REFERENCE

NBS

PUBLICATIONS

NBSIR 87-3066

A111.DE LblflbD

NATL INST OF STANDARDS & TECH R.I.C.

All 102661 860

Goodrich, L. F/Development ol standards OCIOO U56 NO 87-3066 1987 V19 C.1 NBS-P DEVELOPMENT OF STANDARDS FOR SUPERCONDUCTORS

Interim Report

January-December 1985

L.F. Goodrich, S.L. Bray, W.P. Dube, E.S. Pittman, A.F. Clark

Electromagnetic Technology Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303-3328

APRIL 1987

QC

100

. U 5 6

#87-3066 Work performed under Department of Energy contract DE-AI01-76PR06010

1987

NBSIR 87-3066

Research Information Center

National Bureau of Standards

Gaithersburg, Maryland 20899

DEVELOPMENT OF STANDARDS FOR SUPERCONDUCTORS

Interim Report

January-December 19S5

L.F. Goodrich, S.L. Bray, W.P. Dube, E.S. Pittman, A.F. Clark

Electromagnetic Technology Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303-3328

APRIL 1987

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

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

FOREWORD

The ultimate goal of this research program is to develop fundamental

understanding, a technology base, consensual standard definitions, consensual

standards of measure, and standard reference materials for superconductor

parametric measurements. The program areas and elements are based on the

intersection of our expertise and capabilities with recommendations from the

research community and sponsors. The emphasis and priorities are flexible and

the program responds to new problems as they arise.

The key parameters in the commerce of superconductors need standard

definitions and standards of measure. These parameters include critical

current, stability, strain dependence, transient loss, critical magnetic

field, and critical temperature. The standards are voluntary, and developed

by users and producers; we hope thereby to achieve an overall balance of

simplicity and accuracy. The fundamental understanding acquired during the

standardization will be disseminated to the community through open literature

publications. Where appropriate, Standard Reference Materials (SRM’s) will be

developed to promote more-uniform measurements and to serve as a means for

interlaboratory comparisons that will further advance the consensus and evolution of standard test methods. This research program is focused on three

areas: critical current , stability, and SRM's. The original program direction

was to continue with a large part of the research on large conductor critical

current and to begin increasing emphasis on stability and eventually another

critical current SRM. Because of the number of problems and interest in large

conductor critical current, and a small reduction in available funds and

staff, the emphasis has remained on critical current measurements.

A key parameter in the technology of superconducting wire is critical

current; at present there is no standard test method for large conductor

critical current (greater than 600 A) . The measurement problems and parameters must be understood before a new high-current consensual standard test

method can be developed. Such a standard will increase measurement accuracy,

reduce test time, and lower conductor cost.

Superconductor stability is an important parameter in the technology of

large magnets. It ranks in importance near critical current as a design

parameter. A few experiments have tried to measure stability directly, and

several heat transfer tests could be used to acquire data for calculating its

value. If a meaningful and practical test method for stability could be

developed, it would reduce much of the uncertainty and conservatism currently

used in the design of large superconducting magnets, resulting in both savings

in cost and increased magnet performance.

An SRM provides a means for checking the overall accuracy of the complex

systems used to measure superconductor parameters. Traceability of standards

is important to the evaluation of superconductor technology. SRM’s can also

provide interlaboratory comparisons and promote fair competition among

suppliers

.

An effort was made to avoid the identification of commercial product5-, by

the manufacturer’s name, but in some cases these products might be indirectly

identified by their particular properties. In no instance does this imply

endorsement by the National Bureau of Standards, nor does it imply that th<: particular equipment is necessarily the best or worst available for thin

purpose

.

iii
'
CONTENTS

Page

FOREWORD iii

ABSTRACT 1

INTRODUCTION 2

CRITICAL CURRENT 2

Present Capabilities 3

Short Range Variations 3 Persistent Currents *4 Motion *4 Pressure Control 5

Improvements to 3000 A Supply 6

Battery Powered Current Supply 7

User Notes on Digital Voltmeters 13

Test Method for Low Level Voltage Measurement Systems 13

STABILITY 32

Resistivity 32

Copper-to-Superconductor Volume Ratio 3*4 Stability Apparatus 3^

STANDARD REFERENCE MATERIAL (SRM) 35

Nb

3

Sn 35

Large Conductor NbTi 36

Self-Field 36

European Economic Community Wire 36

Nb^Sn Round Robin *40

SUMMARY *40

ACKNOWLEDGMENTS *40

REFERENCES *40

APPENDIX A - THE EFFECT OF ASPECT RATIO ON CRITICAL CURRENT IN

MULTIFILAMENTARY SUPERCONDUCTORS *43

APPENDIX B - QUENCH DETECTOR CIRCUIT FOR SUPERCONDUCTOR TESTING 51

APPENDIX C - PUBLICATIONS AND PRESENTATIONS 55

v

DEVELOPMENT OF STANDARDS FOR SUPERCONDUCTORS

Interim Report

January - December 1985

L. F. Goodrich, S. L. Bray, W. P. Dube, E. S. Pittman, and A. F. Clark

Electromagnetic Technology Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

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 January through December 1985 is

reported. The major effort was the measurement of large conductor

critical current. Other work reported here includes stability and a discussion of possible future Standard Reference Materials.

Key words: angle; aspect ratio; cable; critical current; critical

parameters; measurement; standards; superconductor

1
INTRODUCTION

This report covers the period January 1984 through December 1985. Most of

1984 was devoted to completing the critical current measurements of the small

conductor Standard Reference Material (SRM). The special publication on the

SRM [1] serves as the report for 1984. The sections of this report cover topics other than the SRM and include a few efforts from 1984. Two recent

publications are included as Appendices A and B [2,3]. A complete list of the more than 35 publications resulting from this project is given as Appendix C.

This report covers the measurement of large conductor critical current

and the parameters that affect stability. Experimental concerns, measurement

checks, and results are presented. This report deals mostly with large

conductor critical current. Stability and the next SRM will be phased in

during the following years of this program.

CRITICAL CURRENT

The differences between critical current measurements on large and small

conductors are that some previously negligible parameters become significant

and other new parameters are introduced. The expense of increasing high

magnetic field volume restricts the scale-up of magnets used in critical

current testing, which causes the effects of current injection, current

transfer, and bending strain to become more significant for large conductor

measurements. Our measurements have not indicated any change in the character

of current transfer in large conductors. The self-field of the sample and the

field of the current return path become more significant at the higher

currents. The increased Lorentz force and torque on the sample and magnet

supports can cause additional motion-induced noise and systematic steady state

voltage shifts. Ripple in the current supply becomes more significant at the

higher current levels because filtering is more difficult. Ripple also

affects the sample voltmeter, which has to operate with a very noisy input and

have high common mode rejection. New measurement problems and parameters

occur in cable conductors. The inherent variation of critical current along

the length of a strand will cause intrinsic current transfer and complications

in voltage tap placement and interpretation. These inherent variations can be

due to changes in conductor cross-sectional area, field angle, conductor

aspect ratio, strain, self-field, and possible degradation of the superconducting properties.

All of the large conductor measurements to date have been conducted with

a 10 T radial access magnet. Therefore, some of the observations reported

here are particular to that geometry. The small bore diameter (3.8 cm) of our

9 to 10 T solenoid magnet did not lend itself to large conductor testing.

However the radial access magnet allowed for changing the orientation of the

sample relative to the background magnet which is very useful for large

conductor and cable measurements. We have started procurement of a 1 2 T

solenoid with a 12.7 cm diameter. This large bore solenoid will allow for

testing of longer and higher current samples.

The body of this section of the report is devoted to a number of new experimental concerns that developed with critical current measurements on large conductors. Results of these measurements are given in Appendix A.

2
These new concerns affect the critical current measurements and thus limit the

precision and accuracy of the measurement. To proceed it is necessary to

recognize, understand, and attempt to reduce these effects. The progress that

has been made toward this end is detailed below.

Present Capabilities

During this period, we constructed a sample cryostat with 3000 A capacity

and made our first measurements in this current range. We reduced the amount

of 60 Hz harmonic ripple in our 3000 A power supply. We also increased the

capacity of our low-noise battery power supply from 600 to over 900 A and

reduced the noise level from 0.5 A peak-to-peak to 0.01 A peak-to-peak. We

have also further automated data acquisition and can now measure three voltage

tap pairs at once. This reduced operator fatigue and increased the quantity

of the data. The changing of voltage taps, magnetic field, and angle are still done manually. We had developed automatic data analysis when making the

first SRM. Our data acquisition programs were modified to handle current

transfer voltage, voltage filtering, and the time lag of some volt-meters in

order to be used in large conductor data analysis.

