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| 1978 |
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Front
Cover (1978)
No author
information available
Summary: Not
available |
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Table of Contents (1978)
No
author information available
Summary: Not
available |
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Conference Information
(1978)
No author information
available
Summary: Not
available |
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Editor's
comments (1978)
S. St.
Lorant
Summary: Not available |
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Chairman's
comments (1978)
J. Parker
Jr.
Summary: Not available |
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Keynote
address (1978)
G.
Hurlbert
Summary: Not available |
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RF
superconductivity for accelerators--Is it a hollow
promise?
M.
Tigner
Summary: This review will attempt to
offer a realistic and coherent overview of the failures and
partial successes in harnessing rf superconductivity of
niobium to particle acceleration. Work in materials,
preparation processes, heat transfer, cavity structures and
vacuum electronic phenomena are discussed and put in
perspective. An attempt is made to draw lessons from these
observations and to outline the tasks and opportunities for
the future. |
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Measurements
of superconducting Nb/sub 3/Sn cavities in the GHz
range
P. Kneisel, O. Stoltz and
J. Halbritter
Summary: Superconducting Nb/sub
3/Sn Cavities have potential advantages over rf cavities with
Nb surfaces To test possible applications and to improve the
understanding of Nb/sub 3/Sn coatings on Nb, rf cavities have
been measured between 1.5 and 8K and between 0.1 and 7GHz. The
temperature dependence of the surface resistance R(T)
indicates weak superconducting spots with transition
temperaturesT\min{c}\max{\ast} < 1K
andT\min{c}\max{\ast} \simeq 2.5K. The normal
conducting spotsT\min{c}\max{\ast} \lsim 1K cause
the large rf residual lossesR'_{res} \propto
f^{2}observed up to date. The spots
withT\min_{c}\max_{\ast} \simeq 2.5K cause
temperature dependences ofR'(T)between 2 and 6K,
where R/sub BCS/(Nb/sub 3/Sn) is still negligible. In line
withR_{res} \propto f^{2}, the lowest rf
lossesR_{res} < 2.10^{-9}\Omegaand the highest
field strengthB_{crit} = 83 m^{T}(\wedgeE_{peak} =
29have been observed at the lowest frequency
0.1GHz measured. Surface resistance and penetration depth
measurements have shown that grain boundaries or hydrogen
clusters do not cause the weak spots observed
withT\min{c}\max{\ast} < 2.5K. The origin and
the chemistry of the weak spots withT\min{c}\max{\ast}
\lsim 1K, which cause the largeR_{res} \propto
f^{2}and the lowB_{crit} (T) \simeq const,
are still not clear. They seem related to the Nb/sub 3/Sn
surface. The weak spots withT\min{c}\max{\ast} \simeq
2.5K consist most likely of Nb/sub 6/Sn/sub 2/, which in
cooling below 950/spl deg/C precipitates due to the excess Sn
present in Nb/sub 3/Sn coatings grown in Sn
vapor. |
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Measurements
on Niobium superconductings C band cavities for linear
accelerators applications
V.
Lagomarsino, G. Manuzio, R. Parodi and R.
Vaccarone
Summary: Superconductings cavities
for linear accelerators applications at 4.5 GHz (C Band) have
been constructed and tested. The Niobium cavities were
obtained by electron beam welding of half cavities previously
formed pressing Niobium sheets in a hydraulic press. In such
cavities, operating in TM/sub 010/mode, accelerating field of
26.5 MV/m and peak surface field of 50 MV/m were obtained at a
power level of 30 W/m. The best quality factor obtained at
this field level is Q/sub 0/= 2x10/sup 9/at a temperature of
1.8/spl deg/K. The set of tests on C-Band cavities shows: (1)
our C-Band niobium cavities exhibit quite good R.F. properties
analogous to the ones measured at higher frequencies (X-Band);
(2) limitations on accelerating fields are related only to
R.F. induced magnetic field. The electron loading due to field
emission does not limit the maximum achievable
field. |
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Surface
impedance of superconducting Nb/sub
3/Sn
G. Arnolds, R. Blaschke, H.
Piel and D. Proch
Summary: Microwave cavities
with a resonant frequency of 8 GHz are coated with Nb/sub 3/Sn
by the vapour deposition technique. The surface resistance and
the change of the penetration depth were determinded by
measuring the quality factor and the shift of the resonant
frequency of the cavity in the temperature range from 2 K to
20K. The temperature dependence of the surface resistance can
be described well by the BCS-theory in the temperature range T
< 0.5 T/sub c/, however, the value of the reduced energy
gap /spl Delta//sub 0//kT/sub c/ has to be increased from 1.76
to 2.15. The temperature dependence of the penetration depth
shows significant deviations from the predictions of the
BCS-theory for temperature T < 0.5 T/sub c. The increase of
the reduced energy gap is not sufficient to fit the data but
one has to treat the effects of strong electron-phonon
coupling consistently. Therefore, we calculated the surface
impedance for strong coupling superconductors using an
Einstein model for the phonon density of states. The absolute
value and the temperature dependence of the surface impedance
in the whole temperature range T < T/sub c are discussed
and a comparison with the experimental data is
given. |
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Superconducting
resonators on sapphire
V.
Braginsky and V. Panov
Summary: The results
presented here are concerned with microwave resonators
consisting of sapphire coated with niobium and lead films
10/sub -4/ cm thick having a geometry factor G = 70/spl
Omega/. The Q-values obtained at 2.9 GHz, Q > 2.5 X 10/sup
8/ enabled us to determine the upper limit on theToss tangent
of sapphire to be /spl les/ 2x10/sup -3/ over the temperature
range between 2 and 4.2K. We also measured the residual
surface impedance dependence of the films as a function of
temperature and external magnetic field. With a Q/spl ges/ 2.5
x 10/sup 8/ the eigenfrequency stability of /spl tri/f/f /spl
sime/ 3 x 10/sup -8/ of the niobium-coated sapphire resonator
remains virtually constant over almost a year. Sapphire
resonators coated with Nb/sub 3/Sn make it possible to obtain
cavities which at 1.5K have an eigenfrequency temperature
dependence /spl tri/f/f /spl les/ 5x10/spl -13/ per degree.
Such resonators enable us to construct very stable
autooscillators capable of competing with atomic frequency
standards. We also present an analysis of the upper limit of
the frequency stability of such a typical
auto-oscillator. |
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A
cryogenic microwave switch
D.
Birx and D. Scalapino
Summary: A cryogenic
microwave switch has been developed which has an isolation
greater than 70 db, a throughput of 0.99, and a rise time of a
few nanoseconds. The switch consists of an H-plane T in which
one arm contains a short and a sapphire tube containing He.
The short is fixed so as to isolate the other two arms when
the He is not excited. The high degree of isolation is a
direct consequence of the superconducting properties of the
waveguide. The tube of He is located a quarter guide
wavelength from the T so that when a plasma discharge is
created in it, the other two arms become tightly coupled.
Plasma rise times of order several nanoseconds have been
achieved by applying a high voltage dc pulse across the tube.
The switch has been used to release microwave energy stored in
a superconducting resonator, creating high power microwave
pulses. Here we will discuss the design parameters of the
switch and its observed performance. |
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Frictional
properties of metal-insulator
interfaces
Y. Iwasa, R. Kensley
and J. Williams
Summary: The energy by which
a normal zone may be created in a cryostable composite
superconductor derives almost exclusively from frictional
heating, which originates from the movement of conductor
against insulator. The magnitude of the frictional heat pulse
depends on the mechanical properties of the winding, the
frictional properties of the conductor-insulator interface,
and the dynamics of the slip process. Quantification of the
frictional heat pulse is presently under investigation; this
paper presents preliminary results. |
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Studies
on nucleate boiling crisis of Helium-I in channels of
superconducting magnet systems
V.
Beliakov, V. Shaposhnikov, S. Gorbatchev, I. Michailov and E.
Mikitenko
Summary: The experimental results
of measuring the critical heat flux of Helium-I under natural
circulation and forced flow are described in this paper. The
direction of the experiments under natural circulation depends
upon the real geometry of superconducting magnet cooling
channels. The influence of the nonheated zones and local
hydraulic resistances on the critical heat flux is discussed
and some experimental results are given. The critical heat
flux of saturated Helium-I under forced flow in vertical tube
is measured in a wide range of pressure, quality, and mass
velocities. The expressions have been obtained which describe
the dependency of the critical heat flux upon the different
conditions. |
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General
atomic's superconducting high field test facility and initial
performance
M. Otavka, J. Alcorn,
J. Purcell, W. Chen and W. Toffolo
Summary:
General Atomic has established a high field test
facility whose primary mission is to investigate the J-B-T and
stability performance margins of commercial NbTi
superconductor in the 10 tesla, 4.2 K region. This work is
part of the overall DOE/MFE/MAGNETIC SYSTEMS effort to provide
an adequate technological base for construction of
superconducting toroidal field coils for the next generation
of large tokamak fusion devices. The principal components of
the facility are the coil/cryostat assembly, the helium
refrigerator-liquefier/compressor system, and the gaseous
helium recovery and storage system. The epoxy impregnated,
layer wound main background field coil generates 8 tesla
within its 40 cm diameter bore. The insert background field
coil was layer wound with cooling channels provided by "barber
pole" mylar conductor insulation. Ten tesla is generated
within its 22 cm bore. The initial performance of the facility
will be discussed. Future testing calls for operating test
coils with implanted heating elements to simulate mechanically
induced perturbations. The normal zone growth and recovery
behavior will be observed for various disturbance energies.
This data will then be compared with results obtained from the
transient recovery analysis developed at General
Atomic. |
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The
recovery of superconducting
systems
T. Chilcott and G.
Donaldson
Summary: Superconducting systems,
as proposed for fusion reactors and alternators, must if
possible be encouraged to return to operating conditions after
a transient fault. Electric ac power systems de-energise a
faulty section of line, then re-close. The analogy is useful.
The paper describes measurements and calculations for
superconductors under fault and recovery conditions. In the
laboratory, as in the final installation, recovery may be
initiated in a number of ways; for example the removal of a
continuous heat input could be considered as equivalent to the
removal of external heat flux in a fusion reactor toroidal
field winding. More realistic would be the reduction of
conductor current, simulating the effect of protective action
in a large installation. Laboratory observations are
correlated with computer calculations in which detailed
thermo-dynamic properties of superconductors are used. The
conditions under which the recovery of a large system is
possible are discussed. |
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Pressure
drop measurement on forced flow cable
conductors
J. Lue, J. Miller and
J. Lottin
Summary: Forced flow cable
conductors being developed for use in LCP coils and other
large superconducting magnets utilize supercritical helium
flowing through narrow, uneven channels with large cooling
surfaces. Extensive measurements on pressure drop of a variety
of samples were performed. It is found that the friction
factor versus Reynolds number plots of all the data are
clustered together and behave in a universal way. A factor of
two to three higher in friction factor than the smooth tube
value in turbulent helium flow regime can be expected for this
type of conductor. |
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Low
temperature irradiations of Nb/sub 3/Sn with 14- MeV
neutrons
R. Scanlan and E.
Raymond
Summary: High energy neutron
irradiations have been performed on Nb/sub 3/Sn
superconductors to assess their behavior in a fusion reactor
environment. Irradiations were performed at 4.2 K and property
measurements were made without warming the samples. The
critical current I/sub c/increased with irradiation to a level
about 50% above the unirradiated value at the highest fluences
reached in our experiments. These results are compared with
the results of other low temperature irradiations of Nb/sub
3/Sn. |
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Vapor
formation and heat transfer in liquid helium cooling channels
under transient and steady state
conditions
M. Hilal, J. Dawson,
J. Gonczy, L. Turner and S. Wang
Summary: The
recovery of a normal zone produced in a superconducting magnet
following a mechanical disturbance depends on the heat
transfer rate to the liquid helium cooling channels. One of
the factors which affects the heat transfer to the channel is
the amount of vapor accumulated in the channel during and
following a disturbance. The present work is undertaken to
determine the void fraction in a liquid helium channel and to
study the effect of the amount of vapor in the channel on the
heat transfer characteristics. Heat pulses of 30 to 300 ms
duration are introduced into the simulated conductor, and
observations are made of the resulting temperature response
and of the vapor fraction in the channel. The vapor fraction
is determined by means of a capacitance bridge which responds
to the small difference in dielectric constant between liquid
and vapor helium. For completeness steady state measurements
of temperature and vapor fraction are also
reported. |
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Cluster
test facility for superconducting tokamak toroidal magnet
system development
T. Ando, K.
Yoshida, S. Shimamoto and K. Yasukochi
Summary:
As the step of development of superconducting tokamak
toroidal magnet system, a Cluster Test system is being
constructed. The test facility has two coils, called cluster
test coil (CTC), in a sector position, which provides
back-ground field to a test module coil (TMC). The oval-shaped
TMC has a mean width of 1.5 m and a mean height of 2.0 m. And
it is designed to operate at a peak field of 8 T and an
average current density of 3 KA/cm/sup 2/on the winding space
when CTC, operated with rated current, has 7 T as a peak field
with current density of 3 KA/cm/sup 2/. Experience on the
Cluster Test will provide fabrication techniques and
verifications of computer codes for future toroidal coil
design principles. |
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Nb/sub
3/Sn in 1978: State of the art
W.
Fietz
Summary: Seventeen years after its
discovery as the first practical high field superconductor and
seven years since it was demonstrated in multifilamentary
form, it seems appropriate to ask, "What is the current status
of Nb/sub 3/Sn"? This question is approached from a user's
point of view with particular emphasis on the multifilamentary
form of Nb/sub 3/Sn. Data from national laboratories,
universities, and conductor manufacturers have been compiled
and assessed. The result is a comprehensive picture of past,
present, and future conductor types and their properties.
Applications of these conductors in magnets are reviewed
including both those devices already completed and those which
are being proposed and/or constructed at this time. Finally, a
summary of ongoing research and development programs is
included along with the author's assessment of their impact on
future Nb/sub 3/Sn magnets. |
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The
temperature and magnetic field dependence of superconducting
critical current densities of multifilamentary Nb/sub 3/Sn and
NbTi composite wires
C. Spencer,
P. Sanger and M. Young
Summary: Commercial
NbTi and Nb/sub 3/Sn multifilamentary superconducting wire is
becoming increasing important for use in research and
commercial magnet systems. In both materials the temperature
dependence of J/sub c/plays a major role in the determination
of magnet system operating parameters and design stability
margins. We report here critical current density measurements
as a function of temperature from 4.2 to 19 K and of applied
magnetic field upto 8 T for multifilamentary Nb/sub 3/Sn wire
and for 2 alloys of NbTi superconducting wire. From this data
[/spl part J/sub c(H/sub a//spl part/T] and [/spl part/H/sub
c2///spl part/T]/sub T=T/sub c// can be obtained and stability
criteria and other superconducting parameters of the wires may
be extracted. |
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Properties
and performance of fine-filament bronze-process Nb/sub 3/Sn
conductors
M. Walker, J. Cutro,
B. Zeitlin, G. Ozeryansky, R. Schwall, C. Oberly, J. Ho and J.
Woollam
Summary: In 1975 the Intermagnetics
General Corporation began a comprehensive program funded by
the Air Force Materials labratory to establish the
manufacturing technology of Nb/sub 3/Sn superconductors to
meet a stringent set of design specifications. The processing
of these conductors has been pursued in cooperation with the
General Electric Corporate Research and Development
Labratories. |
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Multifilament
niobium-tin conductors
S. Murase,
M. Koizumi, O. Horigami, H. Shiraki, Y. Koike, E. Suzuki, M.
Ichihara, F. Nakane and N. Aoki
Summary: A
new Nb/sub 3/Sn wire fabrication method has been developed,
improving wire drawing workability and superconducting
properties, such as stability and ac losses. A cross section
of the single filament wire consists of a niobium tube with a
copper sheathed tin rod inside and high conductivity copper
tube outside. These constituents show scarcely any
workhardening. Wires with 54 to 295 filaments were drawn down
to 0.2 mm to 1.0 mm diameter. Heat-treatment conditions to
obtain the highest critical current were clarified as a
function of the tin content inside the niobium tube. The
effect of bend strain in Nb/sub 3/Sn on the critical current
was also examined for samples with different wire diameters
and Nb/sub 3/Sn layer thickness. Losses were measured for
twisted and non-twisted samples by means of magnetization
experiments. Results were compared with calculated values. It
was found that the effective resistivity between Nb/sub 3/Sn
filaments was one order of magnitude higher than that of pure
copper. A coil was constructed using a 1 km long Nb/sub 3/Sn
composite having 258 filaments with 1 X; 2 mm cross section.
The maximum field obtained was 10.65T at 236A in the 6T
backing field by NbTi solenoid. |
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Multifilament
Nb/sub 3/Sn superconductors produced by the E.C.N.
technique
C. van Beijnen and J.
Elen
Summary: Multifilament Nb/sub 3/Sn
conductors are produced by reduction of composites containing
bundles of niobium tubes filled with NbSn/sub 2/-powder and
surrounded by pure copper. Heat treatment at temperatures
between 575 and 675/spl deg/C after final reduction causes the
tin from the NbSn/sub 2/-powder to diffuse into the niobium
tubes, which results in a final Nb/sub 3/Sn-layer at the inner
side of the tube. Two types of experimental wire are produced,
the first type consisting of 4 bundles of 9 tubular filaments,
the powder cores having diameters of about 35 micron, the
outer size of the wire being 1 mm square. The second type
consists of 4 bundles of 45 tubular filaments. This type is
fabricated in 3 sizes: round /spl phi/ 2.3 and square 1.4 and
1.0 mm, corresponding with powder core diameters of 26,18 and
13 microns respectively. Critical current densities in the
Nb/sub 3/Sn-layers reach values of about 4 - 6x10/sup
9/A/m/sup 2/at 8 Tesla and 1,6 - 2,4x10/sup 9/A/m/sup 2/at 12
Tesla. Maximum critical temperatures are about 18.1 K with a
/spl utri/T/sub c/of 0.3 K. |
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Recent
advances in hydrostatic extrusion of multifilament Nb/sub 3/Sn
and NbTi superconductors
E.
Smith, R. Fiorentino, E. Collings and F.
Jelinek
Summary: Recent technical results are
presented relating to the processing of superconductor wire by
hydrostatic extrusion. Included are the reference processing
sequence developed for the Nb/sub 3/Sn system and a discussion
of processing parameters affecting the formability and quality
of tantalum barriers. In addition, a comparison of
manufacturing costs for producing wire by conventional and
hydrostatic extrusion is made. Finally, some aspects of
transferring technology to industry are
discussed. |
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Superconducting
properties and annealing behavior of filamentary Cu-based
composites produced by powder
metallurgy
L. Schultz, R. Bormann
and H. Freyhardt
Summary: Composite wires
with discontinuous Nb filaments embedded in a Cu matrix are
produced by hot extrusion of a powder mixture and subsequent
wire drawing. Nb/sub 3/Sn layers on the surface of the Nb
filaments are formed by a diffusion treatment after plating
with Sn. The peculiarities of the powdermetallurgical
preparation technique and the origin of resistanceless
currents in these materials are discussed. For Cu-Nb
composites, it is shown that the critical currents are
drastically enhanced by a final heat treatment, by which
additional Nb precipitates in the Cu matrix. |
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Mechanical
stresses and strains in superconducting dipole magnets for
high energy accelerators
L.
Greben, E. Mironov and H. Moustafin
Summary:
The paper deals with numerical investigation of stress
and strain distributions in superconducting dipole magnets. A
finite element computer program is developed to calculate
stresses and displacements due to thermal stress,
electromagnetic forces and prestressing of structural
elements. Real mechanical and thermal properties of
superconducting dipole elements are taken into account.
Numerical results on stress and strain patterns in dipole
magnets are presented. |
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Tests
of the superconducting bending magnets
"CESAR"
J. Perot and D.
Leroy
Summary: Two superconducting bending
magnets have been built at the Saclay Laboratory in
collaboration with CERN. They will be installed in a secondary
beam line of the SPS machine at CERN, and because they will be
used also later as a part of a spectrometry experiment, a very
high accuracy of the integrated field (/spl plusmn/ 2x10/sup
-4/) is required. A brief description of the magnets and the
results of the tests are given. |
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Operating
experience with ESCAR magnet cooling
system
R. Byrns, W. Eaton, J.
Carrieri, W. Gilbert, G. Lambertson, R. Meuser, J. Rechen, R.
Schafer and R. Warren
Summary: The ESCAR
magnet cooling system has been successfully demonstrated. This
two-phase helium cooling system includes a CTI-Sulzer
gas-bearing turbine refrigerator with two-stage compression by
oil-lubricated screw compressors, 120 m of 5-cm-diameter
vacuum-insulated transfer line and twelve series-connected
magnet cryostats with weirs for liquid level control. The
refrigeration plant provides up to 1900 W of refrigeration at
4.5 K with a mass flow of 113 g/s. Heat load within the
transfer line has been measured at 0.25 W/m in sub-system
testing. Cool-down times to 4.5 K for the 12 warm-iron magnets
with a cold mass of 2500 kg have been about 12 hours. The
magnet cryostats separate liquid by gravitational extraction
and fill in sequence at a rate of up to 400 /spl ell//hr. A
heater in the transfer line allows adjustment of the inlet
coolant quality (ratio of gas to liquid) to the cryostats;
flow instabilities were not present and could not be induced.
Pressure levels in the cold bore, beam orbit space were below
10/sup -10/torr. Severe pressure transients, incurred as a
result of magnet transitions, have been safely handled both in
terms of refrigerator response and cryostat pressure relief.
Large gas loads at the compressor suction following magnet
transition have not caused overloading or interruption of the
refrigeration plant output. An electrical arc punctured the
helium vessel and allowed liquid helium to flow into the
vacuum space. This was handled by the relief system with no
additional damage. |
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Superconducting
spin tipping solenoids for ZGS polarized beam
facility
S.-T. Wang, K. Mataya,
A. Paugys and C. Chen
Summary: Two
superconducting solenoids have been designed and built for the
Argonne Zero Gradient Synchrotron (ZGS) Polarized Proton Beam
Facility. Each solenoid will have a central field of 8.0 T, an
effective field length of about 1.75 m, and a cold clear bore
of 11.5 cm. It will generate a spin-tipping integral field of
about 14 T-m. The integral field homogeneity over the cold
bore is /spl plusmn/ 4%. The magnet stored energy is about 0.7
MJ. Described in detail is the stability simulation on a high
current density intrinsic stable coil. Comparisons are made
between the simulation data and data obtained from the magnet
performance tests. The cryostat design and construction are
also presented. |
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Design
and development of an ultra-thin solenoid for a high energy
physics particle detector
R.
Smith, R. Niemann and F. Catania
Summary: An
enhanced-stability thin solenoid magnet design is presented.
The details of the high purity aluminum stablized conductor
are discussed. The design details of a special cryostat
constructed for conductor evaluation is presented, and the
aluminum alloy NbTi superconductor under procurement is
described. |
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Magnetization
effects in superconducting accelerator
magnets
W. Sampson, P. Dahl, A.
McInturff, K. Robins and E. Bleser
Summary:
Measurements of the effect of the superconductor
magnetization on the field shape of dipole magnets are
presented. Such effects are most obvious at low fields where
the current capability of the windings is very high and only a
small fraction is used to support the transport current which
produces the field. In most dipoles the fundamental field
component is reduced slightly when the field is increasing and
enhanced by an almost equal amount when the field is
decreasing, The effect on the higher allowed harmonics such as
the sextupole and decapole components is much greater, giving
rise to considerable hysteretic behavior at the low field
levels where the beam is injected into the accelerator. The
measurements summarized in this paper indicate that it is
possible to accurately specify the field from current
measurements if a "standard" energizing cycle is used and the
previous "history" of the magnet is known. |
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Nb/sub
3/Sn dipole magnets
W. Sampson,
S. Kiss, K. Robins and A. McInturff
Summary:
Multifilamentary Nb/sub 3/Sn conductors suitable for
use in accelerator magnets have been under development at BNL
for a number of years. To date three one meter long dipole
magnets have been constructed from braided conductor which had
been reacted prior to winding. The first of these dipoles and
the method of construction has been described in an earlier
paper. The most recent magnet in this series was tested over
the temperature range 4.2 K to 15 K using high pressure gas
cooling. The maximum field 4.8 T, was achieved at 4.2 K. At
higher temperatures the field decreased approximately linearly
with temperature passing through 4.0 T at just under 8
K. |
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Quench
thresholds in operational superconducting
magnets
J. Allinger, G. Danby, H.
Foelsche, J. Jackson, D. Lowenstein, A. Prodell and W.
