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| 1974 |
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Front
Cover (1974)
No author
information available
Summary: Not
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Table of Contents (1974)
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author information available
Summary: Not
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Conference Information
(1974)
No author information
available
Summary: Not
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Editor's
comments (1974)
J.
Rowell
Summary: Not available |
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Chairman's
comments (1974)
C.
Laverick
Summary: Not available |
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Energy
legislation in the 93rd congress--Programs and
personalities
N.
Werthamer
Summary: A review is given of
energy legislation activities of the 93rd Congress,
particularly since the Arab oil embargo, with attention mainly
to energy provision technologies R&D - program goals and
management organization. Relations are pointed out between
Congressional actions on the one hand and Congressional
committee jurisdication and personalities on the other. The
relevance of this legislation for a particular technology,
superconductive electrical transmission, is pointed
out. |
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Energy
R&D policy--Issues and
choices
P.
Craig
Summary: It is a great pleasure for me
to attend the Cyrogenic Engineering Conference today, and to
have the opportunity to discuss with you a few of the many
recents developments in National Energy Policy. There have
been many important energy-related events since this
Conference last met in Annapolis, in 1972. Energy prices have
increased dramatically. Inflation has been rampant in the U.S.
and throughout the world. Floods have taken major tolls in
Bangladesh and in Honduras, and all of the underdeveloped
nations are experienceing dislocations from oil prce increases
so severe that their very existence as nations is threatened,
and widespread starvation will certainly result is major help
is not forthcoming. Technological strides have occurred - not
so dramatic, admittedly, but certain to have major impact upon
the nation, and upon the world in the decades to come. The
United States is organizing to aggressively pursue an expanded
energy R&D program. Formation of the Energy Research and
Development Agency is expected shortly. |
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Superconductivity--A
subject of interest to the atomic energy
commission
D.
Stevens
Summary: Immediately after the
reporting of magnetically hard superconductors late in 1959,
the practical importance of superconductivity to the Atomic
Energy Commission was immediately recognized. The potential
savings of power costs in the High Energy Physics program and
their application in magnetic finement systems in the
Controlled Thermonuclear Research program resulted in the
reorientation a number of magnet development efforts and the
phasizing of research on superconductivity in the fundamental
materials research program of the AEC. It soon became evident
that there was a lot of work to be done, particularly on
materials and ductor development before the true benefits of
superconductivity could be realized. Much of the optimism of
the early 1960's was dampened and the community settled down
to accomplish what had to be done before science and industry
could accept superconductivity with comfort and
reliance. |
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The
electric power research institute's role in applying
superconductivity to future utility
systems
M.
Rabinowitz
Summary: Economics has been the
single most important factor in determining the future of any
new commercial technology in the United States. This criterion
is in need of serious examination in view of the projected
sharply increasing consumption of energy in the next few
decades, particularly in the form of electricity. In order to
make a smooth and meaningful transition from conventional
methods of generating and transmitting electricity, a
coordinated effort between all segments of the private and
public domains will be required. The Electric Power Research
Institute (EPRI) should play a vital role in planning for both
the imminent short term, and long term national electrical
energy needs; and in coordinating efforts to achieve these
vital goals. If, as predicted, the U. S. power consumption
increases by more than a factor of six in the next 30 years,
it should be clear that it is necessary to develop high power
density methods of producing and transmitting electricity.
Superconductivity is the natural prime candidate for a new
feasible technology that can take on this
responsibility. |
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Helium
supply and demand in future
years
C.
Laverick
Summary: Helium can be a resource of
great value to the post industrial world by the turn of the
century if it can be made available in sufficient quantity and
if current research and development efforts on those aspects
of applied superconductivity which hold greatest promise for
significant application can be sufficiently supported and
encouraged in a well coordinated way. It is almost certain
that there will not be enough helium to permit utilization of
all the more significant options to be made available,
difficult choices will have to be made and priorities for use
established. In the next twenty five years, while we wait for
the new technologies to develop, the population of the United
States will increase by about 40 to 90 million people and that
of the world will almost double to reach almost 7 billion
barring some great catastrophe. Land use will double. Demands
for food, energy and resources will multiply, rates of change
and tendencies to global instability will increase. Our
principal immediate challenge is to organize and work so as to
survive as an individual society in these years immediately
ahead, This is the perspective from which we must view the
helium problem and longer term technology
development. |
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Superconductivity
applications in high energy
physics
T.
Fields
Summary: During the past five years,
several very large superconducting bubble chamber and spark
chamber magnets have become operational. The economic and
other factors which have led to the construction of large
superconducting solenoid magnets for high energy physics
research are discussed, and the technological development
which made these magnets possible is summarized. Much of the
recent development work on superconducting magnets for high
energy physics has been directed toward the next generation of
accelerators and beam storage rings. Here the goal is to
develop dipole and quadrupole magnets of high field precision,
low cost, high reliability, capability for slow pulsing, and
minimal sensitivity to radiation heating. The status of this
work and its application to future accelerators is described.
Also, the application of superconducting rf cavities for
linear accelerators and particle separation is reviewed
briefly. |
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Superconducting
materials up to now and into the
future
T. Geballe and J.
Hulm
Summary: "Tool maker, stacker of wheat..
Flinging magnetic curses amid the toil of piling job upon
job." Anybody who has had a magnet quench on him knows what
Carl Sandburg meant when he wrote of Chicago. The question
before all of us today is: Can we lick the magnetic curses and
really apply superconductivity? Certainly the wider the range
of properties that superconductors have the more chance there
is of success. The first portion of the paper will be
concerned with the range of intrinsic properties: the critical
temperatures (T/sub c/), critical fields (H/sub c/), and
critical currents (J/sub c/) In the latter part we will
discuss the fabrication of superconducting materials into
composite wires or cables in forms suitable for producing
magnets, generators, transmission lines, and some of the other
devices that are the subject of this meeting. Structures
concerned with nonpower-related applications such as Josephson
junction devices unfortunately are beyond the present
scope. |
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Role
of superconducting transmission--A subjective
assessment
L.
Fink
Summary: Having been asked to discuss
the potential role of superconducting transmission lines
within utility systems of the future, I find that realism
constrains my comments to within a rather narrow scope.
Despite the valid and necessary role that must be assigned to
quantitative economic and technical analyses in assessing
research priorities for emerging technologies, the fact
remains that the probability of realization for any forecast
scenario remains essentially zero, and that attempts to rough
in a probability distribution by considering a set of scenaria
are far from providing any degree of certainty. .At least some
of us who have had to assess and recommend research projects
would have to admit that often our attempts at quantitative
evaluation have reflected prior heuristic, qualitative
convictions rather than the reverse. |
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Superconducting
rotating machines
J. Smith, J.
Kirtley and P. Thullen
Summary: The
opportunities and limitations of the applications of
superconductors in rotating electric machines are given. The
relevant properties of superconductors and the fundamental
requirements for rotating electric machines are discussed. The
current state-of-the-art of superconducting machines is
reviewed. Key problems, future developments and the long range
potential of superconducting machines are
assessed. |
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Application
of superconductivity in the controlled thermonuclear research
program
G. Hess, E. Ziurys and D.
Beard
Summary: Applications of
superconductivity will be needed in each of the three magnetic
confinement systems being developed in the AEC controlled
thermonuclear research program. Fusion plasmas must be
confined by magnetic fields instead of solid walls. The
tokamak and magnetic mirror confinement systems will need
superconducting magnets to produce the confining magnetic
field. The theta-pinch confinement system will need
superconducting energy storage coils and homopolar machines to
provide energy for pulsed magnetic fields. The AEC is
supporting developments for these three systems but it is not
yet known which magnetic confinement systems will be used in
fusion power reactors. The technology problems associated with
these applications of superconductivity are described in the
paper. |
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Review
of superconducting electronics
R.
Kamper
Summary: This review will sketch the
present state of affairs in applications of Josephson
junctions and SQUIDs to: magnetometry, DC and RF metrology,
detection and amplification of electromagnetic signals,
frequency metrology, noise thermometry and computers. It will
also mention recent progress in super-stable oscillators using
superconducting resonant circuits, pulse transmission lines,
and thin-film deviees to detect radiation or charged
particles. Many of these topics are maturing
nicely. |
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Research
work on superconducting magnet systems in
Germany
W.
Heinz
Summary: Various applications of sc
magnets are pursued in our country. Development in nuclear
physics, especially new developments for dc and pulsed
superconducting beam line magnets are reported. Research work
on magnets for plasma and fusion physics concentrates on
systems studies, toroidal systems, and energy storage devices
and will become one of the most challenging fields for magnet
builders in the future. Industrial applications of sc magnets
are investigated for high speed transportation systems, sc
machinery and magnetic separation devices. Programs and
results are discussed. |
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New
superconducting critical temperatures and
fields
B.
Matthias
Summary: During the past two years,
superconductivity has been discovered in several new crystal
phases compounds. A structure first discovered by Chevrel - et
al. the molybdenum sulfides resembles the A15 structures in
that a large number of compounds new structure become
superconducting. The highest transition temperatures, somewhat
above 15°K, were found for the X = Sn and Fb phases. Odermatt
et al first measured their critical fields in the helium range
to 350kG, and extrapolated from there to above 500kG, Values
which are astonishingly high in comparison to the A15
compounds - these fields are higher than anything ever
reported before. Recently, Foner et al have claimed still
higher values yet. |
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High
field properties of ternary
metal-molybdenum-sulfides
S.
Foner, E. McNiff and E. Alexander
Summary:
Measurements of the upper critical fields are presented
for ternary metal-molybdenum-sulfides as a function of
temperature, T, with dc magnetic fields up to /spl sim/215kG
and pulsed fields to /spl sim/500kG. A nearly linear variation
of H/sub c2/versus T is observed up to the highest dc fields
for all the compounds. The materials (nominal composition),
T/sub c/(measured); (dH/sub C2//dT)/sub T=T/sub c//
(measured), and H/sub c2/(4.2 K) (calculated) for a dirty type
II superconductor assuming no paramagnetic limiting are
respectively: (a) Pb/sub 0.9/Mo/sub 5.1/S/sub 6/: 11.7 K, 60
kG/K, 390 kG; (b) SnAl/sub 0.5/Mo/sub 4.5/S/sub 6/: 14.2 K, 32
kG/K; 275 kG; (c) SnMo/sub 2/S/sub 6/:13.4K, 37kG/K; 290 kG;
(d) PbMo |
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Improved
superconducting properties of multifilamentary niobium
carbonitride wire
M. Ohmer, J.
Wollan and J. Ho
Summary: Transport and
magnetization measurements have been made on 6/spl mu/, 720
filament niobium carbonitride yarn. The magnetization data is
discussed in terms of the hysteretic loss, the temperature
dependence of J/sub c/, the matrix-superconductor bond quality
and the doping effects on the high field J/sub c/'s. Transport
properties are compared to properties obtained from
magnetization data. In addition, weak coupling has been
investigated in low conversion fibers. The V-I curves show
numerous voltage steps, some corresponding to an order of
magnitude increase in resistivity. |
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Neutron
induced disorder in superconducting A-15
compounds
A. Sweedler, D. Cox, D.
Schweitzer and G. Webb
Summary: The effect of
high energy neutron (E>1 MeV) irradiation at 60/spl deg/C
on the superconducting critical temperature, T/sub c/, the
upper critical field, H/sub c2/, the lattice parameter, a/sub
o/, and the degree of Long Range Order has been measured for
Nb and V based A-IS superconducting compounds. Large
reductions in T/sub c/, and H/sub c2/are observed for fluences
up to 5.0x10/sup 19/n/cm/sup 2/. For Nb/sub 3/Al a/sub
o/increases and the degree of Long Range Order is
significantly reduced as T/sub c/is depressed. The results are
discussed in terms of atomic ordering in the A-15
structure. |
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Neutron
irradiation of Nb/sub 3/Sn and NbTi multifilamentary
composites
D. Parkin and A.
Sweedler
Summary: NbTi and Nb/sub 3/Sn
multifilamentary composites have been irradiated with
fast-neutrons at 60 /spl plusmn/ 5/spl deg/C to fluences of
1.2x10/sup 20/n/cm/sup 2/(E > 1 MeV). The NbTi samples show
only a moderate reduction of I/sub c/as a function of neutron
fluence in an applied field of 40 kG. Reductions in I/sub
c/were observed for fluences greater than 3x10/sup 17/n/cm/sup
2/and saturate at 18% for fluences greater than 3-4 x 10/sup
19/n/cm/sup 2/. The Nb/sub 3/Sn composites showed large
neutron radiation induced changes in T/sub c/, I/sub c/and
H/sub c2/. Reductions in T/sub c/were observed for fluences
greater than 7x10/sup 17/n/cm/sup 2/. No measurable changes in
I/sub c/(40 kG) were observed below 10/sup 18/n/cm/sup 2/.
Between 2 and 3x10/sup 18/n/cm/sup 2/, however, there is an
apparent threshold where a very rapid reduction in I/sub c/(40
kG) is initiated. At the threshold the decrease in T/sub c/is
13%. Between the threshold and 1.1x 10/sup 19/n/cm/sup 2/,
I/sub 2/(40 kG) has been reduced to 4% of the unirradiated
value. These changes in superconducting properties in NbTi and
Nb/sub 3/Sn are analyzed in terms of the radiation induced
defects. The impact of the response to irradiation of both
materials on their applications in fusion reactor magnets is
discussed. |
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Effects
of fast neutron irradiation at low temperature on the
properties of Nb-Ti superconductor
wires
M. Couach, J. Doulat and E.
Bonjour
Summary: The object of this work was
to estimate the effects of neutron irradiation at 77K on
commercial stabilized superconductor wires. Irradiation doses
of 10/sup 18/to 10/sup 19/fast neutrons per cm/sup 2/were
explored and the critical current measured in magnetic fields
extending from 2 to 8 Teslas. A reduction in the critical
Current density, depending on the type of the conductor was
observed and for the single core wire a decrease of the take
off voltage indicates a loss of stability. |
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Irradiation
and annealing effects of deuteron irradiated NbTi and V/sub
3/Ga multifilamentary composite wires at low
temperature
E.
Seibt
Summary: To study the effects of low
temperature irradiation on technological type
II-superconductors, NbTi and V/sub 3/Ga multifilamentary
composite wires, the critical current I/sub c/and the
transition temperature T/sub c/were measured before and after
irradiation with 50 MeV deuterons at 10 K and 15 K,
respectively. While the irradiation effects on I/sub c/and
T/sub c/of NbTi are substantially unaffected, the V/sub 3/Ga
wires undergo a reduction in I/sub c/of about 50 % and T/sub
c/decreases from 14.7 /spl plusmn/ 0.1 K to 12.3 /spl plusmn/
0.1 K at a total deuteron flux of 2.7x10/sup 17/cm/sup -2/.
Annealing experiments at room temperature and 100/spl deg/C
show only a small recovery of the superconducting properties
up to 15 %. The field dependence of the volume pinning force
densities P/sub v/was determined and the results are shown to
be consistent with a qualitative dynamic pinning
model. |
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The
influence of low temperature neutron irradiation on
superconducting magnet systems for fusion
reactors
M. Soell, C.v. der
Klein, H. Bauer and G. Vogl
Summary: The
components (insulation, stabilization material and the
superconductor) of a superconducting coil system for a fusion
reactor are exposed to fast neutrons, probably with a dose of
about 10/sup 18/n/cm/sup 2/during the lifetime of a reactor,
and with an energy spectrum comparable with the one of a
fission reactor. Experimental investigations were made on the
influence of low temperature neutron irradiation on the
stability of a superconductor and of the superconducting
properties of NbTi wires with different initial jc and
pre-treatment. The influence on j/sub c/and T/sub c/of Nb/sub
3/Sn was investigated. |
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Preparation
and properties of multifilament niobium carbonitride
superconductor
W. Smith, R. Lin,
J. Coppola and J. Economy
Summary: The
preparation and characterization of a multifilament niobium
carbonitride yarn is reported here for the first time. The
fine diameter multifilament yarn was prepared by chemical
conversion of a precursor carbon yarn. The fibers are
generally composed of both NbC and NbCN phases and exhibit a
T/sub c/of 16-17.5/spl deg/K and an H/sub c2/of 110- 120kG.
I/sub c/values at zero field of up to 130 amperes have been
measured corresponding to a J/sub c/of 10/sup 6/A/cm/sup 2/.
I/sub c/is proportional to the amount of superconductor phase
present as judged by weight gain data. At low conversion
levels an anomaly has been detected in critical current
behavior of the fibers suggesting a non-continuous
superconducting path. Electrical contact to the fibers by
copper coating is discussed showing the importance of
obtaining a low resistance contact. J/sub c/was measured as a
function of magnetic field for both the pure material and
samples doped with silicon using SiCl/sub 4/. The silicon
dopant was found to be effective in reducing the field
dependence of the critical current. |
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Superconducting
properties of thin film niobium carbonitrides on carbon
fibers
G. Pike, A. Mullendore, J.
Schirber and J. Napier
Summary: The
characteristics of a new superconducting wire material,
consisting of a 7 /spl mu/m carbon filament substrate thinly
coated with niobium carbonitride, are reported. These
filaments are coated (/spl sim/ 1000 at a time as a yarn) by a
co-CVD process in which the desired carbonitride is deposited
as a compound. Several coating compositions from NbC to
NbC/sub .75/N/sub .15/have been studied. At each composition
several coating thicknesses from 300 to 3000 /spl Aring/ were
prepared. Although our present coating compositions are not
near NbC/sub 0.3/N/sub0.7/, which has previously been
determined as optimum for this pseudobinary system, we
nevertheless have obtained excellent superconducting
properties. Zero-field critical current densities of 2x10/sup
6/A/cm/sup 2/in the coating have been obtained, and there is
no discernable ac loss at frequencies up to 50 kHz. In
addition to the superconducting measurements we have also
performed chemical, x-ray diffraction, and electron
microscopic analyses on the coated filaments. |
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Properties
of amorphous and microcrystalline
superconductors
W. Johnson and S.
Poon
Summary: Results of x-ray diffraction,
electrical resistivity, critical field(H/sub c2/) and
transport measurements are presented and discussed for bulk
amorphous and microcrystalline transition metal alloys
obtained by liquid quenching. The transition temperature of
the alloys is in the range 1.5 to 4.7/spl deg/K. It is
observed that J/sub c/-H/sub c2/-T/sub c/relations are rather
simple for this class of material. These relations are
compared with the theories of type II superconductors
whereever possible. The high resistance of bulk metallic glass
to radiation damage might render them suitable for magnetic
field applications in high radiation
environments. |
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Superconductivity
in Nb/sub 3/Ge
J. Gavaler, M.
Janocko, A. Braginski and G. Roland
Summary:
The recent report that Nb/sub 3/Ge films can be
prepared with superconducting transition temperatures of over
22K has immediately made Nb/sub 3/Ge a very important
potential candidate for a variety of large scale applications,
such as for example, power transmission lines, or large high
field magnets. Short sample data on sputtered films show that
Nb/sub 3/Ge possesses an extremely attractive combination of
high field properties, competitive with any other presently
known material at all temperatures from zero to T/sub c/. The
development of a process capable of being used to fabricate
Nb/sub 3/Ge in a useful configuration (i.e., long length of
wire or tape) is now being investigated. Chemical vapor
deposition (CVD) is a demonstrated commercial process, which
may be applicable for this purpose. Our initial CVD studies
have yielded films with T/sub c/'s over 21.5K. |
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Sputtered
films
L.
