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| 1967 |
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Front Cover (1967)
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author information available
Summary: Not
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Table of Contents (1967)
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author information available
Summary: Not
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Special Section on Applied Superconductivity -
Preface (1967)
No author information
available
Summary: Not
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Device Applications of Superconductive
Tunneling
B.N. Taylor
Summary:
Although superconductive tunneling has proved to be a
very useful tool for investigating the phenomenon of
superconductivity, it also has many potential device
applications worth further development. Examples of such
devices based on the tunneling of normal electrons might
include an rf oscillator and amplifier, a low-temperature
thermometer, and a generator and detector of microwave
phonons. Examples of devices based on the tunneling of paired
electrons, that is, the dc and ac Josephson effects, might
include a magnetometer, computer elements, a generator and
detector of microwave radiation, and a voltage standard. It is
the purpose of this paper to briefly review normal electron
tunneling as well as the dc and ac Josephson effects. how
these different tunneling phenomena manifest themselves, and
how these manifestations can be useh for these several device
applications. |
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Tunneling and Weak-Link Superconductor Phenomena
Having Potential Device Applications
D.E.
McCumder
Summary: A general physical picture
of superconductor weak-link junctions is outlined with
emphasis on phenomena relevant to device applications. The
total junction current contains a dissipative quasiparticle
component nearly analogous to the current in a normal-state
junction plus an electron-pair component responsible for
superconducting effects such as the Josephson effects.
Stochastic relaxation processes such as have been commonly
associated with vortex motion and flux flow sometimes perturb
the pair current and must also be taken into
account. |
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Superconductive Phenomena in Ultrathin
Films
M. Strongin and O.F.
Krammerer
Summary: Experiments on very thin
films and systems composed of small particles have shown
transition temperatures significantly higher than the bulk
T/sub c/. Of the numerous ideas advanced to explain the high
T/sub c/'s in these films, one of the most exciting is the
suggestion of Cohen and Douglass that superconductive pairing
can occur across dielectric barriers, and thereby lead to high
T/sub c/‘s. Recent results on the r, of thin superconductors
in contact with dielectric barriers, evaporated on cryogenic
substrates, are presented. Some renresentative T/sub e/‘s
obtained in these exneriments are AI/spl sim/5.7°/K. Zn/spl
sim/1.9°K. Sn/spl sim/6°K. In/spl sim/45°K. and Pb/spl
sim/7.1°K. In the-case of Al, Sn, and Zn these T/sub c/’s are
much higher than the values found by Bucked and Hilsch for
films deposited on quartz at liquid-helium temperatures.
Explanations other than the dielectric barrier idea, such as
"amorphorus structure" and the suggestion of Ginzburg that
surface interactions may modify T/sub c/, are also discussed.
In the course of these experiments to investigate high T/sub
c/’s in films it was found that very thin continuous films
(-20 /spl Aring/ or less) could be prepared at cryogenic
temperatures. An investigation of superconductivity in these
ultrathin films showed that T/sub c/ was much lower than the
T/sub c/ of the thicker films. This occurred in all the metals
measured, which included Pb, Al, Sn, and Bi. An explanation of
these results in terms of the destruction of long-range order
in two-dimensional systems due to fluctuations in the
Ginzburg-Landau order parameter will be discussed. By using
this idea and numerical values from the data. estimates can be
made of the possibility of obtaining very high T/sub c/‘s in
films approaching a few atomic layers. |
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Vortex Flow in
Superconductors
G.B.
Yntema
Summary: Electrical resistance
observed in superconductors in the mixed state is interpreted
as a measure of the motion of Abrikosov vortices in a
direction transverse to the imposed net current. Additional
evidence of flow of vortices has been provided by dc
transformer action and by heat transport in the direction of
vortex flow. The connection between the resistive voltage drop
and the flow of vortices is understood in terms of the
superconducting order parameter, which is a complex number
varying in space and time. A vortex, which is formed at one
edge of the sample, moves across it, and is destroyed at the
other edge, has a "zipper" effect on the phase of the order
parameter. On one side of the path of the vortex, the phase is
raised by ?spl pi/ (for a single quantum vortex) ; on the
other side it is lowered by the same amount. This process
makes no net change in the physical state of the sample; yet
it requires a pulse of voltage difference between the ends of
the sample, because the time derivative of phase of the order
parameter is proportional to electrostatic potential (more
generally, to the chemical potential for electrons). A state
of steady flow of vortices thus involves a steady difference
of potential between the ends of the sample. A voltmeter
registers this difference. There is no net induced emf to be
registered. The dissipation associated with the electrical
resistance of a sample in which there is vortex flow occurs in
the form of Joule heating produced by normal (i.e., nonsuper)
currents. Most of this dissipation is in the cores of the
vortices, where the material is at least approximately normal
and where the electric field is strongest. The electric field
in a moving core is partly induced magnetically but is mostly
the gradient of electrostatic potential which is associated
with the rapid changes of order parameter on opposite sides of
the core. A moving vortex not only produces heat but also
carries heat along with it, transversely to the electric
current and to the magnetic field. A plausible model for the
mechanism of this heat transport is based on the-available
excited states of the superconducting system of electrons as
described by BCS. Each available level has a thermal
probability of being occupied. The spectrum of levels
available varies from place to place in the material according
to the local value of the energy gap, which practically
vanishes in the core of each vortex, but is significantly
large between cores. A particular excitation can migrate only
in regions where the energy gap is less than the excitation
energy. Each low-energy excitation is therefore trapped,
rattling about within a definite core. When a core moves, the
trapped excitations are carried along. When a vortex is
eventually destroyed at the edge of the sample, its trapped
excitations are stranded at the last position of the core. As
the gap there goes up, so does the energy of each excitation.
The excitation probability which corresponded to thermal
equilibrium at the orignal energy is excessive at higher
energies. Until the energy becomes so great that the
excitation is no longer trapped, the excitation probability
can readjust only by a net probability of conversion of energy
from the electronic excitation into lattice heat. Similarly,
when a vortex is formed, its core absorbs heat from the
lattice. The net result is transportation of heat from the
location of formation to the location of destruction. The
detailed mechanisms by which forces are applied to vortices
remain obscure. But by thermodynamic arguments we find a force
in the direction of j X B due to net electrical current and a
thermal force in the direction of -VT. In a superconductor in
which the pinning of vortices is slight, we should be able (at
least as laboratory curiosities) to use vortex flow as the
basis of an electrically driven lowtemperature refrigerator
and of a thermally driven source of electrical
energy. |
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Pair-Breaking Mechanisms in
Superconductors
R.D.
Paris
Summary: A review is given of the
theoretical and experimental situation concerning the problem
of superconductivity in the presence of pair-breaking
perturbations. The problem was first considered by Abrikosov
and Gorkov, who advanced a theory which explained the results
of the experiments by Matthias and co-workers on the lowering
of T/sub c/ of superconductors containing small concentrations
of magnetic impurities. The theory predicted further that the
presence of magnetic scattering centers severely distorts the
excitation spectrum of a superconductor and that for
sufhciently large spin concentrations the energy gap
disappears from the spectrum, even at T=0/spl deg/K. It has
since been found that the AG theory can be extended to treat
other pair-breaking situations which lead to second-order
superconducting-normal phase transitions. Examples of these
are the vortex state, the surface sheath state. the proximity
effect, small superconductors in large magnetic fields,
superconductiyity in the presence of high currents, and
superconductivity in the presence of strong Pauli
paramagnetism. In the dirty limit (where the mean free path is
much smaller than the zerotemperature coherence length) the
different pair-breaking regimes are equivalent in that their
behavior is specified by a unified single parameter theory. In
transforming from one pair-breaking regime to another, one
needs only to change the pair-breaking parameter. Experimental
results from the different depairing regimes are presented and
compared with the predictions of the unified
theory. |
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High-Temperature
Superconductors
T.H.
