Scholefeild
Summary: Two
superconducting 20-kG dipole magnets have been designed and
constructed at the National Accelerator Laboratory. They are
of a type called "superferric" since they consist of an iron
yoke and superconducting coil. A 76-cm-long model providing a
4X10-cm aperture operated successfully at 20 kG with a
conductor current density of 36 kA/cmz and a stored energy of
8 kJ. A 3-m-long prototype requiring 17 kA/cm/sup 2/ to reach
20 kG in a 4X13-cm aperture will be tested soon. The iron
yokes of these magnets are refrigerated to 4.2 K.
Niobium-titanium multifilament conductor is used. The coils
are short free and the magnets can be charged to full field in
5 min or less. A 60-cm-long, 4-kG/cm iron-free quadrupole with
a 15-cm bore is being constructed. The field is shaped by coil
sectors of 18/spl deg/ and 36/spl deg/. A conductor current
density of 21 kA/cm/sup 2/ is required to achieve a gradient
of 4 kG/cm. The stored energy will be about 300 kJ at this
gradient. |
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Experimental Superconducting Transformer for
Current Set Up
F. Voelker and R.C.
Acker
Summary: For certain applications one
would like to use relatively few turns of a high current
conductor to make superconducting magnets. High current
conductors introduce the problem of large heat leaks, and also
the problem of a high current power supply. These problems
would be simplified if one could make a suitable current
step-up transformer. A design equation for such a transformer
is presented. An experimental test was performed on a
transformer made of superconducting solenoids which we had on
hand. It had a primary current of 79.2 A, and supplied 670 A
to a small load solenoid. The transformer was operated in two
modes. In the first, the primary current was held constant for
24 h. The magnetic field in the load solenoid was monitored
and the rate of decay in field could be measured. In the
second mode, the primary current was regulated to maintain
constant field in the load solenoid. The time constant of the
secondary circuit was 1.4X10/sup 6/ sec. Such a system could
be operated for a number of weeks without
recycling. |
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Constant Voltage Controller and Transition
Monitor for Superconducting Magnets
D.H.
Lester
Summary: Superconducting magnets with
long charge times require a power supply that will hold a
constant voltage across a magnet during the charge, hold, and
discharge periods. Conventional supplies regulate poorly
during hold and lose control completely during discharge of
the magnet. A simple circuit has been designed to connect
between a magnet and the remote sense input of a conventional
power supply which permits control and regulation at all
times. During the testing of multisection superconducting
magnets, is desirable to know the order and approximate time
interval in which each section undergoes a transition the
normal conducting state. A circuit has been designed which
detects and displays this information. |
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Surface Residual Resistance of
High-Q-Superconducting Resonators
J.
Halbritter
Summary: The
temperature-independent residual surface resistance R/sub res/
of high-Q-superconducting resonators is up till now not well
understood. Experiments in lead-plated copper cavities and
massive niobium cavities (v/spl sime/GHz) have shown R/sub
res//spl alpha//spl omega//sup 1.8/spl plusmn/0.2/, which
excludes normal conducting regions as a cause for the rf
losses. An explanation is presented which can describe the
observed dependencies on frequency and on surface treatment:
The losses are caused by the generation of phonons in fissured
surfaces by locally excited rf fields. These losses are
temperature independent, give R/sub res//spl alpha//spl
omega//sup 2/ and can also explain the observed mode
dependence in lead-plated copper cavities. |
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Analysis of Critical Power Loss in a
Superconductor
M.
Rabinowitz
Summary: A critical power
dissipation resulting from an oscillating magnetic field,
H/sub p/ cos/spl omega/t, can produce a magnetic breakdown
field, H/sub p/' |
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Effect of Tin Additive on Indium Thin-Film
Superconducting Transmission Lines
P.V.
Mason
Summary: We report the effect of adding
up to 23% tin to the indium film of a
thin-film/tantalum-oxide/bulktantalum transmission line.
Addition of tin reduces the velocity and increases the delay
for fixed length by 1.4% for each percent of tin. Agreement
with the predictions of Pippard’s nonlocal theory when
meanfree path is reduced is excellent. Pulse attenuation and
shape degradation are not increased by addition of tin.
Attenuation as low as 10 dB per microsecond of delay was
observed at 1.25 K. The added tin reduces by 50% the
sensitivity of velocity to temperature near T/sub c/, and
improves the reproducibility of velocity from line to line.