Testardi
Summary: New metallurgical phases
and enhanced superconductivity can be obtained using hot
substrate sputtering. Some advantages and limitations of this
technique are discussed. For Nb-Ge studies of T/sub c/,
resistivity, crystallographic structure, compositional
dependences, and impurities are summarized. Although T/sub
c/varies with composition it is not found to be critically
dependent upon exact stoichiometry and, at the same
composition, is considerably higher in the films than in the
bulk. A simple correlation between T/sub c/and resistance
ratio is reported. |
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Electron
beam evaporation synthesis of A15 superconducting compounds:
Accomplishments and prospects
R.
Hammond
Summary: The accomplishments and the
prospects of forming A15 superconductors using electron beam
evaporation are reviewed. The techniques of monitoring and
controlling the evaporation are discussed. The special
advantages to the synthesis of A15 materials are pointed out,
followed by a number of examples. The prospect of the large
scale production of A15 superconductors using electron beam
processing is also considered. |
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Superconducting,
metallurgical and synthesis properties of Nb/sub
3/Ga
G. Webb and J.
Engelhardt
Summary: We present a review of
the superconducting, metallurgical and synthesis properties of
Nb/sub 3/Ga which are important for its potential application.
The superconducting properties to be discussed are T/sub c/,
H/sub c2/(T) and the dc critical current J/sub c/(H). Special
metallurgical features of the Nb-Ga system which put severe
constraints on synthesis techniques of potential practical
importance for this material are discussed. Superconductivity
data for materials synthesized by the techniques of quenching
and annealing, chemical vapor deposition and vacuum
co-evaporation are reviewed. Recent results on the effects of
radiation damage and its recovery by annealing are
reviewed. |
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Improved
J/sub c/in mechanically fabricated Nb/sub 3/Al wires and
ribbons
T. Eagar and R.
Rose
Summary: Nb/sub 3/Al wire which is
essentially all A15 may be fabricated by powder or rod
composite techniques, with subsequent reaction and ordering
anneals. Several of the previously existing problems,
including aluminum loss and Kirkendall porosity, have been
identified and solved, and transverse-field critical current
densities well into the 10/sup 5/amp/cm/sup 2/range can be
produced. However, reproducibility of the critical current
density seems to depend on reproducible control of grain size,
which we have not as yet achieved. The internal oxidation
techniques which succeed for Nb/sub 3/Sn do not appear to work
for Nb/sub 3/Al, for several reasons. A possible solution to
the grain size problem, and to the problem of manufacture of
Nb/sub 3/Al in stabilized form, is to lower the initial
reaction temperature. To achieve the latter we have reduced
the scale of the elemental composite to a required diffusion
distance of about 3000 /spl Aring/. before reaction. We were
thus able to produce substantial quantities of A15 at 1100/spl
deg/C, but for several reasons (see text) we believe finer
composites will be necessary. |
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High-rate
sputtering of Nb-Al-Ge and Nb-Al
superconductors
S.
Dahlgren
Summary: High quality Nb-Al-Ge and
Nb-Al superconductor deposits, up to 1 mm thick, were made at
a rate of 1 /spl mu/m/min using high-rate sputtering
techniques. High-rate sputtering eliminates most of the
fabrication problems generally encountered with these
materials. Reasonable quantities of high-field superconductors
can be made at acceptable cost for practical applications by
high-rate sputtering. Highly reflective metallic mirror
surfaces on these deposits indicate a high quality, even in
thicknesses up to 1 mm. The deposits were completely free of
voids and they did not seem to be overly brittle either before
or after heat treatment. Crystal structures for deposits made
at 20/spl deg/C were not clearly defined, but probably were
body-centered-cubic (bcc). Heat treatment between 550/spl
deg/C and 850/spl deg/C completely transformed the bcc
deposits to extremely small (350 /spl Aring/) grains of the
A-15 crystal structure. The A-15 phase formed by heat
treatment for 1 hr to 5 days at 750/spl deg/C was metastable
and supersaturated with Al and Ge. Deposition and heat
conditions for decomposition of the metastable A-15 phase were
identified. The highest critical temperatures (18/spl deg/K)
were observed only in those deposits containing undecomposed
metastable A-15 phase. To assess practical use of the
sputtered superconductors, 1- to 2-m lengths of ribbon and
filamentary Nb-Al-Ge superconductor were produced on ribbon
and wire substrates. It also was found that an excellent
superconductor-stabilizer bond can be formed by high-rate
sputter deposition of copper stabilizer onto heat-treated
Nb-Al-Ge and Nb-Al superconductors. |
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Bulk
superconductivity above 20 K in Nb/sub
3/Ge
L. Newkirk, F. Valencia, A.
Giorgi, E. Szklarz and T. Wallace
Summary:
The superconducting properties of bulk films(0.02- 0.06
mm thick) of Nb/sub 3/Ge deposited on Cu substrates by
chemical vapor deposition have been studied along with
physical parameters characterizing these deposits. Results of
susceptibility measurements showing superconducting onsets
greater than 21 K and resistivity measurements showing onset
greater than 22 K are presented along with heat capacity
measurements. Data are also presented defining a relationship
between superconducting transition temperature and lattice
spacing throughout the range of 4 - 21 K. In addition, the
effects of substrate temperature and substrate material, as
well as mechanical properties such as substrate adherence are
described. |
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Variation
of T/sub c/of sputtered Nb/sub
3/Ge
A. Ghosh, L. Pendrys and D.
Douglass
Summary: Alloy samples of Nb and Ge
were prepared by simultaneous R. F. sputtering of composite
Nb-Ge targets onto heated alumina substrates. The variation of
the superconducting transition temperature T/sub c/and the
X-ray properties of these specimens were studied as a function
of the alloy composition, in the neighborhood of Nb/sub 3/Ge.
The variation of the sputtering parameters, i.e., the self
bias voltage, the sputtering gas pressure and the substrate
temperature was also studied. Results indicate that high T/sub
c/occur for well-ordered stochiometric Nb/sub 3/Ge alloys. It
was also found that the competing tetragonal phases are
promoted by the highly energetic atoms i.e. at high sputtering
rates. The relative sticking probability of Ge is important at
the higher substrate temperatures. |
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Selective
thermalization in sputtering to produce high T/sub
c/films
F. Cadieu and N.
Chencinski
Summary: To understand the
sputtering process as a means of producing high T/sub
c/metastable superconducting compounds we have investigated
the need for thermalization of the sputtered atoms as they are
deposited on the substrate. We have used low pressure RF
sputtering to produce Nb/sub 3/Ge with T/sub c/of /spl sim/
22/spl deg/K for pressures of 20 mtorr Kr and for 45 mtorr Ar.
To test the hypothesis that a thermalization is required to
effect a high T/sub c/in Nb/sub 3/Ge we have been studying the
sputtering process for systems such as Nb/sub 3/Al where Kr is
used to thermalize the Nb atoms with the minimum number of
collisions and an admixture of Ne is used to optimally
thermalize the Al atoms. A computer simulation of the
sputtering process aids in the choice of optimum parameters.
If the thermalization hypothesis is correct then
co-evaporation should be able to produce high T/sub c/Nb/sub
3/Ge. |
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The
fabrication and properties of Nb/sub 3/Sn superconductors by
the solid diffusion process
M.
Suenaga, W. Sampson and C. Klamut
Summary: It
has been demonstrated that the superconducting compound Nb/sub
3/Sn can be produced at the interface of Nb and a Cu-Sn alloy
when the composite is heat treated at the proper reaction
temperature. This method of Nb/sub 3/Sn preparation was also
shown to be particularly suited to the fabrication of (a)
Nb/sub 3/Sn multifilamentary composite wires for dc and low
frequency applications and (b) low ac loss Nb/sub 3/Sn tapes
for 60 Hz applications. The fabrication method and
superconducting properties of Nb/sub 3/Sn composite conductors
made by this process are reviewed and the metallurgical
aspects of the process are discussed. Since practical
applications of these composites requires large, high current
conductors, methods of producing compound conductors such as
cables and braided ribbons which include strengthening and
stabilizing components are also discussed and some preliminary
results of high current conductors are
presented. |
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Solid
solutions of Niobium-tin for preparing Nb/sub 3/Sn
superconductors
J. Galligan and
J. Tregilgas
Summary: Solid solution alloys
of niobium with concentrations of Sn from 1 to 9 wt% have been
prepared by quenching the solid solution from about 2100/spl
deg/C to room temperautre. These materials are relatively
ductile and can be deformed, by swaging, into the desired
shape, after which the material is reacted at about 800/spl
deg/c. The resulting structure is a mixture of Nb/sub 3/Sn and
Nb, with as much as 45% of the volume containing Nb/sub
3/Sn. |
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Studies
on the multifilamentary V/sub 3/Ga
wire
K. Tachikawa, K. Itoh and Y.
Tanaka
Summary: The stability of the
multifilamentary(MF) V/sub 3/Ga wire under pulsed excitation
and the effect of the aluminum addition to the Cu-Ga alloy
matrix have been investigated. Small coils wound by the MF
V/sub 3/Ga wire and by a similar MF Nb-Ti wire have been
excited with a speed ranging 0.5-200 kOe/sec. The critical
current of the MF V/sub 3/Ga wire coil does not degrade up to
an exciting speed as high as 200 kOe/sec, while that of the MF
Nb-Ti wire coil decreases rapidly in exciting speed higher
than 20 kOe/sec. The instability phenomena, e.g. flux jumps
and training effects are much smaller in the MF V/sub 3/Ga
wire than in the MF Nb-Ti wire. The addition of aluminum in
the Cu-Ga alloy matrix enhances the formation of V/sub 3/Ga
and increases the critical current. The partial substitution
of aluminum for gallium in the Cu-Ga alloy matrix seems to be
promising for increasing the current-carrying capacity and
decreasing the cost of the wire. |
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Potential
fabrication method of superconducting multifilament wires of
the A-15 type
C. van Beijnen and
J. Elen
Summary: A method is described for
production of A-15-type multifilament superconductors, based
on mechanical size reduction of composites of unalloyed copper
with niobium or vanadium cores. These cores are in themselves
hollow and filled with fine grain size powder of an
intermetallic compound of niobium or vanadium, rich in the
second element of the required superconductor. After obtaining
the final dimensions of the ductile composite without
intermediate heat treatments, metal and intermetallic
compounds are converted by heat treatment into the
superconductor wbich is formed as a continuous layer inside
the tublets. In particular the preparation is reported of
V/sub 3/Ga and V/sub 3/Si conductors using V/sub 2/Ga/sub
2/and VSi/sub 2/powder respectively. Critical current density
values of seven filament wire are included, measured at 4.2 K
up to 10 T. |
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Multifilamentary
niobium tin magnet conductors
D.
Larbalestier, P. Madsen, J. Lee, M. Wilson and J.
Charlesworth
Summary: Practical magnet
conductors of multifilamentary Nb/sub 3/Sn have been produced
in a collaborative programme between A.E.R.E. Harwell and
Rutherford Laboratory. The evaluation of these bronze route
conductors is described. Conductors studied range from a 1369
filament all-bronze matrix conductor to 5143 and ~42,000
filament conductors, containing internal high purity copper
protected by diffusion barriers. Filament sizes vary from ~3 -
8 /spl mu/m diameter. The effect of heat treatment conditions
on critical current and transition temperature is presented
and it is shown that overall critical current densities
greater than those available in niobium titanium can now be
produced in multifilamentary Nb/sub 3/Sn magnet
conductor. |
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Superconducting
properties of V/sub 3/Ga composite
wires
D. Howe and L.
Weinman
Summary: Studies of the
superconducting properties of V/sub 3/Ga single filament
composite wires grown by solid state reaction are presented.
The three alloy compositions investigated were: V/Cu-15.4 at.%
Ga, V-6.1 at.% Ga/Cu- 15.4 at.% Ga and V-9.0 at.% Ga/Cu-17.5
at.% Ga. Critical currents were measured at 4.2K in transverse
magnetic fields to 175kG and transition temperature data were
obtained by ac mutual inductance techniques for V/sub 3/Ga
formed at temperatures from 525/spl deg/C-700/spl deg/C.
Growth rates for V/sub 3/Ga were strongly influenced by alloy
composition and formation temperature with more rapid growth
occurring in composite wires with higher Ga contents. In
addition to the more rapid growth rates obtained, a factor of
10 improvement in the critical current density (J/sub c/) also
was exhibited in V/sub 3/Ga grown at 600/spl deg/C in the V-
9.0 at.%/Cu-17.5 at.% Ga composite specimen. The best J/sub c/
observed to date was 10/sup 6/A/cm/sup 2/in a transverse
magnetic field of 100kG for a wire reacted at 550/spl
deg/C. |
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Flux
pinning in Nb/sub 3/Sn multifilamentary
conductors
M. Mathur, M. Ashkin,
D. Deis and B. Shaw
Summary: Magnetization
measurements on diffused reacted samples were made at several
temperatures T, between 4.2K and T/sub c/and magnetic fields H
up to 75 kOe. The samples were distinguished from each other
by different heat treatments and diameter sizes ranges from
200-500 /spl mu/m. From these measurements J/sub c/(H,T) and
F/sub p/(H,T) curves were derived where F/sub p/is the flux
pinning force density. For all samples, F/sub p/(H,T)/F/sub
pmax/(T) vs H/H/sub c2/needs to be described by two functions,
one for T < 8K and the other for T > 8k. This is in
contrast to other materials where flux pinning is usually
described by a single function. |
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Effect
of metallurgical history on 'J/sub c/(5T)' in surface diffused
multifilamentary Nb/sub 3/Sn
A.
McInturff and D. Larbalestier
Summary: Data
are presented of critical current density 'J/sub c/(5T)' in a
5 T perpendicular magnetic field (and 10 T as well for
selected samples) for multifilamentary Nb/sub 3/Sn formed by
Sn diffusion from a Cu-Nb composite's outer surface. The
parameters varied are: composite diameter and Cu/Nb ratio; Sn
percentage; filament diameter; reaction temperature and time;
and different combinations of the above parameters. The
composite diameter varied from 0.254 to 0.08 mm; Cu/Nb volume
ratio from 1.7 to 3; volume percentage of Sn from 2% to 13.8%;
average filament diameter from 2 microns to 7 microns. The
reaction temperature was varied between 550/spl deg/ C to
795/spl deg/ C for different lengths of time, one to several
thousand hours. A three-stage heat treatment was selected to
formulate the basis for the reaction heat treatment map. The
J/sub c/(5T) varied from 10/sup 3/to > 3x10/sup 5/A/cm/sup
2/over the entire composite cross section (Cu-Sn, Nb/sub 3/Sn,
Nb). The optimum performances were 38.8 A in 0.127 mm diam
1.9/1 Cu/(Nb + Nb/sub 3/Sn) composite at 5 T and 78 A in a
0.18 mm diam 3/1 CU/(Nb + Nb/sub 3/Sn) at 5 T. |
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Nb/sub
3/Al formation at temperatures lower than 1000/spl
deg/C
S. Ceresara, M. Ricci, N.
Sacchetti and G. Sacerdoti
Summary: A study
has been made of the possibility of making Nb/sub 3/Al wires
by interdiffusion between Al and Nb at relatively low
temperatures (lower than 1000/spl deg/C). This method should
allow the fabrication of a copper stabilized wire by coworking
Nb and Al composites in a copper matrix before the diffusion
treatment. Results concerning the diffusion temperature and
time de pendence of J/sub c/and T/sub c/are presented. While
J/sub c/(64kG) is quite high, of the order of 1.5x10/sup
5/A/cm/sup 2/, T/sub c/is 15.56/spl deg/K, lower than the
usually reported value. X- -ray measurements of the lattice
parameter are also reported and correlated with the measured
value of T/sub c/. |
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Recent
developments in multifilament V/sub 3/Ga & Nb/sub 3/Sn
wires in Japan
Y.
Iwasa
Summary: The state of development of
multifilament V/sub 3/Ga and Nb/sub 3/Sn wires in Japan is
reviewed with emphasis on the former as it is principally in
Japan that V/sub 3/Ga has undergone the most intensive
development. Commercial sources for these materials are
identified. A brief discussion is presented of the effort on
commercial development of multifilament V/sub 3/Ga wire in the
United States. |
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Ductile
superconducting Cu-rich alloys containing A-15
filaments
C.
Tsuei
Summary: A new approach to a ductile
composite of Cu and an A-15 superconducting compound has been
reported recently. This new metallurgical process consists of
melting the constituent elements and subsequent cold-working
and heat treatment. The superconducting materials prepared by
this technique are composed of about 90 atomic per cent Cu and
a superconducting compound such as Nb/sub 3/Sn or V/sub 3/Ga
which is in the form of fine filaments embedded in the Cu
matrix. The new alloy is superconducting at a temperature
nearly identical to that of the superconducting phase in bulk
form. The current density (J/sub c/) is ~10/sup 4/ to 10/sup
5/A/cm/sup 2/at 4.2/spl deg/K and zero magnetic field (J/sub
c/is calculated by taking the total cross-section of the
specimen). An up-to-date account of the experimental situation
in this field will be given. The discussion includes (1) the
results of varying the alloy composition, the amount of
cold-working and the heat treatment; (2) the transition
temperature and J/sub c/of Sn-coated Cu-(Nb/sub 3/Sn) wires;
(3) J/sub c/as a function of transverse magnetic field; (4)
the phenomenon of J/sub c/enhancement as a result of
mechanical deformation and (5) the basic mechanisms
responsible for the observed superconductivity in these new
materials. |
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Remnant
resistance in TSUEI'S composite
superconductors
A. Davidson, M.
Beasley and M. Tinkham
Summary: The
application of a new superconducting composite wire developed
by C.C. Tsuei for high-field, high-current uses is explored.
The importance of the dish continuous nature of the
superconducting filaments in this material is analyzed. Our
calculations indicate that practical wire may have a remnant
resistivity of 10/sup -14/ohm-cm or less and we report our
experiments to date in which we have tried to measure this
quantity. We also report our observations of quantum coupling
between filaments in this wire. |
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Production
and fabrication of 2500-Lb. Nb-Ti ingots to
rod
T. Cordier and W.
McDonald
Summary: Interest in Nb-Ti
superconducting devices is exploding. This paper outlines the
critical production criteria for this material. Areas
discussed include ingot blending, melting, forging, extrusion
and rod reducing with emphasis on the metallurgical
considerations affecting mechanical properties. Data are
included relating process parameters to TEM finding as well as
R.T. ductility and optical microscopy. The significance of
these starting material considerations is of interest to the
composite wire maker as well as to the device
manufacturer. |
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Multifilamantary
Niobium tin superconductor
tape
P. Brisbin and W.
Coles
Summary: An effective method for the
fabrication of multifilamentary Nb/sub 3/Sn tape has been
developed. Filamentary superconductive paths are produced in
otherwise standard commercial superconductor tape by the
chemical milling of separator slots through the Nb/sub 3/Sn
layer. The present multifilament configuration features a
matrix of ten parallel helical superconducting paths along the
length of the tape; filament width and interfilament
separation are approximately 1.2 mm and 0.4 mm respectively.
Thirty-five multifilamentary tapes 0.4 m long were produced.
