R 78-885 NASA-NBS STUDY 2

THE ROLE OF SUPERCONDUCTIVITY

IN THE SPACE PROGRAM:

AN ASSESSMENT OF PRESENT

CAPABILITIES AND FUTURE POTENTIAL

PREPARED FOR NASA

BY

NATIONAL BOREAO OF STANDARDS

BOULDER. COLORADO

NBSIR 78-885 NASA NBS STUDY

THE ROLE OF SUPERCONDUCTIVITY

IN THE SPACE PROGRAM:

AN ASSESSMENT OF PRESENT

CAPABILITIES AND FUTURE POTENTIAL

Contributions by

F.R. Fickett, C.A. Hamilton, R.E. Harris, R.L. Peterson,

S.R. Stein and J.E. Zimmerman

Prepared for NASA

by

National Bureau of Standards

Boulder, Colorado 80303

NASA Contract Number A-437018(JM)

May 1978

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Edited by

D.B. Sullivan
ABSTRACT

This report describes the results of a study designed to assess the role

which superconductivity might play in the U.S. Space Program. The study was performed by members of the staff of the Boulder Laboratories of the

National Bureau of Standards. Six technical subject areas were considered;

high field magnets, magnetometers, digital electronics, high-frequency

detectors, instruments related to gravitational studies and ultra high-Q

cavities. The study identifies a number of applications of superconductivity which are of potential interest to NASA. Wherever possible, the

devices are related to specific types of space missions.

Keywords: Computers; digital electronics; gravitational studies; high-Q

cavities; infrared detectors; magnetometers; magnets; microwave detectors;

space; superconductivity.

IMPORTANT NOTICE

Certain companies and commercial equipment are identified in this report. This identification does not imply endorsement by the National Bureau of Standards nor does it

imply that the equipment identified is necessarily the best available for the purpose.

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THE ROLE OF SUPERCONDUCTIVITY IN THE SPACE PROGRAM:

AN ASSESSMENT OF PRESENT CAPABILITIES

AND FUTURE POTENTIAL

EXECUTIVE SUMMARY

This report describes the results of a study designed to assess the role which superconductivity might play in the U.S. Space Program. The study was performed by members of the

staff of the Boulder Laboratories of the National Bureau of Standards. Six technical subject

areas were considered; high field magnets, magnetometers, digital electronics, high-frequency

detectors, instruments related to gravitational studies and ultra high-Q cavities. The study

identifies a number of applications of superconductivity which are of potential interest to NASA. This summary presents some of the more prominent opportunities. The identification of an opportunity should be divorced from a suggestion for priority; the setting of priorities

for the space program is not the function of the study group.

Superconductivity has long been singled out as a unique technology, perhaps because of

the dramatic instrument improvements which are made possible by it or maybe because of the

low temperature environment which is essential to the applications. Superconducting devices

can be lumped into two general categories; those which benefit from reduced electrical losses

and those exhibiting improved signal-to-noise performance. The first category includes items

such as magnets and electrical generators. Magnetometers and high-frequency detectors are examples in the latter group. Digital electronics is a good example of an area which benefits from both categories.

There are several superconducting devices which might be considered adequate for space

applications in their present state of development. Both SQUID magnetometers and high-field

research magnets have been developed commercially with excellent success. Some work would be

needed to qualify them for space flight, but that would seem to be a straightforward task.

Other superconducting devices would require varying degrees of development before being put

on space craft. Many items in this class offer performance improvements which simply cannot

be overlooked. One example is the superconducting oscillator which is important for certain

classes of space experiments and for space navigation and ranging. The stabilization of

oscillators with high-Q superconducting cavities is established as the state-of-the-art, and

further advances are expected.

We must interject a few words of caution, lest we get trapped in the credibility problems which were common to the early years of applied superconductivity. Refrigeration of

superconducting instruments for extended space missions is a non-trivial problem. This

subject is addressed at the end of this summary. Also, not all superconducting instruments

will win their competitions with conventional devices. Superconducting high-frequency detectors look promising, but their conventional counterparts are also undergoing further improvements. It is not yet clear which devices will win out. Most likely, superconductivity will

contribute, but to what degree is yet uncertain.

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In each of the following classes, specific applications will be suggested along with the

identification of a superconducting device (or devices) which might be useful for that application. The bold type indicates a specific suggestion which the study group has identified

as possibly important for the space program.

High Field Magnets

Superconductivity offers the only economical means, from an energy standpoint, to the

production of high magnetic fields. Many space applications of such magnets can be suggested

(see Chapter II), but only two are mentioned here. The first application is to cosmic ray

analysis. Elementary particle physicists may be near their dollar limit for high energy

accelerators. Beyond colliding beams, it will probably be difficult to justify significantly

larger machines. Thus we might expect increasing interest from the physics community in the

long-term flight of particle analysis systems, since cosmic radiation consists of a variety

of highly energetic particles. The analyzers will require intense magnetic fields because of

the high energies, and superconducting magnets would appear to be the only reasonable choice.

The need for long-term exposures (to collect sufficient information) provides further impetus

to the selection of superconducting systems since power requirements for conventional magnets

would then be prohibitive.

Magnetic energy storage has been considered for specialized applications on earth but

has not received serious consideration for space flights. Present information indicates that

extremely good weight-to-energy ratios are achievable, but further study of the subject is

suggested. A key consideration for superconducting magnets in space is the weight of the

structural materials, which account for a large fraction of total system weight. Weight is

not presently an important concern of magnet designers. But modern composite technology,

properly applied to magnet structural components, could make impressive reductions in the

specific weight of large magnets.

It is suggested that mater-Lais programs, directed toward magnet weight reduction, be

stimulated by NASA in order to provide a base for optimization of the specific weight of

superconducting magnet systems.

The reduction of specific weights could provide longer term benefits to the space program.

Many of the more ambitious programs for space colonization and travel involve superconducting

magnets (e.g., fusion reactors or MHD generators for energy production, the mass-driver

system, plasma or MHD propulsion engines, etc.).

Low-Frequency Superconducting Sensors

The primary device in this class is the SQUID (Superconducting QUantum Interference

Device) magnetometer. This device has no real competition from conventional devices, either

in sensitivity or bandwidth. The SQUID has been highly developed by commercial interests, in

both magnetometer and gradiometer configurations. Despite this state of development, there

have been no serious proposals for its use in the study of magnetic fields in space. The

iv
preponderance of opinion seems to be that conventional magnetometers are adequate for measurements in space, so that the use of superconducting instruments is not justified. However, it might be pointed out that ten years ago, exactly the same view prevailed with respect to terrestrial geomagnetic measurements, yet today SQUID magnetometers are widely used

for precisely this purpose. The parallel is interesting, but substantive considerations

should prevail.

The measurements of planetary and interplanetary magnetic fields should be evaluated to determine whether the increased sensitivity and bandwidth of the SQUID magnetometer

provides information unobtainable by any other technique.

The bandwidth and sensitivity could offer a means for measuring short range (and short term)

gradients of the interplanetary fields driven by the solar wind. Chapter III describes

such applications in more detail.

Digital Applications of Superconductivity

Digital superconducting electronics is a newly emerging technology of significant promise which must, however, pass the test of real-world applications. The motivation for the

development of this technology is rooted in some very general and fundamental considerations.

The Josephson switch is extremely fast and can potentially achieve switching speeds of 1 ps, but its most attractive attribute is its small dissipation. The level of dissipation is

sufficiently small to allow extremely close packaging of active elements and thus allow

dramatic reduction of intracircuit communication delays. Furthermore, the use of superconducting striplines for this communication provides the opportunity for unparalleled freedom

from dispersion and attenuation of signals.

Interest in the technology is expanding, but the prime driving force behind this technology is a major commercial program to develop a superconducting computer. With some knowledge of the characteristics of the simple logic elements, it is relatively easy to show

that order-of-magnitude (and greater) advances can be made in both speed and capacity. The

increased speed follows from a reduction in size. Overall power dissipation drops many

orders of magnitude below that for conventional computers. All of these changes (increased

speed and capacity, decreased size and dissipation) are in a direction favorable to space

applications. The development of such a computer may be 10 to 20 years in the future, but it

is not too early to consider the opportunities, perhaps revolutionary ones, which may appear.

Given the possibility of taking really powerful computers (perhaps 10 to 100 times the

speed and capacity of today's best machines) into space, what might one accomplish? Numerous

suggestions arise. The processing and compression of complex raw data might provide a means

of conserving the limited power available for infoimation transmission. Weather modeling and

forecasting from data-gathering satellites might be feasible. Intelligent (computer) spacecraft might reduce the need for some difficult manned space missions. The Mars landing

(Viking) was completely computer-controlled because control from earth would have involved

prohibitive delays (signal transit times) between sensing of required corrections and actuation of the desired maneuvers. For more distant planets, computer control of landings, as

v
well as of rover vehicles, will be an absolute necessity. Besides the computer applications

there are a whole realm of improved instrumentation possibilities; analog-to-digital converters, digital filters, fast Fourier analyzers, etc. The opportunity to dramatically improve computer and instrument speed and capacity

should provide new possibilities for space missions.

It is suggested that NASA generate a comprehensive in-house appraisal of the new possibilities for digital electronics provided by superconductivity. The appraisal should

be made available to anyone involved in mission plans and development.

Section IV. 4.0. describes a source of information which might well provide a basis for the

appraisal suggested above. A careful appraisal of the technology would probably identify

specific areas of NASA interest. Stimulation of development, especially in the instrumentation field, would certainly speed progress toward new devices.

Microwave and Infrared Detectors

It is probably fair to state that, in general, superconducting detectors demonstrate no overwhelming advantages over conventional devices. In some instances, the performance of

the superconducting detector represents the state-of-the-art, but never by a wide margin.

For superconducting detectors, development effort has been only modest, at best. This

situation is likely to prevail for the near future.

The space applications of these detectors would probably center on radio and infrared

astronomy and on communications, areas where the best possible noise performance is desired.

Low noise performance almost certainly requires liquid helium temperatures for any type of

detector and indeed, the planned IRAS program supports this contention. Thus, the environment for superconductivity will usually exist for any low noise missions, and superconducting

detectors will be able to compete on an almost equal footing with their non-superconducting

counterparts.

The development of several superconducting detectors should be followed. The Josephson

mixer for millimeter and sub-millimeter wavelengths already performs well. The SUPARAMP

(Superconducting PARametric AMPlifier) could perhaps be operated with success in the millimeter range. The super-Schottky diode (metal layer of the diode is superconducting) with

its attractive noise performance might be extended to frequencies well above X-band. These

devices are receiving modest attention at a few laboratories in the U.S. and Europe. At

present funding levels, the questions on ultimate performance levels might well be answered

during the next five- to ten-year period.

In summary then, superconducting detectors, while offering significant advantages, do

not appear to be a panacea for the problems of all high-frequency instruments. However, the

full story is not yet told. A lot depends on how closely the superconducting and nonsuperconducting devices approach theoretical performance levels. Presently, there appears to be no really strong incentive for large increases in developmental funding levels. The major

question of ultimate performance levels should be settled during the next decade at present

vi
spending levels. However, should some of the superconducting devices (particularly the

junction devices) gain clear advantage, then it would seem essential to stimulate work

directed toward thin-film realizations of the devices, in order to satisfy the stability

requirements demanded by space flight.

Instruments for Gravitational Studies

Superconducting instruments will probably play an important role in many tests of gravitational theories. In general, these experiments require performance which is not yet available, and superconductivity is introduced as a technology which might provide the needed

advances. It is also interesting that some of the devices needed for the tests are also

useful for navigation (gyroscopes, accelerometers, stable oscillators). Thus an investment

in fundamental research could ultimately pay dividends in the form of advanced navigational

instruments

.

The gyroscope-precession experiment is an extremely sophisticated and demanding use of

superconductivity. Development is in an advanced stage, and the system may fly within the

next decade. The gyroscope stability will eclipse that of all others by several orders of

magnitude. A successful flight may lead to interest in navigational applications.

Two other instruments of special note are the superconducting accelerometer and superconducting gravimeter. The accelerometer is being developed as a sensor for gravitational- wave antennas. Projected sensitivity is impressive, and the device might also be considered

for inertial navigation. The gravimeter is an operational device which is already contributing to terrestrial research. The attractive feature of this instrument is its very low drift

level, which makes it the only gravimeter capable of low frequency measurements. It would be

especially interesting to emplace such a gravimeter on the moon or another planet.

The suggestions for experiments on gravitational theories are many, and it is difficult

to select any one for special encouragement. It might be best to let the individual re- searchers develop their proposals to a higher level before any major commitments are made.

Obviously these researchers will need minor funding for this work.

High-Q Cavities

There are many applications for superconducting high-Q cavities, but for space applications the most prominent is the use of such cavities as stabilizing elements in clocks and

oscillators. The best Q for a superconducting cavity achieved to date is 5 x 10^, and the

best stability is 3 x 10 ^. Since it is generally possible to find the center of a reso-

—18 nance line to one part per million, it is reasonable to expect that a stability of 2 x 10

will be achieved using present technology. This might be accomplished within ten years. On

the other hand, large increases in Q values will require major advances in superconducting

technology. The likelihood of it happening within the next ten years cannot be reasonably

assessed.

One intriguing potential space application of these oscillators is their use for ranging. The possibility that gravitational radiation might be detected by more sensitive

ranging to a drag-free satellite should warrant careful consideration. Ranging is important

vii
in many other contexts. For example, superconducting oscillators must surely be considered

if large improvements are needed by NASA's Deep Space Net or the Global Positioning System.

Present performance of the superconducting oscillator is impressive, but the potential for

improvement puts this system into a class of its own. Current research efforts are being directed toward application of these oscillators in

terrestrial experiments. Little effort is directed toward the improvement of Q values and of

clock performance in general. Stimulation of work toward improved performance will have to be driven by applications which require such improvement. It would seem that such improvement would be of significant benefit to NASA.

It is suggested that NASA consider stimulation of further improvements in superconduo. '.no crvities and oscillators for the purpose of improving the measurement of

distance and distance changes in space.

Refrigeration

Refrigeration requirements are without doubt the greatest impediments to broader use of

superconducting devices. For large-scale terrestrial applications, the cost of refrigeration

is a secondary consideration, although it raises the level of sophistication and causes concern about reliability. For small-scale commercial devices, refrigeration requirements

are a primary consideration. To date, most such devices have been cooled with liquid helium

in evaporative cryostats.

Space applications are even more restrictive than terrestrial ones since the reliability

problems which accompany conventional refrigerators cannot be solved by a simple call to the

repair man. For this reason the first cryogenic satellites will use evaporative helium

cooling with cryostats designed for hold times of up to one year.

The development of reliable refrigerators would greatly alter the logistics of superconductivity in space and could tip the balance in favor of some programs.

Further work on refrigerators for space flight is highly recommended. Such coolers

could be applied not only to superconducting instruments, but also to other sensitive

sensors (e.g., infrared sensors).

viii
PREFACE

The U.S. Space Program, under the guidance of NASA, has, from its very inception, been

involved with sophisticated systems which are at the very forefront of science and technology.

Indeed, NASA has often been forced to go beyond the available technology in order to meet the

challenge of its mission assignments.

Thus, it is perhaps essential that NASA keep abreast with advances in superconductivity,

a field which offers real promise for surpassing present state-of-the-art performance for

many areas, including high-field magnets, magnetometers ,' digital circuits, high-frequency

sensors, stable time and frequency references, gravimeters and accelerometers

.

Suggestions for applications of superconductivity in space have been with us for years,

and indeed NASA is actively working on several programs. However, major cryogenic satellites

are only now nearing reality, and the benefits of this new state of matter to space exploration are all in front of us. It thus seems timely to look at this technology in a comprehensive way to determine what potential applications might warrant serious consideration for

space missions.

One might argue that such a study is backwards. Shouldn't one really start with the

desired goal in space and then decide what means will be used to reach it? There is merit to the question, but the answer is simply this: One needs to know what tools are available, and

that knowledge may not be easy to obtain with emerging technologies. Are new mission possibilities developing along with a technology? Can the performance of a space task be improved with a new approach? Can significant cost savings result from a new technology?

When does a concept change from remote possibility to real opportunity? For example, does a ten-year-old assessment of high-field magnets in space have meaning in light of today's state

of the technology? These types of questions suggest that it is prudent to look through one's

bag of tools periodically to find out whether new tools are available. Although one might do

this in a number of ways, this study, done outside NASA, is one reasonable approach. Only

time will permit a measurement of its success. The NBS study group consisted of the editor and the six NBS co-authors of this report.

The study was divided into six technical sections, each relating to a specific type of instrument or instruments and developed by a single member of the group. The study group

sought to present the state-of-the-art in each of the six areas and where possible to identify specific space objectives for the technology. Risky as it is, the group has also

tried to project performance into the future. Such projections are "educated guesses" and

should be treated as such. However, it would be absurd to assume that this study covers the

subject area in a fully comprehensive manner. The emphasis of the report reflects to some degree our individual biases and speculations about future developments. It is hoped, however, that the effort provides a reasonable perspective and a starting point for in-depth

studies of subject areas which might prove of interest to the space program.

The study was sponsored by NASA's Office of Aeronautics and Space Technology and guided

from NASA Headquarters by Mr. Stanley Sadin and from Ames Research Center by Mr. John

Vorreiter. The NBS study group would like to thank Mr. Vorreiter and Mr. Sadin for their

guidance in this effort. We also thank Mrs. Norma Lear for turning our scratchings into a

ix
presentable form and Mr. Nick Sanchez for his work on the graphs and illustrations. Needless

to say, we have received input from many specialists in the scientific community both within

and outside NASA and NBS. The list of such people is too long to include here, but we wish

to express our deepest gratitude for their time and thought, which have added greatly to this

report.
TABLE OF CONTENTS

Page

EXECUTIVE SUMMARY ±±± PREFACE ix

CHAPTER I - INTRODUCTION 1

I. 1.0. Fields of Application 1

I. 2.0. Background 2 CHAPTER II - HIGH FIELD SUPERCONDUCTING MAGNETS 5 II. 1.0. Introduction 7

II. 2.0. General Magnet Applications 7

II. 3.0. Magnets for Space Applications: A General Discussion 8

II. 4.0. Proposals for Space Applications of Superconducting Magnets 15

II. 5.0. Summary and Conclusions 24

II. 6.0. References 26

CHAPTER III - LOW FREQUENCY SUPERCONDUCTING SENSORS 27 III. 1.0. Introduction 29 III. 2.0. The SQUID: Magnetic Field Measurements 31

III. 3.0. Galvanometers, Voltage and Current Sensors, and Wideband Amplifiers 42

III. 4.0. Magnetic Shielding 43

III. 5.0. DC Transformer 43

III. 6.0. Summary and Conclusions 44

III. 7.0. References 45

CHAPTER IV - DIGITAL APPLICATIONS OF SUPERCONDUCTIVITY 47

IV. 1.0. Introduction 49

IV. 2.0. The Superconducting Computer 60

IV. 3.0. Digital Superconducting Instruments 64

IV. 4.0. Source of Additional Information 70

IV. 5.0. Summary and Conclusions 70

IV. 6.0. References 72 CHAPTER V - MICROWAVE AND INFRARED DETECTORS 73

V. 1.0. Introduction 75 V.2.0. Bolometers 76 V.3.0. Super-Schottky Diodes 79 V.4.0. Josephson Devices 81

V.5.0. Non-Superconducting Mixers 88

V.6.0. Josephson Parametric Amplifiers 88

V.7.0. Summary and Conclusions 90

V.8.0. References 93

xi
TABLE OF CONTENTS, cont.

CHAPTER VI - INSTRUMENTS FOR GRAVITATIONAL STUDIES 95

VI. 1.0. Introduction 97 VI. 2.0. Instrumentation 98

VI. 3.0. Tests of Gravitational Theories 104

VI. 4.0. Gravity and Properties of the Planets 110

VI . 5 . 0 . Navigation 110

VI. 6.0. Summary and Conclusions . Ill

VI. 7.0. References 113

CHAPTER VII - ULTRA HIGH-Q CAVITIES 115 VII. 1.0. Introduction 117

VII. 2.0. Superconducting Cavities 118

VII. 3.0. Applications 126

VII. 4.0. Summary and Conclusions 133

VII. 5.0. References 134

APPENDICES 135

I. A. Comments on Cryogenics in Space 137

II. A. Terrestrial Applications of Superconducting Magnets 140

II. B. Current Applications of Superconducting Magnets in Space 147

IV. A. Josephson Tunneling Logic 150

IV. B. Josephson Fabrication Technology 155

References 157

xii
CHAPTER I

INTRODUCTION

The first fifty years following the discovery of superconductivity in 1911 were marked

by intense academic endeavors to understand this new state of matter. Little of practical

interest evolved until the late fifties and early sixties when, in very rapid succession,

high field superconductors were discovered, a microscopic theory was developed and the

Josephson effect was predicted and observed. This flurry of activity marked the birth of

applied superconductivity, a new technology which is just now beginning to provide major new

scientific tools.

Applications of superconductivity have spanned a wide range of technical areas; geophysics, high-energy physics, materials science and medicine, to name a few. In each application it is the capability for vastly improved performance or the ability to perform tasks

which were not previously feasible, which is the incentive for the use of superconductivity.

Many of the technical problems which have found solutions in superconductivity are problems

that also exist in the space environment outside this planet. In addition, space exploration

poses new sets of problems , some of which might be solved through utilization of superconducting devices . This report reviews the applications of superconductivity which might be

useful in the varied and challenging tasks considered for space missions.

In the first portion of this introductory chapter we provide an outline of the study to help the reader locate material which may be of particular interest. We follow with some general comments on superconductivity which might be useful to the uninitiated reader.

Finally, we offer a note on refrigeration, an essential consideration for any superconductive device.

I. 1.0. FIELDS OF APPLICATION

The study has been organized so as to best use the talents of the members of the study

group. Thus, division is made between classes of devices rather than fields of application.

This is not the most effective way to present information to a reader who is interested in,

for instance, properties of interplanetary space. To help remedy this problem we present

here a "roadmap" for those oriented toward a particular field, rather than a device. The

four major categories are subdivided, and references are provided to the six technical chapters which follow.

CROSS-REFERENCE GUIDE — APPLICATION VS DEVICE

Physical Properties of Space and the Planets

Radio and Infrared Astronomy - Chapter V (All) - Chapter VTI (Sec. VII. 3.0.)

Elastic and Vibrational Properties of Planets - Chapter VI (Sec. VT.4.0)

Planetary and Interplanetary Magnetic Fields - Chapter III (Sec. III. 2. 2) Background Temperature of the Universe - Chapter V (All)
Fundamental Questions in Physics

Gravitational Theories - Chapter VI (Sec. VI. 3.0) - Chapter VII (Sec. VII. 3. 4)

Variation in Fundamental Constants - Chapter VI (Sec. VI. 3.5) - Chapter VII (Sec. VII. 3. 4)

Elementary Particle Physics - Chapter II (Sec. II. 4.1)

Information

Data Acquisition - Chapter IV (Sec. IV. 3.0)

Filtering - Chapter IV (Sec. IV. 3. 3) - Chapter VII (Sec. VII. 3. 2)

Computations and Analysis - Chapter IV (Sec. IV. 2.0)

Communications - Chapter V (All)

Logistics of Space Travel and Living

Propulsion - Chapter II (Sec. II. 4. 7)

Shielding - Chapter II (Sec. II. 4. 2) - Chapter III (Sec. III. 4.0)

Re-Entry - Chapter II (Sec. II. 4. 8)

Navigation - Chapter VI (Sec. VI. 2. 5 and VI. 5.0) - Chapter VII (Sec. VII. 3. 4)

Levitated Transport - Chapter II (Sec. II. 4. 6)

Energy Storage and Conversion - Chapter II (Sec. II. 4. 3, II. 4. 4 and II. 4. 5)

1.2.0. BACKGROUND

1.2.1. Superconductivity

For some purposes, it is sufficient to say that a superconductor is a material which

exhibits no electrical resistance below given limits of temperature, magnetic field and

current density. While this might suffice for a discussion of magnet applications, for

example, it is completely inadequate for the other applications considered in this study.

Although this is not the place for a treatise on superconductivity, a brief discussion of some fundamental considerations will draw together the diverse phenomena considered in this

report. An understanding of these fundamentals is not essential for a reading of the study,

but some may prefer to see the thread of commonality in the applications.

Superconductivity is a unique state of matter characterized by an attractive interaction between pairs of electrons. This attractive force is weak and of negligible importance at room temperature, but it becomes very important at low temperatures where the energy

associated with the attraction is comparable to or greater than the energy of thermal vibrations. In this case, the electrons are able to re-order themselves into a state (the

superconducting state) in which they are bound together, a state lower in energy than the

normal state. To excite the system from this state requires the input of a minimum energy A, which is called the energy gap. The superconductor can be described by a mathematical function which is wave-like in character. This wave-like model is important for an understanding

of the Josephson effect and quantum interference.

Perfect conductivity results from this description in the following way. If a current

is passed through a superconductor, then the electrons all attain some added velocity along

the direction of current flow. In a normal conductor, the lattice vibrations and imperfections would result in scattering of some of the electrons and a consequent loss of energy.

2
The normal state thus requires an electric field to sustain a current, ancl energy (voltage

times current) is continuously dissipated. In the superconducting state, the scattering of a single electron is disruptive of the entire bound state and is therefore not favorable (from

the standpoint of energy conservation) . The wave function is said to be coherent (rigid)

,

since any external stimulus must affect the entire body of electrons. Only when the current

exceeds some maximum value, the critical current, will scattering be possible. This occurs when the energy in the driving current overcomes the scattering restriction mentioned above.

At this point, the superconductor reverts to the normal conducting state. It is important to note that no electric field is needed to support the flow of a supercurrent

.

The quantization of magnetic flux in a superconductor also follows from the introduction

of this rigid wave function. For a superconducting ring (a donut) the wave function which

describes the state of the system must be continuous and close on itself in going around the

ring. This means that if there are spatial oscillations in the function, only integral

numbers of oscillations are permitted. These different but distinct wave functions are associated with discrete currents and thus with discrete flux states of the ring. This is

analogous to the requirement which gives rise to discrete energy levels in the Bohr atom. Each atomic orbit is associated with a wave function which has an integral number of spatial

oscillations about the orbit. As with the Bohr atom, the description of flux quantization

requires the consideration of wave-like behavior.

The device which depends on flux quantization is the SQUID (Superconducting QUantum

Interference Device) . The SQUID is a superconducting ring interrupted by a weak connection,

a Josephson junction. This weak connection provides a means for changing the flux state of

the ring and for observing such changes in state. The SQUID is, in essence, a very sensitive

magnetometer

.

A Josephson junction results from any very weak connection between two pieces of superconductor. It may take the form of a tunneling junction, a point contact, or a very narrow

constriction in a superconducting film. The properties of this junction are derived from an interaction between the wave functions in the two pieces of superconductor which are linked.

Current flow through the junction results whenever there is a difference in phase between the wave functions on either side of the junction. A voltage impressed across the junction

causes this phase difference to shift in time, resulting in an oscillatory current flow as the wave functions shift into and out of phase. The high frequency of these oscillations

make the Josephson junction useful for certain microwave and infrared applications.

The Josephson junction is also valuable as a fast logic element (a controllable switch).

The switch is controlled by a weak magnetic field. This field is generated by a current in

a simple control line which crosses the junction. Once induced to switch, the junction

response is controlled by the intrinsic frequency limitations of the Josephson effect. This

limit is set by the value of the energy gap in the superconductors which make up the junc12

tion. This frequency limit is approximately 10 Hz, which corresponds to switching times

-12

of the order of one picosecond (10 s)

.

The diverse phenomena of the superconductive state are seen to result from a rather

small set of underlying principles. This type of approach is perhaps more satisfying than

3
the introduction of a separate explanation for each successive phenomenon which is discussed.

The phenomena associated with the superconductivity are unique and extremely useful, as will

be seen in the text.

1.2.2. Benefits of a Low-Temperature Environment

Unless major materials breakthroughs are achieved, superconductivity will remain a low

temperature phenomenon. However, one should recognize that the low-temperature environment

needed for superconductivity is useful for other reasons as well. For example, the low

thermal noise associated with low temperatures is crucial for infrared astronomy. In fact

the IRAS (Infrared Astronomy Satellite) program has been forced to provide low temperatures

(2 kelvin) in order to achieve low thermal noise.

Besides low thermal noise and superconductivity, other benefits of low temperatures can

be cited. At liquid-helium temperature the expansion coefficient of solids becomes negligible, and this results in extremely good dimensional stability. Thermoelectric phenomena

are reduced in significance, yielding benefits such as elimination of unwanted thermal emf's.

Ionic conduction ceases to cause leakage problems. Failure mechanisms for electronic devices

such as diffusion and electromigration become relatively unimportant. Where heat must be

dissipated, superfluid helium provides almost complete freedom from convective limitations

because of its enormous "effective" thermal conductivity.

Some of these advantages are important to the applications discussed in the study. The

stability of the superconducting cavity stabilized oscillator (Chapter VI) is at least partly

due to the dimensional stability of the cavity. Once in operation, a superconducting com- puter or other digital instrument should be free from failure mechanisms which plague con- ventional systems.

1.2.3. Refrigeration

Refrigeration requirements are without doubt the greatest impediments to broader use of

superconducting devices. For large-scale terrestrial applications, the cost of refrigeration

is a secondary consideration, although it raises the level of sophistication and causes concern about reliability. For small-scale terrestrial devices, refrigeration requirements

are a primary consideration. To date, most such devices have been cooled with liquid helium

in evaporative cryostats.

Space applications are even more restrictive than terrestrial ones since the reliability

problems which accompany conventional refrigerators cannot be solved by a simple call to the

repair man. For this reason the first cryogenic satellites will use evaporative helium

cooling with cryostats designed for hold times of up to one year.

The development of reliable refrigerators would greatly alter the logistics of superconductivity in space and could tip the balance in favor of some programs. A discourse on refrigeration is beyond the scope of this study, but some comments are in order. For space

applications it is particularly interesting to consider use of the low background temperature

of space for radiative cooling. Appendix I. A offers some of our speculations on the refrigeration problem and specifically dwells on the potential of radiative cooling.

4
CHAPTER II

HIGH FIELD SUPERCONDUCTING MAGNETS

F. R. Fickett

II. 1.0. INTRODUCTION. 7 II. 2.0. GENERAL MAGNET APPLICATIONS 7 II. 3.0. MAGNETS FOR SPACE APPLICATIONS: A GENERAL DISCUSSION 8

11. 3.1. Superconducting Magnet Systems 9 11. 3. 2. Advantages of Superconducting Magnet Systems 12

11. 3. 3. Disadvantages of Superconducting Magnet Systems 12

11. 3.4. New Developments 13

11. 3. 5. Other Considerations 14

II. 4.0. PROPOSALS FOR SPACE APPLICATIONS OF SUPERCONDUCTING MAGNETS 15

11. 4.1. Cosmic Ray Analysis 16

11. 4. 2. Shielding of Spacecraft 16

11. 4. 3. Energy Storage 17

11. 4.4. Energy Conversion - Motors and Generators 19

11. 4. 5. Energy Generation 20

11. 4. 6. Levitated Transport 21

11. 4. 7. Propulsion 22

11. 4. 8. Re-entry 24

II. 5.0. SUMMARY AND CONCLUSIONS 24

II. 6.0. REFERENCES 26

5

II. 1.0. INTRODUCTION

In the mid-1960' s NASA, and DOD sponsored a great deal of work to evaluate the possibilities for applying the relatively new science of high-field superconductivity to problems

related to space flight. Many interesting and, often, impractical suggestions resulted.

Much of the impracticality occurred because the technological experience with large superconducting magnets was limited and, furthermore, the superconducting materials available were very crude by present standards. By the time the modern generation of superconducting wires

and magnets came to be, the large NASA programs were no longer in existence, and many of the

ideas had effectively been forgotten. Here, we want to take a fresh look at some of those

ideas, as well as evaluate the overall role that superconducting magnets might play in the

space adventures of the future as they now appear to be developing.

In this chapter we will look at present applications of superconducting magnets, discuss

the advantages and disadvantages of these devices, and attempt to assess possible future developments and their impact on ' applications in aerospace technology. Along the way we will

take a look at the state-of-the-art and discuss research which, we feel, will help us to realize the full potential of these devices. A good bit of what we have to say will deal

with terrestrial applications for two quite simple reasons: first, very few superconducting

magnets have ever flown anywhere, and; second, the applications, construction techniques and

general magnet configurations will not be greatly different regardless of where the magnet is

located.

II. 2.0. GENERAL MAGNET APPLICATIONS

When one looks at magnet applications in a general way, the actual functions in technological applications are quite few in number. In one way or another they all make use of the

ability of magnetic fields to interact with charged particles which are in motion relative to the field. One classification scheme is given in the following list along with one representative application for each class:

1. Deflection, focusing or confinement of charged particles (high energy physics)

;

2. Introduction of energy into systems of charged particles (plasma heating for fusion

devices)

;

3. Deflection of non-magnetic metals (levitated transport);

4. Deflection, suspension or steering of magnetic objects (steering of magnettipped catheters through the brain)

;

5. Inductive energy storage (power plant peak shaving)

;

6. Conversion of energy (motors and generators) ; and

7. Control of chemical reactions (magnetocatalytic effects)

.

7
While normal magnets* are useful in each of these classes, the higher fields and smaller mass

and physical size of superconducting magnets make them quite attractive in all cases. This

is particularly so in aerospace applications where the relatively small mass becomes of prime

importance

.

Large numbers of both large and small superconducting magnets have been constructed in

recent years and used in an amazing variety of applications ranging from high-energy physics

to medicine. In Appendix II. A we describe many of these applications. The magnet systems are in an advanced state of development. We also give a few brief comments on the' current status

and future prospects of each application where appropriate. Obviously this latter assessment

is the authors' alone. A detailed description of many large-scale applications is given in

the book edited by Foner and Schwartz (1974) and in the Proceedings of the Applied Superconductivity conferences and the Cryogenic Engineering conferences. The book also has a number

of chapters describing the worldwide research efforts in large-scale applications as of 1972. In general the trend in research in this field has been to less funding and smaller projects

since then. The exception is the large program to develop the use of controlled fusion of

light nuclei as a power plant concept.

II. 3.0. MAGNETS FOR SPACE APPLICATIONS: A GENERAL DISCUSSION

In Section II. 4.0. we discuss a large number of specific applications which have been

proposed for superconducting magnets in space. Here we outline very briefly some of the

general advantages and disadvantages of these magnet systems. The technology is advancing

quite rapidly, but little attention has been paid to optimizing the systems for space flight

for the simple reason that very little development money has come from NASA in recent years.

The high energy physics laboratories have been the major users, and their requirements are for

higher fields, larger field volumes, reduced energy consumption and short- and long-term

reliability. The resulting magnets (see Appendix II. A), thus tend to be heavy and large. This

is not necessary. If one is willing to make some modest sacrifices in these requirements,

light and small magnets become quite feasible. Research is needed to determine the best ways

to handle the tradeoffs, but no insurmountable problems appear to exist and specific weights

of ^ 25 kilograms per megajoule of stored energy seem to be attainable for large magnets.

As a quick indication of where superconducting magnet technology is now, consider the

following list of parameters for existing magnets and materials:

Largest superconducting magnet - 4.7 m-bore bubble-chamber magnet at CERN, 3.5 T field,

3 coil weight 166 x 10 kg, 830 MJ stored energy

Highest continuous field superconducting magnet - 17.5 T, 3.1 cm bore lab magnet of Nb^Sn

and V Ga by Intermagnetics General Co.

The term "normal magnets" refers to conventional systems which usually have liquid-cooled

copper coils and most often contain large amounts of iron and/or steel.

8
Highest continuous field hybrid (superconducting outer coil, copper inner coil) magnet - 30.1 T, 3.2 cm bore lab magnet by the National Magnet Laboratory at MIT

Highest high field superconductor current density - 10

5 A/cm2 at 20 T in V^Ga, ^ 10

& A/cm2

in Nb^Sn at lower fields

4 2 Typical current density m a superconducting composite wire - 5 x 10 A/cm at 5 T for

2 both NbTi and Nb^Sn (vs 100 A/cm for copper)

.

Clearly, very good, dependable superconducting magnets exist.

II. 3.1. Superconducting Magnet Systems

A superconducting magnet system consists of: a magnet; a container for the magnet (a "dewar" or "cryostat") in which it is thermally isolated from its surroundings and cooled to ^ 4 K; a method of cooling, always a fluid in contact with the magnet, usually liquid helium;

and a power source for starting ("charging") the magnet. The last three of these are described in more detail in the following two subsections as they each represent either an advantage or a disadvantage in the use of superconductors over conventional magnets. Here we wish to briefly describe the parts of the magnet itself. The discussion applies to magnets

of any size or shape, and there are many.

In its simplest form a superconducting magnet is a spool of superconducting wire wound,

with appropriate insulation, on a bobbin of structural material and, frequently, banded on

the outside with more structural material to contain the stresses (hoop stresses) which tend

to expand the coil when it is energized. Smaller magnets and pulsed magnets often are potted

in an epoxy of some sort to prevent wire motion. Larger magnets tend to have a more open

structure to allow flow of coolant throughout the winding for improved heat transfer.

The superconducting material, which is only part of the wire, has the unique property of

exhibiting zero resistance to the flow of direct current, but only below a certain critical

temperature, (T^) , critical magnetic field (H^) and critical current density (J^) . This is

illustrated graphically in Figure 1. Below the surface the material is superconducting,

above it is "normal", a state in which its electrical resistance is much higher than that of

copper. While it is certainly possible that a room temperature superconductor may some day

appear, it is best not to wait for it. All superconductors discovered to date have T^ < 25 K

and, thus, require large amounts of refrigeration, usually at 4 K, for practical applications.

Only two groups of superconducting materials seem to offer any hope for large scale

magnet applications in the near future: the ductile NbTi alloys, which are now used extensively, and the brittle A15 compounds, most likely Nb^Sn or, possibly V^Ga. (In Section

II. 3.4. we discuss the possibilities of other materials in the more distant future.)

Table II-l lists some of the properties of these superconductors. The physics of the various

critical field designations (H , H H ,, H ,) is covered in detail in many texts. We have

c cl c2 c3

included elemental niobium in the table because it is extensively used in cryoelectronic

devices and in superconducting powerline designs — it obviously cannot be used in high

magnetic fields.

9
Figure II-l. The critical parameter surface for a superconductor. Under the surface,

the material is superconducting, above it is a normal (usually poor) conductor.

TABLE II-l

NOMINAL PROPERTIES OF PRACTICAL SUPERCONDUCTORS

Material Crystal T y H „ a Max B J @ 4.2 K

c o c2 c Structure (in use) @ B=4T @ B=10T @ B=17T

2 2 2

(K) (T) (T) (A/cm ) (A/cm ) (A/cm )

5 4 NbTi bcc 10.2 12 8 15 x 10 2 x 10 0 45-48 wt.

% Ti

Nb

3 Sn A15 18.3 22 16 20 x 10

5

2 x 10

5

2 x 10

4

V

3 Ga A15 16.5 22 20 6 x 10

4

6 x 10

4

5 x 10

4

Nb Rhomb. 9.5 0.25 — 0b

0 0

-7 -1 a y = 4tt x 10 T/Am , data at 4.2 K. ° 5 b 7 x 10 at B = 0.

10
When used in essentially ac applications, such as pulsed magnets, superconductors do

show a complex resistance, but it is still many times lower than that of the conventional

normal metal conductors.

An actual commercial superconducting wire is a complex composite usually consisting of

many fine (2-50 urn) continuous strands of superconductor embedded in a pure copper matrix

and traversing the length of the wire, frequently with a twist pitch to the entire filament

bundle. All of these aspects (pure metal matrix, fine filaments and twist) are necessary to provide stability in operation, i.e., to keep all of the superconductor in the superconducting state. In addition, if the wire is to be used in rapidly-changing fields, a high resistivity sheath, usually of copper-nickel is added to break up transverse eddy current paths in

the wire. Furthermore, for reasons of manufacture, tin bronze is often an essential com- ponent of Nb^Sn wires. In this case a tantalum barrier is used between the bronze-superconductor region and the pure copper to keep the tin from migrating into the stabilizer and

increasing its resistivity. Truly, the practical superconductor is a very complex composite.

As you might suspect, it is also a very expensive one I Several dollars per meter is not

unheard of for relatively small diameter (3 mm) Nb^Sn wire. In commercial production, NbTi superconductor is made by drawing down a copper billet

into which rods of the alloy have been inserted. Aluminum is sometimes used in place of

copper. This technique works well, primarily because the NbTi alloys are quite ductile, and

results in a relatively inexpensive wire (^ $0.60/m). Furthermore, in magnet applications

the wire can take quite large strains (> 5%) without destruction of the superconductivity.

Unfortunately NbTi is not acceptable in all applications because of the restriction of its use to maximum magnetic fields of about 8 T, and the more complex Nb^Sn wire becomes necessary. Very little Nb^Sn is actually in use at present.

In very large magnets, stainless steel ribbon or wire is often either co-wound with the

conductor or is bonded to it during manufacture in order to provide additional strengthening

and, thus, decrease the total strain seen by the conductor. This is especially necessary for

the strain-sensitive Nb^Sn conductors.

The amount of structural material necessary to hold the magnet together against the very

large stresses created by the field increases dramatically with the size of the magnet and,

in very large systems, the structure may well represent more than 80% of the total mass. In general this mass must also be cooled to the same temperature as the magnet. The materials

most often chosen for this task are the austenitic (nonmagnetic) stainless steels. There is a great deal of potential here for significant weight reduction by the use of various com- posite materials. We at NBS are currently evaluating the low temperature properties of a number of specially formulated composites for such applications. An estimate of the absolute

minimum total mass, M, of structure required to confine an amount of stored energy, E, in a simple magnet is given by

M = — E, (II-l)

a

where d is the density of the structural material and a is its allowable stress level. As an

9 2 example, titanium stressed to 1.6 x 10 N/m gives d/a = 2.9 kg/MJ (MJ = megajoule)

.

11
II. 3.2. Advantages of Superconducting Magnet Systems

In aerospace applications conventional magnets with water-cooled copper coils and large

volumes of solid iron consuming megawatts of power are not really feasible at all. Thus, if

large field volumes are needed, superconducting magnets appear to offer the only option. For

small fields and field volumes, of course, conventional magnets or permanent magnets are still the choice.

4 2 Superconductors can carry very large current densities, on the order of 10 A/cm

2 2 overall in a magnet. Compare this to ^ 10 A/cm for a copper coil magnet. This number

determines the amount of winding required to obtain a given field in a fixed volume. Furthermore, the density of the superconducting wire is somewhat lower.