Weng
Summary: Superconducting magnets exposed
to intense primary proton beams in high energy physics
applications are subject to potentially extreme heat
deposition. The beam power density, its duration and spatial
distribution, the current density in the superconductor and,
potentially, in the normal metal substrate, as well as the
construction and cooling details of the magnet, are all
relevant parameters. We will discuss an extension of some
earlier work in which 28.5 GeV/c proton beams with up to 50 k
joules of energy were targeted upstream from a 4 m long, 4 T
dipole magnet used to deflect the protons through an angle of
8/spl deg/. Quench thresholds much greater than the enthalpy
limit of the magnet materials were observed. In the beam
exposure experiment described in this paper, intense beams of
1.5 GeV/c protons have been deflected directly into the magnet
coil at relatively steep angles of incidence. The magnet
quench threshold was studied by varying the beam currents and
beam sizes. |
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Special
techniques in the fabrication of coils of high precision
superconducting quadrupole magnets for the CERN ISR
high-luminosity insertion
J.
Billan
Summary: This paper describes two
measuring devices specially conceived in view of the
fabrication by industry of the coils of the eight
superconducting quadrupole magnets for the CERN ISR
high-luminosity insertion. The first device is an automatic
bench for the precise measurement of the SC wire dimensions.
Measurements are performed under a pressure of 20 MPa and with
an accuracy of /spl plusmn/0.003 mm. The second device allows
the immediate detection of any short-circuit which may occur
during winding. It is based on the continuous monitoring and
recording of the d.c. resistance of the whole length of the SC
wire during all winding and impregnation
processes. |
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Design,
fabrication and performance of low current superconducting
beam line dipole
B. Cox, T.
Dillmann, P. Garbincius, L. Kula, P. Mazur, J. Satti, A.
Skraboly and E. Tilles
Summary: A low current
superconducting coil intended for use in a beam transport
dipole magnet was built and tested. The coil design,
fabrication, quench protection, and tests results are
presented. |
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A
magnet system for the time projection chamber at
PEP
M. Green, P. Eberhard, J.
Taylor, W. Burns, B. Garfinkel, G. Gibson, P. Miller, R. Ross,
R. Smits and H. Van Slyke
Summary: A
superconducting solenoid with a conductive bore tube is under
construction for use with the time projection chamber (TPC)
detector at PEP. It will be a uniform induction of 1.5 T over
a 6.3 m/sup 3/volume. Its stored energy will be 11 MJ while
maintaining a radiation thickness of 0.3 radiation lengths for
the coil package. The coil will operate at a current density
of 7x10/sup 8/Am/sup -2/and it will be cooled by force flow
two phase helium in a tube. The final design details are given
here. |
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The
support and cryostat system for doubler
magnets
G. Biallas, J. Finks, B.
Strauss, M. Kuchnir, W. Hanson, E. Kneip, H. Hinterberger, D.
Dewitt and R. Powers
Summary: This paper
describes the support design, and mass production engineering
of the Energy Doubler Dipole Cryostats. The coils, which are
surrounded by a warm iron yoke, must be supported against high
magnetic forces resulting from errors in the concentricity
between the coil and iron yoke. The whole support system must
be further contained within a one inch radial distance. The
support system developed to meet these criteria which is a
combination of heat-sunk G-11 blocks, meets the requirement of
total helium heat leak of 5W per magnet. Data will be
presented on both mechanical and thermal performance. The
unique quality assurance system used by both industrial and
Fermilab fabricators will be discussed. |
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Room
temperature field measurements of superconducting
magnets
R. Peters, L. Harris, J.
Saarivirta and A. Tollestrup
Summary: During
the production of the superconducting magnets for the Fermilab
Energy Doubler/Saver, it is important to closely monitor the
magnetic field quality. Since it is both costly and time
consuming to delay these measurements until after each magnet
has been placed in a cryostat, we are developing a room
temperature measurement system capable of resolving a /spl
utri/B/B smaller than 1x10/sup -5/. |
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Production
measurement of energy doubler
magnets
D. Gross, M. Wake, R.
Yamada, D. Blatchley and M. Price
Summary: We
have measured in recent months 50 Energy Doubler dipoles and 6
quadrupoles using a facility which currently averages complete
measurements of 5 magnets a week (Fig. 1). Dipoles are 22 feet
long with 3 inch diameter beam bore and produce a full field
of 4.5T at 4500A. Quadrupoles are 7 feet long and produce a
field gradient of 99T/m (Fig. 2). In the Energy Doubler, 4.23T
corresponds to 1000 GeV. Magnets are measured individually for
quench behavior, training, a.c. loss, harmonic content,
integral field (gradient) length, vertical plane, longitudinal
field homogeneity. |
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AC
loss in energy doubler magnets
M.
Wake, D. Gross, R. Yamada and D. Blatchley
Summary:
A computerized ac loss measurement system was developed
and more than 50 magnets were measured. The ac losses were
very much dependent on various factors such as conductor
surface and pressure applied to coil windings. Statistical
correlation between quench current and ac loss will be shown.
From the measurements of magnets made of different kinds of
cables, most of the eddy current loss was found to be in the
inner coil of the double layer structure. Among the three
kinds of eddy current loops, the one which comes from the
interconnection between crossing strands was proved to be the
major part of the eddy current loss. Such an eddy current can
be almost eliminated, by the use of a copper oxide resistive
layer between strands. Thus, the Energy Doubler/Saver magnets,
will be operated up to 4.5T with a loss of about 450J/cycle
per each dipole magnet. To estimate real time power loss
instead of total loss throughout whole cycle, a new method was
tried and successful results were obtained. |
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Results
from a superconducting dipole model test
program
V. Vasiljev, L. Dinaburg,
B. Zhukov, P. Kljavin, M. Kosjakin, V. Lebedev, L. Mazalevsky,
A. Nikiforovsky, P. Smirnov, V. Fedorov and M.
Fomin
Summary: The SDM-3 model used
Rutherford-type cable: the individual winding layers were
bonded with epoxy resin and prestressed with metal rings
heat-shrunk onto the windings during the assembly. The SDM-6
model was an improvement on the earlier prototypes insofar as
spiral cooling passages were added to the interior of the
coil. Some technological changes were made to make the dipole
more suitable for mass production. The testing program showed
that these dipoles require relatively little training, exhibit
minimal degradation effects, and are capable of steady state
pulsed operation up to 5.5T. This report briefly discusses
some of the construction features and the test
results. |
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U.S.
- U.S.S.R. cooperation in superconducting power
transmission
V. Levitov and H.
Long
Summary: The cooperative exchange
between the U.S. and the Soviet Union in the field of
superconducting power transmission began in 1971 when a U.S.
delegation representing government and industry met with a
Soviet delegation in Moscow. The following year a Soviet
delegation met in the U.S. and this alternating pattern has
continued to the present. The development and guidance of the
cooperation by these yearly meetings of delegations from the
coordinating committee is described in context with the
evolving U.S. and Soviet national programs to develop
superconducting power transmission. The U.S. national program
is now concentrated in the projects at the Brookhaven National
Laboratory and the Los Alamos Scientific Laboratory while the
Soviet program is concentrated in projects at the G. M.
Krzhizha-novsky Power Engineering Institute, the All-Union
Research Institute for the Cable Industry and the Lenin
All-Union Electrotechnical Institute. These projects are
briefly reviewed along with the supporting projects in other
institutions in both countries. The detailed planning for the
cooperation is carried out by six technical working groups
whose members are drawn from the broad technical community
that supports the national programs. These groups also report
on the technical progress in each country. To facilitate these
reports, five joint technical symposia have been held, one at
each of the delegation meetings except for the 1977 meeting in
Moscow. Highlights of these symposia will be presented. At the
present time the cooperation is focused on joint participation
in both the small scale laboratory studies and large scale
test installations which each country is conducting as part of
its national program. The results from the eight joint
experiments conducted since 1971 will be presented along with
some of the future plans and prospects. |
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Low
AC loss Nb/sub 3/Sn tape for transmission line
applications
P. Brisbin, P.
Swartz and K. Pickard
Summary: A process for
modifying the surface topography of Nb/sub 3/Sn tapes so as to
achieve low ac losses has been further developed by
experimental operation at pilot plant scale. Diffusion process
Nb/sub 3/Sn tapes are treated by etching followed by hot dip
coating with tin. Problems involved in control of the etch
process and the Nb/sub 3/Sn input have been studied for the
purpose of optimizing the product superconductor with respect
to minimum losses in combination with high values of critical
current. Performance capability has been demonstrated by
processing a group of four tapes totaling about 3,000 feet.
The average ac loss for the group, as determined from
measurements at each end of each tape, is at the target
specification: 10 /spl mu/W/cm/sup 2/(500 rms A/cm, 60 Hz, 4.2
K, unclad). The target specification for critical current was
also met. |
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Design
and first stage test of 50-meter flexible superconducting
cable
P. Dolgosheyev, I. Peshkov,
G. Svalov, I. Bortnik, V. Karapazyuk, L. Kubarev, A. Panov,
Yu. Petrovsky and V. Turkot
Summary: A
50-meter flexible superconducting cable model is described,
including a copper-stabilized Nb/sub 3/Sn-tape
current-carrying core and cable-paper electric insulation of
13 mm total thickness which is submerged in cold helium while
under working conditions. The helium-cooled inner channel is
36 mm in diameter and the outer diameter of the
current-carrying core is 75 mm. Helium also flows in an
annular channel between the outer surface of the core and a
corrugated enclosure. The core is incorporated in a flexible
cryostating envelope consisting of 4 corrugated enclosures;
the two middle pipes form an annular channel for liquid
nitrogen which reduces heat leaks to helium. The design of a
110 kV, 12 kA lead was developed to test the flexible
superconducting cable model. For d.c. and a.c. tests of the
model, a technique of inductive current introduction through a
superconducting transformer is offered. A multisection
transformer with a superconducting primary of insulated Nb/sub
3/Sn tape was manufactured, the current-carrying core being
the secondary. Before assemblying, the sections were tested in
a liquid helium cryostat to check the design characteristics
and to determine the critical current in the winding. A.C.
losses (50 Hz) in the transformer's primary were measured. The
cable model was cooled by two improved helium refrigerator
plants, HGU-150/4,5. Current tests were performed at a working
temperature. |
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A
10 m Nb/sub 3/Sn cable for 60 Hz power
transmission
M.
Garber
Summary: A 10 m Nb/sub 3/Sn cable for
60 Hz power transmission has been constructed. The coaxial
cable is of flexible, multiple helix, tape wound form. It has
been designed and instrumented for short circuit current
tests. The inner and outer conductor diameters are
approximately 24 and 30 mm respectively. The cable is
installed in a horizontal cryostat with its ends fixed to room
temperature supports. Measurements have been made of ac
losses, fault currents, current sharing, and recovery from
local quenches. |
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Control
and protection of superconducting power transmission
lines
Ye. Blinkov, I. Yakimetz
and I. Alexeyev
Summary: The paper deals with
the analysis of transient processes in a superconducting cable
under current overloads and the possibility of stabilizing the
superconducting state of the line by means of an automatic
control system. The urgency of employing an automatic control
system for the recovery and retaining of superconductivity in
after-fault conditions is due to the necessity to increase
cable stability under the growing level of short-circuit
currents in the developing power systems and to reduce the
capital costs of the current-carrying system and special
current-limiting devices. The paper defines the operating
conditions of a system of special protection and control with
regard to the specifics of superconducting materials, and
gives the required relations to build such systems. Basing on
mathematical methods a scheme was developed for analog
simulation of the modes of a superconducting cable and for an
automatic control system with a proportional regulator which
uses the valve of thermal energy released in the cable during
current overload as the determining parameter. The results of
a comparative analysis of transient processes in a cable with
and without a regulator show the advantage of the automatic
control system in retaining the superconductivity of a power
line in after-fault conditions. |
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Electric
power losses of current input into superconducting devices
cooled by supercritical helium
V.
Maximov and A. Malykhin
Summary: This paper
deals with the methods and results of calculations to minimize
power losses in cryogenic installations related to current
input into superconducting systems with forced supercritical
helium cooling. Different schemes and modes of cooling the
current leads with a superconducting region at the cold end
are examined. Optimum helium flow rate and minimum power
losses in an idealized and an actual cryogenic unit are
defined. The calculations were made within the working
temperature range of 5-18 K. The paper shows that the choice
of the current lead cooling scheme allowing for minimum power
losses depends on the relation between the temperatures of the
superconducting device and of the superconducting transition
in the superconducting region of the current lead. The value
of minimum power losses decreases with the growth of the
transient temperature in the superconducting
region. |
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Experimental
measurements of the temperature dependence of the critical
current in a prototype section of a superconducting power
transmission cable
Ye. Blinkov,
N. Bendik, J. Hoffer and G. Morgan
Summary:
The current rating for a superconducting transmission
cable can be determined on the basis of the temperature
dependence of the critical rent for the cable core. Therefore,
experimental studies of the core critical current on the scale
superconducting cable section are of practical interest.
Moreover, the studies could confirm the feasibility of
developing the superconducting cables of high current
rating. |
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Test
results of a resistive SC-power switch of 40 MW switching
power at a voltage of 47 kV
A.
Ulbricht
Summary: After a brief description
of the function of an sc. resistive switch in an electrical
network and of the basic design formula, results are presented
of the switch test. Special developments are described which
are necessary for successful switch operation. They include
the control of the high power density in the switch conductor,
the application of high voltage technique to the construction
of all components, and triggering of switches with large
conductor lengths. About 500 power switching operations were
done. Some unexpected results on the stability of the
conductor and the propagation velocity in a normal region are
presented. The attempt is made to explain them. |
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Thin
film superconducting switches
K.
Gray, T. Lenihan and J. Tarczon
Summary:
Although thin superconducting films have been suggested
for use in switching electrical power, virtually no
experimental work has been reported. Our present research
investigates thin films switched by applying a fast pulse of
magnetic field perpendicular to the plane of the film. Short
sections of niobium films, carrying approximately half of
their critical current, were switched completely into the
normal state upon application of pulsed fields as low as 0.02
T (200 gauss). The field pulse has a rise time and length of
order 10 /spl mu/sec, but switching was more rapid. It is
shown that the magnitude of the field, and not its rise time,
cause the switching. Recovery times, upon removing the current
source, were also of order 10 /spl mu/sec. |
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Kirkendall
voids--A detriment to Nb/sub 3/Sn
superconductors
D. Easton and D.
Kroeger
Summary: Multifilamentary composites
with constant Nb/bronze ratio were heat treated for a range of
reaction times. With increasing reaction time, the size of
diffusion (Kirkendall) voids in the bronze matrix increases,
as does the Nb/sub 3/Sn layer thickness. Because of the
resultant changes in mechanical and superconducting
properties, the strain at onset of degradation of critical
current density, J/sub c/, decreases with increasing reaction
time. Thus there is a compromise between strain sensitivity
and current capacity at zero strain as reaction time is
increased. For strains on the order of 0.2 - 1.0 %, the
increased strain sensitivity of J/sub c/in specimens which
were reacted for longer than /spl sim/24 hrs offsets the gain
in J/sub c/resulting from increased Nb/sub 3/Sn layer
thickness. The diffusion voids are also related to thermal
instabilities and crack initiation. |
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The
effect of heat treatment on the superconducting properties of
a multifilamentary Nb/sub 3/Sn
composite
P. Madsen and R.
Hills
Summary: The superconducting properties
of a 2046 filament IMI multifilamentary Nb/sub 3/Sn
superconductor have been investigated. The top of the
superconducting transition increases with time and temperature
of annealing until a fairly steady value near 18K is achieved.
Removal of the bronze increases the transition temperature but
still leaves it depressed in very thin Nb/sub 3/Sn layers.
Microprobe analysis shows that the thicker layers are
stoichiometric and contain only /spl sim/ 0.25 weight %
copper. The critical current increases with time and
temperature of annealing at 630/spl deg/C and 690/spl deg/C,
but longer anneals at 730/spl deg/C and 780/spl deg/C produce
decreases from the maximum values. For the longer anneals at
630-730/spl deg/C the critical current density in the Nb/sub
3/Sn is relatively constant at a given field, but Nb/sub 3/Sn
formed at 780/spl deg/C has a lower current density. Flux
pinning follows a similar dependence on grain size to that
reported by West and Rawlings, and the shape of this graph is
explained by Luhman and Suenaga's suggestion that the stress
produced by the differential contraction of the composite
induces the martensitic transformation and consequently more
pinning centres in the Nb/sub 3/Sn. |
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Reaction
treatment, critical current, transition temperature and bend
properties of a niobium-bronze process multifilamentary
superconductor
D. Martin, M.
Daniel, J. Cutro and R. Schwall
Summary:
Critical current, T/sub c/, and bend properties were
measured on samples of a 0.010" (.025 cm) diameter bronze and
niobium multifilament conductor which were reacted at
650-800/spl deg/C. The conductor consisted of 4453 niobium
filaments in a 13 wt% bronze matrix. The critical currents
exhibit a peak as a function of reaction time at each
temperature. The highest I/sub c/of 98 amperes, corresponding
to an overall current density of 1.8x10/sup 5/Amp/cm/sup 2/at
7 T and 4.2 K was achieved for a reaction at 650C for 7000
minutes. Inductive T/sub c/measurements and critical current
as a function of bend diameter are reported. |
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Stress
induced normal--Superconducting transition in multifilamentary
Nb/sub 3/Sn conductors
G.
Rupp
Summary: In multifilamentary Nb/sub 3/Sn
conductors produced by a solid state diffusion process the
critical current is reduced by the compression which acts on
the Nb/sub 3/Sn due to the stronger thermal contraction of the
bronze. Externally applied tensile stress reduces the
compression and enhances the critical current I/sub c/. I/sub
c/runs through a maximum as a function of the strain of the
conductor. I/sub c/was measured as a function of the strain up
to a flux density of 16 T. Current enhancement factors of
about 2 have been observed for technical conductors near 16 T.
The results can be described by the theory of E.J. Kramer
using a strain dependence of the upper critical flux density
B/sub c2/. In flux densities near B/sub c2/of the stress-free
state the case is possible that the conductor is not
superconducting in the initial state but becomes
superconducting beyond a certain strain and returns reversibly
into the normal state for higher strain. |
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Stress
effects on multifilamentary Nb/sub 3/Sn
wire
R. Bartlett, R. Taylor and
J. Thompson
Summary: Critical current I/sub
c/measurements were obtained on highly stabilized mf Nb/sub
3/Sn wires as a function of heat treatment, stress,
temperature, and applied magnetic field. The ratio of the area
of the copper to bronze core-niobium tube is about 8, and the
filaments are concentrated in the inner 30% of the wire's
cross section. Values of I/sub c/and T/sub c/were determined
for samples subjected to a wide range of heat treatments.
Diffusion reaction times and temperatures in the ranges 16 to
128 h and 700 to 750/spl deg/C provided a number of mf Nb/sub
3/Sn wires having similar I/sub c/characteristics. To some
extent the residual compressive loading on the Nb/sub 3/Sn
wires varied with the particular heat treatment. This loading
arises primarily from the differential contraction of the
remaining bronze and the Nb/sub 3/Sn layer when cooled from
the reaction temperature to the operating temperature. Like
other investigators, we find that, by controlled bending or
stretching of the wires, whereby some of the strain in the
Nb/sub 3/Sn is relieved, the I/sub c/at 14 K is increased by
as much as 30% and the critical temperature is increased by up
to 1 K. The pinning force in strained and relieved wires was
determined from I/sub c/measurements in applied fields up to
10 T. We analyzed changes in the position and height of the
pinning force peak as a function of strain relief using
Kramer's theory of flux pinning. With the Kramer model, by
varying the effective strength and number of pinning sites, we
were able to describe qualitatively differences in the pinning
force curves that were obtained in the as-reacted (strained)
and bent (strain-relieved) states. |
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Strain
dependence of the critical current and critical field in
multifilamentary Nb/sub 3/Sn
composites
J.
Ekin
Summary: High-J/sub c/multifilamentary
Nb/sub 3/Sn superconductors with widely varying amounts of
prestrain and critical field values can be characterized
fairly accurately by a single normalized critical field-strain
relationship. Such a relationship permits first order
prediction of critical-current degradation at arbitrary
magnetic field magnitudes knowing only two parameters for any
conductor, the prestrain and the maximum critical field. Some
of the conductor-fabrication factors affecting the parameters
are considered. |
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Localization
of defects in multifilamentary Nb/sub 3/Sn conductors by an
inductive procedure
G.
Rupp
Summary: A procedure is described to
measure continuously and without damage the local dependence
of the critical current of multifilamentary Nb/sub 3/Sn
conductors of any length. In the superconducting state the
conductor is moved continuously through an area with flux
density B produced for example by a split coil. The magnetic
field of the induced screening currents measured outside the
conductor by Hall probes is a measure of the critical current
of the conductor. The procedure applied to multifilamentary
Nb/sub 3/Sn conductors allows to localize areas of the
conductors with critical current degradation of more than
about 10 %. The conductors can be tested before being wound
into coils. |
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Coil
performance of multifilamentary Nb/sub 3/Sn
conductors
H. Hillmann, H.
Kuckuck, E. Springer, H. Weisse, M. Wilhelm and K.
Wohlleben
Summary: The solid-state diffusion
process was used to manufacture copperstabilized Nb/sub 3/Sn
multifilament conductors. For higher currents also fully
transposed and calibrated flattened cables were fabricated.
The effects of diffusion temperature, filament diameter and
conductor geometry on the critical current density were
studied and values up to 8.6x10/sup 4/A/cm/sup 2/at 10 T were
obtained. Solenoids with graduated windings were manufactured
from long lengths (km) of these conductors by means of the
wind-and-react technique and also by using prereacted
conductors. The solenoids attained the short sample current
usually without training and reached in a free bore of 51 mm
diameter flux densities up to 14 T with NbTi background fields
around 8 T. Excitation speeds up to 1.3 T/s were obtained
without degradation of the quench current. |
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The
metallurgical design of Nb-Ti and Nb/sub 3/Sn multifilament
superconductors
D.
Larbalestier
Summary: Although the
metallurgical properties are only one of a number of important
concerns in the design of a useful multifilament magnet
conductor, they are of basic importance both to the
fabrication process and to the critical current density J/sub
c/. In this paper we discus some of the implications of making
certain metallurgical choices - for example in the Nb-Ti
system high J/sub c/may be obtained over a range of alloy
contents and for bronze route Nb/sub 3/Sn conductors a variety
of bronze to Nb ratios are chosen. Some of the consequences of
the choice are explored with respect to the basic
superconducting properties T/sub c/and H/sub c2/, the strongly
structure sensitive property J/sub c/, the fabricability and
the mechanical stability. |
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Superconducting
devices for digital systems
R.
Keyes
Summary: The performance of digital
electronic systems constructed from semiconductor components
will be limited by power dissipation, high electric field
effects, and interconnection resistance. The low voltages and
power dissipation and the vanishing resistance made available
by superconducting electronics relieve these problems. The
ability of semiconductor devices and superconductors to
satisfy the requirements of digital electronics are compared
in terms of many specific features. Advantages of the low
temperature environment that are independent of the device
employed are also described. |
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Harmony
in science: Superconductivity and high energy
physics
S.
Lorant
Summary: Thirty-one days after the
disclosure of high field superconductivity in Nb/sub 3/Sn, the
bubble chamber group at the Lawrence Berkeley Laboratory began
a program to apply this discovery to high energy physics. On
that day in 1961 a very special relationship was born which,
as subsequent events were to show, proved to be one of the
most fruitful associations in modern science. Given the
well-known high technology content and innovative approach to
problem solving associated with high energy physics, it is
hardly surprising that significant developments in applied
superconductivity took place in accelerator laboratories.
Particle physics requires a bewildering array of
technologically sophisticated equipment: from the instant when
particles are injected into the accelerator through the
acceleration process, beam extraction, separation, and
steering to the instant of collision and analysis of the
interaction products, superconducting devices play a most
important role. We examine each step in this process and not
only describe how the latest advances in superconductivity
have been applied but also discuss why these developments
necessarily took place. It is remarkable that, in spite of
considerable fiscal restraint, high energy physics is entering
a period of major construction activity. Thus if history
repeats itself we are about to witness a flood of innovations
each intended to alleviate some problem brought on by
increasingly expensive power and rising production costs, not
to mention the constant clamor for higher accelerator energies
and greater resolution of the detection
equipment. |
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The
development of standards for practical
superconductors
A. Clark, J.
Ekin, R. Radebaugh and D. Read
Summary: The
program to develop standard measurement practices for
practical superconductors includes the generation of uniform
definitions of terms, the development of standard measurement
techniques, and comparisons of these measurements using
standard reference materials. The initial sets of definitions
have been published and their development will be described.