Short Range Variations

Here, we discuss the large variations in critical current that have been

observed over a very short length of wire. The first report of this is

reproduced here as Appendix A. Refer to the bottom of the fourth page through

the top of the sixth page and figures 5, 6, and 7 of Appendix A. This type of

variation is of fundamental and practical importance, especially in the making

of an SRM. These effects may also be present in cable conductors due to

turkshead compaction. To a much smaller extent, short range variations were

present in the first SRM, which had a round cross section, although it is not

known if these short range variations occurred due to the same mechanism.

These variations in critical current seem to be periodic and associated

with changes in filament deformation. The variation in filament deformation

observed here appears to be different from sausaging [4] in that here change

in deformation occurs collectively (most of the filaments together) rather

than randomly as in sausaging. Also, the extreme deformation was a flattened

filament, rather than a necked filament as in sausaging.

The cause of this periodic variation in filament shape, critical current,

and aspect ratio was the most likely combination of filament twist and

hexagonal filament array [5]. This apparent mechanical instability can be

envisioned by considering that the turkshead will try to remove the filament

twist and the torque will build up in front of the turkshead until the aligned

filaments slip through with extreme deformation.

A key to reducing this effect may be the center copper island [6] or the

local copper-to-superconductor ratio. Compare figures 5 and 7 in Appendix

A. The change in shape of the copper island in figure 7 indicates that it may

be playing an important role in reducing the extreme filament deformation.

3
Persistent Currents

Persistent currents among the sample current return wires caused a time

dependent, partial shielding of the sample from the applied magnetic field.

Four return wires were used on the radial-access-magnet cryostat for a number

of reasons: higher current capacity, reduced net torque on the sample

cryostat, and reduced effect of the magnetic field of the return path on the

sample. The four return wires were soldered together at each end (onto the

sample and onto the bus bar). This resulted in a relatively long decay time

constant of about 25 seconds. A delay of several time constants is required

between each measurement after the cryostat is rotated or the background

magnetic field is changed. This slowed down the measurement process and also

resulted in some points being taken too soon after a change, which compromised

the entire data set.

A solution was to add a small resistance (brass shims, approximately

50 nft) to each return wire between the wire and the bus bar. This reduced the

decay time constant from 25 s to 5 s. The added heating at this location was

not a problem. An effort was made to keep the solder from bridging across the

brass shims, since they were located in the stray magnetic field and the

solder would be superconducting for some sample magnetic fields. These

resistors also helped ensure that the current was shared more evenly among the

return wires.

Two pick-up coils were placed on the sample holder to measure the change

in the magnetic field applied to the sample. These two coils were oriented

about 90 deg. apart in order to measure both components of the magnetic

field. These coils verified the estimated decay time constants and the sample

motion problems.

A further observation was that these return wires can shield some of the

time-changing magnetic flux during a fast ramp (large dl/dt) of the sample

current. The shielding effect seems only to be significant when the sample is

not straight, such as a strand extracted from a cable. In such cases errors may occur in the determination of mutual inductance of the voltage taps which

can result in small errors in the critical current determination. Other sources that result in errors in the determination of mutual inductance are self inductance of the sample, shielding currents in the copper matrix, and

sample motion as the Lorentz force is increasing. Although these other sources are present, the return wires are believed to be the dominant source.

Using multiple return wires has some disadvantages, but the advantages

outweigh them. When doing fundamental studies, it was better not to

complicate the measurements with extra magnetic field corrections. The

reduction in the net force and torque on the sample holder also helped lower

the voltage noise.

Motion

One of the identified experimental problems was motion of the sample and

leads. Voltage noise as large as several microvolts can be introduced by

slight motion of the sample holder. This motion is caused by the increasing

Lorentz force as the current is increased during data acquisiton. The net

force on the sample cryostat is not zero if the current return path does not

4
traverse exactly the same magnetic field profile as the sample path. If the

current sharing among the four return wires is not perfect there will also be

a net torque on the sample cryostat. Additionally, stray Lorentz forces and

mechanical vibrations can cause relative motion of the magnet and sample.

Motion of the leads between the top of the cryostat and the voltmeter input

connections will cause voltage due to the stray magnetic field.

The noise was reduced to less than the intrinsic voltmeter noise with the

addition of a band brake device that was mounted on the top of the magnet.

This brake could be tightened around the sample cryostat with a room temperature screw actuator, thus restricting the relative motion between the

magnet and sample.

The band brake device consisted of a power screw, band, stop, and

stainless steel tube. The stainless steel tube goes from the top of the

cryostat to the top of the magnet. The stop consisted of two aluminum angles

fastened to the top of the magnet so that they each were tangent to the edge

of a G— 1 1 flange on the sample cryostat. The band was a stainless steel cable

that loops around the edge of the G— 1 1 flange on the sample cryostat. Springs

were used to hold the cable away from the G— 1 1 flange when not under tension

to allow the sample cryostat to be inserted. The power screw assembly was also mounted on the top of the magnet and it translated a small torque on a stainless steel tube into a large tensional force in a cable. This force

holds the cryostat against the aluminum angles and reduces the relative motion

between the cryostat and magnet.

Pressure Control

The measurement of the temperature dependence of critical current is best

conducted in a constant temperature bath of liquid helium because of the high

current involved and for better sample stability. This measurement needs to be

done when making an SRM, direct comparisons with other laboratories, and

fundamental studies. Since this testing site has a relatively low ambient

pressure (liquid helium equilibrium temperature about 4.0 K) , most of these

measurements are made above ambient pressure. The easiest way to keep a bath

at constant temperature is to use an automatic pressure controller to control

the vapor pressure. One of the unusual aspects of pressure control during the

critical current measurement is that the gas flow can change by 20 to 30

percent caused by the increased boil-off during the data acquisition or while

ramping the magnet.

Several types of controllers have been used. A good reference is a book

on low temperature laboratory techniques by Rose-Innes [7]. In the past, we have used a commercial Cartesian manostat but it was not able to control the

Dewar pressure during large conductor testing. It could not handle the high

boil-off rate, even with 60 to 80 percent of the flow bypassing the

controller. It was also difficult to set the reference pressure. Therefore i diaphragm type manostat [8] was constructed. The design was similar to the

original reference [9]. The key changes were: a plastic diaphragm, a diaphragm limiting plate with holes to reduce oscillation, a large port with

holes to allow for more gradual opening, and a large reference pressure

chamber so that this pressure does not change as much with flow variation.

5
Increasing the impedance in the exhaust port, decreased the pressure at

the intake port as the flow increased. This was because as the difference in

the port pressures decreased, less inlet pressure was required to lift the

diaphragm off the exhaust port. On the other hand, if the exhaust impedance

was set too high, the system pressure would run away. An empirical setting of

the exhaust pressure that allowed a sufficient range of flow rates was determined. This inverse relationship between inlet pressure and flow rate

partially compensated for the increase in pressure drop across the vapor

cooled leads with increased gas flow, thus, stabilizing the dewar pressure

with respect to gas flow rate. Pressure control to ±133 Pa (±1 mm Hg, ±1.4

mK) was achieved with a helium boil-off rate of 8 liters of liquid per hour.

Improvements to 3000 A Supply

Our high-current power supply is a unit manufactured originally as a 3000

A electroplating supply. The manufacturer supplied a single-section filter.

As received, its noise level was substantial. The noise consisted of two

components; spike voltages that exceeded 160 V peak, and 3 - phase current

ripple on the output of 50 A RMS. Also, the output was coupled to ground

through a fairly low resistance. The effect of this was a ground loop that

resulted in systematic measurement errors.

The spike voltages resulted from rapid switching of the transformer

primary windings through silicon-controlled rectifiers (SCR’s). High voltage

0.2 mF capacitors were connected across the SCR's in order to reduce the

switching speed and, thus, reduce the spike voltages. This resulted in a reduction in the spike voltages by several orders of magnitude. Reducing the

switching speed of the SCR’s increases their power dissipation. Consequently,

reduction of the switching speed is limited by their ability to dissipate

power without damage.

The current ripple presented a much more difficult problem. The filter

was cleaned of varnish at all contact points. To reduce the series

resistance of the 0.5 F capacitor bank, originally connected with 10 gauge

wire, it was rewired with heavy bus bars. The net result was a reduction in

current ripple of the output by more than a factor of 2.