Critical temperature is increased to 6 K for 23% tin, in good
agreement with previous measurements. |
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Magnetization and Susceptibility Measurements of
Polycrystalline Niobium
H. Brechna, M.A.
Allen and J.K. Cobb
Summary: Magnetization
and susceptibility data are presented for niobium rod samples
in the temperature range of 1.2°-8.9°K. The niobium samples
tested had a range of tantalum content from 700 to 4000 ppm.
The samples were annealed and degassed in high vacuum at
temperatures between 2100° and 2300°C. Values of /sub k1/,
/sub k2/, H/sub c1/, H/sub c2/, and H/sub c/ are presented as
a function of tantalum impurity, and these values are compared
with theoretical predictions. |
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Superconducting Microwave Mixers Utilizing
Josephson Junctions
A.J. DiNardo and E.
Sard
Summary: Microwave superconducting
mixers operating at 10-13 GHz utilizing Josephson junctions
have been investigated analytically and experimentally.
Measurements on mixer sensitivity were carried out at 13 GHz
using waveguide-mounted experimental point-contact Josephson
junctions at 4.2 K. Results showed sensitivities of -90
dBm/MHz (10/sup -18/ W/Hz) which included mismatch losses
encountered in the waveguide structure. The Josephson junction
mixing mechanism was analyzed on the basis of small-signal
modulation of the staircase like dc I-V characteristics. It
was shown that a small-signal microwave excitation results in
an ac lateral shift of the regions between steps, such that:
(a) Optimum Josephson mixing occurs halfway between steps. (b)
Mixing occurs between all steps with maximum conversion
efficiency between the zero and first step. (c) Predicted low
conversion loss and possibly internal gain for a matched
mixer. The analytical model was simulated by a digital
computer and the behavior of the Josephson mixer predicted for
various conditions of excitation. The model chosen for the
analysis and the computer simulation is based on the simplest
assumptions concerning bulk-formed Josephson junctions. In the
laboratory an external local oscillator set 60 MHz below the
signal frequency was used to induce the well-known step
structure in the dc I-V characteristic of the point-contact
Josephson junction. Mixing of the Lo and signal was found to
occur between constant voltage steps in agreement with both
analysis and computer simulation. The correctness of the model
is demonstrated by the agreement between the theory and
experiment. |
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Radio Frequency Studies of Superconducting
Helical Resonators (Title Only)
G.J.
Dick
Summary: Not available |
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Superconducting Accelerator Section for a
600-MeV Microtron (Title Only)
J.S.
Allen, P. Axel, A.O. Hanson, R.A. Hoffswell, C.S. Robinson,
J.W. Staples and D.C. Sutton
Summary: Not
available |
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Measured Time and Frequency Response of a
Miniature Superconducting Coaxial Line (Title
Only)
M.P. Ekstrom, W.D. McCaa Jr. and
N.S. Nahman
Summary: Not
available |
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Mechanisms Determining the Critical Current in
Hard Superconductors
W.W.
Webb
Summary: It is well established that
critical current densities in hard superconductors depend on
pinning of the fluxoid lattice by microstructural
inhomogeneities against electromagnetic forces. However, the
connection between the measured critical current densities
which are continuum quantities and the interactions between
the pinning objects and the fluxoid lattice which are virtual
point interactions has presented a rather complex problem that
is the focus of this review. An experimental approach to
diagnosis of the mechanisms and a rather general
phenomenological theory are described and illustrated by a
summary of some studies of hard superconductors containing
high lattice-dislocation densities that provide a high density
of rather weak pinning objects. Conclusions of general
applicability are summarized. |
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Flux Flow and a New Critical-Current
Formula
E.W. Urban
Summary:
An experimental study has been conducted to measure the
flow of magnetic flux through the wall of a specimen of Nb-25%
Zr, thereby to infer the magnetic induction, currents in the
material. It was found that if at some magnetic field the
specimen were heated to above T/sub c/, retooled, then if the
applied field were subsequently changed at a constant rate,
the rate of flux flow into or out of the sample would increase
linearly with time. If measured at a series of magnetic
fields, the rates of these increases, or the flow rate slopes
were found to be a smoothly increasing function of average
applied field. Calculations based on eight published
critical-current density models did not satisfactorily explain
these observations. It was found, however, that the empirical
current-density model J/sub c/=/spl alpha//sub c/[(/spl
mu//sub 0/H/sub c2/-B)/(B/sub 0/+B)] did, in fact, give
excellent agreement with the measured flow-rate behavior. In
this paper the experimental, analytical results are described,
the new critical-current density model is compared with
earlier ones. |
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Flux Jumping in Rings of Niobium Titanium (Title
Only)
S.L. Wipf
Summary:
Not available |
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Flux Flow Effect in a Type II Superconductor
(Title Only)
M.S.