Complete arrays of continuous, fully-defined filaments were
reliably formed. Tapes tested as small pancake coils (6-10T)
demonstrated the integrity and continuity of the matrix and
showed that critical current was sustained in direct
proportion to retained superconductor. Interesting behavioral
differences resulted from filamentization: critical current
anisotropy was reversed, and the slope of the I/sub c/versus H
curve was reduced. Other tests found qualitative evidence of
substantially improved stability in the multifilamentary
tape. |
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Multifilamentary
Nb/sub 3/Sn for superconducting generator
applications
R. Scanlan and W.
Fietz
Summary: The progress achieved to date
of a joint IGC-GE program for developing a practical
multifilamentary Nb/sub 3/Sn superconductor for use in a
superconducting generator is described. The cable approach for
producing a Nb/sub 3/Sn conductor is evaluated, and the
results of bend and tensile tests for various cables are
presented. Fabrication methods for producing long lengths of
multifilamentary Nb/sub 3/Sn superconductor are discussed, and
test data for several small coils are presented. |
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Fabrication
and properties of multifilament Nb/sub 3/Sn
conductors
R. Randall, J. Wong,
D. Deis, B. Shaw and M. Daniel
Summary:
Multifilament conductors have been fabricated in
several configurations employing various combinations of Cu,
Sn, Cu-Sn bronze and Nb. These conductors are in the 8-100 mil
size range and contain 400-13,000 filaments. Filament sizes
range from 8-100 microns in diameter. Heat treatment studies
and their effect upon superconducting properties have been
performed for temperatures in the range 600-800/spl deg/C.
Sample evaluation has included measurements of T/sub c/,
resistivity and J/sub c/up to 150kG. Mechanical tests to
assess allowable bending diameters and metallurgical studies
to determine the distribution of constituent elements
subsequent to heat treatment have been made. These results
will be discussed and compared in the context of optimizing
the properties of these conductors for high field magnet
applications. |
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Composite
conductors containing many filaments of Nb/sub
3/Sn
E. Gregory, W. Marancik and
F. Ormand
Summary: A 342 filament Nb/sub 3/Sn
conductor made by "the bronze" approach is described in some
detail. Larger conductors containing 1045 and 3553 filaments,
respectively, are also mentioned. One of these contains a
barrier layer to separate the bronze matrix from high
conductivity copper which is required for stability in some
applications. A high aspect ratio version of the 3553 filament
conductor has also been made. A 67 507 filament conductor
carrying 1000 A at 12 T, developed for Lawrence Livermore
Laboratories, is shown. It has been made into several
different configurations, one with the size and aspect ratio
of the 3553 filament conductor shows less anisotropy in
current properties with different field
orientations. |
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Multifilamentary
Nb/sub 3/Sn conductor for fusion research
magnets
W. Fietz, C. Henning and
R. Scanlan
Summary: In an attempt to meet the
eventual needs of fusion magnets, a multifilamentary Nb/sub
3/Sn conductor has been proposed consisting of a cabled array
of multifilamentary Nb/sub 3/Sn strands .027 cm in diameter,
surrounding a stainless steel core. This cable assembly would
be jacketed with soft copper to provide an overall rectangular
exterior. For the purpose of testing this concept, a prototype
cable was fabricated in a length of several hundred meters.
The cable was designed to carry 300 amperes at 12T and have a
minimum bending radius of about 6 cm. Fabrication techniques,
short sample critical currents and bending test results will
be presented, and the problems anticipated in scale up will be
discussed. |
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Electroplated
stabilized multifilament
superconductor
V. Agureev, V.
Keilin, E. Klimenko and B. Samoilov
Summary:
This paper describes a method of manufacturing
stabilized multifilament superconductors by connection of
single or multifilament wires together by electroplating
process. The method was tried first in summer 1965 just after
appearing of paper by Kantrowitz and Stekly [1]. The scheme of
the method is shown in Fig. 1. A set of parallel wires was
wound onto a drum and then immersed into an electrolitic bath.
The turns were separated with an insulated wire to prevent
their coalescence. Copper covering of both sides of the set
resulted in a high quality stable conductor. At that time we
considered the method as a successful palliative which allowed
us, however, to test our first stable coil in summer 1965
(Fig. 2). But eventually the advantages of the method became
evident. Some of them are inherent to all compound conductors,
i.e. conductors which consist of a number of smaller cross
section conductors or wires joined in parallel. The advantages
are as following: 1. The superconducting components can be
manufactured by optimum process. They can be tested
preliminary and properly selected. It eliminates the risk of
spoiling great quantity of expensive superconducting alloy at
once. 2. The length of the conductor is not determined by the
size of a metallurgical billet. It is possible to make large
cross section conductors of any necessary length without using
of expensive metallurgical equipment. 3. Superconducting wires
can be fully transposed in such a conductor. |
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The
electrical and thermal conductivities of Cu-4, 10, and 20 at.%
Ni from 1-25 K
R. Linz, A.
Bouley, R. Klaffky and D. Damon
Summary:
Electrical and thermal conductivities of Cu-4, 10, and
20 at.% Ni are presented from 1-25 K. These alloy
concentrations are typical of those being used in some
composite superconductors to minimize electrical coupling
between filaments during rapidly changing fields. Data are
presented for both highly annealed and heavily deformed
specimens. For a given alloy, the electrical conductivities
increase as a result of deformation and can vary by nearly
10%. The thermal conductivities decrease as a result of
deformation and can vary by greater than a factor of two for T
/spl ges/ 4 K. |
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Alternating
field losses in mixed matrix multifilament
superconductors
M. Walker, J.
Murphy and W. Carr Jr.
Summary: Losses were
calculated for and measured at 4.2/spl deg/K in two
mixed-matrix superconductors for the cases of transverse
applied alternating fields superimposed upon zero or 5T
transverse bias fields. The 0.080 in. x 0.160 in. conductors
are composites of copper-sheathed Nb-Ti filaments twisted
every 5 cm within a copper-nickel matrix, where the
copper-nickel to copper to superconductor ratio is nominally
4:4:1. The conductors differ only in the copper-nickel alloy
of the matrix and the associated matrix conductivities,
sigma/sub m/calorimetric measurements were performed with
alternating fields ranging in frequency, f, from 0.3 to 12,800
Hz and from 2500 to 4 oersteds in amplitude H/sub m/. Eddy
current losses consistent with the theory and proportional
toH/sub m/sup 2f/sup 2sigma/sub m and H/sub m/sup 2f/spl
deg/sigma/sub m were observed. The measured losses were found
to be less than the calculated losses and within 45% of the
calculated losses in all cases. |
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Alternating
field losses in a rectangular multifilamentary NbTi
superconductor
J. Murphy, M.
Walker and W. Carr Jr.
Summary: Transverse
alternating field losses on a 1 mm X 1.4 mm monolithic
multifilamentary composite superconductor were measured
calorimetrically at 4.2K for frequencies between 0.1 Hz and
20,000 Hz. The conductor was placed in bias magnetic fields
ranging from zero to 50 kOe and subjected to superimposed
alternating fields with field amplitudes ranging from 10 Oe to
2 kOe. The classical partial penetration hysteresis loss and
full penetration hysteresis loss were observed. Losses were
also measured which can be attributed to eddy currents in the
composite core. At very high frequencies, eddy current losses
were observed resulting from the metallic sheath which
encloses the composite core. The magnitude of the eddy current
loss in the composite core is explained in terms of the
diamagnetism exhibited by the superconducting filaments. The
experimental data presented is analyzed using existing loss
theories which have been appropriately modified using basic
principles to account for the effect of filamentary
diamagnetism at low alternating field
amplitudes. |
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Alternating
field losses in Nb/sub 3/Sn multifilamentary
superconductor
J. Murphy, D.
Deis, B. Shaw and M. Walker
Summary:
Transverse alternating field losses at 4.2K have been
measured from 0.5 Hz to 10 kHz in a Nb/sub 3/Sn
multifilamentary superconductor in bias fields to 5 Tesla. The
0.020 inch diameter sample was prepared by heat treating a Cu,
Nb-1 wt % Zr, CuSn composite at 700/spl deg/C for 20 hours to
form Nb/sub 3/Sn on the inside surface of the annular
filaments. Metallurgical studies have been made to determine
the Sn distribution and to estimate the thickness of the
Nb/sub 3/Sn layer. The I/sub c/-H curve and resistive and
inductive transition curves are presented. The losses are
analyzed with respect to the present loss theories using the
conductor characteristics measured and excellent agreement
between experiment and theory is achieved. |
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AC
losses of multifilament superconductors at frequencies between
1 and 500 Hz
K. Kwasnitza and I.
Horvath
Summary: We report calorimetric
measurements of the ac losses of a small superconducting coil
wound from a filament superconductor designed for low ac
losses (filament /spl phi/ = 8 /spl mu/,number of filaments
1045, CuNi barriers). The coil was pulsed with a maximum dB/dt
= 480 kG/s. Further the ac losses were measured, when the coil
current consisted of a dc- and a superimposed ac component
with1 \leq f \leq 500 Hz. The modification of the ac losses by
the dc component of coil field and coil current was
investigated. |
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AC
losses of Nb/sub 3/Sn
J.
Bussiere, M. Garber and M. Suenaga
Summary:
60 Hz losses were measured at 4.2 K in a number of
Nb/sub 3/Sn rods and tapes produced by the bronze-diffusion
process and in commercial Nb/sub 3/Sn tapes. At low fields
(/spl lsim/900 Oe), losses vary by more than two orders of
magnitude among the different samples and are related to
surface topography. At high fields (/spl gsim/2 kOe) the
variation among samples is much less, and depends mainly on
the bulk critical current density J/sub c/. The losses are
analyzed quantitatively in terms of the critical state model,
including a surface current. |
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Magnetization
and design of multistrand superconducting
conductors
C.
Walters
Summary: Magnetization of cabled and
braided samples which are long compared to their transposition
lengths has been measured in a uniform magnetizing field
directed at right angles to their widths. A separated double
pancake with nonuniform windings was constructed to produce a
suitably uniform radial field in which long coiled samples
could be placed. Magnetization losses have also been measured
in the ISA I magnet at BNL. Results are compared with theories
which are due to Wilson[1] and, basically, Morgan.[2] Also,
design optimization procedures have been defined for the use
of multistrand conductors in magnet windings. |
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Flux-flow-like
state of a Pb-Tl rod in a longitudinal
field
F. Irie, T. Ezaki and K.
Yamafuji
Summary: The remarkable
characteristics of the longitudinal field problem are an
enhancement of the critical current and a paramagnetic effect.
The later has been explained quantitatively to a certain
extent, but the former has not. The existing theories do not
treat a flux-flow state and they ovrain a ciritical current as
a point of transition from static state to normal. Experiments
for this problem, however, show the existence of the resistive
region in V-1 characteristics, which have two kinds of
critical current, i.e. a transition point from static stare to
resistive state (I/sub CR/) and that from resistive state to
normal state (I/sub CN/). Therefore the resistive state of
this problem must be made clear before going to treat these
critical currents. |
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Reduction
of losses at low frequencies and large amplitudes in NbTi
ribbons
A. Lachaine and M.
LeBlanc
Summary: The critical current density
in NbTi ribbons increases considerably and hysteretic (A.C.)
losses decrease by an order of magnitude when a magnetic field
is present along the length of the ribbon. In our work, rather
than feeding a current to the sample through leads connected
to an external supply, we cause persistent transport currents
to flow along the length of the ribbon by induction (by
applying a magnetic field transverse to the ribbon axis and
along its broad faces). The magnitude of the induced currents,
their pattern of circulation and the hysteretic losses are
determined by monitoring the evolution of the magnetic moment
of the ribbon both along its length and along the wide faces
as the transverse field is cycled. We observe that when a
longitudinal field is present, the induced currents adopt
trajectories which are "tilted" and tend to lie along the
total magnetic induction/spl overrightarrow/ B along the flat
faces of the ribbon, hence tend towards force-free
configurations where /spl overrightarrow/ j /spl times//spl
overrightarrow/ B = 0. |
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Anisotropy
of pinning forces in NbTi
K.-P.
Jungst
Summary: For wire samples of cold
worked Nb- 40wt%Ti and Nb-50wt%Ti a strong anisotropy of the
volume pinning force F/sub p/is observed, when the angle
between transport current and external magnetic field is
varied. Good agreement with the experimental results is found,
if F/sub p/depends on f/sub p//sup 2/, where f/sub p/is the
pinning force per pinning centre, and if one takes into
account the finite dimensions of the subbands that act as
pinning centres. For the current-optimized samples, a smaller
anisotropy is seen, and for recrystallized samples the pinning
force is practically independent of the orientation of current
and field, as it was expected. |
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Critical
current density in superconducting niobium
films
R. Huebener, R. Kampwirth,
R. Martin, T. Barbee and R. Zubeck
Summary:
We have measured the critical current at 4.2 K in
Nb-strips of 1 /spl mu/m thickness and different width, w,
prepared by electron-beam vacuum-deposition. The substrate
temperature during the deposition was varied between room
temperature and 800/spl deg/C. The width of the Nb strips
ranged between 20 and 300 /spl mu/m. Whereas up to 600/spl
deg/C the different substrate temperatures yielded about the
same average critical current density, J/sub c/, (critical
current divided by sample cross section) the specimens
prepared with 800/spl deg/C substrate temperature showed a
reduction of J/sub c/by a factor of 4-6. In zero applied
magnetic field the samples prepared with 400/spl deg/C
substrate temperature or lower showed a decrease of J/sub
c/roughly proportional to w/sup -1/2/. Our critical current
data obtained in an applied perpendicular field, for the
high-field regime, were compared with Kramer's theory of flux
pinning which assumes plastic shearing of the flux-line
lattice around individual pinning sites during flux
flow. |
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Magnetic
field microprobe in superconductors carrying a
current
A. Migliori, R. Bartlett
and R. Taylor
Summary: A new technique to
probe local magnetic fields inside superconductors utilizing
the Mossbauer Effect (ME) is reported. The ME nucleus responds
to the local magnetic field resulting in a modified hyperfine
ME spectrum from which the local field and its direction may
be deduced. Concentrations of a few ppm of the radioactive
probe nucleus are sufficient; the allowable depth of the probe
depends on the attenuation of the ME /spl gamma/ ray. For/sup
57/Fe (/sup 57/Co parent) in superconductors the field acts
directly on the nuclear moment and the resulting sensitivity
is only a few kG. For a thin probe consisting of/sup 57/Co in
Pd the response is enhanced by the "giant" localized moment
providing a sensitivity at 1 K of better than 100 G. Results
of such a probe placed at various locations across a
commercial Nb/sub 3/Sn tape carrying a current are reported
and compared with calculations of the field profile using
several models. For thin tape geometries the usual models
prove to be inadequate. |
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Theoretical
and experimental study of magnetic instabilities in
multifilamentary Nb-Ti superconducting
composites
J. Duchateau and B.
Turck
Summary: Classical adiabatic stability
criteria appear to be pessimistic for multifilamentary
composites. The thermal conductivity K, the magnetic and
thermal diffusivities D/sub m/and D/sub /spl theta//, the heat
transfer coefficient h/sub k/play a leading part in the
determination of a stable composite. The differential magnetic
and thermal equations have been solved and typical stability
curves are given for either the slab model or the round
conductor. Measurements of the quenching currents, in
different magnetic, thermal and cooling conditions, for two
meter long non inductive coils are made. The results are in
good agreement with the theoretical predictions. |
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Flux-flow
voltages and faraday induction during guided entry or exit of
flux in hollow superconducting
cylinders
S. Khanna and M.
LeBlanc
Summary: The instantaneous and
integrated voltages detected between pairs of diametrically
opposite contacts attached on the periphery of superconducting
Nb tubes of different wall thicknesses were monitored as flux
threading the walls and the hole is caused to change. For a
chosen change of flux, the signals across a pair of contacts
were found to depend dramatically on the configuration of
leads to the contacts relative to the direction of flux
motion. The persistent currents induced to flow azimuthally
around the walls of the cylinder are made to collapse
partially or completely and the flux retained in or shielded
from the hole and the wall is made to exit or enter by
applying heat at a narrow strip along the length of the tube.
The numerous observations are consistent with the following
picture. Changes of flux in the hole or in the walls generate
a voltage only across the leads which embrace the region where
entry or exit of flux occurs (only changes of magnetic flux in
the measuring circuit excluding the superconducting volume
contribute). |
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Tunneling-current-induced
transitions of superconducting thin
films
A.
Hebard
Summary: The resistive state of a
superconducting film can be induced either by passing
sufficient current along the film or by raising the
temperature above T/sub c/. We have been studying this
transition in aluminum films when they are injected with "hot"
(i.e. nonthermal) phonons and electrons. This injection
efficiently breaks electron pairs in the region near the
injection point. These "hot", electrons and phonons are
produced by tunnel junctions which incorporate or are adjacent
to the Al film under study. This method of current injection
usually results in significantly lower critical currents than
the critical current measured conventionally along the film.
Well-defined and reproducible critical currents together with
I-V traces for the Al film, which under certain conditions are
characterized by a series of constant current voltage steps,
have been obtained with a variety of geometrical
configurations and different tunnel junction
resistances. |
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Effective
resistance of superconducting winding in oscillating magnetic
fields
T. Ogasawara, K.
Yasukochi, S. Nose and H. Sekizawa
Summary:
With the help of careful measurements of the terminal
voltage appearing across a noninductive winding of Nb-Ti wire
we have studied the influence of oscillating magnetic fields
on the transport current. It is possible to discuss the
voltage versus oscillating field characteristics in terms of
the critical state model, and good agreement with experimental
data has been achieved. The analysis shows that the voltage
measurement gives some detailed information on the
distribution of the magnetic induction inside the
superconductor. An effective resistance R/sub eff/appears in
the transport current loop when the amplitude of the
oscillating field exceeds a certain threshold value. The
dependence of R/sub eff/on amplitude and frequency of the
oscillating field and on the wire diameter is discussed and
the application of this analysis to the case of
superconducting solenoids is outlined briefly. |
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Manufacture
and initial technical tests of a high-power d.c. cable with
superconductors
E. Bochenek, H.
Franke and R. Wimmershoff
Summary: The cable
project to be described here in comprehensiv way owes its
basic ideas cryogenic envelope to the suggestions in I964 by
Professor Klaudy. We have extended his concept of a flexible
nic envelope by a new core concept further developed the whole
ideas in such a way that all parts of this cable manufactured
on the machines in general use in the cable industry. Aside
from all materials which are required for the construction of
the cable are standard. There are several reasons for this:
The technological risk should be kept as small as possible,
and the comprehensive experience of the cable industry, which
is reflected in the production engineering, should be utilized
as far as possible. |
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AC
critical currents of commercial Nb/sub 3/Sn
tapes
M. Garber, S. Shen, J.
Bussiere and G. Morgan
Summary: Quench
currents and critical currents have been measured for a number
of commercial Nb/sub 3/Sn tapes at 4.2 K. The measurements
were made at 60 Hz. The observed quench currents in the
as-received tapes are considerably lower than the critical
currents, which can be determined by a new technique after
stabilizing material is added. The temperature dependence over
the entire range above 4.2 K was measured for two of the
samples. The critical current falls off approximately as
1-(T/T/sub c/)/sup 2/. |
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AC
losses of transposed
superconductors
D. Eckert, G.
Enderlein and F. Lange
Summary: Eastham and
Rhodes published results of loss measurements on transposed
superconducting NbTi cables and concluded basing on an
extrapolation to very large numbers of wires that transposed
superconductors could be used favourably in cables for power
transmission. There are some reasons to question the
correctness of their extrapolation. We calculated losses for
transposed superconductors in self field and got results
different from those of Eastham and Rhodes. Loss measurements
were performed the results of which give evidence for the
correctness of our calculations. Our results lead to the
conclusion that the use of transposed cables of irreversible
type 2 superconductors for power transmission is not
advantageous. |
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Development
of a Niobium-copper-invar composite conductor for an AC
superconducting power transmission
cable
W. Beall
Jr.