The excessive conductance to ground was traced to leakage through the

diode rectifier water cooling system. This was remedied by insulating the

cooling system at all points where it contacted the chassis. This resulted in

leakage current having to travel the full length of the cooling pipes which, in

turn, greatly reduced the conductance from the dc output to ground.

Finally, the cooling system, as set up, was thermostatically controlled.

The thermostat sensed the temperature of a heat sink and turned the cooling

system on and off with solenoid valves. These electrically noisy valves produced massive bursts of noise which were picked up by the low level measurement equipment. The solution was to replace the thermostat with a temperature-controlled , continually variable flow valve. This valve is a passive nonelectrical device.

6
Battery Powered Current Supply

The battery powered current supply has been modified in several areas to

improve its performance. First, the current output capability of the supply

has been increased from 600 A to 900 A. Second, the supply’s susceptibility

to ground loop problems has been reduced. Third, the current output has been

linearized and, fourth, the noise level of the supply has been lowered

considerably. This current supply represents the long term evolution of a much simpler design and as such is not an optimal design.

The original design achieved current amplification through a three-stage

common collector transistor cascade. The final stage of the amplifier con- sisted of a parallel combination of two 300 A power transistors. In order to

increase the current capability of the supply, the present design’s final

stage uses six Darlington transistor pairs, each pair consisting of a 25 A and

a 300 A power transistor. This modification resulted in an increase of 50

percent in the peak current capability of the supply. Presently , the current

capacity of the supply is limited by the internal resistance of the battery

cells (two 1350 A/h, 2 V cells) and the limited output capacity of the control

circuit

.

As noted below in the "Test Method For Low Level Voltage Measurement

Systems" section of this report, ground loops are often a source of error when

making low level voltage measurements of the type required in superconductor

testing. In the case of the battery powered current supply, the input ramp

generator is capacitively coupled to ground. Because it is impossible to

eliminate all additional grounds in the circuit and the test apparatus, the

ramp generator required electrical isolation from the remainder of the supply

circuit in order to eliminate the ground loop problem. Consequently, optical

couplers and a separate dc power supply were used at the input stage of the

supply circuit (a 110 V/15 V supply was used prior to the electrical isolation

boundary and 6 V batteries were used after the boundary - see fig. 1). This

modification resulted in a considerable reduction of systematic measurement

errors

.

A major flaw of the original current supply was that its output was nonlinear. Moreover, a linear voltage ramp input (dv/dt=constant ) did not

result in a linear current ramp output (di/dt=constant ) . This nonlinear

behavior resulted in a variable inductive voltage (due to the di/dt variation)

across the sample's voltage taps. For a ramp input, an inductive voltage is

impossible to avoid. However, if this voltage is constant, it is easily

separated from the desired resistive voltage measurement.

The linearity of the battery current supply was greatly improved through

the use of feedback control. Feedback loops were used at two points in the

circuit, one on each side of the electrical isolation boundary. In both cases operational amplifers were used as comparators between the reference and

feedback signals. Ideally, the first feedback signal would be taken from the same point in the circuit as the input to the second comparator. However,

this would defeat the purpose of the optical couplers by creating an electrical coupling between the input and output stages of the circuit.

Consequently, it was necessary to employ a second optical coupler connected in

series with the first as a source of the feedback signal. Inasmuch as the two

optical couplers are not perfectly matched, this feedback signal is not ideal

7
8

Figure

1.

Schematic

circuit

diagram

of

battery

powered

current

supply.
and may result in some residual nonlinearity. The second feedback signal

originates at the output terminals of the six Darlington pairs. Another

operational amplifier, configured as an inverting summer, combines, inverts

and amplifies these six signals. The resulting signal is then amplified and

inverted by a second operational amplifier in the feedback loop and then

applied to the comparator.

Another source of residual nonlinearity is the limited gain of the

comparators. This limited gain is necessary to avoid oscillation, which is a common problem with this type of circuit. Similarly, the capacitors in the

feedback loops of the comparators are necessary to reduce AC gain. Also,

there is a tendency toward parasitic oscillation in the Darlington output

stage. This instability has been effectively controlled by the addition of

the 0.3 mH inductors at the Darlington inputs.

Overall, these modifications have resulted in very good linearity over

the required current range and load conditions. Figures 2, 3, and 4 indicate

the relative linearity of the system. These are plots of the output current's

deviation from linearity (A I, A) vs. the output current (I, A). The plots

show hysteresis with the lower curve corresponds to increasing current while

the upper curve corresponds to decreasing current. The first plot (fig. 2)

shows the total nonlinearity of the system. The two remaining plots, figures

3 and 4, indicate, respectively, the relative contribution to nonlinearity

made by the electrical isolator (circuitry on input side of isolation

boundary) and that made by the remainder of the system. From these plots it

is evident that the majority of the nonlinearity is due to the electrical

isolator. It is important to note that, although these plots have been scaled

in order to accentuate the deviation from linearity, the percentage deviation

is actually quite small. Moreover, at an output current of 800 A the maximum

deviation from linearity is less than 2.5 percent.

As the current supply design evolved and improved it became desirable to

reduce the level of 60 Hz noise which was present at the output. Several

noise sources were identified; the first of which was the input ramp

generator. The use of a passive filter between the ramp generator and the

supply reduced the input noise; however, a comparable reduction in the output

noise was not realized, indicating the presence of another noise source. An

unused terminal (#6 see fig. 1) on the first optical coupler was suspected of

being a significant noise source. Effective filtration of this noise was achieved by connecting a capacitor from this terminal to the emitter (terminal

#4) of the optical coupler. As a result, the noise level has been reduced

from 0.12 A to 0.01 A, peak to peak. The residual noise may be due to

additional pickup and incomplete filtering. Although the current supply has

been greatly improved, further modifications and a new application of the

design are currently being investigated. Specif icly, several methods for

reducing the nonlinearity of the electrical isolation system have been

proposed and are being tested. Also, the reset time (in the event of a sample

quench the supply must be rapidly reset to zero current) was increased because

of the additional filtering within the control circuit. This longer reset

time is undesirable and a remedy is under development. Finally, a new application for the battery powered current supply is being tested. This

application involves using the battery supply in parallel with the 3000 A current supply (see above for information regarding the 3000 A supply) in

order to reduce the amount of current ripple (harmonics of 60 Hz) and i

9
20

Figure 2. Output currents deviation from linearity versus output current

from the entire supply circuit.

10
20

10

0

10

20

30

40

T T T i—

r

i—

r

l—

r

>'AS\^V

-1 1 1 1 1 1 1 1 1 I 1 I I I

I 1 l 1 i 200 400 600 800 1000

CURRENT , A

currents deviation from linearity versus output current from

the input side of the isolation boundary.

1

1
20

10

0

10

20

30

40 J I l I I l I I I I I I i I I I I I—l

200 400 600 800 1000

CURRENT , A

currents deviation from linearity versus output current from

the output side of the isolation boundary.

12
the total output capability of the 3000 A supply. Reducing the current ripple

would decrease the voltage noise and uncertainty of the current. Initial

feasability tests have been made and the results are promising.

User Notes on Digital Voltmeters

Experience using a digital nano voltmeter with an amplified analog output

has revealed some advantages and some cautions. These properties may or may

not be true for all such instruments.

The advantages of digital nano voltmeters are high noise and common-mode

rejection, high voltage gain, and electrical isolation of amplified analog

output. This isolation is achieved using optical coupling between the digital

and analog systems. This isolation can reduce the risk of ground loops. The

high gain is achieved in the digital-to-analog conversion. The anolog output

is a continuous signal constructed by connecting adjacent digital points with

straight line segments.

Two cautions that should be brought to the user’s attention concern time

lag and internal digital filtering. The time lag of the analog-to-digital and

digital-to-analog converters is about 0.6 and can be accounted for in some

computer aided data acquisition systems. To make low level measurements, a lot of filtering and damping are used to make the display steady for visual

readings. This can slow the response time to the point where it is not useful

for even slow direct current sweeps. Additionally, an internal digital filter

is used that switches in and out depending whether the latest reading is

significantly different from the average. This is done so that the instrument

can respond faster to step functions. This filter can be turned off only over

the computer bus and will switch back in on a bus reset or a front panel

filter or damping selection. With the filter in, a smooth voltage increase

with time will appear as a sawtooth waveform because this filter is coming in

and out with time. This is a problem for critical current measurements and

can be a problem in precise steady state measurements such as resistivity. In

the latter case, the filter can give increased statistical weight to outlying

readings. A passive (constant) filter on the amplified signal and/or

statistical curve fitting of the waveform is required for precise

determinations

.