Except for very exotic systems, conventional magnets are limited in the field they can produce to about 2.5 T, the saturation field for iron. Superconductors are capable of producing fields to 15 T or higher. At these fields iron is superfluous. Even at lower fields,

the lack of a requirement for iron has many advantages in machinery applications, allowing more insulation and, thus, higher operating voltages. When large field volumes are necessary, the ability of the superconductor to sustain high fields at the windings means that

modest field values (-v 3 T) can be maintained over volumes of tens of cubic meters, a feat

which is impossible by any other means.

The electrical system necessary to operate large superconducting magnets is very simple,

usually a low voltage (10-100 V) , high current supply (an automotive battery bank is often

used for small magnets) . Once the magnet is energized, a superconducting short, called a persistence switch, can be made across the leads and the power supply removed. The magnet

current will continue to circulate and maintain the field as long as the coil is kept cold.

Conventional magnets often require megawatts of continuous power.

II. 3. 3 Disadvantages of Superconducting Magnet Systems

Nearly all of the potential disadvantages of superconducting magnets for space applications are of a type which can be taken care of by serious investigation and good engineering.

Most of these disadvantages exist today because of the direction which the field has taken

historically, i.e., toward large laboratory systems.

The magnets require cooling to near 4 K. At present this is usually accomplished by

filling the magnet container with liquid helium. If continuous cooling is necessary, it is

provided by refrigerators and liquifiers which only now are becoming highly reliable. Re- moval of 1 W at 4 K requires a refrigerator input power of ^ 500 W at room temperature. In space, radiation of the heat removed could be a serious problem. Refrigerators are large and

heavy, although the entire system weight (refrigerator + power supply + magnet) is still far

below that of a normal magnet system.

A topic which needs investigation is the possibility of operating superconducting magnets in space without providing continuous cooling in the form of liquid helium. It seems possible that, at least beyond earth orbit and with adequate shielding, such operation might

be feasible, particularly if the superconducting materials have critical temperatures in ex- cess of 20 K.

12
The container, or dewar, must completely enclose the magnet and must keep the coolant

for as long a period as possible. Liquid helium costs ^ $5/1. Thus, it behooves one to allow a minimum heat leak into the system. In terrestrial applications the helium is usually

contained in a vacuum-insulated vessel which is, in turn, surrounded by liquid nitrogen

$0.2/1), itself contained in a vacuum vessel. The liquids, the four metal walls and the

standoffs represent a significant weight, but helium boiloff is low, 1-2%/day. Dewars insulated with many layers of aluminized mylar remove the necessity for liquid nitrogen and remove two of the walls, but at the cost of somewhat increased boiloff, although recent

designs have holding times much the same as the nitrogen shielded ones. In any event, the

dewar structure makes access to the magnet occasionally less convenient than in a conventional

system, but this is usually more than compensated for by the larger free bore and field of

the superconducting magnet.

Superconducting magnets contain large amounts of copper (or aluminum) to protect the

superconductor against momentary transitions out of the resistanceless state. This stabilizes

the magnet against transitions induced by fluctuations , and also prevents catastrophic destruction of the energized magnet (or magnet system) when it reverts to the normal state. This stabilization greatly increases the conductor weight and decreases the overall current

density. As the amount of copper is decreased, the magnet becomes more susceptible to a total loss of superconductivity (a "quench") upon excessive shock loading, vibration, field

changes or thermal excursions. This topic is one on which much research is presently

underway.

The very high-field superconductors (Nb^Sn and V^Ga) are brittle intermetallic compounds

and wires containing them are quite susceptible to degradation of their properties at strains ^ 0.6%. This increases the requirement for structural material and increases the weight.

Again, strain- insensitive superconductors can most probably be made, but very little cash is

available for the necessary research and development. For this reason few of the existing

magnets described in Appendix II-A use these materials, concentrating instead on the ductile

alloy NbTi which has a significantly lower critical field.

II. 3.4. New Developments

The improvement of commercially available superconducting wire is occurring quite

rapidly. The production of multifilamentary Nb^Sn conductor on a commercial scale is almost

a reality today. This conductor will offer higher fields and faster charging times as well

as decreased overall weight.

There are many other superconductors about which much has been published. v

3 Ga has

actually been deposited on a tape conductor and used in a magnet. Reports of multifilamentary wire have been made. This material offers higher critical currents than Nb^Sn at very high fields, 18 T. It appears to be the only serious competitor to Nb^Sn in a com- mercial sense. Other materials such as Nb^Ge (T = 23.2 K) ; V„Hf (H = 20 T at 4 K) ,- 3 c 2 c2

Nb_ (Al,Ge) , (H = 41 T at 4 K) ; ternary molybdenum sulfides (H _ ^ 60 T) and a host of

3 c2 cz others are being actively investigated but, to date, have proven to be very difficult or impossible to fabricate in a usable form. If significant improvements in critical current

density at high fields can be achieved with these materials, then their development might

well gain a very high priority. Not enough is known at present to evaluate this possibility.

13
The use of high-strength nonmetallic composites for constraining large, cold magnet

structures and dewars offers the possibility of great improvements in both total mass and

refrigeration requirements, since the strength-to-weight ratio is large, as is the strengthto- thermal conductivity ratio. Also, the fact that rapidly-changing magnetic fields do not cause eddy current heating in the cold structure will further reduce the refrigeration needs

in pulsed systems.

The problem of the cause of training of superconducting magnets should be solved soon. This effect manifests itself as a repeated quenching of the magnet at higher and higher

fields when it is first energized. It is not usually a problem with simple solenoids but

always is with more complex magnets. It probably results from wire motion within the winding

or, less likely, from motion of the superconducting filaments within the wire.

A comment is needed regarding the possibility of a high- temperature superconductor.

Several, predominantly organic, systems have been suggested as possibly superconducting at near room temperature and a lot of basic physics money has gone into investigating them. It seems highly unlikely that any such system will be found which oould be put into commercial

production within the forseeable future. There is, however, the strong possibility of developing superconductors, such as Nb^Ge, to the point where they will be able to maintain

high fields at liquid hydrogen temperatures 20 K) . Use of such, as yet undeveloped,

materials would greatly improve the economics of the magnet systems, but would not greatly

alter their complexity.

II. 3.5. Other Considerations

There are several problems related to space applications of large magnets that are

unique. Again, none appear to be really serious, but they are deserving of special mention.

The effects on humans of magnetic fields of an intensity other than that found on the

earth's surface are not known. This is true both for high-field intensities and for fields near to zero. A certain amount of work has already been done by NASA, DoD and by scientists

in the USSR. Arbitrary field limits have been set for human exposure. The overall conclusion at present seems to be that no serious long term effects occur at any field. However,

there is enough disturbing information from experiments on mice and lower life forms, that more definitive research is probably needed. One concern often expressed is that a significant fraction of the population may be sensitive to magnetic fields, but no one from that

population has ever been tested or even accidentally exposed. The situation was reviewed

recently in some detail by Brechna (1975)

.

The effect of stray magnetic fields on the spacecraft instrumentation and guidance

systems is also a major concern. Some examples from NASA (Golden, 1975) : unshielded photo

tubes are affected at ^ 30 yT, ferrite cores at 0.01 T; relays and solenoids stick at 0.3-0.5 T. At present, the maximum allowable stray field is proposed to be ^ 0.01 T for

spacecraft. Passive shielding of relatively high-field magnets to this level is possible but

may involve large amounts of material. Active shielding by appropriately-placed small superconducting coils is probably a better alternative at higher fields.

14
Another field interaction of concern is that between an on-board magnet and the planetary and solar magnetic fields. This interaction could turn the ship into the world's largest

compass — an undesirable situation in general. An unshielded coil of^0.7mIDx0.1m

long operating at 7 T max field has been calculated to be subject to a torque of ^ 70 Nm in

earth orbit (Pope, Smoot, Smith and Taylor, 1975) . They claim that this must be reduced to

0.07 Nm in order to be acceptable. In their design a second shielding coil is used to null

out the stray fields.

The near zero-g environment of space allows the liquid helium most often used as a coolant to behave in ways which might not be anticipated. Any small g-force created by

rotation of the ship will, of course, move the liquid around and that has created some problems in situations where the liquid enters the fill tube. The IRAS (InfraRed Astronomical

Satellite) system will use a sintered nickel porous plug to resolve this problem. A more

exotic effect involves the magnetic forces on the liquid helium. Helium is diamagnetic and

is thus subject to forces in a field gradient which tend to move it away from the high-field

region. For the magnet just described, these forces can be as high as that produced by 1/3

of the acceleration of gravity. Proper cryostat design, the use of supercritical or superfluid helium and a (heavy) sponge or wick system are all possible solutions depending on how

serious the problem really is.

II. 4.0 PROPOSALS FOR SPACE APPLICATIONS OF SUPERCONDUCTING MAGNETS

Over the last fifteen years, a great many proposals have been brought forth outlining

the potential for use of high magnetic fields over a broad spectrum of aerospace applications. Most of the proposals were good, and the technology of magnet systems was steadily

advancing. But the dollars for advanced space applications were declining, and many of the

ideas faded into obscurity. Here we collect most of these and give a brief description of

the magnet system thought to be required. We cannot assess the feasibility or the desirability

of the proposed use, but we can, and will, make some evaluation of the magnet technology

required. It seems clear that superconductors are the only way to produce fields of reasonable magnitude over even modest-sized volumes in a space environment. Conventional magnets

and their associated power supplies are just too heavy and too large. Unfortunately, even superconducting magnets can be quite heavy and, perhaps, that explains their relative lack of use to date. Clearly, in our present space program, the high magnetic fields may be desirable, but we have done without them. Whether or not the stage will be reached where this

is no longer true is a topic far outside our field of competence. It is a question for the

NASA long-range planners.

Unless stated otherwise, the applications described relate to spacecraft exclusively.

In a few instances the major interest is for military aircraft applications, and these will

be so noted. Of course, a system suitable for aircraft use might well find application in

space with only minor modifications. Some systems, such as fusion reactors, large active

shields and magnets for ore separation are quite plausible for applications in space colonization schemes proposed for the far future. These are also mentioned briefly.

In order to put the proper perspective on this topic, we have included in Appendix II.

B

a discussion of all of the actual instances that we know where a superconducting magnet has

15
left the ground (we exclude several rather dramatic laboratory accidents where the departure

was unintentional). There are only a few such cases and, if we insist on actual space flight

(as opposed to rockets and balloons) they are nearly nonexistent.

11. 4.1. Cosmic Ray Analysis

Several magnets of this type are described in the appendices. Magnetic spectrometers

are accurate, easy to calibrate and provide the means of measuring the sign of the charge of

the particles. Furthermore, they allow a relatively high maximum detectable rigidity

(^ 100 GeV) , but their resolution decreases with increasing energy, and they tend to have

limited geometry factors. Good statistics require long flights, and flight times from seven

days to one year have been proposed for magnet systems.

The magnets needed can be constructed now. The problems associated with long-term

storage of liquid helium and its use in a weightless environment are presently being solved

in conjunction with other space applications of the cryogen. Problems of making the coil

configuration so as to cancel the dipole interaction with the earth's field and to protect

other field-sensitive instrumentation on the satellite are not insurmountable and have

already been considered in some detail in the designs already described.

There are competing detection techniques which are being continually improved and, at present, it looks like the major application for the magnets will be on orbital experiments

of relatively short duration, say to ^ 30 days, in later Spacelab flights. Several groups

are presently considering designs for Shuttle/Spacelab flights.

11. 4. 2. Shielding of Spacecraft

The possibility of shielding spacecraft crews from charged particle radiation by large

superconducting magnets was investigated in detail a decade or more ago. The concept seemed

to offer promise for shielding large volumes to quite high particle energies with relatively

low weight. The designs varied from a simple circular coil creating a dipole field to a convoluted torus which had no field external or internal to the magnet. The former system,

while simple in concept could guard against basically monodirectional radiation and would

subject the crew and instruments to magnetic fields on the order of 0.1 T. Calculation of

expected accumulated dose and other considerations showed the magnet systems to be attractive

for very long manned flights or for very large vehicles, but not so much for the shorter

flights in small ships. This was especially true at the time because superconductors were very expensive and the stabilization and structural material required by the technology made

3 the systems quite heavy. An early NASA study showed that a 30 m volume could be adequately

(but see below) protected by a 4 T magnet of ^ 50-100 MJ stored energy and concluded that the

production of such magnets was within existing capabilities although lighter structure and

3 more reliable refrigeration were much to be desired. A larger volume, 144 m , could be

protected from protons with energy < 1 BeV by a system weighing 150 tons. Most of this is

structure. The recent study on space settlements also considered (and rejected) active

shielding for the habitat, arriving at a magnet weight of 10,000 tons for a 0.5 GeV/nucleon

cutoff. That study (Johnson and Holbrow, 1977) brought out the fact that secondary production of charged particles in the magnet structure by the (unshielded) primary flux above

16
2 GeV/nucleon would require additional mass shielding anyway. They concluded, quite rightly,

that an active shield with a cutoff of 15 GeV would involve unacceptable amounts of structure

to hold the magnet together.

The only shielding magnet ever built was constructed at Lockheed. It was a circular

magnet of 1.8 m i.d. with a 0.1 T central field (1.5 T at the winding). This magnet would

protect a volume of several cubic meters from particles with energies of ^ 100 MeV. The

entire system had a mass of 85 kg, including the cooling system, and could operate for 5-10

days

.

In the future it seems to us that this concept might be reconsidered for certain types

of flights or for space stations. Advances in materials, both superconducting and structural,

and in magnet reliability might change the economies of the situation in particular cases. The very large magnets being produced in the fusion energy program will go a long way toward

demonstrating modern construction techniques and reliability of large cooling systems.

An entirely different concept of shielding was once proposed which would trap an electron cloud around a space station by means of relatively low field (0.2 T) magnets and use

the resulting electrostatic repulsion to shield the station. Apparently the concept did not

prove to be viable.

II. 4. 3. Energy Storage

Storing energy in the field of large superconducting magnets offers a number of advantages over more conventional techniques, such as capacitors and batteries. The first is a significant weight reduction; the low voltage, high current storage mode is also often an advantage, as is the ability to handle rapid discharges and high repetition rates. The possible applications for inductive energy storage range from high field, small (10-

100 kJ) pulsed coils for plasma guns, masers and laser systems, through larger (1-10 MJ)

,

6 8 more slowly pulsed coils for plasma heating to the enormous coils storing 10 -10 MJ, which

have been proposed for electric utility peak shaving systems. Any or all of these applications are quite reasonable for space systems. Furthermore, the economics of competing storage methods are not the same in space (it is hard to store energy by pumping water uphill for

instance), and many of the magnetic systems look very attractive. A case in point is the use

of a warm reinforcement system in which a very large storage magnet is buried in the ground

so that the magnetic stresses can be taken up by the bedrock. This system on earth is calculated to be marginally competitive with other energy storage schemes (Powell, 1974), but

only at very large sizes. In applications on other planets or the moon, the reduced insulation and, thus, refrigeration requirements might make such a system very practical.

Smaller energy storage magnets, charged before launch from, say, a Shuttle vehicle seem

to be extremely practical systems for long-term energy storage for unmanned interplanetary

flights (fuel cells seem the more likely alternative for manned flights) . They obviously

become more efficient as it gets colder, and their specific weight is quite low and decreases

rapidly with stored energy in the 0-100 MJ range. The vacuum environment simplifies the

insulation problems greatly. Also, because of limited interest in the past, very little has

really been done to minimize the specific weight of superconducting energy storage devices.

New superconductors and structural materials make a significant reduction seem quite feasible.

17
The type of magnet deserves some comment. The most efficient design, and the easiest to build is a simple solenoid, but this device has a large associated dipole field, which may be

undesirable in itself and/or may lead to high losses due to inductive heating of nearby

metallic parts when operating in a pulsed mode. A toroidal configuration is the more desirable magnetically but significantly more of a structural problem.

Existing magnets are described in Appendix II. A, but a few comments are in order here.

Pulsed 100 kJ magnets have been made. Pulsed coil tests show that high rates of field change

(10^ T/s) are possible, high coil voltages (10' s of kV) can be handled as can pulse rates

around 5 pps. DC magnets now store as much as 800 MJ, and these are bubble-chamber magnets

not optimized for storage. Existing magnets, again not optimized for storage, have specific

weights as low as 200 kg/MJ (for the winding plus structure at the 10 MJ level) . Studies of

optimum geometries indicate that specific weight values of windings alone could be as low as

3 kg/MJ at the 1000 MJ level for solenoids and 7 kg/MJ for toroidal coils. Increasing the

stored energy to 10^ MJ drops these numbers by about another factor of five (note that dynamite has 0.24 kg/MJ, but its use in a controlled energy-release mode suffers from some technical problems). Of course, the magnet structure begins to represent nearly the whole mass

at these higher values of stored energy, and when this is taken into account the specific

weight tends to be ^ 10 kg/MJ for the high energy systems. Optimizing the coil for energy

storage with minimum mass involves a number of complex trade-offs, but in general a large

winding, thin compared to its radius, and with a relatively low central field is best.

Simple solenoids have a lower specific mass than other configurations. It may well be that an actively or passively shielded simple solenoid still has a mass advantage over a toroid.

For comparison to the above figures, long-life capacitor storage systems have a specific

weight in excess of 1000 kg/MJ. Also capacitors store energy with a density of about

3 3 0.4 MJ/m while in high field magnet systems, this is closer to 30 MJ/m . Battery systems

3 have a specific weight of about 12 kg/MJ and an energy density of ^ 100 MJ/m . On a pure

weight basis, magnets and batteries are comparable. However, the rate of energy removal is

restricted, as is the number of charge-discharge cycles, and the systems are both expensive

and temperature sensitive.

One very serious problem with large energy storage coils is that of switching, particularly where fast pulse times are required. Various superconducting switches have been investigated. The state-of-the-art is a discharge of about 1 MJ in 100 ms or 300 kJ in 2 ms. There is quite a bit of research on this subject because of the possible applications in

laser energy storage and in fusion devices. It is not a trivial problem.

Specific proposed space applications for energy-storage magnets have been very few in

the literature. One is the proposal by Olson (1974) to use a relatively large magnet to create an artificial miniature magnetosphere in space using either an injected plasma

(Shuttle experiment) or the solar wind itself for higher orbits. This "minimag" experiment

would allow studies of magnetospheres on a scale closer to reality than the existing laboratory experiments. It would also give information on the forces exerted on the earth by the

solar wind. No description is given of the magnet required, but the implication is that a

2 m simple coil with a relatively high field of about 9 T at the winding would be adequate.

Experiments would have to be performed far enough from the earth to avoid interaction with

the earth's field which would tend to orient the experimental magnet — and anything attached

to it, like the experimenter.

18
Another application might come in recovering energy produced by decelleration of the mass driver system described below, which is proposed as a means of sending material mined on

the moon to a processing center in orbit.

II. 4. 4. Energy Conversion - Motors and Generators

Superconductors offer significant size and weight advantages in various types of rotating machinery. In terrestrial applications these advantages are overshadowed by economic

considerations and concerns for very long term (30 yr) reliability, which ties the whole

problem up with the uncertainties associated with long-term operation of 4 K refrigeration

systems. It seems quite reasonable that many of these concerns would not be as crucial in

space applications while the reduced mass and size would be quite desirable. In spite of the concerns expressed above, superconducting motors have been built and operated. They are described in Appendix II-A. The state-of-the-art of superconducting machinery for aerospace applications is not as dismal as it is in many other applications of superconductors. This is nearly entirely due

to the Air Force program on airborne machinery. The program is relatively small, but the

fact that it has remained in operation over the last several years has allowed the consideration and introduction of modern materials technology. The situation has been reviewed re- cently (Oberly, 1977) and was one of the topics of an earlier comprehensive evaluation by an AGARD panel meeting (Fuhs, 1972).

Both dc and ac machinery have been investigated. The dc machines operate at high current levels and require ferromagnetic return paths for the flux. They are relatively easy to build because the superconducting windings are stationary. The potential for space applications would probably be limited to rather specific "industrial" operations such as providing power for aluminum ore processing, etc. Superconducting ac machinery, on the other

hand, appears to be promising for a variety of inflight uses. The type most investigated

uses a rotating superconducting field winding inside a stationary armature with a normal

copper winding. The high fields available with superconductors allow removal of the ferromagnetic core of the field windings and the armature teeth. The machines are lighter and

smaller than conventional ones with the same power rating. A comparison (on paper) by Westinghouse of two 1200 MVA (central power station, big!) generators showed reduction in weight

of more than 75% and a length reduction of nearly 50% for the superconducting machine. The

specific weight was 0.53 kg/kVA for the conventional machine and 0.12 kg/kVA for the superconducting one. We have neglected all associated cooling and refrigeration equipment.

Present aircraft alternators run from about 10-500 kVA with a specific weight of ^ 0.2 kg/kVA at the high end and with significant reductions in this value expected as new

materials, such as high-strength composites, become available. In this power range, superconductors do not seem to be competitive, even if stored liquid helium is used instead of a refrigerator. The specific weights required (for a 6 hr flight for a 50 kVA generator) are calculated to be 0.28 kg/kVA with stored liquid and 0.33 kg/kVA with a refrigerator. Refrigerator weight is assumed to be ^ 100 kg/watt of refrigeration.

19
For the power range above 1 MVA, superconducting devices appear to offer significant

advantages over conventional designs, although the ever-present uncertainties regarding the

additional weight required for reliable refrigeration still exist. The Air Force feels that

specific weights for the machinery alone of ^ 0.05 kg/kVA are possible for alternators in the

50 MVA class.

A superconducting generator has been under development for the Air Force for about six

years. It represents the state-of-the-art for airborne systems. The machine is a 5 MVA,

400 Hz, 1200 rpm, 4-pole generator. Its specific weight is ^ 0.09 kg/kVA.

A 1 MVA dc superconducting generator with a 5000 V output has been proposed for use with

a small nuclear power plant to provide electropropulsion for deep space missions (Cooper and

Kuhns, 1966).

The potential for electrical conversion machines in space seems good. A large amount of

materials research and engineering design work is needed if this potential is to be realized.

Other energy conversion systems using superconducting magnets have been proposed involving the use of thermal energy to cycle superconductors or helium in and out of the superconducting/superf luid state which would result in motion of flux provided by a superconducting

magnet which would then be transformed to electrical energy. At present none of these con- cepts appears practical on a large scale.

The transmission of electrical energy by superconducting power lines has received a great deal of attention in recent years. The concepts appear workable for both ac and dc

transmission but the economics on the earth do not look good except at power levels beyond

any current demand. Refrigeration is a major problem for these extended systems. Space

applications appear unlikely, since long distance power transmission by solid conductors is

not one of the primary problems.

II. 4. 5. Energy Generation

Power generation in space is already a reality with nuclear power plants perhaps as large as 100 kW (estimated for the ill-fated Cosmos 954 reactor, which fell in Canada early

in 1978) actually in orbit. The objection to fission plants in space are similar to those

here on earth. Any alternatives should be investigated. Two such options are thermonuclear

fusion and magnetohydrodynamic (MHD) power generation. The first of these is still in the

physics experiment stage, but the experiments are beginning to reach the sizes predicted for

power plants. MHD power generation is a technology now in use. Both systems require superconducting magnets for economical operation even on earth. Both systems seem very promising

for space applications.

The physics of fusion devices seems to require very large magnet confinement systems for

a net energy output and, thus, these power plants would seem to be feasible only for very

large space habitats or planetary installations. Most of the energy from reactors using the

most reasonable fuel (deuterium-tritium) is in the kinetic energy of neutrons which must be

slowed down in a "blanket" in order to recover the energy. Conventional thermal-electric

conversion plants are then necessary. If more advanced fuel cycles become practical, the

energy will reside in charged particles and direct conversion might be possible. The size

and shape and maximum field of the magnets is not yet known. Some terrestrial systems for

large experiments are described in Appendix II. A.

20
In MHD power systems, a very hot (2000° C) ionized gas flows at high velocity between

the poles of a magnet which deflects the charged particles to collection plates on opposite

sides of the channel, resulting in a current output from these electrodes. The source of the

gas is termed a combustor and in most cases it is much like a rocket. The power output is

proportional to the volume of the channel and the square of the applied field. By using

superconducting magnets optimized for light weight, MHD has the potential of having the

smallest specific weight of any power generation system, predicted to be ^ 0.1 kg/kVA at the

10-100 MW level.

Large superconducting magnets for terrestrial MHD systems have been built and are described in Appendix II. A, but these have used cryostatically stabilized superconductors with

copper-to-superconductor ratios of as much as 20:1, resulting in very heavy magnet systems

(2000 kg/MJ stored energy at the 10 MJ level for the NASA-Lewis system). This high specific

weight can be reduced only by operating with less stabilization of the superconductor (Cu:Sc

ratios ^ 2-3) which, in turn, makes the magnet subject to training and sensitive to quenching

by mechanical shock. Oberly (1977) states the situation very well: "The vision of a superconducting magnet in the confined space of an aircraft undergoing training quenches with more

than 1000 I of liquid helium vented in a couple of hours would not inspire pilot confidence."

The problem of designing large, dynamically-stabilized superconducting magnets is by no means

trivial, and it is a matter of the greatest importance for all the magnet applications

described in this report. Stability problems have plagued the few lightweight magnets which

have been built. A contributing factor to this lack of success is that the magnet cannot be

a simple solenoid but must be a rather complex saddle-shaped coil (race track coils can be

used but they require double the mass and volume of conductor, structure and dewar) and

complex coils are much more subject to training than are solenoids. Nevertheless the Air

Force has recently proposed a flyable design with 3.88 MJ of stored energy which will have a specific weight of 335 kg/MJ. The magnet is 68% conductor, 22% structure and 10% dewar.

Note that, even in the best cases, these magnets have much higher specific weights than those

used for energy storage, indicative of the severe penalty paid for using noncircular coils.

Most of the preceding discussion has application to what are called "open-cycle" MHD

systems. Some closed cycle systems using very high temperature fluids heated by fission

reactors have been proposed. They apparently pose too great a danger for use on earth, but

it has been suggested that they be placed in a synchronous orbit and the generated power

beamed to earth by microwaves.

II. 4. 6. Levitated Transport

Magnetically levitated vehicles capable of carrying passengers have already been operated on earth. They offer the potential of speeds well beyond 300 km/hr which is apparently

the limit for wheel-and-axle vehicles. This concept of guided electromagnetic flight would

seem to have great potential for use in transportation systems associated with planetary

settlements. It has, in fact, been quite extensively studied as a means of launching ore from the moon — the mass driver system. More about that shortly.

Levitation is possible using conventional electromagnets or, even, permanent magnets and

ferromagnetic materials. This "attractive" system requires a small gap (^ 1 cm) between the

21 — —
levitated vehicle and the track and is inherently unstable. Both of these considerations

tend to limit che usefulness of this system to low speeds. The repulsive system, and several

other similar suspensions, rely on the repulsion between a large superconducting magnet on board the vehicle and a conducting track to provide levitation. Large gaps 10 cm) are possible and the system is compatible with various desirable propulsion systems. These

propulsion systems are essentially linear motors. The linear synchronous motor, which involves a complex electrically-active track is the one most heard about today, but linear

induction motor schemes have been proposed and, if all else fails, one can always mount a jet engine on the vehicle. In all cases, mathematical models are highly developed, and test

vehicles of various shapes and sizes have been run. The problems seem to primarily have to do with feedback control of various unwanted modes of motion of the vehicle. They are not

trivial, particularly if the vehicle is to transport humans.

The only detailed study of levitated systems for space applications has been made by the

National Magnet Lab at MIT on the mass driver system. It is described in the Space Settlements Study (Johnson and Holbrow, 1977) and in several other papers. Its purpose is to semicontinuously launch small quantities of ore, mined on the moon, from the surface to a catcher in space, where it is loaded onto larger vehicles for transport to the space colony.

The proposed system will launch 10 kg of compacted lunar material at a time from a vehicle

aptly called a bucket. This involves acceleration to 30 g. One to five launches will be

made each second. Each bucket will have relatively small superconducting magnets operating

at ^ 3 T, and be driven by a linear synchronous motor system. Problems of stability and

aiming are quite challenging, but not insurmountable.

The mass driver concept has also been proposed for use as a reaction engine for sending

material from one orbit to a higher one, making it an effective upper stage for, say, transport from a Shuttle vehicle to geosynchronous orbit (O'Neill, 1977). The drive is accomplished by accelerating and launching very small masses (^ 14 g) at the rate of 5 per second.

The economics of the system as presented by the author make it seem most attractive. Furthermore, the magnet coil technology appears well within present day capabilities.

II. 4. 7. Propulsion

There is no real competition for the existing rocket systems for attaining earth orbit.

However, these high-thrust systems are not the most desirable for interplanetary travel.

Very long travel times are necessary. The low- thrust rocket systems, on the other hand,

offer the possibility of reasonable transit times to all but the most distant planets. These

systems have very low thrust, but provide it continuously, and thus they cause the ship to travel in complex, but relatively straight trajectories. High thrust system propulsion

results in trajectories which are portions of conic sections. A number of different systems

have been proposed for low thrust engines: MHD, or plasma, engines have been used by both

the USA and the USSR. Basically they make use of the forces produced by a plasma current

interacting with a magnetic field to accelerate a plasma created by an electrical discharge

in a diverging magnetic field; electric engines use a plasma also, but accelerate the ions by

strong electric fields applied to grids. Thermonuclear rocket concepts use unidirectional

leakage of a reacting plasma out of its magnetic confinement, usually with a parallel hydrogen

22
gas stream into which the enormous energies of the fusion particles are diffused. Laser

drive systems have been proposed in which the plasma is created and driven by a high-power

laser and, in a different approach, a system has been proposed in which lasers mounted on asteroids are aimed at spacecraft with "sails" to capture the energy. All systems except the

very last require relatively high magnetic fields over large volumes, and all serious proposals have assumed the use of superconducting magnets, since specific weight is the con- trolling factor in transit time. The round trip time to Mars for a 10 kg/kW (ratio of total

vehicle mass to engine power) one stage system is 450 days, while a 1 kg/kW system makes the

trip in 150 days. The magnet problems for all of these types of systems have been discussed

above. Again, the desire for very lightweight magnets leads to a need for superconductors

with high current density capabilities and strong lightweight structures. Many of these

systems are described by Moeckel (1969) with a special emphasis on the thermonuclear fusion

rockets. These, of course, suffer from the same problems as the power plants discussed

earlier — no one yet knows if controlled fusion is possible.

The various engines which use nonreacting plasmas all require magnetic confinement,

since the hot plasma should not be allowed to interact with material walls. In addition, the

pulsed plasma acceleration devices require large energy pulses, usually proposed to come from

condenser banks of high specific weight (500 kg/MJ) . It has been proposed that a similar

short term (on the order of milliseconds only) energy storage system might be possible with a "plasma condenser" since a plasma rotating in a constant magnetic field behaves like a dielectric of high dielectric constant. Peschka and Carpetis (1966) have designed several such

systems on paper which store ^ 2 kJ using a 1 T field generated by high current density Nb^Sn

superconductor and having a specific weight of ^ 10 kg/MJ. The magnets do not appear to be

complicated, but the entire system concept may have not proven itself, since nothing more has

been heard about it.

One severe shortcoming of all these pulsed plasma systems should be noted. They are extremely noisy electrically and, thus, are not well loved by the instrumentation people.

A different plasma system is the proposed ion scoop which would use a spaceship-borne

magnet to create a field to channel charged particles from the solar wind or interstellar

space into an electric engine system. The magnet would have to produce a dipole field in

-10 4 excess of the galactic field (^ 10 T) at 10 km from the ship in order to gather usable

quantities of ions. It has been calculated (Matloff and Fennelly, 1974) that this could be

accomplished by a scoop 0.8 km in diameter and 0.4 km long carrying a sheet of supercurrent

11 -4

of ^ 10 A, flowing in a thin layer (< 10 cm) of Nb^Sn. No serious consideration has been

given to the structural problems involved, but this may well take the prize for the largest

magnet system yet proposed.

Let's move back a little closer to reality for our last topic here. It was proposed in

the early 1960's that a superconducting magnet might be wound around a rocket exhaust port in

order to use the axial field to pull the exhaust gases away from the wall and, thus, effectively reduce the thermal transfer to the wall. An experiment was performed by the Navy

using a relatively small model with a 1.5 T superconducting magnet. The thermal transfer was unaffected and the field seemed to interfere with the ignition characteristics and the dynamics of the exhaust gases. A larger field (5.3 T) , produced by a cryogenically cooled

normal magnet, was tried. It worked the same way (end of project).

23
II. 4. 8. Re-Entry

The hot, high-conductivity plasma created upon entry of a hypersonic spacecraft into a planetary atmosphere can potentially be used as the working substance of an MHD braking

system. In such a system the flux could be conceivably caused to at least partially break

away from the body, causing a decrease in the heating. The energy would be transferred to the superconducting magnet on the spacecraft, resulting in increased current in the magnet.

Since superconducting magnets can create fields over a large region, the effective braking

area may be quite large. Furthermore, the braking action would tend to start significantly

further out from the planet than is presently the case. Theoretical investigations and a very few laboratory studies have been carried out, but no conclusions as to the actual desirability of employing such a system were reached.

II. 5.0. SUMMARY AND CONCLUSIONS

Large superconducting magnets are definitely feasible for use in a variety of space

applications. Very high fields and current densities are achievable with relatively low

weight magnets, although few development efforts to date have stressed the weight consideration. The magnets can store and deliver large amounts of energy with a high density and low

specific weight when compared with other systems. Advanced techniques of power production,

such as MHD and fusion, will rely heavily on superconducting magnets. These energy sources

are particularly well suited to space use. The technology of superconducting magnets for terrestrial applications is still advancing rapidly, and one can assume that significant improvements will continue to be made in

superconducting materials and coil design. These developments are being driven by the highenergy physics community, the Department of Energy, the Military and others. The concern for

weight, which is clearly important for space applications, does not presently represent a major concern for magnet designers. Modern composite technology, properly applied to magnet

structural components, could make impressive reductions in the specific weight of large

magnets. This is one clear objective which might be considered by NASA for development

programs

.

The less likely developments, such as the discovery of a high-field superconductor with

a transition temperature above, say, 80 K, should also be kept in mind, since such a material

could greatly alter many of the designs for space stations and colonies. This possibility

appears remote at the present, and thus extensive development work in this direction would

likely not be cost-effective. A second type of materials effort might be considered. In Section II. 3.4. we mentioned a number of ultra-high-field superconductors which have been

discovered in recent years. The development of any of these into practical composite con- ductors could have a significant impact on magnet characteristics (field, weight, size,

etc.). Commercial interests will probably not pursue this development with their own re- sources since the price tag may be 10^ dollars or more. The precise value of improvement in

magnet performance should probably be assessed carefully before such an investment is considered. Such an assessment is beyond the scope of this study.

24
It is difficult from our perspective to place priorities on projects in the space program, but some casual observations may be in order. Elementary particle physicists may be near the limit of dollar expenditures for higher energy accelerators. To obtain information

at substantially higher energies may well require long-term exposure to primary cosmic rays

outside the earth's atmosphere. For particles of this energy, superconducting magnets would

appear to be essential components of analysis systems. Thus, we might expect increasing

interest in the physics community for the long-term flight of large superconducting magnets.

A second application which might deserve further study is energy storage (see Section

II. 4. 3.). Earth-bound systems have been built and tested with good results, but little work

has been directed toward the question of specific weight. Present information indicates that

extremely good weight-to-energy ratios are achievable, but further study of the subject is

suggested. The reduction of weight through use of composite structural materials, as indicated above, would be valuable.

Finally, we suggest that the scientific community at large be made aware that highfield magnet systems of modest size are perfectly reasonable pieces of equipment to use for

experiments in Spacelab. We suspect that many interesting experiments in organic chemistry,

biology and physiology could be done with such magnets in space. We don't mean to slight

physics and materials science, but most researchers in those fields are probably well aware

of the potential for high-field experiments.

25
II. 6.0. REFERENCES

Brechna, H. (1975). Medical Applications and Biological Effects of Magnetic Fields. In Proc. Fifth Int. Conf. on Magnet Technology3 Laboratori Nazionali del CNEN, Frascati,

Italy, 351-366.

Cooper, D.W. and Kuhns, P.W. (1966) . Electrical Generator with a Superconducting Magnetic

Field for Use in a 1-Megawatt Rotating Space Power System. Intersociety Energy Conversion Eng. Conf. Tech. Papers AIM, 298-307.

Foner, S. and Schwartz, B.B. (1974) . Superconducting Machines and Devices: Large System

Applications, Plenum Press, New York.

Fuhs, A.E. (1972). Energetics for Aircraft Auxiliary Power Systems. NATO Advisory Group for

Aerospace Research and Development, AGARD-CP-104, Available from NTIS.

Golden, R.L. (1975). Superconducting Magnets for Space Flight. In Research Goals for CosmicRay Astrophysics in the 1980' s, European Space Agency ESRO SP-109, p 91 (Available as N76-12934)

.

Johnson, R.D. and Holbrow, c. (1977). Space Settlements: A Design Study, NASA SP-413.

Matloff , G.L. and Fennelly, A.J. (1974) . A Superconducting Ion Scoop and Its Application to Interstellar Flight. J. Brit. Interplanetary Soc. 27, 663-67 3. Moeckel, W.E. (1969). Propulsion Systems for Manned Exploration of the Solar System. Astronautics and Aeronautics 7_, 66-77.

Oberly, C.E. (1977). Air Force Applications of Lightweight Superconducting Machinery.

IEEE Trans. Mag. MAG- 13, 260-268.

Olson, W.P. (1974). Potential Experiments with Superconducting Magnets in Earth Orbit.

Space Science Reviews 12, 899-904.

O'Neill, G.K. (1977). Mass-Driver Reaction Engine as Shuttle Upper Stage. Paper presented at AIAA Princeton Conf. on Space Manufacturing, May 1977.

Peschka, W. and Carpetis, C. (1968) . On the Use of Cryogenic Techniques for Space Propulsion

and Energy Conversion. In Propulsion and Re-Entry, (ed. M. Lunc) Gordon and Breach, New

York, 287-304.

Pope, W.L. , Smoot, G.F. , Smith, L.H. and Taylor, C.E. (1975). Superconducting Magnet and

Cryostat for a Space Application. Adv. in Cryo. Eng. 20, 47-60.

Powell, J. (1974). Large Scale Applications of Superconductivity, [see Foner and Schwartz,

1974], Chap. 1.

26
CHAPTFR III

LOW FREQUENCY SUPERCONDUCTING SENSORS

J. E. Zimmerman

III. 1.0. INTRODUCTION 29

III. 2.0. THE SQUID: MAGNETIC FIELD MEASUREMENTS 31

111. 2.1. SQUID Magnetometers and Gradiometers 31

111. 2.1.1. Present State-of-the-Art 34

111. 2. 1.2. Ultimate Potential Performance 37 111. 2. 1.3. Projected Development Rate 37 111. 2. 2. Applications to Magnetic Measurements in Space 33 III. 3.0. GALVANOMETERS, VOLTAGE AND CURRENT SENSORS, AND WIDEBAND AMPLIFIERS ... 42

III. 4.0. MAGNETIC SHIELDING 43

III. 5.0. DC TRANSFORMER 43

III. 6.0. SUMMARY AND CONCLUSIONS 44

III. 7.0. REFERENCES 45

27

II I. 1.0. INTRODUCTION

Although this chapter deals with several low-frequency instruments and devices, most of

the discussion relates to SQUID magnetometers and gradiometers , since these are perceived as

the instruments with the greatest potential for applications in space. Presently, the preponderance of opinion seems to be that conventional magnetometers are adequate for measurements in space, so that use of superconducting instruments is not justified. However, it

might be pointed out that ten years ago, exactly the same view prevailed with respect to terrestrial geomagnetic measurements, yet today SQUID magnetometers are used to advantage by

several groups for precisely this purpose (see, for example, papers by W.D. Stanley and

others in Flynn and Zimmerman, 1978). It is fairly safe to predict that similar developments

will occur in space measurements, that as the analysis and understanding of the available

data improve, needs will arise for more advanced instruments. Also, it is not just that

SQUID magnetometers can perform unique functions because of superior linearity, sensitivity,

and frequency response, but they can do less-demanding jobs better than conventional instruments as well. (For an operational comparison of superconducting and conventional magnetometers, see Zimmerman and Campbell, 1975).

SQUID's are made in two forms (Fig. III-l) . One form comprises a single Josephson

junction in a superconducting loop, inductively coupled to a radio-frequency resonant circuit

and bias oscillator-amplifier system which, in effect, measures the SQUID impedance (see

Zimmerman, Thiene, and Harding, 1970, or Zimmerman and Campbell, 1975). The latter reference

contains a qualitative description more suitable for non-specialists. This form is usually

referred to as the rf-biased SQUID, or simply "rf SQUID".

The other form comprises two junctions in a superconducting loop. The loop is directly

connected to a bias source and amplifier circuit, so the dc properties of the SQUID can be

measured. Although the measurement can be made with either dc or low to medium-frequency ac bias, this device is referred to as the "dc SQUID" (Clarke, Goubau, and Ketchen, 1976).

Figure III-l. Schematic representation of rf and dc SQUID's with bias and signal

amplifier connections.

29
The dc and the rf SQUID 1 s have essentially identical applications, at least in lowfrequency sensing, and their ultimate sensitivities are comparable (Clarke, 1976). Except

for those made by Clarke and his colleagues (Clarke, et al. , 1976), nearly all SQUID's in use

at this time are rf SQUID's. The reason for this is that the single Josephson junction has

been more reliable and easier to make. However, great progress in junction manufacturing

techniques has occurred in recent years, so dc SQUID's may be more widely accepted in the

future. Instruments incorporating rf SQUID's have been commercially available from several

companies for several years. In this report we refer primarily to the rf SQUID, but the dc

SQUID could equally well be used in every case. A SQUID is basically sensitive to magnetic flux, as described below and in the references

given above, and can be used directly as a magnetometer. By the addition of appropriate

coupling coils, it can be adapted to various related functions such as galvanometer, voltmeter, ammeter, current comparator, dc amplifier, VLF antenna, first- and second-derivative

magnetic gradiometers, susceptometer , magnetic resonance spectrometer, and others.

In all but one or two characteristics, SQUID's equal or greatly surpass the performance

of all other types of low-frequency magnetic sensors. They are superior in sensitivity,

frequency response, range, and linearity; and they are competitive in cost, portability,

reliability, and mechanical simplicity. Their main disadvantage, in addition to the requirement for a stable low-temperature cryostat, is that their operation is disturbed by fast

transients and strong high-frequency signals, so they must be more carefully shielded against

these disturbances than other types of sensors. SQUID's are in a state of advanced and continuing development. Their potential importance to technology is indicated by an international conference on the subject held two years

ago, the proceedings of which are available as a book (Hahlbohm and Lubbig, 1977).