Summary: A superconducting ac power cable
has been developed using Nb/Cu/Invar composite conductors and
shields in a coaxial configuration with pressurized. He at 4.7
K as the primary dielectric. Two major concerns for this cable
system were the accommodation of fault currents and the
differential thermal expansion between the conductor assembly
and the cryogenic envelope. We have shown that a Nb/Cu/Invar
composite tube is capable of accommodating the fault currents
as well as the differential thermal expansion. Studies of the
effect of surface finish on the ac losses showed that with
proper surface finish, the losses can be reduced to an
acceptable level for power cable applications. The lowest 60
Hz losses measured at a peak surface magnetic field of 0.1 T
correspond to 43 watts per 3-phase mile for a 138 kV, 3400 MVA
power cable. |
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Investigations
on potential conductors for a D. C. superconducting power
transmission line in a 20 meter test
bed
J. Hoffer
Summary:
The cryogenic and electrical aspects of a program to
evaluate superconductors for a dc superconducting power
transmission line are presented. Results for conductors in
lengths of 20 meters operated at currents up to 10 kA are
described. A battery driven power supply which is capable of
delivering and controlling currents upto 50 kA is discussed.
Specific experiments are discussed which have been designed to
probe the response of the conductors to a variety of thermal
and electrical conditions, such as: thermal gradients over the
length of the conductor, single and two phase flow of cryogen,
loss of coolant flow, operation at temperatures near T/sub c/,
a.c. ripple current, and high current ramping
rates. |
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The
origination of the normal zone in a superconducting cable with
partial cryostatic
stabilization
Ye Blinkov, M.
Kuno, S. Smirnov, N. Bendik and Ye.
Goldenberg
Summary: This paper deals with the
origination and variation in length of the normal zone in a
superconducting cable. A convenient approximate formula for
the propagation velocity of a semi-infinite zone was obtained
assuming a linear decrease of critical current with increasing
temperature; the peculiarities of the normal zone's growth
were studied in a cable with an initial gradient of gas
temperature; the process of expansion and contraction of a
normal zone of finite length is described. Data are given on
the experimental study of the movement of a normal zone in the
three-phase superconducting cable model. |
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Progress
at brookhaven in the design of helium-cooled power
transmission systems
E.
Forsyth
Summary: Following an extensive study
of cryogenic power transmission systems under development in
this country and abroad it was concluded at Brookhaven that a
flexible superconducting ac cable appeared to be the best
choice for an economical high-capacity underground system with
superior technical characteristics. A form of niobium-tin has
been developed for this application with an intrinsic low-loss
performance at 60 Hz. In addition, it has been determined that
the ac characteristics of many commercial forms of Nb/sub 3/Sn
are degraded by additives, these effects are described. A
flexible cable will require a lapped plastic insulation which
must possess necessary properties both at room temperature and
at the operating temperature, these are listed together with a
summary of test results. Several designs for the cryogenic
envelope to contain the cable are under development, the
design options and trade-offs are discussed. Using performance
data based on present knowledge of helium-cooled cables,
systems were designed for an actual utility company
transmission corridor 43 miles long rated at 4800 MVA. These
designs are costed and compared to conventional transmission
facilities. |
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Evaluation
of layered Nb/sub 3/Sn
conductor
R. Schwall, R. Howard
and R. Zubeck
Summary: In the past several
years, considerable progress has been made in developing a low
loss superconductor for use in a 60 Hz ac power transmission
line. In particular, work by Snowden, et al.[1] has indicated
the possibility of exploiting conductor configurations
attainable by thin film evaporation techniques to achieve
critical currents and ac loss characteristics superior to that
of bulk materials. In this paper, we describe instrumentation
which has been designed to test conductors fabricated by the
evaporation technique and present some of our first results on
these materials. |
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Analytical
study of current-sharing at power frequency between a type II
superconducting composite with a normal metal
backing
G.
Morgan
Summary: Current sharing in a
two-layer composite is studied analytically by numerically
solving the 1-dimensional diffusion equation in the normal
metal and the phenomenological critical state model with field
independent critical current density J/sub c/(T) in the
superconductor, with boundary conditions appropriate to a
transmission line at each surface and the interface. It is
found that with a sufficiently low resistivity (/spl rho/)
backing, and reasonable assumptions as to heat transfer, an
appreciable fraction of a current density of several thousand
amps per cm can be carried in the backing with the balance in
a superconductor with high T/sub c/such as Nb/sub 3/Sn. The
dependence of the dissipation on such parameters as thickness,
J/sub c/, /spl rho/ and frequency at constant temperature is
given, and typical loss voltage waveforms
presented. |
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Critical
current densities in Nb/sub 3/Ge between 14 and 21
K
R. Bartlett, H. Laquer and R.
Taylor
Summary: Self field critical current
densities of thick Nb/sub 3/Ge films have been measured in the
liquid hydrogen temperature range 13.8 - 21 K. Bulk critical
current densities J/sub c/above 1.6x10/sup 6/A/cm/sup 2/were
observed at 14 K falling linearly at higher temperatures. The
behavior extrapolated to zero critical current gave a critical
temperature T/sub c//spl sim/ 19.2 K which is /spl sim/1.5 K
lower than the onset T/sub c/determined from the a.c. magnetic
susceptibility. The samples were long sections of copper
tubes, which had been coated on the inside with Nb/sub 3/Ge by
a chemical vapor deposition process. J/sub c/for various high
T/sub c/materials are compared including calculations of
adiabatic stability. Nb/sub 3/Ge holds promise for a dc
superconducting power transmission line operating near 14
K. |
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A
rapidly tunable superconducting
resonator
J. Delayen, H. Yen, G.
Dick, K. Shepard and J. Mercereau
Summary:
Superconducting, high Q rf resonators which are
continuously tunable over at least a factor of three in
frequency have been designed for operation in the range of a
few hundred megahertz. Prototypes of these resonators have
been fabricated of OFHC copper and electroplated with lead. Rf
decay times of 18 msec at 240 MHz have been achieved in these
resonators; the rf losses which dominate the Q are found to
take place in a thin dielectric layer on the superconducting
lead surface. This loss mechanism decreases the decay time
about a factor of 10 from the decay time expected from the
measured rf resistance of the Pb surfaces. Preliminary work on
rapid (less than 1 msec full range) tuning of high Q
resonators, which may lead to a new technique of
electromechanical energy conversion, is
reported. |
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Superconducting
resonator for high frequency - high power
applications
L. Weinman, H.
Murray and R. Vardiman
Summary:
Superconducting helical resonators operating in the HF
band at approximately 16 MHz have been designed and
constructed to demonstrate the feasibility of passing a
kilowatt of power at an operation Q of 1000 (Q/sub DL/= 1000).
Superconducting components are being considered to replace
normal state circuits in communication systems, i.e. antennas
and multicouplers. (A multicoupler is a device using coupled
resonators which permits a number of transmitters to be
simultaneously operated into one antenna.) The niobium helical
resonators were polished and anodized after being fabricated
using an electron-beam welding process. Teflon support
structures were designed to minimize vibrational problems. The
resonator was operated open to the helium bath and the
resulting unloaded Q was in excess of 4x10/sup 6/. A kilowatt
of power was transferred through the resonator when operating
at a Q/sub DL/of 1000. Work is now proceeding on the design
and fabrication of a tunable resonator. |
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A
high-performance Nb helical
cavity
R. Benaroya, B. Clifft, K.
Johnson, P. Markovich and W. Wesolowski
Summary:
A 92-MHz superconducting-Nb helix resonator of
exceptional quality has been thoroughly tested under a variety
of conditions. The unit is a full-scale /spl lambda//2
structure with dimensions appropriate for heavy-ion
acceleration. When operated at a temperature of 1.8K and with
bare (not anodized) Nb surfaces, the low-field Q is 9.4x10/sup
9/, equivalent to a surface resistance of 5x10/sup -10/ohms.
The maximum surface magnetic field is 1200 G and the maximum
surface electric field is 37 MV/m, which corresponds to a
traveling-wave axial accelerating field of 4.6 MV/m. These
characteristics set new performance standards for helix
resonators. A systematic study of the effects of various
surface treatments, including abuses of the cavity, are
described. The tests consist of 24 liquid helium cooldowns, at
4.2K and 1.8K, of the cavity with bare and anodized Nb
surfaces which at various times were electropolished,
oxypolished and heat treated. RF and helium conditioning are
discussed as techniques to get through multipactoring barriers
and extend the maximum obtainable electric
field. |
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On
the role of electrons in RF
breakdown
C. Lyneis, P. Kneisel,
O. Stoltz and J. Halbritter
Summary:
Measurements on two types of S-band single-cell
cavities are reported. In these investigations our interest
has been directed toward questions concerning the location and
mechanisms of magnetic field breakdown in these cavities. The
results show that in TM modes the surface electric fields are
an important factor in the initiation of rf breakdown. Among
the mechanisms for rf breakdown are: localized heating
initiated by multipacting electrons, deterioration of the
superconducting properties through the impact of electrons,
and inherent inhomogeneities in the surface. In order to
reduce the electron loading and subsequent field limitation,
careful treatments have been applied to the surface. In doing
this fields of 81 mT in an electron free mode and 40 mT and 25
MV/m in TM modes were attained in one type of cavity. In the
other type cavity 67 mT and 38 MV/m have been
attained. |
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Superconducting
Nb/sub 3/Sn-cavities
B.
Hillenbrand, H. Martens, H. Pfister, K. Schnitzke and G.
Ziegler
Summary: Microwave critical magnetic
flux densities B/sub c//sup ac/up to 160 mT and higher than
B/sub c1/were obtained with niobium-cavities by special
surface treatments. Breakdown in this flux density region
occurs by a thermal breakdown mechanism in which no
superconducting critical flux density is involved.
Considerable increase in B/sub c//sup ac/may be expected with
high-T/sub c/-superconductors such as Nb/sub 3/Sn. Nb/sub
3/Sn-layers were produced on niobium structures by exposing
them to a saturated Sn-vapour at 1000/spl deg/C. B/sub c//sup
ac/-values up to 40 mT and surface resistances smaller than
10/sup -6//spl Omega/ were measured with TE/sub 011/-cavities
at 9.5 GHz. |
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Effect
of surface metallurgy on the penetration depth and RF
breakdown field of superconducting
niobium
C. Varmazis, T. Luhman,
A. Joshi, O. Kammerer, S. Giordano and M.
Strongin
Summary: The surface metallurgy of
niobium will be discussed including factors such as impurity
distributions at the surface and surface topography.
Measurements of the penetration depth are presented both in
the clean limit, where there is about a 20/spl Aring/ region
of oxygen at the surface, after high temperature heat
treatment, which affects the boundary condition on the GL
order parameter; and in the dirty limit where heating in an
oxygen atmosphere near 10/sup -6/torr can greatly affect the
surface roughness. Auger measurements of the surface
conditions in a 1% Zr in Nb alloy are also presented. A
discussion is given of preliminary results in a special
apparatus where rf breakdown experiments can be performed on
small samples and correlated with the metallurgical factors
mentioned above. |
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On
RF residual losses and phonon
generation
J.
Halbritter
Summary: One restriction against
applying superconducting rf cavities is the achievable rf
residual surface resistance R/sub res/, especially after
surface damage. A theory is presented, where by interfacial
quantum mechanic exchange between conduction band and
localized states, momentum of the shielding current is
transfered to the traps. For rf shielding currents this
oscillating force on the localized states coherently generates
transverse phonons. This transfer of momentum, due to the
overlap of band and trap states, corresponds to an rf residual
resistance which, for the first time, is able to describe
consistently the observed R/sub res/. In addition, the
presented theory explains microscopically the observed
rf-phonon-coupling. |
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The
surface impedance of type II superconductors in the mixed
state
A. Petrovich and R.
Rose
Summary: Using a quarter wave coaxial
resonator containing the superconducting sample as the center
element, the resistive and reactive components of the surface
impedance have been measured on niobium over the entire static
H-field range of the mixed state at frequencies between 70 MHz
and 930 MHz. The samples included one in the plastically
deformed condition and one which had been annealed at 1800/spl
deg/C for 2 hours in a vacuum of 7x10/sup -8/torr; both had
about the same resistivity ratio (/spl sim/14). The two
samples differed considerably in the ratio of the resistive to
reactive component of the surface impedance. The phenomena is
believed to be a consequence of the nature of the electronic
excitations characterizing the fluxiod cores. |
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Microwave
surface resistance of a superconducting Mo-Re
alloy
J. Yasaitis and R.
Rose
Summary: By using a Mo/sub 0.75/Re/sub
0.25/alloy endplate on a Nb microwave cavity (TE/sub 011/mode,
resonant at 11.2 GHZ), an upper bound for the residual surface
resistance of this alloy was shown to be 2.8 /spl mu/-ohm, a
lower bound for the breakdown field was shown to be 102 G. In
view of the fact that these properties were determined at 11.2
GHZ on the first Mo-Re endplate made with no particular effort
devoted to optimization of surface quality, this material is
deserving of further consideration for practical microwave
applications. |
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Investigation
of microwave properties of superconducting Nb/sub 0.4/Ti/sub
0.6/
S. Giordano, H. Hahn, H.
Halama, T. Luhman and W. Bauer
Summary: A
superconducting Nb/sub 0.4/Ti/sub 0.6/test cavity was
constructed to measure the surface losses as function of
frequency in the range from 2 to 8 GHz and as a function of
temperature below 4.2 K. The temperature-dependent part of the
surface resistance agrees essentially with the BCS theory if
the material parameters T/sub c/= 9.8 K, /spl utri/(0) = 1.73
k/sub B/T/sub c/, /spl xi//sub o/= 380 /spl Aring/, /spl
lambda//sub L/= 310 /spl Aring/, and /spl ell/ = 4 to 8 /spl
Aring/ are taken. The magnitude and frequency dependence of
the temperature-independent residual resistance observed in
the niobium titanium cavity suggests no difference in the
behavior of type I and II superconductors. The highest Q/sub
o/= 2.9 x 10/sup 9/was measured in the TE/sub 011/mode at 3.9
GHz. A coaxial cavity with removable sample was constructed
for an in-depth study of rf breakdown and its relationship
with metallurgical parameters as characterized by ac loss,
magnetization, and penetration depth measurements. Peak fields
in solid type-II superconducting cavities are limited by their
low thermal conductivity suggesting the use of films, 0.01 to
0.1 mm thick, on pure niobium or copper. The highest magnetic
field of 120 G was measured on a recrystallized sample
suggesting that the lower critical field H/sub c1/(for this
sample 75 G) does not prevent the application of high-T/sub
c/type-II superconductors to microwave devices. |
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A
superconducting microwave
engine
G. Dick
Summary:
In this paper a new technique of electromechanical
energy conversion is proposed. This technique would make use
of the high Q's attainable in superconducting resonators to
achieve direct mechanical to microwave energy conversion with
a net efficiency greater than 90% a value which is far higher
than that obtained by conventional techniques. In addition, if
surface magnetic and electric field levels are limited by the
critical fields obtained in fixed resonators, power densities
would be achieved which are large enough to make such a
machine a practical means both for generating microwave energy
and for re-converting the microwave again to mechanical energy
for power transmission purposes. |
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Superconducting
cyclotrons
H. Schneider, J.
Fraser and C. Bigham
Summary: The use of high
field superconducting d.c. magnets in cyclotron design makes
possible accelerators that are small and consequently have a
significant cost advantage over comparable iron-magnet
cyclotrons. A general description of superconducting
isochronous cyclotrons is given, with emphasis on the magnet
design requirements. Saturated iron poles to provide flutter
focussing are described. A conceptual design for a heavy ion
energy booster cyclotron for a tandem Van de Graaff is
described, similar cyclotrons for production of proton or
deuteron primary beams and /spl pi/ mesons or neutron
secondary beams can, in view of the low cost, be considered
for dedicated applications such as activation analysis or
cancer therapy. |
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ESCAR-First
superconducting synchrotron, storage
ring
T. Elioff, W. Gilbert, G.
Lamberston and R. Meuser
Summary: The ESCAR
(Experimental Superconducting Accelerator Ring) project is an
accelerator technology development for which the primary goal
is to obtain design and operational experience with a complete
accelerator and storage ring system utilizing superconducting
magnets. Full scale design effort began in July 1974, and
operation is planned in 1977. |
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Results
of magnet prototype evaluation for the fermilab energy doubler
project
B. Strauss, D. Sutter, E.
Ioriatti and W. Habrylewicz
Summary: The
Fermilab Energy Doubler project will require some 744 dipoles
6 meters long and 240 quadrupoles of varying lengths from 1.5
meters to 2 meters. Since earliest conception the Energy
Doubler has been envisaged as an adjunct accelerator located
in the same tunnel as the present Main Ring and capable of
reaching a final proton energy of /spl sim/1000 GeV. To do
this using a lattice similar to the Main Ring's requires a
bending field of 45 kilogauss and, consequently,
superconducting magnets.[1] The magnet fabrication and testing
program was initiated in September 1972, operated the first
test magnets in January 1973, settled on a shell type geometry
by June 1973, tested and evaluated a matched set of dipoles by
November 1973 and operated the first 6 meter, or "20 ft",
prototype dipole by March 1974. The 20 ft dipole did not
perform satisfactorily, reaching less than 50% of design
current and exhibiting excessive training. Consequently, a
redirection of the program channeled further efforts into a
2.5 ft model program to identify and correct the sources of
difficulty and to enable resumption of 20 ft prototype
construction. The first phase of the 2.5 ft program has
included the construction and testing of 12 magnets and is
essentially complete. It has led to the promise of an improved
wire and a slightly more conservative magnet design that is
now being used in the construction of 2 1/2 and 10 ft models.
Events in an intensive development program do not proceed in
logical sequence. While the 2.5 ft model program has been in
active progress, two additional 20 ft prototype dipoles of the
original design and one 7 ft quadrupole have been completed.