Test Method For Low Level Voltage Measurement Systems

It can be difficult to measure the low voltages that are required in

critical current testing. A simple technique to check for voltage measurement

problems in a system is described. This technique will provide information on general system integrity, voltage noise, inductive voltage, thermoelectric

voltage, and false voltages for a new system or when a system is significantly

modified. False voltages can arise from ground loops and common mode

voltages. The common-mode rejection ratio (CMRR) is one of the factors that

indicates the meter’s ability to make true voltage measurements with the

presence of common mode voltage. Some of the false voltage could also depend

on the design of the voltmeter’s input stage.

The test can be done at room temperature without the sample cryostat an i vapor cooled leads. A dummy sample is a copper block that has a cross- sectional area that can easily carry several times the peak current. Estimate

13
the resistance per unit length and calculate what separation would provide a typical voltage. Put two voltage taps with this separation near the center of

the dummy sample. The voltage taps can be staggered so that they are almost

at the same potential. The desired separation is one that allows for the same

voltage and current ranges as in the cr itical-current measurement. In this

case, place the dummy sample directly across the ends of the sample current

supply bus bar. If it is desired to use the sample voltage leads, then place

the dummy sample (as a shunt) across the vapor cooled leads and sample.

The first check is a finite resistance measurement where the resistance

between two voltage taps is measured by sweeping the current from zero to full

value and then back down to zero again. The measured resistance can be

checked with the calculated value. This serves as a check for the system, and

results can be compared to previous or future measurements and other

voltmeters. If the dummy sample is not very long compared to its diameter,

the measured resistance can change because of variation in current injection

at the current contacts. An effort can be made to have uniform and/or

consistent current injection. It may be useful to have two pairs of voltage

taps, one pair with the largest separation allowed by the voltage and current

scales and the other closer to the lower limits of the system. The precision

and repeatability of the measurement are easier to check with the more widely

spaced voltage taps. The voltage noise, inductive voltage and changes in

thermoelectric voltage will become more significant for the closer pair.

Three voltmeters (A, B, and C) were tested on a dummy sample using two

sample current supplies (battery and 3000 A SCR). These two supplies are described in separate sections of this report. Meter A, a digital nano- voltmeter with an amplified analog output, is described in the section

entitled User Notes on Digital Voltmeters. Meter B is an analog nanovoltmeter

with amplified output. Meter C is a digital microvoltmeter that has a lownoise, analog preamplifier (gain of 100) with an external output available.

Examples of this test are shown in figures 5, 6, 7, and 8. All of these

figures show voltage versus current for increasing (lower curve) and

decreasing current (upper curve). Figure 5 was taken using Meter A and a 3000 A power supply. Figures 6, 7, and 8 were taken respectively using Meters

A, B, and C with a battery power supply. The measured resistance was about

1.5 nP with some variation due to changes in current injection at the current

contacts

.

The next test, a zero resistance test, will check for ground loop and common mode voltage problems. This test is the same as the first except the

voltmeter input leads are both connected to the same tap on the dummy sample.

This simulates an ideal superconductor - no resistive voltage regardless of

current. In this case the common mode voltage (noise and steady state) and

ground loop voltage are similar to those that occur during the critical

current measurement. There will still be some inductive voltage but the

increasing and decreasing current portions should average to zero (for

constant dl/dt). If the average is not zero, a ground loop or common mode

voltage problem is evident. This test may be employed during an actual

superconductor measurement. However, as in the dummy sample test, the

voltmeter input leads must be connected to the same tap in order to simulate

an ideal superconductor.

14
5

4

3

>

LU C

CD Cl

i

CD>

1

0

-

1

0 1000 2000 3000

CURRENT , A

Figure 5. Finite resistance test, voltage versus current on a copper block

using voltmeter A and a 3,000 A current supply.

15
VOLTAGE

,

uV

0 200 400 600 BOO 1000

CURRENT , A

Figure 6. Finite resistance test, voltage versus current on a copper block

using voltmeter A and a 900 A battery current supply.

16
VOLTAGE

CURRENT , A

Figure 7. Finite resistance test, voltage versus current on a copper block

using voltmeter B and a 900 A battery current supply.

17
VOLTAGE

,

uV

CURRENT , A

Figure 8. Finite resistance test, voltage versus current on a copper block

using voltmeter C and a 900 A battery current supply with load grounded.

18
The ac and dc common mode voltages on the output of the two current

supplies were measured from the load to ground with and without a ground

connected to the load. The ground used in these tests was a good earth

ground, not an ac power line ground. Without a ground on the load the voltage

reference to ground is through leakage paths. For the 3000 A supply, the

leakage path is mainly through the cooling water. In the battery supply the

leakage path is not well defined but it does exist. The 3000 A current supply

had a common mode voltage of about 6.4 V peak to peak and 1 V dc that was fairly independent of output current without a load ground. With a load

ground, the common mode voltage increased with current from 2 mV peak to peak

and 0 mV dc at low current to 5 mV peak to peak and 80 mV dc at 2500 A. The

battery current supply had a common mode voltage on the order of 0.2 V peak to

peak and 1 V dc that was fairly independent of output current without a load

ground. With a load ground, the ac voltage was independent of current and was

less than 1 mV peak to peak and the dc voltage increased with current from 0

mV dc at low current to 1 9 mV at 800 A.

Examples of this test for ground loop and common mode voltage problems

are shown on figures 9~20 for three different voltmeters and two current

supplies. All of these figures show meter voltage versus current for

increasing (solid line) and decreasing (dotted line) current. The 3000 A

current supply would jump up to about 500 A at turn on which results in a spike or lag in the voltage for this part of the curve. Voltmeter A, figures

9, 10, 11, and 12, was the only meter that passed the zero resistance test on both supplies regardless of load ground, however the noise level is fairly

high (about 100 nV peak to peak). Voltmeter B, figures 13. 14, 15, and 16,

did not pass this test on the 3000 A supply, although it was improved by the

load ground in which case the false voltage was on the order of 100 nV.

Without a load ground, the false voltage was on the order of 1 yV. Voltmeter B has the lowest noise level, 10 nV peak to peak, on the battery supply without

a load ground. Voltmeter C, figures 17, 18, 19, and 20, had the most trouble

with common mode voltage, giving about 230 yV of false voltage on the 3000 A supply without a load ground and 10 yV with a load ground. Voltmeter C had a small problem, 100 nV, with the battery supply without a load ground and was good with a load ground. In all cases the noise level was lower on the

battery supply as expected.

The tests described above only provide a means of determining whether or not a measurement system is producing false voltages. Both common mode

voltages (ac or dc) and ground loops may result in false voltages.

Generalizations about the source of these problems cannot be made because they

are system dependent. However, the value of these tests is not diminished by

this fact. In our case adding a load ground reduced the common mode voltages

and thus reduced the size of the effect.

Opto-isolators or optical couplers provide a good means of reducing

ground loops in critical current measurement systems. Many power supplies,

ramp generators, and voltmeters have a capacitor from the circuit common to

ground. Some instruments have the circuit common connected directly to

ground. Also in some types of testing, such as electromechanical, grounding

the sample cannot be avoided. Optical coupling is now available in

differential preamplifiers and in the amplified analog outputs of some digit r. voltmeters (see user notes on digital voltmeter section). They can also be

incorporated into quench detectors (see appendix B) . Optical coupling can

19
60

40

20

0

20

40

60

i—i—i—

|—i—i—i—

|—i—r—i—|

i—i—i—

|—i—i—

r

_i i i 1 i i i I i i i I i i i I i i i 200 400 600 BOO 1000

CURRENT , A

istance test, meter voltage versus current on a copper

er A and a 900 A battery current supply.

20
60

40

20

0

20

40

60

i—i—i—

|—i—i—i—

|—i—i—i—|

i i r i—

r

200 400 600 800 WOO

CURRENT , A

istance test, meter voltage versus current on a copper

r A and a 900 A battery current supply with load grounded.

21
VOLTAGE

,

uV

CURRENT , A

Figure 11. Zero resistance test, meter voltage versus current on a copper

block using voltmeter A and a 3,000 A current supply.

22
VOLTAGE

0 1000 2000 3000

CURRENT , A

Figure 12. Zero resistance test, meter voltage versus current on a copper

block using voltmeter A and a 3,000 A current supply with load grounded.

23
60

40

20

0

20

40

60

ti—i—

|—i—i—i—

|—i—i—i—

|—i—i—i—

|—i—

r

_i i i 1 i i i I i i i 1 i i i I i i i 200 400 600 BOO 1000

CURRENT , A

istance test, meter voltage versus current on a copper

r B and a 900 A battery current supply.