Responses will be solicited on terms whose definitions are
still in development or are controversial. The progress in the
development of standard measurement techniques for critical
current, transient losses and critical temperature will be
discussed. The different techniques will be compared and the
experimental parameters which must be carefully controlled
will be enumerated. |
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Losses
in multifilament Nb/sub 3/Sn superconductors designed for high
B applications
G. Wagner, S.
Shen, R. Schwall, A. Petrovich and M.
Walker
Summary: In 1975 the Intermagnetics
Feneral Corporation began a comprehensive program funded by
the Air Force Materials Laboratory to establish the
manufacturing technolofy of multifilamentary Nb/sub 3/Sn
superconductors to meet the stringent set of design
specifications: (1) 200 to 500 amperes critical current as
-wound on a one inch bend diamerer at 7 T and 8 K, (2)
sufficient conductor current density to assure an overall
winding current density of 1x10/sup 4/ amps/cm/sup 2/ at 7 T
and 8 k, (3) sufficient copper for stable performance under
the above conditions, and (4) conductor losses which are low
enough during field ramps at rates as high as 10 T per second
to insure a less than 8 K rise in winding
temperature. |
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Interaction
of transport current and transient external field in composite
conductors
S. Shen and R.
Schwall
Summary: Investigations of transient
field losses in superconducting composites carrying transport
current are presented. The magnetization and terminal voltage
of a variety of composites have been measured as a function of
transport current and external field. The losses are analyzed
as a sum of magnetization losses and those due to dynamic
resistivity. Results are presented for slowly changing
external fields where the magnetization losses are purely
hysteretic and for higher /spl dot/B where coupling losses are
important. |
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An
experiment clarifying the distribution of transport currents
in superconducting wires
T.
Ogasawara, K. Yasukochi, Y. Takahashi, K. Yasohama and Y.
Kubota
Summary: The distribution of the
transport current in a twisted multifilament superconductor is
studied experimentally and theoretically for a sample
configuration that simulates the windings of superconducting
magnets. Theoretical analysis shows that the terminal voltage
during a change of the current is directly related to the
current distribution inside the composite conductor. A
strongly non-uniform distribution due to the self-field effect
has been found when the transport current is changed in a
steady field. A time-varying transverse field tends to smear
out this non-uniformity. The experimental results are in good
agreement with calculations except for the high current region
where the appearance of an anomalous resistive state peculiar
to the filamented conductor must be taken into account.
Time-varying external fields have been used to improve
substantially the field stability of a small superconducting
solenoid operating in the persistent mode. |
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AC
loss from the combined action of transport current and applied
field
W. Carr
Jr.
Summary: The ac loss in a superconductor
resulting from the combined action of an alternating transport
current and an in-phase ac transverse field is calculated for
a slab, and normalized in a form which can be applied
approximately to a circular wire. Some results for limiting
cases are also directly calculated for a wire. |
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Magnetic
field dependence of loss-frequency characteristics of
multi-filamentary
superconductors
F. Irie, F.
Sumiyoshi and K. Yoshida
Summary: AC losses
of multi-filamentary superconductors for an ac magnetic field
with a large amplitude, where a strong nonlinearity between
the applied field and the penetrated flux into a
superconducting filament become a problem, have been studied
theoretically. The present theory, based on an effective
complex permeability expressing the hysteresis property of
superconducting filaments, is shown to explain quantitatively
observed magnetic field dependences of loss-frequency
characteristics in Nb-Ti multi-filamentary superconductors. It
is pointed out that due to the nonlinearity the shape of the
frequency characteristics of an eddy current loss is markedly
shifted and deformed from the Lorentz type shown by
Carr. |
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Losses
in superconducting composites under high rate pulsed
transverse field
J. Soubeyrand
and B. Turck
Summary: In case of a plasma
current disruption in a tokomak the conductor of the toroidal
field coil may be subjected to a field varying exponentially
with a time constant as low as a few milliseconds. Expressions
of losses produced by the transverse component of the changing
field can be written in the form W = /spl Delta/ B/sup 2//spl
theta/ / 2 /spl mu//sub 0(/spl theta+/spl tau/) Where /spl
theta/ is the time constant of the coupling currents in the
superconducting multifilament composite and /spl tau/ the time
constant of the magnetic field. The value of the time constant
/spl theta/ evaluated in other theories is discussed For small
/spl tau/, the outer layers of filaments saturate leading to
some decrease of the coupling losses together with the
appearance of hysteretic losses in these layers. Both losses
are calculated for this case. Measured losses are within 20%
of the theoretical predictions, in both two and three
component composites. There is a noticeable decrease of losses
in rectangular composites when the field is parallel to the
broad face of the conductor and the presence of a transport
current does not lead to a significant increase of overall
losses. |
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Losses
in coils for any space and time-variations of electromagnetic
conditions
A. Fevrier, J.
Peninou, J. Renard and J. Maldy
Summary:
Using a circuit model, loss calculations have been
carried out for coils made of twisted filamentary composites
which undergo any space and time-variations of the magnetic
induction and of the transport current. This model has been
tested by measuring losses in solenoids and in "special coils"
which look like small Tokomaks, where the composite which
carries a time-dependent current is submitted to simultaneous
transverse and longitudinal magnetic induction changes.
Experimental and theoretical values are in good agreement
within about 5% to 15% for hysteretic losses and within 10% to
20% for eddy current losses. |
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Losses
in Nb Ti multifilamentary composites subjected to a rotating
magnetic induction
A. Fevrier and
J. Renard
Summary: The purpose of the present
investigation is to study losses in a superconducting
multifilamentary composite submitted to a rotating magnetic
induction. We have designed an experimental apparatus in which
loss measurements are made by measuring the retarding torque
undergone by the sample when it rotates in a magnetic
induction. This torque is proportional to the losses which
take place in the sample. The experimental apparatus
essentially consists of a superconducting split coil
arrangement in which the sample is rotated through a rotating
support fitted out with a strain gauges torque cell. This
equipment may be rotated in the 2.5 to 230 r.p.m. range in a
magnetic induction up to 6 T, which is equivalent to time
dependent magnetic induction up to 145 T/s. Measurements of
losses have been made in Nb Ti multifilamentary composites
which differ in the nature of the matrix, the twist pitch
length, the diameter and the number of filaments. In the low
values range of the product of the angular velocity by the
magnetic induction, experimental results are analyzed with
respect to present loss theories. Calculated values of losses
are very close to experimental ones within 15%. |
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AC
losses in multifilamentary Nb/sub 3/Sn
conductors
J. Charlesworth and P.
Sikora
Summary: We have studied losses at
4.2K in a number of multifilamentary Nb and Nb/sub 3/Sn wires
fabricated by the bronze route. Losses due to alternating
current and to transverse alternating field showed a fourth
power dependence on current or field which is incompatible
with the critical state model, even when the observed field
dependence of the bulk critical-current density is taken into
account. In order to reveal the source of the low losses in
our materials more clearly, we have studied losses with the
field parallel to the filament surface. As expected, the
results show that losses in Nb filaments are low because of
the presence of the Meissner state. However, superficially
similar results for Nb/sub 3/Sn filaments cannot be
interpreted as evidence for an anomalous lower critical field;
a more likely explanation is the presence of a surface
current. Although a simple surface current model cannot
account in detail for the transverse field losses, one
incorporating losses due to surface roughness, field
orientation effects and the field dependence of the surface
current can probably provide an adequate
description. |
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Effect
of trapped magnetic flux on ac losses of Nb/sub
3/Sn
J. Bussiere and J.
Clem
Summary: The effect of trapped magnetic
flux on 60 Hz losses of Nb/sub 3/Sn was investigated for a
number of samples prepared by solid state diffusion and by
reacting niobium with liquid tin. In the presence of trapped
flux all samples showed a significant increase of loss at low
current densities (e.g. from /spl sim/1 to 10/spl mu/W/cm/sup
2/at 500 rms A/cm) and a modest decrease of loss at higher
current densities (e.g. 30% at 1000 rms A/cm). These features
are shown to be consistent with the critical state model
provided one takes into account the field dependence of
surface currents and assumes a considerable decrease of their
magnitude with trapped flux. Even with large amounts of
trapped flux the losses of most samples remain below 10/spl
mu/W/cm/sup 2/at 500 rms A/cm, an acceptable loss level for ac
power transmission applications. |
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SQUIDs--Past,
present, and future
A.
Silver
Summary: The superconducting quantum
interference device (SQUID) was the first superconducting
electronic (SCE) circuit employing Josephson junctions. It has
matured over approximately 15 years and is now the most widely
used SCE device. Although its application is based on a
fundamental periodic response, it has been developed into the
most sensitive linear detection system for magnetic flux. As a
result, SQUIDs are used for such diverse studies as flux
quantization, superconducting properties, thermal noise and
intrinsic fluctuations, geophysics, metrology, biomagnetism,
susceptometry, nuclear magnetic resonance, oceanography,
gravity waves, and relativity. Further improvement in
performance is predicted. Additionally, micro-SQUIDs
(configured in microcircuits) have a bright future as
"ultimate" devices for parametric amplifiers, pulse counters,
A/D converters, shift registers, digital logic, and memory.
This paper reviews the history of this field, emphasizing
recent developments, and projects the future direction of
SQUID electronics. |
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Low
frequency impedance and noise properties of an RF biased
resistive SQUID
R. Giffard, P.
Michelson and R. Soulen Jr.
Summary: A noise
thermometer consisting of a nonhysteretic resistive SQUID has
been used for unusually precise measurements of the low
frequency Josephson linewidth of a heavily shunted
point-contact. The results agree with theories appropriate for
weak links, and support the contention that the pair
fluctuation noise calculated for tunnel junctions is not
present in point-contacts. |
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Limiting
response times in a granular niobium weak
link
R. Peters, T. Refai, S. Wolf
and F. Rachford
Summary: We have measured rf
SQUID characteristics of a granular niobium weak link as a
function of frequency from 20 MHz to 9 GHz. A degredation in
the SQUID performance and a critical current enhancement is
correlated with the a non-equilibrium site decay time as
inferred from the frequency dependence of the critical current
hysteresis. At low frequencies, the electromagnetic response
time extracted from the frequency dependent SQUID modulation
is found to be of the same order of magnitude as the
quasiparticle inelastic scattering time. The electromagnetic
response time is found to decrease at temperatures and
frequencies where the critical currents are enhanced. The
large (an order of magnitude) critical current enhancement
seems to be associated with the granular nature of the weak
link. |
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Microwave
properties of variable thin-film Josephson
bridges
V. Gubankov, V.
Koshelets, G. Ovsyannikov and A. Vystavkin
Summary:
Amplification, down-conversion and detection of the
microwaves in thin-film Josephson variable thickness bridges
of submicron dimensions have been investigated experimentally.
The results for indium and tin bridges are discussed taking
into account the influence of the nonequilibrium electrons in
a weak link region. The same experiments were carried out with
SNS variable thickness Nb-Al-Nb bridges. They showed that the
properties of SNS bridges agreed better with the theoretical
predictions of the resistively shunted junction model in a
wide temperature range. Moreover the parameters of the
Nb-Al-Nb bridges have possessed better stability and
reliability. |
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Superconductive
tunneling devices as millimeter wave photon
detectors
J. Tucker and M.
Millea
Summary: A quantum generalization of
microwave mixer theory predicts that superconductive tunneling
devices may be employed as ultrasensitive low noise photon
detectors at millimeter wave frequencies. |
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A
calculation of interaction between two Josephson
junctions
G. Deminova and A.
Kovalenko
Summary: An interaction between two
Josephson junctions separated by a small superconducting
granule is theoretically investigated on the base of an
approximate solution of the nonlinear Ginzburg-Landau
equation. Voltage-current curves of the junctions are
calculated, boundaries of the region of junction lock-in are
found. |
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Observation
of voltage fluctuations in a superconducting magnet during MHD
power generation
R. Smith, R.
Niemann, M. Kraimer and T. Zinneman
Summary:
Fluctuating voltage signals on the potential taps of
the ANL 5.0 T MHD Superconducting Dipole Magnet have been
observed during MHD power generation at the U-25B Facility at
the High Temperature Institute (IVTAN) Moscow, USSR. Various
other thermodynamic and electrical parameters of the U-25B
flow train have been recorded, and statistical analysis
concerning correlations between the phenomena with a view of
discerning causal interdependence is in progress. Voltage
fluctuations observed at the magnet terminals are analyzed
with special emphasis on magnet stability. |
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Design
and operating experience of the cryogenic system of the U. S.
SCMS as incorporated into the bypass loop of the U-25 MHD
generator facility
R. Niemann, K.
Mataya, D. McWilliams, R. Borden, M. Streeter, R. Wickson, P.
Smelser and N. Privalov
Summary: The design
features and accumulated operating experience, from a
cryogenics point of view, of the United States Superconducting
Magnet System (U.S. SCMS) are presented. The principal
cryogenic system design parameters are enumerated. Details of
the cryogenic aspects of magnet system commissioning, standby
mode, and operation with MHD generators are discussed.
Included are system operation, problems encountered and
corrective actions taken, and measured operating parameters
which include liquid helium boiloff, cryostat pressure and
level versus time, etc. The aspects of the transition between
operation in the laboratory and in an MHD plant are
elaborated. |
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A
superconducting dipole magnet for the UTSI MHD
facility
S.-T. Wang, R. Niemann,
L. Turner, L. Genens, W. Pelczarski, J. Gonczy, J. Hoffmann,
Y.-C. Huang, N. Modjeski and E. Kraft
Summary:
The Argonne National Laboratory is designing and will
build a large superconducting dipole magnet system for use in
the Coal Fired Flow MHD Research Facility at the University of
Tennessee Space Institute (UTSI). Presented in detail are the
conceptual design of the magnet geometry, conductor design,
cryostability evaluation, magnetic pressure computation,
structural design, cryostat design, and the cryogenics system
design. |
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Fabrication
and assembly considerations for a base load MHD
superconducting magnet system
R.
Thome, R. Pillsbury, J. Ayers and T. Hrycaj
Summary:
The development of a Base Load Magnetohydrodynamic
(MHD) Power Generation System will necessitate the evolution
of fabrication, transportation, and on-site assembly
techniques which are new relative to present superconducting
magnets. The magnet considered in this paper consists of an
assembly of two rectangular saddle coils and six racetrack
coils, has a stored energy of 6.7 GJ, has overall dimensions
of 23.1 X 6.4 X 5.8 m, requires 86.7 km of conductor, and
weighs 775,000 kg. The coil external support structure adds
another 1,106,000 kg. The largest single component, a single
saddle coil with sufficient support structure for shipment,
has overall dimensions of 23.3 X 6.2 X 4.0 m and weighs
437,000 kg. The shear size and weight of such components
present many problems in shipment; however, items of
comparable size have been transported and are discussed. This
paper gives a description of the reference design and presents
possible techniques for coil fabrication, component shipment,
and on-site assembly of the superconducting
magnet. |
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Practical
aspects of designing and manufacturing MHD superconducting
base-load magnets in 1988 time
frame
S. Ackerman, R. Randall, E.
Rapperport and C. Roye
Summary: This paper
summarizes a DOE/MIT-FBNML-sponsored study of manufacturing
alternatives and producibility of the "cold structure" design
for three Base-Load MHD Magnet configurations. The study team
consisted of General Dynamics Convair Division (GD), Magnetic
Engineering Associates, and Supercon. Two reference designs
previously created by AVCO, designated as the BL6-P1 Circular
Saddle and the BL6-P2 Rectangular Saddle HD Magnets, were
selected for study. The third configuration, identified as a
CASK Circular Saddle design, is an innovative concept proposed
by the investigators. The three designs have approximately the
same magnet field profiles, active lengths, and winding
current. Fundamental differences include the winding geometry,
and the substructure and superstructure to contain the
windings and resist the Lorentz forces. These design
differences result in differing use of materials, construction
techniques, and assembly procedures. The studies included
selection and fabrication of the superconductor/stabilizer,
the conductor substructure, turn-to-turn and layer-to-layer
insulation, winding approach, and fabrication and assembly of
the superstructure. Manufacturing practicality and projected
costs were the discriminators for tradeoff considerations.
Proposed construction and design alterations permit major
manufacturing operations at a central facility. Remaining
manufacturing/assembly tasks were simplified to allow final
assembly operations on-site. Only known or anticipated (within
five years) manufacturing techniques or facilities were
recommended. Transportation methods and problems were analyzed
and solutions were found for each case. Study results show: 1)
that of the two AVCO designs, the circular saddle
configuration is a more cost-effective design that lends
itself to easier fabrication and on-site assembly, 2) the CASK
Base-Load configuration overcomes many of the manufacturing
problems inherent in the two AVCO designs. |
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Design
criteria for multilayer superconductive
magnets
M.E. Derini, M. Hilal and
R. Boom
Summary: High current round composite
conductors with superconductors near the surface are under
development at the University of Wisconsin. The conductors are
designed for single layer energy storage solenoids and
possibly for solenoids with several layers. The radial and
axial forces in such magnets are obtained by summing forces
between turns. Solenoids with constant tension are achieved by
changing the spacing between conductors in the axial
direction. Multilayer solenoids are designed so that the
tension in the different layers is the same as the required
design value. This design value is chosen to make the
conductors remain in tension. Constant tension designs
facilitate the economic use of force-bearing structure in
energy storage and fusion superconductive
magnets. |
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Superconducting
energy storage magnets
M. Masuda,
T. Shintomi, H. Sato and A. Kabe
Summary: The
energy storage making use of a long time constant of
superconduction coil has been studied as the energy storage
for peak shaving of electric power and power supplies for
fusion reactors and accelerators. The general purpose of the
energy storage is to level a pulsed load. Among many methods
of energy storages this superconducting energy storage has
been considered as the most promising for this purpose. The
related technical problems yet are remained unsolved. One of
the problems is ac less of the superconduction coil come from
its pulsed operataion. The superconductor are severer than
normal conductors. Manyu studies have been done to develop the
superconductors which have small ac losses. |
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Stability
of a force-cooled superconducting magnet made from hollow
conductor
K. Agatsuma, K. Komuro,
K. Koyama and I. Todoriki
Summary: A hollow
superconductor test magnet cooled by forced-flow supercritical
helium has been tested. The steady state stability of this
magnet has been studied analytically in order to get a basis
for design of a big superconducting magnet. The results
suggest that the magnet would be quite stable if the transport
current is held constant below a certain value given by the
simple equation we proposed. |
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Heat
pulses required to quench a potted superconducting
magnet
M.
Superczynski
Summary: The Navy is presently
considering the use of fully potted Niobium Titanium
superconducting magnets for the field windings of electric
motors and generators. These magnets will operate at fields of
6.5 tesla and will be subjected to shipboard shock and
vibration. This shock, vibration or other mechanical
perturbation can result in energy being imparted to the
superconductor within the magnet. If this energy is
sufficiently large, the temperature will rise locally driving
the conductor normal. If the normal zone is larger than the
minimum propagating zone, thermal runaway will occur and a
magnet quench will result. A potted superconducting magnet was
constructed to determine the amount of energy input required
to produce a quench. The magnet was wound from multifilament
Nb-Ti conductor, reenforced with fiberglass cloth, and vacuum
impregnated with epoxy resin. Several heaters were embedded in
the winding and the energy required to drive the magnet normal
was measured at various magnetic fields and current levels.
Energy pulse widths were varied over a wide range to examine
the effects of thermal diffusion and cover a broad spectrum of
possible energy inputs. The results are compared with magnet
operating characteristics and general design limits are
discussed. |
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Analytic
solution for the propagation velocity in superconducting
composites
L.
Dresner
Summary: The propagation velocity of
normal zones in composite superconductors has been calculated
analytically for the case of constant thermophysical
properties, including the effects of current sharing. The
solution is compared with that of a more elementary theory in
which current sharing is neglected, i.e., in which there is a
sharp transition from the superconducting to the normal state.
The solution is also compared with experiment. This comparison
demonstrates the important influence of transient heat
transfer on the propagation velocity. |
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The
initiation and propagation of normal zones in a force-cooled
tubular superconductor
J.
Hoffer
Summary: A numerical analysis has been
performed on the time-dependent equations of heat balance, gas
convection, pressure drop, and mass-flow rate for
supercritical helium gas flowing through a tubular
superconductor. Three dimensional graphs of wall temperature,
gas temperature, and mass-flow rate as functions of position
and time are used to show the evolution of normal zones. In
contrast to other methods of studying stability in
superconductors by a quasi-steady-state analysis of
critical-sized normal zones (minimum propagating zone), our
analysis shows that stability is influenced by both the
magnitude and the time dependence of the disturbance. As the
current is increased in a system subjected to certain types of
thermal disturbances, propagating normal zones may originate
at positions well downstream from the site of the disturbance.
At higher currents, propagating zones may originate both
downstream and at the disturbed site, coalescing into a large
propagating normal zone. With certain types of disturbances
(such as an extraneous heat source over a short length of
conductor), higher critical currents may be reached by fast
current ramping, while with other types of disturbances (such
as self heating in a degraded section of conductor), slow
current ramping leads to higher critical
currents. |
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Conceptual
design of a hollow cable conductor for the large coil
program
J. Chi
Summary:
A conceptual design of a cryostabilized conductor was
developed for the Large Coil Program (LCP) that met all of the
stringent design requirements and constraints. The reference
conductor was selected following parametric analyses involving
the number of strands, the helium (void) fraction and the
copper to noncopper ratio. The number of strands considered
took into account the type of cabling operations that are
possible and the need to provide complete transposition of the
strands to minimize ac losses. The helium (void) fraction
affects the pressure drop and helium pump work required, while
the copper to non-copper ratio affects the helium flowrate
required to meet the stability criterion. The reference
conductor selected consisted of 567 strands of
multi-filamentary superconductors with a copper to non-copper
ratio of 1.92. The stability of the reference conductor
selected was verified by transient recovery analyses using the
TAP-B code. There are a number of uncertainties that affect
the stability of the conductor. They include the heat transfer
and helium flow phenomena within the interstices of the
twisted strands. These uncertainties as well as the heat
transfer and friction factor correlations used in the design
and the various assumptions made are discussed in the paper.
Areas where additional work are needed are
recommended. |
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Superfluid
helium for stabilizing superconductors against local
disturbances
G. Claudet, C.
Meuris, J. Parain and B. Turck
Summary: Heat
can be easily removed from a solid conductor immersed in a
superfluid helium bath at atmospheric pressure, because of its
very large thermal conductivity and a noticeable heat
capacity, similar to that of saturated helium I. As a result,
superconducting composites can withstand large thermal
disturbances without a generalized quench. A series of
non-inductive coils cooled by superfluid helium channels have
been tested to determine if a normal zone created by a small
heater recovers. After the energy is introduced in a few
milliseconds, the development of the normal zone versus time
is measured as a function of different important parameters
including: transport current, external field, initial energy,
thickness of helium channels, size of the spacers. Curves of
critical energies, maximum energies for which a recovery to
the superconducting state is possible, are given. Our results
show the great value of superfluid helium for stabilizing
superconductors submitted to local thermal
disturbances. |
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Asymmetry
of normal zone propagation velocity with respect to current in
a multifilamentary composite
superconductor
R. Bartlett, R.
Carlson and W. Overton Jr.
Summary: We have
observed the phenomenon of destruction of superconductivity by
current in a high current multifilamentary composite
superconductor. This bronze stabilized conductor had
rectangular dimensions of 0.56 cm X 0.17 cm, contained 67,507
Nb/sub 3/Sn filaments of about 5.8-/spl mu/m diam and had a
self-field critical current at 0 K of about 6.1 kA. The
velocity of normal zone propagation was measured on a
56-cm-long sample of this conductor in a vacuum environment.
Currents ranged from 200 A to 900 A and initial sample
temperatures ranged from 14.5 K to 16 K. We observed a
significant dependence of the velocity on the direction of the
current. Because there is essentially zero heat transfer into
a surrounding environment, the effect must relate to an
asymmetry of the fundamental interactions between current and
heat flow in the filaments. |
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Transient
resistive zones in NbTi
composites
M. Sinclair and Y.
Iwasa
Summary: Transient properties of quench
and recovery for long sections of composite superconducting
magnet windings are investigated. Parameters determined
experimentally are used in a theoretical model to predict
voltage vs. time to recover from a temperature perturbation.
The theoretical prediction agrees well with experimental
simulations. |
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Measurements
of stability of cabled superconductors cooled by flowing
supercritical helium
J. Miller,
J. Lue, S. Shen and J. Lottin
Summary: The
concept of stability in superconductors cooled by forced flow
of supercritical helium is somewhat different from conductors
cooled by pool-boiling helium. The crucial point is whether
such a conductor can recover from a large deposition of energy
before the cryogen is heated to a level that prohibits
recovery. In an investigation of stability of forced flow
conductors it is usually necessary to use indirect methods of
heating the conductor initially, such as by an external pulse
coil. However, in using such indirect methods it is essential
to determine accurately the time development and magnitude of
energy deposition. We use ac loss techniques to examine pulse
coil heating and compare those results with extensive
measurements on a specially constructed sample containing an
embedded heater. |
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An
experimental and theoretical study of the effect of local
heating on composite
superconductors
R.