Geballe
Summary: Superconductors with the
highest known transition temperatures to date occur in
P-tungsten phases which are rich in Nb/sub 3/AI. It has been
found possible to raise the temperature of NbzAI (T/sub
3/=18/5/spl deg/K) to above 20°K by replacing some of the Al
with Ge. The low values found for the electronic-heat-capacity
coefficient, and the almost temperature-independent magnetic
susceptibility contrast with the corresponding properties of
other high-temperature /spl beta/-tungsten structures such as
V/sub3/Si. Thus, the location of the Fermi level at a narrow
maximum of the density of states in energy which has been used
as a model to explain the high specific heat and
temperature-dependent susceptibility of V/sub3/S and the other
high-temperature /spl beta/-tungsten structures does not apply
to Nb/sub 3/ (Al, Ge) . No distinctive properties of the
normal state have as yet been measured. The variation of the
transition temperature with valence electron count works well
on a coarse-grained scale for /spl beta/-tungsten structures
but does not account in detail for the observed transition
temperature as a function of nominal composition. The effect
of stoichiometry, order, strain, and the presence of phases
other than /spl beta/-tungsten all play some role in
determining the observed transition temperature. |
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Superconducting Magnets for High-Energy
Physics
H. Brechna
Summary:
Superconducting magnets have established themselves as
useful tools in solid-state physics, magnetooptical
experiments, NMR, MHD, plasma, and other areas of physics. In
high-energy physics only the bubble-chamber physicist has
shown ample interest in using superconducting magnets. Reasons
why there is a reluctance against large superconducting
magnets in combination with high-energy physics experiments
and accelerators are discussed. However, in various areas of
high-energy physics superconducting magnets may be utilized,
such as accelerator, beam-transport, and experimental magnets.
This paper summarizes physical properties of superconducting
systems for experimental and beam transport magnets in
quantitative form. Charging time, field uniformity,
resolution, acceptance, solid angle, improvement in optical
measurements accuracy, and first- and second-order optics for
superconducting magnets, with and without ferromagnetic return
paths, will be compared to room and cryogenic magnets. Summary
of experiences with superconducting magnets (energies>
l0/sup6/J) will be given, as well as irradiation properties of
superconducting type II materials and systems. Expected
irradiation doses in accelerators and their effect on
superconducting systems will be discussed. Lifetime
expectancy, economy of operation, and effect of power failures
are treated. A short section is devoted to possible design of
superconducting magnets with and without ferromagnetic return
paths. |
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Fast Neutron Irradiation Effects in
Superconducting Niobium
S.T. Sekula and
R.H. Kernohan
Summary: The superconducting
magnetization behavior and transition temperatures of single
crystals of Nb were investigated prior to and after a series
of fast neutron irradiations (E> 1 MeV) in the Oak Ridge
Research Reactor at a temperature of 40/spl deg/C. In addition
to increases in the upper critical field H/sub e2/ and small
changes in the transition temperature after irradiation, it
has been found that defects have been created with the ability
to pin magnetic flux as evidenced by an increase in the
nonequilibrium behavior of the magnetization. |
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Superconducting Magnet
Materials
C. Laverick
Summary:
A promising new development in practical high-field
superconductors is the technique developed by Tachikawa of the
Institute of Metals, Tokyo, Japan, for fabrication of V/sub
3/Ga in flexible wire or strip form. This can also be made as
a composite with copper. The short-sample characteristic
obtained is approximately that of Dresent day commercially
manufactured Nb/sub 3/Sn at magnetic fields of up to 150 kG
and is much higher deyond this field. The U.S.S.R. and Japan
are manufacturing three element systems of NbZrTi which are
comparable in properties with currently available NbTi in the
U.S.A., but such systems do not, as yet, appear to offer any
significant advantages over NbTi. At present magnets are being
fabricated from NbTi, Nb/sub 3/Sn, or both, usually as
composite conductors in intimate contact with copper and
frequently with the addition of reinforcing where the
operating stress levels are high enough to merit this. The
properties of these basic superconductors are well known. NbZr
is now obsolescent and current practice favors the use of the
cheaper, lighter, and more ductile NbTi. Several recent and
highly successful large coils have been fabricated from
standard superconductors but improvements in the manufacture
of composites of copperclad NbTi with one or more
superconducting strands and almost any desired current
carrying capacity or size render such stranded conductors
obsolescent. |
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Fully Stabilized Superconducting Strip for the
Argonne and Brookhaven Bubble Chambers
J.
Wong, D.F. Fairbanks, R.N. Randall and W.L.
Larson
Summary: Composite strip conductors
for the two largest superconducting magnets yet to be built
are described. The conductors consist of six bands of Nb48%Ti
alloy buried in 2-in-wide strips of copper. The conductors for
the two magnets have, respectively, critical current
capacities of 3000 and 5880 A. In addition, the quality of the
bond between copper and superconductor is excellent, and the
copper conductivity is high. Data are presented for these and
other characteristics of the strip as manufactured. The strip
conductors are anisotropic in their critical-current behavior
with respect to field orientation. The importance of this fact
in designing magnets economically is pointed out. Performance
results are given for a 24-in. id. test coil containing a ton
of strip. Many strip configurations are possible and can be
easily produced Costs of strip of this type will be
competitive with present materials. |
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Superconductivity Critical Current Densities in
Ti-v Alloys
J.B. Vetrano, G.L. Guthrie,
H.E. Kissinger, J.L. Brimhall and B. Mastel
Summary:
Specimens of Ti-20% V alloy were heat treated to a /spl
beta/-solid-solution condition and then aged for several fixed
times and temperatures below the /spl beta/ transus. X-ray and
electron diffraction were used to determine the composition
and distribution of the metallurgical phases produced by the
aging treatments. These data were correlated with measurements
of the superconducting transition temperature and critical
current density. In order for these materials to carry
resonable supercurrent densities at 4.2°K, it is necessary
that the matrix be rich enough in V for the transition
temperature to be greater than 4.2°K and that there be present
an array of flux-pinning sites. It was found that aging
temperatures of 400°, 5OO°, or 600°C were required to insure
the first condition. The flux pinners produced were /spl
omega/ particles in the case of the 400°C treatment and /spl
alpha/ precipitates in the case of the 500° and 600°C
treatments. In many specimens, the superconducting-to-normal
transition at a given value of applied field was spread out
over a wide range of transport current rather than being
abrupt as is usually the case. |
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Vapor-deposited Nb/sub 3/Sn
Ribbons
H.C. Schindler
Summary:
The electromagnetic performance of high-field type-11
superconductors is significantly affected by structural
factors such as crystal size, crystal orientation, or crystal
strain. Because the vapor-deposition process for depositing
single-phase, polycrystalline, stoichiometric Nb/sub 3/Sn on a
heated moving substrate permits control of these structural
parameters, this technique is used for the deposition of
continuously monitored layers of Nb/sub 3/Sn on long lengths
of ribbon substrate. The thickness of the Nb/sub 3/Sn layer is
controlled and can be readily varied to meet specific magnet
requirements. For small-bore, high-field, layer-wound magnets,
a family of commerical 0.090-in.-wide ribbons has been
developed. For larger-bore magnets, in which "pancake"-type
construction is desirable, a commercial line of +-in.-wide
ribbons is currently being offered. Custom-size ribbons have
been made as narrow as 0.025 in. and niobium stannide has been
deposited on wires as thin as 0.005 in. in diameter. This
paper describes these different ribbons and discusses their
electromagnetic performance. Normalized curves have been
developed for calculation of short-sample critical currents as
a function of volume of niobium stannide. |
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Improved Stabilized
Superconductors
A. Bindari and R.E.
Bernert
Summary: A new theory of the stable
transition of a superconductor between normal and
superconducting states is reviewed and data supporting this
theory are presented. Data on copper-clad Nb-40% Ti are
presented, and the slope for the variation of the critical
current with wire diameter is found to be 1.5 for wire
diameters between 0.005 and 0.05 cm. For copper-clad samples
of this material, voltage-current data agree remarkably well
with the new theory of stabilization. The importance of the
superconducting transitional region is
emphasized. |
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Microstructure Effects in Ti-22 at.% Nb
Conductors
D. Kramer
Summary:
Composite conductors of copper and titanium-niobium,
either cable or solid, have been fabricated with
low-weight-percentage Nb alloys in the range 22-28 at.%
niobium. The fabrication of such alloys and conductors
involves heat treatment in the temperature range 300°-500°C.