24
60

40

20

0

20

40

60 J l l I I 1 l I l l l I i I i 1 i i i 200 400 600 BOO 1000

CURRENT , A

istance test, meter voltage versus current on a copper

r B and a 900 A battery current supply with load grounded.

25
VOLTAGE

,

uV

CURRENT , A

Figure 15. Zero resistance test, meter voltage versus current on a copper

block using voltmeter B and a 3»000 A current supply.
VOLTAGE

,

uV

CURRENT , A

Figure 16. Zero resistance test, meter voltage versus current on a copper

block using voltmeter B and a 3,000 A current supply with load grounded.

27
150

100

50

0

-50

100

150

200

250

300

Zero

t

i—i—

|—

r

j i i I i 200

istance test,

r C and a 900

i—i—

|—i—i—i—|—i—i—i—|—i—i—

r

_J J I 1 I 1 1 I I 1 1 ! I I 400 600 BOO 1000

CURRENT , A

meter voltage versus current on a copper

A battery current supply.

28
0 200 400 600 BOO 1000

CURRENT , A

Figure 18. Zero resistance test, meter voltage versus current on a copper

block using voltmeter C and a 900 A battery current supply with load grounded.

29
300

200

CD<

CD

100 -

0

0 1000 2000 3000

CURRENT , A

Figure 19. Zero resistance test, meter voltage versus current on a copper

block using voltmeter C and a 3,000 A current supply.

30
20

10

0

10 ]- i l i i i i i 1 i i i i i i i i i I i i i i i i i i i 1000 2000 3000

CURRENT , A

tance test, meter voltage versus current on a copper

C and a 3»000 A current supply with load grounded.

31
also be used between the ramp generator and a sample current supply (see

section on battery powered current supply).

STABILITY

There are many theoretical stability criteria which fall into three

categories: flux-jump stability, full cryostability , and limited cryostability. Flux-jump stability is based on the thermal diffusivity of the

superconductor being greater than its magnetic diffusivity, so that a magnetic

flux jump will not cause a thermal runaway event. The cryostability criteria

are based on the power balance equation which compares the disturbance heat

generation and joule heating to the cooperative effects of heat transfer to

the cryogen, heat conduction, and heat storage. Each criterion corresponds to

a solution to this equation with different simplifying assumptions. A magnet

that has full cryostability will recover from a disturbance of any size, such

as an infinite normal zone. A magnet that has limited cryostability will

recover from disturbances of only a limited size.

The goals of this area of the research program are consensual definitions

of terms arid standard test methods. We have started to acquire the data base

needed to create consensual standard test methods for the determination of

stability parameters that are intrinsic to the wire and, thus, are figures of

merit for the wire regardless of the details of the application. These

variables could include resistivity, thermal conductivity, heat capacity,

orientation-dependent surface heat transfer, and stability temperature

margin. The stability temperature margin is calculated using J

C (H,T).

The topics covered here are resistivity, copper-to-superconductor volume

ratio, and critical current as a function of temperature.

Resistivity

We have the capability of routine resistivity ratio measurements of

superconductors. The data acquisition and plotting is computer assisted. The

measurement can be performed in a liquid helium shipping Dewar. The main com- plication of this measurement is determining the low temperature sample resistance just above the superconductive transition temperature in an easy and

routine way. For the low temperature data, the resistance of a carbon

resistor was used as an indicator of temperature. The sample and carbon

resistances were measured at various positions in the shipping Dewar.

A typical set of data is given in figure 21 . The resistance of the

carbon sensor increases with decreasing temperature, so the temperature is

decreasing to the right on the x-axis. There is a variation in the carbon

resistance at which the superconductive transition occurs, because the sample

and carbon sensor are not perfectly thermally coupled and the system is not in

thermal equilibrium. However, this does not prevent a precise determination

of the normal state resistance just above the transition. A more sensitive

plot of these data in the plateau region allows for a resistance determination

to within ±0.1 percent.

32
Figure 21. Sample resistance versus resistance of temperature sensing carbon

resistor (resistance of carbon resistor increases with decreasing

temperature)

.

33
A useful equation was developed which relates the copper residual

resistance ratio (RRR) and the RRR of the composite superconductor . Start

with the following definitions:

H is the composite resistance at room temperature,

C is the composite resistance at low temperature,

p c is the room temperature resistivity of the copper,

P s is the room temperature resistivity of the non-copper material,

A

c is the area of the copper,

A

g

is the area of the non-copper material,

R is the resistance of the copper at room temperature,

x R is the resistance of the non-copper material at room temperature,

w is the RRR of the composite (H/C),

y is the RRR of the copper,

z is the RRR of the non-copper material,

where x = (

p

s Ac )/(pc A

g

).

Then

H = xR/(x+1) and C = xR/(xy+z);

theref ore

w = ( xy+z )/ (x+1

)

and thus

y

= w + (w-z)/x.

The last equations involve dimensionless variables using these

definitions. In the limit that x goes to infinity, y approaches w. In the

limit that z goes to w, y also approaches w. For most composite

superconductors; y, copper RRR, will be greater than w, composite RRR.

Copper-to-Superconductor Volume Ratio

A careful study of assumptions and technique was done on the copper-tosuperconductor volume ratio measurements as part of another program in our group at NBS. This other research is on transient losses in superconductors

for the Air Force Office of Scientific Research [10].

Stability Apparatus

As part of another program an apparatus to study stability has been

constructed and preliminary measurements performed. This other research is on copper stabilizers in superconductor for the International Copper Research

Association by F. R. Fickett. In collaboration, preliminary measurements of

the critical current as a function of temperature are being made.

Calibrations of the thermometers in magnetic field have been made. A

publication is in process.

34
STANDARD REFERENCE MATERIAL ( SRM)

The small conductor (NbTi) critical-current SRM (No. 1457), and further

advertisement of the critical current standard test method (ASTM B714-82) will

help all critical current measurements. As the technology matures, other

critical current and stability SRM's will be useful. Possible critical

current SRM’s that may be useful are: large conductor monolithic NbTi, large

conductor NbTi cable, and small or large Nb^Sn conductors. Magnet designs may

need some settling time and more understanding before a stability SRM can be

developed

.

The cost of producing an SRM involves more than the purchase of material

and its characteri zation. There are costs to refine the characterization

technique to achieve accuracy and precision levels beyond those of an ASTM

standard in order to reduce the overall uncertainty of the SRM. There are also costs associated with preliminary screening of candidate materials in

order to pick the one best suited for this application. Limited funding is

sometimes available from the Office of Standard Reference Materials (OSRM) to

produce SRM’s, but the total cost of development must be passed along to the consumer. The research to develop an SRM must be subsidized with money from

NBS and other interested users such as DoE.

Our choices and direction for the SRM development program will be based

on recommendations and feedback from ASTM committee members (B01.08) and the

research community. Some of the topics to follow address these issues and our present ideas.

Nb

3

Sn

A NboSn SRM may now prove quite useful, although this material presents

additional problems. The problems of current transfer voltage, reaction

conditions, and strain sensitivity would make this very difficult. An

argument can be made that, for these reasons, a Nb^Sn SRM should not be

developed yet. The counter-argument is that it should be done for these very

reasons, assuming it can be done. The difference is in the philosophy of its

usage. Should an SRM check out the measurement system or technique? If a laboratory makes measurements on Nb^Sn then it is dependent on its technique,

including all of the extra measurement problems that Nb^Sn has.

If a NboSn SRM can be produced, it would provide calibration to higher

magnetic fields. With a 12 T magnet on order, we would plan to certify to

that field. The capacity of the conductor would most likely be at or below

600 A at the lowest certified magnetic field, which at this point is thought

to be in the range of 4 to 6 T. This would provide some overlap in field with

the NbTi SRM. As was done with the NbTi SRM, additional non-certif ied

measurements could be made at higher magnetic fields and as a function of

strain. Since these measurements are not done on site, the control, time, an!

statistics are not adequate for certification.

The state of the art in Nb^Sn conductors is not perceived as being as mature as NbTi was when it was developed, especially in terms of critical

current homogeneity. Other concomitant variables are current transfer- l‘ , reaction schedule, irreversible strain, and strain sensitivity.

35
Large Conductor NbTi

The potential benefits of another NbTi SRM would either be much higher

current or higher homogeneity. At this point the possible quantum improvement

with higher homogeneity is not significant. However, there is adequate margin

for increasing the current level with our present capabilities, 3000 A. The

small SRM’s critical current was 300 A at 2 T and 75 A at 8 T. We could

increase this by a factor of ten (or 15 if we start at 4 T). A higher current

SRM would allow for calibration of high current test facilities.

