cations

NBSIR 82-1678

NATL INST. OF STAND & TECH

A111D5 T fl 3 4 T ^

DEVELOPMENT OF STANDARDS FOR SUPERCONDUCTORS

Interim Report

October 1980-January 1982

A.F. Clark, L.F. Goodrich, F.R. Fickett, and J.V. Minervini

Electromagnetic Technology Division

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

JULY 1982

QC

Ud

LOu Work performed under Department of Energy contract DE-AI01-76PR06010

6 and

T3o2 Massachusetts Institute of Technology contract ML-212300

NBSIR 82-1678

National Bureau of Standard

SEP 3 0 1982

DEVELOPMENT OF STANDARDS FOR SUPERCONDUCTORS

Interim Report

October 1980-January 1382

A.F. Clark, L.F. Goodrich, F.R. Fickett, and J.V. Minervini

Electromagnetic Technology Division

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

JULY 1982

Work performed under Department of Energy contract DE-AI01-76PR06010

and

Massachusetts Institute of Technology contract ML-2123Q0

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

CONTENTS

Page

1. INTRODUCTION 1

2. EVALUATION OF PRESENT STATUS 2 2.1 Summary of Major Accomplishments 3

a. Creation of ASTM Subcommittee on Superconductors . . 3

b. Survey of Superconductor Test Techniques in

U.S. and Japan 3

c. International Test Specification Survey 3

d. Round Robin Critical Current Tests 4

e. Comprehensive Experimental Program on Critical

Current Testing 4

f. Critical Current Standard Test Method Developed ... 4

g. Standard Terminology for Superconductivity

Developed 4

h. Definitions of Terms for Practical Superconductors Adopted 5

i. Critical Temperature Measurement Technique

Evaluated 5

j . Preliminary Comparison of ac Loss Measurements ... 5

k. Research Results Disseminated Through More Than

20 Publications 5 2.2 Importance of Current Transfer 6

a. Critical Current Criteria 6

b. Sample Holder and Sample Configuration 6

c. Joints 7

d. Sample Homogeneity 7 2.3 Future Research 7

3. PREPARATION AND DISSEMINATION OF STANDARDS 8

3.1 ASTM Subcommittee on Superconductors 8

3.2 Standard for Terminology 9 3.3 Standard for Critical Current Measurement 12

3.4 Standard Reference Materials 15

4. EXPERIMENTAL PROGRAM 16

4.1 Critical Current 16

iii
Page

4.2 Angular Dependence of Critical Current on Magnetic Field 17

a. Introduction 17

b. Apparatus . 20

c. Experimental Results . . 21

d. Conclusions 36

4.3 ac Losses 36

5. CRITICAL CURRENT MEASUREMENTS: A COMPENDIUM OF

EXPERIMENTAL RESULTS ............ 39

6. REFERENCES 96

APPENDIX A ..... A1

APPENDIX B B1

APPENDIX C Cl

IV
DEVELOPMENT OF STANDARDS FOR SUPERCONDUCTORS

A. F. Clark, L. F. Goodrich, F. R. Fickett, and J. V. Minervini

A cooperative program with the Department of Energy, the

National Bureau of Standards, and private industry is in

progress to develop standard measurement practices for use in

large scale applications of superconductivity. The goal is the

adoption of voluntary standards for the critical parameters and

other characterizations of practical superconductors. Progress

for the period October 1980 through January 1982 is reported.

The major effort was the development of a standard test method

for critical current, the necessary back-up research, and the

coordination of the adoption of the test method and a standard

terminology through the subcommittee level in ASTM.

Key words: critical current; critical parameters; losses; measurement methods; standards: superconductor.

1 . INTRODUCTION

The development of standard measurement practices is essential to the success of any developing technology. In order to help assure success in the

field of large scale applications of superconductivity the Department of

Energy and the National Bureau of Standards have undertaken a program to

establish a uniform terminology and reliable measurement techniques for the

many new aspects of superconductivity that are essential to good design. This

report is the third in a series summarizing the progress in this program which

is jointly supported by National Bureau of Standards and three divisions of

the Department of Energy (Fusion Energy, High Energy Physics, and Magnetohydrodynamics through the Francis Bitter National Magnet Laboratory)

.

The first two reports [6, 21]* summarized the progress in the first two

years. This report covers an extended period ending in January 1982 to

coincide with a change in the contract year. The first report included: a large effort on the standardization of terminology, a preliminary assessment

*Reference numbers refer to chronological listing in § 6, page 96.

1
of measurement capabilities in the United States, the formation of an ASTM

Subcommittee on Superconductors, some preliminary transient loss measurements,

a comparison of critical temperature measurements on practical superconducting

materials, and extensive research on the many factors that affect critical

current measurements. The second year of the program concentrated heavily on

the development of a standard method for measuring critical current and

included contracts to the four U.S. wire manufacturers for research on various

aspects of this measurement. Thus, the second report includes a survey of the

state-of-the-art of critical current measurements, including a round robin and an assessment of the criteria used, reports from the four wire manufacturers,

the final publication of the definition of terms, and the development of a draft standard for the determination of critical current for superconductors

with a critical current less than 600 amperes.

This third report includes an evaluation of the present status (§ 2), a summary of the progress in the adoption of the voluntary standards (§ 3), a review of the experimental effort (§ 4), and (as § 5) a paper reviewing all

the parameters that can affect critical current measurements and an assessment

of their importance. A list of the more than 20 research papers resulting

from this program is included as § 6. The agenda, minutes, and list of

attendees of the August 10, 1981 meeting of the ASTM B01.08 Subcommittee on Superconductors is included as Appendix A. The draft standards for "Standard

Definitions of Terms Relating to Superconductivity" and the "Standard Test

Method for DC Critical Current for Composite Superconductors" are included as Appendices B and C, respectively.

2. EVALUATION OF PRESENT STATUS

It is important in an interactive research program such as this to

continually assess the results with respect to the direction of future efforts.

The final goal is the adoption of voluntary standards for the determination of

design parameters for practical superconducting materials. The progress

toward this goal is summarized in § 3, The Preparation and Dissemination

Standards. In this section we will summarize the background research efforts,

point out significant research problems that have been encountered, and

indicate future directions for the program.

2
2 . 1 Summary of Major Accomplishments

Listed below are highlights of the efforts to date and where more detailed

results can be found.

a. Creation of ASTM Subcommittee on Superconductors

The ASTM Subcommittee B01.08 on Superconductors was created very early in

the program in order to provide not only the means of adopting voluntary

standards but also a forum for feedback on needed research and the practicality of proposed standards. Three meetings have been held and task groups have

developed draft standards for definitions of terms and critical current test

methods which have been modified and adopted by the subcommittee. (See § 3.)

b. Survey of Superconductor Test Techniques in U.S. and Japan

A detailed survey of test techniques applied to superconductors for all

the critical parameters as well as other characterizations in the U.S. and

Japan was performed [6, 21]. It showed that measurements were usually made to

10-15% accuracy at best, few laboratories have routine test facilities,

critical current measurements if wrong are usually low, simple test practices

often lead to dramatic errors, and both producers and consumers of practical

superconductors desired a cooperative effort to develop standard test methods.

c. International Test Specification Survey

An evaluation of test specifications used by forty different laboratories

in the U.S. showed a great diversity [21]. Little agreement was found for

values of test parameters or accuracy, sample mounting techniques were a problem, transfer voltages and strain effects were not understood, accurate

field measurements are badly needed, and the critical current criteria were

not universally accepted.

3
d. Round Robin Critical Current Tests

A round robin test for critical current of three different conductors at

five laboratories was conducted [21], Inaccuracies of 30-40% were common,

probably because no methods or criteria were suggested, the labs were free to

choose their own. (A sixth laboratory recently volunteered additional measurements with no significant change in the results.)

e. Comprehensive Experimental Program on Critical Current Testing

A very thorough testing of all the parameters that affect critical

current measurements was done by NBS and the four wire manufacturers [4-6, 12,

15, 17-22], These effects were evaluated for:

• Sample holder type and mounting technique

• Joints and current transfer for various mounting methods

• Criterion type and magnitude

• Magnetic field angle and homogeneity

• Material type and structure

• Variability in material

• Power supply effects

The results of this research formed the basis for the standard test method as proposed to the ASTM subcommittee.

f. Critical Current Standard Test Method Developed

A draft standard for a test method for determining the critical current

in practical superconductors for less than 600 amperes was developed jointly

by NBS and the ASTM subcommittee. It has been unanimously approved by the

subcommittee and is now in process for a main ASTM committee ballot. (See

§ 3.)

g.

Standard Terminology for Superconductivity Developed

In a series of four publications in the international journal Cryogenics,

detailed and theoretically accurate definitions of terms relating to super4
conductivity were presented for feedback and comment [2, 3, 8, 11]. Suggestions from more than 60 laboratories were incorporated.

h. Definitions of Terms for Practical Superconductors Adopted

Simple and useful terms relating to practical superconductors were

drafted from the four publications above and adopted by the ASTM subcommittee

for use with the critical current test method and any future standards.

i. Critical Temperature Measurement Techniques Evaluated

An experimental comparison of resistive, magnetic, and specific heat

methods of determining critical temperature for practical conductors showed

that they sample different parts of the conductor and can give widely varying

results [6]. Further work was delayed in order to concentrate on other

parameters deemed more important.

j

.

Preliminary Comparison of ac Loss Measurements

A preliminary comparison of several electronic and caloric methods of

determining ac power losses showed a wide disparity of results and severe difficulties based on both experimental complexities and inadequate theoretical understanding [6, 16]. A low level of effort to improve the electronic

methods continues.

k. Research Results Disseminated Through More Than 20 Publications

A primary goal has been to make the results of the research readily

available. Other than these three comprehensive reports to the sponsor, the

program has resulted in 17 research journal publications (eight of which were

papers at major conferences) and four presentations to research advisory

groups (three of which have been published) . The publications are all listed

in § 6

.

5
2 . 2 Importance of Current Transfer

One experimental effect above all others that affect critical current

measurements has shown to be important and even dominant in many experimental

conditions. It is the voltage generated when current must transfer to or from

die superconducting filaments through the resistive matrix. It arises at the

current contacts, any joints in the conductor, or any place that the current

is forced to redistribute within the composite conductor, such as where the

magnetic field changes. This transfer voltage manifests itself in many ways,

some obvious and some quite subtle. These are documented in several of the

publications [4-6, 15, 19, 21-22], but listed below are some of the more important areas with regard to standard measurement techniques, some of which

become critical for large conductor measurements. How to account for these

effects is given in § 5 and also documented in more detail in a forthcoming

paper devoted exclusively to current transfer and its effects.

a. Critical Current Criteria

The real increase in voltage as the critical current is approached is due

to the start of flux flow. Any shift in the baseline from zero is probably

due to a current transfer voltage. A current transfer voltage, because it is

resistive, is proportional to the current although it need not be linear.

This must be kept small and be properly accounted for if an accurate measure

of the true critical current is to be obtained. Following the restrictions in

the ASTM standard test method should minimize the transfer voltage, but the

method also includes a limit on its size (Appendix C)

.

b. Sample Holder and Sample Configuration

In order to minimize the additional effects of current transfer voltages,

the sample must be configured such that the current must be introduced a "long" distance from the voltage sensing contacts. In addition, any redistribution of the current due to a change in the magnetic field, such as changing

from parallel to perpendicular to the electrical current, will introduce

current transfer voltage. (This effect can introduce some long range anomolous

6
voltages, but may also make possible a sample configuration that assures

minimum transfer voltages [19].)

c. Joints

Any physical joint between two superconductors will force a redistribution of the current and thus result in a current transfer voltage [15]. This

will add to the total power used in a magnet, for example, or disturb any

nearby voltage measurements.

d. Sample Homogeneity

If a superconducting filament in a composite superconductor has a nonuniform cross section (e.g. a neckdown) and is operating near its critical

current, it can force a current transfer through the resistive matrix into an adjacent filament. This will also add to a total power, but will also be

important in developing a standard reference material.

2.3 Future Research

Both the sponsor and the ASTM subcommittee have indicated that extension

of a standard test method to larger current capacity conductors should have

first priority. Approximately 6000 amperes has been suggested as the next

appropriate step. An evaluation of the past research for the first standard

test method indicates that for large conductors many of the parameters studied

will have little impact. Some effects, such as current transfer, however,

will only be compounded and must be documented for bigger conductors. In

addition, some of the parameters which have little effect on smaller conductors, such as aspect ratio and self field, will also have to be studied to assure minimum impact on accuracy. There are also some properties unique to

large conductors that will be explored such as stranding or cabling. The

development of a method for small laboratories to be able to determine critical current without extensive equipment will also be pursued.

Other areas of study include a continued low level effort on developing

ac loss measurement techniques, a first look at the problems of magnetic field

7
and field homogeneity measurements with the goal of a critical field extrapolation technique, an evaluation of the state-of-the-art for stability

characterization of superconductors, and exploration of a method for determining critical current as a function of temperature. Of course, implicit in

the program is a continuing effort to coordinate the adoption of voluntary

standards. Included in this will be the development and characterization of a standard reference material for critical current for small conductors funded

by NBS

.

3. PREPARATION AND DISSEMINATION OF STANDARDS

The final goal of this program is the adoption of voluntary standards for

practical superconductors. Two of these standards are well on their way to

acceptance. After the formation of an ASTM subcommittee the first order of

business was the adoption of a standard terminology. The next priority was a standard test method for critical current of small conductors. The progress

in each of these as well as that toward a standard reference material are outlined below.

3 . 1 ASTM Subcommittee on Superconductors

Early in the program it was concluded that The American Society for

Testing and Materials (ASTM) would be the proper format for any voluntary

standards for characterizing practical superconductors. At the instigation of

NBS, the B1 Committee on Electrical Conductors created the Subcommittee B01.08

on Superconductors and an organizational meeting was held at ASTM headquarters

in 1979. At this meeting task groups were organized and the long process

toward voluntary standards was begun [6], At the next meeting, held in

conjunction with the Applied Superconductivity Conference in 1980, draft

standards were proposed for terminology and critical current tests were extensively discussed [21], After appropriate modification and clarification

by more research at NBS and the wire manufacturers these draft standards were

sent to the subcommittee ballot. These were approved by the subcommittee but some negative votes had to be resolved at the next subcommittee meeting and are discussed in the appropriate sections below. The minutes of that meeting

held in August 1981 in conjunction with the CEC/ICMC meeting in San Diego are

8
included as Appendix A. Briefly, NBS researched the points of contention and

proposed modifications or justified acceptance. The subcommittee then voted

to accept, modify, or delete the points in question. Finally, the subcommittee approved both modified standards unanimously.

The adoption of voluntary standards by ASTM is a long and complex process

with many checks along the way. First, the subcommittee must be balanced

between producers and users, and then approval must be obtained at three

levels. The subcommittee ballot must have two-thirds approval with a 60%

return. Any negative ballot must be accompanied by an explanation which must

be resolved before it can go to main committee ballot. The main committee

ballot must obtain 90% approval within a 60% return and again any negatives

must be resolved. The final society approval is a two-thirds affirmative with

a minimum of 50 returns and is principally a canvas mechanism to pick up any

missed negative votes which must again, however, be addressed by the subcommittee .

Thus, the next step for the standards will be for NBS to prepare them in

the proper ASTM style and submit them to the main committee for ballot. The

subcommittee also urged NBS to pursue the development of standard test methods

for larger conductors and for ac losses and a standard reference material.

3 . 2 Standard for Terminology

Terms that were deemed useful for the critical current standard were

selected from the four published articles of precisely defined superconducting

terminology [2, 3, 8, 11] and from the Compilation of ASTM Standard Definitions

(ASTM, Philadelphia, PA, 1979). These were put in a shorter, simpler form

more appropriate to everyday language by the ASTM subcommittee task group and

submitted for ballot. The ASTM Committee on Terminology also assessed the set

of definitions and had three pages of suggested changes which were accommodated

before going to ballot. The subcommittee ballot approved the draft standard

(Appendix B) with a 93% affirmative vote and one negative ballot. Listed

below is a summary of the objections from the balloting procedure and the NBS

recommended resolution.