There are a number of other low-frequency superconducting devices of potential use in some experiments. Superconductivity can provide essentially perfect ac and dc magnetic

-12

shielding, as well as almost zero-field (^ 10 T) enclosures. The zero resistivity of

superconductors is useful for current and flux transformers, and these are commonly used in

conjunction with SQUID's, both for coupling to magnetic fields and their spatial derivatives

as noted above, and for impedance-matching to voltage and current sources. A unique device which utilizes flux flow to provide transformer action (meaning transfer

of power between primary and secondary) at zero frequency and above is called a "dc transformer." It will be discussed only briefly, since it has not reached the point of practical

application.

There are several relatively recent and rich sources of information on low-frequency

superconducting and cryogenic technology, some of which are referred to in this chapter and

which deserve special emphasis, namely: the report on the Stanford relativity experiment

(Everitt, 1977), the book on SQUID's (Hahlbohm and Lubbig, 1977), the Proceedings of the

Cryocooler Applications Conference (Zimmerman and Flynn, 1978), the Proceedings of the Navy

Summer Study on Superconductive Electronics (Silver, 1976), and the Proceedings of the Conference on Future Trends in Superconductive Electronics (Deaver, 1978).

30
III. 2.0. THE SQUID: MAGNETIC FIELD MEASUREMENTS

III. 2.1. SQUID Magnetometers and Gradiometers

A SQUID is a superconducting quantum interference device whose essential topology is

that of a ring or a loop. Its output varies periodically with the magnetic field B applied

perpendicular to the plane of the SQUID loop (Fig. III-2)

.

Figure III-2 Response of a SQUID to magnetic field B. The periodicity B

q is

equal to the flux quantum <J> multiplied by the effective area of

the SQUID loop.

The response is a vector function; there is no response to field components parallel to plane

of the loop. Note that the zero-field peak is exactly like all the others; it has no special

2 distinguishing feature. The effective loop size of a typical SQUID is several mm in area. The flux periodicity is a fundamental constant <f> = h/2e = 2.07 x 10 ^ W. Thus, the field

periodicity B indicated in Fig. III-2 is typically a few tenths of a nanotesla (a "gamma" is

equal to a nanotesla)

.

A SQUID may be used as the sensing element of a magnetometer, but for flexibility in

different applications, it is more common to shield the SQUID and to couple it to the field

by an auxiliary passive superconducting loop known as a flux transformer (Fig. III-3).

Figure III-3. Schematic representation of a shielded SQUID coupled to the field B through a flux transformer. Bias and amplifier circuit is not shown.

The SQUID is shown here symbolically as a circle with a small cross representing the junction. The entire transformer, including the

sensing loop is superconducting.

31
The flux transformer consists of a coupling loop in the SQUID connected by a two-wire

line to an external sensing loop to which the field B is applied. The sensing loop can be

tailored for any desired orientation and field periodicity. The flux-transformer concept is

also very useful for measuring field gradients. A pair of identical sensing loops connected

in series opposition (Fig. III-4) will feed a signal to the SQUID proportional to a component

of the gradient (AB /Ax, AB /Ay, etc.).

To SQUID

Figure III-4. Flux-transformer sensing loops in series opposition to measure magnetic-field gradient, AB /A . x y

The oscillatory nature of the SQUID response vs magnetic field, B, lends itself naturally

to digital recording. An algebraic or "up-down" counter on the SQUID output will give digital

readings of B in units of B . Through the use of a flux transformer, the unit B can be set

° -11

° at any specific value in an enormous range, at least from 10 T to 0.1 T or higher. A B -11 ° smaller than 10 T may require an impractically large sensing loop. Such extremes may be

impractical for another reason also, namely that the maximum reliable counting rate f of max

the system imposes a limit on the size of B^ relative to the time rate of change of signal

dB/dt which the instrument is required to handle, that is: ^— 4"r must be less than f , or

B dt max

B > 4~/ f o dt max

Digital operation of a SQUID only allows readout to within one unit of B^, which might

be, for example, 1 nT. Analog output is achieved by operating the SQUID in a phase-locked

loop (Fig. III-5) , in which a low-frequency reference signal with a peak-to-peak flux amplitude of ^ <j) is applied to the SQUID. The synchronously-detected response is then applied to the SQUID through a feedback resistor, to lock the dc flux in the SQUID onto one particular

peak (or valley, depending on the reference phase) of the response pattern of Fig. III-2).

The voltage across the feedback resistor constitutes the analog output, since the feedback

current in this system is precisely proportional to the applied field B. The limiting sen- sitivity that is ordinarily achieved with analog operation is 10 ^ to 10 ^ B //Hz", so that if

-14 °

B

q is 0.1 nT, one might be able to detect field changes as low as 10 T with a 1 Hz bandwidth.

There is no reason, of course, why the analog output cannot be digitized through the use

of a conventional A/D converter, but as noted above, the SQUID is itself a digital device in

units of B

Q

. Most practical systems, therefore, use this natural digital feature for large

field changes and the analog phase-locked loop for small field changes.

32
RF

OSC. T RF AMPL

LO-FREQ.

OSC.

SYNCHRONOUS

DETECTOR

UP- DOWN COUNTER

ANALOG

"° OUTPUT

SHEILDn

VWAWAVWWFEEDBACK RESISTOR

QUID

LIQUID HELIUM

SENSING

Figure III-5. Block diagram of complete single-component magnetometer, with

digital readout and analog output.

A way of digitizing the signal is to arrange for the phase-locked loop to break lock for

a field change greater than B

q and to reset itself onto the adjacent peak of the response

pattern (Fig. III-2) . This discontinuity in the analog output (Fig. III-5) can be used to trigger the up-down counter. Figure III-6 shows a hypothetical case where the field B has

TIME

Figure III-6 Hypothetical output vs time of an analog magnetometer, with

downward and upward resets at the analog-output limits.

33
increased for a certain length of time, resulting in a downward reset, and then decreased for

a time, resulting in an upward reset. The magnitude of each reset is of course precisely B , which serves as a standard unit to calibrate the record. An example of an actual record with

many such resets is given by Zimmerman and Campbell (1975) . An operational comparison of a SQUID magnetometer with several types of conventional

magnetometers for geomagnetic measurements was conducted by Zimmerman and Campbell (1975).

They concluded that the only instrument with comparable sensitivity is a large-sized induction loop (with an integrator), but the frequency response of the latter is quite inferior,

since it must be tuned for particular ranges of frequency.

Since B for a particular SQUID is a highly reproducible unit, it has been suggested

o that a SQUID is a convenient secondary standard for calibration of other instruments. In

fact, B for thin-film cylindrical SQUID' s can be calculated from the geometry to about a

° 4 part in 10 , but for a primary standard this is not as good as the magnetic resonance instruments can achieve.

Gradiometers are useful for detection and mapping of magnetic anomalies at short to moderate ranges. Present terrestrial applications are detection and tracking of military

targets such as submarines, and biomagnetic measurements such as mapping magnetic fields of

the human heart and brain. The military application is still in the stage of preliminary

development. Measurements on the heart (magneto-cardiography) have reached the stage of

being considered for routine clinical use at several universities, notably MIT (David Cohen),

Stanford (William Fairbank) , Vanderbilt (John Wikswo), and Helsinki Technical University

T. Katila).

The most advanced development of gradiometers for magnetic detection and tracking has

been done at the Naval Coastal Systems Laboratory (Wynn, et al., 1975). Instrument noise

-13

level achieved is about 10 T/m at frequencies in the neighborhood of 1 Hz, with pickup

loops a few cm diameter, 30 cm apart.

The general characteristic of gradiometers which makes them useful for these applications is that they are insensitive to the temporal fluctuations of large uniform ambient

fields such as the earth's field. This permits sensitive mapping of short-range anomalies,

as described above, without interference from fluctuating long-range sources. A superconducting gradiometer was used at NASA Ames Laboratory to map the field due to remanent magnetization of spacecraft on the ground (E. Iufer, private communication).

III. 2. 1.1. Present State-of-the-Art

Here we summarize the approximate quantitative capabilities of present SQUID

instruments.

1. Range: SQUID' s can measure, or be used in, ambient fields approaching the critical

field of the superconducting material, which for niobium is greater than 0.1 T, or 1000

times the maximum earth field. There may be some drift and hysteresis problems at such

high fields, but since this seems outside the scope of this report, we will not pursue

the matter. In a word, for space applications SQUID 's are not limited by their dynamic

range.

34
Sensitivity: The inherent instrumental noise level of present laboratory systems has

—28

been reported (J. Clarke, 1977) to be 10 J/Hz, corresponding to field sensitivities

of the order of 10 ^ T//Ez, frequency- independent above 0.1 Hz or so, with "1/f" noise

below that frequency. SQUID systems used for biomagnetic measurements have demonstrated

-14 i— operating sensitivities in the range o f 1 to 5 x 10 T//Hz (Cohen, 1975; Will lamson,

Kaufman and Brenner, 1977; Zimmerman, 1977). For gradiometers the figure is about

10"13 T/m (Clark, 1977).

To put these figures in the context of the real world, the steady field at the

earth's surface is ^ 5 x 10 ^ I, with fluctuations, depending primiarily on solar

activity, of ^ 10

7 T at frequencies below 0.001 Hz, and ^ 10 T or less at frequencies approaching 1 Hz. The ac field of the human heart, measured close to the chest

wall, is of the order of 10 T, and the so-called alpha rhythm of the brain produces

-12 an ac field near the back of the head of ^ 10 T or less. If the earth could be re- presented as a simple dipole, the magnetic gradient at the surface would be ^ 10 T/m.

Linearity: SQUID instruments are inherently linear. Non-linear effects, of the order

of a part per million, have been seen (R. Clark, private communication) which were

ascribed to ferromagnetic impurities in the associated mounting structures, electronics,

and so on. Presumably, such non-linearities should be no worse than for other types of

instruments. Similar effects were seen when superconducting thin films were used in the

construction of the SQUID' s. With pure or nearly-pure bulk niobium, as is widely used

for SQUID's and flux transformers, the inherent linearity may be of the order of a part

in 10

7 for B up to 0.1 T.

Digital Counting Rate and Slew Rate: As described above, there is a maximum rate of

change of B that a SQUID instrument can cope with. In the "natural" digital mode,

inherent noise limitations permit counting up to the order of 10^ <J> per second, so if

-9 ° -4

B

q for a particular system is 10 T, the system could marginally handle 10 T per

second. This rate of change of field corresponds roughly to what would be seen by a magnetometer loop mounted transverse to the spin axis of a satellite rotating in the

earth's field at 0.1 revolutions per second. In the analog mode of operation, the

ability of the phase-locked loop to hold on a particular peak of the response pattern

(Fig. III-2) is limited in exactly the same way. That is, a simple feedback loop with a

6 dB-per-octave gain curve (a broad-band amplifier with an integrator) can handle a rate-of-change of input field (slew rate) of the order of 10^ <j> per second. However,

in present laboratory and commercial systems the gain curve has been tailored to have more than 6 dB per octave rolloff at low frequencies, which permits handling considerably greater slew rates, approaching 10 4> o per second, for low-frequency input signals.

The restriction to low-frequencies is not too significant since most applications of

SQUID magnetometers are to measurements at low frequencies, dc to a few kHz.

Frequency Response: Present phase-locked loops generally incorporate reference oscillators at 100 kHz or less, and the upper frequency limit of the SQUID, operated in this

mode, is about half that value. Any rf signal with a field amplitude the order of B

q or

35
larger will make the system inoperative, since it will average a complete cycle (or

more) of the response pattern of Fig. III-2. SQUID 's are inherently sensitive to the

12

entire radio spectrum up to 10 Hz. Therefore, SQUID's for use in or near the earth

must be enclosed in an rf shield. For best reliability, such shields are often of such

thickness that the cutoff frequency is just above the highest frequency at which the

system is to be used. A cylindrical room-temperature aluminum shield 10 cm diameter and

1 mm thick has a cutoff frequency of about 140 Hz. The rf shield can also be put inside

the cryostat around the SQUID or around the SQUID and flux transformer. Johnson flux

noise produced by the shield will be seen by the SQUID and may limit the sensitivity,

particularly if the cutoff frequency is low (a thick-walled shield) , but this would not

be a significant operational limitation in most applications.

6. Power Requirements: Bias power requirement of a SQUID is less than 10 W. The associated electronics (rf oscillator and amplifier, reference oscillator, phase-locked

detector, video amplifier, etc.) requires from one to several watts in typical systems.

An up-down counter might require a comparable amount of power. However, there is no inherent reason why these systems should not be designed to operate on much lower power.

7. Drift: Few applications of SQUID's have required measurements at dc or very low frequencies, so that drift has not been a problem, and there is not much "typical" data to quote. Large drifts (and hysteresis as noted above) have been seen when thin-film

SQUID's were operated in field changes of 10 ^ T, but this was eliminated by the use of

bulk niobium. Drift can result from several effects associated with temperature instability, such as thermal emf's and other uncompensated effects in the phase-locked

loop circuit, and variation of SQUID parameters with temperature. The most important

SQUID parameter, (Fig. III-2), might change with temperature, but only by an ex- ceedingly small amount, of the order of a part per million, owing to temperature dependence of the (London) penetration depth.

8. Absolute-Value Measurements: The periodic response of Fig. III-2 contains no clue as to the absolute value of B

q

, and SQUID's are not ordinarily used for absolute-value measurements. Nevertheless, there are in principle several ways the absolute value can be

determined. The most direct is to rotate the instrument about an axis perpendicular to the SQUID pickup-loop axis, so that the peak-to-peak change of field is 2B. Another way

would be to slide a movable superconducting shield over the pickup loop to provide a near zero-field reference. A third way would be to incorporate into the instrument a relatively insensitive auxiliary SQUID (or set of SQUID's) with a value of B

q sufficiently large that the zero-field peak of the response is unambiguously determined from

the approximately known characteristics of the field being measured. Such cascading may

be limited to 2 orders of magnitude per SQUID. The zero-field peak of the auxiliary

SQUID can be used to locate the zero-field peak of the primary highly-sensitive SQUID.

The zero-field peak could also be located by the use of a simple flux-gate or rotating

induction loop magnetometer. The use of an auxiliary SQUID may have other merits,

however, in terms of following fast field changes and transients which the primary SQUID

lock-on system cannot handle.

36
9. Refrigeration Requirements: SQUID' s are generally operated in liquid helium cryostats

at temperatures in the neighborhood of 4 K. The inherent refrigeration power required

for a SQUID and its electrical leads can be made negligible, so the total refrigeration

power is essentially that used by the cryostat itself. Helium evaporation rates vary

from less than one liter per day for cryostats holding a few liters, to 1.8 liters per

day for a 180-liter SQUID cryostat (> 100 days between refills) built for the Naval

Research Laboratory (Davis and Nisenoff, 1977). The latter would be of interest in

connection with possible experiments in space. More to the point are the extensive

analyses and experiments at Stanford (Final Report on NASA Grant 05-020-019) for the

well-known relativity experiment, which envisions an 800-liter cryostat in space with a one-year operating time, and a similar cryostat for the Infrared Astronomy Satellite

(IRAS).

An alternative to liquid helium is the use of a closed-cycle cryocooler to cool the

SQUID and flux transformer. A cryocooler might be relatively compact compared to a sixmonth or one-year liquid-helium cryostat, but it would add significantly to the power

required by the whole system. In any case, a space-qualified cryocooler suitable for

operating a SQUID probably will not be available for several years.

111. 2. 1.2. Ultimate Potential Performance

Increasing the bias frequency of an rf SQUID can give significant improvements in inherent sensitivity, maximum counting and slew rates, and in frequency response. Very high

frequency bias systems have been demonstrated in the laboratory, but are not available com- mercially. It is probable that a factor of ten or more increase in these parameters can be

achieved, a sensitivity to field of 10 T//Hz~ for example. Using a large-area flux transformer sensing loop might provide another factor of ten improvement in field sensitivity, but

possible problems from noise generated by the cryostat and other parts of the system have not

-14

been investigated at levels below 10 T. In the case of gradiometers the sensitivity is proportional to the distance of separation of the pickup loops as well as to the loop dimensions. Within the limits of practical

cryostat sizes, present gradiometers are limited by the inherent SQUID sensitivity. The

usefulness of gradiometers for magnetic anomaly detection, biomagnetic measurements, and

other applications would be greatly enhanced by improved SQUID sensitivity.

The sensitivity of SQUID magnetometers is more than adequate for most applications, and

current efforts are directed primarily toward improving other characteristics such as operating convenience, portability, immunity from transients and rf interference, and slew rate.

111. 2.1.3. Projected Development Rate

Most of the performance characteristics of SQUID' s are more than adequate for terrestrial and space applications. The stimulus for increased sensitivity comes primarily from

the Naval Coastal Systems Laboratory gradiometer development and from a few laboratories

(Cohen, 1975; Williamson, et al. , 1977; and Zimmerman, 1977) using SQUID's to measure brain

fields. In view of this, it may be five to ten years before near-ultimate SQUID sensitivity

is routinely realized.

37
III. 2. 2. Applications to Magnetic Measurements in Space

Magnetic field measurements are of practical and academic interest in many ways. The

prime classical example is the use of a compass needle for navigation on the earth's surface,

and it is not entirely inconceivable to suppose that the magnetic field in space or in the

vicinity of other planets might also be used as a navigational aid. Magnetic measurements

are currently vitally important in geophysical research (e.g., sea-floor spreading and plate

tectonics), in mineral prospecting, and in geothermal prospecting.

Magnetic surveys for geophysics and for mineral prospecting are carried out on the

surface, from the air, and recently by satellite. Measurements of the temporal variations of

the magnetic field at the earth's surface, together with electric field measurements, permit

a calculation of the conductivity of the upper layers of the planetary crust, which in turn

is related to the sub-surface temperature. The estimate of crustal temperature obtained in

this way is of interest in connection with locating geothermal energy sources (see W.D.

Stanley, 1978).

A magnetic sensor satellite (MAGSAT) is scheduled for operation in the near future

(Langel, 1978). The advantage of satellite surveying is that an enormous amount of data on spatial variations can be obtained over the entire planet in a relatively short time. This

first MAGSAT mission will not include a SQUID magnetometer. Assuming success in this mission

one might want to consider the inclusion of a SQUID in similar future programs. The superior

sensitivity of the SQUID could well reduce the averaging time which is necessary to accumulate

a magnetic map of adequate resolution. However, magnetic micropulsations rather than SQUID

sensitivity would in most cases determine the amount of averaging required to map the finer

details of the geomagnetic field. More importantly, the frequency response of the SQUID

could offer an interesting means of studying the interactions between magnetic micropulsations and the ionosphere. A correlation (in time) of micropulsations measured in space and

at the earth's surface provides all the information needed to determine the temporal variations of the current sheath within the ionosphere. The SQUID would provide the best possible

resolution of such time-varying fields.

It is interesting to consider the magnetic field gradients generated by objects on earth

and sensed at a satellite orbit. The magnetic gradient of the earth, considered as a simple

dipole, is ^ 10 T/m at the earths surface. Natural and man-made inhomogeneties (ore

bodies, steel structures, etc.) produce a broad spectrum of magnetic anomalies with much

larger local gradients. SQUID gradiometers are useful for mapping of such anomalies. A

gradiometer, in effect, discriminates against interference from distant fluctuating sources, since the field and the field gradient of a dipole source vary as 1/p 3 and 1/p 4

, respectively,

where p is the distance from the source. We might digress for a moment to note that secondderivative gradiometers, whose response varies as 1/p 5

, are currently seen as the best instrument for cardiography and other biomagnetic studies, which are more or less the ultimate in

"local" magnetic mapping. On the other hand, gradiometers will be of somewhat limited utility

for mapping terrestrial surface anomalies from orbit. For example, a highly-permeable body

(like soft iron) 100 m in diameter in the earth's field would produce an anomaly of ^ 10

_1^ T -19

(field) and ^ 10 T/m (gradient) at an altitude of 200 km. The latter number is far below

the sensitivity of any present or projected gradiometer. If the body were much larger (like

38
1000 m diameter) or strongly magnetized (like 0.1 T) , then it still would not be detected by

a gradiometer at 200 km, but it would be well within the detection sensitivity of a SQUID

magnetometer. The latter would probably be limited by interference from more distant sources

such as solar wind fluctuations and their interactions with the magnetosphere. In order to discriminate against such interference, it would be worth considering two (or more) magnetometers in a pair (or more) of satellites separated by 100 km or so — in effect, a very long

baseline, highly sensitive gradiometer (see Fig. III-7) . With present SQUID magnetometers, a -19 /

gradient sensitivity of the order of 10 T/m /Hz would be realized with 100 km separation.

Presumably, best signal-to-noise ratio for detection of anomalies on the earth's surface

would be achieved when the magnetometer separation is of the same order as their altitude,

e.g. , 200 km.

Satellite Satellite

A B O O—

.

Figure III-7. Suggested system for magnetic survey of the earth. Magnetometers are located in each of the two satellites. The system acts as a very long

baseline gradiometer, thus discriminating against interference from

distant sources.

The magnetic field in space is a fundamentally significant factor in determining the

trajectories of charged particles from the sun and from outer space. For example, the re- latively modest magnetic field of the earth acts as a shield against the potentially damaging

bombardment by cosmic rays, and it has been suggested that the extinction of species like the

dinosaurs might have been associated with a reduction of the earth's field, allowing a lethal dose of cosmic rays.

Except in the vicinity of the planets, the magnetic field is that of the sun. This

field is locked into a plasma of charged particles, primarily protons and electrons, emanating from the sun, generally known as the solar wind. Since there are almost no collisions in

the rarified plasma constituting the solar wind, the magnetic field pattern imposed on the

plasma at the source (i.e., the sun) is locked into it, and plasma and field pattern together

flow out from the sun at an average speed of about 400 km/s. One might, for example, find

current vortices imbedded in the solar wind which would be observable as magnetic field

pulses measured by a stationary instrument. Although the particles do not collide, elements

of the plasma can interact through the electromagnetic field, and so can dissipate energy and

relax toward thermal equilibrium.

39
The rotation of the sun causes an instantaneous picture of a field line to be an Archimedes spiral, like the pattern of water droplets from a rotating lawn sprinkler. The field

lines are further modified by fluctuations and turbulence at the source. A qualitative

picture of these effects, from a review paper by Smith and Sonett (1976), is shown in

Fig. III-8. It demonstrates the complex nature of the solar field, its intimate interaction

with the solar wind, and the effect on charged particles from outer space (cosmic rays).

Figure III-8. Hypothetical instantaneous picture of two representative field lines.

The path of a high-energy proton spiraling around a field line is

indicated, and also the paths of charged particles (cosmic rays) from

outer space (from Smith and Sonett)

.

In the neighborhood of planets and their satellites a variety of complex magnetic effects are seen, depending upon whether or not the body has an ionosphere and whether or not it has

an intrinsic magnetic field. The case of an intrinsic field with no ionosphere is exemplified by Mercury, where the solar wind compresses the field pattern on the windward side and

draws it out into a tail on the leeward side. Conversely, the intrinsic field deflects the

solar wind around the planet and prevents the plasma from impinging directly on it. Venus

has an ionosphere but no (or very small) magnetic field. The solar wind and solar magnetic

field therefore impinge directly on, and are deflected by, the ionosphere, and so the

planet is enveloped much more closely by the solar wind than it would be if it had a large

intrinsic magnetic field. The intrinsic field of the planet Earth deflects the solar wind at distances of the order of 10 earth radii. This interaction and interactions with the ionosphere of solar particles leaking through the magnetosphere result in a variety of bizarre

phenomena, aurorae, radio interference, magnetic storms, and electrojet currents. These

effects have been and will continue to be, intensively studied for both practical and

academic reasons. The moon has neither an atmosphere nor an overall magnetic moment, although it does have a complex pattern of local magnetization, so its effect on the solar wind

40
and cosmic particles is simply to intercept them and create a shadow on the downstream side.

Fluctuations of the solar magnetic field carried along with the plasma induce eddy currents

in the moon. Measurements of the eddy-current contribution to the field near the moon have

been used to infer the electrical conductivity, and from this the temperature profile, inside

the moon, in the same way that natural magnetic field fluctuations at the surface of the

earth are used in geothermal prospecting (see above)

.

These examples show that magnetic field measurements are crucial to the experimental

study of the physics of the solar system. One can predict that this will continue to be the case in the forseeable future, and that by this means various phenomena will ultimately be

better understood. Some examples are the origins and variations of -eunspots, with their

associated intense magnetic fields, the origin or lack of planetary magnetic fields, and in

the case of the earth, the frequent reversals of magnetic field during recent 10 million

years) geologic history, and the interaction of the solar plasma and field with the more

rarified interstellar medium.

5 3 According to numbers quoted by Allen (1973), there are less than 10 electrons/cm

(p 265) in interstellar space (near the galactic plane), and the magnetic field is of the

order of 1 nT (p 269). These values are somewhat smaller than the corresponding numbers for

the solar plasma and magnetic field at the earth's orbit. However, if the latter vary inversely as the square of the solar distance, then they should become comparable to the former

somewhere within the orbits of the outer planets. The dynamic pressure of the solar wind

pushes the interstellar medium out to greater distances, however, so a study of interaction

may be somewhat beyond the capability of present space probes (Mason, 1978; Smith, et al.,

1975).

Since the plasma constituting the solar wind is a nearly perfect conductor, spatial

variations of magnetic field take place over a characteristic minimal distance, the analog of

the London penetration depth in a superconductor, namely

d = m ^ 1/2 m

where m, n and e are respectively the mass, number density, and charge of the particles. For

5 x 10^ electrons per cubic meter, d is about 2 1/2 km. The solar-wind speed being the order

of 400 km means that the characteristic time for field changes is of the order of 0.01 s. However, spatial field variations of the order of 10 nT cannot take place over such short

distances as this would require electron velocities in the current sheet greater than the

velocity of light.

According to Sonett (1978) the characteristic distances for magnetic field change are

5 3 ^ 10 km in interplanetary space and ^ 10 km at the "bow shock" wave where the solar wind

begins to interact with the earth's magnetic field. In either case, the field changes are of

_2

the order of 10 nT, corresponding to current sheets of the order of 10 A/m or current

—10 2 —8 2 6 densities j of 10 A/m and 10 A/m respectively. Assuming a plasma density n of 5 x 10

3 electrons/m leads to drift velocities of

41
2 V = j/ne ^ 10 m/sec for interplanetary space,

and ^ 10^ m/sec in the bow shock.

The gradients represented by the above numbers are 10 T/m and 10 ^ T/m respectively.

For a wind velocity of 400 km. sec the time derivatives are 4 x 10 ^ T/sec and 4 x 10 T/sec.

Belcher and Davis (1971) refer to Alfven waves in the solar wind with wavelengths as short as 10^ km and amplitudes of several nT, indicating gradients of the order of 10 ^ T/m. These

gradients are well below the measurement capability of any existing or projected gradiometer.

However, the solar plasma could in principle sustain much greater magnetic gradients. With

6 3

5 x 10 electrons /m moving at one-tenth the speed of light, the gradient would be about

3 x 10 ^" T/m. This is within the range of present gradiometers , which might therefore be

useful for studying local anomalies of high current density in the solar wind or planetary

ionospheres

.

III. 3.0. GALVANOMETERS, VOLTAGE AND CURRENT SENSORS, AND WIDE-BAND AMPLIFIERS

SQUID's are used in many laboratories as low- impedance galvanometers or galvanometer

amplifiers. An example which might be pertinent to space application is the use of a SQUID

to amplify the output of a thermoelectric element or a bolometer for measuring absolute

radiation levels or small temperature changes. A common application is the use of a SQUID as

a null detector in a cryogenic potentiometer for measuring resistivity of highly-pure metal

specimens

.

The voltage or current sensitivity of a SQUID galvanometer is a function of the inductance L. of the input coil. If the SQUID can detect a magnetic field energy E . of the m _ 2g

min

order of 10 J/Hz (see Section on SQUID magnetometers) , then the current sensitivity is

2 —28

derived from 1/2 L. i . ^10 J/Hz. The practical range of values for L. is perhaps

_g _2 in mm m

10 H to 10 H, although this can be extended either way through the use of an auxiliary

superconducting current transformer. This gives a range of current sensitivities i . from mm

^ 10 A//Hz~ to 10 A/ /Hz". The input coil is assumed to be superconducting, so it has no resistance, and the voltage sensitivity cannot be defined in the usual way. If the input

coil is connected to a signal source whose internal resistance is R

g

, then the minimum detectable source voltage E . is i . R , and the response is exponential with a time constant mm mm s L. /R . If it is required that the circuit respond with a time constant of 1 second, for

example,

in S then the range of values for R and for E . , given a range of 10

-8-2 H to 10 H

_ 8 _ 2 -18

for L. , is 10 Q to 10 Q and 10 V//Hz

s to 10mW

V//Hz, respectively.

Thermal noise in a single loop circuit, such as a source resistance R

g connected to an

input coil L. at temperature T, is given by 1/2 k„T, where k^ is Boltzmann's contant

-23 in

-23 6 (1.4 X 10 J/K) . At 4 K, thermal energy is 5 x 10 J, which is 10 times greater than

the minimum energy the SQUID can detect. Therefore the above numbers for L . and E . are mm mm

academic in the sense that in many practical cases the predominant circuit noise is thermal

noise in the source itself. The effective noise temperature (by the usual definition) of a SQUID galvanometer is of the order of 1 yK. By measuring the thermal noise power from a resistance R

g

, a SQUID can be used as an accurate absolute thermometer from less than a

42
millikelvin up to a temperature at which the resistor melts or otherwise changes its properties. This application was originally suggested and reduced to practice by Kamper, in a version where the source resistance R

g was built into the SQUID itself (Kamper, Siegwarth,

Radebaugh and Zimmerman, 1971)-.

If flat response from dc to high frequency is required, then the time constant L^

n

/R

g must be reduced accordingly. The SQUID noise energy E ^ Af is proportional to the input

circuit bandwidth Af = R /2ttL. and the noise temperature is T = E . Af/k . For example, a

s in n min B bandwidth of dc to 100 MHz would give T^ ^ 10 K, which is still a rather low noise temperature. Such wideband amplification is somewhat outside the context of low-frequency sensors, but it illustrates the versatility of SQUID' s, and also the point mentioned elsewhere that

strong high-frequency signals may interfere with SQUID operation unless rf shielding is

provided. The possibility of using SQUID' s for uhf and vhf amplification was discussed by

Zimmerman and Sullivan, (1977).

III. 4.0. MAGNETIC SHIELDING

An enclosure with superconducting walls a millimeter or so thick provides essentially

perfect shielding against changes of external ac and dc magnetic (and electric) fields.

Also, since magnetic flux can only penetrate a superconductor in integral multiples of the

flux quantum $ , it is theoretically possible, by cooling such an enclosure in sufficiently

low field, that one may end up with absolute zero field inside the enclosure. The technical

obstacles are formidable, however, and zero field has not been experimentally realized in

large (5 0.1 m inside dimensions) enclosures. Trapped residual fields lower than 10 ^ T have been achieved at Stanford, with rather sophisticated techniques (Everitt, 1977).

Very low residual field is essential for highly precise superconducting gyroscopes, as used in the Stanford experiments, and possibly for other applications such as inertial

guidance, force-free satellites, and gravitational wave experiments.

The perfect shielding property of superconducting enclosures (as distinct from the

possibility of zero or very low residual field) is routinely used in SQUID susceptometers and

other laboratory instruments to eliminate externally generated magnetic interference. The

•degree of shielding that is realized depends entirely upon the size and shape of the access

ports. Geometry of the access ports is perhaps the only subtle aspect of the design of

superconducting shields. Arbitrary geometries are probably best analyzed numerically or empirically. For a cylindrical shield with open ends, an externally applied field drops by

about a factor of 30 per shield diameter, inside the shield.

III. 5.0. DC TRANSFORMER

The superconducting dc transformer was invented by I. Giaever (1968). This is a thinfilm device in which flux-vortex flow in a primary film induces a nearly identical pattern of

vortex flow in a secondary film which overlays the primary. By this mechanism dc or ac power is transmitted from the primary to the secondary, and impedance transformations can be

effected by making the films multi-layered. Being a thin-film device, the dc transformer is

43
adaptable to microcircuit fabrication. Suggested uses include impedance matching to low

impedance devices such as superconducting bolometers and other infrared sensors, and the like.

Such applications have not yet been demonstrated, however.

Significant advances in fabrication have been made by J. Ekin (1977), who has developed

thin films with low flux-pinning forces, to improve the efficiency of the transformer.

Shot noise, analogous to shot noise in vacuum tubes and transistors, is produced by the

vortex flow in the dc transformer. Although some measurements of vortex shot noise have been

made on single films, its effect on the ultimate performance of the dc transformer in lownoise circuits has not been analyzed or measured. Such measurements must be made in order to establish the potential usefulness of the dc transformer.

Work on the dc transformer is continuing at NBS/Boulder Laboratories, and there is

considerable literature on the pertinent properties (i.e., flux flow) of thin films.

III. 6.0. SUMMARY AND CONCLUSIONS

The SQUID performance surpasses that of all other magnetic sensors in sensitivity, frequency response, range and linearity. The SQUID is moderate in cost, reliable and portable.

Its use (or non-use) in the space program will certainly be determined by the performance

requirements for particular missions. If the low-temperature environment must be added to an otherwise non-cryogenic mission, then the superior performance needs will have to be justified. However, we believe that it is reasonable to assume that, as one collects more information on magnetic fields in space, the need (and desire) for better temporal and spatial

resolution will provide this justification.

The results from the impending flight of MAGSAT should be enlightening. Will conventional magnetometers provide sufficient detail for the location of magnetic anomalies (perhaps ore bodies)? Detailed study of MAGSAT records should indicate whether increased

sensitivity might reveal significant new information.

We suggest that SQUID instruments, both magnetometers and gradiometers would be of value

in studies of the dynamics of interplanetary and planetary fields. Such instruments could

reveal details concerning local anomalies of high current density in the solar wind or variations in the ionospheric currents of the planets, particularly the earth (see Section III. 2. 2.)

The SQUID as a low-frequency amplifier may offer certain advantages for amplification of

transducer signals. For example, one may need to provide gain immediately following a sensitive cryogenic bolometer (see Chapter V). The SQUID is an excellent low-noise amplifier,

especially for low impedance sources (such as some bolometers) and provides ample gain in the

cryogen with sub-microwatt dissipation. This last fact could affect the lifetime of cryogenic missions, where the heat load on the cryogenic environment must be kept to a minimum.

The SQUID is in a quite advanced stage of development and only needs space qualification

of components to be considered for space missions. This would seem to be a straightforward process, since commercial SQUID systems are already rugged and reliable. The device

sits waiting for a space application which demands its performance.

44
III. 7.0. REFERENCES

Allen, C.W. (1973). As trophysical Quantities, Athalone Press, London.

Belcher, J.W. and Davis, L. , Jr. (1971). Large-Amplitude Alfven Waves in the Interplanetary

Medium, 2. J. Geophys. Res. 76, 3534-3563.

Clark, R.H. (1977). Naval Coast Systems Laboratory, Panama City, Florida. (Private Communication) . Clarke, J., Goubau, W.M. and Ketchen, M.B. (1976). Tunnel Junction dc SQUID: Fabrication,

Operation and Performance. J. Low Temp. Phys. , 25, 99-144.

Clarke, J. (1977). Summary and Conclusions. [see Hahlbohm and Liibbig, 1977], 587-594.

Cohen, D. (1975). Measurements of the Magnetic Fields Produced by the Human Heart, Brain and

Lungs. IEEE Trans. Mag., MAG- 11, 694-700.

Davis, J.R. and Nisenoff, M. (1977). [see Hahlbohm and Liibbig, 1977], 439-484.

Deaver, B. , Jr. (1978). Proceedings, Conference on Future Trends in Superconductive Electronics, AIP Spec. Pub., to be published.

Ekin, J. (1977). U.S. Patent 4,044,298.

Everitt, C.W.F. (1977). editor, Perform a Gyro Test of General Relativity in a Satellite and

Develop Associated Control Technology. Final Report on NASA Grant 05-020-019.

Giaver, I. (1968). U.S. Patent 3, 394,317.

Hahlbohm, H.D. and Liibbig, H. (1977), editors, Superconducting Quantum Interference Devices

and Their Applications. Walter de Gruyter, Berlin.

Kamper, R.A., Siegwarth, J.D. Radebaugh, R. and Zimmerman, J.E. (1971). Observation of Noise

Temperature in the Millikelvin Range. Proc. IEEE, 39, 1368-1369.

Langel, R.A. (1978). NASA Goddard Space Flight Center, Greenbelt, Maryland. (Private Communication) . Mason, P. (1978). Jet Propulsion Laboratory, Pasadena, California. (Private Communication).

Silver, A.H. (1976). editor, 1976 Navy Study on Superconductive Electronics. ONR Report No. NR 319-110.

Smith, E.J. and Sonnett, CP. (1976). Extraterrestrial Magnetic Fields: Achievements and

Opportunities. IEEE Trans, on Geosci. Electronics, GE-14, 154-171.

Sonnett, CP. (1978). University of Arizona, Tucson, Arizona. (Private Communication)

.

Stanley, W.D. (1978). Cryogenic Techniques and Geophysical Measurements. [see Zimmerman and

Flynn, 1978], 205-206.

Williamson, S., Kaufman, L. and Brenner, D. (1977). Biomagnetism. In Superconductor Applications: SQUID's and Machines, (eds. B.B. Schwartz and S. Foner), 355-402, Plenum

Press, New York.

Wynn, W.M., Frahm, CP., Carroll, P.J., Clark, R.H. , Wellhoner, J. and Wynn, J.M. (1975).

Advanced Superconducting Gradiometer/Magnetometer Arrays and a Novel Signal Processing

Technique. IEEE Trans. Mag., MAG-11, 701-707.

Zimmerman, J.E., Thiene, P. and Harding, J.T. (1970). Design and Operation of Stable rfBiased Superconducting Point-Contact Devices and a Note on the Properties of Perfectly

Clean Contacts. J. Appl. Phys., 41, 1572-1580.

45
Zimmerman, J.E. and Campbell, W.H. (1975). Tests of Cryogenic SQUID for Geomagnetic Field

Measurements. Geophysics, 40j, 269-284.

Zimmerman, J.E. (1977). SQUID Instruments and Shielding for Low-Level Magnetic Measurements.

J. Appl. Phys., 48, 702-710.

Zimmerman, J.E. and Sullivan, D.B. (1977). High-Frequency Limitations of the DoubleJunction SQUID Amplifier. Appl. Phys. Lett., 31, 360-362.

Zimmerman, J.E. and Flynn, T.M. (1978). editors, Proceedings of the Conference on Applications of Closed-Cycle Cryocoolers to Small Superconducting Devices, NBS Special Publication SP-508.

46
CHAPTER IV

DIGITAL APPLICATIONS OF SUPERCONDUCTIVITY

R. E. Harris

IV. 1.0. INTRODUCTION 49

IV. 1.1. Difficulties with Present Digital Technology 49

IV. 1.2. Fundamental Principles Governing Digital Logic Devices 50

IV. 1.3. Josephson Logic Gate 51

IV. 1.4. Advantages of Superconducting Technology 53

IV. 1.4.1. Device Speed 53

IV. 1.4. 2. Dissipation 55

IV. 1.4. 3. Superconducting Transmission Lines 55

IV. 1.4. 4. Matched Terminating Resistors 58

IV. 1.5. Fabrication of Superconducting Integrated Circuits 58

IV. 1.6. Superconducting Circuits Already Achieved 59

IV. 2.0. THE SUPERCONDUCTING COMPUTER 60

IV. 2.1. Description of a Hypothetical Superconducting Computer 60

IV. 2. 1.1. Cycle Time 60

TV.2.1.2. Density 61

IV. 2.1. 3. Power Consumption 61

IV. 2. 2. Applications of a Superconducting Computer 63

IV. 2. 3. Present State of the Art 63

IV. 2. 4. Ultimate Achievable Performance 64

TV. 2. 5. Projected Development Rate 64

IV. 3.0. SUPERCONDUCTING INSTRUMENTS 64

IV. 3.1. Important Elements of Instruments 66

IV. 3. 1.1. Current Comparator 66

IV. 3.1. 2. Sample and Hold 66

IV. 3.1.3. Logic Circuits 67

IV. 3. 2. A Few Possible Josephson Instruments 67

IV. 3. 2.1. Sampling Device — Analog 67

IV. 3. 2. 2. Fast Counters 67

IV. 3. 2. 3. A/D Converters 67

IV. 3. 3. Digital Instruments for Real-Time Signal Processing 68

IV. 3. 3.1. Agile Digital Filtering 68

IV. 3. 3. 2. Multi-Channel Spectrum Analyzer 68

IV. 3. 4. Present State-of-the-Art 68

IV. 3. 5. Projected Deployment Rate 70 IV. 4.0. SOURCE OF ADDITIONAL INFORMATION 70

IV. 5.0. SUMMARY AND CONCLUSIONS 70

IV. 6.0. REFERENCES 72

IV. 1.0. INTRODUCTION

Superconducting electronics offers remarkable new capabilities for both ground-based and

space applications. In fact it appears that it will be possible in 10 to 20 years to con- struct a satellite computer having the computing power of today's most powerful ground based

computers . This technology also will make possible new instruments capable of examining

-12

phenomena occurring on a picosecond (10 s) time scale.

Superconducting electronics uniquely offers a variety of properties which make these

revolutionary advances exciting. In computer language the technology consists of a fast

active element (the Josephson junction), an active element with remarkably low dissipation,

and essentially lossless transmission lines which can be terminated by matched loads to minimize reflections and achieve the fastest response. In the language of instrumentation,

superconducting electronics offers ultra-low-noise, high speed of response and immense

bandwidth.

These advantages, when compared with conventional electronics, are the result of some fundamental physical laws and not due to lack of development of conventional electronics. In fact superconducting electronics has achieved these advantages in the infancy of its development, while semiconductor electronics is a most mature technology. For applications requiring

the ultimate in performance, there appears to be no presently known competitor to superconducting electronics, although, of course, some new technology may appear in the future.

Because the technology is so new we begin by discussing fundamental considerations which

appear to suggest that cryogenic technology be used for future digital devices. Secondly we show how the active element in superconducting electronics (the Josephson junction) works and

discuss the technology for fabricating the devices. Finally we review the characteristics of

published circuits, and project the capabilities of future superconducting computers and

instruments

.