The choice between cable and monolithic conductor depends on bias

introduced by the latest magnet design and some questions similar to those

raised on the NboSn, ease versus reality. Most of the big magnet designs now

use cable conductors. Cable conductors are more difficult to measure with the concerns about gauge length, local variations in critical current, and current

transfer among strands. As in Nb^Sn, that may be the reason for developing a cable SRM, if it can be done.

Two other fundamental questions are cost and number be used. The number

of potential users decreases as the current level increases. Also the

material and testing cost would be much higher for a larger conductor.

Self-Field

This topic is an open question: should critical current measurements be

corrected for self-field? This topic is of concern for an SRM but it is also

important for a large conductor test method. Self-field is the magnetic field

produced by the transport current of a conductor. For the first SRM, we did

not make a self-field correction because the same field is present for all

measurements made with transport current. We did attempt to make a smaller

correction for the magnetic field produced by the geometry of the test sample

(inductive coil). This was done to correct to the more ideal case of a long

straight wire.

There are times when a self-field correction is important such as in

comparing the current density of different size wires or single strand to

cable measurements [11]. The effect will be more significant for the larger

conductors and is a bigger source of error whether the correction is done or not. The answer is not obvious.

European Economic Community Wire

We made some measurements of critical current on a wire provided by the

European Economic Community. This was a NbTi wire that was used in their

round robin testing and put forth as a reference material. We were interested

in comparing round robin measurements and determining whether this wire had

short range variation similar to SRM 1457.

This reference material was selected based on its parameters and

availability [12], This is a simple conductor of which many tons had been

produced. The wire has 24 filaments (66 ym diameter) arranged equidistant on

the same radius to reduce internal current transfer problems. Other conductor

parameters are: copper-to-superconductor ratio of 7.44, insulated wire

diameter 1 mm, uninsulated wire diameter 0.94 mm, and filament twist pitch of

36
45 mm. This conductor was believed to have high homogeneity, little variation

in superconductive properties. A round wire avoids differences in critical

current with rotation of the magnetic field vector around the wire axis. A

high copper-to-superconductor ratio will increase cryogenic stability.

Seven European laboratories made measurements on specimens of this wire

[13]. Measurements were made at magnetic fields from 0 to 10 T, but not all

of the participants reported critical currents at all of the integer values of

magnetic field. Vacuumschmelze made additional measurements on specimens

adjacent to the specimen of each participant. Rutherford Appleton Laboratory

made three sets of measurements making a comparison among specimens that were

held on the test fixture with; oil (insulation left on the wire), solder, and

a specimen that was removed and soldered again.

Table 1 has a summary of the critical currents, percentage range of

reading, and number of readings (including the multiple measurements). The

percentage range of critical currents seems to increase with magnetic field

from 2 to 6 T. The smaller than expected range at 8 T is probably due to the

lower number of readings taken there. These data in the range of 4 to 8 T were fit to a linear relationship of current with field. The current and

magnetic field intercepts were used to compare the respective calibrations.

This assumes linear calibration differences. If all of the differences among

these data were due to calibration differences only, the differences would

have to be ±3 percent in current and ±2 percent in magnetic field.

Vacuumschmelze * s measurements on 6 specimens indicated that the wire

properties vary by about ±2 percent in current and ±1 percent in magnetic

field. This type of analysis was not done on SRM 1457 but it would be

similar

.

Some of the conclusions of the European laboratories [13] were that the

range of critical currents were due to variations in: the sample, the method

of mounting and measurement, and the sample temperature. The attempt to

normalize the measurements with the Vacuumschmelze adjacent specimen did not

reduce the deviations, indicating that the sample variation was significant

over the length of a meter. The results of the Rutherford Appleton Laboratory

study indicated a possible progressive 1 percent reduction with soldering a specimen. Also the absolute pressure of the cryostat should be recorded.

The measurements at NBS on this wire were not included in the above

mentioned study, but our results were consistent with theirs. The

measurements at NBS were made on two sample holders in the same background

magnet. The geometry of the two holders were an inductive coil and a short

straight. Table 2 has a summary of these measurements with critical currents

and percentage differences. The coil values are within 2 percent of the

average European value. The difference between coil and short straight is

probably due to a bending strain effect. The bending strain effect was expected to be large because the wire diameter is relatively large and most of

the filaments are off the neutral axis.

Three pairs of voltage taps were placed on the coil sample to determine

the short range variation in critical current. The tap separation of each

pair was 2 cm. Adjacent pairs were separated by about 10 cm. The percentagerange of critical current values increased monotonically with magnetic field

37
Table 1

Summary of critical current measurements by European Economic Community on their reference material.

Magnetic

Field

T

Average

No. of Critical Current, Percentage

Readings A Range

2 15 332.1 4.5%

4 17 225.7 6 . 2%

6 17 1 44.9 10 . 4 %

8 6 68.5 5 . 3%

38
Table 2

Summary of

reference

critical

material

.

current measurements by NBS on European Economic Community

Magnetic

field, T

l c

, A on coil

holder

I c

. A on straight

holder

Percentage

of reference

between coil

and straight

Short range

between

variation

on coil

2 332.5 3^2.6 2.9% 0.62%

4 227.2 234.1 2.9% 0.81%

6 147.4 152.3 3.2% 1.01%

8 67.6 71 .1 4. 9% 1 .39%

39
from 0.6 percent at 2 T to 1 . 4 percent at 8 T. These short range variations

are similar to those observed in the preliminary screening of standard

reference materials at NBS.

Nb^Sn Round Robin

We participated in the USA (DoE) and Japan round robin to measure the

critical current of two select multifilamentary Nb^Sn conductors. This was

the first attempt at international standardization of the measurement of

critical current for this important class of high magnetic field

superconductors . The goal of this round robin was to determine the degree of

agreement among measurements made in different laboratories and identify

sources of any disagreement. Six U.S. laboratories and six Japanese

laboratories made critical current measurements. Additional measurements of

homogeneity and temperature dependence of critical current were performed and

evaluated by NBS for each conductor. This information will be useful in

deciding the course of the NBS superconducting wire Standard Reference

Material program.

We need to make further evaluations before we can draw conclusions. A

discussion of the results and summaries were presented to the community by some of the participants at the Fourth U.S. -Japan Workshop on Superconducting

Materials for Fusion, Feb. 26-28, 1986 in Hedgesville, West Virgina. The

range of measured values for the U.S. participants [14] was 14 percent at 8 T

and 20 percent at 12 T.

SUMMARY

Progress has been made on recognizing, understanding, and in some cases reducing the effect of experimental difficulties that are substantial for

large conductor critical current measurements. The basic characteristics of

the current supply and voltmeter have a major role in the precision and

accuracy of the measurement. A simple test method was outlined which can be

used to test the system integrity. In conjunction with this progress, there

have been two publications during this reporting period, Appendices A and B. Preliminary efforts have been made in stability and future Standard Reference

Materials. The overall result of this work has been an increase in fundamental understanding, technology base, and experimental capabilities.

ACKNOWLEDGMENTS

The authors extend their thanks to W. E. Look for assistance with the

computer; to R. A. Shenko for preparing this report; and to the rest of the

Superconductor and Magnetic Measurements Group.

REFERENCES

[1] L. F. Goodrich, D. F. Vecchia, E. S. Pittman, J. W. Ekin, and A. F. Clark, "Critical Current Measurements on an NbTi Superconducting Wire

Standard Reference Material," NBS Special Publication 260-91 (1984).

40
[2] L. F. Goodrich, W. P. Dube, E. S. Pittman, J. W. Ekin, and A. F. Clark,

"The Effect of Aspect Ratio on Critical Current in Multifilamentary

Superconductors ," Adv. Cryo. Eng. -Materials , Vol . 32, Plenum Press, New

York (1986).

[3] W. P. Dube and L. F. Goodrich, "Quench Detector Circuit for

Superconductor Testing," Rev. Sci. Instrum., 57, 680-682 (1986).

[A] J. W. Ekin, National Bureau of Standards, Boulder, Colorado, private

communication

.

[5] This work was originally presented informally at the 4th Workshop on

NbTi Superconductors (July 1985 Madison, Wisconsin). Discussions are acknowledged, especially with C. King, Intermagnetics General

Corporation, Guilderland, New York.

[6] J. W. Ekin, National Bureau of Standards, Boulder, Colorado, private

communication

.