It has been found that omegaphase precipitation occurs in all
samples. Details as to the size and distribution of omega
precipitates will be given as well as the correlation between
omega precipitation and critical current densities in the
superconductor. |
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Mechanical and Electrical Properties of
Diffusion-processed Nb/sub 3/Sn-Copper-Stainless Steel
Composite Conductors
M.G.
Benz
Summary: Diffusion-processed Nb/sub 3/Sn
has high current density at high magnetic fields. When
combined with copper or copper and stainless steel in the form
of a flat laminated tapelike conductor, it also has
flexibility, low normal-state resistance, and the ability to
withstand high tensile stress This paper considers various
laminated conductors that have been constructed, with emphasis
on mechanical as well as electrical performance
characteristics. Design methods and subsequent test evaluation
for performance characteristics are discussed. |
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Angular Dependence of Critical Currents in
Plasma-plated Nb/sub 3/Sn Strip
Superconductors
W.A.
Fietz
Summary: In many instances it appears
that the limiting performance of a superconductive device is
determined by low-field instabilities rather than by the
high-field current density. If the device is made of
flat-strip superconductors, a further complication may arise
from the fact that in the low-field regions the magnetic field
usually is oriented at some nonzero angle to the surface of
the strip. To see how these conditions affect the performance
of plasma-plated superconductors, we have measured critical
currents in flat strips of Nb/sub 3/Sn in transverse applied
field, varying the angle /spl theta/ between the surface of
the strip and the magnetic field. When transport current is
applied first and the field increased until the specimen
undergoes a normal transition, the relationship H/sub c/(l/sub
T/) = H/sub 0/(I/sub T/) +H/sub 1/(I/sub T/)cos2/spl theta/ is
roughly obeyed, where I/sub T/ is the transport current and
H/sub 0/ and H/sub 1/ are found to be of comparable magnitude.
When the magnetic field is applied first and the transport
current increased to its critical value, virtually no angular
dependence is observed. We tentatively explain this result by
describing the plasma-plated material as a matrix of miniature
hollow cylinders aligned with the magnetic field. Because of
the anisotropy of the plating process the effective wall
thickness of the "cylinders" varies with the specimen
orientation. When the field is applied first, the transport
current can find paths around these "cylinders" which will not
result in locally exceeding the critical current density of
the material until nearly all the specimen is critical.
However, when the transport current is applied first, the
additional shielding currents induced by the magnetic field
are forced to travel in paths which are already carrying
transport current, and locally the critical current density
may be exceeded, precipitating a normal transition before the
entire specimen is critical. Further tests are being carried
out to determine the effect of heat treatment and strip
dimensions on this angular dependence of the critical current.
These results should discriminate between our proposed
explanation and the possibility that the effect is simply a
demagnetization phenomenon. |
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AC Losses in
Superconductors
S.L.
Wipf
Summary: Since the discovery of
technologically interesting high-field superconductors in 1961
more than 50 experimental and theoretical publications have
appeared which are concerned with the relevant ac loss
mechanisms. These papers are reviewed, and certain of the
experimental findings are unified in the light of the present
day understanding. In many cases seemingly disconnected and
even apparently contradictory results are brought into
consonance. In this review, theories and models as well as
experiments are discussed, and brief mention is made of actual
and proposed ac applications. For frequencies with which we
are mainly concerned (< l0/sup4/ sec/sup -I/) and for
fields less than H/sub c/ and H/sub c2/, respectively, for
type I and type II superconductors, ideal homogeneous
materials are loss free. Likewise, in dc background fields up
to H/sub c3/, losses remain negligible if the amplitude of the
ac field stays below a value determined by intrinsic critical
surface current densities. For larger amplitudes cyclical flux
movement takes place inside the superconductor (in the
intermediate or mixed state for type I or II, respectively)
and flux-flow and eddy-current losses appear. In real nonideal
superconductors much larger losses appear. |
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Radio-frequency Losses in the Superconducting
Penetration Depth
J.M. Victor and W.H.
Hartwig
Summary: Trapped flux has been shown
to be responsible for a large part of the residual ac losses
in both types I and II superconductors. The authors have made
theoretical and experimental investigations of losses in the
40-400 MHz range to establish a more detailed low-field model.
The surface resistance r/sub s/, as given by Pippard, is
exceedingly small for Sn, Pb, and other pure metals below
about 0.95 T/sub c/ at these frequencies. As a consequence,
trapped-flux effects provide the dominant losses in
rolled-foil resonant circuits. The theoretical model is simply
Ohmic losses in the normal regions of trapped fluxoids bounded
by the penetration depth and the surface. This gives an added
resistance r/sub h/= v/sub h/ (0) V(t), where the temperature
function is V(t) = (1-t/sup 4/)/sup 1/2/(1--t/sup 2/)/sup 1/.
The magnitude of the loss is predicted to be proportional to
the density of fluxoids trapped, which in turn is assumed
proportional to the background magnetic field for low fields.
The experimental technique consisted of pulse determinations
of circuit Q in the superconducting and normal state as a
function of temperature and weak background magnetic field on
cooling below T,. The most detailed effort was made on pure
tin foil from 60-350 MHz with background H fields up to 6 G.
Data on these and on Sn-In, Pb-Sn, Ta, and Nb samples showed
the function V(t) gave the best fit. The fluxtrapping loss was
linearly proportional to the cooling field. The Pippard
surface-resistance term was separated out along with a very
small residual loss, r/sub 0/. The latter was independent of
field or temperature and could be annealed out. The Pippard
term had a frequency dependence between v/sup 4/3/ and v/sup
3/2/ as expected. These results differed from Haden et al. in
that no break in the decay rate was observed in the pure Sn
circuits. This could be explained by the absence of an
observable fluxoid core transition. The absence of a strong
frequency dependence is consistent with Ohmic loss and ruled
out a dominant hysteresis behavior. The nature of the
flux-trapping loss is seen to make it the principle
dissipating mechanism for most circuit applications since it
resists annealing, does not depefid upon impurities, and even
though greatly reduced by nulling out external fields, may be
partially self-induced due to thermoelectric effects on
cooling. |
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Macrovortex Structure in Hard
Superconductors
Y. Iwasa and J.E.C.
Williams
Summary: The magnetization of hard
superconducting wires, in the presence of a transverse
magnetic field, can be induced to collapse by the application
of a small, local field pulse. Under certain conditions the
collapse may propagate along the wire in steps, leaving behind
a spatially periodic variation in the remanent magnetization
of the wire. The remanent magnetization is associated with
discrete vortices of wire. A phenomenological explanation for
the formation of the macrovortex structure is proposed.
According to this explanation, the energy of magnetization,
which is initially stored uniformly over a finite volume, is
preferentia!ly dissipated locally during the process of flux
jumping, thereby raising the temperature of the region to the
critical temperature. Although the formation of the
macrovortex structure has thus far been observed only in
unclad wire of high pinning strength, it represents a
potentially important mechanism for the concentration of
energy during a collapse of magnetization. |
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Parallel and Perpendicular Magnetic Transitions
of Films and Foils of Pb and Sn
G.D. Cody
and R.W. Miller
Summary: A summary will be
presented of extensive measurements of the parallel,
perpendicular magnetic transitions of Pb, Sn as a function of
temperature (1°K to T/sub c/, for Sn; 1°K to 4.2°K for Pb),
thickness (500 /spl Aring/ to 16 /spl mu/). The results, apart
from their intrinsic interest, can be utilized to extract many
of the superconducting parameters of the metals, including
their thickness, temperature dependence. |
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Granular
Superconductors
R.W. Cohen and B.