Schermer
Summary: Experiments have been
performed where a short section of long, stabilized, NbTi
conductor was driven normal and the voltage was recorded as a
function of position and time. Conductors tested included
samples with internal cooling channels or electrically
insulating coatings. Heater power or pulse energy, conductor
current, and background magnetic field were varied. The data
show a number of interesting effects not previously noted in
experiments where the entire conductor length was heated.
These effects are discussed, as are the detailed comparisons
of the steady-state results with the predictions of numerical
computations. In general, the computation accurately predicts
the power required to form a normal zone as a function of
field and current. Both experiment and theory show that the
temperature interval bracketing the nucleate-to-film-boiling
discontinuity in the heat transfer coefficient can be
traversed smoothly without any corresponding discontinuity in
the curve of maximum wire temperature vs heater power. The
normal zone is hotter and longer than predicted, however,
necessitating modifications to the previously published
results for apparent heat transfer in the film boiling
regime. |
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Cryogenic
stability of composite conductors taking into account
transient heat transfer
W. Nick,
H. Krauth and G. Ries
Summary: To predict
stability behaviour of various conductors under different
cooling conditions, a computer program was developed to solve
the one-dimensional heat diffusion equation along the
conductor using temperature dependent material parameters.
Transient heat transfer is included basing on results of
earlier heat transfer measurements. To check the validity of
these calculations experiments were performed using a copper
stabilized multifilamentary NbTi-superconductor under pool
boiling conditions. The normal transition was initiated by an
ohmic heater. The minimum energy needed to induce quenching as
well as the propagation velocity of the normal zone are then
compared with numerical results and show good
agreement. |
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Measurements
of traveling transition zone along a
superconductor
J. Lottin, J.
Miller, J. Lue and L. Dresner
Summary: The
spatial variation of the temperature in a traveling
superconducting-normal transition zone can provide valuable
information on transient heat transfer in realistic coil
conditions. In this paper, temperature wave curves are
presented for several transport currents corresponding to
propagation and recovery in a high magnetic field. The
temperature profiles have been constructed in the form T = T
(x + vt) by measuring the wave velocity and the temperature at
one point as a function of time. The temperature is measured
directly with differential thermocouples; simultaneous voltage
measurements provide a continuous temperature reference in the
current sharing region. Information is given about
thermocouple calibration in the high magnetic field and about
the temperature and magnetic field dependence of the thermal
conductivity of the sample. |
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Analysis
of propagation of normal zones in immersion cooled composite
superconductors
O. Tsukamoto and
F. Miyagi
Summary: Propagation velocities of
normal zones in immersion cooled composite superconductors are
analyzed. Normal zones of conductors are wrapped with helium
vapour when the heat transfer of liquid helium goes into the
film boiling region. The analysis takes into account the
latent heat necessary for the vapour film zone to spread or
contract. |
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Recovery
of a cryostable magnet following a mechanical perturbation:
One-dimensional and two-dimensional
calculations
L. Turner, S.-T.
Wang and J. Harrang
Summary: When a
cryostable composite conductor carrying current experiences a
heat input from a mechanical perturbation, a normal region
develops which initially propagates and then either collapses
or continues to propagate. A computer model has been devised
to study this phenomenon. The model incorporates initial or
continuing heat input from mechanical perturbations, heat
conducted to the neighboring elements of the conductor and, if
appropriate, heat conducted through insulation to neighboring
turns. Heat is transferred to the helium coolant according to
a specified heat transfer coefficient. If the element of
conductor is in a normal or current-sharing state, resistive
heating also occurs. The (unstable) equilibrium state of heat
generation and conduction has been studied; results agree with
those of a static calculation. The model has been validated
against experimental measurements of response to heat pulses.
A study of the trade-off between cross-sectional areas and
wetted perimeter of a conductor is included, with recovery
current presented as a function of perturbing
energy. |
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Stability
simulations of cryostable superconductors under transient
mechanical perturbations
J.
Harrang, S.-T. Wang, Y.-C. Huang, L. Turner and J.
Dawson
Summary: An experimental technique has
been established to simulate the transient mechanical
perturbation and to measure the minimum propagating current of
a cryostable superconducting coil. The experimental setup is
described. Typical waveforms of potential profiles across the
normal zone and temperature profiles of the normal zone for
both the propagating and recovery conditions are shown. Also
presented are the measured recovery currents versus the
perturbation energy under various external magnetic field
strengths, heater lengths and heating durations. These data
will be discussed and compared with data computed from a
theoretical model. |
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Stability
of the superconductive operating mode in high current-density
devices
S.
Wipf
Summary: The superconductive operating
mode represents a thermal equilibrium that can tolerate a
certain amount of disturbance before it is lost. The basin of
attraction (BOA), in many ways equivalent to a potential well,
is a measure of the size of disturbance needed to lift the
device from the superconductive into a resistive operating
mode. The BOA for a simple geometry is calculated and
discussed. Experimental results are reported, showing how the
concept is used to gain information on the disturbances
occurring in a superconducting device. |
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Volt-ampere
characteristics of composite superconductors under AC
conditions
V. Altov, N. Kulysov
and V. Sytchev
Summary: The behavior of
composite superconductors under AC conditions attracts
widespread attention in connection with recent developments of
a wide range of AC superconducting devices. Particular
emphasis was mainly ced upon investigations of dissipation
processes under a.c.condition where a superconductor
completely retained its superconducting
properties. |
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Multifilamentary
V/sub 3/Ga wire: A critical
review
D. Gubser, T. Francavilla,
D. Howe, R. Muessner and F. Ormand
Summary:
Initial studies of V/sub 3/Ga conductors formed by the
bronze technique coincided with early work on Nb/sub 3/Sn
conductors; however, due to several technological
difficulties, emphasis to date has been on commercial
production of Nb/sub 3/Sn composites. The superior critical
current density of V/sub 3/Ga in high magnetic fields (H>
10T), however, favor this conductor in selected applications.
Improvements in critical current density have been achieved by
using a modified bronze technique whereby the filaments as
well as the Cu matrix are alloyed with Ga. Recent studies of
the temperature and magnetic field dependence of the critical
current density as well as stress effects, and third element
additions are presented. Superconducting applications which
may benefit from the use of V/sub 3/Ga wire will be
emphasized. |
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Composite-processed
V/sub 3/Ga with improved current-carrying capacities in high
magnetic fields
K. Tachikawa, Y.
Tanaka, Y. Yoshida, T. Asano and Y. Iwasa
Summary:
Effects of the simultaneous addition of gallium to the
core and magnesium to the matrix on superconducting properties
and structures of the composite-processed V/sub 3/Ga have been
studied. Single core composites with V-(0-6) at.%Ga alloy core
and Cu-19at.%Ga-(0-0.8)at.%Mg alloy matrix were fabricated
into tapes and heat treated. The simultaneous addition of
gallium and magnesium significantly increases the J/sub c/of
V/sub 3/Ga. Furthermore, the addition of gallium and that of
magnesium increase H/sub c2/(4.2K) by 1-2 T. The J/sub c/of
over 1x10/sup 5/A/cm/sup 2/is achieved at 20 T even for thick
V/sub 3/Ga layers of 5-10 /spl mu/m. The present result
demonstrates the possibility of making a 20 T superconducting
magnet with the stable multifilamentary type V/sub 3/Ga
conductor. |
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Anisotropy
of the critical current in solid solution superconductor
NbTi
K. Best, D. Genevey, H.
Hillmann, L. Krempasky, M. Polak and B.
Turck
Summary: The critical current densities
depend on density, size and distribution of pinning sites.
Changing the round diameter of a superconductor causes an
isotropic change of their distribution. By this an isotropic
change of current densities is caused. Anisotropic change of
the pinning site distribution by flattening causes an
anisotropy of the current density. In the case of the
induction perpendicular to the flat area a decrease of the
critical current density is expected. In the case of the
induction parallel to flat area an increase of j/sub c/is
expected. Monocore NbTi conductors with different densities of
/spl alpha/-Ti precipitates have been flattened with different
aspect ratios and critical current densities for both cases B
\parallel F and B /spl perp/ F have been measured with the
following results: The anisotropy factor j/sub c\parallel/ /
j/sub c/spl perp// depends on the critical current density of
the original round shaped conductor and on the aspect ratio
for high critical current densities it is independent of B.
Decreasing density of pinning sites, which means decreasing
current density causes dependence of the anisotropy factor on
the induction B. |
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Development
of cabled and soldered superconductors with low losses for
nuclear fusion research
magnets
K. Kwasnitza and I.
Horvath
Summary: Three-stage prototype high
current superconducting cables are developed, as they are
needed for the LCP fusion project. Measurements of the pulsed
field losses show the effectiveness of the applied high
resistivity CuNi foils in the cable to reduce the cable matrix
losses. |
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Generalized
critical current density of commercial Nb46.5, Nb50 and Nb53
w/o Ti multifilamentary
superconductors
K. Hwang and D.
Larbalestier
Summary: We have measured the
critical current density J/sub c/of four different commercial
compositions of NbTi over the range of fields up to H/sub
c2/and at reduced temperatures. We have presented the results
in scaled form so that attainable values of J/sub c/in NbTi at
arbitrary fields and temperatures may easily be derived. We
have also measured the effect of post manufacture thermal
treatments on J/sub c/and shown that the change in J/sub
c/scales with a temperature compensated time
factor. |
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Critical
current density in wire drawn and hydrostatically extruded
Nb-Ti superconductors
S.
Alterovitz, J. Woollam and E. Collings
Summary:
Critical current studies have been made on copper-clad
Nb-Ti composite wire prepared under area reductions of 10/sup
2/:1 and 10/sup 4/:1 by hydrostatic extrusion (HE), wire
drawing and HE plus drawing, in a comparative evaluation of
the thermomechanical processing equivalent of
HE. |
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Development
of multifilamentary Nb/sub 3/Sn conductor for fusion
research
S. Shimamoto, K.
Yasukochi, T. Ando, N. Tada, K. Aihara and Y.
Hotta
Summary: A 10kA/13T Nb/sub 3/Sn
conductor, which is intended for use in fusion magnets, has
been developed. The conductor consists of multifilamentary
Nb/sub 3/Sn conductor monoliths with a copper stabilizer and
stainless steel reinforcement monoliths laminated with copper.
The conductor design is based on the results of experiments
with model conductors (1kA/10T and 0.1kA/7T), which show that
a conductor can be reinforced effectively and that
reinforcement can reduce eddy current losses in a
conductor. |
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A
DC-powered Josephson flip-flop
A.
Hebard, S. Pei, L. Dunkleberger and T.
Fulton
Summary: A novel dc-powered flip-flop
logic and/or memory element utilizing two Josephson tunneling
gates has been designed and tested. Circuit resistances R,
critical currents I/sub c/, and fanout inductances L are
chosen so that the gates operate individually in the latching
current-steering mode. However, the gates G1 and G2 are
interconnected in such a way that if, say, G1 is at V/spl ne/0
and G2 is at V=0, a switching of G2 to V/spl ne/0 returns G1
to the V=0 state. The fanout current redistribution, which
accompanies this switching event, occurs with a time constant
of about L/R. Switching back to the initial state is a
symmetric process. Tolerances on circuit parameter values for
proper operation are reasonably wide. |
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Damped
three-junction interferometers for latching
logic
L. Geppert, J. Greiner, D.
Herrell and S. Klepner
Summary: This paper
describes the design, fabrication and characterization of
damped three-junction interferometers suitable for LSI
latching logic. The high speed and low power dissipation of
Josephson interferometers make them ideal devices for logic
applications. However, their inherent high Q gives rise to
resonances which appear as current steps in the quasiparticle
curve. If not controlled, these current steps can interfere
with the proper operation of logic circuits. Studies have
shown that these resonances can be effectively damped by
shunting the interferometer inductance with a small
resistance. In this work the interferometers were made by
placing the interferometer loops parallel to the groundplane
and clustering the Josephson junctions together. This
minimized the loop inductance associated with the
counterelectrode which in turn meant that adequate damping
could be achieved with metallurgically compatible short
resistors connected only to the base electrode. The resultant
structure with two overlying control lines each 2.5/spl mu/m
wide had an area of 0.008mm/sup 2/. The Josephson junctions
were 5/spl mu/m in diameter. The measured damped resonance
characteristics and threshold characteristics (I/sub g/(V/sub
g/) and I/sub m/(I/sub c/) respectively) of these structures
agreed very well with theory. |
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Josephson
edge detector, a novel switching
element
S. Faris and A.
Davidson
Summary: A uperconductive switching
device called a Josephson Edge Detector (JED) is descrived
which uses an interferometer capable of storing several flux
quanta. Unlikeconventional logic interferometers, the new
device does not work by switching between loves and windows of
a threshold curve: rather it takes advantage of extrememly
asymmetric critical points beneath the threshold curve. Thus,
the JED is sensitive to the sign of the time derivative of an
input signal. This is an attractive feature for several
applications. |
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Josephson
junction with lateral injection as a vortex
transistor
K. Likharev, V.
Semenov, O. Snigirev and B. Todorov
Summary:
Long narrow Josephson junctions with current injection
to their long (lateral) sides are examined theoretically.
Injection into a finite number of points as well as
distributed injection are considered. The external magnetic
field effect on the critical current and on the I-V curves is
calculated. It is shown that the junction with current
injection into many points in parallel is an almost complete
analog of a conventional semiconductor transistor, the role of
electric charge carriers being played by Josephson vortices
carrying a single flux quanta. The use of a vortex transistor
as a circuit element of both analog and digital devices is
discussed. |
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Dynamics
of an asymmetric nondestructive read out memory
cell
H. Beha
Summary:
In single flux quantum interferometer memory cells the
information can be stored without bias current, if the
interferometer inductance L is chosen sufficiently large (/spl
lambda/ = 2/spl pi/LI/sub 0///spl Phi//sub 0/ = 2/spl pi/) to
allow for three stable states (-1-, 0-, 1-mode) at zero
control and gate current. In this case the binary informations
are represented by the -1- and 0-mode of the interferometer.
The nondestructive read out operation of a binary "1" is
performed without a transition into another mode or into the
voltage state. The binary "1" corresponds to no sense signal.
Nondestructive read out of the binary "0" is achieved in
switching to the voltage state and in returning to the 0-mode
after the end of the drive currents. In this paper the
dynamics of switching back into the zero voltage state are
investigated by simulations of trajectories in the order
parameter phase plane. It is shown that for realistic
fabrication tolerances the interferometer settles in the
wanted 0-mode and not in the -1- or 1-mode even under severe
gate or control current disturbs, if the maximum Josephson
currents of the two Josephson junctions are unequal (2:1) and
if the McCumber damping factor is sufficiently large (/spl
beta/ /spl approx/ 100). |
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LC
resonance steps in the I-V characteristics of DC
SQUIDS
Y. Song and J.
Hurrell
Summary: The LC resonance induced,
current steps in the IV characteristics of dc SQUIDs have been
studied theoretically and experimentally. Analysis enables us
to derive from these I-V characteristics the device parameters
such as loop inductance, junction capacitance, resistance, and
critical current. These four parameters completely
characterize the dynamics of the symmetric dc
SQUID. |
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Inductance
measurements of superconducting chip-to-package connectors
suitable for Josephson LSI
technology
H. Jones, D. Herrell
and Y. Yao
Summary: It is necessary in any
interconnection scheme used in LSI to be able to pass
high-speed pulses from chip-to-chip through the package with a
minimum of distortion while at the same time maximizing the
number of input/output connectors (I/O's) in large arrays of
densely packed connectors. We have measured the inductances
(both self and mutual) of chip-to-package connectors which
were arranged in a peripheral row around the edge of the
circuit chip. The largest self-inductance measured for an 8:1
ratio of I/O's to ground connectors is low enough so that
signals with a 30 ps time constant propagate through the
connector without significant distortion. The largest
self-inductance ranges from 28 pH for the linear array of
eight connectors between grounds to 19 pH for four connectors
between grounds. These results indicate that the electrical
properties of these connectors are satisfactory for the LSI
technology application of Josephson devices. |
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Superconducting
contacts to p-InAs
M. Millea, A.
Silver and L. Flesner
Summary: Since no
insulating barrier exists between electroplated Pb and the
naturally inverted surface channel of p-InAs, this is an
attractive system for developing superconductor-semiconductor
devices. The electrical and induced superconducting properties
of this surface channel have been investigated. A
gate-controlled planar Josephson junction further requires
that superconductors be separated less than some effective
coherence length within the junction barrier. The differential
resistance between superconducting Pb contacts on p-InAs has
been measured for devices with contact separations down to one
/spl mu/m on substrates with tailored n-type surface
conductance. The differential resistance is dependent on the
voltage between contacts, magnetic field amplitude and
orientation, substrate bias, and temperature. Results show
that the Pb superconductivity influences the semiconductor
surface transport in a manner which suggests the potential for
gate controlled Josephson coupling at realizable contact
spacings. |
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A
simple method of RF-SQUID analysis and its application to a
SQUID with a separate driving
coil
S. Tinchev, K. Likharev and
O. Snigirev
Summary: A simple method is
developed for analysis of rf SQUIDs operating in the
hysteretic mode, for arbitrary values of rf detuning and
plateau number. For plateaus with large numbers, the reactive
component of the interferometer flux is shown to be essential.
The results of theoretical analysis are confirmed by
experiments with a 10 MHz point-contact SQUID. The new method
is applied to studies of a SQUID with a separate coil for
driving flux. The response curve slope in such SQUID can be
somewhat larger than that in the conventional rf SQUID. The
balanced SQUID circuits are also discussed. |
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Strain
relaxation in Pb-alloy Josephson junction electrode
materials
M. Murakami and C.
Kircher
Summary: An investigation has been
carried out of the relationship between strain relaxation in
Pb-alloy Josephson junction electrode materials and the
failure of Pb-alloy/oxide/Pb-alloy junctions during cycling
between 300 and 4.2K. The strain behavior of Pb-12wt%
In-4wt%Au alloy films evaporated on oxidized Si substrates has
been studied using an x-ray diffraction technique. The thermal
expansion coefficient mismatch strain is found to be partially
relaxed upon both cooling to 4.2K and rewarming to 300K. The
amount of strain relaxation depends strongly on film thickness
h, decreasing from 0.5% to 0.2% as h is reduced from 0.5 to
0.1/spl mu/m. The stability of Pb-alloy junctions during
thermal cycling correlates with the observed levels of strain
relaxation. A deformation mechanism map has been constructed
for Pb films from which the dominant strain relaxation
mechanisms can be predicted. From the analysis, two strain
relaxation mechanisms are expected: dislocation glide at high
strain and grain boundary diffusion creep at low strain near
300K. Dislocation lines and hillocks were observed in films
that had been repeatedly cycled, providing supporting evidence
for these mechanisms. It is proposed that junction failure
occurs when non-uniform deformation in the electrodes causes a
high enough local stress to rupture the tunnel barrier,
producing a short. |
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Analog
applications of the Josephson effect: Recent developments and
future prospects
R.
Chiao
Summary: A selected group of
high-frequency analog devices which have received much recent
attention, the 4-wave, the 3-wave paramps, the RSJ and the SIS
mixers, will be reviewed. Present achievements and problems of
these devices will be discussed, comparisons made, and future
prospects will be assessed. |
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Spectral
response of high-quality superconducting point contacts in the
presence of background
radiation
Yu. Divin and F.
Nad
Summary: The dc I-V curves and spectral
response of high-quality superconducting point contacts have
been experimentally studied. The model of these contacts is
suggested to account for their dc properties. These
high-quality contacts have a high sensitivity to microwave and
background radiation. Spectral response referred to the power
absorbed in the contact has been obtained using echellette
grating monochromator. Spectral response curves are in
qualitative agreement with the theory considering the
influence of background radiation on the
contact. |
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Properties
of parametric amplifiers using Josephson junctions with
external pumping
L. Kuzmin, K.
Likharev and V. Migulin
Summary: The signal
and noise properties of two basic types of externally pumped
parametric amplifiers, using Josephson junctions with low
capacitance, are calculated. The first type is a single-
frequency nondegenerate amplifier and second one is a
two-frequency degenerate amplifier, referred to sometimes as
SUPARAMP. Calculations are based on the conventional RSJ model
of the Josephson junction. In contrast to the works published
earlier, the most realistic open circuit approximation is used
for idler combinational frequencies, as well as for the
pumping frequency harmonics. A single-frequency amplifier is
shown to have relatively low noise temperatures (T/sub N/ /spl
approx/ 25T) at signal frequencies of the order of the
Josephson junction characteristic frequency /spl omega//sub
c/. On the other hand, SUPARAMP can have much lower noise but
at lower frequencies, and it behaves approximately as a usual
parametric amplifier with a critical frequency /spl omega//sub
cr/ /spl approx/ 0.2 /spl omega//sub c/. |
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Parametric
amplification in Josephson tunnel junction arrays at 33
GHz
F. Goodall, F. Bale, S.
Rudner, T. Claeson and T. Finnegan
Summary:
Progress on the development of a low-noise
superconducting parametric amplifier operated at liquid helium
temperatures and designed for practical use with a radio
telescope at 33 GHz is reported. Series arrays of small Pb -
Pb oxide - Pb tunnel junctions (~20 /spl mu/m X 20 /spl mu/m)
are coupled in a microstrip configuration and the Josephson
device is operated as a reflection-type amplifier in the
doubly degenerate mode. Various practical considerations in
using such a devise on a telescope are reviewed. In
preliminary experiments, gains of greater than 13 dB have been
observed near 33 GHz. |
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Flux
modulated coherent radiation from arrays of Josephson
microbridges coupled by superconducting
loops
R. Sandell, C. Varmazis, A.
Jain and J. Lukens
Summary: We have studied
the phase locking of two and eight microbridge arrays shunted
by superconducting loops in a dc SQUID configuration. The
difference in phase between adjacent bridges is controlled by
the applied flux, and thus the radiated power is a periodic
function of this flux. With adjustment of the external flux,
the individual bridge voltages can be made to add in phase.
The effect of additional normal shunts on the phase locking is
also investigated. |
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Characteristics
of a Josephson junction harmonic mixer with external
pumping
Yuan
Taur
Summary: Numerical calculations are
carried out on harmonic mixing with an externally pumped
Josephson junction. Bias conditions for optimum conversion
efficiency are studied for a variety of junction parameters.
The predicted noise temperature for harmonic mixing is
compared with that for fundamental mixing at the same signal
frequency. |
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The
super-Schottky diode at 30 GHz
M.
McColl, M. Bottjer, A. Chase, R. Pedersen, A. Silver and J.
Tucker
Summary: The super-Schottky diode
mixer is extended to a frequency of 30 GHz using a barrier
modification technique to alter the metal-semiconductor
interface. Control of the metal-semiconductor barrier,
developed here for the super-Schottky, introduces a new
technological tool for tailoring the properties of junction
devices. Reducing the barrier height of Pb contacts to p-GaAs
increases the conductivity of the junction, allowing a
reduction in area to achieve the diode impedance required for
matching. This area reduction decreases the shunt capacitance,
which is the chief parasitic element of the super-Schottky
diode at high frequencies. A second technique available for
extending such devices to higher frequencies is the
electrolithographically produced multiple contact array
structure. This multiple contact concept requires exceptional
uniformity in deposition of the metal contacts. Such
uniformity is unattainable with conventional electroplating. A
new high-field pulsed-plating technique has recently been
developed to yield Schottky contact arrays with uniformly
theoretical spreading resistances at diameters as small as
1200 A. Arrays with 100 individual diode contacts are now
being routinely produced. The area reduction due to barrier
height lowering, however, has produced individual
super-Schottky diodes whose capacitance is decreased by over
an order of magnitude. This reduction in capacitance implies
that single mixer diodes will achieve a conversion loss of 7
dB, a mixer noise temperature of 6 K, and a video NEP of
5x10/sup -16/W/Hz/sup 1/2/at 30 GHz. |
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Properties
of short superconducting
bridges
S. Artemenko, A. Volkov
and A. Zaitsev
Summary: Short Josephson
junction with a current concentration (for example, a variable
thickness bridge or a point contact) is considered.
Phase-current relation obtained differs significantly from
sinusoidal one. In particular, it allows to explain the
appearance of rf-induced subharmonic steps on the I-V curve
(CVC). One-dimensional bridge (for example, a proximity effect
bridge) and series arrays of such bridges are considered also.
Properties of such bridges are determined by the penetration
of the electric field into a superconductor over a large
depth. |
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High
sensitivity microwave SQUID
J.