[7] A. C. Rose-Innes, Low Temperature Laboratory Techniques, English

Universities Press Ltd., London, 1973.

[8] M. M. Kreitman, "Simple Diaphragm Pressure Regulator for Cryogenic Bath

Temperatures," Rev. Sci. Instrum., 35, 749-750 (1964).

[9] J. E. Ostenson, Ames Laboratory, Ames, Iowa, private communication.

[10] R. B. Goldfarb, "Transient Losses in Superconductors," NBSIR 86-3053,

National Bureau of Standards, Boulder, Colorado (June 1986).

[11] M. Garber and W. B. Sampson, "Critical Current Anisotropy in NbTi

cables," IEEE Trans, on Magnetics, MAG 21, 336, 1985.

[12] European reference material provided by K-J. Best of Vacuumschmelze

,

Hanau, West Germany.

[13] C. R. Walters, Rutherford Appleton Laboratory, Oxfordshire, England,

private communication.

[14] D. C. Larbalestier , University of Wisconsin, Madison, Wisconsin, private

communication

.

41

APPENDIX A

International Cryogenic Materials Conference - ICMC

Cambridge, Massachusetts - August 1985

To be published in:

"Advances in Cryogenic Engineering (Materials)"

Vol. 32, R. P. Reed and A. F. Clark, eds.

Plenum Press, New York (1986)

THE EFFECT OF ASPECT RATIO ON CRITICAL CURRENT IN MULTI FILAMENTARY

SUPERCONDUCTORS *

L, F. Goodrich, W. P, Dube, E. S. Pittman, and A. F. Clark

National Bureau of Standards

Boulder, Colorado

ABSTRACT

Experimental data and discussion are presented on the critical current of

straight superconductors as a function of the orientation of a perpendicular

applied magnetic field. Commercial, multifilamentary NbTi and Nb^Sn

samples were measured in a radial access magnet that allowed an arbitrary

angle setting. The change in critical current was measured at different

magnetic fields to scale the effect for use in a standard test method. For

a NbTi sample, the critical current with the magnetic field parallel to the

wider face of the conductor is higher than that with the perpendicular

orientation. The effect can be as high as 40% for a NbTi sample with an aspect ratio of six. The effect in Nb^Sn is opposite that in NbTi. A

discussion of the most likely cause of the effect, which accounts for the

difference between NbTi and Nb^Sn, is given.

INTRODUCTION

The aspect ratio effect is defined as the variation of critical current,

I , with sample-field orientation in a constant applied magnetic field

perpendicular to the wire axis. The aspect ratio of a conductor or filament

is defined as the ratio of the cross sectional dimensions (wide/narrow).

The emphasis of this study was measurement of how the critical current

varied with the angle of constant applied magnetic field. The most likely

cause of the aspect ratio effect is the asymmetric deformation of the

filaments and, thus, the orientation of the flux-pinning sites.

The aspect ratio effect on critical current is important for most

applications where a monolithic rectangular conductor is used. It may also

be of importance for understanding the critical current of cable conductors

that have been deformed during compaction. The difference in I can be 20’

for a NbTi conductor with an aspect ratio of two (40% for a rat5o of six).

The present American Society^for Testing and Materials (ASTM) critical

current standard test method suggests a measurement orientation with the

magnetic field parallel to the wider face of a rectangular sample unless

otherwise specified, but this orientation is difficult to obtain in the

* Work supported in part by the Department of Energy through the Office of

Fusion Energy and the Division of High Energy Physics.

Contribution of NBS, not subject to copyright.

43
hairpin geometry. Also, in some applications, such as the end turns of a dipole magnet, might require the measurement in the other (or both) orientations .

EXPERIMENTAL

The critical current measurements reported here were made using a straight sample geometry and a radial access magnet. Samples are identified

in Table 1. The critical current was defined as the current at which the

electric field strength was 0.2 yV/cm. The sample was centered in a holder, and four conductors (symmetrically located on a 2-cm diameter

circle) were used for the return current path. This arrangement reduced

the effect of the magnetic field from the return current path and reduced

the net torque on the sample cryostat. The sample was held in, and electrically isolated from, a machined brass rod holder with epoxy. The rest of

the structure was fiberglass epoxy.

Two experimental problems were identified and their effects on precision

and accuracy reduced. The first was voltage noise, as large as several yV,

introduced by slight motion of the sample during acquisition of the voltagecurrent (V-I) curve. The net force on the sample cryostat is not zero if

the current return path does not traverse exactly the same magnetic field

profile as the sample path. Additionally, stray Lorentz forces and mechanical vibrations can cause relative motion of the magnet and sample. The

noise problem was reduced to the 0.1 yV level with the addition of a band

brake device that was mounted on the top of the magnet. This brake could be

tightened around the sample cryostat with a room temperature screw

actuator, thus restricting the relative motion of the magnet and sample.

The second problem was persistent currents in the four parallel return

wires, because the wires were soldered together a£ each end and the magnetic

flux was changed each time the angle was changed. These persistent

currents would cause a time dependent, partial shielding of the sample from

the applied magnetic field. The decay time constant was reduced from 25 s to 5 s by adding a small resistance (brass shims, approximately 50 nft) on the end of each lead. These resistors also helped to ensure that the

current was shared more evenly among the return wires. This reduced the

net torque and effects of the magnetic field of the return path to a negligible level.

The orientation of the sample was changed by rotating the sample

cryostat relative to the background magnet. A pin in a series of circumferential holes at the top of the cryostat and the band brake mentioned above were used to hold the orientation. The zero angle was defined as the

orientation with the magnetic field parallel to the wider side of the

conductor. The zero position in the raw data was only approximate because

of variation in how the sample was mounted in the holder.

Two experiments were conducted to test the apparatus and the effect.

The first experiment was to measure I of a large (I 1200 A at 8 T) round

wire as a function of angle from 0° to 360°. The measured critical currents

were independent of angle within the experimental precision of ±1%. This

result indicated that the apparatus had no artificial angular variation due

to the applied magnetic field adding or subtracting from the magnetic field

of the return leads.

The second experiment was a comparison of measurements made on a monolithic conductor and on a conductor composed of two of these monoliths

soldered together along their length. This comparison was not ideal, but

it allowed a direct comparison of aspect ratio effect and critical current.

There were no indications of current-sharing problems in the I measurements.

44
Table 1. Physical Parameters of the Samples Measured.

Critical Current at 8 T for NbTi and 10 T for Nb^Sn.

Sample Type Conductor Filament I , A AI , % Cu/non Cu

number aspect aspect ratio

1 NbTi round ^ 1.0 1215 < 1 2

2 NbTi 1.28 ^ 1.11 114 5 1.8

3 NbTi 1.93 1.28 to 1.41 350 19 to 29 1.25

4 NbTi 2.0 1.15 to 1.17 700 19.5 04-

5 NbTi 2.0 ^ 1.24 600 17.5 5

6 NbTi 6.0 1.61 to 1.93 2350 42 5

7 Nb

3

Sn 2.0 'V' 1.92 42 15.5 1.63

8 Nb

3

Sn 2.0 * 3.15 1020 19 ?

Within the experimental precision, I of the two-monolith conductor was

twice that of the single monolith anS the aspect ratio effect was the same percentage of I . These results suggested that the demagnetization factor

of the overall conductor did not have a significant effect.

DATA

The general dependence of I on angle was similar for all samples

measured. It was more broad at 0° than at 90° an^ fell between sin 6 and

sin 0. Typical data are shown on Fig. 1 with sin 0 and sin 0 for reference.

The filaments were not perfectly aligned with the surface of the conductor;

some were tilted slightly on either side of alignment. This caused the curve to be flat around 0° and is the most likely cause of the small peaks

observed in some of the samples (see Fig. 2) on either side of 0°.

The shape of I with angle scales approximately as a percentage of I for the magnetic field range of 2 to 10 T (see Figs. 2, 3, and 4). Obviously,

the effect at zero magnetic field must be zero and should approach zero continuously. In general, the effect at 2 T was a little lower percentage

than at higher magnetic fields. The lack of a strong field dependence

suggests that the fluxoid-core size and fluxoid spacing does not have a large effect on the resulting pinning force.

The aspect ratio effect in Nb-Sn (see Fig. 4) is opposite that of

NbTi, but it is comparable in magnitude for similar conductor aspect

ratios. Two types of multifilamentary, bronze process Nb«Sn conductors

were measured, one made using the conventional process and the other with

the jelly-roll process. Results were consistent between these two conductors.