9
ASTM BO 1.08 SUBCOMMITTEE ON SUPERCONDUCTORS

SUMMARY OF OBJECTIONS FROM NEGATIVE BALLOTS

DEFINITIONS

Critical magnetic field strength , H^. Definition is incorrect for but

appropriate for H^* Suggest changing to upper critical magnetic field,

H

c2

Recommend: Change as above.

Critical current density . For twisted conductors it needs to be specified

with respect to orientation of the conductor.

Recommend: No change. Area needed is as defined - that perpendicular to

the principle axis.

Cable . Limited to cables with central cores.

Recommend: Add specific composite conductors, including ones without

central cores.

Composite conductors . Should be types of "materials" not types of

"wire."

Recommend: Change wire to materials.

Matrix-to-superconductor-ratio . This term has no physical significance

and is misleading for specifications. It also is non-linear.

Recommend: Substitute "Volume percent superconductor" defined as that

part of a composite conductor by volume that is superconducting under

normal operating conditions.

10
At the last subcommittee meeting these and other changes were all discussed

and adopted unanimously. The next step will be for NBS to incorporate these

changes and some changes in style required by ASTM and prepare the standard

for the main B1 committee ballot.

As a point of interest, through the entire process of the preparation of

the four articles, creation of the draft standard, and modification of the

standard through the balloting procedure, individuals from more than 60

institutions throughout the world have contributed substantially. These are listed in Table I.

Table I. Organization Providing Input for Standard Definitions

Producers

AIRCO

ALCOA

AMAX

American Magnetics

Brown-Boveri

Imperial Metals Industries

INCO

Users

Air Force Materials

Air Force Aero Propulsion

Argonne

Brookhaven

CEN - Saclay

CERN

CGE - Marcoussi

ERDA - DPR

ERDA - MFE

Garrett AiResearch

General Atomic

General Dynamic

General Electric

Other

Air Force Office of Scientific Rsch.

All Union Research Inst. -Moscow

Battelle

Chiba University

Freie Universitat

Gould

Harvard University

Inst, de Rech. de l'Hydro Quebec

Molycorp

Moscow State University

Intermagnetics General

Kabelmetal

Magnetic Corp. of America

Supercon

Teledyne Wah Chang

Vacuum Metallurgical

Vacuums chmelz

Lawrence Berkeley

Lawrence Livermore

Los Alamos

Magnetic Engineering Associates

National Accelerator

NASA - Lewis

National Magnet

Naval Ship Rsch. and Development

Netherlands Energy Research

Oak Ridge

Rutherford

Stanford Linear

Westinghouse

Naval Research

Nihon University

Rensselaer Polytechnic University

Singer Company

Stanford University

Sumitomo Electric

Union Carbide

United Nations

University of Wisconsin

Wichita State University

11
3.3 Standard for Critical Current Measurement

Following the second meeting of the ASTM Subcommittee on Superconductors

where the draft standard for the test method for determining dc critical

current in composite superconductors was created by combining the best from

the three proposed, NBS addressed and resolved the remaining open questions

and objections. After specific research for these problem areas a comprehensive draft was prepared and sent to the subcommittee members for comment.

Following the incorporation of these comments a draft standard (Appendix C) was prepared for ballot, mailed to the subcommittee, and the resulting vote

was an affirmative 73% with four negative ballots complete with further

objections

.

Listed below are the objections which were raised and the NBS recommendations for action based on further research. These were then considered at

the next subcommittee meeting in August 1981 at San Diego.

ASTM BO 1.08 SUBCOMMITTEE ON SUPERCONDUCTORS

SUMMARY OF OBJECTIONS FROM NEGATIVE BALLOTS

CRITICAL CURRENT

1.2 Let contract negotiations determine which conditions apply not which

ones are inappropriate. (2.)

Recommend: No change. The purpose of the standard is to set the

conditions. Exceptions can always be made.

3.1.1 A bandwidth of periodic and random deviations must be specified.

Suggest 10 Hz to 10 MHz. (3. & 4.)

Recommend: Add "within the bandwidth 10 Hz to 10 MHz."

12
3.1.3 A 20% accuracy for yields more like 2% accuracy in J^. (1.)

Recommend: Change 20% to 12%. (See variable list.)

3.3 Weather variations of pressure result in temperature accuracy of 2%.

Against having to measure absolute pressure. (5.)

Recommend: No change. 99% confidence interval for 1 year is within

±0.5%.

3.4.1 Magnetic field uniformity of 1% not possible for large magnets and long

or helical samples. Suggest 2% is adequate. (1.)

Recommend: Change to ±2%.

3.6 A shunt not in immediate contact with the specimen may make it impossible to measure high current density specimens. The answer should be

the same if properly stabilized. (3. & 4.)

Recommend: Delete sentence in parentheses. Add after specimen "(when

the specimen is in the normal state)."

5.1 Differential thermal contraction restriction would preclude using Nb^Sn

on copper or stainless steel holders. However, it has been shown this

works. (3. & 4.)

Recommend: No change. Strain tolerance must be held.

5.5 A recheck such as this when used with large field variations would be

time consuming and expensive. Suggest a recheck at each field level

and begin with highest field. (1.)

Recommend: Need to discuss. Suggested options: recheck at highest

current level, operational check. Some check is needed.

13
6.1 Suggest listing of all the variables and their effect on the final

accuracy. (2.)

Recommend: Listing is provided but should not be part of the standard.

Remark: This standard is not for unreacted A15 material unless the preparation

is specified and included in the report. (1.) Covered in 7.1.

Remark: More descriptive material is needed - a newcomer would be unable to

make a satisfactory test. (2.)

Remark: Test procedure is not simple or economical. It should be readable by

a purchasing agent. (2.)

Remark: An equation should be added for the current transfer length. (6.)

Add reference to 6.2.1.

The subcommittee, after a great deal of discussion, adopted all of the recom- mendations except they deleted the recheck requirement of part 5.5.

To aid in the above discussion which involved some trade-offs between

accuracy and ease of measurement, NBS prepared a listing of the accuracy

tolerances required in order to result in an overall accuracy of 5%. These

are shown in Table II along with the expected current dependence for each of

the independent accuracies. It is valuable to note that the magnetic field,

strain, and temperature measurements are the dominant sources of inaccuracy, none of which are the primary variable but all of which will have to be

considered for the other superconducting critical parameters.

The next step in the process of adopting a standard test method for

determining critical current will be for NBS to assimilate the subcommittee

changes along with some changes in style for the draft standard to be mailed

out for main committee ballot. Simultaneously, research will be continuing

for the next standard test method for larger conductors with critical currents

from 600 to 6000 amperes.

14
Table II. Accuracy Tolerance for Critical Current Variables

Variable Accuracy, % 1^ dependency AI , %

c

I 1 linear 1

I, PARD 5 unknown 1

E 12 „ n

E a I 1 c (10% L, 6% v)

c

H, magnitude 1 log I = a + bH

c 2.5

H, PARD 1 unknown 0.5

H, uniformity 2 averaged 1.5

H, angle 7° averaged 1

T 0.5 T c a (T* - W 2

£ > bending 0.1 Nb

3

Sn scaling law 2

2 NbTi

£

tensile 0.05 Nb

3 Sn scaling law 2

0.5 NbTi

'J e (AI ) 2 = 4.98

c

3.4 Standard Reference Materials

A promise of support from NBS for the acquisition and characterization of

a standard reference material (SRM) for critical current has been obtained.

It will require estimating the probable number of units to be manufactured,

establishing criteria limits on the various characterization parameters and

pursuing the availability of such a conductor. Once a suitable length of

conductor is located then a complete characterization will be done but it is

intended that the SRM will be certified only for critical current at several

magnetic field values.

15
4 . EXPERIMENTAL PROGRAM

Study of the parameters affecting critical current have dominated the

experimental effort during this period. The major portion of those results

are given in § 5 which will be published as a journal paper in the open

literature. Given below are a brief summary of those results, some work that was not included in § 5 and a short description of some progress on ac loss

measurements.

4. 1 Critical Current

In preparing the standard for measurement of critical current a large

amount of research was required to insure that the various numerical limits

called out in the standard were reasonable and necessary. As a result of the

many experiments performed by NBS and by the wire manufacturers, sufficient

data exist to make a persuasive argument for the particular values chosen for

inclusion in the standard. All of the values are, of course, tied to the

assumption of an overall accuracy of 5% for the critical current determination, and are summarized in Table II in § 3. Detailed results are given in

§ 5 for the phenomena that affect the measurement of critical current which

fall into the three categories listed below.

(1) Those of most serious importance which, it turns out, are also the ones that can be accommodated. Experimental work has tended to concentrate on them in the recent past. They are:

Holder type

Sample mounting

Critical current criterion

Current transfer.

(2) The second group contains aspects of the measurement that are relatively difficult to control, but that turn out to not strongly affect the

I measurement if moderate care is taken in design of the apparatus. They

c are

Joints

Field angle

16
Field homogeneity

Sample current supply

Cabling effects.

(3) The final group consists of parameters associated with the production of the superconducting wire. In general the amount of variation observed

is quite large, but it is not possible to say that this has a detrimental

effect on the value of the critical current determination. The parameters

investigated were:

Filament uniformity

Spool uniformity

Lot /billet /percent superconductor

Copper/superconductor ratio

Aspect ratio.

The last of these has some interesting features that still need to be evaluated, but again there is no direct effect on the determination of I for a given configuration for small conductors.

The paper included as § 5 is a compendium of all these results to date.

It is intended to be an "everything you wanted to know about measuring critical current" paper that is available to anyone.

It was concluded from all the research that the data accumulated over the

past three years and the interactions generated by the ASTM committee have

allowed it to arrive at a set of parameter values for critical current measurements that are technically sound and entirely practical. The resulting

standard is a reasonable first effort and the time has come to put it into

practice.

4 . 2 Angular Dependence of Critical Current on Magnetic Field

a. Introduction

The dependence of the critical current, I c> on the angle, 6, between the

conductor and the magnetic field, H, was measured for three different types of

superconductors; multifilamentary (MF) NbTi, MF Nb^Sn, and tape Nb^Sn. This

17
dependence is important in determining the accuracy tolerance of this angle in

I

c measurements. 1^(0) is a ls° useful in understanding current transfer in

the hairpin geometry where the sample axis changes along its length from

parallel, to perpendicular, to parallel with respect to the uniform applied

magnetic field. The larger I in parallel field gives an additional safety

margin for the ends of a hairpin sample. This advantage is also present in

the long-straight geometry, but not in the short straight or coil geometries.

This is a more complete treatment of these effects which are summarized in

§ 5.

I (6) was measured for five samples to determine its dependence on the

type of sample, filament twist, magnetic field, and critical current criterion,

E^. These samples are listed on Table III. 1^(0) for the three types of

superconductors had very similar shapes, but with different sensitivities.

Two of the samples were MF NbTi; one with a rectangular cross section and the

other round. The shapes of X

c

( 0 ) for these two samples were essentially the same within 5%. The twisted MF Nb^Sn sample was prepared by twisting a length

of sample 3 (untwisted MF Nb^Sn) prior to its reaction. The shapes of 1^(0)

for these two samples were very similar, within 3%, suggesting that twisting

the filaments has little effect. The scaling of 1^(0) with magnetic field was determined in order to interpolate or extrapolate the results to other fields.

The data indicated that the empirical relation that scales the results with

magnetic field is

[I (H) - I (H)]/I (H) = f ( 6) , (1)

C CO CO

where I is the critical current at 0 = 0, I is the critical current at 0 co c and f ( 0

)

is a function of sample and 0 but not magnetic field. In this paper

the angle is zero when the conductor axis is perpendicular to the applied

magnetic field. Scaling was also necessary in order to get agreement between

the two NbTi samples which had different I^'s. This same scaling works for

different critical current criterion where both I and I are calculated

co c using the same E^. Then f ( 0

)

will be very similar for criteria differing by

as much as a factor of ten. All of the data presented here will be normalized

according to Eq. (1).

18
cxb3

COX3H o

•HU

CO to r

i-i 3 OJ 00 00 vO

at a, • • • • • a cx

cx o o o O 1

p-H r-H *-H i“H 3o3 U <—i

CT\ co

CO | X

•H

B • • • & O

Eh o O o o

of tsc o o C3 CT\

S-i QJ CO ooXX

QJ 6 *—1

r-—1

00 OO

Xi CO S -H 3 -H 3 Mh

CNl CNl

*\ CO w X co 3 • • o o -<r o CM o

•H

X

CO S X • • X • 3 B o o o

QJ CO CM B X • •H • O Q o

•H

EHX 3 3

13 •H co 3 CO

Eh ro CO co

JH

X

X coX

ai CO 3 3 X 3

cxX 3

Co 3 03 03 03 H to (3 3 3 3 3 3 3 CX 3 3 O O 3 O p 3 3 Eh 3

CJ

QJ 3

qj 3

r-l QJ CX x B B

cO 3

c/J 3

r—1

CN] co

X

19
b. Apparatus

I (0) of the round NbTi sample (#2) was measured in two ways. First, a series of coil samples was prepared with winding pitches ranging from 1 to

40° . Total sample lengths varied from 10 to 90 cm. The results of the I

c measurements gave X c

( 0 ) of the sample using the 1° data as the zero angle

reference data. A more complete series of measurements were made using a rotating device that allows a single short (3.6 cm) sample to be rotated

through an angle of nearly 120° while it was in the measurement configuration.

This device allowed for the convenient and systematic measurement of all the

samples including the Nb^Sn based superconductors. Comparisons between these

two techniques using sample #2 showed agreement within 1% (see § 5).

The rotating device was made with a housing, two sets of gears, a long

actuator, dial indicator, sample table, and flexible current leads. The

housing was a G-ll tube that supported the gear shafts and fit into the bore

of the solenoidal magnet. Two sets of gears, worm-worm wheel and miter-bevel,

were used to translate and step-down the rotation of the actuator and room temperature dial indicator. The gear ratio was 6.25 turns of the ten turn dial

indicator for a 90° rotation of the sample. The gear lash was only about 1°.

The sample table was made of G-ll and it was fastened to the worm wheel.

Flexible current leads were made using four lengths of sample #2 (round NbTi)

for each current lead. These two leads were tied together to help balance the

Lorentz force and were soldered to a bus bar on each end. The table could be

rotated through an angle of about 120°. It was set up so that it would rotate

from about -20° to +100°, in order to check the zero angle and symmetry.

1^ was measured at a number of set points on the dial indicator of the

rotating device. The set points were every 5° or 10°. The pattern of the

measurements was the zero-angle set point first, then all of the positive

angles, the zero angle again, then all of the negative angles and the zero angle last. I at the zero angle was checked in order to determine if there was any damage to the sample due to quenching or strain. There was only one case where the sample was strained significantly. This was during a measurement at 100° where the Lorentz force pulled the sample (MF Nb^Sn) from the

support and the zero-angle I was then 1.2% higher. Otherwise the value of I

20
at the zero angle was the same to the uncertainty of the I measurement ±0.3%

indicating that the quench protect circuit and support were adequate.

The rotating device worked quite well, but, it had a couple of disadvantages. One disadvantage was that its current carrying capacity was limited to

about 400 A by the flexible current leads. This limited most of the measurements to the lower angles (the angle is zero when the conductor axis is

perpendicular to the applied magnetic field). A couple of measurements were

made on a coil sample holder with the conductor axis parallel to the magnetic

field in order to get some data at 90°. The other disadvantage of the rotating device was the limited sample length which caused some current transfer

problems such as a negative voltages [19] on the NbTi samples and large

current transfer voltage on the MF Nb^Sn. These problems limited the sensitivity of E to 0.2 yV/cm for the MF NbTi and 5 yV/cm for the MF Nb„Sn.

c 3

c. Experimental Results

The zero angle of the rotating device was somewhat uncertain due to the

arbitrary experimental scale and variations in the sample mounting. It was,

however, determined using the measured I as a function of angle around the zero angle and assuming it to be symmetric about the zero angle. Typical data

are shown on Fig. 1. Notice that the curve is symmetric about -0.8°. The

zero-angle shift was determined in this way for all of the samples and in all

cases it was less than 2°. The results of measurements around 90° are shown

on Fig. 2. This curve is fairly symmetric about 89° consistent with its

zero-angle shift. The shape of the curve at the peak is not known. The lines

drawn are only an attempt to estimate the value at the peak. The rest of the

data presented here has been interpolated to round values of angles using the

zero-angle shift and the shape of the curve. This was done in order to

compare the measurements of different samples and different runs.