Hollenhorst and R. Giffard
Summary: We have
measured the characteristics of a novel SQUID magnetometer
pumped at 9 GHz. The device comprises a niobium point-contact
shunting a low impedance superconducting microwave
transmission line which isolates the junction at high
frequencies from a toroidal input transformer. The junction is
matched to a 50 /spl Omega/ coaxial transmission line and the
SQUID is operated in a reflection mode. For low values of the
weak link critical current we observe a modulation of the
phase of the reflected microwave signal periodic in the
applied flux. The modulation persists at high microwave power
levels indicating that the junction response is maintained to
at least 200 GHz. For large values of critical current,
amplitude modulation is observed. We have obtained a
differential sideband power of 230 pW/(/spl Phi//sub 0/)/sup
2/ which is close to the value predicted by a simple model. In
flux-locked loop operation we have obtained an energy
sensitivity referred to the input terminals of 7x10/sup
-31/J/Hz, which is superior to that so far reported for any
other SQUID. |
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Performance
of a resonant input SQUID amplifier
system
M. Simmonds, W. Fertig and
R. Giffard
Summary: We have demonstrated that
the input circuit of a SQUID magnetometer may be resonated at
the signal frequency to obtain enhanced sensitivity. The
improvement which may be obtained over a narrow bandwidth is
limited in principle by the input noise of the SQUID. In
practice the sensitivity on resonance is limited by thermal
noise, and depends on the temperature and Q of the input
circuit. |
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Harmonic
generation and half flux quantum periodicity in the
RF-SQUID
P. Fernandez, C. Salvo,
R. Parodi, A. Siri and R. Vaccarone
Summary:
We studied the RF driven SQUID by a numeric simulation
and experimentally. The amplitude and the phase of the RF
voltage vs RF driving current and DC flux has been obtained
both with the SQUID working in the conventional mode and in a
new operating mode based on the 2nd harmonic generation in the
system. A spectral analysis has been carried out and we report
the characteristics of harmonic and subharmonic generation.
For the even harmonic components, and in particular for the
second one on which the proposed method is based, the V/sub
RF/vs I/sub RF/staircase pattern is replaced by a series of
peaks. We find that the position of the peaks does not depend
on the DC flux level and so they result in a stable marker for
RF level measurements. The 2nd harmonic output, as a function
of DC flux, shows a triangular behaviour with V/sub RF/= 0 at
/spl phi/sub DC/ = n /spl phi//sub 0//2. The dependence of
V/sub RF/on /spl phi//sub DC/is at least as high as in the
conventional operating mode. We have experimentally verified
the harmonic generation and the half flux quantum periodicity
up to the 4th harmonic. The signal amplitude was enough to
allow flux-locked-loop operation. |
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All-refractory
weak-link SQUIDs for use in Josephson logic and memory
applications
D. Jillie and H.
Kroger
Summary: We report on the fabrication
of all-refractory submicron-dimensioned superconductor-normal
metal-superconductor (SNS) weak links between two coplanar
electrodes. This unique geometry allows two or more weak links
to be placed in parallel (dc SQUID) underneath an overlying
control line for use in Josephson logic and memory
applications. The method of fabrication is described and is
amenable to incorporation in large-scale integrated circuit
production. Depending upon geometry, weak links with a broad
range of parameters may be obtained. Resistances are in the
range of 0.01 to 2/spl Omega/ and critical currents range from
a few /spl mu/A to a few mA. When incorporated into dc SQUIDs,
inductances of one or two pH are typically obtained. Data are
presented on the I-V characteristic, the dependence of
critical current on temperature and magnetic field, and the rf
response. In addition to their potential for use in
superconducting microcircuitry, these devices closely
approximate an ideal three-dimensional point contact, and as
such are of basic interest. |
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Niobium
Josephson junctions with doped amorphous silicon
barriers
H. Kroger, C. Potter and
D. Jillie
Summary: Nb-(a-Si)-Nb Josephson
devices have been prepared by rf sputtering. The silicon films
were deposited in an argon-hydrogen atmosphere. Such films can
be activated as either n- or p-type semiconductors by the
incorporation of phosphorus or boron, and the Fermi level of
the material can be moved a considerable fraction of the
bandgap. The Josephson current density of n-type layers is
found to be substantially greater than p-type layers of the
same thickness. |
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The
properties of superconducting Nb/sub
3/Si
D.
Dew-Hughes
Summary: Superconducting Nb/sub
3/Si has been made by the explosive compression of the high
temperature, equilibrium tetragonal Ti/sub 3/P phase,
confirming the work of Pan and his associates at Kiev. The
material has a broad superconducting transition with an onset
T/sub c/ /spl sim/18 K and a mid-point /spl sim/16K. T/sub
c/is considerably reduced by fast neutron irradiation but can
be partially recovered by annealing at 600/spl deg/C. X-ray
results are inconclusive but the new superconducting phase has
been tentatively identified as having the A15 structure with a
lattice parameter /spl sim/5.12/spl Aring/. The upper critical
field H/sub c2/(0) is disappointingly low at 15.5-17 tesla;
this is, however, not inconsistent with highly degraded A15
material. |
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Sputtering
of high T/sub c/Nb/sub 3/Ge and Nb/sub
3/Si
R. Somekh and J.
Evetts
Summary: We present observations of
two effects that relate to the formation of high T/sub
c/Nb/sub 3/Ge and Nb/sub 3/Si A15 phases using a DC getter
sputtering technique. The first is an effect associated with
convection of the sputtering gas which has been studied by
tilting of sample substrates and heater with respect to the
horizontal. The second effect is associated with surface
markings on the films, in particular in the Nb-Si system.
These observations constitute further evidence of the
complexity of this sputtering process and indicate that unless
there is careful control of the experimental conditions, the
deposition parameters and the film properties are likely to
vary locally on a rather small scale. Finally we shall discuss
some of the reservations we have concerning the high T/sub
c/values we observe in the Nb-Si system. |
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Superconducting
properties and structure of Nb/sub 3/Ge samples prepared by
high pressure D.C. sputtering and by solid state
diffusion
B. Letellier and J.
Renard
Summary: Properties of Nb-Ge A15
samples prepared by solid state diffusion and high pressure
D.C. sputtering are reported and discussed in term of
correlations between critical temperature, crystal structure
and conditions of elaboration. In samples prepared by
diffusion, it is found that the A15 phase may be obtained. So
prepared compounds are in thermodynamical equilibrium whatever
the annealing temperature up to 1 750/spl deg/C and the
quenching rate up to 1.6 10/sup 4/K/s. When the annealing
temperature is very closed to the melting point of the
compound, T/sub c/onset may reach 16 K. In samples prepared by
sputtering, it is found that the Nb/Ge ratio is not an
important parameter while the substrate temperature is a very
critical one. The highest T/sub c/'s, close to 22 K are
correlated with the elimination of a defect which is the
hexagonal Nb/sub 2/Ge/sub 3/phase. From both kinds of
experiment it is suggested that, in our samples, oxygen almost
has no effect on superconducting properties in the low T/sub
c/range but seems to be harmful to obtain critical
temperatures in the 22 K range. |
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Critical
current densities of magnetron sputtered Nb/sub 3/Ge
films
R.
Kampwirth
Summary: Nb/sub 3/Ge films
fabricated by magnetron sputtering at substrate temperatures
T/sub s/, from 600 to 850/spl deg/C and film thicknesses of 1
-5 /spl mu/m have been analyzed with respect to transition
temperature T/sub c/, critical current density as a function
of applied field J/sub c/(H), and grain size. J/sub c/at
5Tesla shows a strong dependence on T/sub s/, decreasing by
more than an order of magnitude as T/sub s/increases from 700
to 815/spl deg/C. This decrease will be related to grain
diameter D. Results will be presented which suggest a lower
J/sub c/(H) in thicker films prepared at a fixed T/sub s/is
caused by increased grain diameters as the films grow in
thickness. Evidence will be presented showing these results to
be consistent with grain boundaries being the dominant pinning
mechanism in these films. |
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Microstructure
control in Nb/sub 3/Ge and its effect upon the
critical-current density
A.
Braginski, G. Roland and A. Santhanam
Summary:
We have investigated the correlations between
critical-current density, J/sub c/, and the microstructure of
Nb/sub 3/Ge/sub 3/ films deposited by chemical vapor
deposition. Single-phase A15 samples and samples containing
particles dispersed in the A15 matrix have been pared. owing
films of thickness varying in the range and multilayered films
we controlled the A15 grain size and the Nb/sub 5/Ge/sub 3/
particle size dispersion. For particle sizes approaching the
coherence length of Nb/sub 3/Ge/sub 3/ we have observed an
effective pinning and very high critical-current densities. We
applied our findings to the growth of /spl sim/ 10/super 5/
/spl Aring/ thick films and fabricated layered Nb3Ge tape
conductor ples having J/sub c/ = 10/sup 5/ Acm/sup -2/ at 4.2
K and 200 kilogauss. |
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Effect
of strain on the critical current of Nb/sub
3/Ge
J. Ekin and A.
Braginski
Summary: The strain dependence of
the critical-current density has been determined for composite
tapes of Nb/sub 3/Ge prepared by chemical vapor deposition.
For Nb/sub 3/Ge layers 3-4 /spl mu/m thick deposited on a
nickel-molybdenum-iron alloy substrate, the critical current
monotonically increases when uniaxially strained to about
0.6%. This strain corresponds almost exactly to the
compressive strain that would be introduced into the Nb/sub
3/Ge by the substrate due to thermal contraction during
cooldown after reaction. The increase in critical current was
relatively small, about 6 1/2% at 7T and 5 1/2% at 4T. At
higher strain, the critical current decreased rapidly, falling
by more than 50% at a strain of 0.9%, for example. For Nb/sub
3/Ge deposited on a tantalum substrate, however, the critical
current monotonically decreased, falling by more than 50% at a
strain of 0.4%. The results indicate that Nb/sub 3/Ge can
withstand considerable compressive strain (at least 0.6%), but
fractures at tensile strains of only 0.1 to
0.2%. |
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Critical
currents in Nb/sub 3/Ge based
pseudobinaries
S. Alterovitz, J.
Woollam, J. Engelhardt and G. Webb
Summary:
We have prepared Nb/sub 3/Ge, Nb/sub 3/Ge/sub
1-x/Ga/sub x/and Nb/sub 3/Ge/sub 1-x/Sn/sub x/on heated
alumina substrates using CVD. Midresistive transitions were up
to 21K and transition widths were as low as 0.3K. Critical
currents were measured to 22 Tesla at temperatures from 4.2K
to 19K. Effective upper critical fields B*/sub c2/were
measured by extrapolatingJ\min{c}\max{V2}vs B data.
Preliminary data show that for small Ga additions, B*/sub
c2/increased above the value at x=0. Flux pinning forces vs
reduced field b=B/B*/sub c2/do not obey scaling laws, which we
explain as being due to inhomogeneous material having a
distribution of T/sub c/and B/sub c2/values. |
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High
magnetic field properties of CVD-prepared Nb/sub 3/Ge and
Nb/sub 3/(Ge,X/sup +/)
J.
Thompson, M. Maley, L. Newkirk and R.
Carlson
Summary: Measurements of the field
dependent critical current density, J/sub c/(H), and the upper
critical field, H/sub c2/, have been performed on a number of
Nb/sub 3/Ge samples prepared by the chemical vapor deposition
(CVD) process. These samples, obtained from up to 20 m lengths
of Nb/sub 3/Ge tape, were deposited on various substrates at
temperatures between 835 and 950/spl deg/C. Flux pinning,
provided by the introduction of controlled amounts of second
phase precipitate, Nb/sub 2/Ge/sub 3/, produced good high
field properties with 4.2 K critical current densities on the
order of 8.3x10/sup 4/A/cm/sup 2/at 18 T. Measurements of
J/sub c/(H) performed on these samples in both liquid helium
and hydrogen in fields up to 18.5T have been analyzed in terms
of Kramer's model of flux pinning. Qualitative agreement has
been found with the scaling laws predicted by this theory. The
effect of ternary additions of Si and Ga on both H/sub c2/and
J/sub c/(H) have been investigated as well. Contrary to
expectation, the addition of ternaries into the A-15 lattice
results in somewhat depressed critical characteristics. The
effect of deposition parameters on the high field properties
of both Nb/sub 3/Ge and Nb/sub 3/(Ge,X) will be
discussed. |
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The
role of the large coil program in the development of
superconducting magnets for fusion
reactors
P. Haubenreich, J. Luton
and P. Thompson
Summary: The central element
in the Office of Fusion Energy's development of
superconducting toroidal field magnets for fusion reactors is
the Large Coil Program (LCP). Toroidal field coils in a
tokamak reactor face special problems of heat generation by
pulsed poloidal fields, demands for continuity of operation,
structural design to handle the asymmetric in-plane loading
and the out-of-plane forces repeatedly imposed, and space
competition that makes high current densities desirable.
Several design concepts have been advanced but large coils
meeting tokamak requirements must be built and tested before
an optimal choice can be made. This is being done through the
LCP, in which three U.S. industrial teams are designing and
will build one coil each to a common set of specifications.
Coil specifications and test conditions were chosen to insure
maximum relevance to fusion program needs. Each test coil will
have a 2.5 X 3.5 m D-shape bore, will contain about 7
MA-turns, and must operate at a peak field of 8 T while
subjected to pulsed fields up to 0.14 T in a test stand that
can accommodate up to 6 coils in a compact toroidal array.
Coils by General Dynamics/ Convair and General Electric will
use different NbTi conductors cooled by pool-boiling helium.
The Westinghouse coil will use Nb/sub 3/Sn cooled by a forced
flow of supercritical helium. These coils will be delivered in
1980 and 1981 for testing in the Large Coil Test Facility at
Oak Ridge in a compact toroidal array with three coils from
outside the U.S. These will be produced by EURATOM, Japan, and
Switzerland for testing under an International Energy Agency
agreement. |
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Superconductivity
for mirror fusion
C.
Henning
Summary: Mirror experiments have led
the way in applying superconductivity to fusion research
because of unique requirements for high and steady magnetic
fields. The first significant applications were Baseball II at
LLL and IMP at ORNL, which used multifilamentary
niobium-titanium and niobium-tin tape, respectively. Now the
USSR at Kurchatov is building a smaller baseball coil with a
6.5 mm square multifilamentary niobium-titanium superconductor
similar to the Baseball II conductor. However, the largest
advance in fusion magnets will be used in the Mirror Fusion
Test Facility (MFTF) now under construction at LLL.
Improvements in the technology of the previous LLL experiment,
Baseball II, have been made using new conductor joining
techniques, a ventilated wrap-around copper stabilizer, and
stronger structural welding methods. The MFTF coil winding is
proceeding on a separate former to allow parallel construction
of the main structure. Not only does this shorten the project
schedule to equal that of other conventional constructions,
but a second vacuum barrier is created between the magnet
helium and the plasma environment for reliable operation. In
the future, LLL envisions a superconducting version of the
Tandem Mirror Experiment and a possible hybrid reactor leading
to economical fusion power. |
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MFTF
test coil construction and
performance
D. Cornish, J.
Zbasnik, R. Leber, D. Hirzel, J. Johnston and A.
Rosdahl
Summary: A solenoid coil, 105 cm
inside and 167 cm outside diameter, has been constructed and
tested to study the performance of the stabilized Nb-Ti
conductor to be used in the Mirror Fusion Test Facility (MFTF)
being built at Lawrence Livermore Laboratory. The insulation
system of the test coil is identical to that envisioned for
MFTF. Cold-weld joints were made in the conductor at the start
and finish of each layer; heaters were fitted to some of these
joints and also to the conductor at various locations in the
winding. This paper gives details of the construction of the
coil and the results of the tests carried out to determine its
propagation and recovery characteristics. |
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Fabrication
of the MFTF magnet windings
D.
Deis, C. Henning, R. Hinkle, V. Kopytoff and J.
MacDonald
Summary: The Lawrence Livermore
Laboratory (LLL) is currently in the construction stage of the
Mirror Fusion Test Facility (MFTF). MFTF will be the next
large mirror fusion experiment and employs a large set of
superconducting Yin-Yang coils. These coils contain 54,430 kg
of stabilized NbTi conductor and will generate a peak field of
7.68 T with a stored energy of 409 MJ. This paper presents
details of the design of these coils and the status of the
fabrication. |
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Superconducting
ohmic-heating coil simulation
P.
Thullen, J. Lindsay, D. Weldon and H. Vogel
Summary:
Superconducting coils have been proposed for use as
ohmic-heating coils in the conceptual designs of most tokamak
and reverse field pinch fusion reactors because of their high
energy efficiency. Lack of detailed knowledge about the
behavior of superconducting coils performing the required
bipolar flux swings may delay their application in future
fusion reactors. The Los Alamos Scientific Laboratory has
undertaken a study to show how such coils can be constructed.
This program includes both theoretical and experimental
phases. The work is based to a large extent on the experience
gained during the fast discharge (1 ms) magnetic energy
transfer and storage (METS) program. A series of experiments
in which a typical ohmic-heating current waveform is simulated
have been undertaken using the 300-kJ, 10-kA METS
superconducting coil, a 500-kW, 250-V, 2-kA dc motor generator
set used as a capacitor, and a rectifier power supply. The
coil has undergone full bipolar flux swings to greater than
rated current (/spl plusmn/ 12-kA, 430 kJ) and remained
superconducting. The commutator dc machine has functioned
successfully as a capacitor up to 12 kA. We describe the test
circuits and discuss the components. Terminal characteristics
are presented. Coil energy losses are presented as a function
of the terminal current. These tests support the viability of
bipolar pulsed superconducting coils for use in tokamaks and
other fusion concepts. |
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Conceptual
design of a superconducting Tokamak: "TORUS II
SUPRA"
R. Aymar, G. Claudet, C.
Deck, R. Duthil, P. Genevey, C. Leloup, J. Lottin, J. Parain,
P. Seyfert, A. Torossian and B. Turck
Summary:
The conceptual design of TORUS II SUPRA concerns a
large sized Tokamak of the next generation. It takes into
account constraints coming from the envisaged superconducting
toroidal magnet, but keeps unchanged all the operational
facilities and working conditions which a more conventional
design, i.e. a water cooled-copper magnet, could offer. The
main parameters are R/sub 0/= 2.15 m, a = 0.75 m, B/sub 0/=
4.5 T, I = 1.7 MA. The scientific aims of the device concern
the development, at a multimegawatt level, of plasma heating
methods, mainly wave absorption and the contribution to high
temperature (3-5 keV) Tokamak physics allowed by these methods
: main emphasis is put on tentative profile control of plasma
parameters during the quasi-steady state possible with a D.C.
toroidal magnetic field. |
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Design
and construction of superconducting quadrupole magnets for ion
beam fusion
S.-T. Wang, H. Ludwig
and L. Turner
Summary: A high gradient
superconducting quadrupole has been designed and developed as
the heavy ion beam focussing element in the low velocity
portions of an rf linac for the Argonne Ion Beam Fusion
Reactor. The quadrupole magnets will require an extremely
short magnet coil length (/spl sim/ 20 cm to 30 cm) and
extremely high central gradients (/spl sim/ 100 T/m to 200
T/m). The useful warm bore will be about 4 to 6 cm and the
integral gradient homogeneity should be constant to /spl
plusmn/ 5% over the useful warm bore. Special techniques have
been developed which are especially suitable for multi-layer
coil winding and coil assembly with high average current
density over the coil cross section. A 5-layer quadrupole with
9 cm winding bore has been built and tested to the full
performance of about 100 T/m with little training. The
achieved average current density is 22,000 A/cm/sup 2/at a
peak field in conductor of about 5.0 T. An 8-layer quadrupole
is under construction for a design gradient of 140 T/m over 9
cm winding bore. The peak field will be about 7.2
T. |
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Some
results from the T-7 tokamak superconducting magnet test
program
D. Ivanov, V. Keilin, B.
Stavissky and N. Chernoplekov
Summary: The
first results from the superconducting toroidal coil test
program for the T-7 tokamak are described. The coils, which
have a cold mass of 12 tons and a field volume of 6m/sup 3/,
utilize a force-cooled circulation system. The system was
cooled to 4.5K in 4 days using an HGU-250/4.5 refrigerator
with a nominal capacity of 400W. The coils were initially
charged to 4500A, and then to 4800A, or 75% of the critical
current at 4.2K, which was achieved without quenching. At this
current the magnetic field at the torus axis was 2.4T, the
maximum field at winding 4T, and the stored energy 12MJ. Total
charging time was 5 hours and the discharge time 20 minutes.
The T-7 installation is now being prepared for plasma physics
experiments. |
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Regulated
AC power for Josephson interferometer latching logic
circuits
P. Arnett and D.
Herrell
Summary: We have developed an ac
power distribution scheme which provides switched regulated
power of alternating polarity (no rectification necessary) for
Josephson interferometer latching logic circuits. Distribution
from the power source relies on a tree system of thin film
transformers having single primaries and multiple secondaries
to maintain low current levels and small phase skews
throughout the system. Power signals are clipped on chip to
the desired voltage supply level by series connected large
area Josephson junctions ("regulator" junctions) and
distributed across the chip by voltage busses. Supply
resistors between the regulated voltage bus and the logic
circuits define the operating current level. A detailed design
is presented for a specific interferometer latching circuit
family. We report experimental data on the two major
components of the power system design - the regulators and the
transformers. A series string of four regulator junctions
provides a trapezoidal waveform of 11 mV amplitude at > 66%
logic duty cycle (that portion of the machine cycle in which
logic operations can be performed). Thin film transformers
have their designed inductance (350 pH) and coupling
coefficient ( > .90). |
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The
vortex file: A proposal for a new application of type-II
superconductivity
W.
Bachtold
Summary: Thin-film type-II
superconductors are proposed as a new storage medium for
future Josephson computers. Vortices in thin films have much
in common with magnetic bubbles; storage and propagation can
be achieved with a periodic guiding structure and induced
transport currents. The packing density was found to be
similar to magnetic-bubble density, the maximum vortex shift
rate is estimated to be several hundreds of Mbits/sec, being
at least two orders of magnitude superior to bubbles.
Single-vortex generation and detection schemes are
investigated and it is found that the technology bears
considerable potential, if thin-film type-II superconductors
can be made with sufficiently low pinning
forces. |
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Turn-on
delay of Josephson interferometer logic
devices
E.
Harris
Summary: It has generally been assumed
that the switching dynamics of Josephson logic devices can be
well approximated by treating the Josephson device as a
capacitatively-shunted switch which opens instantaneously when
a control current crosses threshold. In this approximation,
the switching delay is proportional to RC, where R = load
resistance and C = device capacitance. However, we show here
that the above approximation is quite incorrect in the
technologically important case of logic interferometers in
which resonances are suppressed by internal damping. We treat
in detail the case of damped two-junction interferometers with
step-function control current. Analysis of the equations of
motion shows that in addition to the RC delay there is an
initial turn-on delay during which the output voltage is very
small, and which is most pronounced for heavy internal
damping. It consists of the time required for the difference
in phase between the two junctions to increase to a critical
value, followed by approximately the first half-cycle of
growth of the average of the junction phases. The turn-on
delay is a strong function of overdrive, (i.e., the factor by
which the control current amplitude exceeds threshold), being
smallest at large overdrives, and scales as (C/i/sub 0/)/sup
1/2/, where i/sub 0/= junction critical current. Hence it can
be reduced by miniaturization and/or increased junction
current density. For parameter values typical of 5/spl mu/m
Josephson circuit technology, the turn-on delay is on the
order of 10pS at large overdrives (/spl sime/2). |
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Attenuation
in superconducting striplines
R.
Kautz
Summary: Measurements of the Q of
stripline resonators yield values for the attenuation of
Nb-Nb/sub 2/O/sub 2/-Pb striplines typical of those used in
superconducting microcircuits. At 4 K the attenuation between
50 and 500 MHz is proportional to frequency and probably
results from dielectric losses. Near the transition
temperature of Pb, the attenuation begins to show the
frequency-squared dependence associated with superconducting
losses. |
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Planar
SNS microbridges
J. Warlaumont
and R. Buhrman
Summary: Planar SNS
microbridges have been fabricated in the variable thickness
geometry with all dimensions as small as 200 nm. The behavior
of these microbridges, including I-V characteristics and
response to microwave radiation, is described. |
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Fabrication
and characterization of S-N-S planar
microbridges
R. van Dover, R.
Howard and M. Beasley
Summary: We have
investigated the potential of the planar S-N-S geometry for a
high-T/sub c/Josephson device technology, developed
fabrication techniques compatible with integrated processing
and dealt with the problems of processing high-T/sub
c/materials with their sensitivity to impurities and damage.