IV. 1.1. Difficulties with Present Digital Technology

Serious difficulties are being encountered in achieving substantial further increase in

computer speed. Because of the finite speed of light, the maximum dimension of a processor

may be not bigger than say half the distance an electromagnetic signal can travel in one machine cycle time. The Cray-1 is apparently now the fastest available computer, having a cycle time of 50 ns.* Its processor and one million 64-bit words of memory are crammed into

3 only 1.7 m . Its maximum dimension, measured in terms of the time required for light to -9

traverse it, is 5 x 10 s. Within that space are dissipated 115 kW. It is a remarkable

engineering feat to remove that heat.

In order to construct a future 1 ns computer, the diameter should be no more than about

-5 3 3

5 cm (2 in.),** having a volume of 6.5 x 10 m (65 cm ) . As will be shown later, it is

likely that such a computer, constructed with the same technology as the Cray-1, would in

* Within some repetitive calculations the Cray-1 can seem to have a cycle time of 12.5 ns. ** The velocity of propagation is assumed to be reduced by a factor of 3 from that of free

space because of the reduced velocity within microstriplines in the machine.

49
fact dissipate more heat than the Cray. Thus the task of cooling such a computer would be

still more difficult than with the Cray, perhaps impossible.

IV . 1 . 2 . Fundamental Principles Governing Digital Logic Devices

The fundamental principles which demonstrate the advantages of superconducting electronics have been well discussed by Keyes (1969, 1970, 1975, 1977). Indeed, his papers are required reading for those wishing to thoroughly investigate the subject.

Keyes begins by pointing out that the power dissipated by any device is described by

2 V /Z, where V is some average voltage across the device and Z is its impedance. He then

develops the values of V and Z using the following arguments.

In a computer, a signal must progress through a large number of active devices and

transmission lines. If only a small amount of noise is added to the signal at each stage,

the signal will rapidly become lost in the noise. Thus each stage of a computer standardizes

the signal as illustrated in Fig. IV-1. As can be seen, variations of the input signal are

INPUT

Figure IV-1. Illustration of the non-linearity required of each active element in a digital system. Because a wide range of inputs give either a low- or high-level output, noise at the output is reduced. The output is said

to be standardized.

moderated by the non-linear responses; that is, small amounts of noise at the input are suppressed at the output. To achieve such standardization, a non-linear device must be used.

To achieve a non-linear response the input voltage must be greater than kT/e. If V << kT/e,

then the motions of the charge carriers in the device would be affected little in comparison

with their thermally- induced motions, and a linear response would result. For the sake of

simplicity we will assume that V >_ 10 kT/e.

The impedance of the device is obtained by noting that all computers or instruments must

eventually communicate with the outside world at impedance levels of the order of that of

free space. The latter has an impedance of Z^ = 377 ft. For the most efficient power transfer, the device impedance must not be too far from Z

q

. Thus, for our order of magnitude

argument, we use Z

q for the impedance of the device. Therefore, remembering that we have

assumed that V >_ 10 kT/e, the minimum power dissipation is

50
P > (10 kT) 2/Z . (IV-1) — o

2 -4

The minimum dissipation varies as T having a value of 1.66 x 10 W for T = 300 K and

—8

2.95 x 10 W at T = 4 K. A further consideration shows that dissipation will increase with increasing speed of

response. When a device changes state, the voltage across it changes. To develop this volt2 age, energy 1/2 CV must be added. The capacitance C is that of the device. Now when the

device is returned to its original state, the energy is dissipated. Thus the minimum dissipation for a device which is charged and discharged every t seconds is

P = CV 2 /2x . (IV-2)

Hence if a cycle time x is required, then the clear goals of technology are to reduce the

temperature and the capacitance. Capacitance is largely determined by device dimensions. In turn, these are determined by the method of lithography used to produce the device. Thus,

technologies do not differ too much in capacitance. Therefore whenever it becomes difficult

to remove excess heat, the clear advantage will go to the technology which provides the

lowest heat dissipation. As demonstrated above, this might be best accomplished with a low

temperature technology.

A useful way of characterizing the dissipation in a logic gate is through the product of

its power dissipation and logic delay time. The result is roughly the energy dissipated in

a switching process. Fig. IV-2 shows a plot of delay time as a function of dissipation for

a variety of semiconductor and superconductor devices . The diagonal lines represent a constant Px product. It can be seen that superconductor devices have a large advantage

(1000 - 10,000 better Px product).

IV. 1.3. Josephson Logic Gate

Many of the difficulties with conventional digital logic are overcome by superconducting

logic. The active device in superconducting logic, the Josephson logic gate, is formed from

one or more Josephson junctions. A Josephson junction is formed by two superconducting

electrodes, separated by a thin insulating layer only a few tens of angstroms thick. To achieve its remarkable properties, the junction must be cooled to a low temperature 4 K) so that the junction electrodes are superconducting. Junctions have been used in two con- figurations to form Josephson logic gates.

The first is simply a single Josephson junction having insulated control wires over it, as shown in Fig. IV-3. As is discussed in Appendix IV-A, it is the magnetic field produced

by a current flowing through the control wire which causes the junction to change state. The second type of Josephson logic gate is called a Superconducting Quantum Interference

Device, or SQUID. It is composed of two, three, or more small Josephson junctions. A threejunction device is shown in Fig. IV-4. Connected in parallel, these junctions are small

51
Figure IV-2. Delay time as a function of power dissipation for a variety of electronic

technologies. The product of delay and dissipation gives roughly the

energy required for one switching operation. The diagonal lines are for

equal energies. Josephson devices are seen to be as fast as any other

technology, but to produce 1000 to 10,000 times less energy. The energy

—9 —12 —15 —18

units (nJ, pJ, fJ, aJ) are 10 , 10 , 10 and 10 joules,

respectively.

El ectrode

Figure IV- 3. Illustration of a thin film, Josephson junction. Two control lines

are shown above, and insulated from, the junction.

52
Nb Ground Plane

(Ml)

Figure IV-4. Quantum interference Josephson gate composed of three junctions in

parallel. The labels Mn refer to different layers of metallization.

Two control lines are shown at the left. They are not drawn across

the top of the device so as to reveal junction structure. I is a

c control current, and I is the bias current for the gate.

g

enough that the magnetic field produced by the control wire does not affect them directly.

Instead, the switching is induced by the field passing through the loops of superconductors

connecting the junctions. This type of device is more sensitive to magnetic fields than is

the single junction, and is therefore a better candidate for miniaturization. It is ex- pected to be the dominant type of device in the future, although most published work to date concerns the single junction.

IV. 1.4. Advantages of Superconducting Technology

IV. 1.4.1. Device Speed

The intrinsic response time, 7 , of a Josephson junction is determined by the superconducting energy gap, 2A

q

, by the relation,

7 = -h/2A . (IV-3)

g o

-3

For lead, which has an energy gap of 2.5 x 10 eV, x - 0.27 ps. A more serious limitation is due to the intrinsic capacitance C in a device consisting

of two planar electrodes separated by an exceptionally thin insulating layer. An RC time

constant, where R is the junction normal state resistance, determines the response time of

the device. Although data on this time constant is scarce for realistic alloys from which

junctions are made, R can be estimated from information in a patent for an alloy of Pb with

6.5% In. The capacitance C can be approximated from a variety of papers. The resulting

dependence of RC on the current density j and the barrier thickness is shown in Fig. IV-5.

The figure is probably correct to half an order of magnitude. Reliable junctions have been

3 2 Tfebricated with critical current densities up to at least 2 x 10 A/ cm . This corresponds to RC/*^ = 13. Thus, for superconducting lead, RC = 3.5 ps, an extremely low value when com- pared to conventional electronic devices.

53
Response time of Josephson tunnel junction, when limited by RC, as a function of critical current density. R^ is the normal state resistance

of the junction and C is its capacitance. The left ordinate gives R^C

normalized to the intrinsic response time xgo= 2A /e. The right ordinate

gives the time in ps for a lead alloy junction. The solid line (with

circles) is from an analytic expression derived from a variety of papers

and a patent. The x's are measured values from several papers.

54
It may be possible to fabricate reliable devices having much higher critical current

densities, making it possible to achieve the intrinsic device response time. However, as is

usual, other parameters will also serve to make it very difficult to achieve this performance

in a circuit.

IV. 1.4. 2. Dissipation

That Josephson junctions offer dramatically lower dissipation than semiconductor devices

has been discussed in Section IV. 1.2. It should be noted here, however, that low dissipation

is probably a more significant advantage than high speed. We note that in Fig. IV-2, the

Transferred Electron Device (TED) exhibits similar response time compared to a Josephson

junction.

IV. 1.4. 3. Superconducting Transmission Lines

Although the superconducting technology offers a remarkably fast, low-dissipation active

element, there remains another very significant advantage over semiconductor technology —

lossless, low dispersion microstrip transmission lines. In a very fast integrated circuit it is important that high speed signals be propagated

throughout each chip, and from chip to chip, faithfully. Thus, waveguide-like structures are

required. In thin-film technology a convenient structure is the microstrip line. This line

is composed of a conducting ground plane, covered by an insulator, with a narrow line of

conductor on the top. When used properly, this structure will propagate signals with little

loss or dispersion. We will demonstrate, however, that such a line is not satisfactory when

miniaturized for use in a small integrated circuit.

Microstrip lines are often fabricated in larger sizes using printed circuit technology.

We illustrate the characteristics of such a line in Fig. IV-6. The line is assumed to be

0.69 cm wide on a 0.16 cm thick printed circuit board which has a dielectric constant of 3. The line has an impedance of 50 Q. The conductors are assumed to be 0.0075 cm thick. We

show in the figure the attenuation a per unit length and the phase velocity v.. The attenua-

-3 11 tion is shown to rise with frequency to a maximum of about 10 dB/nnn at 10 Hz. This

should be quite acceptable for common-sized PC boards. The phase velocity is seen to be the same for all high-frequency components, but to drop off for low frequencies. It is important

for all frequency components to travel at the same velocity so that waveforms such as pulses

are not distorted as they propagate down the line. The low frequency drop-off can be understood in terms of the ratio of the electromagnetic skin depth 5 to the dielectric thickness

S. The skin depth is given by 5 = Vz/oju a so that it becomes smaller with increasing frequency. At high frequencies when 5 << S the field is confined within the lossless dielectric

and there is negligible dispersion. That is, all frequency components in this range travel

with the same velocity v,. However, as 5 becomes larger than S, the field pattern changes

with frequency and the dispersion becomes substantial, as reflected by the change in v, with

frequency. Examining the graph, it can be seen that signals having a fundamental frequency

component of about 10 Hz or greater can be faithfully propagated by this example line.

When we consider a miniaturized line, however, normal state lines work very poorly if

indeed they can be said to work at all. Consider the example in Fig. IV-7a for a 2 urn-wide

55
i ~i 1 r

FREQUENCY

Figure IV-6. Phase velocity and attenuation a of a room-temperature copper stripline on a printed circuit board. Dimensions are given in the text.

56
Figure IV-7. Phase velocity v and attenuation a for normal and superconducting thinfilm striplines. (a) is for a room-temperature copper line and (b) is

for a superconducting line. Each has impedance 6 fi. Details are given

in the text.

57
line, above an insulator such as SiO having a dielectric constant of 5 and a thickness of

712 X. The line has a limiting impedance of about 6 Q at frequencies above the range of the

graph, a convenient value for superconducting circuits (although this is a normal state

—6

line). The conductors are copper, having a resistivity of 1.6 x 10 fi/cm and thickness of

1000 X. As can be seen from the figure, the attenuation exceeds 1 dB/mm for frequencies

q above 10 Hz. Even more detrimental to its performance, perhaps, is the significant dispersion revealed by v ± over the entire range of frequencies shown. Thus, we conclude that

decreases in width and height of normal state lines, which are required for them to be used

in dense circuitry, render them essentially useless as waveguide structures.

However, superconducting lines of the same size make lossless and dispersionless waveguides up to rather high frequencies. In Fig. IV-7b, we show a and v^ for a superconducting

lead alloy line above an oxidized niobium ground plane. The dielectric Nt^C^ has a dielectric

constant of 29 and a thickness of 740 R. The line shows negligible dispersion up to a frequency of about 3 x 10"^ Hz. The attenuation is satisfactory for propagation within a chip

10 9 up to about 3 x 10 Hz and for propagation between chips up to about 5 x 10 Hz. Further reduction in attenuation is possible by using wider lines which have thicker

dielectrics. Wider lines, used for major transmission lines only, would not seriously degrade the circuit density. Wider lines would be particularly useful for connections between

chips

.

Thus the superconducting transmission line provides significant advantages for computers

and instruments at very high speeds. It also permits the use of one more technique as we see below.

IV. 1.4. 4. Matched Terminating Resistors

When superconducting transmission lines are used, the low loss permits them to be terminated by a matched load. Thus, reflections from the ends of the lines are substantially

reduced. Such reflections might interfere with the devices sending the signal, or at least

reduce the effective rise time of the signal to that time which is required for the reflections to die off. Normal state lines cannot be properly terminated because of the large

distributed losses in the lines themselves.

IV . 1 . 5 . Fabrication of Superconducting Integrated Circuits

For several years published descriptions of methods for fabricating reliable Josephson

junctions have been available. Only recently, however, has this technology moved beyond the IBM Research Laboratories. The technology not only includes processes for fabricating lead

alloy junctions (Pb-Au-In) , but also resistors (Aul^) , and microstrip lines. Furthermore,

special designs for packages which permit fast signals to move on and off a chip have

appeared in the patent literature.

Little is available in the published literature about the yield or reproducibility of

the processes. However, it has been revealed that junctions can survive the order of 500

thermal cycles to liquid helium temperature and back to room temperature. Another report

shows that they change resistance irreversibly when stored at room temperature. The

changes amount to tens of percent per month. However, storage in the freezing compartment of

58
an ordinary refrigerator inhibits changes for periods of many months. Substantial improvements over these capabilities are expected to have been developed already but not published.

Further progress should continue in the future.

What appear to be large-scale efforts to construct a superconducting computer at IBM

suggest that whatever the detailed success of fabrication technology, it is sufficient for

simultaneous operation of multitudes of these devices.

Further details on fabrication technology are given in Appendix IV-B.

IV. 1.6. Superconducting Circuits Already Achieved

A variety of logic and memory functions have already been achieved in superconducting

integrated circuitry. Published results are shown in Table IV-1.

TABLE IV-1

SUPERCONDUCTING CIRCUITS ALREADY ACHIEVED

Type Response Dissipation Number Linewidth References

Time Junctions

(ps) (ym)

Single

Junction

38 0.1 yW 1 1 Jutzi, et al. (1973)

NDRO* 80 3 2 Broom, et al. (1975)

NDRO* 600 3 50 Zappe (1975)

NDRO* 95 2 Jutzi (1976)

SFQ* 50 1 25 Zappe (1974)

SFQ* 100 1 3 Gueret, et al. (1977)

Logic 165 1 50 Henkels (1974)

Logic 170 28 yW 1 25 Herrell (1974b)

Logic 235 40 nW 3 3 Zappe (1975)

Logic 60 17 yW 3 5 Baechtold, et al. (1975)

Logic 42-120 1 yW 3 2.5 Klein, Herrell (1978)

Adder

(1 bit)

500 7 25 Herrell (1974a )

Multiplier

(4 bit)

27000 35 yW x 45 45 25 Herrell (1975)

Memory

(64 bit)

5000 322 13 Henkels (1978)

Shift 6200 20 yW x 8 8 25 Yao, Herrell (1975)

A/D 16000 20 25 Klein (1976)

NDRO - nondestructive readout memory

SFQ - single flux quantum memory, destructive readout

* - these units could form the basis for a random access memory

59
It should be noted for all of the devices having linewidths of 25 um or more that no attempt has been made to achieve maximum speed. Several results do show, however, that delay

times under 100 ps are possible, when linewidths not much bigger than 5 um are used.

It is difficult to project the rapidity with which delay times will be further reduced.

Several points should be noted, however:

1. Reductions in delay time will accompany further reduction in linewidth.

2. Reductions in linewidth, as in semiconductor technology, below about 2 um can be

achieved using electron beam lithography.

3. Because superconducting circuits made using optical lithography already offer so much improvement over semiconductor circuits there may be little driving force for

further reductions in delay time and size for some time.

IV. 2.0. THE SUPERCONDUCTING COMPUTER

IV. 2.1. Description of a Hypothetical Superconducting Computer

The most compelling reason for constructing a computer out of Josephson tunneling logic

is that by taking advantage of its low dissipation, more compact, higher-speed computers can

be constructed. We assume that it will eventually be possible to construct a computer having

a cycle time of 1 ns or less. We consider the implications of this speed on the size, dissipation and memory density required in such a machine. The arguments given are very general.

To be made more specific, a particular machine would have to be designed for a particular

function. In space for example, one might choose to minimize dissipation at the expense of

speed, versatility, or memory capacity. On the ground, dissipation will be a minor problem.

IV. 2. 1.1. Cycle Time

To attempt to justify the assumption that a 1 ns machine can be built, we simply note

that 100 ps simple logic devices have been constructed. In a typical computer a signal must

propagate through 8 to 10 logic levels during one machine cycle. Thus 8 to 10 times 100 ps

gives about 1 ns. A second, and seemingly frivolous, argument is based on the folklore that

a cryogenic device can supplant a room temperature one only if it does something otherwise

impossible or improves the performance of the room temperature device by a factor of ten. We

note that a cycle time of 1 ns is roughly a factor of fifty better performance than the

Cray-1 (the fastest computer constructed to date)

.

Given the intrinsic response time of less than 1 ps for a Josephson junction, it is

conceivable that computers having cycle times in the range of 10 to 100 ps may one day be

built. However, an improvement in speed of one to two orders of magnitude will provide so much more computing power than today's machines that pressures for faster machines may not

develop until after 1 ns machines have substantially penetrated the market.

ISO
IV. 2. 1.2. Density

It is simple to show that the required density in such a machine is well within the

present state-of-the-art. We assume the computer is contained within a sphere of diameter

5 cm. This hypothetical computer is illustrated schematically in Fig. IV-8. If we consider

3 the total machine volume of 65 cm to be one-tenth processor and the remainder memory we can ask for the density required in each.

Let us assume the processor has the same number of circuits as an IBM 370/168 which is

5 3 2 x 10 . Thus a density of 3100 circuits/cm is required. If we further assume that the

integrated circuits are on 0.25 mm-thick substrates (a common value for present silicon

wafers) and that these substrates are spaced by 0.25 mm, we find 20 layers/cm. In addition we assume that packaging limitations decrease the available space by a factor of 10. Thus

2

a 2 we require only 1600 circuits/cm or an area per circuit of 6.3 x 10 cm . One would expect

-4 2 each circuit to occupy no more than 10 cm so this requirement is not restrictive. The

spacing between the wafers is sufficient for the coolant (liquid helium) flow.

In the case of the memory, the requirements are slightly more stringent. Let us assume

g the memory contains one million words, each having 100 bits, or a memory of 10 bits. Using

the same wafer and spacing as before, and assuming that 90 percent of the area is for control

2 2 circuitry, one requires a density of 86,000 bits/cm , or an area per bit of 1100 um . Since

a memory device with non-destructive readout has already been reported having an area of

2 900 um , the requirements for memory density can be assumed not to be a restriction.

In fact a recent U.S. Patent suggests the possibility of constructing a memory cell

2 having an area of only about 10 ym . Such a device would be fabricated undoubtedly using

electron-beam lithography. Allowing a factor of ten for decoding circuitry and packaging,

8 3 our 10 bits could be contained within 5 cm , a package 1.8 cm on a side. Heat removal could

be more difficult with this exceedingly high density.

IV. 2. 1.3. Power Consumption

It is the low dissipation of superconducting electronics which makes possible the high

density computer described above. Of particular significance is that superconducting memory

requires no stand-by power, the information being stored in the form of circulating supercurrents. In these order-of-magnitude arguments, we shall assume that heat removal from the

memory can be neglected.

Heat removal from the processor is also encouraging. We consider our hypothetical

computer to be constructed of the most-recently published logic. A single tunnel junction,

when in the voltage state, carries a current of order 1 mA and a voltage about equal to the

gap voltage of 2.5 mV. Thus it dissipates a few uW. SQUID logic devices having a dissipation of about 1 yW have been reported. We assume for simplicity that each "circuit" in our computer contains ten gates, giving 10 yW for each circuit. Multiplying by the 2 x

10~*

circuits in this machine and 0.5 because we assume no more than half the processor gates are operating at a given time, we arrive at a net dissipation of 1 W, exceedingly small when

compared with conventional machines. However, this heat must be removed at 4 K using an approximately 300 K work input. Carnot efficiency multiplies these powers by a factor of

approximately 75. Furthermore, it is thought that refrigerator inefficiency and heat leaks

61
/Cryogenic

p Container

Memory

i i

1—rn

10 5 2 1 0 SCALE , cm

Figure IV-8. Illustration of a hypothetical superconducting computer, giving

a suggestion of the size of processor, memory and cryogenic container. A space-borne container would probably be designed

differently.

62
can be accounted for by another factor of ten. Thus the dissipation at room temperature

would be about 750 W, still much smaller than conventional machines, such as the 115,000 W

dissipation of the Cray-1.

It therefore appears quite likely that a superconducting computer having the processing

capability and memory of today's most powerful machines would dissipate sufficiently low

power to be flown in space. Its extreme speed also makes it a strong candidate for replacing

many ground-based computers.

IV. 2. 2. Applications of a Superconducting Computer

As with most computers, the range of applications of a superconducting computer is quite

broad. However, because the superconducting computer offers such a major advance in computing speed, it may make possible applications which are more than just evolutionary.

One can think of this range of applications as divided into three areas, each being more

removed from present applications than the preceding.

Applications of the first kind consist of usual applications of computers for logging

data, data reduction and analysis, etc., but with substantial increases in speed. Furthermore, decreases in dissipation may make computing in space more widespread.

Applications of the second kind rely on the increased computing speed and larger memories

which are possible in a superconducting computer. For example, weather modeling and forecasting may become much more timely and accurate given the capabilities of a superconducting

computer

.

Applications of the third kind are those in which a computer performs revolutionary new

tasks made possible by increased speed and memory. One must ask when considering what these

applications might be: What are the implications on artificial intelligence-like tasks of

such a powerful computer? Can one, without waiting for theoretical developments, perform

tasks by brute force? What tasks, which were previously impossible, could be performed in

real time? Could one send such a computer on an interplanetary Voyage, trusting it to analyze data and only selectively return it to earth? Would such a computer reduce the need for man in space? Never before has it been possible to conceive of sending such a powerful

machine, which requires so little power, into space. The implications of these machines

could be profound indeed!

IV. 2.3. The Present State-of-the-Art

The list of achieved superconducting circuits in Table IV-1 gives some suggestion of the

present state of superconducting computer development. All of the papers listed are a result

of a massive program at the IBM Thomas Watson Research Center in Yorktown Heights, New York,

and to a lesser extent at the IBM Zurich Research Laboratories. A great deal of extremely

high-quality research has resulted from this program. Furthermore, it appears that this

program is directed toward producing a prototype superconducting computer as soon as possible, say, within five years. Personnel at IBM will not, however, discuss the aims of their

program. We do know from personal contact that the program is large enough to involve

separate groups with various responsibilities such as materials development, circuit design,

packaging, and lithography. We guess that it may involve 100 professional scientists and

engineers

.

63
IV. 2. 4. Ultimate Achievable Performance

The 1-ns computer described previously probably does not represent the ultimate achievable from this technology. Given the 0.27 ps response time of a single junction, it is

conceivable that a computer having a cycle time below 100 ps is possible. Supporting this

possibility is the fact that even a 1-ns machine does not tax the density acheivable from the

technology. However, it should be noted that while very fast integrated circuits have been

developed, there exists no published work on packaging these circuits in such a way as to maintain speed in a whole assembly of the circuits.

IV. 2. 5. Projected Development Rate

It is possible that a prototype superconducting computer will be demonstrated within

five years. Such a projection is difficult to make since almost all of the work is closely

guarded by IBM. However, within ten years, a commercial superconducting computer may be

available. IBM staff have mentioned in public that this technology may be cost competitive

in medium scale computers, as well as providing revolutionary new capability in large computers. Perhaps mid-range computers will be where IBM's principal thrust will in fact be,

since they have historically left the supercomputer field to other companies.

It should also be emphasized that this technology is much more difficult than previous

computer technologies. The higher speed of response now requires attention, which was not

required in the past, to all interconnections which must be properly matched microstrip lines

with proper terminating resistors. Furthermore, packaging must be done much more carefully

with similar attention to interconnections. These factors may seriously interfere with the

present distribution system of electronics components. That is, it may be much more difficult for one company to make a superconducting integrated circuit for sale to another company

which assembles these circuits into a computer. The circuit manufacturer might necessarily

produce a much larger, more integrated piece of hardware. Because of the more detailed

engineering required, these factors may work to the disadvantage of small companies.

Because a superconducting computer produces such small dissipation, it is a natural

candidate for deployment in space. Of course, the circuitry would have to be made spaceworthy. However, a much bigger task is the construction of a suitable lightweight, reliable

refrigerator. (Refrigerators will not be a problem on the ground, as it is possible to achieve reliability using serveral, and weight is no problem.) The reader is directed to the

Introduction (Chapter I) and Appendix I-A for a discussion of small refrigerators and of

passive cooling.

IV. 3.0. SUPERCONDUCTING INSTRUMENTS

The needs for very fast measurements in military devices and the potential arrival of

very fast computers both require new fast measurement capabilities. That Josephson tunneling

logic is a compelling candidate for this new class of instruments is suggested by its known

high speed and low dissipation, by its past successes in this field (some Josephson instruments are now available commercially) , and by the development of technology for making re- liable thin-film Josephson junctions. The following is a brief review of past and present

64
applications of Josephson devices, many of these applications having originated or having

been perfected at the National Bureau of Standards.

In the past, Josephson junction-based instruments have been characterized as containing

a single Josephson junction. Typically, this junction was a point contact (like a cat's

whisker diode), or possibly a tiny constriction in a thin film. The use of one junction,

rather than more, was largely because photolithographic techniques had not been applied to Josephson devices.

The single Josephson junction was typically incorported into a loop of superconducting

material to form a Superconducting QUantum Interference JJevice (SQUID, see Chapter III). A

SQUID is capable of detecting very small magnetic fields, of order 10 ^ T(10 ^ oersted).

Most of the Josephson junction-based instruments now in use incorporate this device. SQUID

devices are available commercially from at least five companies.

When used directly to detect magnetic fields, or their gradients, SQUID devices are used

for magnetic anomaly detection (such as submarines) , for magnetocardiography (a non-bodycontact method of measuring human heart performance) , magnetoencephalography (magnetic brain

waves), geophysical measurements and prospecting. Additional uses of SQUID 1 s are for rf

power and attentuation standards, ultra-low temperature thermometry, and precise current

comparison. In each case the Josphson device made possible previously impossible measurements, or made a substantial improvement over previous measurements. In addition, single

Josephson junctions, not used in the SQUID configuration, are used to maintain the U.S. legal

volt, and for ultrasensitive broadband or heterodyne detection in the millimeter and far

infrared region of the electromagnetic spectrum. Single Josephson junctions have been used

for microwave and infrared harmonic generation and mixing at frequencies up to 30 THz (10 jam

"..-aveler.gth . The development of lithographic fabrication methods for complex (multi-junction) superconducting circuits provides new opportunities for high-speed devices. Fast instruments

demand a number of characteristics from the technology from which they are formed. In many

ways they are quite similar to very fast computers.

One must consider what is "fast". An instrument should be capable of characterizing

events on a time scale which is shorter than that in which the event occurs. Of course that means as fast as possible, because there are always events of scientific interest which offer

remarkable steel. A r.:re practical measure :f "5 =sr" might re a steed somewhat faster than

events occur in commonly available circuitry, such as in future computers. Thus, instrument

response times of the order of 1 ps would be most useful, since the fastest present instruments are sampling oscilloscopes with response times of 25 ps. Superconducting technology

should be capable ultimately of speeds in the 1 ps range.

A more -comprehensive definition of "fast" might recognize the difference in types of

measurements. For example, it might take more than a second to make an exceptionally high

accuracy measurement of voltage. However, even for a measurement of this sort, the total

time required can be reduced through the use of faster active devices.

Along with high speed is the concurrent requirement of small size. Recall that signals

propagating at the speed of light travel only 300 urn (0.030 cm) in 1 ps. If a variety of

65
sensors in an instrument are to analyze a signal at close to the same time, they must be in

close proximity to one another. The size scale is, of course, determined by the time scale.

If a variety of elements in the circuits are located close together, their dissipation

must be sufficiently small that their operating temperature can be maintained. The heat

produced by conventional very fast devices already prevents more than a small number from

being located on the same integrated circuit chip. The need for an improved technology is

thus evident.

If an instrument is to analyze analog signals, those signals must be accurately transmitted throughout the instrument. Thus, the use of lossless, properly terminated, lines is

of great advantage in a fast instrument.

All of these considerations are, of course, the same as those discussed earlier with

respect to computers. Thus, in the fastest instruments the use of superconducting technology

is compelling.

IV. 3.1. Important Elements of Instruments

There are a variety of possible instruments, but those to be discussed here make use of two simple components, a current comparator and a sample-and-hold circuit.

IV . 3 . 1 . 1 . Current Comparator

A Josephson gate is essentially a current comparator. It is used in its simplest form

in digital circuits where it must determine only whether a signal represents a "0" or a "1".

However, it can also be used in an analog sense as well. If the gate is biased in the zero voltage state, with some current flowing through it, then the gate will switch at some welldefined current flowing through the control line over the gate. One can vary the switching

point by providing a bias current through a second control line over the gate. Thus, an adjustable current comparator results. It should operate as fast as any Josephson gate —

that is, ultimately in less than 1 ps. Such current comparators would be important elements

in many of the devices below or they might be the fundamental element in a fast trigger

circuit

.

IV. 3. 1.2. Sample-anal-Hold

Sample-and-hold circuits are conventionally used to sample a voltage in a very short

time, and maintain it (as in the form of a charge on a capacitor) until it can be characterized by a slower device. A sample-and-hold circuit may also be useful in a superconducting instrument for the same purpose. Alternately, some superconducting instruments may

operate in such a short time that a sample-and-hold circuit will not be necessary. One

possible kind of superconducting sample-and-hold device would simply store a current in a superconducting persistent current loop. The current would be switched in and out of the

loop using a Josephson junction in the loop. The product of the critical current I of the

junction and the inductance L of the loop would have to be sufficiently large, however, since

-15 2 the maximum flux in the loop LI^ is quantized in units of ^ = 2.07 x 10 Wb/m , the superconducting flux quantum. The flux quantum would have to be smaller than the resolution

desired from the sample-and-hold circuit. The speed of response of such a simple circuit

66
would be determined by the time required for the Josephson junction to go from the normal

state to the superconducting state where the flux would be trapped. Times as short as 1 ps might be achieved.

IV . 3 . 1 . 3 . Logia Circuits

One may also use logic in an instrument. A counter, for example, must perform logical

operations. Such elements of an instrument might be quite similar to those in superconducting computers.

IV. 3. 2. A Few Possible Josephson Instruments

IV. 3. 2.1. Sampling Device — Analog

Unpublished reports of sampling devices suggest that such instruments have already been

constructed having a time resolution of a few ps. Such sampling devices are constructed from

current comparators of the type described above. Such devices provide the best means for

characterizing fast repetitive signals.

IV. 3. 2. 2. Fast Counters

It should be possible to construct fast counters using superconducting technology. The

speed of response would be determined by the speed of the circuitry for the least significant

bit. More significant bits would change state more slowly and would therefore not be as important. Careful engineering of this portion of the circuit for the least significant bit

might bring the response time down to a few picoseconds, corresponding to a counting rate of

order 500 GHz. Examples of the use of such fast counting rates might be in high accuracy

clocks or in detecting particles at very high flux.

IV. 3. 2. 3. A/D Converters

In conventional electronics the fastest, and seldom used, analog-to-digital converters

are of the parallel type. That is, the converter contains one current (or voltage) com- parator for every possible level of the input analog signal. Thus, a 4-bit converter might

have 16 comparators (15 comparators if one level is considered as a fixed reference) . Subsequent circuitry then condenses the 16 bits of level information into a 4-bit binary word.

A similar design in superconducting technology would undoubtedly also provide the highest

possible speed. The sampling rate would probably be determined by the time required to store

the digitized signal in preparation for the next one. Sampling rates as high as 10 to 100 GHz might be achieved.

A/D conversion is subject to the limitation that one cannot digitize a signal which is

changing faster than the time resolution of the converter. Sample-and-hold circuits are used

to maintain a signal until it can be digitized. The aperture time t of a sample-and-hold

circuit may determine the fastest signal which can be digitized. It seems unlikely in superconducting technology that a sample-and-hold circuit could operate in less than the intrinsic

response time for a Josephson junction: h/2A = 0.27 ps for lead. During this period the

signal is allowed to change only by less than one part in 2 n

for an n-bit converter.

Assuming a sine wave at frequency f, one finds

67
-1

f max

The maximum sampling frequency fmax

is plotted in Fig. IV- 9 as a function of the number of

bits of accuracy. It is readily seen that a reduction in the aperture time of the sampleand-hold circuit dramatically increases f . Thus achieving the minimum aperture time re- presents a high priority task if extremely high-speed A/D converters are to be realized. A

long range goal might be to reduce t below a few ps. ap

Applications of very fast A/D converters might include communications (where digital

filtering or image processing is used), data acquisition (conversion of rapidly varying

signals to digital form), and very broadband radars.

IV. 3. 3. Digital Instruments for Real-Time Signal Processing

Given a high-speed digital technology, it becomes possible to conceive of real-time

applications of computational techniques which previously were done off-line, or which could

be applied only to very slow signals. One such example involves Fourier analysis of signals.

Such signals might pass through a superconducting A/D converter of the type discussed above,

and then be digitally analyzed using a special-purpose fast Fourier transform processor

designed to perform these calculations at the highest possible speed. Such an FFT processor

could engender a variety of instruments, including fast digital filters.

IV. 3. 3.1. Agile Digital Filtering

Digital filtering is an extremely useful concept, applicable to areas such as communications and noise reduction. Filtering can be done with rapid change in frequency, lineshape, etc., possibly controlled by the incoming signal. One application of this type of

filter might be in the interpretation of frequency coded signals.

IV. 3. 3. 2. Ifulti-Channel Spectrum Analyzer

Superconducting digital technology would enable the construction of an analyzer which

has higher speed than those using conventional technology. Such analyzers are already being

worked on, using conventional technology in the Search for Extraterrestrial Intelligence

(SETI). Such a device could also be coupled with processing, possibly using special purpose

circuits to examine the data in real time, thus reducing the quantity of data which must be

stored or transmitted.

IV. 3.4. Present State-of-the-Art

There already exists a variety of superconducting instruments. The great majority of

them involve point contact or cat's whisker type Josephson junctions. Some superconducting

instrumentation has been built using integrated circuitry in conjunction with the development

of a superconducting computer. For example, analog sampling has been done with a response

time of perhaps a few picoseconds — close to ten times better than with conventional instruments. Ultimate achievable performance for some instruments has been briefly discussed along

with the instrument descriptions (Section IV. 3.0.)

68
2 4 6 8 10 NUMBER OF BITS

Figure IV- 9. Maximum sampling rate for A/D converter as a function of accuracy,

or number of bits. Curves shown are for various aperture times

t of the sample-and-hold circuit,

ap

69
IV. 3. 5. Projected Deployment Rate

The superconducting computer is the dominant force in the development of the technology

Programs for instrumentation development are few, and their sizes are small. However, substantial funding, which would bring other institutions into the field, would speed their

introduction. For example, a small Navy-funded program now exists at the National Bureau of

Standards to develop an A/D converter. Much broader funding of this type is needed if the

introduction of these instruments is to be speeded up. Refrigeration problems are the principal barrier to deployment in space. However,

small, low-dissipation instruments, combined with passive cooling and a helium reservoir,

might remain operational in space for a considerable period of time.

IV. 4.0. SOURCE OF ADDITIONAL INFORMATION

The Office of Naval Research has contracted with International Business Machines Corporation to perform a Josephson technology computer assessment. It is understood that this

assessment may result in a report divided into several parts: An open report, an IBM proprietary report, and a report which will be distributed to only a very small number of peopl

outside IBM. Under terms of the contract, the report will be completed within one year.

IBM has offered to provide at least some of this report to NASA when it is complete.

For further information, contact Dr. W. Anacker at the address below and refer to contract

number N00014-77-C-0179.

Dr. W. Anacker

Thomas J. Watson Research Center

P.O. Box 218

Yorktown Heights, New York 10598

(914) 945-3000

It is highly recommended that NASA acquire this information. IBM has made a large

number of exceedingly high-quality contributions to the field of superconducting electronics

IV. 5.0. SUMMARY AND CONCLUSIONS

Digital superconducting electronics is a newly emerging technology of significant promise which must, however, pass the test of real-world application. The motivation for the

development of this technology is rooted in some very general and fundamental considerations

(see Section IV. 1.2.). The Josephson switch is extremely fast and can potentially achieve

switching speeds of 1 ps. Its most attractive attribute is its small dissipation. The

dissipation is sufficiently small to allow extremely close packaging of active elements and

thus allow dramatic reduction of intracircuit communication delays. Furthermore, the use of

superconducting striplines for this communication provides the opportunity for unparalleled

freedom from dispersion and attenuation of signals.

Broad interest in the technology is expanding, but the prime driving force behind the

technology is a major commercial program to develop a superconducting computer. With some

70
knowledge of the characteristics of the simple logic elements, it is relatively easy to show

that order of magnitude (and greater) advances can be made in both speed and capacity. A

necessary result of increased speed is reduced size, and overall dissipation drops many

orders of magnitude below conventional computers (see Section IV. 2.0.)- All of these changes

(increased speed and capacity, decreased size and dissipation) are in a direction favorable

to space applications. The development of such a computer may be 10 to 20 years in the

future, but it is not too early to consider the opportunities, perhaps revolutionary ones, which may appear.

Several applications can be suggested now. The processing and compression of complex raw data from space experiments would provide a means of conserving the limited power available for information transmission. For example, the mapping of the heavens in any frequency

region involves tremendous amounts of data. A certain amount of data filtering can be

accomplished without a large computer, but a really smart satellite could provide much more

intelligent selection of data. A really powerful computer in space could offer a means to improved weather modeling and forecasting on a global scale. The combination of the acquisition and processing of weather data could simplify and speed up the modeling process, and

thus produce more timely and accurate forecasts.

These applications are founded on established concepts, but it is also exciting to consider more exotic ideas (see Section IV.2.2.). Never before have we been faced with the

potential for such a large computing capability in space. Could such computers assume intelligence-like roles? If so, would the need for some difficult manned space missions be

reduced? These are difficult questions which have certainly been considered before, but

reconsideration might be appropriate in light of the new opportunities which are now

developing.

Other types of fast instrumentation (Section IV. 3.0.) are made possible by this new

technology. Ultra-fast A/D converters, digital filters, counters and spectrum analyzers are just a few of the devices which should be considered. The applications of such instruments

are myriad. For example, improved A/D conversion could improve signal acquisition in the

Search for Extraterrestrial Intelligence (SETI) . Coupled with a superconducting computer,

such A/D converters could perform real-time processing of microwave signals in a very broad

frequency band. This type of system would essentially be an ultra-broadband digital receiver. Broader bandwidth offers larger information capacity and a valuable savings in power

required for transmission.

All of these applications are highly speculative. We assume that commercial interests

will develop the computer, but that is, by no means, certain. Should NASA place a priority

on such developments, it might be wise to assess the probability of success and to provide

some partial stimulus to assure this success. The peripheral development of other digital

instruments will probably accompany the computer effort. No doubt, there are instrument

developments which would be important to the space program and which would then deserve some NASA encouragement.

71
IV. 6.0. REFERENCES

Baechtold, W. , Forster, Th. , Heuberger, W. and Mohr, Th.O. (1975). Complementary Josephson

Junction Circuit: A Fast Flip-Flop and Logic Gate. Elec. Lett. 11, 203-204.

. 2 Broom, R.F., Jutzi, W. and Mohr, Th.O. (1975). A 1.4 mil Memory Cell with Josephson Junctions. IEEE Trans. Mag. MAG- 11, 755-758.

Gueret, P., Mohr, Th.O and Wolf, P. (1977). Single Flux Quantum Memory Cells. IEEE Trans.

Mag. MAG- 13, 52-55.

Henkels, W.H. (1974) . An Elementary Logic Circuit Employing Superconducting Josephson

Tunneling Gates. IEEE Trans. Mag. MAG- 10, 860-863.

Herrell, Dennis J. (1974a) . A Josephson Tunneling Logic Adder. IEEE Trans. Mag. MAG-10,

864-867.

Herrell, Dennis J. (1974b) . Femtojoule Josephson Tunneling Logic Gates. IEEE J. Solid

State Circuits, SC-9, 277-282.

Herrell, D.J. (1975) . An Experimental Multiplier Circuit Based on Superconducting Josephson

Devices. IEEE J. Solid State Circuits, SC-10, 360-368.

Jutzi, W. , Mohr, Th.O., Gasser, M. and Gschwind, H.P. (1973). Josephson Junctions with 1 ym

Dimensions and with Picosecond Switching Times. Elec. Lett. 8, 589-591.

Jutzi, W. (1976). An Inductively Coupled Memory Cell for NDRO with Two Josephson Junctions.

Cryogenics 16, 81-88.

Keyes, Robert W. (1969) Physical Problems and Limits in Computer Logic. IEEE Spectrum, May.

36-45.

Keyes, R.W. , Harris, E.P. and Konnerth, K.L. (1970). The Role of Low Temperatures in the

Operation of Logic Circuitry. Proc. IEEE 58, 1914-1932.

Keyes, Robert W. (1975). Physical Limits in Digital Electronics. Proc. IEEE 63, 740-767.

Keyes, Robert W. (1977) . Physical Limits in Semiconductor Electronics. Science 195,

1230-1235.

Klein, M. (1977). Analog-to-Digital Converter Using Josephson Junctions. 1977 IEEE Int. Solid-State Circuits Conference Digest of Technical Papers, Vol. 20, ieee Cat. No. 77CH

1172-6 ISSCC. 202-203.

Klein, M. , Herrell, D.J. and Davidson, Arthur (1978) . Sub-100 Picosecond Josephson Interferometer Logic. 1978 IEEE Int. Solid-State Circuits Conference Digest of Technical

Papers, to be published.

Yao, Y.L. and Herrell, D.J. (1974). An Experimental Josephson Junction Shift Register

Element. Int. Electron Devices Meeting, Tech. Dig., 145-148.

Zappe, H.H. (1974). A Single Flux Quantum Josephson Junction Memory Cell. Appl. Phys. Lett.

25, 424-426.

Zappe, H.H. (1975) . A Subnanosecond Josephson Tunneling Memory Cell with Nondestructive

Readout. IEEE J. Solid State Circuits, SC-10, 12-19.