The scaling of X

c

( 0 ) with magnetic field using Eq. (1) is shown for all

five samples (see Table III) on Figs. 3-7. The slight irregularities in the

low angle data are due to the I measurement uncertainty. The lack of signic ficant dependence of the normalized I on magnetic field for each of these

samples indicates this empirical relation works very well. This is amazing

21
20

18

16

14

12

10

8

6

4

2

B

Typ

for

30

angle , degrees

:al normalized I as a function of angle around the zero angle

sample #2 at 1 pV/cm and 9 T.

22
Fig. 2. Normalized I as a function of angle around the 90° angle for sample

#5 at 10 yV/cm and 9 T.

23
Ico)

/I

CO

f->o H , T

Fig. 3. Normalized as a function of magnetic field at various angles for

sample #1.

24
Ico)

/I

CO

Vo H . T

Fig. 4. Normalized as a function of magnetic field at various angles for

sample #2.

25
Ico)

/Ico

Hott ' T

Fig. 5. Normalized as a function of magnetic field at various angles for

sample // 3.

26
Ico)

/I

CO

Po H r T

Fig. 6. Normalized I as a function of magnetic field at various angles for

sample #4.

27
o

o

\

o

o

*-—

i

o

2 4 6 8 10

Po H. T

Normalized I as a function of magnetic field at various angles for

sample #5.

Fig. 7.
considering the range of angles, magnetic fields, and thus critical currents

tested. The critical currents at the zero angle as a function of magnetic

field for each sample are given in Table IV. Notice that X for sample #2

changed by a factor of 7.1 over the wide range of magnetic field tested. A

further point not covered by this range of angles and magnetic fields is that

this relation has to break down at low enough magnetic field and high enough

angles since I in parallel field cannot be larger than I in zero field.

This was not investigated because of the limited current capacity and the

interest in the low angle and high magnetic field region.

The scaling of 1(0) with critical current criterion using the same kind

of normalization as Eq. (1) is shown for two samples on Figs. 8 and 9.

Similar data were obtained on the other samples. Notice the lack of

significant dependence of the 'normalized I on the criterion. The value of

c

I for each of these criteria are given in Table V. These data indicate that

co the shape of the voltage-current curve must not change much with angle even though I is changing significantly.

The shape of the normalized I as a function of angle for each of the

five samples is shown by a full log plot on Fig. 10, All of these data were

at 8 T and the criterion listed on Table IV. Notice that the results from the

two NbTi samples are about the same. Also, the results of the tested and

untwisted MF Nb^Sn samples are about the same. The shape of the normalized I as a function of angle for the three types of superconductors are very similar,

but with different sensitivities.

The normalized -I as a function of angle was measured up to 90° for two

of the samples, MF NbTi and MF Nb^Sn. These measurements could only be made

at higher magnetic fields. The data for the MF NbTi and the MF Nb^Sn are shown respectively on Fig. 11 at 9.75 T and on Fig. 12 at 9 T. One of the

NbTi data points, the triangular symbol, was measured on a coil sample holder

at 90°. The rest of the data were taken on the rotating device. Notice that

the curve smoothly continues up to 90°.

29
. 1 1 10

Ec , jj\f/cm

Fig. 8. Normalized I £ as a function of E

c at various angles and 9 T for

sample #2.

30
Ico)

/I

CO

sample #5.

31
Ico)

/I

CO

Fig. 10. Normalized I as a function of angle for all five samples at 8 T.

32
Ico)

/Ico

Fig. 11.

deqre es

Normalized I as a function of angle for sample #2 at 1 yV/cm and

9.75 T.

33
Ico)

/I

CO

an9

le , degrees

Fig. 12. Normalized I as a function of angle for sample #5 at 10 pV/cm and

9 T.

34
Table

IV. I

asa

function

of

magnetic

field

H

ONU

03 c H

M-locooc3

IJH >

QJ 1—I &cti H

Omo

CMmo

CMoso>3- w

CO

<r

in

00

CT\ cn

cn

00

CO

•<r

oo

cn

in

o-s o

r-»

CM

la

03 CO oo

CO

ON

CM om CM o

CM 00

in

in6o>

3- W

35

Sample

#5
d. Conclusions

The normalization according to Eq. (1) works very well in scaling X c

( 0

)

with magnetic field and criterion. I (0) for all of the samples tested had

c very similar shapes, but with different sensitivities. The results indicate

that the sample axis and the applied magnetic field can be misoriented from

perpendicular by as much as 7° with less than a 2% change in 1^. The high

field data indicate that I in a parallel field for a MF NbTi and MF Nb^Sn may

be respectively as much as twelve times and four times larger than the I in a

c perpendicular field.

4.3 ac Losses

A relatively low level effort has continued to assess the measurement

problems for ac losses. Although virtually lossless under dc conditions,

superconductors do have finite losses when operated in an ac magnetic field,

with ac current, or both. Earlier efforts showed that measurements by different techniques, although frequently giving wide disparity of results, can be

shown to agree [6, 16]. It appears now that some of that agreement was fortuitous. The work in this area described below has been mostly supported by NBS.

There have been developed one caloric and several electronic methods of

determining ac power losses. The caloric method utilizes a determination of

the liquid helium boil-off rate as a measure of the additional power lost in a superconductor in ac conditions which necessarily shows up as heat. The

method is straightforward and unambiguous but requires great care in apparatus

design and is not very accurate below a few milliwatts. Several of the electronic methods involve either the measurement of a phase angle or the multiplication of current and voltage signals. Because the very large inductive com- ponent in most superconductors swamps the small resistive (power loss) component

of the voltage signal, this requires very precise voltage phase and magnitude

measurement is required. Magnetic techniques are also possible but have not

been explored as yet.

The existing apparatus [16] was slightly modified to increase the caloric

sensitivity and provide a larger, more definable sample volume, and compari36
sons were tried with the lock-in amplifier and analog multiplication techniques. It was quickly determined that even with an improved lock-in amplifier the zero-setting of the phase was not reproducible to the 0.03 degrees

required as a minimum. The analog multiplication technique was tried with a "Brookhaven" wattmeter which NBS acquired for this study. With this device a "current" and a "voltage" signal from the superconducting sample are amplified

and then multiplied together. The wattmeter is designed to have very low and

calibratable phase shifts in each channel. After experiencing some difficulty

in obtaining reproducible results, the characteristics of the wattmeter were

studied.

Using a precise phase signal and meter available in another group of NBS,

the phase shifts in the amplifier and multipliers were measured. Typical

results are shown in Figure 13 for the voltage channel phase shift as a function of frequency. The clear minimum can be shifted in frequency over a wide range with proper calibration but must be done each time a new frequency

is used. The minimum, although not precisely zero, presumably could be made

the same in both channels to within 0.03 degrees.

Identical signals with a precisely known phase difference were also fed

into the multiplier channels and the output versus phase angle measured. The

output very nicely followed the expected cosine curve but on an expanded scale

near the important 90° point the multiplier output showed significant non-zero

contributions. The conclusion was that the wattmeter device could readily be

used for comparative measurements but was not appropriate for research studies

of ac losses in general.

With borrowed electronics a similar study was done on the multiplication

technique but using digitized signals with a greatly enhanced accuracy. The

phase shifts were measured and it was concluded to be a viable and accurate

technique. Based on this, NBS has acquired the necessary digital processing

electronics for further research. In addition, a new sample cryostat is being

designed that will also permit magnetization studies. It will also provide a sample configuration more comparable to a theoretical model which is also

being developed, and more thermal sensitivity for the caloric method.

37
^ m ^ in ^

lo sa

no c\i <\i

soojBa P

' 1J TL

I

S ^SDLjJ

ca

N

-O

L

0)

-C

X

O

c

©

D

Cr

0

L

U_

38

Fig.

13.

Phase

shift

in

amplifier

section

of

wattmeter

as

a

function

of

frequency.
5. CRITICAL CURRENT MEASUREMENTS: A COMPENDIUM OF EXPERIMENTAL RESULTS *f

L. F. Goodrich and F. R. Fickett

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

ABSTRACT

The results of a program to evaluate the measurement of the critical

current of relatively small (<600 A) practical superconductors are presented.

Experimental data showing the effect of various parameters on the measurement

are given. Specific areas covered are: experimental design and sample mounting; electric field and resistivity criteria; temporal and spatial variations

in the field and current; and temperature and strain effects. The goal of the

presentation is to describe the critical current measurement process and its

pitfalls in sufficient detail to serve as a guide for those relatively new to

the field of practical superconductors.

*Work supported in part by the Department of Energy, Division of High Energy

Physics, Office of Fusion Energy, and Magnetohydrodynamics Division through

Massachusetts Institute of Technology.

(•References, Figures, and Tables in this section are numbered independently

from the remainder of the report.

39
I . INTRODUCTION

Accurate measurement of the critical current, I , of practical superconductors is a subject of concern both technically and in the commerce associated with these materials. The measurement is not a simple one and it is not

unusual to find that two critical current determinations for the same material

disagree by large amounts, often on the order of 20-30%. In an attempt to

explain the source of some of these discrepancies and to promote more uniform

measurements, a cooperative standards program involving DoE, NBS, and ASTM was

begun. Research was undertaken at NBS and by the U.S. wire manufacturers that

would ultimately lead to an ASTM Standard for the measurement of critical

currents below 600 A. The results of the program to date have been pub1 2 lished ’ and a draft standard is now being evaluated by the ASTM membership.

In the course of the program many measurements were made at NBS to evaluate

the effect of various experimental parameters on the critical current determination. In this paper we present the results of these measurements and

discuss their effect on the measured critical current.

Our objective here is to present adequate information so that a technically knowledgeable person can make an accurate measurement of the critical

current of a commercial superconductor. This task involves the construction

of a proper apparatus and the use of a correct measurement technique. The

effect of the most important parameters (e.g. field orientation) on the

results are treated in detail in separate sections; usually with data showing

the result of variations in the parameter on the measurement.

We deal here only with practical superconductors, usually multifilamentary niobium- titanium (NbTi) or niobium-tin (Nb^Sn) wire with a copper

matrix, although Nb^Sn tape conductor is mentioned occasionally. The general

construction is shown in Fig. 1 and a cross section of a complex conductor in

Fig. 2. Most practical conductors are not round (and sometimes not monolithic)

and this leads to a whole new set of field orientation effects. Most of what

we have to say will, of course, apply to any superconductor, but some of the

topics (e.g. matching of thermal contraction) will require reevaluation for

conductors of significantly different composition from those investigated

here. Also it is important to realize that commercial superconductors are subject to significant variability in their properties due to variations both

2 in basic materials and in processing from one wire lot to the next. Thus,

frequent measurements of critical current are required in order to maintain

40
uou

CJ3eo

CJ

J-l

Cl) a.3

CO3uC

CD

•H3ST3

a)

4-J

c/i •H

CD

CD +-> i—1 'H

CL 01 e o

•H CL

01

gO

nj o

CH -H O H

DCl 3 2 0

•H 4-J

4-> O

CJ3 r—I

5-4 (3

4->

a

01 -H 3 CL o

bO

•H

41
Tantalum Barrier

^Cu-Sn Bronze Unreacted l\lb Core

Reacted

iNbfSn

3553 (187x19) Filaments

5^m Filament Diameter

Fig. 2. A complex composite superconductor based on Nb^Sn.

42
quality control and the measurement technique should reflect this by being as easy as possible to use. The basic data from which the critical current of a practical superconductor is determined is a plot of voltage, V, as a function of current, I, the

V-I characteristic. It is assumed that other parameters such as applied

field, temperature, strain, and displacement are constant during this measurement. A typical real V-I characteristic is shown in Fig. 3. The curve should

be reversible (taking into account any dl/dt voltage); irreversibility is an

indication of potential problems such as sample heating. The most common

types of sample holders and magnets are shown schematically in Fig. 4, the

reasons for choosing one configuration over another is discussed in Section

IV. The V-I characteristic of Fig. 3 shows one problem immediately — a unique critical current value is difficult to define here. This problem is

resolved pragmatically by the selection of a "critical current criterion,"

either a specific value of electric field, (voltage per unit length),

measured along the wire or an effective resistivity (a line of some specific

slope, E/J, in ft* cm where J is the current density). The philosophy of the

selection process and its effect on the results is discussed at length in

Section V. Further problems arise in that the low current part of the curve

in Fig. 3 is actually showing a phenomena, known as current transfer, that is

determined by the sample mounting and/or measurement configuration and is not

intrinsic to the wire. It arises as currents flow through the resistive

matrix between superconducting filaments in response to changing conditions

(see Section V) . Its presence does not necessarily degrade the accuracy of

the measurement, but it may. The high current portion of the curve does

represent an intrinsic property of the superconductor, the flux flow voltage,

that arises from the motion of flux vortices across the superconductor. There

is an empirical relationship that shows the voltage in this region to be

proportional to some power of the current; more about that later.

Another aspect of practical conductors that needs mentioning is their

stability, the amount of disturbance, either mechanical or thermal, that the

conductor will tolerate before thermal runaway (or quench) occurs. This

property is controlled by the addition of relatively pure metal, most often

copper, to the conductor composite. More copper results in greater stability,

but a lowered overall current density for the conductor which is frequently

undesirable in magnets. Superconductor stability is a complex topic, but one

43
VOLTAGE

, %

of

Full

Scale

Fig. 3. Typical V-I characteristic for a commercial superconductor (multifilamentary Nb^Sn at 7 T) . The curves show the structure of the curve at different voltage detection levels.

44
Critical Current Measurement

Short Straight Coil

Long Straight

. 4. Common sample holders and configurations used for critical current

determination

.

45
that has been frequently discussed in the literature. The degree of stability of a wire should not greatly affect the measured critical current, but the

actual experiment is more difficult to perform on marginally stabilized wires.

The rest of the paper is divided into five major sections. Section II

treats the design of our apparatus, and Section III the choice of materials

for the test program. In Section IV we discuss the effect of various parameters on the design of critical current experiments. Section V treats the

experimental measurements and their interpretation. The results are sum- marized in Section VI.

46
II. APPARATUS

There is an important distinction to be made immediately. In this

section we describe our apparatus constructed for the rather complex and

sensitive experiments needed to evaluate various aspects of critical current

measurement. In later sections we will be discussing the design of experimental systems that may well be less complex than ours. In general, we feel

that any system that takes into account the applicable constraints of the

later sections will be adequate for critical current measurements with an

inaccuracy of ±5%.

A schematic diagram of our apparatus is shown in Fig. 5. Each of the

parts is discussed in more detail below. Regardless of the specific configuration chosen, however, the system will measure the appropriate critical

current to better than ±5% and reproduce a given measurement to better than

0.5%.

Sample Holders

Sample holders of each of the types shown in Fig. 4 were constructed,

since one of our goals was to determine the limits of useful application for

each type. In addition, numerous holders representing modifications of these

were made for specific tests. Some examples are: coils with variable turnsper-centimeter to evaluate field angle effects and a rotating table system for

the same purpose; flat bottom hairpin holder to evaluate the effect of small

radius of curvature; various configurations of the short straight current

leads, used in studying current injection effects; and holders that allow

placement of voltage taps on opposite sides of the sample while still maintaining support against the Lorentz force. In each holder, except those designed

to measure orientation effects, the applied magnetic field is essentially

perpendicular to the sample axis, at least between the voltage taps. This is

the orientation in which the minimum critical current will be measured and,

thus, represents the limiting case for most applications.