Lubell
Summary: Not available |
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Achievement of Very High Critical Currents in
Type II Superconductors (Title Only)
W.E.
Timms and M.A.R. LeBlanc
Summary: Not
available |
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Noise Analysis for Amplifiers with
Superconducting Input
V. Radhakrishnan
and V.L. Newhouse
Summary: A method is
developed to adapt the conventional definition of the noise
temperature to amplifiers with superconducting inputs. In
particular this method is applied to a crossed-film cryotron
amplifier with superconducting input to calculate its noise
temperature. We then compare it with that of other amplifiers
and masers. In addition to the noise temperature, a new
criterion is evolved here: to estimate the figure of merit of
amplifiers and detectors. This involves a new parameter called
herein the action factor, which is the product of the smallest
energy measurable by a device, with its smallest possible
response time. The action factor and the noise temperature are
worked out for the cryotron and a few other amplifiers and
detectors for comparison. The advantage of the cryotron
amplifier is brought out. |
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Noise Thermometry with the Josephson
Effect
R.A. Kamper and J.E.
Zimmerman
Summary: Thermal noise causes a
random frequency modulation of the self-oscillation of a
Josephson junction, and the temperature of the noise source
can be determined by analysis of the generated signal. We show
that a thermometer based on this principle would be
theoretically capable of measuring temperatures in the
microkelvin range, and describe a prototype thermometer which
has recorded noise temperatures down to 0.075 K. |
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Characteristics of Photodielectric Optical
Detectors Using Superconducting
Cavities
W.H. Hartwig and J.J.
Hinds
Summary: Superconducting cavities
loaded with various semiconductor crystals form a unique class
of digital optical transducers. There are several
low-temperature photodielectric effects in semiconductors.
They are used to tune superconducting cavity resonators and
have been demonstrated as laser detectors and for optical
feedback control of the resonant frequency. The free-carrier
PDE is due to inertia forces on mobile carriers and the
trapped carrier PDE is due to polarizability changes on
entering a trap. Recent work by Hartwig, Hinds, and Khambaty
(unpublished) shows most semiconductors display mixtures of
the two effects. The present state of knowledge supports a
preliminary design rationale for radiation detectors. This
paper establishes the link between the several photodielectric
mechanisms, the properties of the superconducting cavity, and
accepted standards of qualifying a radiation
detector. |
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Sputtered Thin-Film Superconductor-Semiconductor
Tunnel Junctions
W.H. Keller and J.E.
Nordman
Summary: Thin-film tunnel junctions
using a semiconductor rather than an oxide barrier have been
fabricated. Niobium, used for the bottom layer, and the
semiconducting center layer were deposited using rf sputtering
techniques, while the top layer was evaporated lead.
Sputtering was done from high-purity single-crystal slices of
Ge, Si, and GaAs and from polycrystalline InSb.
Superconductive tunneling was observed in most samples. The
semiconductor tunnel junctions were somewhat light/heat
sensitive and also exhibited a nonlinear I-V characteristic at
voltages greater than the superconducting energy gap to a
greater extent than comparable oxide devices. Excess current
was noted in the devices at voltages below the superconducting
gap and the measured gap of the niobium film was found to be
reduced from that of pure material. Although initial devices
were inferior to good oxide junctions, present results show
that techniques of rf-bias sputtering and sputter etching have
strong possibilities for the production of deposited insulator
junctions. |
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Observation of Single Fluxon Drift Through a
Superconductor
K.T. Burnette and V.L.
Newhouse
Summary: This paper reports what is
believed to be the first direct observation of the drift of
single fluxons through a superconductor. This has been
achieved by using an ultrasensitive cryotron amplifier to
sense the changes in persistent current which occur in a
superconducting film loop when a fluxon is drifting into or
out of the loop. The loop is deposited on an insulated
superconducting ground plane; this decreases the inductance
sufficiently so that the current changes induced by single
fluxons entering or leaving the loop can be sensed. It has
been found possible to nucleate single fluxons by current
pulsing narrow films which are superimposed over but insulated
from the loop. The fluxons are found to be nucleated at the
intersections of the edges of the film loop with the
superimposed films, and then swept into or out of the loop by
the Lorentz force exerted by the loop current. The fluxon
velocity component normal to this current has been measured
for tin and is found to be less than a factor of two different
from the values predicted by certain of the present theories
of fluxon motion. |
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High-Current ac Losses in Large Superconducting
Niobium Tubes
R.W. Meyerhoff and W.T.