We have used both Nb/sub 3/Sn and Nb for the banks and Cu for
the normal bridge. Device with a normal link less than 1
micron long exhibit Josephson effects over a wide temperature
range 0 < T < T/sub csns/. We discuss the electrical
properties of these bridges and evaluate the potential of this
geometry for high resistance (R/sub n/= 1 - 10/spl Omega/)
devices. The I-V characteristics are compared to the
predictions of a simple TDGL model. |
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Properties
of high-resistance superconducting microbridges based on lead
alloy films
M. Feuer and D.
Prober
Summary: We have fabricated
high-resistance, type-II microbridges with submicron
dimensions and well-characterized lead alloy films, and
studied their Josephson-effect properties. Bridge resistances
of 2-18 /spl Omega/ have been achieved in uniform-thickness
structures which are 2000-20,000 /spl Aring/ wide. Critical
currents of narrow bridges near T/sub c/are found to agree
with theory for uniform depairing. The microwave response of
the bridges agrees with theory at low power levels, but
self-heating limits microwave steps to voltages < 375 /spl
mu/V. Specific predictions for heating limits of alloy
microbridges in the variable thickness geometry are
presented. |
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Fabrication
and barrier diagnostics of superconductive tunnel junctions on
Nb-Sn and V-Si
D. Rudman, R.
Howard, D. Moore, R. Zubeck and M. Beasley
Summary:
The procedures found to lead to good tunnel junctions
on the Al5 superconductor Nb-Sn are described. The use of an
acetic acid atmosphere and/or oxidized Si layer barriers is
shown to be particularly effective. The role of excess Sn in
good oxide barrier formatiom on bare Nb-Sn is discussed. These
results are related to earlier work using oxidized Si barriers
on V/sub 3/Si. |
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Experiments
on Ge-Sn barrier Josephson
junctions
E. Hu, L. Jackel, R.
Epworth and L. Fetter
Summary: Supercurrents
have been observed through thick (/spl sim/500/spl Aring/)
barriers of co-evaporated Germanium - tin mixtures. The
Critical current density is a strong function of the barrier
composition and thickness. For 600/spl Aring/ thick barriers,
a change in composition from 20% Sn to 50% Sn yields a factor
of 10/sup 6/change in current density. Typically the critical
current of the junctions decreases by a factor of 10 for every
80/spl Aring/ increase in barrier thickness. In general, the
low current-density junctions show I-V characteristics which
are well described by tunneling. High current-density
junctions usually have characteristics which, depending on
junction preparation, indicate either tunneling, or
metal-to-metal conduction. By using smooth base-electrodes it
is possible to make junctions with barriers as thin as 400/spl
Aring/ that are effectively pinhole free and whose I-V
characteristics show true tunneling. The relatively thick
barriers of these junctions allow a substantial decrease in
the junction capacitance per unit area compared to oxide
barrier junctions. This lower capacitance allows the
fabrication of junctions with non-hysteretic I-V
characteristics having critical current densities as small as
10/sup 3/A/cm/sup 2/. Using small-area, non-hysteretic
junctions, we have made a high-sensitivity D. C. SQUID with an
intrinsic energy resolution of 10/sup
-31/Joules/Hz. |
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Preparation
of Nb/sub 3/Ge superconducting tunneling
junctions
R. Buitrago, A.
Goldman, L. Toth and R. Cantor
Summary: A
procedure in which Auger electron spectroscopy (AES) is used
to control barrier thickness has been developed for the
fabrication of Nb/sub 3/Ge-Al/sub 2/O/sub 3/-Pb tunneling
junctions. Both high-resistance and low-resistance Josephson
junctions have been made using this technique. A similar
procedure has been used to form junctions of V/sub 3/Si
crystals. |
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Tunneling
studies on thin film Nb-Al
alloys
J. Yeh and C.
Tsuei
Summary: Relatively high quality
Josephson tunnel junctions have been fabricated using e-beam
coevaporated Nb-Al thin film alloys as the base electrode.
Alloys of various Al concentrations, and deposited at several
substrate temperatures, have been studied. Structure analysis
indicates that these alloys contain metastable disordered bcc
and/or amorphous phases. Low excess tunneling currents and a
small barrier dielectric constant (/spl epsilon//sub r/~5) can
be achieved by oxidizing an aluminum layer which is deposited
on the base electrode as the tunnel barrier. |
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A.C.
properties of Nb-Nb/sub x/O/sub y/-Pb Josephson tunnel
junctions for 2e/H
measurements
V. Lacquaniti, G.
Marullo and R. Vaglio
Summary: In order to
improve the Italian voltage standard via the Josephson effect
at IEN some results have been obtained in the study of the
a.c. properties of Nb- -Nb/sub x/O/sub y/-Pb junctions. Our
devices, characterized by a very high geometrical definition,
usually exhibit a resonant frequency of about 10.5 GHz with a
length in the microwave direction of 0.52 mm. A fine
adjustment (better than 0.5%) of the resonant frequency of a
single junction can be achieved after testing at 4.2 K by
means of a photolithographic procedure. This technique makes
easier to couple junctions in series with the microwave
signal. Some properties of the radiated junctions have been
studied by means of "Q" measurements obtained by both the
analysis of self-resonant modes behavior and the junction
response to a microwave signal. Q values as high as 130 at 4.2
K has been evaluated. The results, also discussed in
comparison with data derived from Auger spectroscopy, give
interesting information about the influence of the dielectric
barrier and its metallic interfaces on the coupling to the
microwave. Measurements of 2e/h at 5 mV level have been
carried out successfully. |
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Granular
niobium and high T/sub c/niobium nitride variable thickness
weak link SQUIDs
S. Wolf, E.
Cukauskas, F. Rachford and M. Nisenoff
Summary:
SQUIDs employing short (~1 /spl mu/) ultra thin weak
link sections of "granular" niobium and niobium nitride have
been prepared on cylindrical quartz substrates by rf and rf
reactive sputtering, photolithography and anodization. These
SQUIDs have demonstrated near ideal intrinsic noise
characteristics when biased at 20 MHz. Niobium nitride devices
have operated with this nearly intrinsic noise at temperatures
up to 12.5 K. Current-phase relation and flux entry versus
applied flux were measured on one niobium SQUID. Results
indicate a nearly [if not exactly] sinusoidal current-phase
relation from T/sub c/to almost a degree below T/sub c/and
flux entry noise in good agreement with the intrinsic flux
noise theory. Biasing these devices at 9.2 GHz gives a
degraded response which can be related to relaxation effects
in the weak links. Thermal noise switching between the
superconducting and dissipative states was also observed at
9.2 GHz and could be fit by a simple model. |
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Amplitude
modulation of proximity effect weak
links
R. McNamara and J.
Mercereau
Summary: By a careful examination
of the size of the microwave induced step structure in the dc
current-voltage (I-V) characteristic of proximity effect weak
links at low frequency (<4 GHz), it has been determined
that the amplitude of the microwave Josephson current
decreases exponentially with increasing voltage. This
amplitude modulation of the Josephson effect depends on the
magnitude of both the rf and dc voltages and is interpreted in
terms of voltage induced pair breaking within the link rather
than thermal heating. Application of this effect to produce a
three terminal Josephson device is introduced. |
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Muffin-tin
cavities at X-band for linear accalerator
application
H. Padamsee, M.
Banner, J. Krichgessner, M. Tigner and R.
Sundelin
Summary: A series of 2-cell, /spl
pi/-mode, standing wave accelerator cavities operating at 8.6
GHz has been fabricated from thin (0.5 mm) Nb sheet metal. The
cavities are of the rectangular geometry, muffin-tin variety.
Of six cavities, four achieved effective accelerating fields
greater than 17 MeV/m; the other two reached 11 and 14 MeV/m.
The accelerating gradient in the best cavity was 21.8 MeV/m,
corresponding to a peak rf magnetic field of 1400 Gauss. Q/sub
0/values in all cases were greater than 1x10/sup 9/. No
significant electronic activity was observed in any of these
cavities. As a preliminary undertaking, several 1-cell X- band
cavities have also been fabricated using the same technology.
Similar statistics for accelerating field values were obtained
and the peak rf magnetic field achieved in the best 1-cell
cavity was 1600 Gauss. A summary of the fabrication and
preparation techniques together with test results are
presented. |
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Superconducting
bar-loaded resonator for electron accelerator
application
H. Padamsee, M.
Banner, P. Kneisel and M. Tigner
Summary:
Superconducting rf structures hitherto used for
electron accelerator application have been plagued by
multipactoring problems. For storage ring application, where
operating frequencies are likely to be restricted to the UHF
band (e.g., 500 MHZ), multipactoring may impose even more
severe limitations in performance. In the course of searching
for an rf structure with more favorable multipacting
properties, it was decided to investigate the "ridged
easitron". The cavity design is a variation of the crossbar
structure. A rectangular ridged waveguide is loaded with a
pair of bars at half-wave-length intervals. All the bars are
oriented parallel to the beam plane and the ridge runs along
the (waveguide) walls that intercept the bars. Rf properties
measured in model copper cavities are discussed. Test results
are presented on the superconducting performance and
multi-pactoring behavior of a 2-cell S-band Niobium cavity
operating in the standing wave, /spl pi/-mode. |
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Characterization
of Nb/sub 3/Sn diffusion layer (A-15)
material
V. Diadiuk, J. Bostock
and M. MacVicar
Summary: Microstructural
characterization of Nb/sub 3/Sn diffusion layers grown by
Sn-vapor reaction with single crystal Nb substrates has been
carried out using Auger and X-ray spectroscopy, X-ray and
reflection electron diffractometry, and scanning electron
microscopy. The layers exhibit preferred crystallographic
orientation traceable to the underlying Nb-substrate
orientation. The <111>-Nb-substrate layers differ
substantially from all other-Nb-direction layers. In
particular, they exhibit surface-like composition profiles
over several hundred angstroms into the bulk, a very rough
micro-structure including numerous microcracks, and a surface
superconducting transition temperature of < 1 K.
Non-<111>-Nb-substrate layers with 1 /spl mu/m columnar
grain structure and surface T/sub c/'s /spl sim/ 18 K appear
to have breaking stresses far in excess of bulk
polycrystalline Nb/sub 3/Sn. |
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High
fields in superconducting Nb/sub 3/Sn accelerating
structures
G. Arnolds, R.
Blaschke, H. Piel and D. Proch
Summary:
Several X-band three-cell and nine-cell accelerating
structures were fabricated from bulk niobium and coated with a
Nb/sub 3/Sn layer by the vapour diffusion technique. To
determine the quality of the Nb/sub 3/Sn layer the temperature
dependent surface impedance was measured in the range 25 K
/spl geq/ T /spl geq/ 1.5 K at low field level. From this data
material parameters as T/sub c/, /spl utri//kT/sub c/, /spl
lambda//sub 0/and /spl rho//sub n/were calculated. The high
field behaviour of the cavities was investigated for
temperatures 4.2 K /spl geq/ T /spl geq/ 1.5 K. Maximum
surface fields of 45 mT were reached corresponding to an
accelerating field of E/sub acc/= 6 MV/m in the /spl
pi//2-mode. The results of low and high field measurements
with respect to the fabrication and treatment of the Nb/sub
3/Sn-layer are discussed. |
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Mechanical
and superconducting properties of Nb/sub
3/Al
G. Webb
Summary:
Nb/sub 3/Al can be quenched from under a solidus into
the low T/sub c/ductile BCC structure. In the BCC structure a
variety of cold working operations can be performed on it
without fracture. Later annealing converts this material to
the high T/sub c/brittle A-15 structure. |
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Studies
of electron beam coevaporated Nb/sub 3/Sn composites: Critical
current and microstructure
R.
Hammond, B. Jacobson, T. Geballe, J. Talvacchio, J. Salem, H.
Pohl and A. Braginski
Summary: The critical
current pinning force has been measured as a function of the
magnetic field for a variety of microstructures of Nb/sub
3/Sn. These include Nb/sub 3/Sn structures with columnar
grains, codeposited Nb/sub 3/Sn-Al/sub 2/O/sub 3/structures
with equiaxed grains, and codeposited Nb/sub 3/Sn-Cu
structures with equiaxed grains. The peak in the pinning force
is compared with the grain size. For grains or columns larger
than 400 /spl Aring/ an inverse grain size dependence is
found. For smaller grains the pinning force appears to
decrease. A possible detrimental effect of copper on Nb/sub
3/Sn is pointed out. |
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Properties
of reactively sputtered superconducting
films
J.
Gavaler
Summary: A review of the
applicability of reactive sputtering to the synthesis of
several A15 and B1 superconductors was performed. Critical
temperature, T/sub c/, of Nb/sub 3/Ge and V/sub 3/Si made by
this method were found to be slightly lower than the highest
reported values. T/sub c/'s of V/sub 3/Ge and NbC films were
higher than found in bulk samples. Nb-Si films were
nonsuperconducting. The preparation of pseudo-binary alloys of
the Nb-V-Ge, Nb-V-Si and the Nb-Ge-Si systems resulted in no
improvements in T/sub c/'s. Fabrication of layered structures
consisting of Nb-C-N and nonsuperconducting layers was
investigated. |
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Neutron-irradiation
effects in the pseudobinary compound Nb/sub 3/(SnGa): Changes
in T/sub c/, I/sub c/, and flux
pinning
C. Snead and M.
Suenaga
Summary: Specimens of single-core
wires of Nb/sub 3/(SnGa) have been neutron irradiated at
~100/spl deg/C to fluences of 2.6 x 10/sup 19/n/cm/sup
2/(E>1.0 MeV). Measurements of I/sub c/at 4.2 K in
transverse magnetic fields to 22 T, and measurements of T/sub
c/were made. The interest in this material (~1 at.%Ga in the
Nb/sub 3/Sn layer made by the "bronze process") stems from the
greater critical-properties' values (T/sub c/=17.4K, H/sub
c2/~22.5 T) relative to those of Nb/sub 3/Sn. Results are
given for two different specimen reaction temperatures, 700
and 750/spl deg/C. Increases in I/sub c/for low-fluence (/spl
Les/3x10/sup 18/n/cm/sup 2/) irradiations similar to those
observed for Nb/sub 3/Sn were observed, but with greater
relative increases for the specimens (750/spl deg/C) that had
the lower initial current density. Decreases in I/sub c/at
higher fluences scaled with the measured decreases in T/sub
c/similar to behavior of Nb/sub 3/Sn. Increases in H/sub c2/of
the lower-temperature-reacted specimens were much smaller than
those observed in Nb/sub 3/Sn. In the
higher-temperature-reacted specimens, the lack of any H/sub
c2/increases at all suggests that some kind of limiting
mechanism is active. For neutron fluences above ~ 5 \times
10/sup 18 n/cm/sup 2/the limiting of H/sub c2/is not in
evidence. |
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Preparation
of Nb/sub 3/(Sn-In) wires by a ductile fabrication
process
R. Akihama, K. Yasukochi
and R. Roberge
Summary: High critical field
and high critical current Nb/sub 3/(Sn-In) multifilamentary
wires were fabricated using an improved Sn base alloy. Nb rods
with one or nineteen holes were impregnated with a ductile
Sn-Cu-In alloy and reduced to final size wire by swaging and
cold drawing without intermediate annealing. Addition of In
and Cu to the Sn substantially improved the high field
critical current and upper critical field. At 4.2 K, the
nineteen core wire reacted at 900/spl deg/C for 9 hours has an
overall critical current density of 10/sup 4/A/cm/sup 2/at 18
T and an extrapolated upper critical field of 25.5
T. |
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High
field properties of diffusion-processed Nb/sub 3/(Sn - In)
tapes
R. Akihama, K. Yasukochi,
H. Okubo and Y. Iwasa
Summary: Upper critical
fields and critical currents of diffusion processed Nb/sub
3/(Sn-In) tapes were measured at 4.2 K as a function of
reaction temperature, reaction time, and the concentration of
indium in the matrix. These tapes were fabricated with pure
niobium steps rather than with indium doped niobium tapes, as
was the case reported previously. The highest H/sub
c2/achieved with the present samples is 28 T, which is
comparable with the best value obtained in the previous
samples with indium doped niobium. Above 12T, these samples
have higher critical currents than commercial Nb/sub
3/Sn. |
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Some
improvements in laves phase high-field
superconductors
K. Inoue, T.
Kuroda and K. Tachikawa
Summary: Improvements
in superconducting properties and workability have been
achieved both in V/sub 2/(Hf,Zr) and in V/sub 2/(Hf,Nb),
through the effects of additional elements. The V/sub
2/(Hf,Zr) can be fabricated into a conductor form by a
composite diffusion process in which a composite of vanadium
sheath and Zr-Hf alloy core is cold-worked and then heat
treated. The addition of hafnium to the vanadium sheath
considerably improves the critical current density J/sub c/,
and moreover it makes the fabrication of the composite easier
by reducing the difference in workability between the sheath
and the core. The multifilamentary V-(1-2)at%Hf/Zr-25at%Hf
wire may be practically useful for generating magnetic fields
up to 15T. The V/sub 2/(Hf,Nb) ternary alloys containing
hafnium less than 17at% can be fabricated by direct plastic
deformation at room temperature. The titanium addition to the
V/sub 2/(Hf,Nb) significantly improves both the
superconducting properties and the workability. In the
cold-rolled V-19at%Hf-6at%Ti-8.3at%Nb, transition temperature
T/sub c/of 9.9K, upper critical field H/sub c2/(4.2K) of about
22T and overall J/sub c/(4.2K) of 1x10/sup 4/A/cm/sup 2/at 13T
are obtained. |
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Practical
Nb/sub 3/Al wires: Experimental behaviour of small scale
solenoids
S. Ceresara, M. Ricci,
G. Pasotti, N. Sacchetti and M. Spadoni
Summary:
A number of preceding papers showed the practical
possibility of realizing copper stabilized Nb/sub 3/Al wires,
both in mono and multifilamentary configuration. Results are
now presented concerning the experimental behaviour of small
scale solenoids made by the wind and react method. Details of
the construction technique are given in the following. A
comparison between short sample characteristics and the
experimental results of the small scale solenoids has been
made. As a general result a reliable performance of these
solenoids has been obtained, thus giving reasonable confidence
on larger scale magnets based on this A-15
material. |
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Two
superconducting storage
rings-ISABELLE
J.
Sanford
Summary: This paper summerizes the
general features of the desin and the status of the project at
present time. It brings up to date the results reported aat
the National Particle Accelerator Conferece in March
1977. |
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Progress
report-fermilab energy doubler
A.
Tollestrup
Summary: This report will update
the Progress made on the Fermilab Energy Doubler/Saver. Since
the last report made to this Conference in 1976, over 100, 22
foot long coils have been constructed and studied. About 50
complete magnets have been constructed in the Magnet
Fabrication Facility by techniques that routinely turn out 5
magnets/week and can be easily expanded to 8/week. The tooling
is such that it can be easily duplicated. |
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BNL
superconducting storage ring magnet
update
A. McInturff, E. Bleser,
P. Dahl, J. Kaugerts, K. Robins and W.
Sampson
Summary: This report updates the
various performance data, design and specifications for the
storage ring and experimental series dipole and quadrupole
magnets as well as the working line and correction coil
systems. The working line system includes the closed orbit
dipoles, the quadrupole trim, sextupoles, octapoles, decapoles
and duodecapoles. These are the magnets in the standard
subunits of the Brookhaven National Laboratory Intersecting
Storage Ring Accelerator "ISABELLE". There have been 14 full
size single layer dipole prototypes 4.25 to 4.75 m long
constructed. There have been two 1.5 m long full size
quadrupoles built. A one meter double layer dipole was built
in order to obtain high field > 6.0 T data. A subunit of
the original 200 X 200 GeV version of the ISABELLE lattice was
assembled to study systems operation. (ISABELLE now has the
design goal of 400 X 400 GeV.) This subunit utilized the
proposed ISABELLE form of refrigeration, namely "high pressure
helium gas forced flow". The array composed of two dipoles and
a quadrupole with auxiliary working line and correction coil
systems was serially connected both electrically and
cryogenically. One of the major design goals, that of the
magnets being protected by the ability to absorb their own
energy, proved to be attainable. This is accomplished without
external intervention, i.e. external extraction or driving the
magnets normal by an active external circuit. This simply
means the magnets L/R time constants during a quench are short
enough to prevent thermal damage or run away. There are two
series of prototypes. The first one is the so-called
"standard" which will become the ring magnets for "ISABELLE"
when the machine design is frozen. The second is that in which
various characteristics and/or limits of various parameters
are explored. Various parameters such as maximum ramp rate and
other /spl dot/B dependent phenomena are bracketed even though
the present standard parameters are workable. There has also
been obtained in the last two years operational data on four
large (25 cm cold aperture, 2.5 m long) pool boiling dipoles.
They have been an integral part of the High Energy Unseparated
Beam guiding particles to the MPS (multiparticle spectrometer)
of the AGS (30 GeV alternating gradient synchrotron,
ISABELLE's injector). |
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Fermilab
doubler magnet design and fabrication
techniques
K. Koepke, G.
Kalbfleisch, W. Hanson, A. Tollestrup, J. O'Meara and J.
Saarivirta
Summary: During the last year, the
Fermilab Doubler magnets have benefited from a development
effort to upgrade the performance of the superconducting
magnets. This paper presents the results of this effort. The
design philosophy and the fabrication techniques used on
current magnets will be discussed, along with innovative
laminated tooling which has been designed to give Fermilab a
two dipole a day production capability. Specific topics to be
discussed are coil geometry, coil winding techniques, coil
clamp collars, buss geometry and insulation, integral quench
heaters for quench protection, coil twist, coil helium
irrigation and assembly techniques that assure azimuthal
preload and accurate coil size. |
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Design,
construction, and operation of 12 ESCAR bending
magnets
G. Lambertson, A. Borden,
J. Cox, W. Eaton, W. Gilbert, J. Holl, E. Knight, R. Main, R.
Meuser, J. Rechen, R. Schafer, F. Toby and F.
Voelker
Summary: The goal of the ESCAR
(Ecperimental Superconducting Accelerator Ring) Project was to
build a rapidly and operate a small, state-of-the-art, rapid-
pulsing proton synchrotron using superconducting magnets. The
magnetic field quality and beam tube vacuum were to be
sufficiently good to permit operation as a storage ring for
high current beams. The intereating systems aspects involved
in a complete cryogenic accelerator were felt to be as
challenging as imporatant as the development of reproducible
high-performance superconductiong magnets. |
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Development
and production of superconducting resonators for the argonne
heavy ion linac
K. Shepard, C.
Scheibelhut, P. Markovich, R. Benaroya and L.
Bollinger
Summary: The first six niobium
split-ring resonators for the Argonne Heavy-Ion Energy Booster
have been completed. The average performance at 4.2K is an
accelerating gradient of 3.7 MV/m or an effective accelerating
potential of 1.3 MV per resonator for an rf input of 4
W/resonator. The resonators are constructed in part of an
explosively bonded Nb-Cu composite material which performs
well for rf surface fields of at least 200 G. In initial
tests, the resonators frequently exhibit thermal instability
at E/sub a/< 3 MV/m because of several types of microscopic
surface defects. The methods used for locating, identifying,
and removing these defects are discussed. |
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A
superconducting synchrotron power supply and quench protection
scheme
R. Stiening, R. Flora, R.
Lauckner and G. Tool
Summary: The power
supply and quench protection scheme for the proposed Fermilab
6 km circumference superconducting synchrotron is described.
Specifically, the following points will be discussed: 1, the
46 MW thyristor power supply; 2, the 3 X 10/sup 8/joule
emergency energy dump; 3, the distributed microprocessing
system for the detection of quenches; 4, the thyristor network
for shunting current around quenched magnets; and 5, the
heaters internal to the magnets which cause rapid propagation
of quenches. Test results on prototype systems are
given. |
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Superconducting
generator technology-an
overview
J.
Edmonds
Summary: Application of
superconducting technology to the field windings of large ac
generators provides virtually unlimited field capability
without incurring resistive losses in the winding. Several
small-scale superconducting generators have been built and
tested demonstrating the feasibility of such concepts. For
machines of much larger capacity, conceptual designs for both
300 MVA and 1200 MVA have been completed. The results of those
studies lend credence to the viability of such technology on
the larger scale. The development of large, superconducting
generators will require a substantial, sustained effort to
produce a machine with performance and reliability equal to,
or greater than that of conventional generators. However, the
potential benefits include increased efficiency, improved
steady-state and transient stability, potential for higher
voltage machines, improved I/sub 2//sup 2/t capability,
potential for growth to higher ratings and voltages, reduced
size and weight, as well as reduction or elimination of onsite
generator fabrication. The Electric Power Research Institute
(EPRI) will fund a 300 MVA generator. Designed, engineered and
fabricated as a turbo generator, the superconducting machine
is to be installed in a powerplant, tested and operated in
concert with a prime mover, the steam generator and the
auxiliary support systems of the powerplant. This method of
demonstration will eliminate the more conventional method of
evaluating new generator technology and will provide answers
to the viability of operating a superconducting machine and
its cryogenic handling systems in a fulltime, demanding
environment. |
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Alternative
technologies for superconductor
fabrication
R.