Nearly all of the holders are made from fiberglass-epoxy matched to the

sample for relative thermal contraction as discussed below. Several of them

are shown in Fig. 6. High current NbTi bus bar (>700 A at 8 T) was used for

the current contacts. These were epoxied into grooves in the sample holder

with a high temperature version of a commercial filled epoxy. This technique

allows repeated soldering of samples without loosening of the bus bar.

47
Fig. 5. Electrical systems for critical current measurement.

48
49

Fig.

6.

Actual

sample

holders

of

the

types

shown

in

the

drawing

of

Fig.
Sample Voltage Detection and Current Supply

As shown in Fig. 5, the sample voltage is read by an analog nanovoltmeter

and, after filtering, is plotted on the Y axis of an X-Y recorder (some early

comparisons were made between this system and a digital multimeter with an analog output). The input cord of the nanovoltmeter has copper clips that are manually clipped to the appropriate set of terminals on the wire bundle from

the sample holder, which may contain as many as sixteen wires. This is the

only junction between the sample and the nanovoltmeter input. Isothermal

conditions were maintained by putting small bags of lead shot under and on top

of the junction.

The sample power supply for most measurements was a series-regulated

600 A battery driven by a commercial ramp generator. For comparison studies

and the evaluation of ripple and noise, a 600 A silicon-controlled rectifier

(SCR) regulated supply was used. In each case standard current shunts (±0.25%) were used to measure the current and/or drive the Y axis of the recorder.

Current leads into the dewar were commercial 600 A vapor-cooled leads.

The most sensitive of the possible configurations, the nanovoltmeter and

battery power supply, allows the measurement of voltages as low as ten nanovolts. This is a much greater sensitivity than required for conventional

critical current measurements, with the possible exception of the evaluation

of wire used in magnets for nuclear magnetic resonance (NMR) measurements.

The desire that the V-I curve be reversible for all measurements on all

samples requires a quench-protect circuit. The circuit diagram of our quench

protect (adopted from a Los Alamos National Laboratory design) is shown in

Fig. 7. The voltage across the entire sample at the current bus bars is

detected and used to shut down the current ramp. The level at which this

shutdown occurs is adjustable by means of a front-panel dial. This circuit is

not absolutely necessary. Especially in the case where many measurements are

to be made on similar wires an experienced operator can easily prevent thermal

runaway after a few samples have been quenched.

Magnet Systems

Both types of magnets shown in Fig. 4 were used in these experiments.

The simple solenoid was a 10 tesla magnet with a 3.8 cm bore. The split-pair

magnet used for the long straight configuration measurement had a maximum

field of 7 tesla and a radial access port of 1.5 cm (5 cm bore). Both magnets

50
20K-2M

20K-2M

H

=)CL

51

Fig.

7.

Circuit

diagram

for

a

quench

protection

system.

The

resistors

are

marked

with

units

of

ohms

and

the

capacitors

with

units

of

picofarads

unless

otherwise

noted.
have been calibrated to a field accuracy of ±0.2% and a reproducibility of

±0.1%. At very low fields trapped flux could degrade these numbers significantly, but none of the measurements reported here involve that field region.

52
III. MATERIALS

As mentioned earlier, one of the goals of the research described here was

to develop supporting data for a "standard test method" for general laboratory

use in the measurement of critical current. In this endeavor we limited our

consideration to commercial superconductors with high field critical currents

less than about 600 A, since this represents a practical limit for many laboratories. The materials used for our tests are described in Table 1. Sample

numbers in the text and figures refer to this table.

Table 1. Critical Current Test Materials

Sample

Number Type

Dimensions

(mm)

Number

of filaments

Twist

(cm

1

)

Copper to

non-coper ratio

Approximate

Critical

Current

at 8 T,

4 K. (A)

1 Rectangular NbTi 0.53 x 0.68 180 0.79 1.8 114

2 Round NbTi 0.64 180 0.79 1.8 128

3 Round Nb^Sn 0.70 2869 0 1.7 132

4 Tape Nb^Sn 0.2 x 2.3 — — 1.6 172

The first wire listed in the table is one that has been used for numerous

experiments at NBS over the years. The second wire is similar to the first

except it is round. This wire was added to the group when the effect of the

aspect ratio of rectangular wires discussed below was observed. The multifilamentary Nb^Sn is a conventional wire that we react after forming; note that

the filaments are untwisted. The lack of a twist pitch helps considerably in

understanding the intricacies of current transfer along and across the wire as discussed below. The tape conductor is most helpful as a "worst case" sample

in the evaluation of field orientation and current ripple effects.

Many lengths of each of these conductors were used throughout the tests

reported here. Thus, as already mentioned, some sample-to-sample variation in

properties is to be expected. Our experience indicates that, with careful

sample selection, this should not result in more than about a 2% variation in

critical current among samples of a given material as used here. This source

of uncertainty could be minimized by use of a carefully characterized standard

reference material and this possibility is now being explored ai 53
IV. EXPERIMENT DESIGN

This section and the following one present the bulk of our data on the

various factors that influence the correct determination of critical current.

In each instance we discuss the implications of the data for the design and

operation of an experimental apparatus. The general details of cryogenic

apparatus construction are not treated here, but excellent texts on the

4 subject are available.

In this section we present data related to the design and construction of

the experimental apparatus. It is probably possible to construct a "perfect"

apparatus for the determination of I , but the restrictions of the real world

make this attainment unlikely. Most of us must take into account the limitations of space, existing equipment, time, and money. With this in mind, we have tried to present a design philosophy that allows choices to accommodate

such restrictions while still maintaining the overall measurement accuracy.

At the least, we hope that the data presented will indicate the consequences

of various design choices.

Sample Holders

The major sample holder types are shown in Fig. 4. Table 2 gives a comparison of the various types. The terms used have already been introduced,

but they will also be discussed in detail below. It should be stressed that,

Table 2 . Sample Holder Comparisons

Sample geometry Advantages Disadvantages

Short straight simple geometry

solenoidal magnet

high contact heating

high current transfer

insensitive criteria

Long straight low contact heating

moderate criteria

simple geometry

some current transfer

split-pair magnet

Hairp in low contact heating

moderate criteria

solenoidal magnet

some current transfer

moderate geometry

Coil low contact heating

low current transfer

sensitive criteria

solenoidal magnet

complicated geometry

54
in some circumstances, the effects listed as disadvantages may, in fact,

preclude the use of that holder for measurements on a specific material. In

all cases the sample should be fit closely to the holder to avoid buckling in

the field, this is especially important for the coil geometry. The use of

small amounts of varnish or grease to hold the sample in place is usually

acceptable, but see the comments below on heating.

Typical V-I data taken with the various holders are shown in Fig. 8. The

plot is made on a full-log grid to illustrate that the curve usually has two

regions that must be considered. The high current behavior is representative

of flux flow and is intrinsic to the superconductor. The data in the lower

current region illustrate the varying role of current transfer in each design.

This current transfer voltage must be subtracted in some manner in order to

arrive at the intrinsic V-I curve from which I is determined. It is obvious

c from the figure that this problem becomes more acute as the electric field

criterion decreases. Table 3 and Fig. 9 illustrate how the critical current

determination at various fields and criteria is affected by sample holder

type. A current transfer correction has been made to these data. Notice (see

Fig. 8 and Table 3b) that, when the current transfer voltage is the same size or larger than the criterion used, the possibility exists of over- or underestimating the current transfer part which may lead to large errors. Clearly the choice of a sample holder requires some serious thought.

Similarly, many of the problems vanish when a proper choice is made. It

should be noted that most data presented here are for Nb^Sn — the situation

is generally much less severe for NbTi, as shown in Table 3, because the

current transfer voltage is generally lower. However, at very low criteria

(M.0 nV/cm), the possibility of error from this source must be considered as can be seen in the table.

Voltage and Current

The two problems to be resolved with regard to voltage and current leads

are: how to get current into the sample and properly distributed among the

superconducting filaments in as short a distance as possible; and, how to

arrange the voltage taps so as to make the most meaningful measurement. Both

of these problems are far more complex with a multifilamentary superconductor

composite than they would be with, say, a simple copper wire. Again, the

complexity is greatest for the Nb^Sn conductors.

55
E

f

/4V/cm

100

Fig. 8. Full log plot of representative I-V characteristics (multifila

mentary Nb

3 Sn at 7 T) for different sample holders.

56
data that defines zero is that from the coil sample, I cQ are shown for two values of the electric field criter 1 or'

10

3) with

The base

Results

57
The large sample currents used in these tests are usually brought into

the cryostat by means of commercial helium-vapor-cooled leads. Current is

monitored by a conventional, low-resistance, series shunt at room temperature.

In the cryostat, the leads connect to a superconducting current bus of much

higher capacity than the samples (1^ at least double). Monolithic rectangular

NbTi conductor is handy for this purpose unless flexible leads are needed, in

which case a bundle of smaller wires is appropriate. In most geometries, an added measure of protection is achieved because the current bus is parallel to

the magnetic field in the high field region.

Transfer of the current from the bus into the sample is a complex process

involving: the length and resistance of the joint the size, distribution,

6 and twist pitch of the sample filaments; and the details of current transfer

between the filaments of the sample. ^ Large voltages can arise due to these

mechanisms that may overwhelm the intrinsic V-I data. In the papers cited, a large amount of data is presented on each of these topics. It can be summarized by saying that a perfectly adequate situation will be achieved if the

current contact length is at least ten times the maximum transverse dimension

of the sample. For conductors with twisted filaments this contact length

should also be greater than one twist pitch. These requirements are easily

met for all sample configurations except the short straight. There it is

usually possible to bend the sample into a "flat-bottom hairpin" shape (or, in

the case of multifilamentary Nb Sn, to react to this shape) such that a

^ 6 reasonable length lies along the bus bars. This approach helps to an extent.

It is important that none of the free wire between the bus bars be bent, i.e.,

the entire bend region should be soldered to the bar (except for a multifilamentary Nb^Sn sample that has been reacted to this shape or if the bend of a NbTi sample is not too severe).

Even with reasonable current contact lengths, if the voltage taps are too

close to the contact, large voltages related to the current transfer may be

seen. The situation is significantly worse when marginal current contact

lengths are used. The data shown in Fig. 10, taken on a coil sample of our Nb^Sn wire, illustrate all of the above points. These data were obtained by

making a set of measurements of the voltage between adjacent taps on the

sample and then cutting the sample in the current contact region and repeating

the measurements. The value used for the distance, x, of the pair of voltage

taps from the current contact was obtained by an iterative process that takes

58
E

,

/iV/cm

IO

'10

10-"

<£*& Q,

IO” 12

IO

" 13

O.l 0.3 I 3 10 30

X, cm

Fig. 10. Full log plot of the electric field (or effective resistivity) due

to current transfer as measured at various distances, x, from the

current contact. Each curve shows data for a different length

current contact, l , between the sample and the bus bar. The wire

is multifilamentary Nb^Sn measured at 4 T and 160 A (note the

intrinsic I at 1 nV/cm is ^186 A)

.

c

59

Q.

•cm
into account the electric field as a function of position. The points on the

plot at the largest x were taken on voltage taps positioned around the center

of the coil sample. In general the data indicate that length of contact on

the bus bar is not as effective in minimizing these voltages as is length of

free sample beyond the contact before the first voltage tap.

The behavior illustrated by the data in Fig. 10 represents a complex

aspect of practical superconductors that has been the subject of much study.

A rule of thumb which will keep the current transfer voltages small in any

arbitrary conductor is to make the free length between the current contact and

the first voltage tap greater than the conductor's current transfer length,

2

L = (0.1/n) 1/2

(p /p *) 1/2

d (1) m

Here p is the resistivity of the interfilament material, d the diameter of m

the filament region, n characterizes the sharpness of the superconductor's

resistive transition (see below) , and p* is the resistivity equivalent to the

criterion used to determine the critical current. For critical current

measurements at a p* of 10

^

ft*cm, a free length 30 times the transverse

dimension d will typically be more than adequate for Nb^Sn:bronze conductors.

The required length becomes greater as the critical-current criterion becomes

more sensitive, however. Once again, the problems are minimal with NbTi:copper

where the required length is typically 15 to 20 times less than for Nb^Snrbronze

conductor. These general requirements on the free sample length are conservative. Free lengths shorter than the expression for L may well do the job,

especially if the current contact is long, as shown in Fig. 10. Measurements

made at low voltage levels may give strange results (including negative

voltages) in the case of very short current injection contacts.

Another aspect of the current transfer problem is that the redistribution

of current among the filaments can occur near a change in magnitude or angle

of the applied magnetic field as well as adjacent to a joint. This was observed in obtaining the data shown in Fig. 8. The long geometry is an example of the sample in a field gradient. The total length of sample //3 (Nb^Sn) measured in this geometry was 22 cm (550 d) , which put the ends of the

sample in a very low field region (5 cm bore) . A number of voltage taps were placed along the sample and a current transfer voltage was observed adjacent

to the joint and in the bore region, however, there was a region (a few

60
centimeters long) in between these two where the current transfer electric

field was very small (less than a few nV/cm) even up to the I of the bore

region. A profile of the current transfer electric field in the field gradient

region indicated gradual increase of E with increasing field through this

region, then a slight decrease in the bore region. This same kind of separation of these two causes was also observed in the hairpin sample where instead

of a field gradient region there is a change in angle between the sample and

the magnetic field. Both of these observations indicate that no matter how

long the low or parallel field region is, there can be a current transfer

voltage in a temporal or spatial field variation and in fact the high perpendicular field region is necessary to complete the distribution of the current

among the filaments. This is another advantage that the flat-bottom hairpin

has over the curved bottom hairpin. In all cases the removal of the current

transfer voltage adjacent to the joints from the measurement region is advantageous. The rule of thumb result given above is still valid; however, its

application to these aspects of current transfer is more complex because of

their distributed nature.

Protection of the sample against burnout during a quench has already been

discussed and a quench-protect circuit was shown in Fig. 7. When many samples

with highly resistive matrix materials (CuNi is sometimes used) must be

measured, a reasonable alternative or adjunct to such a circuit is the use of

a resistive shunt in parallel with the sample, usually just soldered across

the bus bars. The shunt, usually of copper with a residual resistivity ratio

(RRR) ^200, conducts very little of the current when the sample is superconducting and most of it when the sample is normal. In determining the actual

current sharing between the sample and the shunt, the contact resistance of

the various joints involved must be considered. It is not uncommon to solder

the sample directly to the shunt. This practice is acceptable in most cases,

but it should be realized that it can make an intrinsically unstable superconductor (i.e., filaments too large, filaments not twisted enough, or not enough

stabilizer) appear to be stable — a misleading result.

Voltage taps are usually soldered to the sample. Attachment to the bus

bars is sometimes used for relatively crude measurements. Care should be

taken to avoid creating stress concentration points. The measured voltage is

most often used to calculate an electric field or resistivity which is then

compared against a chosen criterion to define 1^. Proper use of these

61
criteria requires attention to the concept of effective length . The problem

is illustrated in Fig. 11. It arises when a part of the sample between the

voltage taps is not in perpendicular field, a situation that is common for the

hairpin geometry. As the sample approaches its critical current, the

appropriate voltage develops on a-a. Dividing this voltage by the tap

spacing, SL , gives the (nearly) correct electric field. However, if the

cicl voltage measurement is made on tap sets bb or cc a value nearly the same as that on aa will be seen because the sample lengths in parallel field have not

yet developed any voltage — they are not yet near to their critical current.

Thus an electric field calculation using data from the cc taps and the

measured distance between them along the wire would give a misleading

result — a very low electric field. Thus, it is necessary to find an effective length that can be used in calculations. This can only be done by

using a "standard" wire that has been measured by some other technique.

The problem persists in a less drastic form all the way around the curve

of a round-bottom hairpin sample because of the changing orientation of the

wire with respect to the applied field. For example, a measurement made with

probes located at opposite ends of a diameter on a round-bottom sample holder

had a true effective length of 35% of the actual distance along the wire.

Note that this length is even smaller than the perpendicular projected length.

The basic conclusion is that the round-bottom hairpin geometry is not one to use if precise data are needed, but can be adequate for comparisons.