Beall Jr.
Summary: Loss measurements were
made at 60 Hz on 6-m lengths of 1- and 3-cm-diam
superconducting niobium tubes used as the inner conductor of a
coaxial line. These tubes carried transport currents up to
1700 and 5300 A, respectively. The losses were found to fit
the relation P=/spl conint/AE/sub c1/(H/H/sub c1/)/sup n/,
where /spl conint/ is the frequency, A is the surface area,
E/sub c1/ is the energy loss per unit surface area per cycle
at H/sub c1/, the lower critical magnetic field, and H is the
peak magnetic field at the surface of the superconductor. The
exponent n equals 3 for H /spl les/ H/sub c1/ and n /spl ges/4
for H /spl ges/ H/sub c1/ . The ac losses showed a 25% linear
increase with temperature from 4.2 to 5 K. For the case of an
eccentric coaxial line, where the current and magnetic field
vary about the circumference of the superconductor, the ac
losses were found to agree with those predicted from
calculated values of the current and magnetic field
distributions. In particular, when the peak surface field is
less than H/sub c1/ and R is the ratio of the maximum to
minimum surface magnetic fields about the circumference of the
superconductor, the losses were a factor (3R/sup 2/+2R+3) /4R
greater than those found at an equivalent current for a
concentric configuration. |
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Superconducting Power Transmission by Low
Voltage Cable (Title Only)
M.F.
Merriam
Summary: Not available |
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Superconducting Transmission Lines for
High-Capacity dc Circuits
B.C.
Belanger
Summary: The hysteresis losses in
commercial type-II superconductors preclude the use of these
materials for ac superconducting power cables. While recent
work has shown that ac cables employing high-purity Nb may
have acceptably low losses, it is clear that dc cables would
better exploit existing superconductivity technology and make
possible the use of commercial NbTi and Nb/sub3/Sn.
Coincidentally, there has been a lively interest in dc
transmission in the electric power industry due partly to
advances in SCR converters and partly to increased emphasis on
system stability. In this paper the relevance of these
developments is discussed. Several alternative designs for
superconducting dc cables utilizing commercial superconductors
are discussed and design and cost calculations are presented.
Areas where additional experimental work is needed are
identified. |
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Development of a Superconducting Cable for
Transmission of High Electric
F. Moisson
and J.M. Leroux
Summary: The opportunities
opened by the use of cryoresistive and superconducting
materials in underground transmission systems have led C.G.E.
and L’Air Liquide to undertake, in close cooperation, a
cryocable program which started in 1966. A first set of
problems associated with the development of cryogenic cables
deals with the cable system: design, safety, terminal
equipment including leads, cryogenic equipment, refrigerators,
and problemsrelated to overload capability and reliability. A
second set concerns the cable itself, i.e., scientific and
technological problems associated with the conductor, the
electrical insulation, and the thermal exchange between
conductor and helium. We gained useful experience on the
design problems and on the technological problems involved in
the construction of a cryoconducting cable. A 20-m aluminum
cable cooled down to 25°K with pressurized helium flow was
built and tested with 3500-A dc under 20 kV; results are
presented. On this model we solved the following types of
problems. First, mechanical problems concerning cooling down
of the cable, thermal contraction of the pipes, electrical
insulation and conductors, construction of an invariable cable
constituted by elementary helicaly wound conductors were
solved. Second, thermal problems of reduction of heat leaks,
conception of thermal insulation, and segmentation of vacuum
jackets were solved. Third, electrical problems of design of
300°-25°K leads were solved; this problem of losses at both
ends is, in proportion, more important for short model than
for long cable. Finally, refrigeration problems of helium and
nitrogen flows, thermal shields and design of refrigerators
(optimal capacity and spacing) were solved. In order to solve
problems concerning the cable itself, research has been done
on superconducting materials, electrical insulation and heat
exchange. |
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Design Features of ac Superconducting
Cables
H.M. Long and J.