One of these, 20 ft dipole #2, has been successfully operated
in the forced flow liquid helium pump loop.[2] A 7 ft warm
iron quadrupole has also been tested successfully and would be
adequate in present form for use in the Doubler
project. |
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Magnets
for the ZGS superconducting stretcher
ring
J. Purcell, S. Wang, R.
Niemann, K. Mataya, H. Ludwig and J. Biggs
Summary:
Dipoles and quadrupoles have been developed for use in
a 12.5 GeV storage ring at the ZGS. The winding bore of these
magnets is 10.16 cm with a useful aperture of 7.6 cm. The
magnets produce a high quality field (/spl plusmn/0.1%
nonuniformity including the ends) that requires no correction
at any field level. An operating current of 190 A at the
design field of 30 kG allows the use of a very low heat leak
cryostat design. A 3 meter length of magnets is expected to
require only 2 liters helium per hour. Construction of 5
prototype modules, consisting of 2 dipoles and 2 quadrupole
each, complete with cryostats, is almost
completed. |
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Use
of superconducting self-correcting harmonic coils for pulsed
superconducting dipole or multipole
magnets
A. Dael, F. Kircher and
J. Perot
Summary: Thanks to the zero
resistance of a superconducting wire, an induced current in a
closed superconducting circuit is continuously exactly opposed
to its cause. This phenomenon has been applied to the
correction of the field harmonics of a pulsed magnet by
putting short-circuited superconducting coils of particular
symmetry in the useful aperture of the magnet. After a recall
of the choices to be done for the main characteristics of such
devices [1], the construction of two correcting coils
(quadrupole and sextupole) is described. Experimental results
about magnetic efficiency and time behaviour are given; they
are quite encouraging since the field harmonicshave been
reduced by one or two orders of magnitude. |
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High
field superconducting magnets for accelerators and particle
beams
J. Allinger, G. Danby and
J. Jackson
Summary: Experience in designing
precision superconducting magnets for fields up to 60kG will
be described. Realizable construction tolerances and their
impact on field accuracy is discussed. For dipole fields up to
60kG or more, rectangular coil window frame type magnets will
be compared with circular or elliptical coil designs. In all
cases, the same superconductor current density versus maximum
field performance is assumed. The comparison will include
field quality and correction required as a function of
aperture size, stored energy, ampere turns required, and
overall magnet size. In quadrupole design the impact of the
allowed superconductor current density being roughly inversely
proportional to peak field is severe. For gradients up to one
Tesla/cm or greater, similar comparisons for different types
of quadrupole construction is made. |
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Operating
experience with a superconducting magnet in a primary proton
beam
J. Allinger, G. Danby, B.
DeVito, S. Hsieh, J. Jackson and A. Prodell
Summary:
Two superconducting dipole magnet modules have been
operated successfully at BNL since October 1973 to bend 30
GeV/c protons from the AGS by 8/spl deg/ in the new North Area
beam to the 7-ft. Bubble Chamber. The magnets have been
energized for over 1000 hours, and, of that time, have been
traversed by the proton beam with beam intensities up to
5x10/sup 12/protons per pulse, for about 750 hours. Operation
of these series connected magnet units, each 2 meters long at
35kG has been routine even when they were intermittently
subjected to beam heating of several hundred joules per pulse
due to other operational beam difficulties. Experiments were
also performed where copper blocks were inserted 2 meters
upstream, resulting in very large interaction rates in the
beam tube inside the units. A magnet quench occurred with
target thicknesses such that energy approaching 1kJ per pulse
was being absorbed in the system. These are the first
superconducting magnets in operation in a primary beam on
which extensive radiation heating tests have been
performed. |
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Engineering
design & analysis of large superconducting particle
analysis magnets
J. Heim and R.
Fast
Summary: Superconducting magnet
technology is one phase of applied superconductivity where
significant electrical power savings may be appreciated.
Furthermore, these power savings maybe gained without
sacrificing reliability of operation or initial capital costs.
This paper describes the design and construction of 4 large
superconducting dipole magnets which are being used at Fermi
National Accelerator Laboratory to conduct high energy physics
experiments. Two of these magnets have been built and operated
continuously for several months prior to installation in beam
lines. Two larger superconducting dipoles are presently under
construction and both magnets will be completed this year. All
magnets are designed to operate continuously without special
attention consuming approximately 10% of the power which would
be demanded by a conventional magnet. The engineering concept
and construction techniques are described. |
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Superconductive
energy storage for large
systems
R. Boom, B. Haimson, G.
McIntosh, H. Peterson and W. Young
Summary: A
summary report of a three-year study of superconductive energy
storage for large utility systems is presented. The preferred
conceptual design choices include: large, thin-walled
solenoids, 1. 8 K cooling, TiNb in aluminum composite
conductors at 5 tesla, bedrock structural support for both
axial and radial forces, and a three-phase Graetz (ac/dc)
bridge converter interface to the power grid. Preliminary
estimates show that capital costs are given by $/kW = 40 + 125
(P/1000)/sup -1/3/(t/2)/sup 2/3/Where P is the average power
in MW and t is the discharge (peaking) time in hours. A
typical operating loss is 10 /spl sim/ 15% of the stored
energy. The concept is technically feasible requiring only
present day technology. |
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Will
superconducting magnetic energy storage be used on electric
utility systems?
W.
Hassenzahl
Summary: As the cost of fossil
fuel has increased and the load factors on electric utilities
have decreased, the need for efficient, reliable energy
storage systems has increased. Although pumped hydro storage
is now used extensively on those utility systems having the
appropriate resources nearby, it is only 65% efficient.
Superconducting magnetic energy storage which promises to be
more than 90% efficient and easily sited may become a
competitive energy storage technology. A comparison of the
various energy storage systems is presented in terms of
performance on electric power systems, and cost. Emphasis is
given to the various technologies involved in the development
of large superconducting magnets. A brief review of the Los
Alamos Scientific Laboratory program on superconducting
magnetic energy storage is included. |
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The
Fermilab cryogenic energy storage
system
F.
Mills
Summary: A superconductive magnetic
energy storage system has been designed to smooth the
pulsating power requirement of the Fermilab accelerator. The
system stores 1 MWh and operates at peak power level of 200 MW
on a 10 sec cycle. The magnetic system employs cryogenically
stable superconductors and a flux-forcing normal secondary.
The power system uses phase controlled thyristor
rectifier-inverters similar to those employed in the main
accelerator power supply. |
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Inductive
shielding for pulsed energy storage
magnets
R. Moses and J.
Ballou
Summary: The design of a shield
winding to be placed around a superconductive magnet is
discussed. By operating the shield and main winding in
parallel, it is possible to transfer energy to and from the
magnet system without exposing the superconductive coil to a
changing field, thereby eliminating ac losses in the
superconductor and stabilizer. Such units have applications
for CTR devices and for load leveling the power demand of the
main ring at the Fermi National Accelerator
Laboratory. |
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External
field reduction of superconducting energy storage
solenoids
J. Ballou and R. Moses
Jr.
Summary: The external multipole field of
a superconducting solenoid is evaluated for large units buried
in bedrock. Methods to partially cancel these fields are
discussed. It is shown that external magnetic field
"pollution" can be reduced at modest system
expense. |
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Cellular
concrete - A potential load-bearing insulation for cryogenic
applications?
T. Richard, J.
Dobogai, T. Gerhardt and W. Young
Summary:
The need for low cost, low thermal conductivity, high
strength insulation suitable for cryogenic applications is
becoming more evident. An investigation of the potential of
cellular concretes to fulfill this function has been
initiated. A review of the thermal and mechanical
characteristics of foamed plastics and cellular concrete is
presented along with relative cost comparisons. Test data from
preliminary investigations is presented to define the
influence of material constituents, density and temperature on
the mechanical and thermal response of cellular concrete.
Specimen densities range from 0.64 to 1.44 gr/cc. The
influence of temperature variations from 22/spl deg/C to
-196/spl deg/C is reported for selected
densities. |
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10
kA, 300 kJ magnetic energy transfer and storage (METS) test
facility
C. Swannack, D. Blevins,
C. Harder, J. Lindsay, J. Rogers and D.
Weldon
Summary: The theta-pinch fusion test
reactor proposed within the controlled thermonuclear research
program at the Los Alamos Scientific Laboratory will demand
that approximately 250 MJ be delivered into the plasma
compression coil in one millisecond at a repetition rate of
four per hour. Multiple modules of superconducting inductive
storage will supply this pulsed energy. A Magnetic Energy
Transfer and Storage (METS) facility has been constructed to
test low-loss superconducting inductive energy storage
modules, superconducting switches, and developmental high
voltage circuit breakers. The storage module has been tested
to 12.5 kA with 386 kJ stored energy. |
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A
300 kJ pulsed superconducting energy storage
coil
W.
Punchard
Summary: Magnetic Corporation of
America has designed and is presently building a 300 kJ
superconducting pulsed energy storage coil. The coil is a
solenoid and will operate at a central flux density of 2.0
Wb/m/sup 2/and a current of 10 kA. The critical current as
measured by extrapolation along the peak field load line is 15
kA. The structure which is completely non-conducting has been
designed mechanically so that the coil may be operated at 12
KA. The windings are partially potted and are cooled on one
face of the braid. It has been designed to remain
superconducting throughout the 10 s charge period and the 2 ms
discharge period. The repetition period is 30 s. |
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Program
for development of toroidal superconducting magnets for fusion
research
H. Long, R. Brown, R.
Derby, R. Kernohan, M. Lubell, J. Luton and P.
Walstrom
Summary: Toroidal fusion devices
will require superconducting magnet systems of greater size
and complexity than heretofore constructed. A program
employing the talents of a number of organizations for the
development of superconducting magnets for toroidal fusion
devices has been inaugurated. The objective of this program is
to demonstrate the suitability and reliability of toroidal
superconducting magnets for fusion research devices and
experimental power reactors (EPR) in a time compatible with
the AEC Controlled Thermonuclear Research (CTR) program goal
for EPR construction during early 1980's. Since much of the
superconductivity expertise resides now in AEC laboratories,
the focus of the program is in these laboratories, but
industrial and consultative participation is an integral part
of the plan. |
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25
tesla hybrid magnet
P.
Cheremnykh, G. Churakov, B. Rozhdestvenski, B. Samoilov and N.
Chernoplekov
Summary: In 1966, E.G.Komar and
D.B.Montgomery put forward independantly an idea of a hybrid
solenoid. A hybrid solenoid generating a stationary field up
to about 25T in a 28mm working bore has been designed and
built jointly by the Kurchatov Atomic Energy Institute and by
the Efremov Scientific Research Institute of Electrophysical
Apparatus. The inner section of the hybrid sistem represents a
conventional water-cooled coil , while the outer coil is made
of a superconductor. The results of an extensive series of
tests on the hybrid magnet together with its power supply and
protection system indicate a close agreement between
experimentally observed and theoretically calculated main
parameters and demonstrate a stable and reliable operation of
the installation. |
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Mechanical
forces in superconducting windings of toroidal magnet
systems
W. Gauster and W.
Stoddart
Summary: A new computer code is
briefly described which can be advantageously applied to
superconducting toroidal systems with complicated coil shapes.
The guiding idea is to use the same geometry for the current
elements and the stress model. Furthermore, a short review
shows that more work is necessary for elucidating the
influence of mechanical stress on the critical current density
of the superconductors and on the physical properties of the
non-superconducting stabilizing material. Values for the
centripetal force acting on each torus coil can be easily
calculated when the "1/r approximation" for the magnetic flux
density is assumed. The influence of the magnetic flux leaking
out between the coils is discussed and it is shown that for
tori with large numbers of coils (say, 32 to 64) and with not
too large aspect ratios (say, 1.5 to 2) the above mentioned
approximation for the calculation of the centripetal forces
yields values which are only a few percent too large. Finally,
the consequence of a coil failure in a toroidal magnet is
discussed under the assumption that each superconducting coil
operates in persistent mode. Due to the re-arrangement of the
currents and of the magnet fields, centripetal forces and
perpendicular force components result whose asymptotically
reached values are appreciably larger than the static values
calculated for a torus with one coil missing. |
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Test
results of Nb/sub 3/Sn ribbons for the Princeton D coil test
program
J. Kaugerts, J. File and
J. Willard
Summary: A previously described D
coil test program has been modified. Details of a smaller
Nb/sub 3/Sn D coil test program are described. Cusp coil tests
have been made with several Nb/sub 3/Sn composite ribbons.
Measurements of both the quench and recovery currents as a
function of magnetic field component perpendicular to the wide
edge of the ribbon are presented. |
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Diamagnetic
forces in superconducting
magnets
R. Stevenson and D.
Atherton
Summary: We have previously reported
damage to two large superconducting solenoids caused by axial
expansion forces and radial compression forces. These forces,
which can be greater than and are of opposite sense to the
classical Lorentz forces, we have attributed to the
diamagnetism of the composite superconductors. Some
irregularities experienced in the operation of force cooled
superconducting magnet systems appear to be readily understood
by considering diamagnetic forces. It seems likely that
several other previously unexplained occurences in the
operation of large superconducting magnets will also
eventually be attributed to this effect. We describe how
diamagnetic properties of superconducting strips produce
repulsive forces on opposite edges of the strips. At the end
pancake and innermost layer of a solenoid, these forces are
seriously unbalanced and may override the Lorentz forces. We
discuss how the magnitude of these forces may be estimated and
how their effects may be minimized. We suggest that the stress
analysis of large superconducting magnets currently in
operation or in design be reviewed to consider the relevance
of diamagnetic forces to their safe operation. |
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The
control of 'training' in 'racetrack' shaped superconducting
magnets
V. Edwards, C. Scott and
M. Wilson
Summary: This paper describes a
series of experiments on small straight-sided ('racetrack')
magnets designed to investigate the influence of the force
support structure on training. In windings typical of those
used in large magnets we have demonstrated that training can
be eliminated by correct clamping of the windings and that it
can be re-introduced by poor clamping. This behaviour has been
found for several quite different types of winding
impregnation and has also been confirmed at temperatures down
to 2.3 K. |
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Low
temperature properties of potting and structural materials for
superconducting magnets
G.
Hartwig
Summary: The embedding of
superconductors is one component determining the behavior,
such as training charcteristics or degradation, of
superconducting coil assemblies. Mechanical, thermomechanical
and electrical properties of potting or structural materials
are discussed. A review of low temperature measurements of
parameters, such as elastic moduli, work of fracture, thermal
contraction and conductivity, specific heat and breakdown
voltage on unfilled, filled or reinforced epoxy resins is
given. |
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Composite
superconductor design for large
magnets
R. Hay and J.
Tarrh
Summary: The correct specification of
superconductor characteristics is of fundamental importance to
the performance and optimization of a magnet. This paper
presents design criteria for NbTi multifilamentary composite
superconductors for large magnets. Expressions are developed
relating conductor configuration to current density, magnetic
field, superconductor area, temperature, thermal stability
criteria, and magnet protection. Optimization of these
conductor characteristics in a magnet includes consideration
of current density regionalization, or grading, to minimize
the required amount of superconductor without impairing
performance. Standardization of conductors is
proposed. |
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Flux
diffusion losses in stabilized
conductors
M. Hilal and R.
Boom
Summary: The "ac" losses in normal metal
sheaths around superconductor composites are calculated. Three
cases are considered: (1) the composite region is assumed to
have a finite constant resistance, (2) flux lines are assumed
to move from the normal metal into the superconductor at
constant velocity, and (3) complete equations of motion and
continuity of flux are used in both the normal and
superconductor region. Case (1) is incorrect since it predicts
a decrease In losses for an increase in rate of change of
applied magnetic field, in contradiction to case (3). Case (3)
is, of course, complete and accurate but complex enough to
require numerical solutions. Case (2) however is only in error
by a few percent for charging rates in the order of one
tesla/h. Case (3) can be applied for any charging rate as long
as the superconductor filaments are transposed, not just
twisted. |
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Multipole
magnetic field trapping by
superconductors
M.
Rabinowitz
Summary: Multipole magnetic fields
up to 17,600 Oe were trapped transversely to the axes of
solid, hollow, and split hollow superconducting cylinders.
Dipole, quadrupole, and sextupole magnetic fields were
permanently stored with high fidelity to the original fields
using Pb, Nb, and Nb/sub 3/Sn. The advantages of this
technique are mahifold: 1. Only one pattern magnet has to be
made, whose field, regardless of configuration, may then be
inexpensively replicated ad infinitum. 2. There is a saving of
space, weight, and material. 3. There is a saving of
electrical power, as the only power loss is due to heat
conduction. 4. The stored magnetic field can remain extremely
stable over long periods of time. 5. The magnetic field can
persist for long durations even during power curtailments or
shutdowns. This paper will explore the advantages and
limitations of this technique, experimental results, geometry
effects, and possible applications. |
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Superconductivity
to the rescue
R. McCracken, D.
Richied and T. Toohig
Summary: There are many
large, useful electromagnets in service in Laboratories
thru-out the world that are faced with common problems;
incipient failure due to age, ever present desire for higher
fields and staggering escalation of power costs. Conventional
rewind is generally not feasible due to magnet configuration
and continuing high operating costs. This paper presents a
unique engineering approach for the modification of existing
exciting coils to operate in a superconducting mode using
proven state of the art techniques and considerable cost
savings over the fabrication of new superconducting coil
packages using more conventional coil packaging. Typical cost
analyses show the attractive use of such
proposals. |
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Multifilamentary
niobium tin solenoids
D.
Larbalestier, V. Edwards, J. Lee, C. Scott and M.
Wilson
Summary: This paper describes the
construction and performance of several small solenoids wound
from filamentary composite wires produced by A.E.R.E.,
Harwell, U.K. The Nb/sub 3/Sn is formed in the wire after
winding by a high temperature treatment. The maximum field
obtained was 12.2 Tesla, produced by placing a 9.9 Tesla
Nb/sub 3/Sn solenoid in the backing field of a 6.5 Tesla Nb-Ti
solenoid. The working current densities are substantially
greater than those obtained with Nb-Ti. |
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Superconducting
Nb/sub 3/Sn solenoids operating at 15 - 16.5
T
W. Fietz, E. Mains, P. Swartz,
E. Knopf, W. Markiewicz and C. Rosner
Summary:
Present state of the art in tape wound Nb/sub 3/Sn
solenoids is the generation of magnetic fields above 15T in
clear bore diameters up to 65 mm. Details of the construction
and operation of magnets of this type, especially the first
one which achieved 15.8T in a 26 mm bore at 4.2K are given.
The operation of this solenoid at reduced temperatures in a
pumped helium bath is described. The solenoid was operated at
various temperatures to below 3K where a maximum field of
16.5T was observed, limited only by mechanical strength. The
prospect of achieving even higher fields at 4.2K and lower
temperatures is discussed in terms of the properties of Nb/sub
3/Sn and practical design considerations. |
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The
design of a 2T superconducting solenoid for magnetic catheter
guidance
J. Hale, N. Pierce and
M. Hoenig
Summary: For several years, members
of this Laboratory, in cooperation with the Neurosurgical
Service at Massachusetts General Hospital, have been engaged
in a research program directed toward the perfection and use
of a magnetically guided catheter system for use in the
cerebrovascular system. Until recently, this work was carried
out using a conventional iron-core, water-cooled
electromagnet. This paper describes the design, construction,
and operating parameters of a superconducting magnet system
for medical use. This design was carried out with the idea
foremost in mind of producing a neurosurgical tool that was
easy to use in a hospital environment. This required the use
of a vacuum insulated liquid helium dewar that could operate
in any orientation while helium transfer was taking place, and
a flexible, vacuum insulated helium transfer line. The system
is completely self-contained, the only external connections
being to the dc power source for magnet excitation, and 110
vac for housekeeping functions. |
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Cooling
of a system of superconducting magnets by means of pumped
subcooled liquid helium
P.V.