Magnetic Field

Magnetic field is an expensive commodity and it is, thus, the parameter

that usually determines the design details of the critical current measurement

apparatus with regard to size and shape. Most high field laboratory magnets

are simple solenoids with bore diameters of <10 cm for 8 tesla NbTi magnets

and <5 cm for 10 tesla Nb^Sn/NbTi compound magnets. The region of 1% field

homogeneity is usually on the order of a few centimeters.

The major design problem related to field configuration, once the size of

things is fixed, is the angle between the conductor and the field. There are

two aspects to this problem: 1) the effect of a lack of perpendicularity

between the sample axis and the field; and 2) for rectangular conductors, the

effect of the angle of an accurately perpendicular field with respect to, say,

the wide face of the sample.

62
Q Q

Fig. 11. Hairpin sample geometry to illustrate the concept of effective

length (£ = length between taps a and a)

.

63
The first of these concerns was investigated in two ways. First, a series of coil samples were prepared with winding pitches varying from 1 to

40°. Total sample lengths varied from 10 to 90 cm. The results of the critical current measurements with these samples are shown by the open circles in

Fig. 12. A more complete series of measurements were made on all samples

using a rotating table device that allows a single short (3.6 cm) sample to be

rotated through a total angle of nearly 120° and the critical current measured

at as many points as desired. These results are also shown in the figure.

Two features are obvious: the sample can be misoriented by quite a bit

without seriously degrading the measurement accuracy (at least 8° before a 2%

increase in I occurs); and, beyond a certain point the critical current

increases very rapidly with field angle. The data for all samples presented

here was taken in a field of 8 T, but data taken in 4 and 6 T had a very

similar angular dep endence when normalized to (I - I )/I (where I is the

c co co co critical current with the field perpendicular to the sample axis) which

suggests this magnetic field scaling for each sample. This magnetic field

scaling was also observed for sample 2 (NbTi) at 9 and 9.75 T (note that 1^ =

349 A at 4 T and I = 49 A at 9 . 75 T) . Also, there was very little change in

these curves over a wide range of critical current criteria for each of these

samples. In addition, different types of NbTi conductor give similar behavior.

The behavior of the multifilamentary Nb^Sn is somewhat different, but the

effect is similar to that for the Nb^Sn tape. A twisted multifilamentary

Nb^Sn sample (prepared by twisting a length of sample 3 prior to its reaction) was also measured and its angular dependence of I was very similar to the

untwisted sample. For each of the rectangular conductors, the field is

parallel to the wide face during this measurement. If one attempts to explain

the observed angular dependence, the first guess at the shape would be

I c /l

c o

= 1/cosO considering the perpendicular projection of the applied field.

The NbTi I *s increased faster with angle than this and the Nb_Sn I 's

c 3 c increased more slowly. Figure 13 is a full-log plot of the data showing the

large angle (conductor and field closer to parallel) data as well. The leads

to the rotating table device limited the measurement to currents less than

about 400 A, but higher field data on the twisted multifilamentary Nb^Sn

sample and sample 2 indicated that the curves on Fig. 13 can be extrapolated

to 90°.

64
0 15 30 45

ANGLE , degrees

Fig. 12. The effect of angle between the conductor axis and the applied field

on the critical current determination. The angle is zero when the

conductor axis is perpendicular to the applied field. I is the

critical current at the zero angle. Data shown were taken with

variable pitch coils (marked coil) and with a short-sample rotating

table device. The numbered samples are described in

65
(IcLJ i l l i l I L_ L I

i illil

3 10 30 100

Angle, degree

Fi e . 13 . Full log plot of data shown on Fig. 12, Including some larger angle

data. I is the critical current at the zero angle.

66
The effect of field orientation with respect to the faces of the rectangular conductors was investigated using a long straight sample in the radialaccess magnet. The field is always normal to the conductor axis in these

tests. The results are shown in Fig. 14 where I is the critical current

co with the field parallel to the wide face of the conductor. Measurements at 3

and 7 T (7 T data shown in Fig. 14) indicated that (I - I )/I was a good

c co co magnetic field scaling of this effect. There are several interesting features

here. The most important from the standpoint of experimental design is that

moderate care in positioning the sample will result in good data when the

measurement configuration has the field parallel to the wide face. This

configuration is the one most commonly encountered in the high field region of

simple magnet windings. On the other hand, the configuration that has the

field normal to the wide face has a stronger angular dependence for the

critical current and, thus, measurements in this position will require significantly more care in sample orientation. The figure shows another interesting effect in that the angular dependence of the critical current of the tape

conductor is opposite to that of the rectangular multifilamentary conductor.

This is probably due to a difference in the pinning mechanism of these two

types of superconductors. This topic of aspect ratio and its effect on I is

c a very interesting one and is currently under study at NBS.

Strain

Proper assessment of the sources and effects of strain on the sample is one of the most critical aspects of experimental design. The problem is

complicated because sample strain may arise from many sources including:

differential thermal contraction of the apparatus, both static and dynamic

(due to different cooling rates) ; sample mounting; and Lorentz force. In

addition, the complex nature of the composite conductor makes detailed evaluation of the strain in the twisted filaments quite difficult. A recent

g comprehensive review of this topic has been published by Ekin and should be

consulted for details on the effects of strain within the superconducting

composite.

An important feature of strain effects is the extreme difference between

NbTi and Nb^Sn. Niobium- titanium conductors are generally very strain tolerant. Their critical current is not degraded significantly (1%) until the

overall tensile strain reaches ^0.5%. Further straining to 2% gives only a

67
12

10

8

6

4

2

0

-2

-4

—• s

“I

. 14.

» s g i —r

i"—T”T

ANGLE , degrees

Dependence of I on rotational angle of field around the conductor

axis for rectangular conductors at 7 T. The zero angle is defined

where the field is parallel to the wide face of the conductor and

I is the critical current at the zero angle,

co

68
g 10% degradation of 1^. Niobium-tin, on the other hand, is very sensitive to

strain as illustrated by Fig. 15 from the review just mentioned. The intrinsic strain is that existing in the Nb^Sn itself and may have components from

the conductor matrix material. Thus, a particular sample may lie on either

side of the peak when under no external stress. In any event, it is clear

that strains as small as 0.2% may result in significant changes in I c

* One

practical result is that samples of Nb^Sn conductor must usually be formed to

the shape required for the test before they are given the final reaction heat

treatment to form the intermetallic compound (to avoid introducing bending

strain)

.

Another aspect of the strain dependence of critical current that is

important for measurements and applications near the upper critical field is

that the strain dependence becomes a strong function of magnetic field. This occurs for both NbTi and Nb^Sn, but is probably only significant at reasonable

strain values for the latter. Data illustrating this effect is shown in

9 Fig. 16 from recent measurements by Ekin. Recent theoretical and experi9 10 mental work * has led to a mathematical description of this "strain-scaling"

behavior and these papers should be consulted if measurements at high fields

(above about 10 T) are contemplated.

Strains induced by differential thermal contraction of the sample holder

with respect to the sample may be large because the holder is most often a nonmetallic and these materials contract much more on cooling than do the

metals. The two most common solutions are to make a metallic sample holder or

to use a nonmetallic composite, usually epoxy-fiberglass. The former approach

is not recommended because of the problems associated with electrical insulation. The fiberglass-epoxy materials, especially NEMA G-10, are useful if

proper attention is paid to the large thermal expansion anisotropy. Typical

data taken in our laboratory"^ are shown in Fig. 17 for both G10 and Gil

fiberglass-epoxy as well as the much less expensive (and less well characterized) cotton phenolic. Data are also given in the reference for both NbTi and

Nb^Sn commercial multifilamentary conductors. They fall within the range of

the "warp direction" curves. Thus, for example, the best way to make the

bottom segment of a round-bottom hairpin is by using a piece of rolled-cloth

fiberglass-epoxy tube with its axis normal to the centerline of the apparatus.

Attachment of bus bars and such to these nonmetallic holders is easily

accomplished with filled epoxies that also match the thermal expansion

69
CM

O

cm mmma ow.

«*— CO

o

'cn

c

70

Fig.

15.

Effect

of

intrinsic

strain

on

the

critical

current

of

commercial

multifilamentary

Nb„Sn.

From

reference

8.

I

is

the

maximum
Critical

Current

(Amperes)

Fig. 16. Observed change in the effect of strain on the critical current of

Nb_Sn conductor at fields near the upper critical field. From

reference 9.

71
thermal

expansion,

percent

Fig- 17. Thermal contraction of nonmetallics used in sample holder construction. From reference 11.

72
reasonably well. Both the fiberglass-epoxy and the filled epoxies can withstand the heating required for making soft solder joints. An insulating

electrical varnish soluble in alcohol-toluene is used for holding down voltage

leads and other low stress, nonpermanent applications.

The Lorentz force is a serious source of strain because of the high

currents and fields involved in critical current testing. The force on a conductor carrying 600 A in a 10 T field is 60 kg per centimeter of length.

Also, because of the form of the dependence of I on H, this force reaches its

maximum at relatively low fields for practical conductors. The problem is

usually solved by laying the test sample in a fairly tight-fitting groove in

the holder and orienting the sample with respect to the field so that the

force is directed into the holder. There are experiments, such as rotation

tests in the long straight geometry, where this solution cannot be used. In

that case either a special holder is needed or one must use varnish or grease

to hold the sample in place and the contraction precautions observed. This

solution also has the potential of restricting the transfer of heat from the

sample to the bath resulting in the self-heating effects described below.

Temperature

There are numerous ways by which heat can be introduced into the test

specimen. Some of them have already been mentioned, such as contact heating,

heating due to current transfer between filaments, and the intrinsic heating

that occurs near 1^. In addition, rapidly varying the sample current or the

magnetic field can introduce eddy current heating. The specific heat of all

common materials is extremely low at liquid helium temperature and, thus, a small energy input can result in a large temperature excursion. The use of

varnish and/or grease on the test sample for mechanical stability may result

in lowered thermal stability as mentioned above. Reasonable care in making

joints is generally all that is required for preventing thermal runaway.

There is a more subtle thermal effect that is more common. In this case,

the V-I data are still "normal" in appearance and behavior, but the value of

I is reduced by 1-4% depending on the criterion used (greatest reduction for

high criteria) and on the magnetic field (greatest reduction for lower fields)

Detection of this situation requires a careful examination of the V-I curve in

the flux flow region. As we will see shortly, the voltage there is approximately proportional to In

. Values of n cover a wide range in general, but for

73
a given material n is reasonably constant. When the heating effect occurs,

the value of n changes dramatically, becuase 1^ is a function of temperature,

which in turn is a function of where the sample is on the V-I curve. A

typical example from our Nb^Sn data: n = 23 for a normal run, but for a run with a relatively light coat of grease on the wire n >_ 30 at the higher

electric fields (within a factor of 5 of quench). A practical technique for

avoiding this problem is to determine the sample voltage at quench and use a criterion that is well below that (factor of 5 or 10).

74
V. EXPERIMENT EXECUTION AND DATA ANALYSIS

The successful execution of a critical current measurement requires

consideration of a number of points that do not exist in conventional resistivity work. Not the least of these is the definition of critical current.

This is not a philosophical question, but a rather practical one involving the

choice of an appropriate criterion of electric field or resistivity. Similarly, the voltages caused by currents travelling through the nonsuperconducting matrix must be treated with care in the data analysis. These problems

and numerous other, more mundane ones are the subject of this section.

V-I Curve Measurement

The sample voltmeter shown in Fig. 5 may be either digital or analog. We

have used both. The analog meter is a commercial nanovoltmeter . If a battery

power supply is used for sample current, the noise level is down to ^2 nV and

this combination provides the greatest sensitivity. However, when an SCR

regulated power supply is used, the SCR switching noise reduces the sensitivity of the nanovoltmeter to ^100 nV. In this case it offers no advantage over

the digital voltmeter. Our measurements of I using the two meters (with the

SCR current supply) agreed to within 0.5%.

The choice of a criterion to define the point on the V-I curve where the

critical current is reached is most often determined by the potential application of the conductor. Large, high field conductors may use relatively high

criteria, while conductors for NMR magnets may require very low values.

The criterion may be stated in terms of either electric field or resistivity. The electric field criterion indicates the voltage drop per unit

length of the superconductor in the flux-flow state at the critical current.

Typical values range from 0.1 to 10 yV/cm for conductors with critical cur- rents of less than 600 amperes. The resistivity criterion refers to the

effective resistivity of the superconductor in the flux-flow state, i.e., the

voltage drop per unit length divided by the current per unit area. Typical

values for the resistivity criterion range from 10 to 10

^ ft* cm using the

total cross-sectional area of the conductor. One problem with the resistivity

criterion is deciding what cross-sectional area to use to calculate the

resistivity. The case of a NbTi conductor is fairly simple, either the total area of the conductor, or that of the NbTi alone is used. For a Nb^Sn conductor the area used may be: the total area of the conductor; the non-copper

75
area (which may include the area of the diffusion barrier, bronze, Nb, and

Nb^Sn) ; the area of the Nb and Nb^Sn; or the area of the Nb^Sn alone. The

determination of some of these areas involves extensive metallography and

statistical techniques, which means it is difficult and very time consuming.

For our multifilamentary Nb^Sn superconductor, the total cross-sectional area

is about 14 times that of just the Nb and Nb„Sn. The critical current values

J -11

determined at 7 tesla using a resistivity criterion of 10 ft* cm and these

two areas differ by about 19%.

We choose to use the electric field criterion. It does not penalize

highly stabilized conductors and it can easily be converted into an equivalent

resistivity criterion using the total cross-sectional area of the conductor.

One should note that a given electric field criterion does not correspond to

the same resistivity criterion at all values of magnetic field. At high

fields a given corresponds to a larger value of resistivity than at low

fields. This could be a problem very near to the upper critical field, H

£

(because every conductor will show a finite critical current for any value of

E ) , but most practical applications are restricted to upper fields of 0.8

or less.

The effect of choice of criterion on the value measured for the critical

current is shown in Fig. 18 for NbTi and in Fig. 19 for Nb^Sn. These plots

illustrate how I scales with criteria and magnetic field where I is the

critical current at 0.1 yV/cm for NbTi and 1 yV/cm for Nb^Sn. The irregular

shape of the curves at the lower criteria reflect the uncertainty of those

values. These data were taken using a coil sample in order to minimize the

current transfer voltages that must be subtracted out at the lower criteria.

Clearly, there is a systematic variation with criterion as one would expect

from the shape of the V-I curve. For both conductors there is only a slight

dependence on field for values likely to be encountered in a reasonable test

(use of NbTi at fields >8 T is not common at 4 K, nor is the use of criteria

<10 nV/ cm)

.

The behavior shown in the figures is another manifestation of the power

law dependence

E a

r n

( 2 )

76
Fig. 18. Dependence on the critical current of NbTi conductor (# 1) on magnetic field for various electric field criteria (1, 2, 5 sequence).

The vertical axis is normalized to I at 0.1 yV/cm, I

c co

77
The vertical axis is normalized to I

c at 1=0 yV/cm, I cq

.

78
in the flux flow region that was mentioned earlier. The sharper the transition (large n) , the smaller the effect of choice of criterion on the critical

current value obtained. Evaluation of n is not a critical part of a normal

experiment, but knowledge of its expected behavior is helpful in analyzing

problems, such as sample heating. In addition, a well-characterized n of a particular sample could be used to extrapolate to a lower criterion than can

be achieved in another measurement. Selected data from our samples are given

in Table 4. Very similar values of n will be obtained if one uses critical

current data based on a resistivity criteria. The NbTi data (sample #1) are plotted in Fig. 20. The uncertainty in the value of I at the lower criteria

cause the irregular shape of n calculated using those values. The conclusion

to be drawn here is that each material has a value of n associated with it

that depends somewhat on the criterion chosen and on the magnetic field. The

value decreases, usually slightly, with increasing field. Values of n generally lie between 20 and 100. Variations of 10 to 20% in the value of n do not

reflect significant change in the shape of the V-I curve. Significantly

higher or lower ones nearly always indicate a problem in the measurement.