Notaro
Summary: Alternating current
superconducting cables which can be readily integrated into
utility systems require both a superconductor with low
characteristic ac losses and a high-performance cryogenic
system to minimize cost of heat pumping the thermal losses to
the atmosphere. Moreover, the resulting cable systems must
operate with the long-term reliability expected of utility
systems. Recent publication have treated the measurement of
the superconducting material properties and the conceptual
electrical and cryogenic design a superconducting cable. This
paper will discuss a cable configuration and examine its
electrical and cryogenic design features with principal
emphasis placed upon the relationship of these features with
present technological capabilities. Fabrication and
installation feasibility and the expected levels of
performance and reliability can be inferred from the practical
operating cryogenic systems which are used a guide for the
selection and design of the cryogenic enclosure for the
cable. |
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Comparison of Superconducting and Semiconducting
Bolometers
C.L. Bertin and K.
Rose
Summary: Superconducting tin bolometers
are now reasonably well understood, and the results of
calculations of performance are given over a large range of
sheet resistances (0.01-1000 /spl Omega//spl square/ at
4.2/spl deg/K). Comparison of superconducting bolometers with
semiconducting ones show that tin and germanium are equivalent
in performance. The superconducting devices have the
additional advantage of simplicity of sample preparation and
lower impedance levels permitting higher operating speeds if
desired. |
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Nuclear Radiation Detection Using a
Superconducting Resonant Cavity
C.W.
Alworth and C.R. Haden
Summary: Nuclear
radiation is detected using a superconducting resonant circuit
operated at 380 MHz. The technique uses a doped semiconductor
crystal placed on the stub of a reentrant cavity. The cavity
is lead plated and cooled to 4.2°K, such that it becomes
superconducting. The cavity then becomes the frequency
controlling element for a voltage variable oscillator. Nuclear
radiation impinging on the crystal causes a change in the
relative dielectric constant, which in turn causes the
resonant frequency of the cavity to change. The crystal
exhibits a peculiar trap property at 70°K or below, which
allows any electron-hole pairs generated in the crystal to be
indefinitely trapped. The output frequency of the resonant
circuit is then proportional to the total dose received by the
crystal. Frequency shifts of the order of 4.051 kHz/min are
observed in CdS, for a /sup 14/C source of 1/spl mu/
Ci. |
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Material-properties Analyzers Using
Superconducting Resonators
J.J. Hinds and
W.H. Hartwig
Summary: A low-temperature
material-properties analyzer, using a superconducting
microwave resonant cavity, is discussed. Placing semiconductor
or dielectric material samples in the cavity perturbs the
resonant frequency, absorbed microwave power, and cavity Q.
Additional perturbations occur when the sample complex
dielectric constant is altered by a thermal, nuclear
radiation, or optical stimulus. In samples such as Si, GaAs,
CdS, and CdTe, these perturbations have been used to determine
such material properties as relaxation time, lifetime, Fermi
level, trap ionization energy, trap density, capture cross
section, freecarrier density, and trap population. A
contactless experimental technique similar to the thermally
stimulated conductivity experiment is proposed. The
contactless ac measurement system is shown to be sensitive.
accurate. useful with randomlv shaued or powdered samples, and
applicable to many types of insulators and
semiconductors. |
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High-Frequency Limit of the Josephson
Effect
D.G. McDonald, K.M. Everson, J.S.
Wells and J.D. Cupp
Summary: Coherent
detection of applied radiation via a Josephson current step
has been achieved with an applied frequency approximately a
factor of 3 higher than heretofore reported. The detected
signal is at 2.5 THz 118 /spl mu/) . The inadequacy of
existing theory to predict the frequency dependence of the
Josephson effect is discussed. |
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Characteristics of NbN Dayem
Bridges
M.A. Janocko, J.R. Gavaler, C.K.
Jones and R.D. Blaugher
Summary: Dayem-bridge
weak links have been fabricated by sputter etching niobium
nitride thin films having Tc’s of /spl sim/15 K. These
junctions exhibit a negative resistance region extending to
4.5 mV at 3.2 K, in which are seen self-induced subharmonic
current steps and structure near the energy gap voltage.
Temperature dependence of these features and effects of
applied microwave radiation are discussed, and possible
explanations of the negative resistance region and of the
self-induced step structure are given. |
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Superconducting Microstrip High-Q Microwave
Resonators
A.J. DiNardo, J.G. Smith and
F.R. Arams
Summary: Superconducting miniature
microstrip resonators, operating at X band, have been
constructed using vacuum-deposited lead on low-loss sintered
alumina substrate. Unloaded Q's as high as 200,000 and 500,000
have been measured at 14.3 GHz at 4.2 and 1.8 K,
respectively. |
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