Arend, S. Stoy and D. Richied
Summary: In the
course of developing the refrigeration system for the Fermi
National Accelerator Laboratory energy doubler, a new concept
was developed for the cooling of long, superconducting magnet
systems. The concept depends on: a) the use of pumps for the
transport of liquid helium and refrigeration, and b) heat
transfer between a stream of boiling helium and one of
subcooled liquid helium for maintenance of a constant
temperature of the fluid surrounding the magnet windings. In
order to verify the validity of the concept an experimental
program at Fermilab was started. This paper describes the
experimental installation and some of the test results
obtained during a number of test runs. |
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Dense
supercritical-helium cooled superconductors for large high
field stabilized magnets
M.
Hoenig and D. Montgomery
Summary: We have
developed an analytical model of a dense, supercritical
helium-cooled superconductor, stabilized to withstand
short-term instabilities. The model allows for the absorption
of energy from transient instabilities and Joule heating
during current sharing. Coolant heat transfer parameters are
optimized with respect to geometry and superconductor critical
temperatures. Data is presented to demonstrate the advantages
of a novel extended surface conductor bundle. This work has
been directed at large, high-field magnets in the 5 to 9 Tesla
range. Conceptual designs of a toroidal "Tokamak" coil of 6
meter minor diameter and a 10 m. long X 2.5 m. diameter
saddle-wound "MHD" magnet are presented. |
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Flow
instabilities in gas-cooled cryogenic current
leads
P. Thullen, R. Stecher and
A. Bejan
Summary: Gas-cooled cryogenic
electric current leads which are operating at high ratios of
current to mass flow are commonly used to supply electric
current to superconducting magnets. Loss of coolant flow under
such operating conditions will result in destruction of the
lead and possible damage to the magnet itself. A flow
instability, which results in loss of coolant flow, can occur
in current leads due to the dependence of the kinematic
viscosity of the coolant on the local temperature. This paper
discusses the physical basis of the instability and presents
an analysis which is used to describe the operation of current
leads used in the EPRI-MIT 2000 kVA superconducting
alternator. Details of the lead design and test results are
included. |
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A
thermally switched flux pump
G.
Homer, P. Houzego, C. Scott and M. Wilson
Summary:
This paper describes a superconducting power supply
(flux pump) of the transformer rectifier type in which the
rectification is produced by thermal switching. The supply is
intended for use with a superconducting magnet. At an
operating current of 1000A the pump can charge a magnet at
about 20 W with an efficiency greater than 93%. In separate
battery tests the pump has developed 38 W at a current of
1500A. Design of an improved 1500A, 500 W supply is in
hand. |
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A
slow cycling flux pump using digital
control
T. Droege, J. Purcell and
S. Wang
Summary: A slow cycling flux pump has
been constructed where operation is controlled by digital
logic driving a high power operational amplifier. Hall sensors
allow closed loop control of the secondary currents to enable
switching the heater driven power cryotrons at the optimun
time. Operating efficiency of 87% has been
achieved. |
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Adjacent
conductor field corrections to high critical current short
sample measurements
G. Miranda
and J. Rogers
Summary: We have measured the
critical current of several superconductors to 24 kA in
magnetic fields to 4.5 T. The samples were hairpin shaped with
the straight section under test in a perpendicularly applied
field. The proximity of the straight parts causes the field of
the current in one leg to change the applied field in the
other leg. We have corrected the measured results for this
adjacent conductor field (ACF). These corrections result in
data for uniform applied fields. One conductor was used in a
300 kJ magnet. This single-layer, edge-wound coil was rated
near 9.8 kA using the uniform field data. Under test the
magnet reached 12.5 kA giving no sign of approaching the
critical current. The "excess current" is due to the field
gradient across the conductor in this coil. One contribution
is due to the variation of j/sub c/with the field value within
the conductor. A second contribution results from the position
dependent anisotropy of the filaments. A third possibility is
to change the definition of I/sub c/. We have chosen the
definition to be the current at which the sample generates an
electric field of 3.5 /spl mu/V/m. If 46 /spl mu/V/m were
chosen, I/sub c/would rise approximately 0.86 kA at 17
kG. |
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Investigations
for the development of superconducting power
switches
K. Grawatsch, H. Kofler,
P. Komarek, H. Kornmann and A. Ulbricht
Summary:
The transmission behaviour of NbTi-foils,
NbTi-multifilament wires in CuNi-matrix and of NbN-sputtering
sheets have been investigated and compared under different
switching modes. Suitability for switching of 50 Hz-a.c.
current has been proofed. Based on the results at 1200 A
switch for a 12 kJ-superconducting storage coil which was the
basic step in a project for the development of superconducting
storage systems has been built. The switching time with and
without load current has been investigated for different
amplitude of current pulses. |
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Superconducting
switches using radiation induced
quenching
R.
Schmieder
Summary: The use of penetrating
radiation such as X-rays or high energy electrons to cause
quenching of a superconducting short across an inductive
energy store, thereby switching the persistent current into an
external load, is examined. The basic equations describing
this process within the two-fluid model are presented, and
estimates of the size of megampere switches are made. The
ratio of switching energy to switched energy is derived, and
X-ray vs. electron impact quenching is discussed. Several
geometries for practical switches are presented. |
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Development
of a superconducting switch for magnet energy storage
systems
J. Lindsay, D. Blevins,
H. Laquer, G. Miranda, J. Rogers, C. Swannack and D.
Weldon
Summary: Some of the problems
encountered in constructing superconducting switches for
magnetic energy storage systems are discussed. Empirical rules
are given which assure that a switch is driven fully normal
when triggered, and some of the possible geometrical
arrangements for switches are considered. Results of energy
transfer experiments from 20 kJ and 300 kJ superconducting
energy storage systems using superconducting switches are
given. |
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Performance
characteristics of the U.VA. superconducting wind tunnel
balance
R. Humphris, R. Zapata
and C. Bankard
Summary: Initial operational
characteristics of a wind tunnel electromagnetic balance
utilizing superconducting coils are reported. Both d.c. and
a.c. superconducting coils are used for balancing and
measuring 3-component aerodynamic forces on simple models in
Mach 3, room temperature flow. Liquid helium boil-off
measurements corresponding to a wide range of operating
conditions are presented together with results of systematic
a.c. losses scaling experiments using various pancake coils
wound with superconducting tape. This unique prototype
facility is primarily for studying the practical feasibility
of using superconducting magnetic suspension techniques for
aerodynamic testing and accumulating the knowledge and
expertise required for extrapolating these techniques to
large-scale facilities. |
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The
application of superconductivity to magnetic
separation
P.
Marston
Summary: Although magnetic separation
may become the first industrial application of
superconductivity, its advantages do not appear to be as great
as many of us had anticipated. |
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Comparison
and optimization of lift and drag forces on vehicles levitated
by eddy current repulsion for various null and normal flux
magnets with one or two tracks
J.
Hogan and H. Fink
Summary: Lift and drag
forces are compared for various null and normal flux
superconducting magnet configurations where each coil consists
of a pair of infinitely long parallel wires separated by a
fixed distance. The nullflux configuration has a minimum in
the drag to lift ratio for a particular value of I/sup 2//W
(I=magnet current, W=vehicle weight) when the vehicle cruises
freely at constant speed. This calculation takes into account
losses due to non-uniform eddy-current density distribution in
the solid track. Results indicate that the null-flux
configuration is the most efficient design, followed by the
normal-flux, single track configuration. All other schemes are
less efficient for a thin track configuration. A general
method is outlined for calculating the zero-torque
case. |
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Superconducting
levitated high speed ground transportation project in
Japan
T. Ohtsuka and Y.
Kyotani
Summary: An outline of the magnetic
levitation project in Japan is given. Research has been
concentrated on levitation system utilizing electromagnetic
repulsive force between a normal metal track and on-board
superconducting magnets. Development of levitation magnets
together with other items basic for a levitation system is
given. |
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U.S.
department of transportation program in magnetic suspension
(repulsion concept)
J. Reitz and
R. Borcherts
Summary: The TMLV Technology
Program is concerned with the use of magnetic levitation for
suspension of high-speed ground vehicles traveling at speeds
of 480 km/h. The program is configured to permit comparative
evaluation of two competing MAGLEV concepts -- repulsion and
attraction. The program has two major tasks: conceptual design
of a passenger-carrying system which meets a specified ride
quality objective, and design and test of a high-speed (480
km/h) rocket-propelled test vehicle capable of providing
engineering data relevant to the aforementioned conceptual
design. The high-speed tests will be carried out at the Naval
Weapons Center, China Lake, California. This paper will
discuss both parts of the program, with emphasis on the
superconducting magnets used in the levitation/guidance
systems. |
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Superconducting
magnet with tube-type cryostat for magnetically suspended
train
H. Kimura, H. Ogata, S.
Sato, R. Saito and N. Tada
Summary: In order
to develop a compact and light-weight superconducting vehicle
magnet for a magnetically suspended high speed train, a
superconducting magnet with a tube-type cryostat has been
constructed and tested. The shape of the superconducting coil
is a rectangle, which has a length of 1.5 m and a width of 0.6
m, and coaxial vessels of the cryostat cover the coil winding.
The liquid helium vessel and the outer vacuum vessel of the
cryostat have an outside diameter of 8 cm and 14 cm,
respectively. The experimental results and the evaluation for
the magnet are described. |
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A
review of the magneplane
project
C. Tang, W. Harrold and
R. Chu
Summary: The Magneplane is a
magnetically propelled and suspended vehicle for high speed
ground transportation. The design represents an attractive
transportation option applicable to various transportation
corridors. The purpose of the paper is to highlight various
development efforts associated with the project and to report
recent results of the study. |
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Superconducting
maglev and LSM development in
canada
D. Atherton and A.
Eastham
Summary: Studies on the use of
superconducting magnets for levitation, synchronous propulsion
and guidance of high speed intercity ground transportation in
Canada are reported. For a 100 passenger vehicle weighing 300
kN, a levitation height of 22 cms at 480 Km/hr. is obtained by
the interaction of eight 3.85x10/sup 5/amp turn 100 X 30 cm
magnets with eddy currents induced in two 80 X 1 cm aluminum
guideway strips. In low speed sections, aluminum thickness is
graded to 3 cms to maintain total drag (aerodynamic and
magnetic) almost speed independent. The variable speed LSM
uses fifty 5x10/sup 5/amp turn 40 X 150 cm magnets on a half
pitch of 45 cms. The motor, with split 3-phase guideway
windings energized in 5 Km block lengths, has 72% efficiency
and 0.82 power factor. A favoured guidance scheme for a
flat-topped elevated guideway (minimizing snow accumulation)
uses the interactions of the propulsion magnets with flat
null-flux loops overlying the LSM windings and with the edges
of the levitation strips. This scheme produces a lateral
stiffness of 10/sup 6/N/m and a maximum guidance force equal
to vehicle weight. The Canadian test facility, presently being
commissioned, uses full scale magnets and a 7.6 m diameter
wheel rotated about a vertical axis with a maximum peripheral
speed of 100 Km/hr. |
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J-8
superconducting D.C. machines - Concerning mainly civil marine
propulsion but with mention of industrial
applications
A.
Appleton
Summary: This paper discusses the
current status of superconducting, d.c. motors and generators
with particular regard to civil marine propulsion. It
commences with a brief account of the factors which help to
ensure reliability and proceeds to enumerate the advantages
which are expected from superconducting d.c. marine propulsion
systems. Following a mention of a practical demonstration of
the latter, now under test at IRD, the paper concludes with
assessment of the general market for the machines. Also,
included in the paper is some mention of industrial
applications for these new machines. |
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A
high speed superconducting
rotor
J. Parker, R. Blaugher, A.
Patterson, P. Vecchio and J. McCabria
Summary:
As a result of the predicted savings in weight and
volume offered by superconducting field windings, the USAF
initiated a technology advancement program on superconducting
machines. The initial phase of this program was devoted to the
preliminary design and analysis of certain critical components
judged to be essential in constructing a high speed, high
power superconducting generator suitable for airborne
application. As a final feasibility test, the most critical
components, i.e., the dynamic cryogenic seals, power leads,
and superconducting field winding, were to be assembled into a
complete working rotor and tested under full operating
conditions. This paper will review the overall design,
fabrication and final test results of a prototype 12,000 rpm
4-pole superconducting rotor. The main emphasis will be placed
on the design, fabrication and test of the superconducting
coils, and on the description of the cryogenic cooling scheme
for the 4-pole coil structure. |
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Superconducting
three element synchronous A.C.
machine
L. Boyer, J. Chabrerie,
A. Mailfert and M. Renard
Summary: There is a
growing interest in a.c. superconducting machines. Several new
concepts have been proposed for these machines in the last
years. One of the most promising seems to be the "three
elements" concept of Pr. Smith, allowing to cancel the torque
acting on the superconducting field winding and thus
overcoming some of the major contraints. This concept leads to
a device of induction-type generator. In the present paper a
synchronous three elements superconducting a.c. machine is
described, in which a room temperature, d.c. fed rotating
winding is inserted between the superconducting field winding
and the a.c. armature. The steady-state machine theory is
developed. After having the flux linkages established, the
torque expressions are derived. The condition for zero torque
on the field winding, as well as the resulting electrical
equations of the machine are given. In the last part of the
paper, the theoretical behaviour of the machine is studied,
using phasor diagrams and assuming for the superconducting
field winding either a constant current or a constant flux
condition. |
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Protection
of superconducting field windings for electrical machines by
the use of an inertial free electromechanical
shield
N. Dagalakis and J.
Kirtley Jr.
Summary: This new machine has two
independent rotors, the driven inner rotor with the
superconducting field winding and the free rotor which serves
as the electro-mechanical shield. At steady state both rotate
at synchronous speed, but during a fault transient the shield
acts as a low-pass filter protecting the field from the large
pulsations in torque and magnetic field. The new machine is
described and its steady state and transient performance are
outlined. |
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Multipole
superconducting electric motors for ship
propulsion
P. Thullen, T. Keim
and J. Minervini
Summary: While a great deal
of attention has been paid to two-pole superconducting
synchronous machines, very little analysis of low speed,
multipole superconducting synchronous machines has been done.
Such machines may prove desirable as drive motors in ship
drive systems. This paper presents the results of an analysis
which assumes a motor of sufficient size that the airgap may
be considered to be flat. A power output expression is given
which shows the effects of machine geometry and superconductor
characteristics on machine size. Based on this expression, a
40,000 hp 120 rpm motor is sized, and the resulting machine is
compared with a conventional ship drive motor. The comparison
illustrates possible size reductions through the application
of superconductivity. |
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Superconducting
magnetic systems and electrical
machines
I.
Glebov
Summary: The use of superconductors
for magnets and electrical machines attracts close attention
of designers and scientists. During the last ten years the
developments were carried out in the All Union Research
Institute for Electrical Machinery (USSR) to create
superconductive magnetic systems, commutator motors, homopolar
machines, topological generators and turbogenerators with
superconductive field wind ings. All the machines are
tentative experimental ones and serve as a basis for further
developments. |
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Magnetic
field in a system with superconducting
shields
V. Shakhtarin, S.
Pylinina and M. Rizhikov
Summary: The problem
of calculating of the magnetic field in magnets with
superconducting shields is considered. As an example of
calculating the magnetic field of a rectangular cross section
coil with two shields is analyzed. The results are compared
with experimental data. |
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Coherent
effects in series arrays of proximity effect superconducting
bridges
D. Palmer and J.
Mercereau
Summary: Planar series arrays of up
to 500 proximity effect superconducting bridges have been
fabricated with sufficient uniformity to respond coherently to
incident radiation (10-20,000MHz) without separate biasing. By
varying the geometry, resistance for the arrays can range from
.1-50/spl Omega/. Despite this high total impedance, low
frequency coherent response to external radiation is limited
only by single bridge voltage noise. Low frequency (30-500MHz)
coherent internal voltage oscillations of current biased
arrays have been detected and confirm that the oscillating
voltage signal grows linearly in bridge number while the
voltage noise increases only as the square root. An array of n
elements responds to incident high frequency radiation in
first approximation as though it were a simple Josephson
junction but quantized in units of n(h/2e). Coincident
operation to high frequency depends strongly on small (/spl
leq/ 1/spl mu/) bridge separation. High resistance, coincident
operation, and exponential dependence of critical current on
temperature make these arrays good candidates for voltage
standards, coherent and bolometric radiation detectors, and
cryogenic rf sources. |
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Observation
of microwave synchronization of thin film microbridge
arrays
D. Jillie, J. Lukens and
Y. Kao
Summary: We report on the
characteristics of thin film microbridge Josephson junctions
fabricated using electron beam lithography and discuss the
techniques used to fabricate these junctions, which are
typically 0.3/spl mu/ square. Extensive measurements have been
made on both individual junctions of tin and indium and on
small arrays of these junctions. Synchronization of current
biased series arrays by an applied microwave field has been
achieved. This synchronization appears as a zero differential
resistance across the entire array at a voltage of MV/sub
J/where M is the number of elements in series and V/sub J/is
the Josephson voltage hv/2e. Furthermore we have observed a
systematic variation with microbridge dimension of the
microwave enhancement of the critical current. |
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Coherent
AC Josephson effect in a bulk granular superconducting
system
M. Yu and A.
Saxena
Summary: A three-dimensional
close-packed array of Josephson junctions was successfully
produced in the form of a bulk granular superconductor. The
granular system is a tightly compressed solid of oxide coated
small tin particles in the thousand /spl Aring/ range. The
three-dimensional array of Josephson junctions thus formed
showed coherent Josephson oscillations when external rf
radiation was imposed, rf induced constant voltage steps were
observed on its I-V characteristics at voltages a few thousand
times the Josephson voltage hv/2e. The external rf radiation
also induced Stable dc voltages on unbiased bulk samples,
suggesting a practical mode of microwave detection. The
analogy of this bulk superconducting system with a magnetic
spin system will be discussed. |
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Radiation
detection by coherent Josephson phenomena in agglomerated tin
films
W. Ayer and K.
Rose
Summary: We have previously reported
radiant energy detection by superconducting thin films in both
a thermal (or bolometer) mode and a current mode. We now
report evidence that at least some current-mode detection is
due to Josephson behavior associated with agglomeration in tin
films. The temperature dependence of the critical current
agrees with results for Dayem bridges, and we observe
microwave-induced steps in the current-voltage characteristics
which have the proper dependence on frequency and power. At a
constant film resistance the current-mode responsivity shows
structure (dips) in its dependence on bias current which
shifts with microwave frequency in agreement with the
Josephson theory. The films appear to behave like an array of
weak links in the "correlated" state as observed by Clark and
explained by him and Tilley. |
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A
superconducting niobium powder detector for submillimeter
radiation
A. Kovalenko and G.
Leshchenko
Summary: Recently a number of
works were published,in which it was shown that very sensitive
detectors of microwave range may be created using granular
superconductors. For example, in [1] in x-band a responsivity
of 3 .10/sup 5/ V/W was achieved, the NEP is estimated to be
7.10/sup -15/ W.Hz-1/2. According to [2] ,for broad band
detection the NEP is 1.10/sup -14/ W.Hz-1/2 at /spl lambda/ =
3 cm and 5.10/sup -13/ W.H2-5 at /spl lambda/ = 4 mm.