Rose
Summary: Superconductor fabrication for
large magnetic devices has largely been accomplished by
continuous multifilamentary fibre methods based on extrusion
of composite billets followed by wire drawing. This paper
reviews other methods aimed at achieving the same
superconducting properties, but with other advantages. The "in
situ" technologies are really older than the mechanical
methods, which in fact were developed as a result of questions
raised by early "in situ" research. The technical
possibilities of the "in situ" approach were shown by Tsuei,
who used rapidly cooled Nb-Cu and Nb-Cu-Sn alloys which were
subsequently drawn into wire. High current densities are
attainable in these alloys due to percolation through a
contiguous structure of superconducting phase. By careful
microstructural control the critical current densities can
approach in magnitude those of continuous fibre bronze-matrix
composites. Similar microstructures may be developed from
powder composites, either of mixed powders or liquid
infiltrated powder or fibre structures. Controlled directional
solidification results in highly regular, oriented multiphase
structures which may be processed to form superconducting wire
composites. All of these methods are capable of high current
densities, and at least some offer superior mechanical
properties. In general, the use of phase transformations or
powder methods to produce composition modulations on a fine
scale has resulted in a class of materials with potentially
superior technical properties. |
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On
the micromechanics of multifilamentary superconducting
composites
S. Cogan, D. Holmes,
I. Puffer, T. Eagar and R. Rose
Summary: The
mechanical properties of multifilamentary Nb/sub 3/Sn
composites depend on several interrelated effects. The elastic
modulus is influenced by crystallographic texturing as well as
internal stresses. The textures as they develop in the
as-drawn composite are typical of FCC and BCC metals, as is
the oblate filament shape of the Nb filaments, which can be
understood in terms of the orientation restriction of slip
systems in such materials. Jc versus strain measurements on a
12.25% Nb composite with 1.2 micron diameter filaments show
that zero strain Jc's are obtainable well beyond 1.0% strain
on the first cycle. However, the internal residual stresses
are relieved by applied strains well below 1% and subsequent
Jc vs. strain performance is markedly impaired. The possible
advantages of ultrafine filament composites are
discussed. |
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Effects
of stress and strain on the critical current density of "In
Situ" multifilamentary superconducting wires in high magnetic
fields
R. Roberge, S. Foner, E.
McNiff, B. Schwartz and J. Fihey
Summary: The
effects of stress (or strain) on the critical current density
of "In Situ" multifilamentary superconducting wires are
evaluated in high field for axially stressed specimens. The
overall critical current density shows no degradation at 14
tesla for stress levels of /spl sim/700MPa for a Cu-36 wt %Nb-
20 wt % Sn material. Preliminary results on the effects of
large cyclic stresses are also presented. Prestress models are
discussed and compared with the "In Situ" results and
previously published experiments. |
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Superconducting
Cu-Nb/sub 3/Sn composites produced by cold extrusion of fine
powders
R. Flukiger, S. Foner, E.
McNiff, B. Schwartz, J. Adams, S. Forman, T. Eagar and R.
Rose
Summary: Multifilamentary Cu-Nb
composites were produced by cold P/M processing. Elementary
powders of /spl leq/ 40 /spl mu/m diameter were first cold
extruded and subsequently drawn to wires or ribbons. Cu-Nb/sub
3/Sn composites were produced by electrochemical Sn plating of
the Cu-Nb composite, followed by a reaction annealing at
temperatures between 650 and 850/spl deg/C. After
cross-sectional reduction by a factor R = 500, a sample of the
composition Cu-30%Nb (20 at%Sn) had a J/sub c/value of 10/sup
4/A/cm/sup 2/at 12 T, which is comparable to the values
obtained by other techniques. This cold P/M technique is
adaptable to a wide range of superconducting systems and
appears to be promising for industrial
applications. |
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Nb/sub
3/Sn composites with submicrometer
filaments
D. Finnemore, J.
Verhoeven, E. Gibson and J. Ostenson
Summary:
New fabrication techniques have been developed to
produce a homogeneous array of very long - very thin filaments
of Nb/sub 3/Sn in a Cu matrix. With the use of chill casting
and drawing techniques one can produce a superconducting
composite wire 0.025 cm in diameter which has approximately
500,000 Nb/sub 3/Sn filaments uniformly distributed across the
cross section. Typically the filaments have a ribbon shape
about 200/spl Aring/ thick and 4000/spl Aring/ wide. Critical
currents based on the total area are greater than 10/sup
6/A/cm/sup 2/in zero field and greater than 10/sup 4/A/cm/sup
2/at 14 Tesla so the performance compares favorably with
commercial tape or commercial multifilamentary wire. Special
features of the material are the ease of fabrication, the low
reaction temperatures (500/spl deg/C), strain tolerance, and
the strength of the material. |
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AC
losses in "in-situ" Nb/sub 3/Sn-Cu composite
conductors
A. Braginski, G.
Wagner, J. Bevk and J. Harbison
Summary:
Alternating current loss measurements were performed on
"in-situ" Nb/sub 3/Sn-Cu multifilamentary untwisted ribbon and
wire samples. A twisted wire sample was also characterized.
The Cu/Nb atomic ratio was 7 and the tin concentration was in
the range from 2.4 to 6.9 at. %. The annealing temperature was
550/spl deg/C for 60 or 288 hours. Critical-current densities
were in the range from 5 to 10x10/sup 9/A m/sup -2/at 0.2
tesla, 4.2 K. Total losses at 4.2 K have been determined
electronically in zero bias field, the frequency range of 20
to 1600 Hz and ac fields up to 0.25 tesla. The results
indicate that untwisted samples behave as filamentary
conductors with large filaments, one to two orders of
magnitude above the real filament size. The effective
transverse resistivity of the matrix is very low and eddy
current losses correspondingly high. |
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Degradation
mechanism of Nb/sub 3/Sn composite wires under tensile strain
at 4.2K
T. Luhman, M. Suenaga, D.
Welch and K. Kaiho
Summary: Bronze-processed
Nb/sub 3/Sn composite wire conductors exhibit changes in their
superconducting parameters when strained in tension. This
paper describes a detailed study of the effect of strain on
critical current and an analysis by optical and SEM techniques
of crack formation in the Nb/sub 3/Sn layer under strain. The
effect of strain history on both reversible and irreversible
changes in critical current and the roles of differential
thermal contraction induced residual strains and of Nb/sub
3/Sn cracking are discussed. |
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Design
studies of superconducting
generators
S. Minnich, T. Keim,
M. Chari, B. Gamble, M. Jefferies, D. Jones, E. Laskaris and
P. Rios
Summary: An Electric Power Research
Institute sponsored study of superconducting, central-station
turbine-generators of 300 and 1200 MVA rating has been
completed at General Electric. A unique construction concept
is used for the superconducting field winding. The goal of
this construction is to ensure that the winding can be
energized to the current and flux density predicted by the
superconductor short-sample data, without the "training"
instabilities frequently encountered in large coils. The
remainder of the rotor components includes an aluminum
radiation shield and a compound, steel-aluminum
electromagnetic shield, designed to withstand short circuit
electromagnetic forces. The study was focused on generator
model configurations which were proportioned to have a low
transient reactance, approximately 0.2 per unit. The critical
clearing times calculated for these configurations were
comparable to those of conventional units, the low reactance
offsetting an inherently lower rotor inertia. These generator
configurations are estimated to have about half the size and
weight, and about half the losses of conventional units of the
same rating. Transient characteristics are described,
including the effect of the rotor shielding configuration on
rotor damping. |
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System
considerations for airborne, high power superconducting
generators
H. Southall and C.
Oberly
Summary: The design of rotating
superconducting field windings in high power generators is
greatly influenced by system considerations. Experience with
two superconducting generators designed to produce 5 and 20
Megawatts has resulted in a number of design restrictions. The
design restrictions imposed by system considerations have not
prevented low weight and high voltage power generation
capability. The application of multifilament Nb/sub 3/Sn has
permitted a large thermal margin to be designed into the
rotating field winding. This margin permits the field winding
to remain superconducting under severe system operational
requirements. System considerations include: fast rotational
start-up, fast ramped magnetic fields, load induced transient
fields and airborne cryogen logistics. Preliminary selection
of a multifilament Nb/sub 3/Sn cable has resulted from these
considerations. The cable will carry 864 amperes at 8.5K and
6.8 Tesla. |
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Three
phase induction motor with a superconductive cage
winding
H. Brechna and H.
Kronig
Summary: The basic idea behind the
induction motor with a superconducting cage winding is to
combine the advantages of the induction motor (starting
torque, ease of operation, little maintenance) with those of
the synchronous motor (load independent speed, output power
control by means of the load angle) at the additional cost of
refrigeration power. |
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Design
limitations for superconducting field magnet arrays on large
hydrogenerators in the absence of
shielding
S.
Kuznetsov
Summary: This paper concentrates on
the electrical dimensioning of an entirely new class of
superconducting machines - the slow speed, multipolar
hydrogenerator; with special reference to the design of field
windings incorporating state-of-the-art Nb-Ti superconductor.
The conceptual machine used to illustrate the limitations on
the field array is a 60 Hz, 48 pole unit with a stator rating
of 400 MVA; overall characteristics are compared with a
conventional 615 MVA hydrogenerator. A specific
superconducting cable shape is recommended for the field
winding along with maximum charging rates, winding
inductances, and inter-magnet attractive forces. A 230 kV
stator winding is presented for investigating the large radial
and attractive forces acting on the superconducting rotor as a
function of load. Although the LHe refrigeration requirements
and the abnormally low synchronous reactance of the example
400 MVA machine tend to obscure its commercial value, the
limitations governing the field winding are applicable to
turbogenerator type superconducting rotors. |
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First
electrical tests on a 500 kW hypersynchronous
alternator
Y. Brunet, J. Mazuer
and M. Renard
Summary: The design and the
construction of a 500 KVA hypersynchronous cryoalternator were
presented early in the litterature. The interest of this
electrical configuration is explained. The development of the
experimental program is detailed. Operation, electrical and
cryogenic tests performed on the prototype are
reported. |
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MIT-DOE
program to demonstrate an advanced superconducting
generator
J. Smith, G. Wilson and
J. Kirtley
Summary: The objective of the
program is to demonstrate improved characteristics for large
central station superconducting generators. Based on the
EEI-EPRI program at MIT, new concepts have been devised,
analyzed and selected for a 10 MVA demonstration machine. The
present status of the project is given and the design of the
machine is outlined. |
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The
effects of irradiation on the copper normal metal of a
composite superconductor
J.
Williams, C. Klabunde, J. Redman, R. Coltman and R.
Chaplin
Summary: This report presents a new
body of magnetoresistance data for copper that were obtained
in neutron irradiation experiments at 4K and 330K. These data
are combined with previously obtained results on initial
damage rates and saturation effects to yield a projection of
total resistivity vs neutron dose (expressed in displacements
per atom) for copper in service at < 10K in a magnetic
field. |
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Effect
of subchannel flow velocities on the stability of hollow cable
conductors
J.
Chi
Summary: Forced flow, helium cooled
compacted twisted strand hollow cable conductors have been
proposed for use in a number of large superconducting coils.
The twisted strands and the compaction process result in
non-uniform void distributions that are expected to affect the
stability of the conductor and the helium pump work required.
For the design of a large coil for the Large Coil Program
(LCP), it was postulated that the flow velocity in the
smallest subchannel controls the recovery of the conductor
initially driven normal by a fault condition. However, the
helium pump work and the resultant refrigeration load are
determined by the nominal mass velocity. Consequently, for the
design of hollow cable conductors, a relation between the
minimum subchannel mass velocity and the nominal mass velocity
is needed. Parallel channel models were developed and
theoretical relations were derived between subchannel mass
velocities and the nominal conductor mass velocity. The
results show that the nominal mass velocity is several times
greater than the subchannel mass velocities required for
conductor recovery. The implications of this work with respect
to optimum conductor configurations are discussed; and further
research and development work required to better understand
subchannel flow effects are suggested. |
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Recovery
capability of multifilamentary superconductors with
non-uniform void distribution
A.
Lee
Summary: The effects of several abnormal
conditions resulting from a possible non-uniform void
distribution in a multifilamentary and cabled superconductor
on the cryogenic recovery capability of a triplex, a seven
strand bundle and a 19 strand bundle were studied. For a
triplex, if only one of the three strands was cooled, the
maximum recoverable initial normal temperature is reduced by
1/3 of the value for the nominal case in which all the three
strands are cooled equally. If the outer six strands of a
seven strand bundle were cooled and the center one was not
cooled, the recovery capability of this bundle is just
slightly below that of the nominal triplex. If only the outer
12 strands of a 19 strand bundle were cooled, the bundle would
not recover once it is driven normal. The effects of the cold
and stagnant helium located in the space between the strands
and of the insulation thickness are small. |
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Critical
boiling, vapor block, and prospects for control of
miniquenches in channel-cooled
magnets
S.
Sydoriak
Summary: A correlation to /spl
plusmn/13% is given for the wide-ranging data of 7 groups who
have measured steady-state critical nucleate boiling heat
flux, CHF, from a channel wall to helium flowing at known mass
flux G. There is no discernible trend in any of the plots of
CHF deviation with respect to the six relevant experimental
parameters. To achieve quench control in a partially quenched
multichanneled magnet, the average mass
flux\bar{G}must exceed G, due to vapor block in the
quench-affected channels. Equations derived here
for\bar{G}/Gare based on both laminar and turbulent
flow models. In a proposed method of quench control, isolated
miniquenches (involving up to one full turn in separate layers
of the conductor) are eliminated by a calculable flow of
coolant provided by an immersed circulating pump in parallel
with the magnet channels. Prospects for success are enhanced
by the fact that in typical forced-flow channel boiling, the
first (breakaway) and second (recovery) CHF's are reported to
be identical, in strong contrast to typical pool boiling. It
is also noteworthy that among systems designed to provide
(steady-state) CHF, the one considered here is unique in
providing a transient heat flux that is greater than the
critical steady state flux. |
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Stabilization
of large superconducting magnets in superfluid
helium
S. Van Sciver and O.
Christianson
Summary: Stability of large
superconducting magnets cooled with superfluid helium (He II)
is examined. Impetus for this research is the Wisconsin
Superconductive Energy Storage magnet system, which proposes
to be operated at 1.8 K. A survey of steady state heat
transfer in He II is presented in the context of magnet
stability. In addition, new results from an experiment to
study the transient response of He II show substantially
better heat transfer than that of normal helium (He I). For
example, a step-function application of heat flux at a rate of
2 W/cm/sup 2/can be absorbed by the helium for about five
seconds before film boiling begins. This time delay is caused
by the required temperature rise of the He II adjacent to the
hot spot. A comparison between the enthalpy rise in the helium
and the applied energy shows a close correlation. These new
results are discussed in terms of superconducting magnet
stability. |
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Tests
in helium to verify cryostability and
replenishment
R. Krause, E.
Christenson, R. Bradshaw and R. Tatro
Summary:
The cryostability analysis performed by General
Dynamics Convair Division, of its pool-boiling conductor pack
design for the Large Coil Program (LCP) required knowledge of
the helium heat transfer and replenishment characteristics.
This data was not available for the long vertical channels
with complex helium flow passages. A test specimen was
designed to simulate the flow and heat transfer
characteristics of a conductor pack for use in the LCP
superconducting magnet being designed by General Dynamics. The
test program was conducted in a 200-liter liquid helium dewar.
The test package simulated the flow in a single row, five
layers high, and was tested in both vertical and horizontal
orientations. The results showed that in the vertical
position, steady-state heat transfer was nonhysteretic, with
breakaway flux of 0.40 watt/cm/sup 2/. In the horizontal
position, steady-state heat transfer was hysteretic, with
breakaway flux of 0.40 watt/cm/sup 2/and recovery flux of 0.27
watt/cm/sup 2/. In both horizontal and vertical tests, helium
replenishment was sufficient to avoid dryout at steady-state
fluxes up to 0.8 watt/cm/sup 2/. This testing technique, to
verify the stability analysis for a large pool boiling magnet,
was inexpensive compared to the cost of the
magnet. |
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Maximum
attainable toroidal magnetic fields for
tokamaks
M.
Soell
Summary: A calculation method of
determining the maximum permissible magnetic field values for
superconducting toroidal magnet systems in tokamak reactors is
described. Simple scalings for the maximum magnetic field
values B/sub max/are found. The results of mechanical stress
calculations by the finite element method are introduced into
the formulas for calculating B/sub max/, yielding considerable
changes compared to the B/sub max/values calculated without
the "finite element correction". |
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A
toroidal field magnet system for
NUWMAK
S. Hong, P. Michaelson, I.
Sviatoslavsky, W. Young and R. Boom
Summary:
A conceptual design of a toroidal field coil system is
presented for NUWMAK, a compact fusion power reactor. The
magnet major radius is 5.13 meters, the minor radius is 3.84
meters and the maximum field in the coil winding is 11.5
Tesla. The magnets are to be cooled with 1.8 K superfluid
helium to allow operation at the highest fields present using
ductile NbTi instead of brittle Nb/sub 3/Sn as the
superconductor. Various advantages and disadvantages of this
design are compared to those of an optional Nb/sub 3/Sn magnet
design operating in 4.2 K pool boiling liquid helium. There
are only eight superconducting TF coils, allowing excellent
access for maintenance, and the resulting field ripple is
trimmed with close-in normal metal coils. These trimming coils
do not encircle the plasma but are saddle shaped, and so can
be changed out without disturbing the plasma chamber or the TF
coils. |
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The
influence of annealing and mechanical deformation on the
superconductivity and structure of DC-sputtered Nb/sub
3/Ge
J. Gregory, J. Bostock and
M. MacVicar
Summary: The relationship of
T/sub c/to film composition, lattice parameter, ordering, and
mechanical deformation in Nb/sub 3/Ge has been investigated.
Particular attention has been focused on annealing effects in
these films, both those occurring during the deposition as
well as post-fabrication effects. We find that inadequate time
or temperature of annealing during or after deposition does
not allow the maximum possible T/sub c/for a given composition
to be obtained, while excessive annealing induces the
precipitation of second phase Nb/sub 2/Ge/sub 3/with a
concomitant decrease in T/sub c/. The presence of second phase
material accelerates the decrease of T/sub c/with annealing.
Four-point bending measurements show T/sub c/'s of the films
deposited on Al/sub 2/O/sub 3/substrates are unchanged up to
strains ~.1%. The breaking strain of these Al5-ceramic
composites increases upon addition of a Au layer to the
composite. |
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Enhanced
transition temperature and upper critical field in V-Al/Cu-Ge
composite superconductors
K.
Tachikawa, H. Sekine and K. Togano
Summary:
Superconducting V/sub 3/(Ge,Al) composite tapes were
produced by the solid-state diffusion between a V-Al alloy
core containing 0-23at.%Al and a Cu-Ge alloy matrix containing
5 or 9at.%Ge. The transition temperature,T/sub c/, and the
upper critical field,H/sub c2/(4.2K), especially the latter,
are significantly increased with increasing aluminum
concentration in the vanadium core from 7K and 2T at 0at.%Al
to 12.5K and 18T at 23at.%Al, respectively. According to the
H/sub c2/versus temperature curve, H*/sub c2/(0) of /spl
sim/32.5T, Pauli-paramagnetic limitation parameter of /spl
sim/2.0 and spin-orbit scattering frequency parameter of /spl
sim/0.85 were obtained for the V-23at.%Al/Cu-9at.%Ge specimen.
The composite specimen with thinner V-Al core shows more
significant increase in T/sub c/by the heat treatment. The
T/sub c/, the H/sub c2/and the structures of the arc-melted
V/sub 3/(Ge,Al) specimens were also investigated to compare
with those of the composite specimens. |
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Anisotropy
of optimized and not optimized technical NbTi
superconductors
K. Best, D.
Genevey, H. Hillmann, L. Krempasky, M. Polak and B.
Turck
Summary: In addition to anisotropy
experiments with monocore superconductors having the same
final cold work condition but different numbers of
precipitation heat treatments, experiments with filament
conductors having identical numbers of heat treatments but
different final cold work condition have been performed.
Additional results of twisted and not twisted high filament
conductors are presented and discussed. The results are in
agreement with those of monocore conductors but additionally
at optimized and over optimized conductors an unexpected
increase of j/sub c/in the case B/spl perp/F for low aspect
ratios was observed. |
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Suitability
of NbTi containing up to 2 Wt.% Ta for use in fabrication of
superconductors
C. Curtis and W.
McDonald
Summary: Procurement specifications
for the widely used superconducting alloy Nb46.5Ti have a
maximum Ta content limitation of 1000 ppm by weight. To study
the effect of up to 2% Ta by weight in a NbTi alloy, three
Ta-modified Nb46.5Ti ingots containing 0.5, 1 and 2 wt.% Ta
nominal were produced. Fabricability of the modified alloys
compared to standard Nb46.5Ti during normal fabrication for
0.125 inch diameter wire is discussed. At 0.125 inch diameter,
a portion of each of the four types of wire was clad with OFHC
copper and drawn to final composite diameters of 0.010 inch
and 0.004 inch. Results of metallurgical studies conducted on
in-process material are discussed. Results of short sample
tests to determine critical current density for the 0.010 inch
and 0.004 inch monofilament samples, including the standard
Nb46.5Ti alloy for comparison, are presented. |
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On
the superconducting transition temperature of A-15 "Nb/sub
3/Si" synthesized by ion
implantation
M. Clapp and R.
Rose
Summary: A new technique has been
applied towards the synthesis of metastable A-15 Nb/sub 3/Si,
namely ion implantation of Si into an A-15 substrate. The
substrate material was A-15 Nb/sub 3/Al/sub 0.9/Si/sub 0.1/.
Its surface was depleted of Al by a diffusion anneal. The Al
deficiency was then replaced with Si by sequential Si
implantations at different doses and energies. This produced a
surface with a disordered bcc Nb structure. During subsequent
heat treatments, the surface layer recrystallized epitaxially
on the A-15 substrate into A-15 Nb/sub 3/Al/sub 0.2/Si/sub
0.8/. In an effort to optimize the superconducting transition
temperatures of the implanted layer, the stoichiometries,
recrystallization anneals and ordering anneals were varied.
The (Al + Si) concentration was varied in ~sim1% steps by
varying the length of the diffusion anneal and keeping the Si
dopant profile constant. Recrystallization anneals were varied
from 800 to 980/spl deg/C and were followed by ordering
anneals of 500 to 700/spl deg/C. The highest T/sub c/obtained
was 5.6K. The low T/sub c/could be intrinsic to Nb/sub 3/Si,
but may also be due to structural or stoichiometric defects in
the implanted layer. |
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Peak-effect
of NbN diffusion wires under hydrostatic
pressure
W. Sattler and E.
Saur
Summary: Superconducting NbN wires with
stoichiometric composition show a pronounced maximum in the
critical current curves just below B/sub c2/. This behavior is
called peak-effect. For the first time critical currents of
such samples have been measured under hydrostatic pressure as
a function of the magnetic induction. There is no significant
change in the critical current at lower fields up to 8 T,
where the peak starts to arise. A small but systematic shift
of the peak maxima to lower inductions with increasing
pressure is observed. Also the upper critical inductions B/sub
c2/change slightly under pressure to lower magnetic
inductions. In the reduced plot of the quenching curves (I/sub
c/vs.b=B/B/sub c2/) the peak maxima for different pressures
occur at constant reduced induction b/sub
p/=0.8. |
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Superconducting
behavior of Nb/sub 3/Ge/sub 1-x/Ga/sub x/films obtained by
chemical vapor deposition
R.
Bergner and V. Rao
Summary: Films of Nb/sub
3/Ge/sub 1-x/Ga/sub x/(x = 0 to 1) with the A-15 structure
were obtained on hastelloy, copper and quartz substrates by
chemical vapor deposition. The bulk alloys of composition
Nb/sub 3/Ge/sub 1-x/Ga/sub x/were first chlorinated and
subsequently reduced by hydrogen to form the thin films. The
ratio of H/sub 2//Cl/sub 2/flow rates and the residence time
of the gas were optimized to obtain single phase A-15 films.