Abeles
Summary: The superconducting
transition temperatures T/sub c/ of some metals prepared in
the form of small grains are appreciably higher than the
ordinary values. The extremely small grain size gives rise to
very short mean free paths and critical magnetic fields up to
two orders of magnitudes larger than the thermodynamic
critical fields. The critical fields and resistivities can be
explained by a model which assumes tunneling between grains.
Experiments indicate that the transmissivities of the
tunneling barriers are large (/spl cong/10/sup -1/). Thus the
grains are strongly coupled to each other, a finding which
would appear to rule out enhancement of T/sub c/ due to
quantization of electronic motion expected in small isolated
systems. Other possible enhancement mechanisms discussed are:
Josephson tunneling, surface enhancement of the
electron-phonon interaction, and an increase in the electronic
density of states due to atomic disorder. The relation between
the somewhat unusual properties of granular superconductors
and current application of thin films will be
discussed. |
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Radiant-Energy Detection by Superconducting
Films
C.L. Bertin and K.
Rose
Summary: Superconducting films offer
promise as fast, sensitive detectors at intermediate
frequencies from millimeter waves to near infrared. The
superconductor’s marked temperature dependence of resistance
ahout its transition temperature suggests that it would have a
high responsivity as a bolometer, and operation at cryogenic
temperatures suggests that a low NEP can be achieved. In
addition, placing the sample in the liquid-helium bath should
result in very short response times (/spl tau/) on the order
of 10 nsec. Films have been studied with sheet resistances
from 200 /spl Omega//sq to > 1000 /spl Omega//sq. These
films achieve NEP /spl ap/ 10/sup -9/ W/spl middot/Hz/sup
-1/2/ as bolometers with /spl tau//spl ap/20 nsec. There is an
enhanced microwave response, NEP/spl ap/ 10/sup -9/ W/spl
middot/Hz/sup -1/2/ with ?spl tau//spl ap/ 20 nsec. which is
especially pronounced for high- resistance films. This mode
shows promise for use in the farinfrared and submillimeter
region. The bolometer mode compares favorably with other
bolometers. |
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Peak-Effect Studies on Niobium
Nitride
E. Maxwell, B.B. Schwartz, H.
Wizgall and K. Hechler
Summary: Observations
of the temperature dependence of the peak effect in Nb-N
between 4.2/spl deg/ and 1.35/spl deg/K indicate that the
field at which pinning effects set in is independent of
temperature in this range. It is suggested that the repulsion
between neighboring flux bundles acts to enhance flux pinning
in the peak region. |
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Flux Flow and Thermal Stability of Stabilized
Superconductors
W.F. Gauster and J.B.
Henricks
Summary: Test equipment has been
designed in such a way that complete test cycles with bare or
insulated compound conductor samples in unrestricted
liquid-helium flow can be easily obtained. Sample currents up
to 2000 A, fields to 80 kG can be used. An evaluation of the
recovery points yields magnetoresistance data for the
stabilizer material. Using these stabilizer resistance values,
it is possible to split the total current into the
superconductor, the stabilizer components. The tests show
that, in general, the take-off points cannot be explained by
the transition from nucleate to film boiling of .the liquid
helium. The V vs Z curves in the flux-flow zone seem to
indicate a heat-transfer law in the form Q=h*/spl tau//sup n/
with n/spl sime/2 5 in the nucleate boiling regime. Tests with
poorly stabilized samples (either bare or insulated) can be
explained by assuming that parts of the sample are in the
superconducting state, while other parts are in the normal
state or in the flux-flow state. |
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Cryoelectric Content-Addressable Memories Using
Flux-Shuttling Nondestructive
Readout
S.A. Keneman
Summary:
The feasibility of large capacity content addressable
memories (CAMS) using the flux-shuttling nondestructive
readout (NDRO) technique and designed to handle the multiple
response problem is investigated in this paper. A 4X4 CAM
incorporating flux-shuttling cells and crossed-film cryotron
logic circuitry was designed, fabricated, and tested. On the
basis of the results of the tests performed, a highly eficient
CAM was designed. The ultimate performance limits of the type
of CAM designed are considered. |
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The Anisotropic Structured Cell: A Practical
Cryoelectric Storage Element
R.A.
Gange
Summary: The realization of a fast
random-access memory through application of cryoelectric
technology is an attractive goal. The substantially perfect
diamagnetism which characterizes the superconducting state
permits the design of storage elements of very high packing
density. The very low power densities and the extreme
nonlinearity in the transition between the superconducting and
normal states promise very fast operation of these elements
with no half-select or delta noise. In addition, the number of
memory positions that can be sensed appears to be quite large.
In this paper, I describe the design concepts and performance
of a three-wirer cryoelectric storage element called the
Anisotropic Structured Cell. Included is a comparison of the
performance of the ASC storage elements as compared to loop
cells of earlier design, and a discussion of the sense
amplitude, signal-to-noise ratio, cell-to-cell isolation,
operating tolerance, and operating current levels of
high-density (13 200 cells/in./sup 2/) ASC arrays of 1/4
million bit capacity. The paper also contains a first-order
analysis of the use of these ASC elements in a large and
practical high-speed cryoelectric memory organized in
accordance with the hybrid, a, b, system. |
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A Multi-Aperture Cryogenic Storage
Cell
J.L. Mundy and V.L.
Newhouse
Summary: Two types of
superconducting memory cells are now being considered: the
continuous-film memory and cryotron-switched storage loops.
The former suffer from low cell operating range compared to
cell-to-cell variation. The cryotron cells, although having a
wider operating range, must be switched very rapidly to have a
detectable output signal. A recent development in cryogenic
sensing called "Current Stretch" has allowed more freedom in
the design of superconducting memory cells. With current
stretching the information is transformed to a persistent
input current in a cryotron amplifier. The signal produced
when the cell is switched is then proportional to the magnetic
flux change rather than the rate of change of flux. This
feature eliminates the usual memory requirements of
fast-access current risetime, pickup noise cancellation, fast
cell switching, and the need for a separate sense line with
cryotron-accessed memories. A new class of cell which takes
advantage of this technique will be described. It consists of
a cryotron shunted across a loop, one side of which has a
lower inductance and critical current than the other. If the
cryotron is activated while current is injected into the cell,
a persistent current is created in the loop. Cell-flux changes
are detected through one of the cryotron access trees using
current stretching. The operating tolerance of these cells is
only limited by their geometry. They have no metal-to-metal
contacts, use only three metal layers including the ground
plane, need no sense line, no noise cancellation, and can be
switched arbitrarily slowly. Cells can be fabricated as small
as 10 mi1/sup 2/ and operating ranges as large as 40% have
been observed. The cell speed is on the order of 50 nsec with
a single crossover access cryotron. |
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Tunneling Cryotron
Flip-Flop
J. Matisoo
Summary:
We have constructed tunneling cryotron flip-flops, have
observed current transfer from one leg to the other, and have
measured the time required for current transfer to be 2 nsec.
The results are in agreement with the model previously
proposed. The tunneling cryotron flip-flop consists of two
devices in parallel, linked by a wholly superconducting path.
A current is fed to the parallel devices; the current is
steered from one device to the other by application of a
control pulse. The measurements have been made on a loop of
1.16 in. in length (L= 1.08X 10/sup-10/H), containing two 10
mil diam junctions, having zero-field critical currents of 21
mA and 29 mA, respectively (in operation these are biased to
near maxima on the gain curves, at 33 mA and 42 mA). The
measuring apparatus is similar to that previously described.
In operation, a current of 20 mA is transferred from one
device to the other by the application of a 2 mA control
pulse, a gain of 10. The time required for this current to
transfer, /spl utri/t/sub transfer/, is 1.4 nsec /spl les/
/spl utri/t/sub tranfer/ /spl ges/2.0 nsec, with an
uncertainty of at most 0.4 nsec. According to the previously
proposed model of a constant gap voltage driving the loop
inductance and which does not include thejzbnction transition
times the transfer time for 20 mA of current should be 2 nsec.