Zappe, H.H. (1975). Quantum Interference Josephson Logic Devices. Appl Phys. Lett. 27

,

432-434.

Zappe, H.H. (1977). Josephson Quantum Interference Computer Devices. IEEE Trans. Mag. MAG13, 41-47.

72
CHAPTER V

MICROWAVE AND INFRARED DETECTORS

C. A. Hamilton

V.1.0. INTRODUCTION 75 V.2.0. BOLOMETERS 76

V.3.0. SUPER-SCHOTTKY DIODES 79

V.4.0. JOSEPHSON DEVICES 81

V.4.1. Josephson Oscillator Sources 82

V.4.2. Josephson Mixers for Heterodyne Receivers 83

V.4.2.1. Josephson Mixer with Internal Local Oscillator 83

V.4.2. 2. Josephson Mixer with External Local Oscillator 84

V.5.0. NON-SUPERCONDUCTING MIXERS 88

V.6.0. JOSEPHSON PARAMETRIC AMPLIFIERS 88

V.7.0. SUMMARY AND CONCLUSIONS 90

V.8.0. REFERENCES 93

73

V.1.0. INTRODUCTION

Microwave and infrared detectors and receivers are widely used in space applications.

These applications include ground-based communication and radioastronomy receivers as well as satellite receivers. Mapping of background radiation, for example, is an important application requiring the ultimate in receiver sensitivity. The success of a proposed program to search for signals from extraterrestrial life may well depend on the sensitivity of the

receivers used. In all of these cases, improved sensitivity yields benefits by making possible

greater data transmission rates, reduced observing time or smaller antennas.

There is a wide variety of superconducting devices currently being developed for micro- wave and infrared detection. In many cases, these devices offer comparable or improved

performance over their conventional counterparts. However, because these devices are relatively new, often unstable, and not always fully understood, few if any are currently being

used in operational receivers. It is the purpose of this chapter to review all of the

promising superconducting devices which are being developed or contemplated for use as micro- wave or infrared detectors.

The most intensive efforts toward developing superconducting microwave and infrared

detectors have been based on the Josephson effect. The characteristics which make Josephson

effect detectors attractive are (1) a very high degree of nonlinearity , (2) low temperature,

12 low noise operation, and (3) demonstrated frequency response to 4 x 10 Hz. Used as video

detectors or mixers, Josephson junctions have achieved sensitivities which equal or surpass

conventional devices. In the mixing mode of operation, the required local oscillator (LO)

power is very low and may be generated externally or internally using the Josephson selfoscillation. The latter case has the advantage of simplicity (no LO source required) but

resolution is limited by the linewidth (several hundred megahertz) of the Josephson oscillation.

The Josephson self-oscillation may also be used as a microwave source. The linewidth of

such a source could be narrowed by coupling it to a cavity with a sufficiently high Q.

Although the power available is rather small it may be useful at frequencies above 100 GHz

where other sources are rare. Efforts are also underway to increase the power output by the use of synchronized arrays of Josephson devices.

All the Josephson detectors have some common disadvantages. They are, for the most

part, very low impedance devices and are therefore difficult to couple to radiation fields.

In order to control parasitic elements, the most successful devices have generally been of

the point-contact variety. In their present form, these devices are probably not sufficiently stable for space applications. A third problem arises from the fact that the Josephson mechanism operates at very low power levels. This results in low-level saturation and a correspondingly small dynamic range. The rapidly developing technology of thin-film Josephson integrated circuits may provide solutions to all of these problems. For example, a

1 um-square thin-film junction fabricated by electron-beam lithography with a current density

2 of 2000 A/cm should have an upper cutoff frequency of about 1000 GHz and a resistance of

100 P.. Series arrays of such junctions could be used to optimize input coupling as well as

to increase the saturation level.

75
Parametric amplifiers are another very promising application of the Josephson effect for

microwave and infrared receivers. Although experimental and theoretical investigations of

Josephson parametric amplifiers are relatively recent, the results are impressive. As with

the Josephson detectors, fabricating stable devices, impedance matching, and low-level

saturation remain problems.

Two other very promising superconducting detectors which are not based on the Josephson

effect are the super-Schottky diode and the superconducting transition edge bolometer.

Super-Schottky diodes have achieved record sensitivities both as video detectors and mixers.

As with the Josephson mixer, the super-Schottky mixer requires very little LO. power and has a somewhat limited dynamic range. Considerable effort has been devoted to the development of

superconducting composite bolometers in which the functions of radiation absorption and

temperature measurement are separated. An aluminum film near its superconducting transition

temperature is used as the thermometer. When operated at about 1 K, such devices have

achieved nearly ideal performance.

The following sections describe in more detail superconducting bolometers, super-Schottky

mixers, and Josephson sources, mixers and parametric amplifiers. The description includes

(1) the physical mechanism, (2) theoretical limits on the ideal performance for a practically

realizable device, and (3) the current state-of-the-art for the superconducting device as well as its non-superconducting competitors.

A bolometer is a thermal detector consisting of a radiation-absorbing element and a resistance thermometer. It produces an electrical signal by changing its resistance when it

is warmed by absorbed radiation. For applications requiring the ultimate in sensitivity,

the use of bolometers is usually limited to the far-infrared region (20 um to 1 mm) at frequencies which are too high for heterodyne detectors and too low for quantum effect detectors.

They are typically broadband devices with a slow response time (1 ms to 1 s) . Cryogenic

bolometers, especially those made from doped germanium, are widely used as sensitive infrared

detectors.

Superconducting bolometers, although not in wide use, have been extensively studied.

Before describing specific superconducting bolometers, a brief review of the fundamental

performance limits is useful (see for example, Adde and Vernet, 1977). The principal parameters describing a bolometer are C (joules/kelvin) the heat capacity, G (watts /kelvin) the

thermal conductance to an external heat sink and R^ (volts/watt) the voltage (or current)

responsivity . The bolometer time constant t is equal to C/G. Since the various noise con- tributions are independent in a bolometer, they can be added quadratically to yield a system

NEP (Noise Equivalent Power)

:

The first term is the Johnson noise associated with the bolometer resistance R. The second

term is thermal noise from statistical fluctuations in the bolometer temperature T. The

V.2.0. BOLOMETERS

NEP 2 = 4kTR

(V-l)

76
third term represents the detector response to fluctuations in the background temperature

T . 0£ is the detector emissivity, A its area and Q the viewing solid angle. The fourth

Bterm represents any additional excess noise sources in the bolometer by a noise voltage

Y„(V//Hz) . A high-performance bolometer must have a high responsivity to minimize terms one

and four and it should be shielded as much as possible from unwanted backgound radiation to minimize term three. This usually involves a He-cooled mount and optics and perhaps cooled

filters to restrict the input bandwidth. G and especially C should be as small as possible

to minimize term two and at the same time achieve a reasonable response time x = C/G. Since

G can usually be made arbitrarily small, the ultimate sensitivity of a bolometer with a given

t is determined by the heat capacity C. This ultimate sensitivity for an ideal bolometer is

2 1/2 given by the second term of Eq. (V-l) , i.e., NEP = (4kT G)

Bolometers have been built which are based on several different superconducting properties. The transition edge bolometer makes use of the rapid variation in resistance of a thin

film at its superconducting transition edge. The recent discovery of the importance of a good thermal contact between the film and its substrate has led to major improvements in the

performance of this device (Clarke and Hsiang, 1975a). The SNS bolometer is based on the

temperature dependence of the critical current in a superconductor-normal metal-superconductor (SNS) junction (Clarke, et al. , 1974). The extremely low impedance of SNS junctions

(^ 10 ^ Q) necessitates the use of a SQUID (Superconducting QUantum Interference Device)

amplifier. A third device, known as the SIN bolometer (Clarke, Hoffer and Richards, 1974),

makes use of the temperature-dependent quasiparticle tunneling current through a superconductor-insulator-normal metal junction which is biased at a voltage below the superconducting

energy gap. The construction of all three types of bolometers is similar to that shown for

the transition edge bolometer in Fig. V-l.

Figure V-l. Typical structure for a composite bolometer (after Clarke, et al., 1978).

Radiation absorbed in the bismuth film heats the sapphire substrate

causing a change in the resistance of the aluminum superconducting

transition edge thermometer.

77
This device consists of a thin sapphire substrate with an optically absorbing bismuth coating

on one side. On the other side are an electrical heater for calibration purposes and an aluminum film which acts as the thermometer. Electrical leads and mechanical support are provided by four indium-coated nylon threads. Temperature stabilization of the mount

.-6 10 K//Hz) at the frequency of operation is provided by a combination of active control

and a massive heat sink. Electrical NEP is determined by measuring the response to electrical

heat dissipated in the bismuth heater. Since, in practice, the bismuth film will absorb only

about half of the incident radiation, the reported electrical NEP's for these devices are about half of what could be expected for radiation detection.

Table V-l shows the performance which has been achieved with the various types of

bolometers. The listed performances are all based on a negligible noise contribution from

background radiation. Except in applications with very low backgound radiation, any of these

bolometers would be background-noise limited. Thus, the use of superconducting bolometers is

justified only in applications with very low background radiation. The transition-edge

bolometer has a superior performance to the SNS and SIN devices. Since it is easier to construct and can be used with a conventional FET amplifier, it is the preferred device at the present time.

TABLE V-l

BOLOMETER PERFORMANCE

Bolometer NEP Theoretical

Type NEP

(W/v^L?) (W//Hz) (W/K) (s)

Op.

Temp.

(K)

References

SIN

SNS

Trans.

Edge

Ge

Al Trans. Edge

Ga;Ge

In:Sb:Ge

2. 5 X lO"15 Clarke, et al. (1978)

5 x lO"

15

1. 1 x lo"

15 io"8

3 1 5 Clarke, et al. (1974)

lO"

14

2. 5 X io" 15

5 x 10

-8

0 13 1 5 Clarke, et al. (1975)

7 x lO"

13

4. 2 X lO"

13

2 x 10

-4

0 0003 4 2 Coron, et al. (1975)

.7 x lO"

15

1. 3 x io"152 x IO"

8 0 08 1 27 Clarke, et al. (1978)

5 x lO"15

2. 2 X io"15

6 x IO"

8

0 025 1 2 Nishioka, et al. (1978)

6 x 10

-16

3. 4 X io"16

1 7 x IO"

8

0 006 0 35 Nishioka, et al. (1978)

2 1 /? a The theoretical limit is NEP , = (4kT G) th

b No results are reported for operating SIN bolometers

The last three entries of Table V-l show that there has been considerable progress

recently. Bolometers with very nearly ideal NEP's have been built using both superconducting

and non-superconducting thermometers. Further advances depend almost exclusively on the

78
ability to build devices with smaller and smaller heat capacities. Thus, superconductivity

does not offer the promise of any breakthrough in bolometer performance. It does offer,

however, a very simple, sensitive and low thermal mass thermometer which can be used with the new composite bolometers.

V.3.0. SUPER- SCHOTTKY DIODES

Diodes in the four forms: semiconductor-normal metal (Schottky diode), semiconductorsuperconductor (super-Schottky) , superconductor-insulator-normal metal (SIN) and superconductor-insulator-superconductor (Josephson junction or JJ) have all been fabricated. Each of

these devices has a nonlinear I-V characteristic and therefore can be used as a high-frequency mixer or detector. In addition to normal electron tunneling, the Josephson devices

have a pair-tunneling current which greatly complicates their operation. One of the effects

of the pair current is to up— convert low frequency fluctuations to the signal or i.f. frequencies, thus adding noise to the mixer output. The use of JJ's as mixers is treated in

Section V.4.2. of this report.

The presence of the superconducting energy gap gives the SIN and super-Schottky diodes a much higher degree of nonlinear ity than the conventional Schottky diode. The SIN device,

however, relies on electron tunneling through a very thin oxide barrier. This thin barrier

gives rise to a large capacitance which makes impedance matching at high frequencies very

difficult. The super-Schottky diode has a much lower and wider barrier and therefore can achieve the same current density with a much smaller capacitance. This property, together

with its high degree of nonlinearity, makes the super-Schottky diode a very promising mixer

element. Experimental results so far support this conclusion.

Super-Schottky diodes (Vernon, et al., 1977) consist of an array of 5 urn diameter Pb

dots electroplated onto heavily doped p-type GaAs. Electrical contact is made by a blunted

whisker which touches at the edge of two or more dots. As a result of the high doping level,

conduction is primarily by electron tunneling, and for voltages less than the superconducting

energy gap, the I-V characteristic is highly nonlinear.

The I-V characteristic of either a conventional Schottky or a super-Schottky diode can be approximated by

I = I e

(SV)

. (V-2)

o

The parameter S is a measure of the nonlinearity and is therefore of central importance in

determining the sensitivity of the diode as either a video detector or mixer. At 1 K,

S = 300 for a conventional Schottky, and S = 10,000 for a super-Schottky diode. If the same conversion loss could be obtained, this implies a 40:1 improvement in sensitivity for the

super-Schottky diode.

Figure V-2 is a curve of the intrinsic conversion loss of a super-Schottky diode re- sulting from the frequency conversion process. The absicca is el/kT where 1 is the superconducting energy gap and e is the charge of an electron. For a Pb super-Schottky diode at

1 K, this intrinsic loss is 4 dB. In practice, additional losses are contributed by the

7 s
diode capacitance and series spreading resistance. At X-band this parasitic loss is predicted to be about 3 dB.

T/Tc

0.4 0.2 0.1

o

°0 5 10 15 20 eA/kT

Figure V-2. Intrinsic conversion loss of a super-Schottky diode as a function

of the ratio of the energy gap eA to the thermal energy kT (after

Vernon, et al., 1977). In practice parasitic losses often con- siderably increase this loss.

Experiments at 9 GHz and 1.1 K have yielded a single sideband mixer noise temperature

T = 6 K and a conversion loss L = 7 dB. This is the lowest microwave mixer noise tempera- m s ture yet reported. For receiver applications, the noise temperature of the super-Schottky

mixer is comparable to parametric and maser amplifiers, while its bandwidth is considerably

greater. The saturation level of the super-Schottky diode can be estimated to be about

—8 5 P - PT

_ - 10 W. This corresponds to a dynamic range of less than 10 for T = 6 K and

s LO c j b m

B = 1 GHz. This relatively small dynamic range may limit the usefulness of super-Schottky

mixers in some applications.

The parasitic loss of these devices increases as cj 2 and therefore presents a very

serious problem for higher- frequency operation. Efforts to reduce the spreading resistance

by increased doping of the GaAs have not yet been successful because of the onset of an additional leakage current. Progress may be made by using a parallel array of smaller diodes

and by the use of other semiconductors in which higher conductivities can be achieved.

The use of a semiconducting barrier between two superconductors (Schyfter, et al., 1977)

may be an attractive alternative to the present super-Schottky designs. This device has the

strong nonlinear ity resulting from the superconducting energy gap, the low capacitance

characteristic of a thick barrier device, and no loss from spreading resistance. Although

such devices have been fabricated, no serious effort has yet been made to develop them as mixers.

Current and projected performance of super-Schottky mixers (Silver and Hartwick, 1977) are plotted on Fig. V-3. If the projected performance at higher frequencies is achieved, the

super-Schottky diode may be the best mixer available. The exceedingly low local oscillator

—8

requirements (10 W) will also be an important advantage at the higher frequencies.

80
WAVELENGTH (mm)

10

2 10

1 10° 10" 1 10" 2

FREQUENCY (Hz)

Figure V-3. Theoretical and achieved mixer noise temperatures as a function

of frequency.

V.4.0. JOSEPHSON DEVICES

Josephson junctions are highly nonlinear low-noise devices. The origin of the non- linearity can be seen from the two familiar Josephson equations for an ideal junction:

I(t) = I sin<f> (V-3)

c and

g-$V(t>. CY-4)

2e where — = 483 MHz/ijV. Equations (V-3) and V-4) show that a very small voltage across the

Tl junction leads to a rapidly oscillating current through the device. This property can be

exploited for high-frequency generation, mixing, detection, and parametric amplification.

Excluding parasitic effects, the inherent cutoff frequency of Josephson devices is a function

of the superconductors used and is given in Table V-2.

81
TABLE V-2

PROPERTIES OF A FEW SUPERCONDUCTORS

Material Transition

Temperature

(K)

Energy

Gap

(mV)

Cutoff

Frequency f (GHz)

Sn

Pb

Nb

NbN

Nb

3 Sn

Nb

3 Ge

3.7

7.2

9.2

16

18

21

1.15

2.4

2.8

4.8

5.4

6.4

440

910

1060

1800

2100

2400

At frequencies above f^, the Josephson effects die out rather slowly (1/f). It may

therefore be possible to use Josephson devices at frequencies as high as 10 f . For example,

harmonic mixing has been demonstrated at frequencies up to 4000 GHz 4 f ) using a Nb point

contact junction (McDonald, et al. , 1974).

V.4.1. Josephson Oscillator Sources

Equations (V-3) and (V-4) show that, if a constant voltage V is applied across a junction, the current will oscillate at a frequency 2eV/h. This suggests the use of a Josephson

junction as a high-frequency generator. In practice, very low power and large linewidth make

it quite difficult to utilize this high-frequency oscillator. Consider, for example, a typical 1 Q Josephson junction dc-biased to produce a 50 GHz output (^ 100 uV) . The maximum

2 -9

power which can be coupled out of such a device is on the order of V /8R - 10 W. Furthermore, Johnson and shot noise in the junction lead to voltage fluctuations which broaden the

linewidth of the oscillation. At 4 K this broadening is on the order of 1 GHz for a 1 Q

junction. Both the available power and coherence can be improved by coupling the junction to

a high-Q cavity. Output powers of 10 ^ W with a linewidth of 4 MHz have been observed for

this configuration (Dayem and Grimes, 1966). Future heterodyne receivers using Josephson or super-Schottky mixers with very low local oscillator requirements may be able to utilize a Josephson source as the local oscillator. This potential is greatest at very high frequencies where conventional sources are rare. Such a receiver design has been proposed

(Silver, 1975) but not yet demonstrated.

Arrays of Josephson junctions in close proximity have been observed to oscillate co- herently (Palmer and Mercereau, 1975) . Carefully designed arrays in a very high-Q structure

82
offer the promise of producing significant narrow band power. Work in this direction is

currently being pursued with electron beam lithography, and the results are encouraging

(Sandell, et al., 1978).

V.4.2. Josephson Mixers for Heterodyne Receivers

The most commonly used figure of merit for heterodyne receivers is their single sideband

system noise temperature

T = L. sys 1TM +T

i.f.

/T1 (V-5)

is the input loss, is the mixer noise temperature, T\ ^ is the noise temperature of

the i.f. amplifier and n is the mixer conversion efficiency. For moderate conversion

efficiency (n > 0.2), it is usually possible to reduce ^ so that the system noise is

principally determined by T^. If the background noise < T^, the rate at which information

can be received is inversely proportional to the system noise temperature. Thus it is clearly

desirable to achieve the lowest possible T,,. M

The large cutoff frequency and low operating temperature of Josephson mixers make them

attractive as a front-end component for low noise mm-wave receivers. Josephson mixers can operate with an external local oscillator or in a mode where a single junction acts as both

local oscillator and mixer (internal LO) . Both of these cases will be considered.

V.4.2.1. Josephson Mixer with Internal Local Oscillator

If a Josephson junction is biased at a voltage V

q and a signal frequency f is applied,

the junction will generate all of the frequencies nf

Q

± mf where f = 2 eV^/h. Thus, if the

bias is precisely set to V

q = (h/2e)/(f^ ± f ), the junction will act as both mixer and

local oscillator, i.e., the signal to be detected at f mixes with the self-oscillation f s o to produce an output at f . ^

. Thus, the local oscillator is tuned by adjusting the dc bias

and a separate local oscillator is not needed. At submillimeter and far-infrared wavelengths

the availability of an internal local oscillator easily tuned over a wide frequency range is a considerable advantage.

The theoretically obtainable noise temperature for Josephson mixers with either an external or internal local oscillator is roughly equivalent (see Fig. V-3) . However, in

mixers utilizing the internal LO, the large linewidth of the self-oscillation requires that

the i.f. be broadband and makes the mixer unsuitable for detecting narrow band (e.g., com- munication) signals. Its use will probably be limited to applications needing only moderate

frequency selectivity such as radio astronomy and infrared spectroscopy.

Reducing the LO linewidth by resonant techniques is not practical because the junction

must also couple effectively at the signal frequency. However, since the Josephson linewidth

Af is proportional to the impedance at the junction squared (Af

q ^ R

2

) , it may be possible

to reduce the linewidth by connecting a low impedance shunt across the junction. Such a shunt will short out the low frequency fluctuations which broaden f but it must have sufficient inductance so as not to short out the i.f. frequency. Satisfying both of these

conditions may prove to be difficult.

83
The results which have been achieved with internal LO Josephson mixers are shown in

Table V-3 and on Fig. (V-3) . At the present time these results fall well short of the

theoretical predictions. This is probably because the broadband nature of the internal local

oscillator severely limits the usefulness of the device. As a consequence development efforts

have been minimal.

V.4.2.2. Josephson Mixer with External Local Oscillator

If both a local oscillator (f^) and a signal (f

g

) are applied to a Josephson junction,

all of the frequencies £fTn + mf + nf are generated (f is the Josephson frequency). The

J_jO so o frequencies for which n = 0 are not subject to the line-broadening characteristic of f . Thus, the addition of an external local oscillator solves the line-broadening problem inherent to the Josephson internal LO mixer.

The operation of the Josephson mixer with an external LO is entirely different than that

of conventional mixers and can be understood with reference to Fig. V-4 as follows: The

application of local oscillator power (^tq) to the junction produces a series of constant

voltage steps in the I-V curve. These steps are separated in voltage by multiples of hf /2e

and their amplitude is a function of the applied power. When a signal at f is applied, the

effect is roughly equivalent to an amplitude modulation of P at the difference frequency

J_iU f . £

. For signal frequencies f < f Xsee Table V-2) the conversion efficiency can be greater

than one and because of the high order of the nonlinearity, efficient harmonic mixing is

possible.

I

I V

Figure V-4. The I-V curve of a Josephson mixer with externally applied local

oscillator power. Application of signal power causes the curve to vary between the solid and dashed lines at the difference frequency (f )

.

34
TABLE V-3

MIXER PERFORMANCE

Type Frequency SSB Noise Conversion Operating References

Temp. Tm Loss Temperature

(GHz) (K) (dB) (K)

JJ

Internal LO

JJ

External LO

Super Schottky

Cooled Schottky

Uncooled Schottky

Varactor Down

Converter

GeCu

Photo conductor

40 - 260

36

95

115

130

300

450

9

15

33

100

15

85

115

115

30,000

6000

54

93

140

180

220

500

6

186

196

300

437

420

500

275

5000

Gain of 1.3 dB

20

2.4

59.5

11

5.4

4.6

5.5

1.4

1.8

4.2

4.0

4.2

1.1

% 9

18

15

77

300

300

300

300

Avakjan, et al. (1975)

Taur, et. al. (1974)

Kanter (1974)

Taur and Kerr (1978)

Claassen & Richards (1978)

Edrich, et al. (1977)

Blaney, et al. (1978)

Vernon, et al. (1977)

Weinreb and Kerr (1973)

Weinreb and Kerr (1973)

Kerr (1975)

Weinreb and Kerr (1973)

Kerr (1975)

Kerr (1975)

Weinreb (1978)

Teich (1969)

85
Considerable experimental and theoretical effort has been devoted to the study of this

type of Josephson mixer (Claassen and Richards, 1978). At low frequencies, f << f (the

cutoff frequency), the theoretical value of has been found to be approximately 40 T where

T is the effective junction temperature. This effective temperature may differ from the bath

temperature because of junction heating. The reasons for the excess noise are complex and are related to the fact that the junction self-oscillation can convert low-frequency fluctuations into the signal and i.f. bands. At frequencies f >_ f , the noise temperature

2 ° increases as f . This behavior, assuming a typical Nb junction, is plotted in Fig. V-3

.

Because of their low capacitance and nearly ideal non-hysteretic I-V curves, point

contact junctions have been used almost exclusively in Josephson mixing experiments. J?jst of

these devices are adjusted during the experiment to achieve the desired I-V curve. Th; s mode

of operation is clearly impractical for field-usable or satellite-based receiver. Some

very recent work with a permanently adjusted point contact has shown that it is possible to fabricate mixer junctions which are moderately immune to vibration and can be thermally

cycled many times (Taur and Kerr, 1978). The mixer configuration used with this permanentpoint contact is shown in Fig. V-5. The contact is made between a Nb wire and foil attached

to metallized substrates which are glued to a. third base substrate as shown in Fig. V-5 (a).

Choke patterns on the substrates reflect a proper reactive impedance for optimum coupling to the junction. The junction assembly is placed across a reduced height waveguide mount. This

particular device has achieved a single sideband mixer noise temperature of 140 K at 115 GHz. This and other similar results with Josephson mixers are tabulated in Table V-3 and shown as the square points on Fig. V-3. Generally, the experimental results are within a factor of

two of the predicted performance for Josephson mixers. In the frequency range 30 to 300 GHz

their performance is the best reported to date. Although noise temperature is perhaps the

most important figure of merit for a mm-wave mixer, other parameters which must be considered

are conversion loss, instantaneous bandwidth, saturation and LO power requirements. Conversion loss for Josephson mixers is at least comparable to that for conventional mixers and in at least one case (Taur, et al., 1974), conversion gain has been reported. LO power require-

-9

ments are exceedingly low 10 W) . However, since mixers begin to saturate when the

signal power is comparable to LO power, Josephson mixers have a low saturation level. This

also implies a relatively small dynamic range. The instantaneous bandwidth of Josephson

mixers is in principal very large. However, if the mixer is exposed to a warm background, it

is often necessary to restrict the input bandwidth to avoid saturation from the background.

The use of resonant circuits to improve the signal coupling will also restrict the bandwidth.

As in the case of Josephson sources, the fabrication of arrays of thin-film junctions

offers improved performance. Although the noise temperatures cannot, in principal, be

lowered with arrays, improvements in impedance matching, bandwidth and saturation level

should be possible.

86

V.5.0. NON-SUPERCONDUCTING MIXERS

Because of their reliability and ease of operation, Schottky diodes are used almost

exclusively in current microwave and millimeter wave receivers. The performance of both

cooled and uncooled Schottky mixers is shown in Fig. V-3 and is clearly not as good as the

Josephson devices. However, the results for Schottky mixers represent measurements for

operational receivers while the Josephson data are mostly laboratory measurements.

The recent development of high-frequency (19 - 25 GHz) very low-noise (6 K) maser amplifiers at the Jet Propulsion Laboratory (JPL) has made practical the use of a Schottky varactor as a frequency down converter (mixer) . Although varactors can have a very low noise

temperature, their conversion loss is about f

g /f^

f

• F° r typical i.f. frequencies, this

conversion loss is unacceptable. However, when used with the JPL maser as an i.f. amplifier,

varactor down converters have achieved a system noise temperature comparable to the Josephson

devices (Weinreb, 1978). Furthermore, their theoretically obtainable performance exceeds

that of the Josephson mixers (see Fig. V-3).

V.6.0. JOSEPHSON PARAMETRIC AMPLIFIERS

A Josephson junction can be operated at zero bias so that it behaves as a nonlinear

inductor for currents less than the critical current. The nature of the nonlinear inductance

can be seen by rewriting the Josephson equations as follows:

dt

Lj(I)I

L sin 1 (I/I ) Lj(I) = m

C (V-6)

L _ =

J 2el

c

An ideal Josephson junction is thus a nonlinear inductor with a characteristic inductance L^

at zero bias. Josephson parametric amplifiers based on this mode of operation have received

the most thorough experimental and theoretical study and have become known as SUPARAMPS

(Superconducting Unbiased PARametric AMPlif ier)

.

These amplifiers have the advantages of simple construction, very low pump power re- quirements and wide instantaneous bandwidth. Wide bandwidth relative to conventional paramps

is possible because the self-coupling reactance in the SUPARAMP is small and there is no need

to resonate it out (Feldman, Parrish and Chiao, 1975). The principal disadvantage of the

SUPARAMP is its low output saturation level.

Figure V-6 shows the configuration for a typical reflection type SUPARAMP. Operating as 0

a three-photon device, one pump photon decays into a signal photon and an idler photon giving

f = f g + f^. A more successful mode of operation uses two pump photons to produce one

88
PUMP SOURCE

SIGNAL IN -SIGNAL OUT

TO RF MIXER

Figure V-6. (a) Microwave configuration for a reflection-type SUPARAMP and

(b) the detail of the microstrip circuitry (after Feldman,

et al. , 1975)

.

89
signal photon and one idler photon, 2£ = f + f^. The success of this four photon doubly

degenerate device depends on the fact that the unbiased Josephson junction is symmetric to a physical inversion. The high degree of nonlinearity of the Josephson junction normally

creates a large number of active idler frequencies which dissipate the signal power. High

gain requires that these parasitic frequencies be suppressed. The unique symmetry of the

four-photon device causes parasitic currents at frequencies ilf + mf to vanish when SL + m

p p is an even integer. The stripline stub at A/12 is used to short out currents for I + m = 3 so that the lowest frequencies at which loss occurs are on the order of 5 f . For reasons which will become clear shortly, Fig. V-6b indicates a series array of Josephson elements

rather than a single device. The theory of the SUPARAMP is well developed and predicts that

depletion of pump photons by the amplification process leads to saturation at an input

temperature

Tsat = 1 ' 4 N2v

GHz

r P

"5/4

(5 ° /Z

o )F

'

where N is the number of series-connected junctions, v^

Hz

is the frequency in GHz, V is the

peak gain and F is a function of the coupling. For typical values N = 1, Vq^

z ~ 33, = 100

(20 dB gain) and F = 1, the saturation temperature is Tsat

= 0.14 K. Achieving a practical

saturation temperature therefore requires a large number of series-connected junctions.

Since Josephson junctions typically have a resistance on the order of 1 ft, the use of a series array also simplifies the impedance matching problem.

Saturation in the SUPARAMP is most appropriately characterized by a maximum output

brightness temperature (Feldman, 1977). This output temperature cannot be exceeded regardless of the input temperature. Although this saturation is a severe limitation at present it

may be less of a problem in space applications where background temperatures are usually low.

The development of SUPARAMPS with larger I

cR products and perhaps the use of rectangular

arrays should substantially increase the achieved saturation levels. A summary of recently

reported results with Josephson paramps is given in Table V-4. The last three entries show

state-of-the-art results for conventional paramps and masers. It is fairly clear that at the

present time the SUPARAMP does not offer any significant advantage over more conventional

devices

.

V.7.0. SUMMARY AND CONCLUSIONS

It is probably fair to state that, in general, superconducting microwave and infrared

detectors demonstrate no overwhelming advantages over conventional devices. In particular

instances the performance of the superconducting detector represents the state-of-the-art,

but never by a wide margin. For superconducting detectors, the magnitude of development

efforts has been only modest, at best, and that situation will likely prevail for the near

future.

The space applications of these detectors would probably center on radio and infrared

astronomy and on communications, areas where the best possible noise performance is desired

Low noise performance almost certainly requires liquid helium temperatures for any type of

90
TABLE V-4

PARAMETRIC AMPLIFIER PERFORMANCE

SUPARAMP - 30 tunnel junctions in series (Wahlsten, et. al . , 1977)

10 GHz operating frequency

12 MHz bandwidth

24 dB gain

_g

10 W pump

1.7 x 10^ K maximum output brightness temperature

SUPARAMP - 80 or 160 series microbridges (Chiao and Parrish, 1976)

33 GHz operating frequency

3.4 GHz bandwidth

15 dB gain

20 ± 10 K SUPARAMP DSB noise temperature

220 K DSB system noise temperature

SUPARAMP - Single point contact (Taur and Richards, 1977)

36 GHz operating frequency

50 MHz bandwidth

11 dB gain

50 K noise temperature

Cooled (18 K) varactor paramp (Edrich, 1977)

47 GHz operating frequency

300 MHz bandwidth

22 dB gain

100 K system noise temperature

50 K noise contribution from paramp

Uncooled varactor paramp (Okean, DeGruyl and Ng, 1976)

15 GHz operating frequency

500 MHz bandwidth

25 mW pump power

75 K system noise temperature

JPL maser amplifier

19 - 25 GHz

50 - 230 MHz bandwidth

6 ± 2 K system noise temperature

91
detector, and indeed, the planned IRAS program supports this contention. Thus, the environment for superconductivity will usually exist for any low noise missions, and superconducting

detectors will be able to compete on an almost equal footing with their non-superconducting

counterparts.

Besides the need for further improvements of performance, there is a need for development of more stable and long-lived devices. Point contacts may never be adequate for space

flight, and even the present thin-film devices suffer failures. This situation may improve

as the lithographic thin-film technology, generated by digital electronic development (see

Chapter IV), becomes more widespread. Particular importance should be attached to the application of electron-beam lithography to the fabrication of high-frequency detectors. Photolithography, while useful in some cases, will probably not provide the very small dimensions

required for high frequency work. While thin-film devices should, in principle, work as well

as the point contacts, developmental effort will be required to demonstrate this.

There are several conclusions to be drawn from a study of Fig. V-3. First, the superSchottky diode warrants further development. The big questions involve the suppression of

parasitic losses and extension of work to higher frequencies. The Josephson mixer is clearly

ahead of the conventional Schottky diodes __and the varactor down converter, but the predictions

for an improved varactor down converter are better than the theoretical Josephson limit. The

questions here are clearly unsettled, and more work should be done on both devices.

The SUPARAMP, while not up to the performance levels of some non-superconducting devices,

has certain attractive features. Prime among these are the wide bandwidth, low pump power

requirement and very simple construction. Even with the very modest effort on development of

the SUPARAMP, performance is quite respectable (see Table V-4) and further improvements are expected. The low pump power required of this device may be a key advantage for extended

space missions where heat load in the cryogenic environment is of vital concern. This

advantage also holds for the Josephson mixer.

Apparently, the key question in further bolometer development centers around the re- duction of heat capacity. The selection of thermometer, superconducting or not, is primarily

a question of convenience and simplicity. The superconducting transition-edge thermometer

fairs well in this respect.

To summarize then, superconducting detectors, while offering significant advantages

(both real and potential), do not appear to be a panacea for all high-frequency instruments.

However, the full story is not yet told. A lot depends on how closely the superconducting

and non-superconducting devices approach theoretical performance levels. Presently there

appears to be no really strong incentive for large increases in developmental funding levels.

The major questions should be settled during the next decade at present spending levels.

However, should some of the superconducting devices (particularly the junction devices) gain

clear advantage, then it would seem essential to stimulate work directed toward thin-film

(lithographic) realizations of the devices, in order to satisfy the stability requirements

demanded by space flight.

92
V.8.0. REFERENCES

Adde, R. and Vernet, G. (1977). High Frequency Properties and Applications of Josephson

Junctions from Microwaves to Far-Infrared. Superconductor Applications: SQUIDs and

::.:.-!:: >:cs. eds., B.B. Schwartz and S. Foner, Plenum Press, New York. 249-320.

Avakjan, R.S., Vystavkin, A.N. , Gubankov, V.N. , Migulin, V.V. and Shtykov, (1975). Millimeter Waveband Signal Conversion in S-c-S Josephson Junctions with Self Pumping. IEEE

XA,~11, 838-840.

Blaney, T.G., Cross, N.R. and Jones, R.G. (1978). Performance of a Josephson Heterodyne Receiver at 450 GHz. Proa, of Conf. on Future Trends in Superconductive Electronics

,

Charlottesville, Virginia (to be published).

Chiao, R.Y. and Parrish, P.T. (1976). Operation of the SUPARAMP at 33 GHz. J. Appl. Phys.

47, 2639-2644.

Claassen, J.H. and Richards, P.L., (1978). Point Contact Josephson Mixers at 130 GHz.

Proceedings National Radio Science Meeting, to be published.

Clarke, J., Hoffer, G.I. and Richards, P.L. (1974). Superconducting Tunnel Junction Bolometers. Rev. Phys. Appl. 9, 69-71.

Clarke, J. and Hsiang, T.Y. (1975a). Low Frequency Noise in Superconducting Sn Films at the

Transition Temperature. IEEE MAG 11, 845-847.

Clarke, J., Hoffer, G.I., Richards, P.L. and Yeh, N.Y (1975b) A Superconducting Transition

Edge Bolometer. Low Temp. Phys. - LT14, Vol. 4, eds. M. Krusius and M. Vuorio, American

Elsevier, New York. 226-229.

Clarke, J., Hoffer, G.I., Richards, P.L. and Yeh, N.H. (1978). Superconductive Bolometers

for Submillimeter Wavelengths. J. Appl. Phys., to be published.

Coron, N. , Dambier, G. , Leblanc, J.L. and Moalic, J. P. (1975). High Performance FarInfrared Bolometer Working Directly in a Helium Bath. Rev. Sci. Inst., 46, 492-494.

Dayem, A.H. and Grimes, C.C. (1966). Microwave Emission from Superconducting Point-Contacts.

Appl. Phys. Lett. 9, 47-49.

Edrich, Jochen, Sullivan, D.B. and McDonald, D.G. (1977). Results Potentials and Limitations

of Josephson Mixer Receivers at Millimeter and Long Submillimeter Wavelengths. IEEE

MTT-25, 476-479.

Edrich, Jochen (1977). A Cryogenically Cooled Two-Channel Paramp Radiometer for 47 GHz. IEEE MTT-25, 280-285.

Feldman, M.J., Parrish, P.T. and Chiao, R.Y. (1975). Parametric Amplification by Unbiased

Josephson Junctions. J. Appl. Phys. 46, 4031-4042.

Feldman, M.J. (1977). The Thermally Saturated SUPARAMP. J. Appl. Phys. 48, 1301-1310.

Kanter, H. (1974). Josephson Junction Mixer Using an External Local Oscillator. Rev. Phys.

Appl. 9, 255-262.

Kerr, Anthony R. (1975). Low Noise Room Temperature and Cryogenic Mixers for 80-120 GHz.

IEEE MTT-23, 781-787.

Lukens, J. and Sandell, R. (1978). private communication. SUNY at Stonybrook.

McDonald, D.G., Petersen, F.R. , Cupp, J.D., Danielson, B.D. and Johnson, E.G. (1974).

Josephson Junctions at 45 Times the Energy Gap Frequency. Appl. Phys. Lett. 24,

335-337.

93
Nishioka, N.S., Richards, P.L. and Woody, D.P. (1978). Composite Bolometers for Submillimeter Wavelengths. Applied Optics, to be published.

Okean, H.C., DeGruyl, J. A. and Ng, E. (1976). Ultra Low Noise, Ku-Band Parametric Amplifier

Assembly. IEEE MTT-S, International Microwave Symposium, 82-84.

Palmer, David William and Mercereau, J.E. (1975). Coherent Effects in Series Arrays of

Proximity Effect Superconducting Bridges. IEEE MAG-11, 667-670.

Sandell, R.D., Varmazis, C. , Jain, A.K. and Lukens, J.E. (1978). Study of the Properties

of Coherent Microbridges Coupled by External Shunts. Proa, of Conf. on Future Trends

in Superconductive Electronics, Charlottesville, Virginia (to be published).

Schyfter, M. , Maah-Sango, J., Raley, N. , Ruby, R. , Ulrich, B.T. and VanDuzer, T. (1977).

Silicon-Barrier Josephson Junctions in Coplaner and Sandwich Configurations. IEEE MAG13, 862-865.

Silver, A.H. (1975). Superconducting Low Noise Receivers. IEEE MAG-11, 794-797.

Silver, A.H. and Hartwick, T.S. (1977). New Horizons in Receiver Technology. Report No. SAMSO-TB-77-205, Air Force Systems Command SAMSO.

Taur, Y. , Claassen, J.H. and Richards, P.L. (1974). Josephson Junctions as Heterodyne

Detectors. IEEE MTT-22, 1005-1009.

Taur, Y. and Kerr, A.R. (1978). Low Noise Josephson Mixers at 115 GHz Using Recyclable Point

Contacts. Proceedings National Radio Science Meeting, to be published.

Taur, Y. and Richards, P.L. (1977). Parametric Amplification and Oscillation at 36 GHz Using

a Point-Contact Josephson Junction. J. Appl. Phys. 48, 1321-1326.

Teich, M.C. (1969). Homodyne Detection of Infrared Radiation from a Moving Diffuse Target.

Proc. IEEE 57, 786-792.

Vernon, F.L., Jr., Millea, M.F., Bottjer, M.F. , Silver, A.H. , Pedersen, R.J. and McColl, M. (1977). The Super-Schottky Diode. IEEE MTT-25, 286-294.

Wahlsten, S., Rudner, S. and Claeson, T. (1977). Parametric Amplification in Arrays of

Josephson Tunnel Junctions. Appl. Phys. Lett. 30, 298-300.

Weinreb, Sander and Kerr, Anthony R. (1973). Cryogenic Cooling of Mixers for Millimeter and

Centimeter Wavelengths. IEEE SC-8, 58-63.

Weinreb, S. (1978). Low Noise Millimeter Wave Parametric Down-Converters. Proceedings

National Radio Science Meeting, to be published.

94
;ha?tz?. vi

instruxents tzz- z-?:-::zz.-~z e~:::=

R. L. Peterson

VI. 1.0. INTRODUCTION 97 VI. 2.0. INSTRUMENTATION 98 VI. 2.1. Accelerometers 98

VI . 2 . 2 . Gravimeters 100

VI. 2. 3. Superconducting Bearings 101

VI. 2. 4. Superconducting Gyroscope 102

VI. 2. 5. Inertial Guidance and Navigation Systems 103

VI. 3.0. TESTS OF GRAVITATIONAL THEORIES 104

VI. 3.1. Detection of Gravitational Radiation 104

VI. 3. 2. Equivalence Principle 107

VI. 3. 3. Gravitational Red Shift 108

VI. 3. 4. Geodetic and Mass-Current Precession of a Gryoscope Spin Axis 108

VI. 3. 5. Measurement of the Gravitational Constant G. Cavendish Experiments 109 VI. 4.0. GRAVITY AND ELASTIC PROPERTIES OF THE PLANETS 110

VI. 5.0. NAVIGATION 110

VI. 6.0. SUMMARY AND CONCLUSIONS Ill

VI. 7.0. REFERENCES 113

95

VI. 1.0. INTRODUCTION

Of all the known forces of nature, gravity might be called the first-born, but the least

well understood. It has been said that if we knew as much about electricity as we now know

about gravity we wouldn't yet have the electric light bulb. The reason for our relative lack

of knowledge is that gravitational forces are so very much smaller than the others. Isaac

Newton developed his theory of gravity based upon earlier observations of planetary motion.