Therefore it was interesting to investigate experimentally the
responsivity of granular superconductors in submillimeter
range. |
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Observations
on small tunnel junctions: Anomalous critical-current
temperature dependence, periodic structure, and microwave
detection
J. Yeh and D.
Langenberg
Summary: Observations on very
small Pb-Pb oxide-Pb tunnel junctions prepared by a new method
are reported. The zero-voltage Josephson critical current is
found to increase, then decrease with decreasing temperature.
The current-voltage characteristics show very prominent
periodic structure. The junctions have microwave (X-band)
power sensitivities of 10/sup -14/W, more than two orders of
magnitude better than conventional-size
junctions. |
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Effects
of laser irradiation on weak link
devices
R. Janik, L. Morelli, N.
Cirillo, J. Lechevet, W. Gregory and W.
Goodman
Summary: Experimental results will be
presented on the effects of optical radiation impinging on
superconducting weak links and films. In-house fabricated weak
links of various materials showed a lowering of critical
current values dependent on radiation intensity. Also,
specially modified commercial SQUIDS showed a lowering of both
dc bias and rf bias required for SQUID operation as the
optical intensity was increased. We will discuss the
possibility of using high Tc materials for SQUID fabrication
and the "tuning" of these devices via optical
irradiation. |
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A
superconducting thin-film low-energy particle
detector
J. Hoyle, R. Humphris
and J. Boring
Summary: Techniques for
constructing a superconducting thin-film particle detector
have been developed. Detecting surfaces were fabricated by
evaporating a 500 /spl Aring/ tinindium film on a glass cover
slide and scratching to obtain a 1 micron strip. At a point
just below the T/sub c/and at current densities near I/sub c/,
pulses due to single incident Ar, Ar+, He, and He+ particles
were observed. Pulses were observed for helium and argon down
to 200 eV and 150 eV respectively and up to approximately 800
eV. The response of the detector to ions and neutral particles
was identical with near 100% efficiency. Calculated values for
the size of normal regions of the film due to incident
particles were of the same order of magnitude as those
measured. Pulse height analysis indicated no dependence on
incident particle energy. It was concluded that this lack of
pulse height dependence on incident energy was due to an
intrinsic fluctuation phenomenon of the superconducting
film. |
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Measurements
of the magnetic fields produced by the human heart, brain, and
lungs
D. Cohen
Summary:
Magnetic fields produced by organs of the human body
are being measured in the M.I.T. shielded room, using both a
SQUID magnetometer and second-derivative gradiometer.
Measurements of the field around the human body can yield new
information about the organs which generate current, not
available to surface electrodes, and also about organs which
contain foreign, ferromagnetic particles. Magnetocardiograms
of normal and abnormal heart subjects are being analyzed and
visually displayed in order to assess their information
content. Magnetoencephalograms recorded from normal and
abnormal brain subjects are also under analysis. Measurements
have been made of magnetite dust in the lung, with two
potential medical applications: the first is the use of pure
magnetite dust as a deliberately inhaled tracer (harmless) for
pulmonary diagnosis; the second is the assessment of the
amount of asbestos accumulated in the lungs of heavily-exposed
workers, since most asbestos (harmful) occurs with adhered
magnetite. |
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Advanced
superconducting gradiometer/Magnetometer arrays and a novel
signal processing technique
W.
Wynn, C. Frahm, P. Carroll, R. Clark, J. Wellhoner and M.
Wynn
Summary: Recent developments in
superconducting magnetic gradiometer technology have led to
the construction of advanced ultrasensitive
gradiometer/magnetometer arrays. Details of construction
techniques and data showing operational capabilities are
presented. The most recent of the gradiometer/magnetometer
arrays simultaneously measures five independent spatial
gradients of the magnetic field and three vector components of
the magnetic field. The measured signals from this array are
subjected to a novel signal processing technique which
provides detailed information about the magnetic signal
source. |
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Superconductivity
applied to gravimetry
J. Goodkind
and R. Warburton
Summary: A gravimeter has
been developed which uses the levitation of a superconducting
sphere by the field of persistent current magnets.
Instabilities in the field have been reduced to the point
where environmental noise on local gravity is greater than the
instrument noise. In present results the persistent currents
are decaying at a rate of 2 parts in 10/sup 7//yr. Concurrent
measurements of magnetic field and gravimeter signal allows
for correction of this drift. The device is being applied to
geophysical problems through measurements of: secular
variations in local gravity, tides of the solid earth, effects
of oceans and atmosphere on local gravity, and normal modes of
the earth. |
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A
SQUID readout system for a superconducting
gyroscope
J.
Hendricks
Summary: A design of a read out
system for a superconducting gyroscope to be used in an
orbiting Gyroscope Relativity Experiment is discussed. The
"London Moment" of the superconducting rotor, which lies along
the spin axis of the rotor, will be measured with a SQUID type
magnetometer. The SQUID will be built around the gyro rotor,
with a very close spacing to give an inductance between 10/sup
-8/and 10/sup -9/Hy. A SQUID of this design should resolve
10/sup -4//spl phi//sub o/. The angular resolution of the
gyroscope will then be /spl Delta//spl theta/ = 3.5 /spl
times/ 10/sup -3 arc-second, which is sufficient for the
intended experiment. |
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Cryogenic
direct current comparators and their
applications
R. Dziuba and D.
Sullivan
Summary: Cryogenic direct current
comparators, utilizing superconducting shields and
Superconducting Quantum Interference Devices (SQUIDs), provide
current ratios of up to 100/1 or higher with accuracies of
/spl lsim/ 1 /spl times/ 10/sup -9/ and current resolutions of
/spl lsim/ 6 /spl times/ 10/sup -11/ A. Two types of
comparators differing in the shielding arrangement of the
ratio windings are described. One type consists of unit
windings inside a seamless Pb tube; the other consists of
multi-turn ratio windings within an overlapping toroidal
superconducting shield. For both shielding configurations,
SQUIDs serve as flux sensors for the comparators. The
application of these comparators to the measurment of
resistance ratios is described. |
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Applications
of a zero-frequency superconducting inductance
bridge
R. Meservey, P. Tedrow and
D. Paraskevopoulos
Summary: An inductance
bridge has been constructed which operates at zero frequency
to compare superconducting inductors. The bridge uses a point
contact SQUID as a current null detector. The unknown and
standard inductors are at liquid helium temperature, but the
bridge is balanced by resistance adjustments at room
temperature. The present bridge can detect a change of 1 part
in 10/sup 4/in a /spl mu/H inductance and has various
advantages over ac bridges. The bridge can be applied in a
number of ways. Currently we are using it to determine the
kinetic inductance, penetration depth and critical current of
thin film circuits. The bridge is particularly adapted to
measuring the quantum phase difference vs current of
current-dependent inductances. Susceptibility and precision
measurements of inductance are other possible applications.
Although the bridge was developed to measure small
inductances, the principle can be easily applied to larger
inductances and may be useful in large scale
applications. |
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A
reliable DC squid made with tunnel
junctions
J. Clarke, W. Goubau
and M. Ketchen
Summary: D.C. SQUIDS of
cylindrical geometry have been fabricated using Nb-NbO/sub
x/-Pb tunnel junctions. The SQUIDs can be thermally cycled
repeatedly and cooled in the earth's magnetic field without
significant loss of performance. Flux resolution of 3 /spl
times/ 10/sup -5/ /spl phi//sub o///spl sqrt/Hz above a
frequency of 1 Hz has been achieved through optimization of
electronic circuitry and careful shielding. Fluctuations and
drift in the temperature of the helium bath are found to be
the major source of long-term SQUID noise. With a simple
temperature stabilizer, drift has been reduced to less than 8
/spl times/ 10/sup /-5/ /spl phi//sub o//hr over a three hour
period. |
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Superconducting
magnetometer for high resolution susceptibility
measurements
M. Cerdonio and C.
Messana
Summary: We have developed a
superconducting susceptometer operating in temperature range
up to 230/spl deg/K. The instrument is capable of routine
susceptibility measurements on frozen solutions and
suspensions, powders, crystals; a single helium fill up,
lasting about 8 hours, allows about 50 susceptibility
measurements at various sample temperatures. Sensitivity is
6x10/sup -11/e.m.u./cc for a sample volume of 0.5 cc and is
independent of temperature. The overall accuracy of the
instrument for sample temperatures upto 230/spl deg/K is
3x10/sup -9/e.m.u./cc, temperature independent, in a typical
applied field of 20 gauss. |
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Current-phase
relations and noise in rf biased
squids
L. Jackel, T. Clark and R.
Buhrman
Summary: An investigation has been
made of the effect of the weak link current-phase relation on
noise in rf biased SQUIDs. Non-sinusoidal current-phase
relations have been observed in various weak links and these
non-sinusoidal relations have been correlated with
significantly increased intrinsic noise in the SQUID ring. The
current-phase relation is also found to affect the amplitude
of the rf SQUID ring dissipation. The result of a rf SQUID
system noise analysis shows that, due to increased intrinsic
noise and reduced ring dissipation, the minimum attainable
noise for a SQUID ring having a very non-sinusoidal
current-phase relation is considably greater than for a ring
with a sinusoidal relation. |
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Low
noise, permanently adjusted UHF SQUID
magnetometer
T. Clark and L.
Jackel
Summary: We report the development of
a simple 450MHz SQUID magnetometer, utilizing a symmetric
two-hole permanently adjusted point contact SQUID ring. One
hole of the SQUID ring is coupled to the center conductor of a
resonant superconducting coaxial cavity. A superconducting
coil, inserted into the other hole, allows us to couple an
external flux into the SQUID system. The sensitivity is an
order of magnitude better than that attained with identical
SQUID rings biased at /spl sim/10MHz. We show that this
improvement can be achieved simply and relatively
inexpensively, and without sacrificing the excellent flux
coupling attainable with such SQUID rings. |
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Fabrication
and performance of NbN thin film planar
squids
T. Fujita, S. Kosaka, T.
Ohtsuka and Y. Onodera
Summary: Planar SQUIDs
with a Dayem bridge and two holes have been fabricated by
sputter etching NbN thin films deposited epitaxially on MgO
substrates and their performance has been tested in a
conventional mode of operation at 15 MHz. It has been found
that the devices operate with satisfactory reliability and
their operating temperature covers over a wide range nearly
from the superconducting transition temperature of the basic
films e.g./spl sim/15 K down to the pumped liquid helium
temperature. The fabrication technique and the result of the
performance test are presented, and the bridge characteristics
and possible application of the device are discussed from a
technical point of view. |
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On
some aspects of superconducting quantum interferometer
optimization
B. Vasiliev, V.
Danilov and K. Likharev
Summary: A problem is
considered of optimizing the following parameters of the
single-junction quantum superconducting fluxmeter: 1. The full
range output signal of the magnitometer /spl utri/A. 2. The
derivative of the signal characteristic\gamma =
|\frac{\partialA}{\partial\phi_{e}}|3. The minimum
detected flux change /spl delta//spl phi/ For the hysteresis
case (\ell=2\piL_{R}I_{o}\phi_{o}^{-1} > 1) the
following results have been obtained. In the resistive mode
(q= wL_{R}R^{-1} < 1) the maximum value /spl
utri/A equals
(\phi_{o/2})[\Delta_{k}Q(\piL_{R}C_{T}^{-1}]^{1/2}where
the maximum value of the function\Delta_{k}(\ell)is
about unity, when /spl ell/approx 1.5. A satisfactory
agreement has been obtained between the measured and
calculated dependences of /spl utri/A on k and C/sub
T/parameters for the fluxmeter having a pumping frequency of
about 10 MHz and the high-Q circuit. With detuning /spl x/o
conditioning either a triangular or trapezoidal waveform
pattern, /spl gamma/ is as usually given
by\gamma_{o}=\frac{w}{k}\sqrt{\frac{L_{t}}{L_{R}}}If
the detuning is great and negative a signal amplitude may take
a rectangular form, and /spl gamma/ in this case is much more
larger than /spl gamma//sub o/. The experimentally obtained
value\delta\phi =10^{-11}Gs.cm/sup 2/is near enough
to the known theoretical estimation. Calculations indicate
that for a nonhysteresis case one may have the following
formulae:\gamma_{\max}/\gamma_{o} = l/q \gg
1and(\delta\phi)_{\min} \simeq 1.2 (L_{R}/\ell)
(4\Delta fk_{B}
T/R)^{1/2}giving(\delta\phi)_{\min}/(\Deltaf)^{1/2}
\approx 10^{-14}Gs.cm/sup
2//\sqrt{Hz}when/spl ell//spl ap/1,L_{R}=
10^{-10}H (not taking into account the noise of the
amplifier). The magnetometers were employed to search for the
electrical dipole moment of the electron in the macroscopic
experiment, in low temperature paramagnetic thermometry and
also to test the degree of magnetic field shielding with a
hollow superconducting screen. |
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Some
aspects of the dynamics of Josephson junction circuits and
devices
T.
Fulton
Summary: A descriptive review of some
of the familiar and not-so-familiar behavior of Josephson
devices was given making use of mechanical analogs. Previously
unpublished material on the two-dimensional junction analog
and noise properties of the resistively-shunted junction model
is given here. |
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Storage
and detection of single flux quantum in Josephson junction
devices
P.
Gueret
Summary: It is shown both by computer
simulations and experimentally that a single Josephson
junction has memory and can therefore be used for information
storage. Means of reading-out the information content of such
a memory element are demonstrated. Finally, memory operation,
writing and reading, is described as a direct application of
these concepts. |
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A
1.4 mil/sup 2/memory cell with Josephson
junctions
R. Broom, W. Jutzi and
Th. Mohr
Summary: An integrated loop-memory
cell with miniaturized Josephson-junction dimensions (/spl
sim/5 X 5 /spl mu/m/sup 2/) and very high Josephson current
density (30 kA/cm/sup 2/) has been made in a 2 /spl
mu/m-minimum line-width lead-alloy technology. The storage
currents are easily set up with coincident word and digit
pulses. Nondestructive read-out, write and half selection has
been successfully performed. The deduced current transfer time
during write is below about 80 ps. |
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Characteristics
of in-line Josephson tunneling
gates
S. Basavaiah and R.
Broom
Summary: Characteristics of in-line
Josephson gates have been computed, using numerical
techniques, for a number of differing junction length to
Josephson penetration depth ratios. These are verified by
systematic experimental results, where the agreement between
theory and experiment was found to be good. |
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Barrier
formation in lead-based tunnel junctions studied by surface
techniques
A. Emmanuel, G.
Donaldson, W. Band and D. Dew-Hughes
Summary:
A process is described for the production of Pb(In) -
oxide barrier - Pb Josephson tunnel junctions. The junctions
are predictable in resistance to within 30%, and have
excellent leakage, storage and ageing properties. Critical
currents and magnetic behaviour agree well with theory.
Investigation of the deposited films by surface techniques
shows that the presence of In suppresses the formation of
'hillocks' which occur during annealing and during sputter
etching. The distribution of In through the films is
investigated by Ion Scattering Spectroscopy, which shows In
enrichment at the free surface. There is some, as yet
inconclusive, evidence that In plays an important role in the
barrier formation (oxidation) process. |
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Single-crystal
silicon-barrier Josephson
junctions
C. Huang and T.
Duzer
Summary: We have made and tested
1250/spl Aring/-thick silicon-barrier Josephson junctions. The
junctions tested show supercurrent and have junction
characteristics which may be useful for high-frequency
applications. A detailed junction fabrication process and the
effects of surface oxides on the junction characteristics are
described. |
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High-speed
switching and logic circuits using Josephson
devices
H. Chan, W. Lum and T.
Duzer
Summary: We have investigated the use
of Josephson devices as switching components for computer
circuits. One part of this work is a study of the switching
properties of the superconductor-semiconductor-superconductor
junction. The switching speeds of a single junction and a
basic memory cell, computed by numerical methods, are
comparable with the corresponding values for oxide-barrier
junctions. Having a much lower Q than the conventional
oxide-barrier junction, it should not have the previously
experienced difficulties caused by junction resonances. The
damping of plasma oscillations in the semiconductor junction
is also more effective, leading to a shorter switching time. A
new type of logic gate employing Josephson devices has been
investigated. This circuit would automatically unlatch after
each operation, thus requiring no external means for
resetting. Numerical analysis predicts subnanosecond
switch-reset operation for appropraite parameter choices. The
qualitative behavior of the circuit has also been shown with a
mechanical analog. |
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Application
of integrated circuit technology to the fabrication of large
numbers of niobium based Josephson
junctions
S. Owen and J.
Nordman
Summary: A batch fabrication
technology is described for the production of small niobium
based Josephson junctions. This technology uses thermally
oxidized silicon wafers as substrates, allowing maximum usage
of silicon integrated circuit techniques and equipment.
Patterns are produced in rf sputtered niobium films by sputter
etching through masks generated by "step and repeat"
photolithographic techniques. Typically over 600 junctions are
fabricated at one time. Measurements of voltampere curves,
Josephson current versus magnetic field, and self resonant
step structure were made. |
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Advances
in superconducting quantum electronic microcircuit
fabrication
R. Kirschman, H.
Notarys and J. Mercereau
Summary: Standard
microelectronic fabrication techniques have been utilized to
produce batch quantities of superconducting quantum electronic
devices and circuits. The overall goal is a fabrication
technology yielding circuits that are rugged and stable and
capable of being fabricated controllably and reproducibly in
sizeable quantities. Our progress toward this goal is
presented, with primary emphasis on the most recent work,
which includes the use of electron-beam lithography and
techniques of hybrid microelectronics. Several prototype
microcircuits have been successfully fabricated. These
microcircuits are formed in a thin-film parent material
consisting of layers of superconducting and normal metals, and
use proximity-effect structures as the active circuit
elements. |
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Niobium
microbridges for SQUID
applications
L. Holdeman and P.
Peters
Summary: Niobium microbridges
developed for SQUID applications are discussed. We can
reliably produce superconducting films down to a thickness of
50 Angstroms using rf bias sputtering techniques. Bridge
patterns of submicron width are produced through electron
lithography with a scanning electron microscope/flying spot
scanner combination. The transition from macroscopic leads to
the microbridge is made with a multiple exposure technique.
The pattern is reproduced in the superconducting niobium films
by sputter etching. |
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Preparation
of superconducting weak links in molybdenum films by ion
implantation
E.
Harris
Summary: We have implanted patterns of
N+ and/or S+ ions into Mo films so as to produce structures in
which two heavily-doped Mo regions with relatively high
transition temperature T/sub c/are connected by a short (~
1/spl mu/m) lightly-doped Mo region with lower transition
temperature T'/sub c/. Near T'/sub c/the lightly-doped region
acts as a weak link between the heavily doped regions and
passes a supercurrent having an oscillatory "diffraction
pattern" dependence on magnetic field similar to that of
Josephson tunnel junctions. |
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Parametric
amplification with self-pumped Josephson
junctions
H.
Kanter
Summary: The parametric nature of
supercurrent in Josephson junctions may be exploited for
amplification of high frequency signals in several modes of
operation, where in each case the reactance variation is
provided by the internal oscillations due to the average
contact potential, a)One mode is negative resistance
amplification with a single idler. This mode is entirely
equivalent to that generally employed in conventional
parametric amplifiers with varactor diodes. b)Negative
resistance amplification with several idlers at further
combination frequencies. This mode of operation is typical for
the resistively shunted junction model, c)Amplification by
upconversion, which is conceptually employed in the rf SQUID
used in magnetometry. Experimental verifications of these
various modes are described and their suitability for low
noise amplification discussed. The upconverter experiment
permitted the measurement of device noise by variation of the
temperature of the input termination. An upper noise limit of
15/spl deg/K is established. This experiment demonstrates that
in high frequency application of active self-oscillating
Josephson junctions the fundamental noise achievable is not
greatly in excess of the thermal limit. |
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Superconducting
low noise receivers
A.