One implication of this excellent agreement is that
thejzrnction transition time is appreciably smaller than the
previously measured upper bound of 800 psec. Additional
operating details will be discussed. |
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Electronic Grey Matter
R.D.
Parmentier and A.C. Scot
Summary: One of the
most exciting long term goals of Electrical Engineering is to
construct an electronic machine with functional ability
similar to that of the human brain. A brief inspection of the
brain indicates the enormous difficulties involved. The brain
is overlaid with a thick covering or covtea of grey matter
comprising approximately 10/sup 10/ neurons or active elements
plus additional regulating circuitry in the cerebellum. The
electrical engineer setting out to duplicate such a gadget is
faced with three stark problems: (1) Wired in cost per active
element. The cost of 10/sup 10/ at a penny each (wired in)
would be $100 million for hardware. (2) Power dissipation per
active element. The power dissipation of l0/sup 30/ neurons at
1 /spl mu/W each would be 10 kW. (3) Design scheme. Some plan
of construction is obviously necessary before one begins to
build something with 10/sup 10/ working parts. If the design
requires every interconnection to be correctly made, it will
be impossible to carry it through. The main purpose of this
paper is to suggest that there is now the possibility of
overcoming these difficulties using tunnel junctions between
superconducting metals as the active elements. These devices
are analyzed in detail and it is demonstrated both
analytically and experimentally that neuristor pulse
propagation is possible. |
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Dynamic Fault Tolerance in Cryoelectric
Arrays
J.P. Pritchard Jr and B.G. Slay
Jr.
Summary: Properties peculiar to
superconductive phenomena and derived devices permit operation
of cryoelectric arrays containing failed elements. This fault
tolerance extends throughout the operational life of the array
and is therefore referred to as dynamic fault tolerance. A
cryotron-gated, superconductive loop possesses three stable
states: two with circulating current of opposite polarity and
a third without circulating current. The latter can be paired
with either of the circulating states to form a binary logic
device which will fail safe to the noncirculating current
state under all failure modes except short-circuit formation.
Analysis of failure origin and technology redefinition has
eliminated short circuitry as a failure mode in completed
arrays. The requirements for implementation of dynamic fault
tolerance are developed and illustrated in terms of successful
experiences with a 20, 000 gate cryoelectric array
objective. |
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Frequency and Time-domain Analysis of a
Superconductive Coaxial Line Using the Two-fluid
Model
W.D. McCaa Jr. and N.S.
Nahman
Summary: A miniature 80.ft
superconductive coaxial line (Nb inner conductor 0.01 in.
o.d., Pb outer conductor 0.034 in. i.d.,
polytetrafluoroethylene dielectric) has been analyzed in terms
of the Gorter-Casimir two-fluid superconductivity model,
considering the anomalous and classical cases (without
relaxation effects) for the normal components. The dielectric
and the metal flux trapping losses are assumed to be
negligible. The 10/sup 6/-10/sup 12/ Hz line attenuation and
the 1-100 psec step responses are presented for the
temperature range of 2.3°-10°K. Predictions are compared with
the experimental results of several independent investigators
It is concluded that if the dielectric and the metal
flux-trapping losses are negligible, then (1) The model
provides bounds between which the responses of physically
realized lines will fall, and (2) The normal conductivity of
the conductors must be very good if the maximum bandwidth and
minimum risetime are to be obtained. |
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Some Rate-Dependent Aspects of Flux Jumping in
Nb-25% Zr Tubes
F. Rothwarf, D. Ford, G.
Articola and G.P. Segal
Summary: A series of
experiments have been performed on rate dependence of flux
jumping in tubular samples of Nb-25% Zr using pulsed magnetic
fields. The field H/sub 1/ for which the first flux jump
occurred was studied as a function of the rate of change of
field H/sub 1/ at the time of the flux jump. The range of
H/sub 1/ was 10/sup 1/ |
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Proposed Ammeter Using Flux
Quantization
R.
Meservey
Summary: The possibility is
discussed of using flux quantization in superconductors as the
basis of an ammeter and more generally as a fundamental
standard of electromagnetic measurement. A particular design
is proposed and in this context some of the fundamental design
problems of this sort of instrument are considered. It is
concluded that an instrument of this type could measure a
current in terms of the flux quantum, length, and time to an
accuracy approaching 1 part in 10/sup 6/. The limitations and
advantages of the proposed instrument are discussed and
comparisons are made with present current-measuring
instruments and some conjectures are made of the future
potential of flux quantization instruments of this
kind. |
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Uses of High-field Superconducting Magnets in
Mossbauer Spectroscopy
R.L.
Collins
Summary: Two areas of Mossbauer
spectroscopy are benefited by the availability of high-field
superconducting magnets: chemical-structures studies and
research into magnetically ordered systems. The electric field
gradient (e f.g.) tensor at the Massbauer nucleus is a measure
of the distortion of the surrounding charge cloud from cubic
symmetry, and this reflects the disposition of strong ligands.
The most common Miissbauer nuclides are /sup 57/Fe and /sup
119/Sn. The e.f.g. tensor produces a splitting into two lines,
but this does not yield the two parameters of the tensor. Upon
addition of a magnetic field (30 Kg+), the remaining
degeneracies are removed and a characteristic distortion
ensues. This is the only method for extraction of the e.f.g.
parameters from powdered samples, Theoretical and experimental
examples of this technique will be displayed. |
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Measurement of the Surface Resistance of
Superconducting Lead at 2.868 Ghz
H.
Hahn, H.J. Halama and E.H. Foster
Summary:
The surface resistance of superconducting lead at 2.868
GHz was determined from a measurement of the unloaded Q of a
TE/sub 011/ cavity. The results were compared to the
analytical expression for the surface impedance of pure
superconductors in the Pippard limit, which was worked out by
Abrikosov el al. Adequate agreement was found by writing the
reduced surface resistance in the form of r= A r/sub
AGK/+r/sub 0/, with r/sub 0/ being a residual resistance and A
a constant. The best fit was obtained for A =3.0, r/sub
0/=2.2X10/sup -5/ and the gap parameter /spl utri/ (0) =
1.97k/sub b/T/sub c/. |
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Microwave Studies of Thin Superconducting
Films
D.A. Soderman and K.
Rose
Summary: The results of linear microwave
transmission measurements on superconducting tin films are
presented. The effects of residual reactances are noted and a
comparison is made with calculations based on the BCS theory.
Residual reactances are found to have a substantial effect on
the nonlinear transmission measurements. They allow a clear
separation of regions where superelectron depairing and
"domain growth" dominate. Linear transmission measurements in
the presence of a perpendicular magnetic field suggest the
"domain growth" is the result of fluxon
penetration. |
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The Pinning Potential and High-frequency Studies
of Type-II Superconductors
J.I. Gittleman
and B. Rosenblum
Summary: The electrical
impedance of type-II superconductors as a function of
frequency is discussed in terms of a "pinning potential." Many
flux-pinning properties of the mixed state at any frequency
can be treated in terms of such a potential and a simple
phenomenological extension of the Bardeen-Stephen flux-flow
theory. The form of the potential can be determined by various
dc and high-frequency experiments. There is a critical
frequency above which the impedance for subcritical currents
becomes the impedance of the "ideal" mixed state. Much below
this frequency the impedance is zero. This critical frequency
is given by: /spl omega//sub 0/=2/spl pi/c/spl rho//sub n//spl
alpha///spl phi//sub 0//sup 1/2//H/sub c2/H/sup 1/2/. At
microwave frequencies, almost all effects of flux pinning are
absent, and the properties of the "ideal"mixed state are
observed even in the most strongly pinned material. Such
measurements have been used to determine the effective mass of
the flux tube. |
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The Superconducting Magnet for the ANL 12-ft
Hydrogen Bubble Chamber
J.R.