This theory held up remarkably well until the early 1900' s when Einstein developed his theory

of general relativity, largely on philosophical grounds. This theory was verified to a certain extent within just a few years in three ways, and thereafter because it was con- sidered such a profoundly beautiful theory, it was not much questioned until about the early

1960's. It was at about this time that it became clear that space technology would allow more tests of general relativity than people had ever imagined before. This realization

provided a great stimulus for critical thinking about Einstein's theory and for the invention

of many new theories of gravity.

An example of an alternate theory is Dirac's "large-number hypothesis," which is itself

philosophically grounded. This theory has among its consequences that the gravitational

constant G in fact should decrease with time. Now in general relativity G is a constant;

therefore if a test were to show that G is decreasing, this would have profound implications,

bearing on the age of the universe and its evolution. So, in the scientific community, it is now considered to be very important to make very good tests of the theory of general relativity to see whether in fact it is correct. Some tests have already been carried out to rather great precision, and others are in progress or in the proposal stage. Those for which

superconductivity is making or can make a contribution are discussed in the following sections. To repeat, it is the advent of space technology that has made possible these new

tests of gravitation theories, and it is clear that the new results will come in the main

from space exploration and instrumentation.

The superconducting cavity stabilized oscillator is a key instrument for the test of

gravitational theories. The importance of this device is such as to warrant separate coverage in the following chapter. However, for cohesiveness , the discussion of tests of gravitational theories (Section VI. 3.0. in this chapter) will cover such applications. Cross

referencing will be used to tie these two chapters together.

The measurement of the time-variation of the local gravitational field at the surface of

a planet is also of considerable interest. Such measurements provide useful information

about the planet interior as well as the planet's interactions with other objects in space

(moons, planets, etc.). Superconductivity already plays a useful role for such measurements

on earth (especially for long-term variations) and deployment of such instrumentation on the moon or other planets would provide interesting information.

Because of their relation to gravity, inertial guidance and gyroscope guidance are discussed in this chapter. While serious proposals for the application of superconductivity to such guidance have not yet been made, the needs for improved autonomous navigation in the

future may prompt thought in this direction.

97
VI. 2.0. INSTRUMENTATION

V1 . 2 . 1 . Accelerometers

Strictly speaking, an accelerometer measures accelerations. However at the present time

the term accelerometer is also used for those instruments which are capable of detecting very

small displacements, such as the displacements of the ends of the large cylindrical antennas

used in the detection of gravitational radiation (see Section VI. 3.1.).

Acceleration measurements per se are made by sensing the force necessary to keep a proof mass in place with respect to the accelerating frame, using Newton's second law to calculate

the acceleration from the known mass and the measured force. The force is often a spring or

a string under tension or some manifestation of electromagnetic or electrostatic forces. The

conventional force rebalance accelerometers are electo-mechanical devices, and are typically

designed for a given range of acceleration. We shall not further describe these conventional

accelerometers except to point out the existence of a recently-developed low-g accelerometer

which has the acronym MESA. This accelerometer was specifically developed for sensing small

accelerations on space craft and will be tested and calibrated in Space Lab II. The measurement of acceleration in the MESA is accomplished by sensing the motion of the proof mass, which is a berylium cylinder, with respect to the instrument case. Electrostatic forces are generated to oppose the motion of the cylinder in a direction to restore it to its null or zero input position.

It seems clear that a superconductive analog of MESA could be designed and used profitably in special situations. Any vehicle with cryogenics on board might well use superconductive rather than electrostatic positioning and sensing. For example, the proof mass might

be a hollow superconducting sphere. Superconducting coils might be used for generating the

magnetic force for positioning the sphere, and a SQUID magnetometer might be used for sensing

the position of this sphere. As noted below, the projected performance of a SQUID for such

position sensing is expected to be excellent.

Many types of superconducting accelerometers, used as displacement sensors, have been

proposed. Basically, they are all the same. That is, they use a pendulum which swings

within some type of electrical system, thereby changing the properties of that electrical

system, and that change is detected. There are both resonant and non-resonant types of

accelerometers

.

Perhaps the best-known superconducting accelerometer (at the moment) is the one developed at Stanford University (Paik, 1976) specifically for the purpose of detecting gravitational waves in conjunction with large cylindrical antennas. This instrument has been under

development for several years. At 3 mK, which is the temperature at which the entire system

is hoped eventually to operate, this accelerometer is projected to detect (ideally) a motion

-20

between the ends of the cylinder of 3 x 10 m in a 1 Hz bandwidth. A superconducting

niobium diaphram oscillates between two coils which are themselves made of superconducting

material and carry a persistent current. The oscillating diaphram modulates the inductance

of these coils, thereby changing the current. This current change in turn is detected by a SQUID magnetometer. This particular accelerometer is designed to resonate at the resonance

frequency of the cylinder, about 1 kHz.

98
Another type of accelerometer which has been proposed has been called a flux gradient

accelerometer (Hoffman, Douglass, Gram and Lam, 1976). It consists of a pair of cylindrical

magnets in an opposing field configuration mounted rigidly to the end of the cylinder and

hence vibrating with the cylinder. A pendulum with coils mounted on it hangs between the two

magnets. The magnetic flux through the coils changes as the pendulum moves relative to the

magnets, and this induces a voltage. A SQUID can be used to measure the flux change if the

-20

coils are superconducting. The projected sensitivity is 9 x 10 m for a 1 Hz bandwidth.

A preliminary version has been built but the device is not nearly as well developed as the

tunable diaphram device mentioned previously.

Another accelerometer has been proposed in which a superconducting mass is supported by

a superconducting magnetic field which is rigid with the case. An acceleration causes an inductance change which is detected, for example, by a SQUID. A variation of this, which

will offer a more rapid response time, consists of tuning the support circuit for the superconducting mass at a high resonsance frequency, for example, 30 MHz. The voltage across the

tuned circuit is measured and fed back to hold the mass at a constant position. The acceleration is proportional to the feedback current. This proposal might offer sensitivity to -19

motions of 10 m at 1000 Hz. Yet another idea for a superconducting accelerometer is based upon the use of superconducting re-entrant cavities. In this design (no publications yet) , which has been con- structed in a preliminary version, the cavity is mounted rigidly to the end of the bar whose

motion is to be detected. The superconducting cavity is driven at its resonance by an oscillator (about 600 MHz) . To the rear end of the cavity is mounted a small mass in the

shape of a cylinder. The mechanical resonant frequency of the rear wall with its mass is

about 10 Hz, much smaller than the approximate 1000 Hz resonant frequency of the large

antenna cylinder. When the antenna vibrates, the relative motion between it and the small

cylinder within the cavity changes the resonant frequency of the cavity, which change is

detected by conventional techniques. However, this idea has been found to be inadequate

because of the instabilities in the oscillator. These instabilities change the resonance

curve so that the motional effect is obscured. An improvement on this concept is to use a second identical re-entrant cavity back- to-back with the first, fed by the same oscillator.

If the signals from the two cavities are subtracted, the oscillator instabilites are eliminated to first order, but it is found that they are not eliminated entirely. Unambiguous

gravitational wave detection with this system is still not possible. One needs a still

better oscillator and the developers of this type of accelerometer say that this idea will

not be successful until an oscillator with the capabilities of the superconducting cavity

stabilized oscillator (Capter VII) has been developed.

A variant of this idea is to replace the two cylinders which are mounted back-to-back

at the rears of the two re-entrant cavities, by a single cylinder which is levitated magnetically. This cylinder will be hollow with a hole in the side. Through the hole is fed a wire making a loop in the interior of the cylinder, levitating it. This cylinder passes

through openings in the ends of the two superconducting cavities and is thus , for all practical purposes, freely suspended, and should offer greater sensitivity. But as in the preceding case, a more stable oscillator is needed for successful operation.

99
Finally, we shall mention a non-resonant accelerometer which has been called a singleaxis superconducting accelerometer (Fairbank, et al. , 1974). In this design a straight

superconducting wire carrying a current passes through a horizontal niobium spool , levitating

it. The accelerometer case is attached to the end of the antenna and moves with it. The

spool remains fixed in space. A superconducting inductance is mounted on the case and its

inductance is modulated because of the relative motion of the case and the spool. The inductance can be made part of a resonant circuit so that the resonant frequency changes with

4 the motion of the bar. A prototype model has been built with a Q of greater than 10 for

-17

the varied inductance, enabling measurements of deflections of the order of 10 m at 2 K. This is one of the older proposals. We have not heard of any recent extension- of it and

believe development on it has stopped.

For comparison with the above displacement sensitivities, the strain gauges used by

Weber in the initial gravitational wave detection experiments could detect displacements of

-16

the order of 10 m.

VI. 2. 2. Gravimeters

A gravimeter is an instrument in which the gravitational force acting on a mass is

balanced by an elastic restoring force. Gravimeters may be called seismometers, but by

convention the term is reserved for the measurement of periods greater than about 100 s. All

the gravimeters using a mechanical spring for the elastic restoring force tend to creep under

stress. This creep normally gives rise to irregular drifts. The best mechanical room tem-

—8

perature gravimeters are the La Coste gravimeters. These have drifts of about 10 g per

day, which makes them unsatisfactory for measurements of phenomena whose variation is less

than this. The fortnightly earth tides provide one example for which the La Coste gravimeters are inadequate. These motions are now being measured routinely by superconducting

gravimeters

.

In the late 1960 's a superconducting gravimeter was developed by two groups using essen- tially the same design (Goodkind and Warburton, 1975 and Tuman, 1974). In this instrument a hollow niobium sphere which acts as a perfect diamagnetic body is supported by the extremely

stable magnetic fields and gradients produced by the persistent currents in a set of superconducting coils. As long as the magnetic field is smaller than the critical field of the

superconducting elements of the instrument, the magnetic restoring forces do not give rise to magnetic creep of measurable magnitude. The instruments are believed to be entirely free of

magnetic creep because of the cylindrically symmetric construction which is all of aluminum

and maintained at liquid helium temperature. In the superconducting gravimeters the signal

-9 -11 drift is less than 10 g per day; better than 10 g per day is theoretically possible but

not yet achieved.

A number of superconducting gravimeters are now being deployed throughout the U.S. in an earth-tide measurement program. These gravimeters use capacitive sensing of the position of

the ^superconducting sphere. Other similar gravimeters, however use a more sensitive SQUID

readout system.

The superconducting gravimeters developed to date are also more sensitive than the best

La Coste mechanical gravimeters at frequencies lower than a few cycles per hour. At two cycles per month, for example, a superconducting gravimeter is 100 - 1000 times more

'sensitive

.

100
In addition to the earth-tide measurement program, the superconducting gravimeters have

been used to search for gravitational effects not contained in general relativity, specifically, evidence for a preferred reference frame and a possible variation of the gravitational constant. Upper limits were placed on the possible values of these effects. Also, as we will mention in Section VI. 3. 0. , a superconducting gravimeter has been used in an effort

to detect the influence of gravitational radiation on the earth's vibrational behavior.

Another area of potential use lies in earthquake prediction. The so-called dilatancy theory

predicts surface rises as high as 1 m per month in some cases (Goodkind and Warburton, 1975),

which would correspond to a change in g of about 3 x 10 ^ g per month. Because of the great

stability of the superconducting instrument it is apparent that it could be very useful here.

A La Coste gravimeter was placed on the moon in the Apollo 17 expedition. The primary

purpose of that experiment was an attempt to detect gravitational radiation by looking at the

moon's vibrations. However, the gravimeter did not function properly. The La Coste gravimeters are very sophisticated and complicated mechanical devices having springs, levers, and

hinges. Since a superconducting gravimeter is an inherently simpler instrument, it might be

appropriate to consider its use in future studies of the moon and planets.

Further development of superconducting gravimeter design is proceeding very slowly at present, if at all. At the moment, the groups are primarily interested in using their present instruments to make physical measurements. However, it is apparent that renewed development effort could be worthwhile since the sensitivities of the present instruments are some

two orders of magnitude less than their theoretical sensitivities.

VI . 2 . 3 . Superconducting Bearings

There are very many applications in space where frictionless rotational or translatory

motion of a shaft or cylinder would be useful. Examples include the chopping wheels used

with infrared devices, servo-controlled tipping plates, rotating knife-edge detectors, the

proposed equivalence principle concentric cylinder experiment (in space) , and even an ex- periment on a rotating bucket of liquid helium. A particularly intriquing idea would be the

use of bearings for a dilution refrigerator in space. Earth-based dilution refrigerators

require gravity to effect the phase separation used for operation of the refrigeration

cycle. A rotating cryogenic chamber would allow the phase separation in space to be effected

centrifugally.

Efforts have recentl}7 begun on developing superconducting bearings of real technological

utility, having small machining tolerances and high reliability. Of course several devices

described in this report use superconducting levitation and accurate position sensing, for

example the accelerometers and the gravimeters. But a systematic study of the engineering

requirements for bearing performance specifically has not been made until recently. Many

experiments (already proposed) could probably be improved and other experiments might become

possible with the availability of a properly engineered superconducting bearing. Conventional bearings encounter severe trouble at cryogenic temperatures because of lubricant

problems and heat dissipation.

101
Most of the work on superconducting bearings to date has been on the translatory mode

of the bearing, and this by a single group (Worden and Everitt, 1974) , so far as we know.

The work has been done in connection with an equivalence principle experiment in space,

which will be discussed in Section VI. 3.2. In the apparatus now under development, the

levitating magnetic field for the shaft or cylinder is provided by a series of wires wound

back and forth in a cylindrical cradle. This reverses the current direction in adjacent

wires so that the net magnetic field at large distances is zero, which is important for

reducing the influence of the field in other parts of the space vehicle. The coils must be

wound to very high tolerance to operate properly, since very large variations in the normal

force can result from very small deformations in the support coil. Since the force is

repulsive and decreases with the distance of the cylinder from each point in the surface of

the cradle, the bearing is automatically self-centering and dynamically stable.

It is important not to confuse the type of levitation for these superconducting bearings with that used in superconducting generators (see Section II. 4. 4.) or the superconducting train. The train carries high-field superconducting magnets and runs over an alumi- num track. At high speed the repulsive force between moving magnets and eddy currents in

the track causes levitation; thus that bearing is inherently dissipative. The superconducting generators are being developed at MIT and several industrial companies. They substitute

a rotating superconducting magnet for the armature winding of the conventional generator.

Although such generators could use superconducting bearings, there has been little incentive

for this, since a transition to room temperature has to be made at some point to couple the

generator to the power source. Superconducting bearings is an area of great potential "leverage" and has not received

significant funding to date. A few years of development would probably result in bearing

specifications of wide utility in space. The development to date has shown no insuperable

problems, but clearly there is a long way to go before superconducting bearing design becomes standard. Included among the research still needed are: investigation of materials

and construction techniques (magnetic contaminants have degraded performance) ; fabrication

of bearings with lower eddy current dissipation and better manufacturing tolerances; and

analysis of different possible configurations, such as thrust bearings, linear bearings,

load bearings, and bearings with stiff and loose support.

VI. 2. 4. Superconducting Gyroscope

Work on the superconducting gyroscope has been supported almost entirely by NASA. The

work began in about 1963, and has resulted in many conference reports and very many publications. Since the work has been so thoroughly described (Everitt, 1974; Lipa, Fairbank

and Everitt, 1974) we shall not give many details here but simply make a few comments for

the sake of completeness.

This device is without question the most sophisticated of the devices described in this

report, and its design seems to be nearing completion. Among the advances needed in the

state of the art of gyroscope technology were the following: A gyroscope with a drift lower

by a factor of at least 10

6 than the best conventional gyroscopes was essential. A telescope to provide a directional reference to a star to 10

3 arc seconds without drift for a

102
year was needed. A means to sense the pointing of the spin axis of the gyroscope to the same level of accuracy was needed (this has been accomplished by means of superconducting

loops and detectors). Magnetic shielding to remove extraneous magnetic fields essentially

to the zeroth magnetic quantum state was required. A servo system for drag compensation of

-9

the satellite to maintain the net acceleration field acting on the gyros below about 10 g was required (this has been accomplished through the development of the drag-free satellite,

which has been shown to be good enough for this particular experiment). Also, a dewar for a full year's operation at liquid helium temperature was required (this should be demonstrated

in the IRAS program)

.

Briefly, the superconducting gyroscope consists of the following elements: A 4-cm

diameter quartz sphere is given a superconducting niobium coating. This sphere is electrostatically supported by an active 3-axis servo system. Changes in the spin axis direction

are detected and read out by a SQUID and superconducting loops, which link the lines of

magnetic flux that result from the so-called London moment of the spinning sphere. The

Marshall Space Flight Center and Stanford University are designing alternative readout

systems (Hendricks, 1975). Marshall is designing the SQUID readout system directly into the

gyroscope housing, whereas Stanford uses a superconducting loop in the housing with a SQUID

external to it. The largest remaining problem seems to be that of spin-up. At the earth's

surface large electrostatic forces must be used to counteract the effects of gravity, and

this makes the testing of the spin-up of the gyroscope a very difficult one. It is possible

that some of this testing will be carried out on a space shuttle. Finally, theoretical work

has been recently published on the torques on the sphere rotating in a magnetic field, and on

the trapped-flux spin-down torques, and these have been found to be negligible.

VI . 2 . 5 . Inertial Guidance and Navigation Systems

Gyro-stabilized platforms are the heart of the usual inertial guidance and navigation

systems. The accuracy of such systems depends ultimately on the accuracy with which the axes

of the platforms can be maintained fixed in inertial space. Requirements at the extremes of

operation are: (1) drift-free operation over long periods of time, for example, on the

submarines in Navy usage, or on deep space satellites in the space program; and (2) accurate

performance for brief periods of time in an environment of severe accelerations and vibrations, as in missile or rocket systems. The drift of the present navigational gyros is now

of the order of 1 arc sec per hour (Section VI. 5.0). Compare this drift with the drift of the

superconducting gyroscope under development, which is designed for less than 0.05 arc sec per

year. This comparison should be made with caution. The superconducting gryoscope will have

the advantage of operation in zero gravity. A significant portion of its advantages would

surely be lost if it were to be operated in the earth's gravitational field.

Twenty years ago, a proposal was indeed made to build a navigation system based upon a superconducting spinning sphere suspended in stable equilibrium by magnetic fields. Obviously this design is quite similar to those that have been discussed elsewhere in this

report, for example, the gravimeters which are now very successful instruments. It is

apparent that the superconducting gyroscope already basically developed for the general

relativity program (Section VI. 2. 4.) could also be used as a navigational gyro.

103
Another type of inertial reference system based upon superconductivity was proposed

about a dozen years ago in a patent (Clauser, 1965). In this design a microwave field is

injected into a spherical superconducting cavity. Clauser argues that this field will maintain its orientation in space as the sphere rotates. A detector placed above an iris which

is initially coincident with one of the node lines of the microwave field inside the cavity

would then give a non-null reading when the sphere rotates. The angle necessary to bring

back the null measures the deviation from the original course. A system of two or three such

spheres gives complete directional information. A very high-Q cavity may be needed so that

the field will not dissipate during the time of the necessary guidance. Aside from practical

problems, we are not totally convinced that the principle is valid. The idea, which we mention for completeness, apparently has not been followed up, for we have not found anything

further in the literature on it, but it might be worthy of further analysis.

VI. 3.0. TESTS OF GRAVITATIONAL THEORIES

VI. 3.1. Detection of Gravitational Radiation

Most theories of gravitation predict that gravitational waves should exist. These waves have not yet been unambiguously detected, but hold a great deal of popular interest. Work

first seriously started in this area in about 1960, and in the late 1960 's the first reports

of the possible detection of gravitational radiation were published by Joseph Weber at the

University of Maryland. He used a large aluminum cylinder (now called a Weber antenna) to absorb the gravitational radiation, and piezoelectric strain transducers to detect the re- sulting internal oscillations of the cylinder. Because signals were observed frequently,

Weber instrumented two such antennas separated by about 1000 km, and searched for coincident

signals. He reported an excess of coincident signals over chance, and ruled out other sources of excitation such as cosmic rays, seismic disturbances, or electromagnetic disturbances. Weber's techniques and results touched off a world-wide effort to repeat his

experiments with various modifications.

However, none of the other groups have yet reported seeing enough coincidences to make

them think they have seen gravitational radiation. The physics community today generally

believes that what Weber saw was not gravitational radiation. There are two main reasons for

this. One can calculate the intensity of a gravitational wave which would have excited the

Weber antennas above the noise level. This intensity is several orders of magnitude higher

than that which is reasonably expected from various sources of gravitational radiation, for

example, the collapse of a star. The other reason, of course, is the fact that no one else

has seen excess coincident signals, using supposedly more sensitive instruments. Nevertheless, the physics community almost unanimously believes that gravitational radiation has to be there, and many groups are proceeding with great vigor in constructing yet more sensitive

Weber-type antennas.

The most important of the ideas today is that of lowering the temperature of the an- tenna to much less than 1 K; one group has announced a design goal of 3 mK. We will discuss

this particular antenna in more detail because it is the one which uses superconductivity in

several ways. Specifically, it uses a superconducting coating of niobium-titanium on the

104
surface of the aluminum cylinder in order that the cylinder can be levitated by superconducting magnets. This has been successfully accomplished. A superconducting accelerometer

has also been built (see Section VI. 2.1.), of such a sensitivity that the combination of this new accelerometer, with the excellent vibration isolation provided by the magnetic levitation

and the reduction of thermal noise (proposed 3 mK temperature) suggests that this particular

antenna will ideally be some six orders of magnitude more sensitive than the original Weber

antenna. The sensitivity to relative motion of the two ends of the cylinder in an ideal case -20

would be about 10 m. This system has not yet been put into operation although various

parts of it have been tested. The cylinder has been brought to about 4 K and the superconducting accelerometer has measured the 4 K thermal noise. The apparatus is surrounded with

superconducting shielding to eliminate extraneous magnetic fields.

Although not all groups are opting for magnetic levitation, there are two principle

advantages to the magnetic support. For one thing, the support force is almost uniform

across the length of the cylinder, so that there is no mode selection. In the mechanically

supported versions there necessarily have to be nodes at the support points. Secondly,

magnetic support provides probably the highest degree of isolation from vibrations which are inherent in the earth. A third advantage is that a magnetically suspended cylinder is

relatively immune to heat leaks, an important consideration for a cooled cylinder.

Other designs and techniques for detecting gravitational radiation have also been

proposed. For example, a hollow square gravitational wave antenna has been pr*^osed.

Aluminum disk antennas have been built. Detection of the induced beat frequency between two crossed rotating dumbells has been proposed. It has also been pointed out that &t> far as the

cylindrical antennas are concerned, one does not really have to have a very massive cylinder

because a higher Q value (smaller damping) than is possible with aluminum can be achieved by

using a single crystal dielectric. With a higher Q value one can get away with smaller mass. Sapphire, for example, has been proposed for such an antenna. Laser interf erometry is yet

another technique. Here the displacement between two mirrors is measured by means of a shift

of fringes and a sensitivity of 10 ^ m has been achieved. This can certainly be improved.

The laser interferometer is a non-resonant antenna, that is, broad-band, and should be capable of detecting frequencies up to more than 20 kHz. (The cylinder antennas are usually

designed to detect best at frequencies near 1 kHz.)

Ranging between "free" masses is yet another method which has been proposed for the

detection of gravitational radiation. The principal advantage of such a system is the long

baseline which is made possible by the use of electromagnetic ranging between the masses. In

fact, baselines comparable to the gravitational wavelength can be contemplated, and this can be many thousands of kilometers. Theory says that the antenna strain sensitivity should be

proportional to the length of the baseline and this is why the method is attractive. This is

also a broad-band detection method and may be applicable to long-period gravitational waves;

for example, from one second to days or more. One of the masses could be a satellite and the

other the earth. In this case, one would simply have a transponder aboard the satellite and

use Doppler ranging to the satellite from the earth. In other suggestions, one might have

two drag-free satellites with laser ranging between them. In the former method — tracking a

105
single space craft from the earth — the dominant noise currently is oscillator instability.

That is, the frequency stability of the oscillator over the ranging delay time, which can be more than ten minutes for interplanetary distances, is the important factor. Calculations

show that a frequency stability to one part in 10"^ would be necessary for a successful

experiment. However, no oscillators of such stability exist at the present time. Both the

hydrogen maser clock and superconducting cavity oscillators now have frequency stabilities

better than 1 part in lO^, and development is continuing. The present stability of a superconducting cavity oscillator (see Chapter VII) is still well below its theoretically

projected limit.

We now turn to a different technique for gravitational wave detection. It can be shown

theoretically that a gravitational wave sweeping over the earth, the moon or any other

spherical body will excite certain modes of oscillation. However, seismic noise, always

present on the earth, may forever prohibit detection of gravitational wave excitation of the

earth; a vertical seismometer experiment was only able to establish an upper limit on the

radiation intensity. The moon is considerably less noisy. The Apollo 17 crew placed a mechanical gravimeter on the moon, but unfortunately it did not work properly, and no definitive conclusions about gravitational wave detection could be made. Emplacement of a superconducting gravimeter (see Section VI. 2. 2.) is an attractive alternative.

It is true that certain even oscillatory modes of spherical masses will be able to absorb energy from the incident gravitational wave whereas the adjacent odd modes will not

absorb energy. One group (Tuman, 1974), using a superconducting gravimeter, has observed

that the earth oscillations do in fact exhibit an asymmetry between the odd and the even modes when they have been excited: the even modes have higher energy content. Earthquake

mechanisms are thought to excite both types of modes rather symmetrically. Of course, there can be mechanisms, perhaps deepseated earthquakes, which would give rise to the asymmetry.

Nevertheless, it is intriguing to suppose that gravitational radiation is being absorbed.

The investigators have been able to estimate the absorption cross-section of the earth for

gravitational radiation, and from that have been able to estimate what the intensity of the

supposed radiation would be. This intensity turns out to be only an order of magnitude or so less than the intensity that would have had to exist in the Weber experiments of the late

1960's and early 1970' s. At present, however, most scientists would not be willing to say

that gravitational radiation has thus been detected.

We have found no sentiment whatsoever among the investigators designing and operating

Weber antennas for putting these bars into space. They feel that the present techniques for

vibration isolation will be sufficient and that the advantage of having a bar floating in

space would not offset the complications arising from having the bar essentially out of one's

control after it has left the earth. Nevertheless, there are some advantages for a bar in

space which, for the record, we will simply point out. Of course the most obvious is freedom

from earth noises. For example, the antenna should be completely free from seismic noises

and reasonably far away from earth-based electromagnetic interference although it is possible

that very large scale electrical storms could affect the very sensitive instrumentation

onboard a satellite. In space the unsupported bar would need no vibration isolation. There

106
are no acoustic paths for a freely floating bar, except the small amount through the helium

exchange gas; but such vibrations would be very small and of high frequency — the bars of course have a low resonant frequency. Furthermore, because the bar is unsupported there

would be no bending or torsional modes which could be excited if the bar had supports, thus

confusing the results. Supports also induce stresses which can give rise to acoustic processes within the bar. We should point out that there may be considerable future interest in

putting a bar in space if the latest generation of low-temperature antennas now under con- struction prove unsatisfactory in some way.

Sources of gravitational radiation include the gravitational collapse of a star, rotating

binary stars, rotating or pulsating neutron stars, and black holes. Objects falling into a black hole should have a peak frequency of about 1 kHz if the object falling in is about 10 sun masses. In all likelihood not all sources have yet been conceived.

VT.3.2. Equivalence Principle

The equivalence principle states that inertial mass and gravitational mass are the same

thing, and is foundational to Einstein's theory of general relativity. The first modern

attempt to measure the possible inequivalence was made in 1964 and had a sensitivity of about

11 12

3 parts in 10 . A subsequent experiment pushed the inequivalence limit to 1 part in 10

Both experiments were earth-bound. Although it may seem that these are very sensitive

tests, there is still a great deal of interest in continuing the measurements, not only as a check of gravitational theories, but because some scientists are now speculating that the

weak-interaction force may well violate the equivalence principle. As is well known, the

weak-interaction force violates parity conservation, which all other forces obey. The weak

interaction" is now believed to be consistent with the equivalence principle at the 1% level.

Recently it has been proposed to repeat the earth-bound test in space, with the expec14 tation that the inequivalence limit can be pushed to better than 1 part in 10 (Worden and

17 18 Everitt, 1974), with a theoretical limit between 1 part in 10 and 1 part in 10 . The

basic idea is to use two different materials, as in the earth experiments, but shape them in

the form of concentric cylinders. These will be placed in space and positioned by superconducting magnets. Inequivalence would result in a small relative displacement of the two masses, which would be detected with a SQUID magnetometer (see Chapter III) . There are three

advantages of doing the experiment in space. First, the driving acceleration should be three

orders of magnitude larger than in the comparable earth-bound case. Second, the experiment

would be isolated from seismic vibration, which is the limiting factor in the earth-based

experiments. Finally, random gravity gradients due to the motion of large masses near the

laboratory are practically eliminated in a satellite. To achieve their most reeent design

goal of 1 part in 10"^, the investigators propose cryogenic techniques throughout. Lower

temperatures will of course result in lower thermal noise and greater sensitivity. Superconducting shielding will be used, and as already mentioned, superconducting magnetic re- straints will control the position of the test masses, and a very stable and sensitive

position readout will be made by SQUID' s. Another advantage of low temperature is that there

is better mechanical stability — thermal expansion and mechanical creep are very much

smaller. The problem of levitating the cylinders is much more complicated in the earth's

gravitational field than it will be in space. Hence, the space shuttle might be used for

107
testing that part of the design. (See also Section VI. 2. 3. on superconducting bearings for

further comments on this experiment.)

VI. 3. 3. Gravitational Red Shift

The equivalence principle affects electromagnetic radiation and manifests itself there

as the gravitational red shift. Two well-known investigators (see, for example, Will, 1974)

have asserted that the red shift experiment is not a strong test of general relativity,

arguing that the red shift can be deduced from conservation of energy together with elementary quantum theory and the results of other equivalence principle experiments* But this

conjecture is not proven and red shift experiments are still considered important. In fact,

a measurement of the gravitational red shift to second-order in the solar potential is re- garded by many scientists as one of the most important gravitational measurements which could

be done.

This is the reason for the interest in a close solar probe. The first-order shift at a —6 —13

distance of four sun radii is 0.5 x 10 , whereas the second-order shift is 5 x 10 , according to general relativity. The elapased time for a space craft to orbit half-way about

the sun, changing its direction by 180°, is sixteen hours, and the clock stability required

over this time interval is a few parts in 10^ , which is about at the present limits of the

hydrogen maser clock. The relativistic theories which are challenging the general theory of

relativity have the same first-order shift but differing second-order shifts, and a good

measurement here would be able to eliminate some of the competing theories. A clock with a stability of 10 ^ could be read down to about 1% of the second-order term. Continued re- finement of the hydrogen maser might achieve this stability, but the frequency-stabilized

laser and the superconducting cavity oscillator also hold promise of extending clock stability

—16

to the 10 level (see Chapter VII).

Red shift measurements have already been made in the earth's gravitational potential.

The first of the more modern experiments was a measurement over 75 vertical feet and con- firmed the predicted red shift to about 1%. Subsequent measurements on board rockets have

-4

reduced this to 2 x 10 . Space flights using oscillators with great frequency stability

will make possible much greater sensitivity. For example, the European Space Research

Organization has proposed placing a hydrogen maser clock on board a space craft in a highly

eccentric orbit around the earth. However, it is apparent that if the vehicle has cryogenics

on board, an improved superconducting cavity oscillator with yet greater stability might also

be used.

VI. 3. 4. Geodetic and Mass-Current Precession of a Gyroscope Spin Axis

In 1960, Schiff pointed out that a well-constructed gyroscope placed in an orbiting

satellite about the earth, could provide two sensitive tests of the general theory of relativity. Two effects cause a precession of the gyroscope spin axis — the geodetic effect,

which is due to the motion of the gyroscope around the earth, and the motional or masscurrent effect, which is due to the rotation of the earth beneath the gyroscope. The

motional effect can only be measured with a superconducting gyroscope (see Section VI. 2. 4) ; no

108
other instrument or technique has yet been proposed with the required sensitivity. The

gyroscope experiment will also test those theories «vf gravitation which predict no masscurrent drag, in contra-distinction to the general theory of relativity.

VI . 3 . 5 . Measurements of the Gravitational Constant G. Cavendish Experiments

In Einstein's general theory of relativity, the gravitational constant G is truly a constant, but it is not in several competing theories, the best known of which is Dirac's

large-number-hypothesis , which predicts that G decreases in time since the matter of the

universe is receding. There are several variant theories, but all place the expected rate of

change of G close to the observed rate of the expansion of the universe, which is 5.6 ± 0.7

parts in 10"'""'" /year . Occultations of stars by the moon have been observed for some 20 years

with the use of atomic time and these observations show that if G and only G among the

fundamental constants is changing, then the relative rate of change of G is -7.2 ± 3.7 parts

in lO^/year (Van Flandern, 1976). More recent similar analyses, however, give a larger un- certainty, with the range of possible values for the change in G spanning zero (Calame and

Mulholland, 1978). Laser ranging to the moon may someday be able to distinguish between

competing theories that assume some change of G, although the earth-moon tidal interactions

might complicate interpretation of the results. Radar observations of Mercury would perhaps

provide the best means for estimating the possible time variation of G over the next few

years. The placement of long-lived transponders on Mercury's surface could improve the

measurements.

The traditional Cavendish experiment measures G itself, and at the moment is accurate to

4 5

1 only about 3 parts in 10 , with a hope of perhaps 1 part in 10 . Measurement of the effects

of the variations of G are much more accurate as we have seen in the above discussion. Those

measurements do not involve superconductivity. However, a new type of Cavendish experiment

(already in progress) does involve superconductivity and is hoped to have a precision of a

12

few parts in 10 /year in measuring the time rate of change of G (Van Flandern, 1976). The

idea of replacing the mechanical suspension and torsion forces in the traditional Cavendish

experiment by magnetic suspension and torsion has been discussed informally. Whether precision

could be significantly improved is not known, and thus it is much too early to discuss whether

the alternative theories to general relativity, which predict certain corrections to G,

may be checked through measurements of G. In another proposal a Cavendish apparatus would be orbited in a high eccentric orbit,

where the sun's potential can vary by as much as 1 part in lO''. The earth's orbit provides

a variation of only 1 part in 10"^ in the sun's potential. A Cavendish experiment with a sensitivity of 1 part in 10^ would be required to detect, with 10% precision, the variations

in the local value of G predicted by several gravitational theories (due to the sun's changing Newtonian potential along the earth's orbit). Thus the orbiting Cavendish apparatus

would not need to be nearly as sensitive as the earth-based apparatus.

109
VI. 4.0. GRAVITY AND ELASTIC PROPERTIES OF THE PLANETS

Measurements of the local gravitational field and its variations are made by seismometers

and gravimeters which detect the motion of the surface of a planet. Most knowledge of the

earth's interior has been obtained by studying the motions of the surface in the seismic

frequency range (periods of less than about 100 s) . Instruments which measure longer periods

are called gravimeters. Both instruments give several types of information about the planets.

For example, the normal modes are thereby elucidated and the amplitudes of the tidal motions

can be measured. The effects of the oceans and the atmosphere of planets having such can be

determined. Just a few years ago, it was hoped that earthquake prediction could be facilitated with seismometers. Earthquake scientists have recently become more cautious about this,

however. It is possible that a network of superconducting gravimeters (see Section VI. 2. 2)

could be useful. Until the development of these gravimeters, it was not possible to measure

periods of longer than about a day. The superconducting gravimeter now measures effects such

as the fortnightly earth tides (driven by the moon) very accurately (^ 1%) . The theories of

gravitation might also be checked by precise measurement of these tidal periods. It has been

proposed that gravitational radiation might also be detected with use of gravimeters (see

Section VI. 2. 2.).

VI. 5.0. NAVIGATION

Navigation is perhaps the oldest science, and at the present time uses very many techniques. We shall mention only those having some application to space technology.

The Navy has, for many years, used navigation satellites to assist its forces in determining their positions. A recently developed fuller exploitation of the received Doppler

signal has reduced error determination of position to about 40 m per knot of ship velocity.

The Department of Defense is very active in developing a satellite navigation system for triService capability. DOD is especially interested in four technical capabilities: (1) that

of maintaining very accurate clocks in space over long periods of time; (2) determining and

predicting satellite orbits to within ten feet; (3) predicting and compensating for variable

delays in the ionosphere; and (4) eliminating ambiguity or signal degradation caused by

multi-path signal transmission.

NASA, of course, has since its inception, been heavily involved in navigation technology.

Because of its relevance to other parts of this report, we shall mention here NASA's Orbiting

Astronomical Observatory (OAO-C, or Copernicus). This satellite, launched in 1972, used a 3-axis, precision, inertial grade gyro system. The drift rates of the gyros were observed in

orbit and remained within a band of about 16 arcsec/hour, or about 1 arcsec/hour when the

effects of known disturbances were considered. Regarding deep-space navigation, NASA's

MARK-I navigation, involving ground-based radio tracking and flight path control, was adequate to meet the needs of various lunar, Venus, and Mars missions in the 1960's. During the

1970' s, MARK-II navigation involving a combination of ground-based radio and on-board optical

measurements, but still utilizing ground-based control, was and is considered adequate for

such missions as the Viking in 1975, the Mariner, Jupiter and Saturn missions of 1977, the

110
Pioneer to Venus mission of 1978, and the Mariner to Jupiter and Uranus mission of 1979. In the 1980' s and beyond, however, reaction time becomes more important for many of the advanced

missions under consideration by NASA. Examples of these include precision asteroid or comet

fly-bys, probe delivery, outer planet satellite tours, and others. It is thus necessary to develop for the 1980' s a MARK III navigation system which can use on-board computation and

control to avoid both signal propagation delays and certain earth-based delays in data

processing and decision making during time-critical mission phases. Thus, autonomous navigation is now becoming important to NASA interests.

Navigation in space depends, among other things, upon the use of accelerometers (see

Section VI. 2.1.) and gyroscopes (see Section VI.2.4.). We have not encountered any serious

recent proposals for improving navigational procedures by introducing superconducting techniques. Nevertheless, it is apparent that should the need be felt in the future, superconductivity has a great deal to offer, both in the greatly improved sensitivity in acceleration

measurements and in the directional measurements offered by superconducting accelerometers

and gyroscopes. To add some perspective to the projected performance of the superconducting

gyroscope, consider the following triangulation. For the base distance between the earth and

the moon, the error associated with a drift of 0.05 arc sec per year (projected drift of

superconducting gyroscope) is less than 100 m per year.

VI. 6.0. SUMMARY AND CONCLUSIONS

Superconducting instruments may well play an important role in future experimental tests

of gravitational theories. In general, these experiments require performance which is not

yet available, and superconductivity is introduced as a technology which might provide the

needed advances. It is also interesting that some of the devices needed for the tests are also useful for navigation (gyroscopes, accelerometers, stable oscillators). Thus an investment in fundamental (superconductive) research could ultimately pay dividends in the form of

advanced navigational instruments.

The gyroscope-precession experiment (see Sections VI.2.4 and VI. 3. 4.) constitutes an extremely sophisticated and demanding use of superconductivity. Development is in an advanced stage and the system may fly within the next decade. To be successful the gyroscope

stability will have to eclipse that of all others by several orders of magnitude. No doubt a successful flight will lead to interest in navigational applications.

Accelerometers based on superconductivity is another area which deserves attention (see

Sections VI. 2.1.). The two proposals which seem most promising are the tunable diaphragm

and the flux gradient accelerometers. With projected sensitivities (displacement) in the

-20

range of 10 m, these devices could find many uses. NASA might want to consider further

stimulation of development work which is currently directed toward application as sensors for

gravitational wave antennas

.

Other experiments which deserve careful study include the proposed test of the equivalence

principle (Section VI. 3. 2.) and tests for changes of the gravitational constant (Section VI. 3. 5).

The superconducting oscillator is a prime condidate for further developmental work as it

111
offers significant opportunity for improvements in ranging and navigation. Improved ranging,

for instance, might offer a means of detecting gravitational waves as they perturb the re- lative distance between a satellite and the earth. This type of application is discussed in more detail in the following chapter.

112
VI. 7.0. REFERENCES

Bertotti, B. (1974). editor, Experimental Gravitation. Academic Press, New York.

Calame, 0. and Mulholland, J.D. (1978). Lunar Tidal Acceleration Determined from Laser

Range Measures. Science 155 , 977-978.

Clauser, M.U. (1965). Superconducting Inertial Reference System. U.S. Patent No. S3 1763 519.

Everitt, C.W.F. (1974). The Gyroscope Experiment. I: General Description and Analysis of

Gyroscope Performance. [see Bertotti, 1974], 331-360.

Fairbank, W.M., Boughn, S.P., Paik, H.J., McAshan, M.S., Opfer, J.E., Tabor, R.C., Hamilton,

W.O., Pipes, B., Bernat, T. and Reynolds, J.M. (1974). Search for Gravitational Radiation Using Low-Temperature Techniques. [see Bertotti, 1974], 294.

Goodkind, J.M. and Warburton, R.J. (1975). Superconductive ty Applied to Gravity. IEEE

Trans. Mag. MAG- 11, 708-711.

Hendricks, J.B. (1975). A SQUID Readout System for a Superconducting Gyroscope. IEEE

Trans. Mag. MAG 11, 712-715.

Hoffman, A.W. , Douglass, D.H. , Gram, R.Q. and Lam, C. (1976). Flux Gradient Accelerometer

:

Tests on a Working Model. ?.e-: . Sci. Instr. 47j 1441-1444.

Lipa, J. A., Fairbank, W.M. and Everitt, C.W.F. (1974). The Gyroscope Experiment II: Development of the London-Moment Gyroscope and of Cryogenic Technology for Space. [see Bertotti,

1974], 361-380.

Paik, H.J. (1976). Superconducting Tunable-Diaphragm Transducer for

Sensitive Acceleration Measurements. J. AppZ. Fhys. 47, 1168-1178.

Tuman, V.S. (1974). In Search of Low-Frequency Gravitational Radiation. [see Bertotti,

1974], 543-572.

Van Flandern, T.C. (1976). Is Gravity getting Weaker? Sci. Amer. 334, 44-52.

Will, C.M. (1974). The Theoretical Tools of Experimental G ravitation [see Bertotti, 1974],

1-110.

Worden, P.W., Jr. and Everitt, C.W.F. (1974). The Gyroscope Experiment III: Tests of the

Equivalence of Gravitational and Inertial Mass Based on Cryogenic Techniques. [see

Bertotti, 1974], 381-402.