Films of composition Nb/sub 3/Ge/sub .75/Ga/sub .25/exhibited
a very sensitive dependence of T/sub c/on the H/sub 2//Cl/sub
2/flow rate. T/sub c/of Nb/sub 3/Ge/sub .75/Ga/sub .25/was
found to increase very rapidly with decreasing lattice
parameter. The highest T/sub c/obtained for Nb/sub 3/Ge/sub
.75/Ga/sub .25/films was 18 K. Films of composition Nb/sub
3/Ge/sub .5/Ga/sub .5/and Nb/sub 3/Ge/sub .25/Ga/sub .75/had
relatively low T/sub c/(4 to 8 K). The dependence of T/sub
c/on lattice parameter for the various compositions was found
to be in good agreement with the universal defect behavior of
A-15 compounds proposed by Noolandi and
Testardi. |
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Synthesis
and magnetic field properties of transition metal substituted
A-15 systems
F. Cadieu and N.
Chencinski
Summary: We have been synthesizing
high T/sub c/systems based on Nb/sub 3/Ge in which another
transition metal atom, either Mo, Ti, or Zr, is partially
substituted for Nb. Systems have been prepared by selectively
thermalized sputtering onto heated substrates. The systems
investigated allow changes in T/sub c/to be observed due to a
variation in the electron per atom ratio as well as due to
atom size effects. Superconducting transition temperatures are
measured inductively which allows the identification of
homogenous samples. Sample composition analysis is obtained by
quantitative X-ray fluorescence. For the (Nb/sub .99/Zr/sub
.01/)/sub 1-x/Ge/sub x/system measurements of dHc/sub 2// dT
for fields up to 10 T for the resistive T/sub c/onset point
went from 4.1 /spl plusmn/0.4 T/K to 2.6/spl plusmn/0.3T/K as
varied from 0.24 to 0.19. At x=0.23 the resistive T/sub
c/onset was 18.9k (H=0). Extrapolation to 0 K for no Pauli
paramagnetic limiting gives H/sub c2/(T = 0 K) = 54 /spl
plusmn/ 5 T. |
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Structure
design of the westinghouse superconducting magnet for the
large coil program
F. Domeisen,
D. Hackworth and L. Stuebinger
Summary: In
the on-going development of superconducting toroidal field
coils for tokamak reactors, the Large Coil Program (LCP)
managed by Union Carbide Corporation will include the design,
fabrication and testing of large superconducting coils to
determine their feasibility for use in the magnetic fusion
energy effort. Structural analysis of the large coil is
essential to ensure adequate safety in the test coil design
and confidence in the scalability of the design. This paper
will discuss the action of tensile and shear loads on the
various materials used in the coil. These loads are of
magnetic and thermal origin. |
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Optimization
of the high field critical current density in MF Nb/sub 3/Sn
for magnet use
S. Hong and D.
Larbalestier
Summary: We here report new high
field critical current measurements on FM Nb/sub 3/Sn
conductors together with some bend test data on a conductor to
be used for a 15 tesla magnet system. The results show that
zero stress J/sub c/values of 280 A/mm/sup 2/(measured over
the bronze+Nb cross section) at 15 tesla are obtained. These
high values have now been obtained for 3 different composites
from 3 different sources, suggesting that it should now be
possible to specify high J/sub c/from filamentary Nb/sub 3/Sn
with some confidence. We have also made measurements of J/sub
c/under tensile stress in the field range 13-16 tesla,
obtaining values of over 500 A/mm/sup 2/at 15 tesla,
approximately twice as large as those obtained in zero stress
measurements. We also report on some room temperature bend
test measurements for which no degradation in J/sub c/was
observed until outer filament strains of at least 0.5% were
reached. |
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Braided
multifilamentary Nb/sub 3/Sn hollow superconductor and its
magnet
K. Agatsuma, K. Komuro, K.
Koyama, I. Todoriki, Y. Naganuma, O. Kohno and T.
Saito
Summary: Nb/sub 3/Sn hollow
superconductor and its magnet has been made possible by a
unique fabrication process. The hollow superconductor with
250344 Nb/sub 3/Sn filaments has been fabricated to have outer
dimension of 5 X 6 mm and inner one of 2.4 X 3.4 mm. By
"simultaneous conductor assembly and coil winding method",
effective strain induced in Nb/sub 3/Sn filaments can be far
smaller than apparent strain. The 5 mm thick hollow conductor
has been coiled into a magnet with inner diameter of 150 mm,
which is equivalent to 3.3 % apparent strain, without any
degradation. A test module magnet with four double pancake
coils has yielded satisfactory results as
expected. |
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Developments
in Nb/sub 3/Sn forced flow conductors for large
magnets
P. Sanger, E. Adam, E.
Gregory, W. Marancik, E. Mayer, G. Rothschild and M.
Young
Summary: The production of a Nb/sub
3/Sn forced flow conductor requires the development of the
techniques for reliably fabricating long lengths of compacted
cable in a stainless steel jacket. Development is needed in
two major areas: 1) cabling and preliminary compaction of the
cable and 2) wrapping, final compaction squaring of the full
conductor. The configuration of the Nb/sub 3/Sn strand is
presented and its performance characteristics are described.
Early investigations have been made using handmade cables and
the sample length was limited to 5 m by the compaction
techniques used at the time. Machine made copper cable has
been completed and differences are also described. Finally
developments in the continuous wrapping and compacting process
will be discussed. |
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Cryostability
in force-cooled superconducting
cables
M. Hoenig, A. Montgomery
and S. Waldman
Summary: In past experiments
with force cooled NbTi cabled conductors we discovered that
cryostability was essentially independent of coolant velocity.
In order to verify these results we performed experiments
using a 3m long cable with a Nb/sub 3/Sn superconductor. The
paper presents our new experimental results in comparison with
previous data. |
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Multifilamant
Nb/sub 3/Sn multistrand cable performance using supercritical
helium
A. Montgomery and M.
Hoenig
Summary: We have measured the effect
of change of supercritical helium flow on stability of a
cabled multifilamentary (MF) Nb/sub 3/Sn conductor. We varied
the flow of supercritical He through the cable and measured
the ability of the cable to absorb single pulses of magnetic
energy while the cable was carrying about 73% of its measured
critical current in an applied 9.0T magnetic field. We have
quantified the pulse energies in terms of the thermodynamic
behavior of the helium fluid within the cable. Varying fluid
mass flow rate G in the range 0 /spl leq/ G /spl leq/ 36
g/cm/sup 2/s, we have observed stability of the MF Nb/sub 3/Sn
cable which is apparently as independent of G as were
previously investigated MF NbTi cables. Thus our present and
previous findings in 3m length MF superconductor cables
suggest the dominance by flow independent heat transfer
mechanisms, rather than the flow generated turbulence
previously considered dominant. |
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Lattice
braided superconductors
M.
Janocko
Summary: A technique has recently
been developed for the braiding of large numbers of
superconducting wires into fully transposed, rectangular cross
section conductors of low aspect ratio. The technique is
derived from textile industry lattice braiding used to
manufacture square cross section braids which are not,
however, fully transposed. The void structure of the lattice
braid cross section is superior, for axial forced flow
cooling, to folded flat braids and to triplex cables. The
lattice braided conductor also has an effective transposition
length which is much shorter than its actual transposition
length, and, because of the geometry of the wire path through
the cross section, axial magnetic field coupling is cancelled
out over lengths which can be only fractions of the actual
transposition length. A prototype braid sample has been
fabricated from insulated superconducting wire which exhibits
the advantages expected. Application of this type of conductor
to large force-cooled and/or pulsed coils is
discussed. |
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Cryostatic
influence of formvar coatings on sizing of the stabilizer in
superconductive NbTi-Cu: Conductor bundle
effects
S. Caspi, J. Lee, Y. Kim,
C. Chuang, R. Allen and T. Frederking
Summary:
In continuation of single-conductor studies, quench
onset data of quasi-steady operation are reported for a
formvar-coated composite (Nb48Ti/Cu) in bundle geometry
(conductor thickness /spl ap/ 0.2 cm, approximately square
cross section). Overall thermal conductances of the heater
simulation technique applied are of the order 0.1 W/(cm/sup
2/K), comparable to single-specimen results, around 4 K and
below the lambda transition in agreement with model
predictions. Considerable deterioration however occurs below 3
K and down to the lambda temperature during operation in
near-saturated He I. Consequences for magnets of intermediate
energy density are discussed with emphasis on the
modifications necessitated by coatings. |
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Investigations
of a steel reinforced NbTi superconducting flat cable for
toroidal magnets
E.
Seibt
Summary: Studies of the mechanical and
electrical properties of a steel reinforced NbTi
superconducting flat cable (8 kA/8 T at 4.2 K) to be used in a
toroidal magnet system (TESPE) are presented. Young's moduli
and tensile strength values of cable samples and single
components were derived from stress-strain characteristics and
compared with rule of mixtures calculations. Measurements of
mechanical cyclic fatigue performed at room and liquid helium
temperatures up to 10/sup 5/cycles within the elastic strain
range show different effects for NbTi/Cu single strands.
Electrical measurements were performed in order to get
information about changes in critical current and resistivity
due to static and cyclic stress loading. For static strains
less than 0.5%, no degradation in critical current was
detected, but the residual resistivity ratio of the cable
conductor was reduced by about 40 %. In addition, the training
behavior was studied of prestrained and thermally treated
samples. |
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Development
of NbTi conductor for high field
applications
H. Segal, K.
Hemachalam, T. De Winter and Z. Stekly
Summary:
The processing parameters of NbTi for use in magnetic
fields above 10 Tesla at temperatures of 4.2 K and lower have
been optimized. In particular, the time, temperature and
number of heat treatments as well as the amount of cold work
reduction have been analyzed for approximately 100 samples.
Two alloy compositions, namely Nb-46.5 wt. pct. Ti and Nb-55
wt. pct. Ti, were used in this program. In all cases, the
lower titanium concentration alloy had the superior
performance at high fields. Superconductor critical current
densities greater than 1x10/sup 5/A/cm/sup 2/were measured for
Nb-46.5% Ti in fields as high as 11 Tesla at a temperature of
1.5 K. Such a current density is typical of fields of 6 or 7
Tesla at 4.2 K. Many of the results at reduced temperature
compared very favorably with those of Nb/sub 3/Sn at 4.2 K.
For example, at 12 T and 2 K, conductor current densities of
2.7x10/sup 4/A/cm/sup 2/are found for NbTi. At 12 T and 4.2 K,
a typical conductor current density of Nb/sub 3/Sn is less
than 2.5x10/sup 4/A/cm/sup 2/. A small coil containing 100
meters of wire was operated in the bore of a Bitter magnet. At
1.5 K the coil operated without quenching with a maximum field
at the windings in excess of 14 Tesla. Based on these results,
NbTi operating at reduced temperatures appears to be a good
alternative to Nb/sub 3/Sn for conductor in high field
magnets. |
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Evaluation
tests for large scale multi-filamentary Nb/sub 3/Sn
superconductors
N. Tada, K.
Aihara, Y. Hotta, Y. Ishigami and H. Moriai
Summary:
Six different configurations for multifilamentary
Nb/sub 3/Sn conductors for large scale magnets with high
magnetic fields were developed and tested. These conductors,
which could carry approximately 1kA at 13T, were cryogenically
stable, reinforced superconductors 4.5mm X 10mm in size.
Critical currents, stability and the effects of stress were
measured in short sample tests. Conductor stability was
related to barrier configuration. At stresses above 250-800
MPa the critical current was significantly degraded, with the
magnitude of the reduction depending on reinforcement
techniques and the construction of the conductor. A monolithic
conductor which was composed of filament groups surrounded by
barriers, each of which was composed of 331 filaments, and
which was about 30% stainless steel by total volume was found
to be suitable for large scale magnets with high magnetic
fields. |
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Production
system for conductor of the mirror fusion test facility
magnet
W. Marancik, E. Adam, E.
Gregory, E. Mayer and P. Singh
Summary: A
production system has been designed, constructed, and operated
that is capable of processing Mirror Fusion Test Facility
(MFTF) stabilized superconductor. The MFTF conductor consists
of an embossed strip, with uniformly spaced holes along its
length, wrapped around and soldered to a 6-mm-sq multifilament
superconductor core. The embossing provides an internal
extended heat transfer surface as well as additional copper
stabilization. The conductor can be made at a rate of 100 m/hr
in single lengths of up to 600 m each. About 3000 m of
conductor per week can be made on a one-shift basis. Three
separate production lines have been constructed: a
stabilizer-sheath fabrication line, a plating line, and a
sheath wrapping and soldering line. Each of the three lines is
capable of operating independently and simultaneously and is
instrumented for control and inspection of the
product. |
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50-kA
conductors for tokamak poloidal field
coils
J. Wollan, J. Rogers, B.
Zeitlin, M. Walker and T. DeWinter
Summary:
The Los Alamos Scientific Laboratory (LASL) is building
a prototype 20-MJ, 50-kA superconducting poloidal field coil
to demonstrate feasibility for tokamak poloidal field coil
systems. The coil must be capable of being swung from +7 T to
-7 T in one second. It must carry 50 kA at 7 T, be cryostable,
but have low losses under pulsed operation. LASL contracted
with Magnetic Corporation of America (MCA) and Intermagnetics
General Corporation (IGC) to produce 7.6 m of prototype 50-kA
conductors and longer lengths of the various subcable units.
The cable designs and the manufacturing experiences of the MCA
cable are discussed. Energy loss measurements on the subcables
of the MCA cables and on prototype IGC conductors are
presented and discussed in terms of projected losses for the
20-MJ coil. |
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On
the preparation of "In Situ" multifilamentary superconducting
wires
R. Roberge and J.
Fihey
Summary: The "In Situ" technique for
fabricating superconducting multifilamentary composites is
reviewed. The casting technique is described. Our previously
reported observation of a sharp transition in the critical
current density at a critical concentration is supplemented by
the observation of a sharp increase in the hardness for a
niobium concentration of around 15 wt %. Also observed is a
sharp transition in the J/sub c/at 12 Tesla as a function of
Nb concentration again at about 15 wt %. The "In Situ"
technique has now achieved overall J/sub c/'s comparable to
those of commercial multifilamentary composites. Furthermore
the mechanical properties of "In Situ" materials have improved
strength. We are exploring production at larger scale by a
continuous casting process. |
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30-MJ
superconducting magnetic energy storage (SMES) unit for
stabilizing an electric transmission
system
J. Rogers, H. Boenig, J.
Bronson, D. Colyer, W. Hassenzahl, R. Turner and R.
Schermer
Summary: Electric power systems that
have major loads and generation centers separated by large
distances may experience low-frequency power oscillations.
This type of oscillation has occurred on the Pacific AC
Intertie that connects southern California and the Pacific
Northwest. A separate, almost parallel, dc-transmission line
also connects these areas. The Bonneville Power
Administration, which operates this transmission system, has
overcome the instability by controlling the power transmitted
on the dc-transmission line. A 30-MJ (8.4-kWh) superconducting
magnetic energy storage unit with a 10-MW converter could also
provide damping for this instability. The conceptual design of
the 30-MJ coil and the cryogenic and electrical components of
the system are described. The system is to operate at a
maximum current of 5 kA and will modulate the AC Intertie at
0.35 Hz. Discharge will be controlled to retain a minimum
stored energy of 20 MJ to limit cyclic strains in the coil and
ac losses in the conductor. The conductor will be made of
multistrand-copper and copper-matrix, multifilament NbTi
superconducting wires on a stainless steel
mandrel. |
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Applied
superconductivity activities at
siemens
G.
Bogner
Summary: A 35 m long semiflexible
single phase ac test cable for 110 kV and 10 kA with full load
terminations was successfully tested with respect to its
cryogenic and electrical behaviour. Within the development of
two-pole generators with output ratings from 1000 to 3000 MVA
the electrical losses of appropriate field winding conductors
are determined under transient operation conditions. The
characteristics of a rotating bath cooling are investigated
with special emphasis on stability problems. Experimental
studies on structural rotor materials are also being carried
out. In the course of our work on magnetic levitation a 17
tons test carrier has been levitated several times at an air
gap of 100 mm and speeds up to 150 km/h. At present a linear
synchronous motor with superconducting field windings and
active track is just before completion and will go into
operation late 1978. With regard to the future European fusion
program and the US-DOE Large Coil Task (LCT) we have initiated
activities on the development and fabrication of large
D-shaped magnets. |
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Operational
characteristics of a flux pump
O.
Mawardi, A. Gattozzi and H. Chung
Summary:
The mechanism of switching that is taking place in a
moving flux spot in a superconducting foil has been
investigated in details. The structure of the current,
temperature and magnetic field in the leading edge of the spot
has been derived theoretically. The resulting distributions
for these field variables have then been used to estimate the
effective inductance and resistance of the spot. These
parameters allow the determination of an equivalent electric
circuit for the flux pump as an electric generator. The ratio
of the inductance to the resistance of the spot decreases with
the inverse square of the velocity for small velocities. On
the other hand, the external current generated by the flux
pump first increases with the velocity, it then reaches a
maximum and eventually decreases inversely with the velocity.
Design parameters, such as efficiency and dependence of the
output current on the velocity for the pump, have been
evaluated. These theoretical predictions were found to be
verified by experimental observations which are also presented
in this paper. |
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Helium
requirements for a superconducting power
network
M. Hilal, E. Stone and S.
Van Sciver
Summary: A review of the helium
requirements for future superconductive energy technologies is
presented. The first section of the paper includes a survey of
commercial sized units of five energy related devices:
superconductive magnetic energy storage, magnetically confined
fusion reactors, superconducting power transmission lines,
magnetohydrodynamic units, and superconducting ac generators.
Results of the survey are discussed in terms of requirements
for typical size units on a per unit power basis. Estimates
are based on published system studies and private information
provided by various workers in the field. In the second
section, the helium inventories for several superconducting
power networks are presented. Assuming typical size units, the
helium required to supply a megawatt of power to a load is
estimated. The results indicate that large helium quantities
are needed for all phases of superconductive energy
generation, storage and transmission. |
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Performance
study of inductive output cryotronic
converters
A. Dronov, V. Ignatov
and A. Misyulin
Summary: A matrix method of
electric circuit analysis was used for the theoretical
performance study of a converter based on both a fullwave
rectifier and a cyclic superconducting transformer with two
cryotrons. The solution for the loop current matrix allows to
calculate the transient commutation process in any cryotronic
converter under any shape of supply voltage. Expressions are
given for the converter maximum current under load conditions.
The experimental study of the commutation processes was
conducted on the model of a
cyclic-superconducting-transformer-based cryotronic converter
at the working current of 800 A in the secondary coil ratio of
the primary and secondary 800:1, of the superconducting
transformer without a ferromagnetic core. In the cylindrical
cryotrons the switching element is made of a bifilaly wound
Pb-3.8% Sb tape and is placed between the control winding and
the superconducting shield. In resistive state the valve
resistance is 5.5x10/sup -3//spl Omega/ at 4.2 K. A
superconducting solenoid consisting of four disk coils
connected in parallel served as the converter load, its
inductance being 0.15x10/sup -3/H. The measurements of
currents in the secondary circuits and voltage drops on the
cryotron valves were made by Hall probes and potential probes,
respectively. The commutation processes were studied in
conditions of pumping the current into inductive load and
energy recuperation accumulated in the load through the
superconducting transformer into its primary. The paper offers
current oscillograms for the windings of control and voltage
drops on the valves of both cryotrons, and also oscillograms
for the transformer's primary and secondary and the
load. |
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Performance
tests of a 1.5 MJ pulsed superconducting coil and its
cryostat
S.-H. Kim, S.-T. Wang,
W. Praeg, C. Krieger and M. Lieberg
Summary:
To demonstrate cryostability of large pulsed
superconducting ohmic heating (OH) coils for Tokamak power
reactors a 1.5 MJ coil was developed at Argonne National
Laboratory (ANL). The coil was first charged up to 11.2 kA dc,
which is the critical current of short sample cables at 4.5 T.
Above the critical current, an unbalance voltage of the coil
increased smoothly indicating stable current sharing between
the superconducting filaments and the copper stabilizer. The
coil was then pulsed with a 7 MW power supply. The highest
ramping rate was 11 T/s. AC losses, mainly eddy current losses
in the copper stabilizer, were proportional to /spl dot/B/sup
2/ with about 2.5 kW at 9 T/s. After 3000 pulses no
degradation of the coil performance has been observed. A
non-metallic cryostat was developed for testing of the coil to
avoid eddy currents in the cryostat. |
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Conceptual
designs of 50 kA 20 MJ superconducting ohmic heating
coils
S. Singh, J. Murphy, M.
Janocko, H. Haller, D. Litz, P. Eckels, J. Rogers and P.
Thullen
Summary: Two conceptual designs of 20
MJ superconducting coils are described which were developed to
demonstrate the feasibility of an ohmic heating system. Both
NbTi and Nb/sub 3/Sn superconductors were considered for both
7 tesla and 9 tesla maximum fields. Cabled and braided
conductors were investigated and the braided conductors are
identified as the best alternative due to their high operating
current densities and because of its porosity. The coils are
designed to be cryostable for bipolar operation from +7 tesla
to -7 tesla and from +9 tesla to -9 tesla maximum fields
within 1 second. The structural design addresses the
distribution of structure and structural materials used in the
pulsed field environment. Both the immersion cooled (pool
boil) and the forced flow cooled coils are
described. |
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SLPX-Superconducting
long-pulse tokamak experiment
D.
Jassby, J. File, G. Bronner, J. Clarke, H. Johnson, G. Martin,
J. Murray, M. Okabayashi, W. Price, P. Rogoff, C. Singer, L.
Stewart, J. Bundy, S. Gralnick, T. Luzzi, J. Marino, D.
Sedgeley, P. Eckels, P. Gaberson, J. Murphy, R. Hay, K. Lind,
C. Paulson, G. Fuller and A. Gaines
Summary:
The principal objectives of the SLPX (Superconducting
Long-Pulse Experiment) are: (1) to demonstrate quasi-steady
operation of 3 to 5 MA hydrogen and deuterium tokamak plasmas
at high temperature and high thermal wall loading, and (2) to
develop reliable operation of prototypical tokamak reactor
magnetics systems featuring a toroidal assembly of high-field
niobium-tin coils, and a system of pulsed niobium-titanium
superconducting poloidal-field coils. This paper describes the
status of the engineering design features of the SLPX, with
emphasis on the magnetics systems. The toroidal-field coils
have an aperture of 3.1 m X 4.8 m, and can operate with a
maximum field at the conductor of 12 T. The superconducting
poloidal field magnetics system consists of a pulsed NbTi
central solenoid, and a set of d.c. NbTi equilibrium-field
coils. The entire machine is enclosed in an outer vacuum
container equipped with re-entrant ports that provide ambient
access to the room-temperature plasma vessel. |
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A
10 tesla toroidal field magnet system for general atomic's
PGFR concept
J. Alcorn, J.
Purcell and W. Chen
Summary: General Atomic's
concept for a reactor compatible superconducting toroidal
field coil is presented. The concept employs bath cooled,
copper stabilized NbTi conductor to generate 10 tesla at a
nominal temperature of 4.4 K. The emphasis of this paper is
upon the coil configuration, stability criterion, cryodynamic
performance, and support of magnetic loads. The guiding
principles of the design are fabrication economy, reactor
compatibility, and operational reliability. |
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Quench
protection for a 2-MJ magnet
J.
Taylor, M. Alston-Garnjost, P. Eberhard, G. Gibson, M. Green,
B. Pardoe, M. Pripstein, R. Ross, R. Smits and P.
Lecomte
Summary: A superconducting solenoid
with conductive bore tube has been used at energies up to
1.9-MJ to test various methods of quench protection. The
methods all involve shifting the main coil current to the
conductive bore tube and include (1) allowing the quench to
evolve naturally, (2) interrupting the primary circuit while
providing a varistor used as a shunt across the coil, and (3)
turning the entire magnet normal by dumping a short pulse of
current from a capacitor bank through the
windings. |
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A
microprocessor based superconducting magnet protection
system
R. Di Gesare and M.
Hennessy
Summary: Historically,
superconducting magnets have been constructed with little
regard to instrumentation. Several large expensive magnet
systems have fauled due to inadequate instrumentation.
usually, it is such an effort to produce an operating cryostat
and magnet that little effort is expended in the remaining
task to protect the magnet from human negligence and
improbable but possible devaastating faulure modes. Several
examples of this are documented in the exellent report by BNL.
Fortunately, the time has comewhen this has been realized and
several recent, large, superconducting magnet systems are
being built with adequate instrumentation. |
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The
inductive quench propagation in a NbTi-magnet as the
dominating effect for the transient current distribution in a
NbTi-Nb/sub 3/Sn hybrid magnet
P.
Turowski
Summary: The transient behaviour of
a NbTi-Nb/sub 3/Sn hybrid magnet after a quench was
investigated. The NbTi-coil showed a fast decay time of the
magnet current due to an inductive quench propagation
mechanism. High internal voltages were the consequence of the
graded current density. In the magnet system, protected by
shunt resistors, the NbTi-coil became the dominating part in
the transient events due to the fast development of internal
resistance and the resulting fast field decay. |
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Author
Index (1978)
No author
information available
Summary: Not
available |
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