Purcell
Summary: The large Argonne magnet is
scheduled for completion in late 1968. The bore of this magnet
is 16 ft and the distance between pole faces is 10 ft. Iron is
used for the magnetic return path. Major components and
materials have been ordered and fabrication of the complete
system is proceeding on schedule. Constrnction has started on
the container. A 2-ft bore magnet has been wound from the
conductor to be used in the large magnet and tested. This
small magnet, which weighs about 2000 lb, performs very well.
A model has been built to study problems associated with the
coil structure, such as cooldown. maintenance of clamping
forces, and general mechanical integrity. |
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High-Homogeneity Tape-Wound Superconducting
Coil
C.D. Graham Jr, H.R. Hart Jr. and
E.F. Mains
Summary: A 100 kG superconducting
magnet with field uniformity better than 1 part in 10/sup 4/
over a 1 cm diam sphere has been constructed from composite
Nb/sub 3/Sn superconducting tape. Data showing the performance
of the coil are presented. The field profiles depend both on
the magnitude of the field, on the field history; field
uniformity is considerably reduced at low fields after an
excursion to higher fields, because of trapped flux. Neither
the field nor the field profile at a given current agrees with
the values calculated for a copper coil of identical geometry.
The discrepancy results from the anisotropic diamagnetic
properties of the superconducting tape winding, is
semiquantitatively understood from an analysis based on the
critical-state model. The construction of the coil is
described, particularly the electrical connections between
modules and the methods used to obtain field
uniformity. |
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Measurement Techniques for Superconducting
Coils
E.F. Mains and C.D. Graham
Jr
Summary: This paper describes some of the
experimental methods we have used in buiiding high-field
superconducting coils. Field-level measurements have been made
using Hall probes, magnetoresistance probes, and search coils.
Copper magnetoresistance probes are inexpensive, simple, and
reproducible enough for routine testing; such probes can be
built into a coil. Reproducibility is better than the normal
limits of panel meters or X-Y recorders. The search coil and
fluxmeter (or ballistic galvanometer) is used as a primary
standard for calibrating other probes. Absolute accuracies of
about 1/2% are attainable by calibration against an NMR probe.
We have determined field ztniformity by direct measurement of
the field gradient. A small coil is oscillated /spl plusmn/1
mm at 10 Hz in the magnetic field. The ac voltage from the
oscillating coil, which is directly proportional to the field
gradient, is amplified and detected with a lock-in amplifier.
Numerical integration of the gradients gives high-resolution
field plots. This straightforward apparatus can detect a field
gradient of 1 G/cm in a 100 kG field. Our coils are
constructed in modules, with each module consisting of a pair
of disks or pancakes wound from superconducting tape. It is
obviously useful to detect which module in the coil is
limiting. For this purpose we use a set of shunt resistors
across the modules, and record the voltage across the shunts
on an oscillograph as the transition from superconducting to
normal state occurs. In this way, one can deduce where the
normal region originates in the coil, and can tell something
about the way in which it propagates. |
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Stabilized, Levitated Superconducting
Rings
J. File, G.D. Martin, R.G. Mills
and J.L. Upham
Summary: A levitated,
stabilized superconducting ring is described. The ring, wound
of 3310 turns of Nb-25% Zr 10-mil wire, has been operated with
currents up to 16 A. It has been floated, with its associated
Dewar, independent of all physical connections, for periods
exceeding 7 h. Ring stabilization criteria and a system to
maintain the ring position are presented. |
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A Partially Stable 90 kG Composite
Superconducting Solenoid
S.D. Lindenbaum
and J.F. Becker
Summary: We have designed and
constructed a superconducting solenoid, composed of an outer
coil of A-25 (Nb-25 at.% Zr) wound on an inner coil of T-48R
(Nb-48 at.% Ti) The solenoid has a 1/2 in. bore, a length of
4.5 in., and o.d. of 4.3 in., and produces an axial field of
90 kG with a maximum current of 2.5 A(II/I= 3.6 kG/A) . A
discussion of coil-training effects is included. |
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High-Field Superconducting
Magnet
W. Wurz
Summary:
A large superconducting magnet has been built which
makes it possible to maintain magnetic fields of up to 20 kG
for several hours within a horizontal room-temperature work
space of 10 cm diameter. The id. of the superconducting coil
is 16.5 cm. High stability of the magnetic field has been
achieved by winding laminated sheets, half-copper,
half-stainless steel, between the layers of the coil. The
current density is about 20,000 A/cm/sup 2/. Access to the
superconducting coil is provided by a new cryogenic seal,
which makes the exchange and rearrangement of the coils
possible, and, consequently, permits different field
shapes. |
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Progress in Superconducting Beam Handling and
Accelerator Structures Since November
1966
P.G. Kruger and J.N.
Snyder
Summary: At an earlier meeting a
full-scale superconducting quadrupole which was under
construction at Brookhaven National Laboratory was described.
The results of test on that quadrupole will be described here.
Also, at the meeting a year ago calculations were reported
concerning the possibility of constructing a FFAG field with
Nb/sub 3/-Sn, for a 200 BeV accelerator. Those calculations
assumed a circular cross section for the geometry of the coil.
Since then the calculations have been extended to include an
elliptical cross section for dipole, quadrupole, and
diquadrupole superconducting structures. One advantage for the
elliptical geometry is that it reduces the cost of the
necessary Nb/sub 3/-Sn by about a ratio of (a+b)/2r where a
and b are the semimajor and minor axes of the ellipse and r=
a, the radius of the circle for the circular case. Results of
the calculations for dipoles, quadrupoles, and diquadrupole
fields in elliptical cross-section geometries will be
presented. These can all be used in accelerator designs with
separated function elements or with combined function elements
provided the elements are not pulsed. Pulsing produces losses
which are not well understood at present. |
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The Transient Stabilization of Nb/sub 3/Sn
Composite Ribbon Magnets
J.R. Hale and
J.E.C. Williams
Summary: The transient
thermal behavior of a section of composite superconducting
ribbon which has suffered a sudden rise in temperature has
been investigated in detail by computer simulation. Three
equations describing a one-dimensional flow of heat from the
composite, through an insulating film and the subsequent
diffusion of heat into a metal foil, have been programmed for
simultaneous numerical solution. The effectiveness of the
interleaving in stabilizing the composite has been found to
depend strongly on the thermal conductivity and thickness of
the insulating film, as well as the thermal properties of the
metal foil. Also, values of the initial sudden temperature
rise due to a collapse of magnetization have been calculated.
Estimations, based on the numerical results: are made of the
effectiveness of various interleaving materials. |
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High-Efficiency Superconducting Homopolar dc
Generators
D. Atherton
Summary:
The immediate requirement for flux pumps appears to be
a design capable of producing 10/sup 4/ A at 100 W and high
efficiency to energize large stabilized magnets. We have,
however, successfully operated homopolar superconducting dc
generators and also stabilized solenoids in helium vapor
rather than liquid, and it appears reasonable to suggest that
large magnets could be energized during the latter stages of
cooldown, so that the power requirements for reasonable
energization times may be less than this. Two designs for
homopolar superconducting dc generators will be discussed and
their performances reported on; both use permanent magnet and
iron rotors. The first design has produced 1600 A and 20 W.
The most significant feature of the design is the magnetic
circuit used to generate in excess of 6 kG in the normal spot
crossing the sheet. This enables reasonable power to be
generated in a low-speed (300 rpm) design. There are, however,
large losses attributed to sloshing of liquid helium. The
second design is evolved from the iirst by enclosing the rotor
in a vacuum container; special cooldown procedures are
required. High efticiency is obtained (measured in excess of
100% at the time of writing, the excess being due to
measurement error rather than a triumph over the second law of
thermodynamics!). The design promises heavy currents and
reasonable power outputs. The design allows topological
manipulations of the sheet to allow adjustments of the
current-carrying capacity of the sheet or output voltage (and
hence power) or output impedance (and hence
efficiency). |
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Automatic Superconducting
Switches
H.L. Laquer
Summary:
The design and testing of a simple automatic
superconducting switch or current limiter is described.