113

CHAPTER VII

ULTRA HIGH-Q CAVITIES

S. R. Stein

VII. 1.0. INTRODUCTION 117

VII. 2.0. SUPERCONDUCTING CAVITIES 118

VII. 2.1. Fabrication 118

VII. 2. 2. Oscillator Stability 120

VII. 2. 3. Oscillator Design 122

VII. 2.4. Oscillator Performance 125

VII. 3.0. APPLICATIONS 12 6 VII. 3.1. High Power Devices 126

VII. 3. 2. Filters 127

VII. 3. 3. Transducers 12g

VII. 3.4. Oscillators and Clocks 129

VII. 3. 5. Special Devices 132

VII. 4.0. SUMMARY AND CONCLUSIONS I33

VII. 5.0. REFERENCES 134

115

VII. 1.0. INTRODUCTION

A resonant circuit has a strong response near a certain frequency, called the resonant

frequency ( V

Q

) > and a weak response for very different frequencies. Such circuits are needed

in order to separate a desired ac signal from signals at other frequencies. The bandwidth

(Av) is a measure of the selectivity of the resonance. However, since most applications

place requirements on the relative width of the resonance (Av/v

q

) , the most commonly used

figure of merit for a resonator is its quality factor or Q which is equal to v q /Av. The Q is

defined as the ratio of the energy stored in the resonator to the energy dissipated during

one radian of the oscillation. Thus high Q implies both great selectivity and small power

loss.

The first electrical resonators consisted of coupled lumped reactances, the stored

energy oscillating between the electric field of a capacitor and the magnetic field of an inductor. Resonant cavities were introduced by W. W. Hansen in 1938. They usually consist

of a volume of any shape almost completely enclosed by high conductivity walls and are necessary at high frequencies because of the difficulty of fabricating lumped circuit elements.

Cavities have the advantage of reasonable dimensions and extremely high Q for the frequency

range from 100 MHz to more than 10 GHz. The factor which limits the Q is the surface resistance of the metallic walls, there being no need to use dielectric materials. Since the

conductivity of metals increases as the temperature is decreased, the Q can be improved by

operation at cryogenic temperatures. The improvement obtained in this manner in normal

metals is severely limited by the onset of the anomalous skin effect which halts the increase

in conductivity when the electronic mean free path becomes smaller than the skin depth. For usual microwave frequencies, the rf surface resistance of superconductors is small compared

to what can be achieved with normal metals. It decreases exponentially as the temperature

approaches absolute zero, until it is limited by non- intrinsic effects. At the present, it

is possible to obtain Q's in excess of 10"'""'", which is an improvement by a factor of about 10^

compared to a room temperature copper resonator.

The applications of superconducting cavities can be divided into two categories which

stress different, but related properties of superconductivity. High power applications such

as electron accelerators, proton and ion accelerators, particle accelerators and rf plasma

confinement use superconductivity in order to achieve the low levels of power dissipation

required for continuous operation. On the other hand, a variety of low-power applications

make direct use of the extremely narrow linewidth. The cavities can be incorporated into

high stability oscillators with unsurpassed performance, which can be used as flywheel oscillators in physics experiments directly involving clocks, or in large systems which require

precise timing such as Doppler tracking, satellite navigation, and radio astronomy. The

cavities can be used directly to filter noisy signals, to impedance match between devices

having very large impedance differences, and to store energy from very weakly radiating devices. Superconducting cavities can also be used as transducers for a variety of physical

quantities. They make very sensitive displacement detectors since the frequency changes in

proportion to length. They have also been used to measure materials properties in solid

117
dielectrics, semiconductors and liquid helium, and to measure the thermal expansion of

niobium.

In addition to these applications there are also special uses which aren't easily categorized. Of course, superconducting cavities have been and will continue to be an important

vehicle for the study of rf superconductivity. They can also be used in certain novel and

unique ways such as in cesium frequency standards and electromagnetic gyroscopes.

Extensive general reviews of superconducting cavities have been written by Maxwell

(1964) and Pierce (1974). Shorter reviews of applications were written by Hartwig (1973) and

Turneaure (1972). This report will stress devices and instrumentation based on superconducting cavities and their applications. The reader may refer to the earlier reviews for more

complete background information and to the paper by Barnes, et al. (1971), for a description

of the specification and measurement of frequency stability. Since superconducting cavities

are common to all the devices and applications and superconducting cavity oscillators are used in most of them, these topics will be discussed first, followed by discussions of

specialized devices and applications.

VII. 2.0. SUPERCONDUCTING CAVITIES

VII. 2.1. Fabrication

The response of a normal metal at room temperature to microwave electromagnetic fields

is adequately described by Ohm's law. When the metal is cooled, the conductivity increases

as a result of the increase in mean-free path. One is prevented, however, from achieving a very high Q by the anomalous skin effect which occurs when the mean-free path becomes com- parable to the skin depth. The behavior of a superconductor is quite different. At microwave frequencies the resistance of an ideal superconductor decreases exponentially as the

temperature is reduced below the superconducting transition temperature. In real resonators

this behavior does not extend to zero temperature. Instead, a loss mechanism which is independent of temperature is always observed. The result is that the Q of the resonator becomes

constant, or residual, below some temperature. Figure VII-1 illustrates this behavior in the case of a niobium resonator in the TMq^q m°de at 8.4 GHz (Turneaure and Weissman, 1968). The

intersection of a horizontal line corresponding to a residual Q with the theoretical curve determines the highest temperature at which that resonator may be operated with a Q equal to the desired value.

Microwave resonators in a variety of modes are suitable for use in stable oscillators.

However, there are design criteria which are specific to superconducting cavities. Because

of the very low losses in the walls of the resonator, minor imperfections such as assembly

joints or small amounts of foreign substance such as a surface oxide layer can seriously

degrade the Q. Assembly joints may be located where there is no rf current in the desired

mode. In the case of niobium, current-carrying joints may be fabricated by electron-beam

welding.

Niobium and lead are the two superconductors which have shown the most promise for use

in high stability oscillators. These two materials are available commercially in sufficient

purity, and techniques have been developed which permit complex resonator shapes to be

118
TEMPERATURE (K)

Figure VII-1. The unloaded 0 of an X-band ^q-^q mode niobium cavity.

(Turneaure and Veissman, 1968).

fabricated from either material with surfaces which are clean, microscopically smooth, and

relatively unstrained. The transition temperature of niobium is 9.25 K, and that of lead is

7.19 K, which means that a required Q can be achieved at higher temperatures than for most

other materials. There are known superconductors with higher transition temperatures than

niobium. Several laboratories are studying Nb^Sn, but the results so far are not as good as for pure niobium.

For the best frequency stability, niobium is the preferred material. Niobium resonators

have exhibited the lowest residual surface losses and are unsurpassed in terms of mechanical

stability and surface cleanliness. O's higher than 10"'""'" have been achieved. Two methods

have been developed to manufacture the resonator from the bulk metal. In the first process,

a cylindrical resonator is machined in two pieces and assembled by electron-beam welding. It

is then alternately fired in ultra-high vacuum at about 1900° C and chemically polished.

More recently a technique has been developed which does not require ultra-high vacuum firing:

The surface is prepared by electro-polishing followed by anodizing. This process potentially

makes high-quality niobium cavities available to many more laboratories. However, there is some evidence that the anodized surface degrades in time and this is a possible disadvantage

for oscillator applications.

Lead is an excellent material if the Q needn't be as high as achieved with niobium. If

the cavity mode must be restricted so there is no normal electric field at the walls, Q's in excess of lO"*" may be achieved. The primary advantage of lead is that high quality surfaces

119
can be made in almost any laboratory using standard electroplating techniques. Unlike niobium,

high-quality lead surfaces have not been successfully produced when the resonator has been

machined from the bulk material. It has recently been suggested that resonators be fabricated

from a solid piece of dielectric such as sapphire by plating the outer surface with a superconductor. Unpublished experiments indicate that Q's as high as 10 have been obtained by

this method. It is possible that resonators made this way may be mechanically more stable

than traditional designs but no experimental evidence is available yet.

VII. 2. 2. Oscillator Stability

For the purpose of this discussion, the frequency fluctuations of any oscillator based

on a superconducting resonator can be separated into two categories: Statistical fluctuations around the center of the resonance and perturbations of the resonant frequency itself.

The second category determines an ultimate performance level for any oscillator which is

relatively independent of its design. It includes temperature, power level, and mechanically

induced frequency shifts. Since most superconducting oscillator research has been performed

with solid niobium cavities and the best results have been obtained with these cavities, this

discussion of performance will relate most directly to solid niobium resonators fabricated by

this technique.

Two independent effects transduce a temperature change of the microwave resonator into a shift of its resonant frequency. Thermal expansion or contraction changes both the mechanical

and the electrical length of the resonator while the variation in the penetration of the rf

magnetic fields with temperature changes the effective electrical length. The penetration

depth varies exponentially with temperature and is the dominant effect above 1 K. The co-

-9 -10

efficient of the fractional frequency is 6 x 10 /K at 1.75 K and 4 x 10 /K at 1.3 K. In- side of a vacuum can contained within a dewar it is possible to do excellent temperature

regulation. With a single-stage regulator drift rates of 10 ^ K/week have been observed

while the fluctuations from second to second were too small to be observed. The ease of

temperature regulation in a cryogenic environment helps to compensate for some of the added

complexity.

Any oscillator system operates with some energy stored in the resonator. Because of the

radiation pressure of these fields and the dependence of the surface reactance on the rf

field level, there is necessarily a static frequency shift of the superconducting resonator.

Some measurements indicate that the frequency offset is proportional to the stored energy.

The total fractional frequency shift observed in one experiment was 10 for 10 ' J of

stored energy. The total frequency offset determines the size of the frequency fluctuations

which result from oscillator power fluctuations. Frequency fluctuations from this source can

be reduced by decreasing the operating power level but only if a lower signal-to-noise ratio can be tolerated, otherwise power regulation is necessary.

The third major perturbation of the resonant frequency of a superconducting cavity re- sults from mechanical strains. For a TM

Q10 mode cavity, resonant at 8.6 GHz, the static

stress due to the force of gravity produces a fractional frequency shift of 1 x 10 ^ from the zero strain value. Consequently, changes in either the acceleration of gravity or the

120
orientation of the superconducting cavity result in frequency shifts of the resonance. For a TM n mode resonator, maintained in a fixed location, the variations in gravity are not

-14

significant, but the angular coefficient is 1 x 10 per arc second. This sensitivity

permits many factors to be transduced into short-term, diurnal, and long-term frequency

shifts of the resonator. Another possible effect of the stress of gravity on the resonator

is creep of the niobium. The creep process is not well understood at these temperatures and

has not been measured.

Elastic deformation of the resonator due to mechanical vibrations produces significant

fluctuations in the center frequency. Studies of the most rigid solid niobium cavities have

shown the frequency stability in an oscillator system to be limited by vibrations for averaging times between 10 ms to 10 s. Some vibrations are coupled to the resonator from the

laboratory but the most significant vibrations which have been observed were due to boiling

cryogen (liquid nitrogen) used to maintain the low temperature in the dewar.

If the center frequency of the resonance is sufficiently constant, then other sources of

noise will determine the ultimate stability of the superconducting oscillator system. The

frequency fluctuations about the center of the resonance are highly dependent on the design

of the particular oscillator, however a lower limit corresponding to the case where all noise

sources are filtered by the resonator can be determined. If the per curbing noise is white,

then the phase of the oscillator does a random walk. The one-sided spectral density of the

phase fluctuations, in a form appropriate for microwave resonators, is given by

2

v s.(f) =

IcT (VII-1)

2P

a % %

where is the operating frequency, k is the Boltzman constant, T is the absolute temperature, P is the power dissipated in the load, and Q and Q are the external and loaded Q's

a

10 9 respectively. In the case of a superconducting cavity with = 10 , QT = 5 x 10 , P = 10

-3

W, v = 10

10

Hz, and T = 1 K,

EL a o

-20 2

S ff) = 10 Hz/f .

The active element in a practical oscillator will dominate the thermal noise. In this case T must be interpreted as the effective noise temperature of the device. Such noise tempera4 tures vary from approximately 20 K for varactor parametric amplifiers to more than 10 K for

a transferred-electron device.

Another important limitation on the stability of a superconducting oscillator is additive noise which results from a white noise voltage generator at the output of the oscillator. In an ideal oscillator the additive noise is due to output buffer amplifiers or a user device. The spectral density of the phase fluctuations is

Sff) = kT'/2P , (VII-2)

cp a

where T' is the effective noise temperature of the circuitry which sees the output of the

-3

oscillator. If the effective noise temperature is 300 K and the available power is 10 W,

then

121
— 1 fi S A (f) = 2 x 10 /H

In this case the additive noise dominates the oscillator spectrum for Fourier frequencies

greater than 0.07 Hz. Under the same conditions, the rms fractional frequency fluctuations

are given by

ay

(x) = 8.3 x 10

-21

1/2

-20

4.3 x 10 f.

1/2

where f, is the noise bandwidth of the measurement system, h

Equations (VII-1) and (VII-2) show that both the random walk of phase and the additive

noise can be reduced by increasing the available power; however, this technique is limited by

several factors. The nonlinearity of the resonator couples amplitude and phase modulation

and may ultimately limit the stability. If this is not a problem, then at some field level

the resonator breaks down. Finally, high power levels may exceed the dynamic range of the user device such as a mixer in a super-heterodyne receiver.

VII. 2. 3. Oscillator Design

Because of the tremendous stability potential of superconducting resonators a variety of

techniques have been used to construct superconducting oscillators (see Fig. VII-2) . The

goals of this research have varied. Some oscillators have been constructed to illustrate

feasibility, some to accomplish modest stability goals for further research on superconducting resonators, and others to achieve the ultimate frequency stability over some range of

Fourier frequencies or averaging times. As a result, the achieved frequency stability for

each technique is probably a poor indication of its potential performance. Instead of making

such a comparison, this chapter will outline some of the advantages or disadvantages of each

method from the point of view of achieving the best possible frequency stability.

The techniques discussed in this section use the superconducting resonator in three

different ways — as the sole resonator of an oscillator circuit, as an auxiliary resonator

to stabilize a free-running (noisy) oscillator, or as a filter which provides no feedback to the source. Figure VII-2b illustrates how an oscillator may be realized using a superconducting

resonator and a unilateral amplifier. Oscillation can occur when the amplifier gain exceeds

the losses and the total phase shift around the loop is a multiple of 2tt rad. Automatic gain

control or limiting is necessary in order to produce oscillations at the desired power level.

The resonator may be used in either transmission or reflection, but the transmission mode is

preferable because the insertion loss of the resonator suppresses spurious modes of oscillation which do not lie in its pass bands. This technique has received considerable attention because of its simplicity. The only element which needs to be located in the dewar is

the superconducting cavity which can be connected to the room-temperature amplifier by long

lengths of transmission lines. However, this virtue is its major detraction when state-ofthe-art frequency stability is desired. Changes in the phase length of the transmission

lines produce proportional frequency shifts. If A<|> is the phase change from any source, the

fractional frequency shift is

122
OUTPUT OUTPUT

RESONATOR

RESONATOR

PHASE SHIFTER

PHASE SHIFTER

(b) OUTPUT

AMPLIFIER

©- T

OUTPUT S DISCRIMINATOR

(c)

RESONATOR AMPLIFIER <

(d) 0- 3 OUTPUT

ISOLATOR RESONATOR

(e)

Figure VII-2. Block diagram illustrating several superconducting frequency

sources: (a) negative resistance oscillator, (b) loop oscillator, (c) cavity-stabilized oscillator, (d) stabilizedvoltage-controlled oscillator, and (e) passive filter.

Av/v = A<f>/2Q . (VII-3)

J_j

The phase changes due to factors such as thermal expansion and vibrations are sufficiently

large in a cryogenic system that they totally dominate the short-term stability and drift of

such an oscillator.

One possible solution to this problem is to use an amplifier which functions in the same low-temperature environment as the resonator and is connected to it by a short rigid transmission line. It has been proposed to use a cryogenic travelling wave maser in a unilateral

amplifier design. Alternatives are tunnel diode amplifiers and varactor diode parametric

amplifiers. Both of these devices function by generating a negative conductance at the

resonator frequency. Since they are bi-lateral, they can simply be connected to the superconducting cavity through an impedance transforming network as shown in Fig. VII-2a. When

the negative conductance of the amplifier exceeds the positive load conductance of the

resonator, oscillation results. The major advantage of the tunnel diode oscillator is that

it requires only dc bias power for operation. On the other hand, there are several disadvantages. Shot noise in the tunnel junction limits currently available tunnel diode amplifiers to an effective noise temperature of 450 K at 9 GHz. In addition, the very low operating voltage limits the theoretical output power to 1 mW at 10 GHz from commercially available

123
devices (having peak current less than 20 mA) . If other problems were solved, these two difficulties could limit the frequency stability of the tunnel diode superconducting oscillator. In contrast, cooled parametric amplifiers have demonstrated 20 K noise temperatures,

and room temperature non-degenerate parametric oscillators have produced more than 100 mW at

9 GHz.

The most widely studied technique for realizing a superconducting oscillator has been

the stabilization of a free-running oscillator with a superconducting resonator. One technique for accomplishing this, called cavity stabilization, is shown in Fig. VII-2c. The

oscillator is injection-locked by the power which is reflected from the superconducting

resonator. The stabilization factor, which is the ratio of the free-running oscillator

frequency fluctuations to the cavity-stabilized oscillator frequency fluctuations, is given

by the ratio of the Q of the superconducting cavity to the Q of the free-running oscillator.

Equation VI 1-1 shows that the best possible performance reduces to that of an oscillator

built with the superconducting cavity as its only resonator. There are two major disadvantages to this technique. First, room-temperature oscillators such as klystrons and Gunneffect devices have extremely high noise temperatures. And second, the frequency offset from

the center of the resonance is proportional to the line length between the oscillator and the

cavity, just as in the loop oscillator.

The most successful superconducting oscillator technique to date is the use of active

feedback to stabilize a voltage-controlled oscillator. The superconducting resonator is the

frequency sensitive element of a discriminator which generates an output voltage proportional

to the frequency difference between the oscillator and the center of the superconducting

cavity resonance. Although this system also has a long path length between the room- temperature oscillator and the superconducting cavity, it is possible to design the discriminator so that the dependence of the oscillator frequency on this path length is greatly

reduced. This is accomplished by using phase-modulation sidebands on the carrier frequency

to provide the reference for locating the plane of the detuned short of the superconducting

cavity. Despite the fact that this technique also uses a noisy room-temperature oscillator,

its performance is not limited by this fact. This is true because in such a system it is

possible to greatly multiply the phase vs frequency slope of the resonator by using external

amplifiers. In this way, the frequency fluctuations of the free-running oscillator may be

reduced until the performance level, determined by the microwave detectors, is reached.

Figure VII-2e shows a superconducting resonator being used to filter the output of an oscillator. This application is particularly important when the oscillator is to be used as

a source for frequency multiplication. For example, a state-of-the-art quartz crystal

oscillator may be multiplied to 0.5 THz before the carrier is lost in the phase noise pedestal. However, if the same oscillator is filtered by a passive superconducting cavity with

9 loaded Q equal to 2 x 10 , it could in principle be multiplied to 100 THz. The conclusion of the above discussion is that two types of superconducting oscillators

appear most promising for improved stability: stabilization of a VCO and the all-cryogenic

oscillator. The fundamental limitations of the two devices are similar so the most important

differences at this time are the practical problems of implementation: the VCO stabilization

system has all the critical elements outside the dewar where they are readily available for

124
adjustment and experimentation, but they are necessarily sensitive to problems of temperature

fluctuations and vibration. On the other hand, the active oscillator is compact and totally

contained in the highly controlled cryogenic environment. It will, however, present some new

technical difficulties such as heat dissipation and device parameter fluctuations.

VII. 2. 4. Oscillator Performance

The best superconducting oscillator frequency stability to date has been obtained by

stabilizing a free-running VCO using the system of Fig. VII-2d. The measured time domain

stability of a single superconducting oscillator is shown in Fig. VII-3 (Turneaure, 1977).

10 13

\

10

2

-14 o ALLAN VARIANCE • HADAMARD VARIANCE BANDWIDTH = 10 Hz

10

:

-15

10

' 2

16 _ 10"

\ o = 1x10

14/t

o o •

o I

NOISE FLOORS

a = 3x1 0" 1

6

SAMPLING TIME, i(s)

Figure VII-3. The fractional frequency fluctuations of a superconductingcavity stabilized VCO (Turneaure, 1978).

4 The rms fractional frequency fluctuations in a 10 Hz bandwidth decrease with averaging time

approximately as 5 x 10 ^ s/t, reaching a noise floor of 3 x 10 for times longer than

10 s. Measurements in the frequency domain indicate that the random component of the phase

fluctuations is white for Fourier frequencies between 10 Hz and 50 kHz: the spectral density

-13 2 of the phase fluctuations is S (f) - 7 x 10 rad /Hz. For Fourier frequencies below 300 Hz,

<f> there are several peaks in S^(f) due to coherent frequency modulation of the oscillator by

i> mechanical vibrations. The largest of these bright lines have rms amplitudes of approximately

8 x 10 rad, which is consistent with the stability observed in the time domain. The

125
long-term behavior of the superconducting oscillator was measured via a comparison with an ensemble of cesium frequency standards. The result of a fit to a model including linear

-14

drift was a frequency drift rate of 1 x 10 /day for the best-performing superconducting

oscillator system.

Although this performance is excellent, there are several applications which need even better long-term or short-term frequency stability. Since factors which now limit superconducting oscillators do not appear to be fundamental in nature, further research should

result in significant progress. The superconducting-cavity, negative-resistance oscillator

and the superconducting-cavity maser oscillator provide an opportunity to minimize external

perturbations, since all their critical components are contained within the very stable

cryogenic environment, perhaps making it possible to come closer to the frequency stability

limits determined by thermal noise and the filtering action of the superconducting resonator.

At the present time, throughout the world, there are four groups which actively pursue

the development and improvement of superconducting oscillators. There are another three to six groups which are interested or just starting projects. The two active groups in the

United States have combined funding of less than $100 K per year for this research. As a —1

6

result, progress has been slow since 1974 when a frequency stability of 6 x 10 was first

achieved. A conservative projection of the potential of superconducting oscillators within

the next decade would be a stability of 10 ^ at 10 ms with a noise floor of 10 ^ . The rate

of progress would probably double with increased funding and the involvement of additional

researchers in this field.

VI I. 3.0. APPLICATIONS

All of the uses of superconducting cavities which will be discussed here result from

their low electromagnetic losses. However, substantially different advantages can be gained

from this one property. These are applications where it is desirable to reduce the large

amounts of power which would be dissipated in conventional devices. A second group uses the

very high Q to realize exceptional filter characteristics, such as narrow bandwidth and high

ratio impedance transformation. The low losses mean that practically any object placed in a superconducting cavity will determine its performance. Thus many materials properties may be

accurately transduced as a frequency with high resolution and low noise. As a result of

their state-of-the-art performance, superconducting oscillators are beginning to find a wide

variety of applications either directly as clocks, as components of oscillator systems, or in

a variety of physics experiments involving clocks and time. Finally, there are some special

applications which are rather unique and are discussed separately from all the others.

VII. 3.1. High Power Devices

The performance of conventional linear electron accelerators is limited in the areas of

duty factor and accelerated electron-beam current, energy stability, and energy resolution by

the power which is needed to maintain the accelerating fields. For the linear accelerator at

g SLAC (Stanford Linear Accelerator Center), 5 x 10 W is required for an electron to reach an

126
energy of 20 GeV in a distance of 3000 m. In order to limit the average energy consumption

-3

to acceptable levels, the accelerator is operated with a duty factor of about 10 . The

power required by superconducting cavities is about 10^ times smaller. Taking into account

-4

the efficiency of heat extraction at 2 K, which is 5 x 10 , a superconducting accelerator

would require only 0.2 percent of the power needed by a conventional machine. The superconducting accelerator could be operated continuously with improvements in all the areas which were mentioned above. Similar advantages are obtained in the case of proton and heavy

ion accelerators.

Many accelerators produce secondary particle beams with very high momentum and duty

factor. Conventional rf particle separators require as much as 10 MW of rf power for these

high-momentum particles, but suitable high-power microwave tubes are not available and the

separators must be operated with low duty factor. Superconducting-cavity particle separators

would make continuous operation possible. One has been built and installed at CEKN.

A potentially very important application which is beyond the present state-of-the-art of

superconducting technology is the confinement of high temperature plasmas. The small losses

in superconducting cavities could make an important contribution towards achieving the breakeven point in the generation of thermonuclear fusion power from an rf confined plasma. The

4 confinement problem requires peak surface magnetic fields of 10 Oe within the microwave

cavity; this is one order of magnitude higher than has been achieved to date.

VTI.3.2. Filters

Perhaps the most obvious application of high Q superconducting cavities is as narrow

band filters. Below approximately 100 MHz, lumped element resonators are preferred, but at higher frequencies various types of cavities are used: helical structures between 500 MHz

and a few GHz; low-order mode structures from a few GHz to 10 GHz or more; and Fabry-Perot

resonators at higher frequencies. In addition to the obvious advantage of very narrow bandwidth, for example, 0.1 Hz at 10 GHz, cavities have very pure resonance modes. The geometry

can often be chosen so that spurious modes are separated by an octave or more. In contrast,

quartz crystal filters have spurious modes usually spaced only 0.1 percent from the desired

mode.

Superconducting cavities can be mechanically tuned over a wide range, but the instability

introduced by the tuning mechanism counteracts the most important property of the resonators.

Very fine, stable tuning can be achieved by controlling the temperature of the resonator —

the total fractional tunability is on the order of 10 ^ . Similar tunability can be achieved

by coupling an electronically variable reactance to the cavity. Another method which yields

-4

a tuning range of 10 is optical tuning by the photodielectric effect. This has been realized by placing a high-resistivity semiconductor wafer in the gap of a quarter-wave re- entrant cavity where the rf electric field is very high. When a light beam is directed on

the semiconductor, its dielectric constant changes, thereby producing a large shift in the

cavity resonant frequency. Step changes in the frequency can be made in less than 10 ms, but the presence of the semiconductor degrades the band-width of the filter: a Q of 10 at

1 GHz has been achieved for such a device.

127
The narrowband, tunable filters which can be realized with superconducting cavity filters

may be applicable to some communications or radar receivers. However, it is more likely that

they will be used in highly specialized devices. For example, a superconducting resonator

may be used to obtain strong coupling between an evaporated Josephson junction and an electromagnetic field. This is particularly valuable when the junction is used as an oscillator.

If such a junction is not incorporated into a resonant structure, the large shunt capacitance

shorts out high frequency voltages. The theoretical maximum output power from a Josephson

junction is 0.58 IV, where I is the critical current and V is the bias voltage. Theoretically,

a typical junction can emit about 10 ^ W at X-band, but only about one percent of this is

observed from waveguide-coupled evaporated junctions. In addition to performing the required

impedence matching to the junction, the superconducting resonator also provides narrow-band

filtering of the output signal. Unfortunately, this property detracts from the major advantage of the Josephson junction oscillator — it has a tuning sensitivity 10^ times greater

than conventional oscillators. In order to preserve this property, it will be necessary to use superconducting cavities with extremely agile frequency tuning.

Superconducting resonators may also be useful as high-ratio impedance transformers for use with superconducting antennas. This application also has the disadvantage of narrow

bandwidth. Superconducting antennas are not considered advantageous over conventional

devices (at least for today)

.

VII. 3. 3. Transducers

Superconducting cavities make attractive transducers for a variety of quantities because

they introduce negligible perturbations and their output is usually in the form of a an easily and accurately measured frequency. Since the frequency of a cavity resonator is

approximately inversely proportional to one of its dimensions, it is very natural to use such

a resonator to measure changes in that length. The resolution is limited by the frequency

instability of the resonator; the best achieved stability of superconducting cavity oscil-

—16

lators corresponds to a random noise level of 3 x 10 cm. This performance can be improved, in principle, by designing the resonator so the controlling dimension is very small.

It has been predicted that a quarter-wave re-entrant cavity would permit the resolution of

-17

10 cm for a one-second integration time. However, it has not been demonstrated that the

necessary Q can be obtained with such a resonator design. There is an interest today in

superconducting cavity length transducers for the detection of gravity waves. In such an

experiment, the cavity itself could be used as the antenna, but more likely two cavities

would be coupled to a traditional bar antenna in a way which would cancel a substantial

fraction of the frequency noise in the exciting oscillator.

Substantial use has been made of superconducting cavities to measure a variety of

properties of materials at low temperature. Tue technique is to place a sample within the

resonator and to measure either the frequency or Q as a function of the parameters of

interest. The advantage of this technique is that it does not require ohmic contacts and can be used with randomly shaped, powdered or liquid samples. Dielectric constants very near

-9

unity and loss tangents as small as 10 can be measured because of the high stability and

very low losses of the superconducting cavity itself. Many semiconductor properties have

128
been measured including relaxation tine, lifetime, Fermi level, trap ionization energy, trap

density capture cross section, free carrier density and trap population.

Some properties of liquid helium have also been studied this way. The thermal expansion

-9

has been measured with a sensitivity, in terms of fractional density change, of 4 x 10

This sensitivity is sufficient to yield quantitative data concerning the dispersion relation

for thermal phonons in liquid helium. The damping of oscillations of the liquid helium

through a small orifice can be studied by using the frequency of a superconducting cavity to sense the level of liquid helium within it. This data has been used to study the quantization of vorticity in superfluid helium.

Several interesting and useful devices can be implemented using superconducting cavities.

A thermometer can be made for the temperature range from 0.25 to 0.6 K by filling a cavity

3 with He vapor in equilibrium with the bulk liquid. Changes in density, which are reflected

in frequency changes, are interpreted in terms of the temperature of the gas. The accuracy

of such a thermometer is estimated to be 0.2 percent. A nuclear radiation detector can be

made by placing a properly doped semi conductor crystal on the stub of a quarter—wave reentrant cavity. Below 70 K, the charge carriers created by the absorption of radiation are trapped for very long times at sites in the forbidden band. As a result, the frequency of

the cavity shifts in proportion to the total absorbed dose. A detector for low levels of

light can be implemented in a similar way, only in that case the frequency shift results from

the zz fielec zz zz effect.

VII. 3. 4. Oscillators and Clocks

The excellent spectral purity and medium term stability (up to about one day) of superconducting oscillators have numerous applications. In the following discussion, these are divided into three categories: oscillators used as components of instruments; oscillators

used as clocks to provide timing functions for complex instrument systems; and oscillators

used to perform experiments based directly on clock performance.

Oscillators with very good spectral purity (short term stability) are important elements

of many instruments and measurement techniques. One example is the use as the starting

oscillator for frequency multiplication from microwave to infrared or higher frequencies. The

2 process of multiplication by an integer n increases the phase noise power by n . ^nis creates

a severe practical problem because once the integrated white phase noise becomes comparable

to 1 rad, it is no longer possible to identify the coherent signal component. Superconducting

oscillators have two important advantages in this application: they can operate at frequencies at least as high as 10 GHz, and theoretically they can produce a signal whose spectral purity is limited by the characteristics of the multiplier. For example, it czs cse~c predicted thai a state-of-the-art commercial 5 MHz quarts crystal oscillator way be multiplied to 0.5 TEz before the carrier is lost, but the same signal when filtered by a 10 GHz

10 superconducting cavity with Q = 10 can be multiplied directly to 100 TEz. Certain types of radar also depend critically on the spectral purity of their local

oscillator. Return signals from nearby stationary clutter mix with the phase noise sidebands

and limit the signal to noise ratio of the true Doppler signal. For example, if a 1 GHz

radar has target velocity detection down to 40 m/s and Doppler bandwidth of 10 kHz, then in

129
order to achieve 80 dB sub-clutter visibility it is necessary to use a local oscillator whose

phase noise is more than 120 dB below the carrier for Fourier frequencies greater than 200 Hz. A third application for short term stable oscillators is as flywheel oscillators in

atomic frequency standards. Since the time domain frequency stability, a (t ) , cannot improve

faster than 1/x, the medium term performance of the standard is limited by the flywheel

oscillator if its stability is worse than that of the atomic frequency discriminator at the

attack time of the feedback loop. Quartz crystal oscillators do not degrade the performance

of current atomic standards, but if expected improvements in these standards are made, then

improved flywheel oscillators will be needed and superconducting oscillators are a possible

candidate. Flywheel oscillators are also used for autotuning hydrogen masers, a process

whereby cavity pulling and spin exchange frequency shifts are simultaneously reduced. Present

autotuning systems utilize a pair of masers, one of which could be replaced by a superconducting oscillator, thereby realizing a significant cost reduction.

Superconducting oscillators can also be used to provide time for complex instrumentation

and measurement systems such as radio astronomy and radar ranging. The desirability of

superconducting oscillators for these applications range from cost savings to the potential

for significant improvements in performance.

.

Very long baseline interf erometry (VLBI) using independent clocks may have the following

clock performance requirements for certain types of experiments: initial 1 ps synchronization of the start of recording; total time error of less than 1 ns over a five hour observation period to insure that all the data have the same initial offset error; and sufficient

coherence to guarantee that the recorded signals can be cross correlated. For a 10 GHz

system and an observation time of one hour, the coherence requirement is met by an oscillator

with a noise floor a = 5 x 10 ^ . The required noise floor decreases inversely with both

y the operating frequency and the observation time. The current requirements are met by both

hydrogen frequency standards (active and passive devices) and superconducting oscillators.

The active masers are very expensive (y $250 K) , whereas the other two devices appear to cost

only about one-third as much.

Various types of navigation systems need state-of-the-art clocks. Since both the navigation requirements and the techniques are somewhat flexible, it is difficult to place fixed

requirements on clock performance. Typical performance goals for two navigation systems are discussed here.

The NASA Deep Space Net (DSN) utilizes a network of radar stations to track spacecraft

which have left earth orbit. The current capability of the system is ^ 5 m range resolution

and 1 yrad angular resolution. Planners foresee the need for approximately an order-ofmagnitude improvement in resolution for some missions which will be flown in the early 1980' s, such as the Jupiter Orbiter (JOP) . The clock stability requirements depend on the method of

range measurement. One approach, which has been suggested, is to replace most of the co- herent (two-way) Doppler ranging with non-coherent (one-way) Doppler measurements. The

latter technique, also called wideband VLBI, has the advantage that it substantially reduces

the tracking time for accuracy comparable to current coherent tracking methods. However, it

places the most stringent requirements on clock performance. When daily calibrations are

used, the frequency must be constant to 1.5 x lO

- "^ over one day. Weekly calibrations are

130
preferable to reduce cost and calibration time, but the frequency stability requirement

becomes 3.2 x 10 ^ over one day.

An experiment has been proposed to use the DSN to detect gravitational waves. The

passage of a gravitational wave pulse past the earth and spacecraft produces an identifiable

signature in the range information. This experiment places the strictest stability requirements on the frequency standards. To do a feasibility experiment the frequency stability

must be 3 x 10 ^ for the duration of the experiment, 40 s to 4000 s. This can be achieved

with a state-of-the-art superconducting oscillator system. An experiment, sufficiently

sensitive to detect theoretically predicted gravitational pulses, would require frequency

—18

stability of 3 x 10 . It is probable that a superconducting oscillator using a cavity with

Q = 10^ would eventually be capable of reaching this performance level.

The Global Positioning System (GPS) is a multisatellite system intended to provide

earthbound navigation with a precision of 10 m. Each satellite carries high-stability clocks

which need to keep time to 10 ns over 10 days. Several hydrogen frequency standards are being

developed for this purpose. Depending on future system requirements, superconducting oscillators could become desirable for this application.

There are several fundamental physics experiments which are based directly on superconducting oscillator performance. Two of these, a red shift experiment and a fundamental

constants experiment, are performed by comparing the frequency of a superconducting oscillator

to the frequency of an atomic standard based on a hyperfine transition such as cesium or hydrogen. In the red shift experiment, one looks for a term in the frequency ratio that has

a period of one solar day. Because of the earth's rotation in the sun's gravitational field,

various theories of gravity predict

vhyperfine

V super conducting

= A 1 + 10

12

(r - y T ) cos (2irt/l solar day)

where T q and T^ are zero for any metric theory of gravity such as the general theory of

relativity. With available standards it would in principle be possible to set an upper limit

—3 1 —8

of 10 on T q -

~2 T^. The Eotvos experiment already places a limit of 10 on T q and may

place a limit on T^. If instead of analyzing the data for diurnal effects, they are fit to a linear drift

model, then the same experiment may be analyzed to yield an upper limit on the time rate of

change of the fine structure constant. The ratio of the frequencies of a hyperfine standard

to a superconducting oscillator is

Vhyperfine _, (ml o nDTTnnnnri"! n o ^ ' V super conducting

where m is the mass of the electron, M is the mass of the nucleus and g is the gyromagnetic

ratio of the nucleus; B is a constant. By comparing a superconducting oscillator to a cesium

-12

standard for 12 days it has been determined that (l/a)(da/dt) is less than 4 x 10 /year

with 68 percent probability. The quality of this experiment was determined by the data link

connecting the laboratories where the standards were located. Significant improvements may

result from a comparison of superconducting oscillators and hydrogen standards in the same

131
laboratory. Although astronomical and geophysical measurements have set a tighter upper

limit, they have the disadvantage of averaging possible changes over periods on the order of

10 percent of the age of the universe.

A laboratory experiment has been proposed to use a superconducting resonator excited by

a superconducting oscillator to measure the Lense-Thirring effect — the dragging of inertial

frames by a rotating mass. A toroidal superconducting waveguide is centered on the rotation

axis of an axisymmetric object. The wave travelling in the same direction as the rotating

body takes less time to complete one round trip than the counterrotating wave.. As a result,

the interference pattern rotates around the waveguide. For a 5000 Kg mass with 50 cm radius

3 rotating at an angular velocity of 2 x 10 rad/s, the angular velocity of drag has been

estimated to be 2 x 10 ^ rad/s. The required waveguide Q is 5 x 10^ and the stability of

the exciting oscillator must be 1 x 10 ^. This stability is probably achievable, but it is

difficult to say whether such high Q's can be achieved. There are many other problems at least as difficult as these, making this experiment extremely problematic.

VII. 3. 5. Special Devices

Occasionally superconducting cavities can be used to solve some unusual problems. One

example of this is the reduction of the "cavity phase shift" problem in cesium beam frequency

standards. At the present time, the most significant limitation in the accuracy of cesium

beam frequency standards is the extent to which the phase difference of the rf fields in the

two interaction regions can either be nulled or measured. If superconducting cavities were

used, the variations of the microwave phase across the apertures of the cavities would become

very small and it would be possible to measure the intercavity phase shift with high precision. This approach is not being tried at the present time.

Another novel suggestion is a superconducting cavity gyroscope. One possible configuration is the toroidal microwave cavity which was described earlier in the discussion of the

Lense-Thirring effect. The position of the nodes in such a cavity experiences a phase shift

proportional to the rotation speed of the cavity and the frequency of the exciting radiation.

This is just the Sagnac effect which is used today in laser gyroscopes. Because of the

difficulty in optical detection of fringes, laser gyroscopes are constructed today in the

form of ring oscillators. These devices must be biased in order to overcome a dead zone at low rotation rates; systematic errors introduced by the bias degrade the performance of the

gyroscope. The superconducting version of such a gyroscope is less sensitive by a factor of

4 5 10 to 10 because of the wavelength difference. However, this is offset by the fact that it

is possible to detect a much smaller fraction of a fringe at 10 GHz than at visible frequencies.

Consequently, it may be possible to construct a Sagnac-type gyroscope (using superconducting

cavities) which operates in the interf erometric mode with no dead zone and is competetive in

sensitivity with the best mechanical gyroscopes.

132
VII. 4.0. SUMMARY AND CONCLUSIONS

There are desirable applications for superconducting cavities with Q's up to 10 and

— 18

for superconducting oscillators with stabilities as good as 3 x 10 . The best Q achieved

11 — 16

to date is 5 x 10 and the best stability is 3 x 10 . Since it is generally possible to find the center of a resonance line to one part per million, it is reasonable to expect that

-18

a stability of 2 x 10 will be achieved using present technology, and this might be accom- plished within ten years.

The most intriguing potential space application of these oscillators is their use for

ranging. The possibility that gravitational radiation might be detected by more sensitive

ranging to a drag-free satellite should warrant careful consideration.

Cavity-stabilized oscillators must surely be considered if large improvements are needed

by NASA's Deep Space Net or the Global Positioning System. Present performance of the superconducting oscillator is impressive, but the potential for improvement puts this system into

a class of its own. Most work in progress on these oscillators is directed toward application of the devices

to real problems without concern for further major improvements in performance levels.

Stimulation of work toward improved performance will have to be driven by applications which

require such improvement. This might be considered by NASA if oscillator (clock) improvement

is rated as a high-priority task.

133
VII. 5.0. REFERENCES

Barnes, J. A.. Chi, A.R., Cutler, L.S., Healey, D.J., Leeson, D.B., McGunigal, T.E., Mullen,

J. A. Jr., Smith, W.L., Sydnor, R.L. Vessot, R.F.C. and Winkler, G.M.R. (1971). Characterization of Frequency Stability. IEEE Trans. Instrum. Meas. IM-20, 105-120.

Hartwig, William H. , (1973). Superconducting Resonators and Devices. Proc. IEEE 61,

58-69.

Maxwell, E. (1964). Superconducting Resonant Cavities. In Progress in Cryogenics, Vol. 4,

(ed. K. Mendelssohn), Academic Press, New York, 124-158.

Pierce, J.M. (1974). Superconducting Microwave Resonators. In Methods of Experimental

Physics, Vol. 2, (ed. R.V. Coleman), Academic Press, New York, 541-595.

Turneaure, J. P. and Weissman, I., (1968). Microwave Surface Resistance of Superconducting

Niobium. J. Appl. Phys. Z9j 4419-4427.

Turneaure, J. P. (1972). The Status of Superconductivity for RF Applications. In Proc. 1972

Applied Superconductivity Conference, Anapolis, Maryland, 621-630.

Turneaure, J. P. (1978). From paper by Stein and Turneaure. Proceedings, Conference on Future Trends in Superconducting Electronics, AIP Spec. Pub., to be published.

134
APPENDICES *

I. A. COMMENTS ON CRYOGENICS IN SPACE 137

II. A. TERRESTRIAL APPLICATIONS OE SUPERCONDUCTING MAGNETS 140

II. A. 1. Existing Magnet Applications 140

II. A. 2. Applications in the Near Future 144

II. A. 3. Applications in the Far Future 146

II. B. CURRENT APPLICATIONS OF SUPERCONDUCTING MAGNETS IN SPACE 147

II.B.l. Operational Superconducting Magnet Systems 147

II. B. 2. Systems That Almost Made It 149

IV. A. JOSEPHSON TUNNELING LOGIC 150

IV. B. JOSEPHSON FABRICATION TECHNOLOGY 155

REFERENCES 157

Roman numerals refer to the chapter associated with the particular appendix.