Current

The usual measurement technique involves establishing a constant magnetic

field and ramping the sample current to trace out the V-I curve. Generally

the ramp rate chosen is relatively slow (a few minutes to I ) , but even under

these conditions voltages may be induced in the leads to the sample voltage

taps during sensitive measurements. The technique for evaluating the magnitude

of this effect is to stop part way up the V-I curve and start the current

decreasing at the same rate for a few amperes. The (usually small) loop

traced out measures the dl/dt signal. The effect can be minimized by care- fully twisting all voltage lead pairs and providing minimum area at the sample

itself. This latter consideration is especially important in the inductive

coil geometry, where the voltage leads should be co-wound with the sample.

Ripple or SCR spikes on the current supply can make measurements at very

low criteria impossible, as we discussed in the last section. At more reasonable criteria, however, these disturbances have no significant effect on

the results, as shown in Fig. 21, which compares data taken with a zero-ripple

battery supply to those with a commercial 600 A SCR supply. The agreement

between these two was within the precision of the measurement (±0.2%) except

79
H-o H »T

Fig. 20. Behavior of the exponent, n , as a function of magnetic field for

various ranges of the electric field criterion. The sample is NbTi

(// 1) in a coil configuration.

80
H'O H »T

Fig. 21. Relative critical current values determined using a battery power

supply (I ) and an SCR regulated one (I ). Samples are identified

in Table S.

81
for the Nb^Sn tape sample. The Nb^Sn tape was expected to be the worst case

in this comparison because of the higher ac loss of this type of sample and it

may be showing this effect, but it was only 0.5% at 7 T.

Magnetic Field

There are sources of extraneous signals related to time variation of the

magnetic field such as ripple and drift of the magnet supply. These are rarely a serious problem in a conventional measurement. The prescription for

minimizing them is, as above, to create a minimum loop area in the various

leads. Coil samples offer the greatest potential for problems here and, for

very precise work, a noninductively wound sample may be required.

Because of the shape of the curve of critical current vs. magnetic field

for practical conductors, small errors in field measurement can result in

relatively large errors in the critical current. The dependence of I

(E^ =0.1 yV/cm) on magnetic field for our test samples is illustrated in

Fig. 22. In preparation for a series of measurements the field and field

profile of the magnet should be measured. The field should be known to ±1%

and should not vary by more than that over the measurement region of the

sample, i.e., between the voltage taps.

The concept of self field is often mentioned. This field is the one that

is produced by the sample current. Figure 23 illustrates the self-field

effect on a representative row (a-e) of twisted filaments in a conductor.

Suppose that, if the filaments were not twisted, these five filaments could

carry a total of 100 A (16, 18, 20, 22, and 24 A) with a flux-flow electric

field of 1 yV/cm. If the filaments are twisted the critical current of each

twisted filament will depend on its position relative to the magnetic field

profile (greatly exaggerated to show this effect) . Then current above 88 A

will have to transfer through the normal matrix to redistribute among the

filaments and an additional electric field will result depending on the twist

length and current transfer characteristics of the conductor. The additional

current transfer voltage will cause a reduction in the critical current for a given criterion, i.e., the total electric field will reach 1 yV/cm at a current less than 100 A. This is analogous to the bending strain effect where

the spatial strain causes a spatially dependent current density. In conductors

of the size considered here the magnitude of the self-field effect is small.

Furthermore, the effect is similar in both the I measurement and in most

c

82
Fig. 22. Magnetic field dependence of the critical current for the samples of

Table 1.

83
MAGNETIC FIELD

POSITION

22A 24A

\ \ \

\ \ \

'

V5J XS

20A 22A 24A

Fig. 23. Schematic of self-field effect showing a representative row of

twisted filaments in a conductor, their critical currents and

exaggerated field profile.

84
applications. In very large conductors made up of cabled and/or fully transposed strands in various cross-sectional shapes, the effect could be important

when attempting to predict the behavior from that of a single strand.

Temperature

The effect of bath temperature on the critical current is one of the

least well appreciated aspects of this type of measurement. It is our con- tention that a meaningful report of I data for commercial use requires a statement of the sample temperature to ±0.5%. Usually the assumption is made

that the sample temperature is that of the bath and, in most cases, that seems

to b ? adequate. The temperature of the liquid helium depends on the ambient

pressure. The small additional pressure in the dewar caused by hydrostatic

pressure and the use of vapor-cooled current leads is not usually significant

(<, 10 mK) but could become so if the leads are partially blocked. Typical

atmospheric pressure variations over the course of a year (typhoons, hurricanes, and tornados excluded) translate to bath temperature variations of

about ±20 mK. To avoid problems it is recommended that the pressure in the

dewar be monitored during the measurement. The bath temperature can then be

12 determined from standard tables.

Some justification is required for the above "hard line" on this subject.

Extensive data have recently been presented on both NbTi and Nb„Sn by two

13 14

J groups. ’ The basic result of these papers is that the critical current is

observed to be related to the temperature by

I a (T

" - T) (3)

c c

* * where T^ is the bulk critical temperature at a given field. T^ is usually

determined by extrapolating I (T) (for temperatures near the temperature of

j. interest) to zero. The intercept is defined as T . At high fields the value

/V A of T can be quite low. Data on multifilamentary wires of NbTi give T

C 13 14

C

5.4 K at 8 T and 8.4 K for multifilamentary Nb^Sn at 12 T. Especially at

high fields, the variation in bath temperature from day to day can be a

JUA significant fraction of the quantity T^ - T and thus have a large effect on the I measurement,

cAnother temperature-related consideration is the minimization of thermoelectric voltages that occur because of the large temperature gradients

85
encountered by the voltage leads in a typical experiment. These voltages can

be greatly reduced by proper thermal anchoring of the leads in the region of

the sample^ and by the use of continuous copper wires from the sample to the

voltmeter input. This involves making feedthroughs for the top plate of the

cryostat. Using this technique the thermoelectric voltages can be reduced to

the order of several tens of nanovolts and, more importantly, they tend to

remain constant over the time required to make a measurement.

Current Transfer

This topic has been mentioned several times already. It can be a problem

in even the best designed experiment in that it introduces current-dependent

voltages of significant size. Frequently one has no choice but to try to remove these extraneous signals during the data analysis. The voltages arise

because current flows through the normal metal stabilizer of the superconductor in the process of attaining an equilibrium distribution. The situations that lead to this current flow may result from a number of causes. Current entering the sample from the bus bar is a major source of transfer

voltage (such as that encountered in the short geometry) as illustrated for

the low current parts of the V-I curves in Fig. 8. In addition, transit

through a region of field gradient (such as that encountered in the long

geometry) or, similarly, a change in the direction of the current path with

respect to the field (such as that encountered in the hairpin geometry) will

cause current redistribution regardless of the length of the sample in the low

or parallel field region. The situation is further complicated by the twist

of the superconducting filaments that may result in negative current transfer

voltages, i.e., the transferring current is apparently flowing against the

voltage gradient. While all this has been explained in the references previously listed, it is another matter to properly handle data that contain

these voltages.

Simply put, the data analysis problem is to subtract off the current

transfer voltage so that the chosen electric field criterion can be applied to

the intrinsic V-I curve. When the current transfer voltage is a small fraction of the total signal at I , there is clearly no problem. Also, for

relatively insensitive measurements, such as are often encountered in routine

testing, the current transfer voltage can usually be approximated by a straight

line at low current and thus easily extrapolated out to the I region. In

86
other cases where the voltage is not linear with current and is of significant

size, a reasonable approach is to plot log V versus log I similar to Fig. 8

which should result in a break in slope between the current-transfer region

and the intrinsic flux-flow region. The lower current transfer portion can then be approximated by a straight line (power law) and subtracted from the

data. This was done for the data presented here and the slope of log V versus

log I in the current transfer region was as high as 2 to 3. If a large number

of samples are involved, this technique may not be feasible and a change in

experimental design is suggested. In any case, however, when the current

transfer voltage is large compared to the criterion, it is easy to over or underestimate the current transfer contribution and get erroneous results.

This is illustrated by the variation in I shown in Table 3 for the different

c sample holders (see Fig. 8 also). If very precise data are required it is

best to go to a coil sample.

87
VI . SUMMARY

The data and discussion presented in the previous sections should allow

one to construct an apparatus and carry out a successful critical current

measurement. Clearly, several levels of sophistication can be achieved and

the choice is really one of money, time, and needs. Table 5 provides a summary of the suggested accuracies for the various parameters and their

effect on the final measurement. In some cases (e.g., uniformity and angle of

H) these figures assume that the conditions are not at the extreme limit for

the total length of the sample between the voltage taps. The resulting error from all sources listed is <5%.

Table 5. Tolerance for Critical Current Variables

Variable

Suggested

Uncertainty, %

Resultant

AI , %

c

I 1 1

I, ripple 5 1

E 12 1

c (10% £, 6% V)

H 1 2.5

H, ripple 1 0.5

H, uniformity 2 1.5

H, angle 7° 1

T 0.5 1

e , bending 0.1 Nb Sn

2 NbTi

2

e, tensile 0.05 Nb„Sn

0.5 NbTi

2

We should reemphasize that the work presented here is for relatively

small conductors with I < 600 A. The very large conductors now available

present a few additional problems that have not been addressed such as com- plicated strain configurations, internal motion, large self fields, and

88
probably complex voltage patterns in the flux flow region. In any event, the

average laboratory is not equipped to handle conductors of this size and those

few that are, should be well aware of the problems.

There are two further developments that we feel would contribute greatly

to the critical current measurement. A more thorough understanding of stability in practical composites and the availability of a "standard" multifilamentary superconductor for evaluating new apparatus and maintaining the

accuracy of frequently used systems. Both of these topics are now being

investigated at NBS

.

89
VII. ACKNOWLEDGMENTS

The help and support of the remainder of the Group at NBS, especially

A. F. Clark and J. W. Ekin are gratefully acknowledged. The ASTM committee

B01.08 on Superconductors has provided guidance and served as a sounding board

for many of the ideas presented here. The support and interest of D. Sutter

and D. Beard of DoE and P. Marston of MIT made this work possible. Ms. V. Grove

once again made production of the finished manuscript a painless operation.

90
VIII. REFERENCES

1. F. R. Fickett and A. F. Clark, "Development of Standards for Superconductors, Annual Report FY79," NBSIR 80-1629 (December 1979).

2. F. R. Fickett, L. F. Goodrich, and A. F. Clark, "Development of Standards

for Superconductors, Annual Report FY80," NBSIR 80-1642 (December 1980).

3. H. Brechna, Superconducting Magnet Systems (Springer-Verlag , New York,

1973)

.

4. R. B. Scott, Cryogenic Engineering (D. Van Nostrand, New York, 1959).

5. L. F. Goodrich and J. W. Ekin, "Lap Joint Resistance and Intrinsic

Critical Current Measurements on a NbTi Superconducting Wire," IEEE

Trans. Mag., MAG-17 , 69 (1981).

6. L. F. Goodrich, J. W. Ekin, and F. R. Fickett, "Effect of Twist Pitch on Short-Sample V-I Characteristics of Multifilamentary Superconductors,"

Advances in Cryogenic Engineering - Materials, ^8 (to be published).

7. J. W. Ekin, "Current Transfer in Multifilamentary Superconductors,

I. Theory," J. Appl. Phys . b9_, 3406 (1978).

8. J. W. Ekin, "Mechanical and Properties and Strain Effects in Superconductors," in: Superconductor Materials Science: Metallurgy, Fabrication, and Applications , edited by S. Foner and B. B. Schwartz (Plenum

Press, New York, 1981) p. 455.

9. J. W. Ekin, "Strain Scaling Law for Flux Pinning in Practical Superconductors, Part 1: Basic Relationship and Application to Nb^Sn Conductors," Cryogenics _20, 611 (1980).

10. J. W. Ekin, "Strain Scaling Law for Flux Pinning in Practical Superconductors, Part 2: Application to NbTi, V^Ga, Nb^Ge, Nb^Al, and Nb-Sn

Based Ternary Superconductors," Cryogenics 2/1 (1982) to be published.

11. A. F. Clark, G. Fujii, and M. A. Ranney, "The Thermal Expansion of

Several Materials for Superconducting Magnets," IEEE Trans. Mag. MAG-17 , 2316 (1981).

12. R. D. McCarty, "Thermophysical Properties of Helium-4 from 2 to 1500 K

with Pressures to 1000 Atmospheres," NBS Technical Note 631 (November

1972)

.

13. C. R. Spencer, P. A. Sanger, and M. Young, "The Temperature and Magnetic

Field Dependence of Superconducting Critical Current Densities of

Multifilamentary Nb^Sn and NbTi Composite Wires," IEEE Trans, on Mag.

MAG- 15 , 76 (1979).

91
14 . W. Schauer and F. Zimmerman, "Temperature Dependence of the Critical

Current and Pinning Behavior for Nb^Sn Filamentary Superconductors," Adv.

Cryo . Eng. 2_6, 432 (1980).

15. J. G. Hust, "Thermal Anchoring of Wires in Cryogenic Apparatus," Rev.

Sci. Instr. 41, 622 (1970).

92
Table 3a. Comparison of Critical Current Data for Various Sample Geometries

as a Function of Magnetic Field and Criterion for a NbTi conductor.

(Entries give percent difference with respect to the coil I

c

*)

Field, T

Coil, I 455.2 350.7 284.6 234.3 190.4 150.4 111.3 72.3

B

o

’ c > Long — -1.3 -1.1 -0.8 -0.6 -0.3 — -—

rH

II Hairpin 0.6 0.5 0.6

» 0.6 1.0 1.0 1.4 —

aw Short 0.3 0.6 0.7 0.8 1.4 1.5 2.1 2.3

6

>

3-

!

1 o

II w

Coil, I

c 431.5 332.0 269.2 221.4 179.8 141.8 104.6 67.0

Long -1.4 -1.3 -1.0 -0.6 -0.4 —

Hairpin 1.3 1.2 1.4 1.4 1.8 1.7 1.9 —

Short 0.3 0.8 0.9 1.1 1.5 1.5 2.1 2.5

E

o>

3- o

o

II ow

Coil, I

c 411.1 315.7 255.4 210.5 170.6 134.0 98.2 62.2

Long — -2.6 -2.7 -1.9 -0.6 -0.3 —

Hairpin 0.2 2.0 3.0 0.7 1.4 2.2 -0.2 —

Short -2.2 0.6 1.3 0.8 2.1 2.9 2.7 4.4

93
Table 3b. Comparison of Critical Current Data for Various Sample Geometries

as a Function of Magnetic Field and Criterion for a Nb^Sn Conductor.

(Entries give percent difference with respect to the coil I

c

*)

Field, T 2 3 4 5 6 7 8

g

u Coil, I 539.1 397.4 307.9 247.3 202.9 167.4 139.4

L. Long -0.4 -1.4 -1.4 -1.6 -1.6 -1.3 —

Or—

1

II ow

Hairpin -2.2 -2.9 -2.7 -2.9 -3.3 -3.5 -3.7

Short -2.5 -2.4 -2.3 -2.3 -2.4 -2.3 -2.2

£ Coil, I 480.4 354.8 275.5 221.4 181.4 149.7 124.6

u

c >

3. Long 2.2 0.7 0.3 0.0 -0.2 0.1 —

r-H

II Hairpin -2.8 -2.3 -1.9 -2.4 -2.6 -2.9 -3.2

uw Short -10.7 -8.4 -6.2 -4.4 -4.7 -2.7 -1.3

Bo Coil, I c 416.8 308.8 239.3 191.8 156.8 129.5 107.6

>

Long 5.2 5.6 4.4 4.8 4.2 4.5 — r-H o

ii

CJ W

Hairpin -13.5 -12.3 •11.1 -12.5 -9.3 -10.0 -8.1

Short -18.9 -17.5 -15.8 -15.3 -17.7 -14.0 -11.5

94
Table 4. Values for n of the Test Sample

Range of

E, yV/cm

NbTi rectangular

sample #1

NbTi round

sample #2

Nb

3

Sn MF

sample #3

Nb^Sn tape

sample #4

Field \

T

*

• oh

•

i

—

1

o 0.10.01

0.01

.001

1.0

0.1

0.1

.01

10

1.0

1.0

0.1

0.1

0.01

10

1.0

1.0

0.1

0.1

.01

2 43.0 47.7 43.5 55.4 84.2 20.0 16.2 17.4 73.1 150 279

3 42.1 45.3 31.8 57.2 80.2 20.3 16.6 16.4 71.4 173 261

4 41.3 45.2 34.2 56.7 75.0 20.7 16.4 16.5 72.4 162 208

5 40.6 46.8 41.6 53.7 68.7 20.8 16.0 15.6 70.0 151 191

6 40.4 45.9 39.1 50.7 61.8 20.6 15.8 15.4 65.4 143 184

7 39.1 43.2 36.3 45.5 53.5 20.6 15.9 14.4 59.4 126 166

8 36.9 37.5 31.2 36.8 43.3 20.5 15.6 13.4 52.4 109 150

9 30.0 30.9 27.0 27.1 31.4 20.4 15.3 12.4 45.1 89 107

9.5 25.0 24.2 24.5 — — 20.2 15.0 12.0 41.0 82 96

10 19.1 18.5 19.4 — — 20.0 15.0 11.4

95
6.