Reliable switches having switching currents up to 670 A at 4°K
have been built for use with a flux pump. |
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The Design of an 88-kG, 20-in.-bore
Superconducting Magnet System
E.J. Lucas,
T. De Winter, J. Lauerence and W. Coles
Summary:
The integrated design of a large superconducting magnet
system includes mutually compatible solutions to mechanical,
thermal, and electrical problems. This paper reports on the
results of such a design and some of the analytical and
experimental work that was carried out to provide data for
these solutions. The system consists of a set of bour 51 cm
bore 5 T solenoids, two of which, when operating as a close
pair, will produce a central field of 8.8 T. Each of the four
solenoids is divided into three concentric sections. The inner
sections are composed of twenty pancakes wound with a
stabilized Nb/sub 3/Sn ribbon, while both the middle and outer
sections are wound with a stabilized Nb-Ti square conductor.
The over-all current densities in each of the sections are
5000 A/cm/sup 2/ for the inner, 4540 A/cm/sup 2/ for the
middle, and 6450 A/cm/sup 2/ for the outer. |
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Results of Tests on Models for an 88-kG,
51-cm-Bore-Diameter Solenoid
Z.J.J.
Stekly, E.J. Lucas, T. De Winter, B. Strauss, F.D. Salvo, J.
Laurence and W. Coles
Summary: As part of a
program to build an 88 kG, 51-cm-bore solenoid, two model
coils were built to test the conductor characteristics as well
as the final coil-construction techniques. The first coil was
wound with a square conductor 0.218 cm on a side, with many
strands of Nb-Ti. The coil winding i.d. was 15.24 cm, the o.d.
was 22.3 cm, and the length was 10.16 cm. This coil was built
using small cooling passages, and its construction was such as
to simulate the techniques to be used up to 80 kG in the
full-size magnet. The second coil was wound with steel
reinforced Nb/sub 3/Sn of over-all dimensions 1.27 cm by
0.0292 cm and had a winding i.d. of 12.2 cm and an o.d. of
31.1 cm, and consisted of two pancakes. Its purpose was to
simulate the high-field windings of the full-size coil. Both
coils were instrumented with heaters to determine their
stability characteristics. The performance of each coil is
discussed with regard to stability, charge rate, and quench
characteristics. |
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Problems in Designing Nonaxisymmetrical
Superconducting Magnet Systems
W.F.
Gauster and D.L. Cuffey
Summary: At the Oak
Ridge National Laboratory a large superconducting Bmin
facility "IMP" (Injection into Microwave Plasma) is under
construction. In order to build this system in an economical
way, optimization methods for designing nonaxisymmetrical
magnet systems have been developed. A discussion of these new
design methods and of a series of unusual technological tests
(for instance, experiments with superconducting test coils in
cusp arrangement, exposed to external magnetic fields) is of
general importance for the design of nonaxisymmetrical
superconducting magnet systems. |
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The Stabilization of High-Current-Density
High-Field Superconductive Magnets
E.R.
Schrader, P.A. Thompson and W. Coles
Summary:
This paper is specifically aimed at the types of
problems and their solutions for large, high-field magnets
which require high-current-density operation for reasons of
size and economy. It is apparent that magnet stability can be
varied to some extent to suit the specific use to which the
magnet is put without sacrificing vital packing factors. The
treatment in this paper will include some aspects of the
methods used and illustrations from tests on magnets with
Nb/sub3/Sn ribbon lengths ranging from 0.7 to over 90 km. A
magnet wound with over 90 km of ribbon for the Lewis Research
Center consists of 22 modules arranged in five concentric
rows, the placement of the modules determined by requirements
of axial strength, cooling, and placement of versions of
Nb/sub 3/Sn ribbon. The magnet was connected such as to permit
powering of up to four groups of modules with separate
supplies. The highest field attained is 135 kG in the center
of the 6-in. bore (140 kG at the inner windings) ; this using
three power supplies. The electrical system is designed to
provide combination of voltage and current ramping for each of
the four powered sections. Upon going normal, the power
supplies are disconnected and the magnet sections are shorted
to permit circulating currents with slow field decay. The
required degree of stabilization is achieved by a combination
of silver-plating on the ribbon, insulated sheets of copper
interleaving between each layer, copper strips distributed
across each layer and shorted to the turns, and large
cross-section copper turns which act as secondaries. This
combination of stabilizing means had been determined chiefly
by empirical methods. |
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Photodielectric Detector Using a Superconducting
Cavity
G.D. Arndt, W.H. Hartwig and J.L.
Stone
Summary: Photoinduced free-carrier
processes in Si and Ge have been studied in superconducting
cavities to exploit their excellent frequency stability and
negligible dissipation. Using the narrow bandwidth and very
high unloaded Q, it is possible to detect changes in the
complex dielectric constant by observing the shift in resonant
frequency. Simultaneous measurement of relative power
absorption gives additional and confirming data. An increase
in frequency of several kHz/mW and a linear power absorption
is observed for low values of light. At higher intensity both
responses become nonlinear due to the growth of the plasma
frequency and nonuniform carrier density distribition. For a
525 /spl Omega//spl middot/cm sample of n-type Si in a cavity
with resonances at 290 and 810 MHz. data analvsis showed the
thermal carrier densitv at 4.2/spl deg/K was log cm/sup 3/,
relaxation time was 1.44X 10/sup -10/ sec, the Fermi level was
0 00305 eV above the honor level, and the un: illuminated
plasma frequency was 148 MHz. The product of conversion
eifficiency and free-carrier lifetime was approximately 10/sup
-10/ sec, indicating the effect could be used for a
wide-bandwidth optical detector. By repeating the experiment
at different frequencies, temperatures, and wavelengths, the
theoretical agreement holds and considerable insight can be
gotten into recombination and trapping processes, lattice
collision and mobility effects, and the changes in these
phenomena with radiation and stress of many
kinds. |
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Superconductive Microwave Meander
Lines
D.A. Gandolfo, A. Boornard and L.C.
Morris
Summary: This paper describes the
fabrication, testing, and application of superconductive
meander lines. The meander line is a slow-wave circuit which
consists of a number of parallel tapes uniformly spaced and
connected in series. The tape length is equal to one-quarter
wavelength for the center frequency of the line. Microwave
signals propagate in the form of TEM waves along the tapes.
Interaction between adjacent tapes leads to a dispersion curve
which is in general nonlinear, but which possesses some nearly
linear regions. Thus, either linear or dispersive performance
may be obtained. Meander lines find application as slow .wavc
elements in low-noise amplifiers such as traveling wave
masers, as linear delay lines, or as dispersive delay lines in
pulse-compression filters. The present investigation deals
with lead meander lines fabricated by the photoetch process.
Reflection from and transmission through the meander line were
measured by conventional techniques at room temperature,
77/spl deg/K, and 4.2/spl deg/K. A striking decrease in
insertion loss was noted when the line became superconductive.
Insertion loss and VSWR of the superconductive line were
rapidly fluctuating functions of frequency, generally
increasing and decreasing together. At numerous frequencies
between 4.0 and 4.5 GHz, a 30 nsec delay line had insertion
loss of 0.5 dB and VSWR less than 2. (Most of this insertion
loss was probably due to nonsuperconductive input lines inside
the cryostat.) For a comparison, a copper meander line.
identical in geometry to the superconductive line was operated
at 4.2/spl deg/K, and was found to have insertion loss that
was as much as 20 dB greater than that of the superconductive
line. Meander delay lines should readily lend themselves to
operation in a closed cycle refrigerator. |
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Relaxation Oscillations in Josephson
Junctions
F.L. Vernon Jr and R.J.