135

APPENDIX I. A.

C0:2SNTS ON CRYOGENICS IN SPACE

The use of superconducting devices in space requires cryostats to maintain stable low

operating temperatures. Two cryogenic space vehicles, for IRAS and for the Stanford relativity experiment, are currently in the final design stages. Both of these use evaporating

liquid helium to maintain low temperatures. This will probably be the conventional approach

to cryogenics in space for several years to come. It is generally agreed, however, that ultimately small closed-cycle refrigerators (cryocoolers) must be developed for both space and terrestrial applications of superconducting

devices. The reasons for this opinion are technical, economic, and undoubtedly, intuitive,

and it would be beyond the scope of this report to discuss them here. "Applications of

Closed-Cycle Cryocoolers to Small Superconducting Devices" was the subject of a conference

held recently in Boulder (Zi —erma~ and Flynn, 1978) . The proceedings of this conference are available as NBS Special Publication SP-508. The document is the first of its kind in com- bining papers on the current state-of-the-art of superconducting devices and of closed cycle

cryocoolers, as well as up-to-date thinking about combining the two technologies.

To summarize, present practice (and for some time to come) is to use liquid-helium or possibly solid-hydrogen, cryostats in space. Within five or ten years, perhaps, one may

envision versatile space—qualified closed-cycle cryocoolers capable of long unattended con- tinuous or intermittent operation. The remainder of this section is a discussion of a third

possibility for cryogenics in space, namely passive radiation cooling. Radiation cooling has

already been used for infrared sensors operating at 50 K or above, but here we will consider

the possibility of achieving temperatures in the neighborhood of 10 K or below.

A number of authors observe that the stellar radiation in our galaxy has about the same magnitude as the 2.7 K cosmic background. They estimate that the zodiacal light also has

about the same magnitude. Unless there are other heat sources of greater magnitude not taken

into account, it follows that a black body in interstellar space should reach an equilibrium

temperature of no more than 4 or 5 K. Throughout most of interplanetary space, the equilibrium temperature should be almost this low if the body is effectively shielded from the sun, if it is not too close to a planet, and if unforeseen heat sources such as heating by micrometeorite impacts are not too large. Shielding from the sun and reflection of sunlight and

re-radiation from the planets will be discussed below. The equilibrium temperature of a body

can be lowered if its surface can be made a good reflector at the sub-micrometer wavelengths

of solar and stellar radiation, but a good absorber (i.e., emitter) at the sub-millimeter

wavelengths of blackbody radiation at a few K. The possibility of radiation cooling should not be taken too seriously without further

study of the thermal heat loads and mechanical design. Although it seems that radiation cooling

of superconducting devices may be possible in interplanetary and deep space probes, the re- quirements of such a mechanism are probably not compatible with "ordinary" spacecraft.

In this regard, it may be noted that the two cryogenic vehicles currently contemplated, namely

the Infrared Astronomical Satellite (IRAS) and the Stanford relativity experiment, are already

137
highly specialized to satisfy the requirements of the low-temperature devices, suggesting

that similarly specialized spacecraft may also be conceived.

For satellites operating near the earth or other planets, it is hardly conceivable that

radiation cooling can be effected. The alternatives are liquid-helium or solid-hydrogen

cooling, or closed-cycle cryocoolers, as noted above.

The numerous disadvantages of liquid and solid cryogens and cryocoolers (high-cost,

power requirements, complexity, unreliability, vibration, etc.) are strong incentives to consider radiation cooling in any situation where it might conceivably be made to work.

If the cold package (incorporating thin-film SQUID' s and other microcircuitry , for

example), is envisioned to be a thin plate, for large radiating surface and small mass, the

time rate of change of temperature at low temperatures may be approximated as

dT 2oTk dt dDC

Cv

= (1943 J/mol-K)dDT 3 /6 3

-8 2 4 where a is the Stefan-Boltzmann constant (5.67 x 10 W/m K ), d is the thickness, D is the

molar density, is the specific heat, and 9 is the Debye characteristic temperature of the

material, and the approximation is good for T considerably smaller than 8, but not so small

that the electronic (or other) terms in the specific heat become comparable to the lattice

term. For a black aluminum plate 1 mm thick, the cooling time constant is about nine hours,

so the plate would effectively cool off in less than two days. The cooling rate would be

considerably greater above 100 K, and smaller below 10 K, since 9 for aluminum is about

375 K, and the electronic specific heat is appreciable at very low temperatures.

2 2 The power radiated by a black surface at 10 K is 0.57 mW/m , or 1.1 mW/m for both sides

of a thin plate. A SQUID operating at a bias power level of 10 ^ W would therefore require

2 a radiating area of only 1 mm at 10 K. The prospects of radiation cooling a superconducting device in the neighborhood of the

earth or any of the inner planets seems remote. The power radiated and reflected by the

earth is of the order of

OJ 1300 W (8000) 2 /4 r „ , -7 . 2 p T

- = t -hrz ~~rz = 5.2 x 10 W/m

s 4tt 4 m 93 x 10

where u> is the solid angle, and the expression is evaluated at a distance of one orbit radius

away (93 x 10^ miles) . Thus, in most of interplanetary space the sun would be the main heat

2 source, of the order of 1 kW/m at the earth's orbit. This would have to be reduced by a

7 factor of 10 or so if the superconducting package is to be radiation-cooled to 10 K. A reflecting disc used as a sunshade would reduce the power by the factor (1-r) and also by a factor proportional to the solid angle intercepted by the disc at the package. Both factors

may easily be of the order of 10 or greater. As few as three or four discs in series should

provide the necessary shielding.

Heat conduction along the supports to the cold package can be made small if one en- visions an open structure with the cold package supported only by members in tension (i.e.

,

138
wires or cords) . For example, a mass of 1 kg at 4 K might be supported by six spun nylon

cords, extending off in various directions to give kinematic stability, tied to an intermediate assembly at 20 K. If each cord is 10 cm long and each is calculated to break at 10 G

2 loading (10 kg), then the total cross-sectional area of the six cords should be 0.7 mm , and

the total heat flow is 6 yW. For comparison, the heat radiated by a black surface is 570 yW/m'

2 at 10 K and 14 yW/m at 4 K. For spun glass with the same loading the heat flow would be

about 9 yW and for stainless steel about 60 yW. The data from which these were calculated

are from a technical report edited by R. Stewart and V. Johnson (1961) . For nylon, glass,

and stainless steel, the thermal conductivity integrals from 4 to 20 K were given as 0.008,

6 6 0.020 and 0.16 W/cm, respectively. Tensile strengths were taken as 9 x 10 , 14 x 10 and

ft 9 18 x 10 g/cm . It is envisioned that the cold package, in the form of a thin plate, and the sun shields

would be spaced along an axis pointed toward the sun so that each shield completely intercepts the radiation from the next warmer shield which would otherwise impinge on the cooler

shields

.

There are various ways by which the cold package and the sun shields can be rigidly

assembled by tension members only. For example, the shields might be conical, with the apex

of each cone extending somewhat inside the rim of an adjacent cone, and appropriate cords,

like the spokes of a bicycle wheel, connecting the two. We conclude this commentary by stating the obvious — if superconducting devices are to

be used in space in any number of applications, a large development effort on cryocoolers or other means of -air.taining low temperatures will be required. It is suggested that a modest

beginning of the effort should be encouraged now.

139
APPENDIX I I. A.

TERRESTRIAL APPLICATIONS OF SUPERCONDUCTING MAGNETS

Superconducting magnet applications here on earth are subject to some boundary conditions which are more severe than exist for aerospace uses. Of course, the inverse is also

obviously true, but those conditions have already been discussed at some length. On earth,

nearly all of the tasks proposed for superconducting magnets are capable of being carried out_

by normal magnets, albeit with enormous expenditures of energy. Furthermore, the economics

of capital investment versus operating cost cloud the issue somewhat, as does the (well

justified) conservative attitude of much of our industry, and occasionally, a psychological

reluctance to get involved with a "new" technology. Also, it is still true that operation of

a large superconducting magnet facility requires a much greater competence on the part of the

staff than a similar system of normal magnets would. The major difference is the necessity

of using various cryogens to cool superconducting magnets, and this situation seems unlikely

to change in the near future. With our current knowledge, and given the perversity of nature,

it seems that a commercially viable room temperature superconductor is not in the cards for a

long, long time.

The discussion here is not intended to cover all projects, there are too many, but to give an idea of the very wide range of applications in which superconducting magnets are being used.

II. A. 1. Existing Magnet Applications

In this section we discuss systems which have been built and used. In most cases the use is either in scientific laboratories or is very experimental. There are no "every day"

commercial uses of superconducting magnets. In nearly all cases, the conductor is NbTi.

High Energy Physics Magnets

This application pretty much accounts for all the large superconducting magnets in the

world. It has been the major source of money for conductor development and of personnel for

all phases of the superconductor business. Even so, the great majority of machine and beam

control magnets are still normal.

The higher field strengths and lower energy consumption available with superconductors

has recently led to the construction of several very large systems, such as the Energy

Doubler at Fermilab and others which are described in the next section, since they are not

yet completely operational.

Large superconducting dc dipoles have been built and operated since about 1973 at Brookhaven National Laboratory. One such, a two-module 8° bending magnet has a 3.5 T central

field and stores a total of 300 kJ. The magnet is cooled by a 75 W refrigerator. Another

large system is the high energy beam line at Argonne National Laboratory which was operational in 1976. It has ten superconducting dipoles and two superconducting quadrupoles. The

magnets are contained in four separate cryostats with a total length of > 13 m. The dipoles

operate at 2.6 T and the quadrupoles give gradient fields of 0.31 T/cm.

140
A muon channel magnet system was put into operation in 1974 at the Swiss Nuclear In- stitute. It is an 8 m long iron-clad solenoid. Cooling for the magnet is by forced flow of

supercritical helium. The sixteen coils in the system store a total of 1.6 MJ and give a 5 T central field. This system has proven to be nearly maintanence-free over several years of

operation.

The ability of superconductors to create quite large fields (1-2 T) throughout very

large volumes has been exploited in the construction of four large bubble-chamber magnets.

These dc magnets are simple coils heavily stabilized with copper. The two largest of these are the one at the National Accelerator Laboratory (4.27 m ID, 3.0 T @ 5.0 kA, 400 MJ) and

the CERN-BEBC magnet (4.72 m ID, 3.5 T @ 5.7 kA, 830 MJ)

.

Many other superconducting high energy physics magnets have been made that have had only

limited success and have not yet been accepted to any degree by the machine users. In this

category are a number of very sophisticated pulsed dipoles and quadrupoles. They have shown

a significant sensitivity to thermal and mechanical shock, as well as a tendency to "train"*

excessively. In nearly all instances magnets for high energy physics research are manufactured in-house. The industrial capability is so limited at present that this is a matter of

necessity.

A related application of superconductors has been in the shielding of charged particle

beams from the machine fields through 2-4 m segments of their path. This technique has been

employed at CERN to lead particles into the bubble chamber through the large magnet. In this

application, the beam travels in a superconducting tube which excludes flux from its interior

by the Meissner effect, thus creating a permanently field free region.

Fusion Energy Devices

A large superconducting magnet was built and put into operation in 1971 as a plasma

physics mirror confinement experiment at the Lawrence Livermore Laboratory (LLL) . It is

called a "baseball" magnet. The name arises from the shape of the coil winding, which looks

like the seam on a baseball. The magnet stores about 10 MJ, has a 1.2 m "bore" and a maximum

field at the conductor of 5.5 T. Other smaller superconducting plasma physics devices have been built at Oak Ridge National

Laboratory (IMP), NASA-Lewis (Bumpy torus, 12-3 T coils) and a small (0.5 m bore) mirror at NASA-Lewis (8.8 T) . Two devices, using superconducting rings levitated by normal and superconducting magnets, were developed at Princeton and LLL. Most of these are described in the

proceedings of the 1972 Applied Superconductivity Conference.

* Training, also mentioned in Section II. 3. 4, involves multiple excursions into the normal

state as the magnet is first energized. Usually a large amount of liquid helium is vaporized

in the process. After each of these "quenches" the magnet can be energized to a higher

field. Magnets showing 20 - 50 quenches before reaching maximum field are not rare. After

reaching field the magnet usually can be discharged and recharged without training as long as

it is not warmed up.

141
Research Magnets

For sheer numbers, this class of magnets is probably the largest. Nearly all physics

research laboratories have a superconducting magnet of some sort. The magnets tend to be

relatively small in size and to be simple solenoids or to have a Helmholtz configuration.

Typical is 2-5 cm bore, 7-10 T, 20-30 kg, 100-200 A at full field. They are now wound from

multif ilamentary wire, usually NbTi, but multif ilamentary Nb^Sn is slowly appearing on the

wire market. Unlike their earlier counterparts, these magnets boast charging times to full

field of less than a minute and show very little in the way of training. The construction of

these magnets for sale is a tough business and only a very few small companies exist at present

.

In this group one should also include the very large magnets at places like the National

Magnet Laboratory. The largest continuous field magnets yet made (25.4 T and 30.1 T) were recently (1976) constructed there. They are both hybrids which consist of a large superconducting coil with a water-cooled copper coil insert.

Motors and Generators

Several medium size (to ^ 3 MW) superconducting motors, both ac and dc, have been built

and operated since the first 50 hp motor was made in 1966. To our knowledge only one has

actually been put into service in a real system. The Fawley motor, a 3250 hp dc (2.4 MW)

device, was first tested in 1971 and operated a water pump with some success for an extended

period of time. Both the U.S. and British Navies (and likely others) are interested in

superconducting propulsion systems as they offer a great amount of flexibility in control as well as reduced weight. These systems are usually dc. The largest presently under construction, for delivery in 1977, is a 2.2 MW (3000 hp) homopolar system for the U.S. Navy.

To our knowledge, no machinery has yet been installed on a ship, although the British were

planning a test in 1977.

Another marine application which has been proposed and tested with a small normal magnet

system, is a magnetohydrodynamic engine in which an electric current induced through the sea water interacts with a magnetic field to propel a large (2000 ton displacement) submarine to quite high speed, ^ 90 km/hr is projected.

Many words have been published on the feasibility and desirability of superconducting ac generators. They would be about half the size and one-quarter the weight of their normal

counterparts. The largest of the normal devices (^ 2500 MVA) is now limited by size — any

larger and it won't fit under the railroad bridges. In spite of all this only a few superconducting machines have been built (it isn't easy), and only one, the 3 MVA MIT alternator

has been run in a power grid, and that as a synchronous condenser. The application of superconducting generators in the power system appears to be a long ways off.

Magnets for MED

As we have already mentioned, magnetohydrodynamic power generation on a large scale is a natural application for superconducting magnets. The magnets are of large size, and thus

relatively easy to wind compared to the airborne systems, and the fields required at present

are not inordinately high (^ 4 - 5 T) , although it has been suggested that higher field

142
devices might help the economies significantly. Unfortunately there are other serious problems with MHD that have kept it from power plant applications. A large magnet, finished in

1976, for the USSR by Argonne National Laboratory has just been used in the U-25 MHD facility

in Moscow. The magnet system is ^ 4.4 m long with a magnetic length of 2.5 m. The maximum

on-axis field is designed to be ^ 5 T and the magnet stores 20 MJ. A recent NASA-Lewis study

has considered a system of similar energy but with somewhat reduced size and weight.

An earlier, smaller magnet was built some years ago by AVCO, but it appears that it was never used in an actual MHD system. The worldwide effort in large MHD systems is significant. Whether they will see widespread use or not depends in large part on the future of

fission reactor systems. If fission systems are de-emphasized, MHD offers a possible re- placement at perhaps slightly higher total cost.

Industrial Magnets

Superconducting magnets appear to be well suited for numerous jobs of ore separation, or even scrap recovery, due to the availability of large high field and high field gradient

regions which allow a large throughput of material, but they have been little used in spite

of some effort (primarily by the MIT National Magnet Laboratory) to convince the industry of

their usefulness. In fact, the only magnet of this sort, of which we are aware, is a relatively small one used to remove iron oxide discoloration from kaolinite which is used for

coating printing papers. The mineral industry is not greatly interested yet, but as the

quality of the taconite ores decreases, the higher fields will begin to seem more desirable.

Separation of nonf erromagnetic scrap by eddy current induction is a grand idea, but

experiment has shown that the strong size dependence of the force experienced by a piece of

scrap seems to preclude this approach at least for the near future.

Medical Applications

The use of large, high gradient magnetic fields, produced by more or less portable

superconducting magnets, to steer magnetically tipped catheters through the cranial arteries

has led to the possibility of repairing aneurysms, etc., inside the brain. These and other

clever applications of magnets have been used successfully by a team consisting of members

of the staffs of the National Magnet Laboratory and Massachusetts General Hospital.

Levitated Transport

Superconducting magnets offer the possibility of levitating an already moving vehicle

above a track which may be made up of other magnets or possibly a pure metal in which induced

eddy currents create the repelling field. This "guided flight" removes the constraints

imposed by wheels and axles, making speeds in excess of 600 km/h feasible. The Japanese

National Railway in 1972 operated a model large enough to carry people (3.5 tons), and the

Germans (Siemens, Erlangen) have also run a vehicle of significant size on a circular track.

The work on such systems in the U.S. has nearly completely vanished for a couple of reasons: mass transit, regardless of speed potential doesn't have a real good reputation in the U.S.

for some obscure reason; and the suspension system requires such a large amount of feedback

control in order to produce an acceptably smooth ride that it may not be practical in reality.

143
II. A. 2. Applications in the Near Future

Clearly, the more promising of the applications just described will be evolving as time

goes on. In addition, there are a few superconducting magnet applications which appear

almost certain to become reality within the next decade. In fact some of them are already

well along in construction but have not yet been put into service.

High Energy Physics Magnets

A number of large accelerator and storage ring systems have been proposed and a few are under construction. Two, which are progressing well at present, the Energy Doubler at Fermi

Lab and the Experimental Superconducting Accelerator Ring (ESCAR) at the Lawrence Berkeley

Laboratory will serve to give a feeling for the scale of such projects.

The Energy Doubler will use 1000 (!) large superconducting magnets (800 dipoles and 240

quadrupoles) in a 6 km tunnel. The magnets have NbTi windings and the dipoles have central

fields of ^ 4.5 T. A large number of prototype magnets have already been built and tested.

The ESCAR system will use 24-4 T NbTi superconducting dipoles each ^ 1 m long. They are now in production at the rate of about one a month. Tests of a pre-production prototype were very successful.

These two examples should serve to indicate that this business of superconducting magnets is not just someone's wild dream. Large complex magnets can be built successfully in

large numbers. Refrigeration systems adequate to cool these monsters are now available. A

detailed description of these and many other high energy physics experiments using superconducting magnets is given by Reardon (1977)

.

Fusion Energy Devices

g Magnetic confinement of the hot fusion plasma (^ 10 K) in a toroidal configuration has

proven to be one of the more promising approaches to achieving net fusion energy production,

but the plasma physics and the machines both have a long way to go. At present it seems certain that high fields with large volumes will be necessary, and that requires superconducting magnets. The USSR has a fusion device of the tokamak design which uses superconducting magnets, but to our knowledge it has never operated successfully. A large mirror machine

physics experiment (MFTF) is being constructed at the Lawrence Livermore Laboratory. It uses

a heavily stabilized (8:1, Cu:SC) NbTi conductor. The maximum field at the conductor will be

7.5 T and the stored energy in the magnets will be 500 MJ. The magnets have a fairly complex

winding geometry called a yin-yang pair.

A program at Oak Ridge National Laboratory (ORNL) called the Large Coil Program, is

funding the construction by three industrial contractor teams of three superconducting coils

of the type to be used for the toroidal field coils in large tokamak devices. The purpose of

the project is to prove that such coils can be made successfully and to provide coils for

tests of a large toroidal magnet facility. They are not expected to be used in an actual

fusion device. Several foreign countries have also agreed to contribute coils to the program. The coil design is flexible within certain guidelines. The magnets are to be D-shaped

with a 2.5 x 3.5 m winding bore and produce a field of 8 T at 10-16 kA current. The windings

will be heavily stabilized. Two of the coils are using NbTi conductor with poolboiling of

144
helium as the cooling mode; the third will use Nb^Sn and be cooled by forced flow of supercritical helium. The coil deliveries are presently scheduled for installation in a test stand at ORNL in early 1980.

Energy Storage Magnets

A number of inductive energy storage systems using superconducting magnets have been

built and tested recently, but they are included here rather than in the previous section

because, to our knowledge, they have never been used in an actual application. The largest

systems now built and tested are at the Los Alamos Scientific Laboratory (LASL) . There

started out to be three of them, one was quite successful. They store about 300 kJ in a 2.2 T NbTi magnet operating at 10 kA. Charge and discharge times are 30 s and 10 s respectively. Coils 0.5 MJ) are being developed for plasma heating by induction in fusion

devices; capacitor banks now serve this function, but will be impractical for much larger

machines. Similar coils are proposed for providing the large pulses of energy required by

high power laser systems in a variety of applications.

Giant underground superconducting magnet systems storing 10^ - 10^ MJ have been the

subject of detailed designs by the University of Wisconsin and LASL. These large systems are primarily proposed for peak-shaving utility applications.

Magnets for MED

Superconducting magnets for full scale MHD power plants are now being studied at the

National Magnet Laboratory with Department of Energy Funding. Two magnets of this class, one

for an Engineering Test Facility (ETF) and an actual Base Load (BL) plant magnet were designed by Magnetic Corporation of America. Both magnets have a 6 T field at the channel and

^ 9.8 T at the windings. NbTi at ^ 4.2 K seems to be the conductor choice for both designs.

The ETF magnet is 7.0m long and stores 483 MJ, and the BL magnet is 16.0 m long and stores

4.5 GJ. In both cases the magnet system is ^ 65% structure and the specific weight is

656 kg/MJ for the ETF magnet and 437 kg/MJ for the BL magnet.

Medical Applications

A large toroidal superconducting magnet system to allow the use of mesons in cancer therapy has been constructed and tested at the Stanford Linear Accelerator Center (SLAC)

,

but to our knowledge it has not yet been used in treatment. A similar system is being

developed at the Swiss Nuclear Institute.

Industrial Magnets

It has been shown at the National Magnet Laboratory that it is possible to remove a range of pollutants from water by "seeding" it with fine magnetic particles and then passing

it through a mesh of fine ferromagnetic fibers (steel wool) located in a modest magnetic

field. The non-magnetic impurities are collected by the magnetic seeds which, in turn, are magnetically removed. Again the high field volume necessary for an acceptable throughput

requires that the magnets be superconducting. As best we can tell almost nothing has been

done on this system recently, but there is no indication that there is any problem with the

concept

.

145
II. A. 3. Applications in the Far Future

Clearly, anything suggested here must be taken with much skepticism, so we will keep it

short. It seems certain that fusion energy in one form or another will become a major source

of power after the turn of the century and, at present, it seems that superconducting magnets

will be used for plasma confinement in these devices. Other applications, such as the control of chemical and biological reactions of various sorts are possible, but so much needs to be learned that no prediction is possible. One development which would, of course, change

much of the way we look at high field applications would be development of a viable high

temperature superconductor, but that doesn't look very promising.

Much concern has been expressed over the continuing availability of helium, since the

major source is natural gas, and there is no longer a U.S. program of conservation. The

"experts" apparently cannot agree on whether or not this could become a problem in the far

future. One could probably assume that it will, although development of superconductors

capable of high field operation at liquid hydrogen temperatures, is a very real possibility

and would result in a much lower demand on the helium reserves.

146
APPENDIX II. B.

CURRENT APPLICATIONS OF SUPERCONDUCTING MAGNETS IN SPACE

In this appendix we present a description of the few superconducting magnet systems

which have actually been built for application in space or near to it.

II.B.l. Operational Superconducting Magnet Systems

In this description of actual systems we go into somewhat greater detail than in earlier

sections because: (1) there are so few magnets that doing so is no problem, and (2) the

relative simplicity of the systems which work is encouraging and needs to be stressed.

Orbital Systems

The only two superconducting magnet systems to achieve earth orbit were small coils

flown by the USSR on Cosmos 140 in 1967 and on Cosmos 213 in 1968.

3 The first of these was a very small (12 cm field volume) double solenoid with a maximum

field of ^ 1.4 T in a more or less conventional, nitrogen-shielded transport dewar cooled

with 6.5 I of supercritical helium. The flight lasted for a total of 32 hours. It was

observed that introduction of a very small artifical gravity (about 0.001 g) , directed opposite to the normal direction, caused increased boiloff due to cold fluid entering the dewar

throat and warm gas in the magnet region, but the magnet did not quench. The magnet was

charged and put into a semipersistent mode prior to launch. Some conclusions from this

flight were: that, (1) the large static and vibrational loads at launch did not affect the

solenoid in any adverse way; (2) supercritical helium could be used as a coolant with no real

problems and; (3) small gravitational forces can seriously affect the operation of the system.

The second flight carried a larger split coil with a 1.7 T central field in a 10 cm bore, used as a magnetic analyzer for cosmic radiation. The system mass was 16.6 kg and the

container was similar to the one flown previously. The superconductor used in both systems was the alloy 65-BT (probably NbTi) . In this experiment the dewar was filled with two-phase

helium, and overpressure by the boiloff during flight (which only occurred when the satellite

began rotating six hours into the flight) was used to create the supercritical phase. At some point, ^ 17 hours into the flight, the magnet apparently quenched. The conclusion seems

to be that the dewar system needs to be carefully constructed to operate well both in weightlessness and under the complex conditions imposed by the flight. There seemed to be no problem with the magnet itself. The experiments are described by Anashkin, et al. (1969).

Balloon-Borne Systems

The use of superconducting magnet spectrometers on high altitude balloon flights has

become about as routine as anything gets in this business. The first flight was made in

1969. Nearly twenty successful flights have been made in this country involving four different magnets. The magnets tend to be simple coils of rectangular cross section wound from

NbTi which is stabilized with copper. The systems are quite similar. In one, the central

147
field is ^ 3.0 T. The system uses a nitrogen-shielded liquid helium dewar holding about 80 I

j of helium and will operate for about one day without refilling. The total package weighs

150 kg and is designed to take 5 g shocks from any direction.* The magnet current is 120 A

at full field and the magnet is charged, prior to flight, from a 2 V battery source. Charg- J ing and transfer to persistent mode is a slow operation requiring several hours. Stored

energy in the magnet is 576 kJ. These systems have been described recently by Golden (1975).

Magnet Systems in Rockets

Two superconducting magnet systems have been investigated for use on rocket flights. To the best of our knowledge only one was flown. That one was built at AVCO for flight on an Apache rocket. The NbTi magnet was a small end-corrected solenoid with a 7 cm bore and

15.3 cm long. It developed 3.0 T central field at 55 A current. It was to be charged before

launch and put into persistent mode for at least a two-hour period. The coil was to be able I to withstand 50 g acceleration without quenching. The system mass was to be less than 23 kg. In the course of the program a smaller test coil was actually subjected to 37 g's at frequencies from 100-2000 Hz without quenching. Rumor has it that the actual coil was tested to 50 g's and delivered to NASA-Goddard.

k Magnet Systems in Aircraft

No such systems have actually been , flown. In the text of Chapter II we discussed some of

the problems of very specialized military systems. Otherwise, in general, there is every reason to believe that any system which will work in the laboratory will work on board an aircraft.

Summary: Operational Systems

Very few superconducting magnet systems have operated in space. Those that have were relatively small in size, not complex in design, and operated at fields well below the capability of the conductor. The cooling systems were quite conventional. For the most part, no changes from conventional magnet design were made. Limited testing indicates that acceleration and vibration are not severe problems for these heavily stabilized magnets.

There is no question but that modern superconducting magnet systems are capable of

performance under much more demanding conditions than those already encountered. If such

high-performance systems are to be actually used, and the chances are that they will, now is

the time to begin to evaluate what the limits of performance in space applications truly are. Comparison of the parameters of the magnets just described with those of the existing terres- l trial magnets in Appendix II. A. should be sufficient to convince even the most skeptical that

| superconducting magnets represent a basically untapped source of potential assistance for a large number of space activities.

* In fact, one of the Lawrence Berkeley Laboratory balloon magnets maintained its field

through a descent which ended in a crash on a mountainside, severely damaging the gondola

shell.

148
II. B. 2. Systems That Almost Made It Much of the history of superconducting magnet applications in or near space involves a series of rather heartbreaking scenarios in which good magnets were built, or almost built,

and tested, but never flown for reasons having nothing to do with the magnet which in nearly

all cases was perfectly adequate for the proposed task. Here we describe these magnets, but

space does not allow much of a discussion of the reasons behind their failure to fly.

Orbital Systems

A prototype magnetic spectrometer magnet was designed and built to be used on the High

Energy Astronomical Observatory (HEAO) series of flights. The first of the three flights

in this series has been launched, the second goes in 1978 and the third a year later. There

will be no superconducting magnets on any of these flights since it was decided to use a smaller launch rocket, which resulted in the removal of the heavier of the planned experiments. The dewar system was designed for a hold-time of one year and, in fact, has since

proven to be capable of this extended containment in ground-based tests. The NbTi magnet

itself and several rewound versions are described by Pope, Smoot, Smith and Taylor (1975).

Apparently only one coil was built; two are needed for the spectrometer. It produced a field

of 7.0 T at the winding with a current of ^ 120 A and a stored energy of ^ 750 kJ. The magnet

mass was 150 kg and the coil had an inner diameter of 68 cm and was 9.5 cm long.

A smaller (30 cm), but similar, magnet system was being developed at Marshall for shuttle

applications, but it now seems unlikely that this system will actually be used on any immediate

flight.

Batloon-Bome Systems

A large magnet with a complex saddle shape was built about 1967 by the Lawrence Livermore Laboratory (Taylor, et al., 1967) and successfully tested at that laboratory. It had a 1.1 T central field, aim bore, a maximum current of 870 A and a stored energy of 1.2 MJ. The coil and dewar weighed 1630 kg. Aluminum alloy was used as the structural material to minimize the weight. This magnet is probably a reasonable representation of the breed. The

distribution of mass is as follows: coil form 20%; helium container 8%; conductor 31%;

structural shell 17%; dewar 24%.

Magnet Systems in Rockets

A coil was to be made at LASL for an ionized plasma experiment in about 1972. The

magnet was quite large (275 kg, 43 cm bore, 3.5 T) , but the forces were low (less than 6 times

the force produced by gravity) . Modeling and test coils indicated that such a system would be

possible, but it was apparently never built.

149
APPENDIX IV. A.

JOSEPHSON TUNNELING LOGIC

Josephson tunneling logic is based on individual switching devices called Josephson

junctions (after Brian Josephson who first theoretically predicted their properties) . Such a junction is formed by two superconducting electrodes separated by a thin insulating layer,

only a few tens of angstroms thick. To achieve its remarkable properties the junction must

be cooled to low temperatures (4 K) so that the junction electrodes are superconducting.

A Josephson junction has a time-averaged current-voltage (I-V) characteristic as shown

in Fig. IV.A-1. The dashed line shows the I-V curve of the junction when it is not superconducting. Not shown in this characteristic are oscillations in the current which occur at

Typi cal ly

% 1 mV

Figure IV.A-1. Time-averaged current-voltage characteristic of a Josephson junction.

the dashed line is the I-V curve of the junction when it is not

superconducting

.

2e frequency f = — V, where V is the average voltage across the device, e is the charge on an electron, and h is Planck's constant. That typical oscillations are at very high frequency

2e

is clear since — = 484 GHz/mV. For our purposes these oscillations are at such a high

frequency that they can only be observed if extreme care is taken. These oscillations must,

however, be considered in careful analysis of high-speed device operation.

A particularly unique feature of the Josephson junction is its ability to carry current

with zero voltage drop. We see this reflected in the portion of the I-V curve along the

current axis. When biased at points along this portion of the curve the device dissipates

exactly zero energy. It is a superconductor.

There is a maximum zero-voltage current I . If we try to raise the junction current m

above I , voltage will appear across the junction. At average voltages above zero, but below

150
the upward bend in the I-V curve at E , the junction carries very little current. Above E

g g the I-V curve approaces the linear one of a non-superconducting junction, a pure resistance

To see how such a junction is operated as a switching device — a Josephson tunneling

gate — one attaches it to the bias supply as shown in Fig. IV.A-2. The gate is the "X."

V

Figure IV.A-2. Bias supply for Josephson gate.

When biased on the zero voltage part of the I-V curve, as shown by point a_ in Fig. IV.A-3.A,

all of the voltage V supplied by the battery drops across the source resistance R . No

s voltage appears across the junction.

a II

V

A B C

Figure IV.A-3- Operation of Josephson gate (latching).

To cause the junction to switch, one applies a magnetic field to it, which reduces Im

(to be explained in greater detail later) below the junction current at point a_. In practice

the magnetic field is produced by passing a current through a control wire (usually a thin

film) directly above the junction. The current through the wire is called the control current. Once I is reduced below a, that point becomes unstable and the junction switches to m —

point b_ along the dashed line as shown in Fig. IV.A-3B. If the control current is reduced,

I returns to its original value, but the junction remains at point b. The junction is said m —

to "latch" at point b. To return to point a, one reduces the voltage V, essentially to zero,

151
and then raises it again. The path followed by the junction is shown in Fig. IV.A-3C. It is

also possible to construct "non-latching" junctions which return to point a_ as soon as the

control current is reduced and I rises. m

Josephson tunneling gates of the type described above can have remarkably fast switching

times: as fast as a few picoseconds from point a. to point b_. Measured switching times from

a to b are limited by present measurement techniques to around 30 ps. Switching times of

about 80 ps have been achieved for switching from a to b and back to a.. In addition to rapid switching (and perhaps more important) the Josephson tunneling gate

offers exceptionally low dissipation. At point a, no power whatsoever is dissipated; at —6

point b_, typical dissipation is of order (1 mA) x (1 mV) = 10 W. This is three to four

orders of magnitude lower than the dissipation of most semiconductor devices. A revealing

comparison was given in Fig. IV-2 (in Chapter IV). Low power is crucial when large numbers of

devices are combined to form a large computer or even when smaller numbers of devices are assembled for ultrafast measurements.

Thus we have seen how the state of a Josephson gate can be changed by a control current

passing through a wire which is electrically separated from the junction itself. A latching

junction can be returned to its original state (point a) by momentarily reducing the bias.

Non-latching junctions return directly to their original state when the control current is

reduced.

Two questions remain. How does one perform logic and how does one transfer the result

of a logical operation to a succeeding junction? To begin with performing logic, one notes

that, as shown in Fig. IV- 3 (in the main text of Chapter IV), two or more control lines may

be placed above a junction. The maximum zero-voltage current I through the junction is

determined by the sum of the magnetic fields produced by the control currents. Thus, I is

determined by the sum of the control currents.

One next needs to examine how I depends on the control currents (or equivalently the m

magnetic fields they produce). We have illustrated this dependence in Fig. IV.A-4. It can

Maximum zero-voltage current

Control current

Figure IV.A-4. Operation of Logic

152
be seen that a control current of either polarity will reduce I . Let us assume that the m

amplitudes of the control currents are described by the arrows in Fig. IV.A-4. If the junction is biased at point a, applying a control current through one control wire moves the

system to point x. The junction does not change state because x is still less than the

maximum zero-voltage current. However, if control currents are applied through both control

wires simultaneously, the state of the system moves to y. The junction does switch, since

the maximum zero voltage current I has been reduced below the operating current of point a. We can make a truth table of the results:

Control Current 1 NoYes

NoYes

Control Current 2 No

No

Yes

Yes

Junction Switches?

No

No

No

Yes

It can be seen that the output of the junction (its state) is the logical "and" of the input

control currents. Thus logic has been directly performed by a single Josephson tunneling

gate.

It is also possible in an analogous manner to perform a logical "or" by making a junction in which the curve shown by Fig. IV.A-4 is skewed. Logical "not" can also be performed

in a somewhat more involved way. Thus, all fundamental logical functions can be performed,

making possible the construction of computers and digital instruments.

The final subject, not yet discussed, is the method by which the state of a junction may

be transferred to a subsequent junction. As illustrated in Fig. IV.A-5, the junction is

connected to a terminated transmission line. Part of the transmission line rises above the

Control

wi re \

/

To Gate Supply

A/W>—

Transmission

Li ne

Output

of

second junction

Figure IV.A-5. Transfer of information to subsequent junctions.

153
next junction and is a control wire for that junction. When the first junction switches to

b, a time-averaged voltage appears at the ends of the transmission line and a current therefore propagates down it, acting as the control current for the next junction. The second

junction therefore switches to point b_. The resistance in the transmission line terminates

it and dissipates the magnetic energy stored in it, if the first junction switches back to state a. Thus, the control current for the second junction drops back to zero whenever the

first junction reverts to a state a_.

154
APPENDIX IV. B.

JUNCTION FABRICATION TECHNOLOGY

Josephson tunneling logic devices have become serious candidates for a variety of applications to measurements and computers because of substantial progress which has recently been

made in fabricating reliable junctions. The progress has been principally in two areas: the use of alloys for the electrodes, and improved barrier formation techniques.

Josephson junctions for practical use need to be made of superconductors which have

transition temperatures above the boiling temperature 4 K) of liquid helium at atmospheric

pressure. Thus further cooling by pumping on the helium is unnecessary. There are only two elemental superconductors which meet this criterion: lead and niobium. Niobium is difficult

to work with because it oxidizes so readily, making control during barrier formation difficult. Thus, for the moment, lead is the most used material.

Lead junctions suffer substantially because, after lead is cooled with liquid helium, it

undergoes compressive stresses when it is rewarmed. These stresses lead to the growth of

hillocks or bumps in the lead films. The hillocks are of more than sufficient size to destroy the insulating barrier in the junction. Thus lead junctions, once cooled, are destroyed if allowed to rewarm. This clearly renders them useless for field application.

On the other hand, lead alloy junctions have been demonstrated to be resistant to hillock growth, and to survive repeated cooling and heating without failure. The lead alloy is

deposited through sequential evaporation of thin films of pure materials. The layered film

which results is then homogenized by heat treatment at modest temperatures of about 70° C. Presently preferred materials are lead, alloyed with indium and gold.

Fabrication of reliable barriers depends both on the use of the alloy films and on the

method of oxidation. The alloy described above forms an oxide which is enriched in indium

compared with the composition of the alloy. The second electrode is made of materials which

do not react with oxygen as readily as indium, thus inhibiting migration of indium in the

barrier into the second electrode. The presently preferred materials for this alloy are lead

and gold. The indium oxide barriers, formed using these alloys appear to have a lower barrier

height than pure lead. Thus, for a given tunneling current density the indium barrier can be

thicker than that for pure lead, a desirable result which enhances long term stability of the

barrier.

Oxide layers can be produced in a controlled way using an rf sputtering process. One

slowly sputters away the surface of the first electrode, in an oxygen atmosphere. At the same time, because of the presence of the oxygen, the surface continually reoxidizes. Eventually a dynamic equilibrium is reached in which the rate of sputter removal of the surface

equals the rate at which the surface reoxidizes. Because the process is very sensitive to changes in the sputtering conditions such as rf voltages and oxygen pressure, these parameters can be used as sensitive controls of the resulting oxide thickness. Time is not an important parameter, since as soon as dynamic equilibrium is attained, there are no further

changes in the oxide thickness.

155
To make complete Integrated circuits, one must also be able to deposit resistors and

insulators for crossovers and microstrip lines. The resistors are made from Aul^, an intermetallic which is present in the superconducting alloy, so that interdif fusion between

resistor and superconductor is not substantial. Two types of insulator are commonly used.

Niobium is often used as a ground plane and then anodized to form a ttt^O,. insulating layer on

its top surface. SiO is used in other places where insulators are needed (e.g., between

junction and control lines, at stripline crossovers, etc.).

/

156
REFERENCES

Anashkin, O.P., Kurnsova, S.S. , Konyakhina, S.S., Lagarikev, A. A. , Razorenov, L.A. , Sidvakina,

T.M. and Fradkin, M.I., (1969). Trial of Superconducting Magnetic System with Field

H - 20 kOe on Board an Artificial Earth Satellite. Kosmitcheside Issledovaniya 7j

786-793. Translation available as N71-11903

Golden, R.L. (1975) . Superconducting Magnets for Space Flight. In Research Goals for CosmicRay Astrophysics in the 1980 's, European Space Agency ESRO SP-109, 91 (available as N76-12934)

Pope, W.L., Smoot, G.F., Smith, L.H. and Taylor, C.E. (1975). Superconducting Magnet and

Cryostat for a Space Application. Adv. in Cryo. Eng. 20, 47-60.

Reardon, P.J. (1977). High Energy Physics and Applied Superconductivity. IEEE Trans, on Magnetics MAG- 13, 704-718.

Stewart, R.B. and Johnson, V.J. (1961) . A Compendium of the Properties of Materials at Low

Temperature (Phase II) . WADD Technical Report 60-56, Part IV, Wright Patterson Air

Force Base, Ohio.

Taylor, C.E. Nelson, R.L. , Henning, CD., Taylor, A.R. and Harvey, A. (1976). A Lightweight,

1 -Meter-Diameter , 11 kG, Superconducting Magnet System for a Balloon-Carried Cosmic Ray

Experiment. Proa. 2nd Int. Conf. on Magnet Tech., 617, (Rutherford Laboratory, Chilton,

Berks, England).

Zimmerman, J.E. and Flynn, T.M. (1978). Applications of Closed-Cycle Cryocoolers to Small

Superconducting Devices, National Bureau of Standards Special Publication SP-508.

157 <3 U.S. GOVERNMENT PRINTING OFFICE: 1978 —777-067/1257 REGION NO

NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 78-885

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

The Role of Superconductivity in the Space Program:

An Assessment of Present Capabilities

and Future Potential

5. Publication Date May 1978

6. Performing Organization Code

724

7. AUTHOR(S)

D.B. Sullivan, editor

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 7240187

11. Contract/Grant No. NASA Contract No.

A-437018(JM)

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Aeronautics and Space Administration

400 Maryland Avenue SW

Washington, D.C. 20546

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or /ess factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

This report describes the results of a study designed to assess the role

which superconductivity might play in the U.S. Space Program. The study was performed by members of the staff of the Boulder Laboratories of the

National Bureau of Standards. Six technical subject areas were considered;

high field magnets, magnetometers, digital electronics, high-frequency

detectors, instruments related to gravitational studies and ultra high-Q

cavities. The study identifies a number of applications of superconductivity

which are of potential interest to NASA. Wherever possible, the devices

are related to specific types of space missions.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name: separated by semicolons

)

Computers; digital electronics: gravitational studies; high-Q cavities;

infrared detectors; magnetometers; magnets; microwave detectors; space;

supp.rmnm]rr.ivir.v.

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