REFERENCES

1. A. F. Clark and J. W. Ekin, "Defining Critical Current," IEEE Trans, on Mag. MAG- 13 38-40 (1977).

2. R. L. Powell and A. F. Clark, "Definitions of Terms for Practical Superconductors, 1. Fundamental States and Flux Phenomena," Cryogenics 17 , 697-701 (1977).

3. R. L. Powell and A. F. Clark, "Definitions of Terms for Practical Superconductors, 2. Critical Parameters," Cryogenics 137-141 (1978).

4. J. W. Ekin, "Current Transfer in Multifilamentary Superconductors. I. Theory," J. Appl. Phys. 49, 3406-3409 (1978).

5. J. W. Ekin, A. F. Clark, and J. C. Ho, "Current Transfer in Multifilamentary Superconductors. II. Experimental Results," J. Appl. Phys. 49 , 3410-3412 (1978).

6. F. R. Fickett and A. F. Clark, "Development of Standards for Superconductors, Annual Report FY 79," NBSIR 80-1629, National Bureau of Standards,

Boulder, Colorado (Dec. 1979).

7. F. R. Fickett and A. F. Clark, "Standards for Superconductors," Proc.

1979 Mech. and Mag. Energy Storage Contractors’ Review Meeting, Conf790854 U.S. Dept, of Energy, Washington, D.C. (Dec. 1979).

8. D. T. Read, J. W. Ekin, R. L. Powell, and A. F. Clark, "Definitions of

Terms for Practical Superconductors: 3. Fabrication, Stabilization, and

Transient Losses," Cryogenics 19_ , 327-332 (1979).

9. F. R. Fickett and A. F. Clark, "Development of Standards for Superconductors," Proc. 8th Int’l. Cryo. Eng. Conf. (ICEC8), IPC Science and Technology Press (1980) pp. 494-498.

96
10 . F. R. Fickett and L. F. Goodrich, "NBS Superconductor Standardization

Program," Proc. 1980 Superconducting MHD Magnet Design Conference, A. M.

Dawson, ed., (Report published by Bitter National Magnet Laboratory, MIT,

Oct. 1981) p. 87.

11. F. R. Fickett, S. B. Kaplan, R. L. Powell, R. Radebaugh, and A. F. Clark,

"Definitions of Terms for Practical Superconductors: 4. Josephson

Phenomena," Cryogenics 20_, 319-325 (1980).

12. G. Fujii, J. W. Ekin, R. Radebaugh, and A. F. Clark, "Effect of Thermal

Contraction of Sample Holder Material on Critical Current," Adv. Cryog.

Eng. Vol. 26 , Plenum Press, New York (1980) p. 589.

13. G. Fujii, "Present Practices in Japan for the Measurement and Definition

of Various Superconducting Parameters," Tech. Report ISSP Univ. of Tokyo,

Ser. A, No. 1063 (July 1980).

14. A. F. Clark, "Development of Standards for Practical Superconductors,"

Proc. Superconductivity Technical Exchange Meeting, IAPG, PIC-ELE-SC

209/1 (1980), p. 103.

15. L. F. Goodrich and J. W. Ekin, "Lap Joint Resistance and Intrinsic

Critical Current Measurements on a NbTi Superconducting Wire," Proc. 1980

Applied Superconductivity Conference, IEEE Trans, on Mag. MAG- 1

7

, 69

(1981)

.

16. R. Radebaugh, G. Fujii, D. T. Read, and A. F. Clark, "A Standards Program

for ac Losses in Superconductors," Proc. IVth International Congress on Refrigeration (1980), Comm. A 1/2-10 (1980)., pp. 1-4.

17. H. R. Segal, Z. J. J. Stekley, and T. A. DeWinter, "Development of

Critical Current Measurement Standards," Proc. 1980 Applied Superconductivity Conference, IEEE Trans. Mag. MAG-1

7

, 73 (1981).

97
18. G. Fujii, M. A. Ranney, and A. F. Clark, "Thermal Expansion of Nb^Sn and

V^Ga Multifilamentary Superconducting Cables, Fiberglass-Epoxy and

Cotton-Phenolic Composite Materials," Jap. J. Appl. Phys. 2J3, L267

(1981)

.

19. L. F. Goodrich, J. W. Ekin, and F. R. Fickett, "Effect of Twist Pitch on Short-Sample V-I Characteristics of Multifilamentary Superconductors,"

Adv. Cryog. Eng. - Materials , Vol. 28 , Plenum Press, New York (1981),

p. 571.

20. A. F. Clark, G. Fujii, and M. A. Ranney, "The Thermal expansion of

Several Materials for Superconducting Magnets," IEEE Trans, on Mag.

MAG-17 , 2316 (1981).

21. F. R. Fickett, L. F. Goodrich, and A. F. Clark, "Development of Standards

for Superconductors: Annual Report FY 80," NBSIR 80-1642, National

Bureau of Standards, Boulder, Colorado (Dec. 1980).

22. L. F. Goodrich and F. R. Fickett, "Critical Current Measurements: A

Compendium of Experimental Results," Cryogenics, to be published.

98
APPENDIX A

ASTM

Subcommittee B01.08 on Superconductors

Monday, August 10, 1981, 3-6 pm

Council Room, Town & Country Hotel, San Diego, CA

AGENDA

1. Description of ASTM Standards process A. F.

2. Status of draft standards, vote results A. F. a) definitions of terms

b) measurement of critical current

3. Research report F. R.

L. F.

4. Task committee discussion of negative ballots

a) definitions S. J.

b) critical current R. E.

Discussion of new tasks A. F. a) large conductor critical current

b) ac losses

c) critical field

d) stability

e) monitoring adopted standards

Clark

Clark

Fickett and

Goodrich

St. Lorant

Schwall

Clark

A1
Minutes - ASTM B01.08

Subcommittee on Superconductors

August 10, 1981 3:00 pm

Council Room, Town and Country Hotel, San Diego, CA

The meeting was called to order at 3:00 pm by the Chairman, A. F. Clark.

After introductions, he outlined the approval process for any draft standard

through the subcommittee ballot, main committee ballot, and finally the full

ASTM society ballot and the requirements at each level and methods for resolving negative votes. The responses on subcommittee balloting on the two draft

standards on definitions and critical current were as follows:

a) Definitions of terms for Superconductors

65% return 93% Affirmative

1 negative ballot, some comments and also suggestions from the ASTM

Committee on Terminology

b) Critical current measurement for conductors for less than 600 amps

70% return 75% Affirmative

4 negative ballots (one late) and several comments

A summary of the objections from the negative ballots was prepared by the NBS

staff along with recommended actions to be considered by the subcommittee

(copy attached)

.

Fred Fickett (NBS) then outlined briefly all of the back-up research

performed by the wire manufacturers, DoE, and NBS as resource information

available for the subcommittee deliberations.

The task group chairman for the definitions, Steve St. Lorant (Stanford

Linear Accelerator) , led discussion of the criticism and corrections to the

draft standard for definitions. The one negative ballot was easily resolved

by changing H to H

? and several other suggested changes were considered

as improvements. T^e changes were adopted unanimously by the subcommittee.

The task group chairman for critical current. Bob Schwall (Intermagnetics

General Corp.), then led the discussion on the negative ballots and other

suggested changes to the critical current draft standard. These were done

point by point through the standard and changes were either adopted or denied.

Major changes included adding a bandwidth specification for periodic and

random deviations, improving the criteria accuracy, relaxing the field

homogeneity requirement, clarifying the role of a sample protection shunt

during the measurement, and deleting the recheck requirement. Several

non-negative ballot comments were also considered resulting in some minor

additions for clarification. The changes were then reviewed and adopted

unanimously by the subcommittee.

A2
Steve St. Lorant and Bob Schwall were then urged to provide focal points

for feedback on any problems or additions to the definitions and critical

current standards, respectively. The NBS staff will incorporate the changes

and send the revised standards to ASTM for the main committee ballots.

New tasks were discussed and it was decided that large conductor critical

current, ac losses, and stability should take priority. The next step in

critical current was decided to cover up to about 5000 amps and Bruce Strauss

(Magnetic Corporation of America) agreed to provide a review of the state-ofthe-art to this level. Loss measurement research is being initiated at NBS

under the partial sponsorship of DoE and this work and stability will be

discussed at the next meeting. Because of travel restrictions it was decided

to continue to hold meetings in conjunction with superconductivity-related

conferences whenever possible.

A3
List of Attendees

ASTM B01.08 Subcommittee on Superconductors

Name Address

Don Beard DoE, Germantown, ML)

Karl-Juergen Best Vacuumschmelze GmbH, Hanau, FRG

A1 Clark NBS , Boulder, CO 80303

Fred Fickett NBS, Boulder, CO 80303

Loren F. Goodrich NBS, Boulder, CO 80303

Moyses Kuchnir Fermilab, Batavia, IL 60510

Bob Remsbottom Univ. of Wisconsin, Madison, WI 53706

Hank Riemersma Westinghouse Electric Corp. , Pittsburgh, PA 15235

Phillip Sanger Airco Superconductors, Carteret, NJ 07008

Bob Schwall IGC, P. 0. Box 566, Guilderland, NY 12084

Mark Siddal Teledyne Wah Change, P. 0. Box 460, Albany, OR 97321

Steve St. Lorant SLAC, Stanford, CA 94305

Kate Stohlman Supercon Inc., 9 Erie Drive, Natick, MA 02043

Bruce Strauss MCA, Waltham, MA 02254

George Wagner Westinghouse Electric Corp., Pittsburgh, PA 15235

Bruce Zeitlin IGC, P. 0. Box 566, Guilderland, NY 12084

A4
APPENDIX B

DRAFT STANDARD FOR TERMINOLOGY

ASTM Designation B XXX-82

Standard Definitions of Terms Relating to

SUPERCONDUCTORS

This standard is issued under the fixed designation B XXX; the number immediately following the designation indicates the year of original adoption or, in

the case of revision, the year of last revision. A number in parentheses

indicates the year of last reapproval.

aspect ratio - ratio of the longer to the shorter transverse dimensions

of a rectangular composite superconductor. NEW

barrier - any material limiting passage through itself of solids,

liquids, semisolids, gases, or forms of energy such as ultraviolet light.

F 17, F-2*

braid , n - a narrow tubular or flat fabric produced by intertwining

strands of materials according to a definite pattern. D 123, D-13

cable (concentric lay conductor) - conductor constructed with a central

core surrounded by one or more layers of helically laid wires. Several types

are as follows:

compact round conductor - a conductor constructed with a central core surrounded by one or more layers of helically laid wires and formed into final

shape by rolling, drawing, or other means. conventional concentric conductor - conductor constructed with a round

central core surrounded by one or more layers of helically laid round wires.

The direction of lay is reversed in successive layers, and generally with an increase in length of lay for successive layers.

equilay conductor - conductor constructed with a central core surrounded

by more than one layer of helically laid wires, all layers having a common length of lay, direction of lay being reversed in successive layers.

parallel core conductor - conductor constructed with a central core of

parallel-laid wires surrounded by one layer of helically laid wires.

rope-lay conductor - conductor constructed of a bunch-stranded or a concentric-stranded member or members, as a central core, around which are laid one or more helical layers of such members.

unidirectional conductor - conductor constructed with a central core surrounded by more than one layer of helically laid wires, all layers having a common direction of lay, with increase in length of lay for each successive

layer.

unilay conductor - conductor constructed with a central core surrounded

by more than one layer of helically laid wires, all layers having a common

length and direction of lay. B 354, B-l

*The numbers indicate that the definition is taken directly from the

Compilation of ASTM Standard Definitions, 4th edition, 1979 and cite the

standard in which it is used (e.g. D 2864) and the subcommittee with

jurisdiction (e.g. D-27). If new for this standard it is designated NEW.

Bl
component - an individual piece or a complete assembly of individual

pieces. F 303, F-7

composite conductor - a conductor consisting of two or more types of

material, each type of material being plain, clad, or coated; assembled

together to operate mechanically and electrically as a single conductor.

NEW

composite superconductor - a conductor incorporating superconductive

material. Some types are as follows:

composite conductor - see composite conductor.

coreless conductor - a conductor constructed of one or more layers of

helically laid wires and formed into final shape by rolling, drawing, or other means

.

tape - a conductor constructed in the form of a flat ribbon or strip.

tubular conductor - a conductor constructed in the form of a tube.

hollow conductor (also tubular conductor) - a conductor in which the

individual elements are disposed about one or more hollow passages, the

direction of which is along the axial length of the conductor. NEW

composition - the quantity of each of the components of a mixture;

usually expressed in terms of the weight percentage, or the atomic percentage

of each of the components in the mixture. E 7, E-4

conductivity, electrical - the ratio of the current density carried

through a specimen to the potential gradient paralleling the current. This is

numerically equal to the conductance between opposite faces of a unit cube of

material. It is the reciprocal of resistivity. D 2864, D-27

conductor - a wire or combination of wires not insulated from one another, suitable for carrying an electric current. B 354, B-l

critical current, I - the maximum electrical current below which a

Q

superconductor exhibits superconductivity at some given temperature and

magnetic field. NEW

critical current density , J - the critical current divided by the total

cross-sectional area of the conductor. NEW

critical magnetic field strength , H

^ - the maximum magnetic field below

which a superconductor exhibits superconductivity at zero current and

temperature. NEW

critical temperature , T - the maximum temperature below which a superconductor exhibits superconductivity at zero magnetic field and current.

NEW

critical transition temperature - see transition temperature.

current, constant , I - the steady current which is located in a winding

and which produces a magnetostatic condition. A 340, A-6

current density (cd) - current per unit area. B 374, B-8

B2
diamagnetic material - a material whose relative permeability is less

than unity.

Note - The intrinsic induction, B„, is oppositely directed to the

applied magnetizing force H. A 340, A-6

filamentary (multifilamentary) superconductor - a composite

superconductor consisting of at least one superconductive wire embedded in a matrix. NEW

flux density - see magnetic induction.

fully transposed conductor - a conductor in which every strand occupies

every relative position in the conductor at regularly specified intervals

along its length. NEW

magnetic field strength - the measured intensity of a magnetic field at a point, usually expressed in. E 269, E-7

magnetic flux ,

cj) - the product of the magnetic induction, B, and the area

of a surface (or cross section) , A, when the magnetic induction B is uniformly

distributed and normal to the plane of the surface.

<J> = BA

where

<j> = magnetic flux,

B = magnetic induction, and

A = area of the surface.

Note 1 - If the magnetic induction is not uniformly distributed over the

surface, the flux, cj), is the surface integral of the normal component

of B over the area.

cj) = // B-dA

s Note 2 - Magnetic flux is a scalar and has no direction. A 340, A-6

magnetic flux jump - the collective, discontinuous motion of magnetic

flux lines in a superconductor, produced by mechanical, thermal, magnetic, or electrical disturbances. NEW

magnetic flux pinning - the trapping of magnetic flux lines at

defects in the superconducting material. NEW

magnetic induction (also flux density) , B - that magnetic vector quantity

which at any point in a magnetic field is measured either by the mechanical

force experienced by an element of electric current at the point, or by the

electromotive force induced in an elementary loop during any change in flux

linkages with the loop at the point.