Penderson
Summary: Investigation of the
characteristics of Josephson tunneling in tin-tin-oxide-tin
junctions showed that pulse voltages with sharp risetimes were
being generated. Pulse amplitude across the junction remained
constant and equal to the energy gap voltage V/sub g/, but
pulse frequency could be changed over a 25: 1 range by a
change in bias voltage from 0 to an upper limit approximately
equal to V/sub g//2. The oscillation mechanism is that of
switching between the pair-current state of the junction at
V=O and the quasiparticle-current state at V= V/sub g/. The
external circuit maintains the switching cycle and determines
the output frequency characteristics. Using the L/R time
constant of the external circuit and the assumed basic dc I(V)
curve of the junction in the absence of relaxation
oscillations, calculations were made that accurately describe
the observed dc I(V) curves, frequency, and waveforms,
including their dependence on -bias voltage and magnetic
field. |
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Performance of Superconducting Oscillators and
Filters
J.L. Stone and W.H.
Hartwig
Summary: With improved understanding
of loss mechanisms in superconducting resonant circuits (SRC),
it is possible to design a variety of devices which take
advantage of the high Q. Superconducting surface resistance.
trapped-flux loss, and residual losses which are annealable
can all be related to the superconductor itself. Dielectric
dissipation is well enough understood to permit this source of
loss to be minimized. Coupling and radiation losses can be
made negligible. SRC’s are described with a variety of
geometries to fill the range from 10 MHz to 10 GHz Type-I
superconductors provide the highest Q’s and are readily
fabricated by several techniques. The superconducting
resonator can be tuned accurately in frequency by temperature
control of the surface reactance as well as the dielectric
constant of liquid helium. An external magnetic field can be
used to tune the bandwidth as well as the resonant frequency.
A quarter-wave reentrant cavity is ideally suited for use as
an optically tuned frequency-control element. This is
accomplished using the photodielectric effect in
high-resistivity semiconductor wafers which terminate the
stub. Wide-bandwidth optical communication detectors are
described using semiconductors with short recombination
lifetime. Oscillators have been built which are optically
tuned, and others which have frequency stabilities as good as
commercial frequency counters. Use of SRC’s to study material
properties at low temperature is emphasized. |
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Physics of Preparation of Josephson
Barriers
W. Schroen
Summary:
Josephson barriers of the tunneling type are usually
prepared by oxidation in room environment, but they degrade
after storage at room temperature for a few days. This paper
describes a preparation technique based on an oxygen glow
discharge which renders stable, reproducible superconductive
tunneling devices (STDs) . The barrier may also consist of
organic molecules (KPR, monocarbonic acids) ; lead serves as
the superconducting metal. Simplified models are used to
discuss the main features and advantages of the glow-discharge
technique. Extensive investigation has shown that arrays of
STDs can be fabricated using the photoresist technique
developed by Pritchard et al. The current-voltage and
magnetic-field characteristics of these arrays are examined;
it was found that they did not change during array storage at
room temperature for more than 240 days and repeated
temperature cycling from room temperature to 4.2/spl
deg/K. |
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A High-sensitivity Superconducting
Detector
J.E. Zimmerman and A.H.
Silver
Summary: Weakly connected
superconductors can function as very low power sensors by
utilizing the coherence properties of superconductors. One
configuration which has produced significant success
incorporates a superconducting point contact and a small
resistance element in a low-inductance superconducting
circuit. When the critical supercurrent (io) of the point
contact is of the order of /spl phi//sub 0//L, (/spl
phi/=h/2e, L=inductance of the ring), that circuit exhibits
coherent quantum behavior. Used as a parametric amplifier, it
has been used to measure ultralow voltages, limited by the
thermal noise fluctuations in the resistance element R. The
voltage seneitivity is approximated by 8kTR//spl phi//sub0/,
where k is Boltzmann’s constaht and T the absolute temperature
of the resistance R. We have measured 4X10/sup -16/ V across
1.7X10/sup-10/ /spl Omega/ at 4.2/spl deg/K with a
signal-to-noise ratio approaching 10. The operation of this
sensor follows from the Josephson oscillation of voltage
biased superconducting point contacts. A dc voltage VO across
the resistance of the circuit produces an oscillating current
at the angular frequency /spl omega//sub j/= 2/spl pi//V/sub
0///spl phi//sub 0/ and an oscillating voltage across the
point contact at /spl omega//sub j/. Upon supplying an rf or
microwave current at frequency /spl omega//sub D/, the point
contact mixes /spl omega//sub j/ and /spl omega//sub D/. Using
a homodyne detector at /spl omega//sub D/ of bandwidth larger
than /spl omega//sub j/, we can retrieve the oscillation at
/spl omega//sub j/. Direct measurement of this frequency /spl
omega//sub j/ yields V/sub 0/. Such superconducting circuits
may be useful as cryogenic thermometers, as thermal radiation
sensors, and in low-level spectroscopy. |
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Frequency Conversion Using Weak Multiply
Connected Josephson Junctions
F.
Rothwarf, H.M. Krisch and D. Ford
Summary: A
Josephson tunneling device essentially consisting of a
lead-tin solder bead encircling a 5-mil niobium wire has been
used to produce some frequency conversion effects. Thin spots
in the oxide coating of the niobium wire give rise to two or
more parallel weak-contact regions between the solder and the
niobium. A typical voltage vs junction current plot displays
an antisymmetric tunneling curve with zero voltage across the
junction until a certain critical current I/sub jc/ is
supplied. A field current I/sub H/ flowing through the niobium
wire produces a magnetic flux which links the multiply
connected region(s) formed by the penetration depths of the
solder and the niobium and the distance(s) between the
parallel weak contacts. The critical current I/sub jc/ is a
periodic function of the magnetic flux threading the multiply
connected region(s), the modulation period being proportional
to one flux quantum. When the device is biased with a junction
current I/sub j/ in the interval I/sub jc//spl les/I/sub
j//spl les/3I/sub jc/, a modulated dc voltage appears across
the junction as I/sub H/ is varied. The period of the
oscillations is independent of the junction bias I/sub j/. The
number of cycles /spl utri/N produced for a given absolute
change |/spl utri/I/subH/| defines the junction constant K/sub
j/ of the device. K/sub j/ depends upon the geometry of the
device and the penetration depths of the superconductors used
in its construction. If a time varying I/sub H/ is applied to
the niobium wire an output frequency /spl conint//sub 0/ can
be obtained, given by the relation /spl conint//sub 0/= K/sub
j/ | dI/sub H//dt |. For a triangular I/sub H/ the device acts
as a frequency multiplier giving /spl conint//sub 0/=4K/sub
j/l/sub Hm/ f/sub in/, where I/sub Hm/ is the current
amplitude of the triangular waveform and fin its frequency. A
sinusoidal I/sub H/ gives rise to a frequency-modulated
output: /spl conint//sub 0/=2/spl pi/K/sub j/I/sub Hm//spl
conint//sub in/ | cos2/spl pi/.spl conint//sub in/t |. Other
input waveforms yield other frequency-modulated
outputs. |
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Superconductive Pressure
Contacts
G.K. Gaule, J.T. Breslin and
J.J. Winter
Summary: Superconductive contacts
are nearly as old as superconductivity itself; in 1914, only
three years after he had discovered superconductivity,
Kamerlingh-Onnes found that a Pb-Pb contact carried
supercurrents. Significantly, extreme cleaning of the contact
surfaces was not necessary. Systematic studies were not
undertaken until 1932, when Hohn and Meissner investigated
Pb-Pb, Sn-Sn, and Sn-Pb contacts at various temperatures and
magnetic fields. Their contact supercurrents reached fractions
of an ampere. By deliberately depositing insulating layers on
Ta contact surfaces, Dietrich in 1952 found strong evidence
for small supercurrents through thin (/spl sime/15/spl Aring/)
layers of this kind. In a theoretical study in 1962, Josephson
found that such currents can result from a special kind of
weak coupling between the two wave functions representing the
superelectrons in the two contact member. Josephson weak
coupling may also take place via a normal conducting layer; in
fact Meissner, in 1958 had shown experimentally that
interposed normal layers up to 3000 /spl Aring/ (and also
ferromagnetic layers up to 100 /spl Aring/) do not prevent an
otherwise superconducting contact from passing small
supercurrents. |
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