Silver
Summary: It is now possible to develop
a new class of receivers for the millimeter and shorter
wavelength region by application of Josephson and other
electron tunneling phenomena which occur in superconducting
materials. This development represents a major advance in
receiver sensitivity and will be applicable over a wide
frequency interval. Operating in the conventional heterodyne
detection mode this receiver has a theoretical noise
temperature as low as 20 Kelvin at 100 GHz. Three basic
components required are: a superconductor-semiconductor
Schottky diode mixer; a Josephson junction phase-locked
multiplier local oscillator; and a Josephson parametric
amplifier. The component and system design and limiting
factors will be discussed. |
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Noise
in Josephson effect mm-wave
mixers
J. Claassen, Y. Taur and
P. Richards
Summary: We have recently shown
that point contact Josephson junctions can function as
millimeter wave heterodyne mixers with conversion gain. The
best results achieved thus far show a single sideband
conversion gain of 1.3 and a mixer contribution to the system
noise temperature of 54K. Both of these results are /spl sim/
5 times better than the best published figures for cooled
Schottky barrier diode mixers operated at the same frequency.
The measured noise for a variety of junctions can be expressed
as a universal function of the normalized rf frequency /spl
Omega/ = /spl hbar//spl omega//2eI/sub c/R. It is about a
factor 2 larger than the calculated noise arising from the
thermal noise in the junction shunt resistance R. The noise
calculation was done for the resistively shunted junction
model using an analog junction simulator. |
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Power
spectrum of an injection-locked Josephson
oscillator
C. Stancampiano and S.
Shapiro
Summary: Previous experiments have
shown that a Josephson oscillator, exposed to a weak
narrow-band input signal, exhibits behavior characteristic of
an injection-locked oscillator. When in lock, Adler's theory
of injection locking describes the experimental observations
reasonably well. This paper extends the range of applicability
of the theory to the out-of-lock regime where a spectrum of
output frequencies is observed. Obtaining the theoretical
output power spectrum requires solving a differential equation
having the same form as the equation describing the
resistively shunted junction model of Stewart and of McCumber.
Experimental measurements of the output spectrum of a nearly
locked Josephson oscillator are shown to be in reasonable
agreement with the theory. Additional results discussed
briefly include the observation of a frequency dependence of
the locked Josephson oscillator output, and experiments in
which a Josephson oscillator-mixer was injection locked by a
weak signal at the if. |
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Self-synchronous
parametric amplification using Josephson
junctions
M.
Yu
Summary: A new way to use Josephson
junctions for parametric amplification with phase control is
proposed. The dc current bias of the junction is adjusted such
that the Josephson oscillation automatically synchronizes with
the input signal and oscillates at twice the signal frequency
with the proper relative phase. The impedance of the junction
then has a negative resistive component which can be used for
amplification. Extension to other phase sensitive circuits is
also discussed. |
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Perturbation
treatment of mixing in Josephson
junctions
M. Levinsen and B.
Ulrich
Summary: We consider a current biased,
resistively shunted josephson junction irradiated at two
frequencies. The perturbation technique introduced by
Aslamasov and Larkin is used in the calculations. Both signals
are treated as perturbations. The second order calculation
yields the size of the mixing steps at V/sub plsm/=h(/spl
omega//sub 1//spl plsm//spl omega//sub 2/)/2e. As in the
case of a single frequency we show that subharmonic mixing
steps are absent. The amplitude of the voltage oscillation at
the difference and sum frequencies is shown to be non-zero at
all voltages. We calculate the microwave resistance for one
frequency /spl omega//sub 2/to third order in the
perturbation. There are negative resistance regions near V/spl
plsm/ (as well as near V/sub 2/= h/spl omega//sub 2//2e). Near
V, the negative resistance region appears for bias voltage V
just above V/sub -/, while near V the region appears for V
just below V/sub +/. This means that when an incident
frequency mixes with a cavity mode the mixing step at V/sub -/
will be inverted compared to the cavity step
itself. |
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Energy
storage and subharmonic oscillations in Josephson
junctions
D. Dempsey, M. Levinsen
and B. Ulrich
Summary: The energy stored in
the magnetic and electric field near a superconducting point
contact is typically the same magnitude as the coupling energy
which produces the Josephson effect in the weakly coupled
superconductors. These energies are usually of order one
electron volt. One consequence of energy storage in both the
electric and magnetic field is that the junction can oscillate
at a fundamental frequency /spl omega/=2eV/hn, where n is an
integer. The dynamics of these subharmonic oscillations have
been studied for a model in which the magnetic and electric
energies are represented as being stored in an inductance and
a capacitance respectively. The model was studied numerically
for various biasing conditions, and the behavior compared to
experimental data. A simple analytic approximation was
developed that gives physical insight into the mechanism that
creates the subharmonic oscillations. By use of an electrical
analog model, we demonstrated that these subharmonic
oscillations can phase lock with an externally applied
signal. |
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Electrodynamic
approach to studying processes in systems with weakly coupled
superconductors
V. Bystrov, V.
Dmitriev, V. Pavlyuk, N. Sherstyuk and O.
Tretyakov
Summary: Using the Josephson effect
in radio schemes whose compnent sizes are of about the
operatin wavelength, requires rigorous methods for studying
wave processes in weakly coupled superconductors. The analysis
of electromagneric fields in such schemes assists in
understanding their electrodynamics and getting information
necessary for instrument designing. |
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Design
of stable thin-film Josephson tunnel junctions for the
maintenance of voltage
standards
T. Pech and J.
Saint-Michel
Summary: The behaviour of
Josephson tunnel junctions is theoretically investigated in
order to determine the junction preparation parameters which
allow one to obtain a junction, which, when subjected to
microwave radiation produces a convenient constant-voltage
current step structure at high bias voltages. Major junction
characteristics, like geometrical dimensions, normal tunnel
resistance R/sub n/, critical current I/sub o/, are taken into
account, as well as various coupling conditions between
junction and microwave radiation. Stable Nb-oxide-Pb junctions
having various electrical characteristics for rather different
geometrical dimensions have been prepared and tested. The
experimental results are in good agreement with our
calculations. |
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Coupling
between Josephson junctions and
microstriplines
T. Finnegan, J.
Wilson and J. Toots
Summary: A promising
method for microwave coupling to thin-film Josephson devices
via microstripline techniques has been developed which has
significant advantages over more traditional waveguide
techniques. In particular, direct determination of intrinsic
junction cavity parameters such as the geometrical resonance
frequencies and Q values are made practical and compact
cryogenic multi-octave microwave holders readily realized. The
results of coupling experiments with Pb-Pb oxide-Pb and Nb-Nb
oxide-Pb tunnel junctions are discussed and applications of
these results to the design and construction of shielded
single junction 2e/h devices and small multi-junction arrays
are described. |
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Influence
of an external radiation on a Josephson oscillator and phase
locking effects
G. Vernet and R.
Adde
Summary: The frequency conversion signal
of a Josephson junction operating as an oscillator mixer is
not observable when the intermediate frequency is too low. To
test experimentally the possibility of phase-locking effects a
superconducting point contact oscillating at f/sub 0/ ~ 9 GHz
is irradiated with an external radiation of frequency f/sub
1/between 8 and 18 GHz. The Josephson oscillation level and
linewidth are measured as a function of the power level P/sub
1/of the external signal at f/sub 1/with /spl delta/f = |f/sub
0/-f/sub 1/|as a parameter. With large /spl delta/f values,
the variations of the Josephson oscillation and linewidth are
the consequence of the V-I characteristic modification. With
low /spl delta/f, the operating point in the VIC is in the
induced step curvature and the observed variations show that
progressive phase looking occurs. |
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Experimental
investigation on parametric excitation of plasma oscillations
in Josephson tunnel junctions
C.
Bak, B. Kofoed, N. Pedersen and K. Saermark
Summary:
Experimental evidence for subharmonic, parametric
excitation of plasma oscillations in Josephson tunnel
junctions is presented. The experiments described are
performed by measuring the microwave power necessary to switch
a Josephson tunnel junction biased in the zero voltage state
to a finite voltage state. |
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Non-Josephson
radiation from the cavity containing a superconducting point
contact junction
A. Vystavkin, V.
Gubankov, L. Kuzmin, K. Likharev, V. Migulin and V.
Semenov
Summary: The results of experimental
investigation of "non-Josephson" radiation from the X-band
cavity containing the Nb-Nb superconducting point contact
junction are discussed. Experimental dependences of the
amplitude and frequency of "non-Josephson" oscillations on the
bias voltage across the junction and modifying of the
I-V-characteristic (IVC) of the junction under action of these
oscillations are presented. The comparison of the results with
theoretical calculations is made. It is shown that the
influence of fluctuations on phenomenon characteristics is
considerable. |
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Millimeter
waveband signal conversion in S-c-S Josephson junctions with
self-pumping
R. Avakjan, A.
Vystavkin, V. Gubankov, V. Migulin and V.
Shtykov
Summary: It is known [1-5] that
nonlinear properties of the Josephson junctions can be used
for parametric amplification and conversion of oscillations in
the millimeter and submillimeter waveband region. There are
two different modes of operation in this case: (a) with
external heterodyne and (b) using the intrinsic Josephson
oscillations as a pump signal (self-pumping mode of
operations). |
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Low
frequency noise in Josephson
junctions
J. Clarke and G.
Hawkins
Summary: Using a high resolution
SQUID voltmeter, we have measured the spectrum of low
frequency voltage fluctuations across a thin-film Josephson
tunnel junction biased at a constant current I greater than
the junction critical current I/sub c/. We find that the
frequency dependence of the voltage spectrum V/sup 2/(f) may
be accurately represented by the power law V/sup 2/(f) /spl
propto/ f/sup -1/ over the frequency range of our data:10/sup
-2/ < f < 10 Hz. The dependence of the magnitude of the
spectra at any single frequency upon the value of the bias
current I and upon the sample temperature T supports our
hypothesis that the observed voltage fluctuations arise from a
modulation of the junction critical current I/sub c/by
equilibrium, thermodynamic temperature fluctuations in the
active junction volume. We are able to interpret our
measurements in terms of the semi-empirical theory of Clarke
and Voss for the low frequency fluctuation spectrum of systems
obeying a diffusion equation. This interpretation provides
design criteria which may prove useful in reducing the level
of long-term drifts in systems employing Josephson tunnel
junctions. |
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Low
frequency noise in superconducting Sn films at the transition
temperature
J. Clarke and T.
Hsiang
Summary: We have measured the noise
power spectrum of tin films evaporated on glass substrates
biased at the transition temperature. The spectrum is close to
1/f between 0.1 and 1000 Hz. The noise is found to be
correlated over a correlation length (D//spl pi/f)/sup 1/2/,
where D is the diffusivity of tin film. We have also studied
samples in which a thin layer of aluminum was predeposited
before tin evaporation. The aluminum layer appears to greatly
reduce the thermal boundary resistance between tin and the
substrate. The amplitude of noise for such samples is greatly
reduced below 10 Hz, where it is almost independent of
frequency. These results are in accord with the thermal
diffusion theory proposed by Clarke and Voss. They should have
important implications on future construction of
superconducting bolometers. |
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Noise
measurements on proximity effect
bridges
S. Decker and J.
Mercereau
Summary: Audio frequency noise
density measurements have been performed on weakly
superconducting proximity effect bridges using a cooled
transformer and room temperature low noise preamplifier. The
noise temperature of the measuring system is approximately
4/spl deg/K for a 0.9/spl Omega/ resistor. Noise density was
measured as a function of bias current and temperature for the
bridges. Excess noise above that expected from Johnson noise
for a resistor equal to the dynamic resistance of the bridges,
was observed in the region near the critical current of the
device. At high currents, compared to the critical current,
the noise density closely approaches that given by Johnson
noise. |
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Phase
slip, dissipation, Bernoulli effect, parametric capacitance,
and other curious features of the Josephson
effect
J.
Zimmerman
Summary: The details of the
Josephson effect in thin metal bridges are difficult to derive
quantitatively, and many papers have been written on the
subject attempting to describe quantitatively such qualitative
features as irreversible phase slip, instantaneous 2/spl pi/
phase slip, variation of order parameter in time and space,
Bernoulli effect, and others. It does not seem to be generally
recognized that all of these qualitative features are
universal attributes of the Josephson effect and occur also in
tunnel junctions. In addition, when a steady voltage is
impressed across a junction, a term in the electric field is
set up (in addition to the Bernoulli field) which has a rather
complex temporal and spatial variation. |
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Comments
on the roles of the cosine term and the reactive part of the
quasiparticle term in the Josephson tunneling
current
R.
Harris
Summary: A short summary of the theory
of Josephson tunneling in the presence of a time-dependent
voltage is given. It is shown that neither the reactive part
of the quasiparticle current nor the cosine term contributes
directly to the time-averaged current. However, non-zero
frequency components of these two terms can interact with a
circuit in such a way as to alter the voltage and therefore
the time-averaged current arising from the quasiparticle and
sine terms. The importance of the reactive part of the
quasiparticle and cosine terms is considered briefly at high
frequencies. |
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Extension
of the operating range of superconducting
microbridges
T. Klapwijk and J.
Mooij
Summary: We have reduced the
self-heating of microbridges by attaching the actual thin (0.1
/spl mu/m) bridge to a thick (1 /spl mu/m) film. Two different
fabricating procedures have been followed, based on scratching
with a diamond point. Contrary zo expectation, based on
previous experimental evidence, Sn microbridges as long as 0.7
/spl mu/m and 1 /spl mu/m wide were found to show microwave (9
and 35 GHz) induced steps, periodically varying with power, in
a temperature interval 0.3 < T/T/sub c/< 0.97. Good
results were also obtained with Al. These results support the
idea that previous experiments by various authors have been
considerably influenced by self-heating, not taken into
account in their interpretation of the results, and that it is
possible to use microbridges for device applications at
temperatures far below T/sub c/. |
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Dynamic
properties of variable thickness
bridges
K. Likharev and L.
Yakobson
Summary: We investigate the
time-dependent behaviour of the variable-thickness bridges
(VTB) and give expressions for the excess current, for the V
cos /spl varphi/ term and for the upper frequency boundary of
the Josephson effect. We discuss the validity of the results
obtained for different weak links: cross-scribed bridges,
proximity effect bridges and whiskers. On the basis of
previous analysis the properties of memory element QUANTRON
are studied. Then it is shown that the properties of
many-terminal bridges allow one to combine high parameters of
an externally-pumped down-converter and the simplicity of a
self-pumped down-converter. |
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High
impedance point contact Josephson
junctions
H. Tolner and C.
Andriesse
Summary: We describe point contact
junctions with normal state resistances of 10/spl Omega/ -
10k/spl Omega/, made from electropolished Nb parts, which
couple well to millimeter radiation. The response is governed
by fluctuations. The principal effect of external radiation is
a change in the dynamic resistance. An approximate expression
is given for the differential resistance at zero current,
based on a linear combination of the effect of fluctuations
and of microwave radiation. Experimental data show significant
deviations from the RSJ model. The results are in qualitative
agreement with numerical calculations by Kalashnik et al. We
argue that in our junctions zero-point fluctuations are
dominant, caused by the very small capacitance of about 10/sup
-15/F. |
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Observation
of sine modulated bessel behavior in microwave biased
SQUIDs
F. Rachford, C. Huang, M.
Nisenoff and S. Wolf
Summary: The amplitudes
of the signal lobes ("Bessel Maxima") observed in several
types of thin film SQUIDs excited at 9.2 GHz exhibit
pronounced periodic intensity modulation. This behavior is
consistent with the simplified flux entry model proposed by
Mercereau in the limit of /spl tau/ ~ T where T is the
shortest time between successive flux entry into loop and /spl
tau/ is the flux passage time. From our results, /spl tau/ was
deduced to be ~10/sup 12/ seconds for both proximity effect
and narrow constriction devices. |
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Detailed
measurements of the response of an rf SQUID in the regime
LI/sub c/ < /spl Phi//sub o//2/spl
pi/
R. Rifkin, D. Vincent, P.
Hansma and B. Deaver Jr.
Summary: Using a
superconducting ring containing a Nb point contact inductively
coupled to a resonant circuit fed with an rf current source,
(typical rf SQUID configuration) we have measured the tank
voltage as a function of frequency in the regime LI/sub c/
< /spl Phi//sub 0//2/spl pi/. For LI/sub c/ << /spl
Phi//sub 0//2/spl pi/, the curves of tank voltage versus drive
current for various values of external flux /spl Phi//sub x/
through the ring agree with the calculations of Hansma. For
higher critical currents, there is an asymmetry in these
curves for frequencies above and below /spl omega//sub 0/, the
resonant frequency of the circuit. Calculations using an
expanded theory including the effects of a conductance G/sub
o/(1+/spl alpha/ cos /spl phi/) and of screening flux give
good agreement with the data. In principle /spl alpha/ can be
determined from these measurements, however the cos /spl phi/
term and the screening flux produce qualitatively similar
effects. A quantitative determination of /spl alpha/ requires
a precise knowledge of G/sub o//I/sub c/. Using values of
G/sub o/and I/sub c/from independent impedance measurements,
the data yield a negative value of /spl alpha/ in agreement
with another experiment. |
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Enhancement
of the critical current of superconducting bridges by
microwave radiation
Y. Latyshev
and F. Nad
Summary: The phenomenon of the
enhancement of the critical current was found in
superconducting bridges when microwave radiation effected on
them. In these and followed papers the dependence of the
ciritical current on incidentpower was investigated basically
at the frequencies /spl sim/10 GHz. |
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Observation
of the Josephson effect in Nb/sub 3/Ge dayem
bridges
M. Janocko, J. Gavaler
and C. Jones
Summary: Dayem bridge Josephson
junctions have been successfully fabricated for the first time
from sputtered thin films of high-T/sub c/Nb/sub 3/Ge.
Microwave induced current steps have been observed in the IV
characteristic of one of these junctions at a frequency of 10
GHz at temperatures between 20.9 and 21.0K. This junction was
fabricated by a mechanical scribing technique instead of
sputter etching. Sputter etched junctions also exhibited
Josephson steps, but only at much lower temperatures, due to a
depression of T/sub c/by the sputter etching
process. |
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Fabrication
and Josephson properties of Nb/sub
3/Ge
R. Laibowitz, C. Tsuei, J.
Cuomo, J. Ziegler and M. Hatzakis
Summary:
Sputtering techniques have been used to fabricate
Nb/sub 3/Ge films on amorphous substrates which exhibit
superconductivity up to about 21 K. Of the many sputtering
parameters that can be varied, the most important appear to be
target geometry and composition and substrate temperature.
Initial results of analyzing the composition indicate that a
Ge-rich phase nucleates during the initial stages of the
deposition. Josephson microbridges have been fabricated from
these films and microwave-induced steps in the I-V curves have
been observed in micron-sized bridges. An interpretation of
the criticality of the bridge size in terms of a vortex state
is given. |
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Author
Index (1974)
No author
information available
Summary: Not
available |
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