Note 1 - If the magnetic induction, B, is uniformly distributed and

normal to a surface or cross section, then the magnetic induction is:

B3
where

:

B = magnetic induction,

(f> = total flux, and

A = area.Note 2 - B. is the instantaneous value of the magnetic induction and

B is the maximum value of the magnetic induction. A 340, A6

m

matrix of composite superconductor - the continuous longitudinal phase of

a pure metal, a polyphase alloy or mechanical mixture that is not in the

superconducting state at the normal operating conditions of the embedded

superconductor. NEW

mixed matrix of composite superconductor - matrix composed of more than

one component. NEW

normal state - the thermodynamic state in which a superconductive

material no longer exhibits any of the characteristics of the superconducting

state. NEW

quench - the abrupt and uncontrolled loss of superconductivity produced

by a disturbance. NEW

stabilizer - a metal, but not necessarily the matrix, in electrical

contact with a superconductor, to act as an electrical shunt in the event that

the superconductor reverts to the normal state. NEW

strand , n - one of the wires of any stranded conductor. B 354, B-l

stranded conductor - a conductor composed of a group of wires, usually

twisted, or of any combination of such groups of wires. B 354, B-l

superconducting - adjective describing a material exhibiting

superconductivity NEW

superconducting state - the thermodynamic state in which the material

exhibits superconductivity. NEW

superconducting transition - that combination of temperature, T, electric

current density, J, and magnetic field, H, values at which a transition from

the superconducting to the normal state takes place.

Note - A representation of this relationship is shown in the Figure 1.

NEW

superconductive - adjective describing a material exhibiting the

characteristics of normal conductivity, but which shows superconductivity

under appropriate conditions. NEW

superconductivity - a property of many metals, alloys and chemical

compounds by virtue of which their electrical resistivity vanishes and they

become strongly diamagnetic under appropriate conditions. NEW

superconductor - a material that exhibits superconductivity under

appropriate conditions. NEW

B4
tape - a composite superconductor in the form of a flat ribbon or strip.

NEW

temperature - the thermal state of matter as measured on a definite

scale. E 41, G 3

temperature, absolute - (a) temperature measured on the thermodynamic

scale, designated as kelvins (K)

.

(b) Temperature measured from absolute zero (-273.15°C or -459.67°F).

The numerical values are the same for both the Kelvin scale and the ideal gas

scale. D 1356, D-22

transition temperature (also critical transition temperature) - the

maximum temperature below which a superconductor exhibits superconductivity at

a given magnetic field and current. NEW

transposed conductor - a composite conductor in which filaments or strands occupy different relative positions about the conductor axis in a regular manner. NEW

transposition length - the length in which a filament or strand returns

to its original relative position in a transposed conductor. NEW

trapped magnetic flux - the magnetic flux retained in a superconductor

when the applied magnetic field is reduced or reversed in direction.

NEW

twist , n - the number of turns per unit length made by a filament or strand about a conductor axis. NEW

twist pitch (also twisted length) - the length in which a filament or strand returns to its original relative position in a twisted conductor.

NEW

twisted length - same as twist pitch.

twisted conductor - a composite conductor in which the filaments or strands are displaced about the conductor axis. NEW

volume percent superconductor - that percentage by volume of a composite

superconductor which is superconducting under normal operating conditions.

NEW

wire - a rod or filament of drawn or rolled metal whose length is great

in comparison with the major axis of its cross section. B 354, B-l

B5
Current

Density

B6

Figure

1.

Transition

surface

from

the

superconducting

to

the

normal

state.
APPENDIX C

DRAFT STANDARD FOR CRITICAL CURRENT

ASTM Designation B XXY-82

Standard Test Method for

DC CRITICAL CURRENT OF COMPOSITE SUPERCONDUCTORS

This standard is issued under the fixed designation B XXY; the number immediately following the designation indicates the year of original adoption or, in

the case of revision, the year of last revision. A number in parentheses

indicates the year of last reapproval.

1 . Scope

1.1 This method covers the procedure for the determination of the dc

critical current of composite superconductors.

1.2 This method is intended for use with superconductors having a critical current of less than 600 amperes under test conditions and at magnetic

fields of less than 0.8 of the upper critical magnetic field.

2 . Applicable Documents

2.1 ASTM Standards

B XXX Definitions of Terms for Superconductors.

ASTM Standards on Metallic Electrical Conductors - Volume 23 (1982).

3 . Summary of Method

3.1 A direct current is applied to the superconductor specimen and the

voltage generated along a section of the specimen is measured. The current is

increased from zero and the voltage-current characteristic is generated. The

critical current is defined as the current at which a specified electric field

is exceeded in the specimen.

4 . Significance and Use

4.1 The critical currents of composite superconductors are used to

establish design limits for applications of superconducting wires. The

operating conditions of superconductors in these applications determine much

of their behavior and tests made with this method may be used to provide part

of the information needed to determine the suitability of a specific

superconductor

.

4.2 Results obtained from this method can also be used for detecting

changes in the superconducting properties of a composite superconductor due to

processing variables, handling, aging, or other application or environmental

conditions. This method is useful for quality control, acceptance, or research

testing if the precautions below are observed.

Cl
4.3

The critical current of composite superconductors depends on many

variables. These variables need to be considered in both the testing and the

application of these materials. (Ref. 1)

4.3.1 Test conditions such as magnetic field, temperature and relative

orientation of specimen, current and magnetic field are determined by the

particular application.

4.3.2 The test configuration may be determined by the particular conductor through the tolerances required by sections 8.1 and 8.4.

4.3.3 The specific critical current criterion may be determined by the

particular application.

4.3.4 It may be appropriate to measure a number of test specimens if

there are irregularities in testing.

4.4 A precaution is needed in the interpretation of results when the

reference line of the V-I curve (8.5, 8.5.1) has a finite slope. The current

transfer correction is to be used to correct for a true current transfer.

Voltages may occur from other sources.

4.4.1 A current transfer voltage will result from having a voltage tap near (near is determined by resistivity of the matrix and electrical field

criterion) to a current contact, or having a gradient in the magnetic field near the region between voltage taps or having a field-sample orientation

change near the region between voltage taps. (Refs. 1, 2, and 3)

5 . Terminology

5.1 Refer to B XXX Definitions of Terms for Superconductors for general

terminology for the field of superconductivity.

6 . Safety Precautions

6.1 Very large direct currents with very low voltages do not necessarily

provide a direct personal hazard, but accidental shorting of the leads with

another conductor, such as tools or transfer lines, can release significant

amounts of energy and cause arcs or burns. Care must be taken to isolate and

protect current leads from shorting. Also the stored energy in superconducting

magnets commonly used for the background magnetic field can cause similar

large current pulses or deposite large amounts of thermal energy in the

cryogenic systems causing rapid boil off or even explosive conditions.

6.2 The use of cryogenic liquids is essential to cool the superconductors

to allow transition into the superconducting state. Direct contact with cold

liquid transfer lines, storage dewars, or apparatus components can cause immediate freezing, as can direct contact with a spilled cryogen. Normal

safety precautions for the handling of cryogenic liquids must be observed.

7 . Test Specimen

7.1

The test procedure is intended for specimens with a critical current

of less than 600 amperes under test conditions.

C2
7.2 There shall be no joints or splices in the test specimen unless

otherwise specified.

7.3 The test specimen should, wherever possible, have the same residual

strain state as the final product.

8 . Procedure

8.1 The (maximum) bending strain, induced during mounting of the specimen,

shall not exceed 0.1% for Nb Sn (and other brittle materials) or 2% for Nb-Ti

(and other ductile materials). The tensile strain, induced by the differential

thermal contraction of the specimen and holder, shall not exceed 0.05% for

Nb Sn (and other brittle material) and 0.5% for Nb-Ti (and other ductile

material). (Ref. 4)

8.1.1 Pre-reaction forming of brittle conductors to the test configuration may be required.

8.1.2 Matching the thermal contraction of the specimen and specimen

holder may be required (Ref. 5). Suitable materials for construction of the

specimen holder are NEMA G-10 and G-ll with the specimen in the plane of the

fabric (Ref . 6)

.

8.2 Solder voltage taps to the specimen in accordance with the limits in

8.1, 8.3, and 10.7.2.

8.3 Measure the distance along the specimen between the voltage taps, L,

to an accuracy of 10% or 50 mm, whichever is smaller.

8.4 The critical current, I , shall be determined by using an electric

field criterion, E^, of 100 yV/m unless otherwise specified.

8.4.1 There are other criteria that could be used (resistivity, power),

but E^ is considered to be an expedient criterion (Ref. 7).

8.4.2 The specified E may be calculated on the basis of any other

criteria if so desired.

8.5 Record the V-I characteristic of the test specimen under test

conditions

.

8.5.1 A valid V-I characteristic shall give I to a precision of 2% for

both increasing and decreasing current. If a number of I measurements are to

be made on a specimen at the same temperature, this current reversal test has

to be performed for only the lowest magnetic field to be reported.

8.6 Draw a straight line through the lower current (less than 0.8 of the

resulting I ) portion of the V-I curve to serve as a reference line (see

Fig. 1). Determine I by finding the point on the V-I curve where the voltage,

measured relative to £he reference line, is LE . c

C3
8.6.1 A finite slope of the reference line may be due to current

transfer (Refs. 1, 2, and 3) and in that case the line serves as an approximate correction to this effect. A valid determination of I requires

that the voltage of the reference line at I must be less than LE . c c

9 . Report

9.1 Identification of test specimen should be made by the manufacturer's

lot number. This number should insure unique identification. Subsequent

processing not identified by the lot number should be reported.

9.2 The following test conditions shall be reported:

9.2.1 Test magnetic field,

9.2.2 Test temperature,

9.2.3 Length between voltage taps and total specimen length,

9.2.4 Test configuration (geometry, angle between the specimen axis and

the magnetic field, orientation of specimen with respect to magnetic field if

the specimen is rectangular)

.

9.3 Modified tolerances (per 10.2) shall be reported.

9.4 The value of 1^ and shall be reported.

9.5 For routine tests, report only such of the items above as apply.

10 . Precision and Accuracy

10.1 The suggested tolerances listed below of the many variables affecting the critical current should provide an accuracy of 5% on test specimens

having a critical current of less than 600 amperes under test conditions and

at magnetic fields of less than 0.8 of the upper critical magnetic field. The

individual test should have a precision of 2%.

10.2 Because of the large number of variables that affect the critical

current (Ref. 1), the range of composite superconductors and the testing techniques, all of the tolerances listed below may not be considered appropriate

or reasonable to obtain in all cases. In these cases, the appropriate sections

may be modified. Any such modification shall be made part of the report.

10.3 The critical current shall be determined from a voltage-current

characteristic measured with a f our-terminal technique.

10.3.1 The current source shall provide a current having a maximum

periodic and random deviation of less than ± 51 at I within the bandwidth

10 Hz to 10 MHz.

C

10.3.2 A four-terminal standard resistor, with an accuracy of at least

1%, shall be used to determine the sample current.

C4
10.3.3

A recorder and necessary pre-amplifiers, filters and/or voltmeters shall be used to record the V-I characteristic. The resulting record

should allow determination of E to an accuracy of 12% and the corresponding

current to an accuracy of 1%, with a precision of 0.5%.

10.4 A quench protect circuit may be necessary to allow the positive

completion of step 8.5.1 (Ref. 1).

10.5 A dewar will provide the necessary environment for measuring I . Unless otherwise specified, the specimen shall be measured immersed in liquid

helium. The liquid temperature shall be reported to an accuracy of ± 0.5%.

10.6 A magnet system shall provide the magnetic field to an accuracy of

1% and a precision of 0.5%.

10.6.1 The magnetic field shall have a uniformity of ± 2% over the

length of the specimen between the voltage contacts.

10.6.2 The maximum periodic and random deviation of the magnetic field

shall be less than ± 1%.

10.7 The test fixture shall provide adequate support for the specimen

and orientation of the specimen with respect to the magnetic field.

10.7.1 The specimen support is adequate if it allows for the positive

completion of step 8.5.1.

10.7.2 The angle between the specimen axis and the magnetic field shall

be determined to an accuracy of 7° for the length of the specimen between the

voltage taps. Unless otherwise specified, the angle shall be 90 ± 7°.

10.7.3 In the case of a rectangular specimen, the magnetic field shall

be parallel to the wide face of the specimen unless otherwise specified. The

angle between the magnetic field and the wide face shall be reported to an accuracy of 7° for the length of the specimen between the voltage taps.

10.7.4 The test configuration of the specimen (straight, hairpin,

bifilar coil, pancake coil, or solenoidal coil) will be chosen by the tester

unless otherwise specified.

10.8 A shunt may be used to protect the specimen when the specimen is in

the normal state as long as less than 1% of the current will flow in the shunt

at I . The shunt will not be in immediate contact with the specimen unless

otherwise specified.

References

1. Goodrich, L. F. and Fickett, F. R. , "Critical Current Measurements: A

Compendium of Experimental Results," Cryogenics, to be published.

2. Ekin, J. W. , "Current Transfer in Multifilamentary Superconductors.

I. Theory," J. Appl. Phys. U9_, 3406 (1978).

C5
3. Ekin, J. W. , Clark, A. F. , and Ho, 3. C., "Current Transfer in Multifilamentary Superconductors, II. Experimental Results," J. Appl. Phys. 49 , 3410 (1978).

4. Ekin, J. W. , "Mechanical Properties and Strain Effects in Superconductors,"

Chapter 7 in Superconducting Materials Science , edited by S. Foner and B. Schwartz, Plenum Press, New York (1981) p. 455-509.

5. Fujii, G. , Ekin, J. W. , Radebaugh, R. , and Clark, A. F. , "Effect of

Thermal Contraction of Sample Holder Material on Critical Current," in:

Adv . Cryog . Eng . 26 , Plenum Press, New York (1980), p. 589.

6. Clark, A. F., Fujii, G. , and Ranney, M. A., "The Thermal Expansion of

Several Materials for Superconducting Magnets," IEEE Trans. Magnetics,

MAG-17 , 2316 (1981).

7. Powell, R. L. and Clark, A. F„, "Definitions of Terms for Practical

Superconductors, 2. Critical Parameters," Cryogenics _18, 137 (1978).

sis®*, m

e

3

>*

t-» m

«3M1 J3

jbm

CURRENT

Fig. 1. Schematic representation of the composite superconductor's V-I

characteristic in two regions: 1) Intrinsic characteristic showing the usual

resistive transition as I approaches I and 2) current-transfer characteristic

exhibiting a linear region at low current. The reference line described in

section 8.6 is shown as the dashed line.

C6
NBS-114A (REV. 2-BC) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR 82-1678 August 1982

4. TITLE AND SUBTITLE

Development of Standards for Superconductors

5. AUTHOR(S)

A. F. Clark, L. F. Goodrich, F. R. Fickett, and J. V. Minervini

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No,

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City . State. ZIP)

Department of Energy

Washington, D.C. 20545

Massachusetts Institute of Technology

170 Albany Street

Cambridge, Mass. 02139

10. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

A cooperative program with the Department of Energy, the National Bureau of

Standards, and private industry is in progress to develop standard measurement

practices for use in large scale applications of superconductivity. The goal is the

adoption of voluntary standards for the critical parameters and other characterizations of practical superconductors. Progress for the period October 1980

through January 1982 is reported. The major effort was the development of a standard test method for critical current, the necessary back-up research, and

the coordination of the adoption of the test method and a standard terminology

through the subcommittee level in ASTM.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s)

critical current; critical parameters; losses; measurement methods; standards;

superconductor

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

[jjj] Unlimited

118 | | For Official Distribution. Do Not Release to NTIS

r 1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

15. Price 20402.

[TT] Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $12.00

USCOMM-DC 6043-P80
.