A Nb^Sn tape measured previously3 had an aspect ratio effect in the same direction as these multifilamentary Nb^Sn conductors.

The amount of asymmetric filament deformation, filament aspect ratio, was determined from photographs of the wire cross sections. A large number

of filaments (in some cases all of the filaments) were averaged by summing

the ratio of width to thickness and then dividing by the number of filamt: r to be averaged. These ratios are listed in Table 1.

45
CRITICAL

CURRENT

,

Fig. 1. I versus angle for a NbTi

conductor with an aspect

ratio of 6, sample #6.

Fig. 2. Percentage change in

I versus angle for

NfjTi sample #5.

Fig. 3. Percentage change in I versus angle for NbTi

sample #3.

ANGLE , degrees

Fig. 4. Percentage change in

I versus angle for

N^Sn sample #7.

Some filament aspect ratios of some wires varied by more than a factor

of 2 for cross sections separated by less than a few centimeters along the

length of the wire. Two such cross sections of the same wire are shown in

Fig. 5. Further observation indicated that this variation might be occurring periodically along the conductor. As a result of this, another sample was prepared with a number of closely spaced voltage taps to look at short

46
range variation in I and aspect ratio effect. Figure 6 is a plot of I versus position along the wire using adjacent voltage taps separated by

about 0.23 cm. Because of the close tap spacing (low voltage with E = 0.2

yV/cm) and I variation (quench current determined by the lowest I ) a number of repeat determinations were made.

The variation in I was about 12% for the 0° data and 10% for the 90°

data. The variation aiso looked periodic for both 0° and 90°. There was a high point for both the 0° and 90° data, starting from the left, approximately every third point. There was also a variation in the aspect ratio

effect along the wire. Consider the third position: the 0° point was relatively low and the 90° point was relatively high, this resulted in a small aspect ratio effect. The situation was reversed for the fifth

position, which results in a large aspect ratio effect and alternates along

the wire. The aspect ratio effect varied from 19 to 29%.

The cause of this periodic variation in filament shape, I , and aspect

ratio was most ^ikely the combination of filament twist and hexagonal

filament array. Filament deformation was extreme when the flats of the

hexagonal filament array were parallel to the minor dimension, where the

filaments were on top of each other. A transverse view of the filaments

etched from the matrix indicated that the wavelength of the extreme filament

deformation was about 0.28 cm, which was about 1/6 of the twist length.

The length of the flattened filament was about 0.1 cm. The fact that the

tap separation was about the same as the deformation wavelength, apparently

resulted in a beating of the two, forming a third artificial wavelength in

the variation of I . The measured 1 values were consistent with a 0.28 cm

c c wavelength but the actual amplitude of the variation may be larger.

Measurements with a smaller tap separation, on the order of 0.05 cm, would

have been very difficult because of the extremely low voltages.

Fig. 5. Photographs of two different

cross sections of the same NbTi wire, sample // 3.

Fig. 6. Variation of I

Q

versus position for 0°

and 90° at 8 T and

Ec =0.2 yV/cm for

sample //3.

47
The cross sections of other rectangular conductors were examined more

closely considering this periodic filament deformation. All of the others

measured showed some change in deformation as indicated by a change in the

filament array. Two cross sections of the same wire are shown in Fig. 7.

Notice the change in the shape of the center copper island and difference

in filament deformation. Voltage tap and microscope studies did not reveal

another case as extreme as Fig. 5, but there was always some variation.

The physical parameter that may have been unique to the sample shown in

Fig. 5 was the low copper to superconductor ratio 1.25/1.

Some measurements were made on a seven-strand NbTi cable, which had

been compacted into a rectangular cross section. A photograph of the cable cross section is given in Fig. 8. A strand changes from almost round in

the middle top position to deformed at the corner and side positions of the

cable. Despite current sharing among the strands, V-I curves on short

sections of individual strands gave some indication of the local 1^.

A number of voltage taps were placed along individual strands to

measure I for different cable positions. Three regions of the cable were

measured: an "overall" region encompassing two cable twist lengths of a strand, a "top" region encompassing only the portion of a strand on top of

the cable, and a "side" region encompassing the comers and side portions.

The shorthand notation for these three regions from here on will be:

overall, top, and side. Voltage taps for the top and side measurements

were placed on the top and bottom (wider faces) near the corner of the

cable on a given strand. One of these voltage taps was used for both the

top and side measurements. This placement caused most of the corner region

to be included in the side measurement. Thus, all strand regions were

included in either the top or the side measurements.

The cable I data had a combination of aspect ratio and self field

effects. The self field of the conductor vectorially added to the applied

magnetic field, which resulted in different magnetic fields for different

angles and cable positions. The I of three regions of the cable versus

angle are given in Fig. 9. The overall I , circles, had some structure

approximately every 45° but the variation was less than 2%. Combining the

aspect and self field effects, the top I as a function of angle should

have been ordered in magnitude as follow!; 180° > 0° > -90° and 90°. The

side I should have been ordered in magnitude as follows; -90° > 90° > 180°

and 0°. In fact, these experimental data and data on two other strands were consistent with this ordering. The only difference was a splitting of

the lower critical currents. This was caused by a mixture of top and side

characteristics due to the finite width of the transition from top to side.

Notice that I for -90° top was greater than 90° top because I for -90°

side was at a maximum. Also, I for 180° side was greater than 0° side

because I for 180° top was at a maximum,

c
Fig. 8. Photograph of the cross section of a NbTi cable.

NbTi cable, see Fig. 8.

The size of the aspect ratio and self field effects can be estimated

with the assumption that the self field effect can be removed by averaging

the critical currents taken 180° apart. Using this assumption at 8 T, the

top has an aspect ratio effect of 8% and a self field effect of 3%. The

side at 8 T has an aspect ratio effect of 14% and a self field effect of

6%. At 4 T, the aspect ratio and self field effects respectively are: top

6% and 5%; side 19% and 10%.

DISCUSSION

The most likely cause of the aspect ratio effect is the asymmetric

deformation of the filaments and thus the orientation of the flux-pinning

sites. If the magnetic fluxoid is more closely aligned with the pinning

^ site, the pinning force will be larger than in the nonaligned orientation.

Extending to the whole conductor cross section, the total pinning force

will be anisotropic. In the case of NbTi, the pinning sites (precipitates,

dislocations, and impurities) have a preferred orientation parallel to the

wider side of the filament (the wider side of the overall conductor) thus

the higher critical current with the magnetic field in that orientation.

For Nb_Sn, the pinning sites (grain boundaries) have a preferred orientation

perpendicular to the wider side of the filament, due to columnar grain

growth; therefore, Nb^Sn has the opposite orientation effect. Even though

there is a mixture of equiaxial and columnar grain growth, the presence of some columnar grains apparently causes the total pinning force to be

asymmetric

.

The effect in Nb^Sn is opposite that of NbTi, but it is comparable in

magnitude for similar conductor aspect ratios. Due to the relative hardness, the Nb filaments in the bronze matrix (Nb^Sn) is deformed more than

the NbTi filaments in the copper matrix. However, as mentioned above, th»

mechanism is different for the two materials and the resulting effect on I is similar.

49
The lack of a strong field dependence of the percentage change in I with angle suggests that the fluxoid-core size and^fluxoid spacing do no?

have a large effect on the resulting pinning force . The samples measured

in this study had a minimum amount of area reduction in going from a round

to a rectangular cross section. However, for thin, highly deformed pinning

sites, the change in fluxoid-core size with magnetic field could cause a strong field dependence of 1^ with angl^ for NbTi. A sign reversal has

been observed in rolled NbTi conductors between low magnetic fields (I

higher at 0°) and high magnetic fields (I

c higher at 90°).

In order to make these results more general, an empirical relationship

between the size of the effect and some physical parameter was sought.

With a predictive relationship, the size of the effect in more complex

conductors, such as compacted cables, could be estimated. The aspect ratio

of the filaments was the obvious choice. However, as shown above, there was too much variation for even a simple estimate. There might also be a dependence on the manufacturing technique. These data exemplify the

magnitude and shape of the aspect ratio effect.

CONCLUSIONS

The aspect ratio has a significant effect on the critical current of

rectangular monolithic and compacted cable NbTi and Nb„Sn conductors. A

20% change in critical current with angle in a constant applied magnetic

field is typical. The most likely cause of the aspect ratio effect is the

asymmetric deformation of the filaments and thus the orientation of the

flux-pinning sites. The effect in Nb~Sn is opposite that of NbTi due to

the different kinds of pinning sites in the two materials. Aspect ratio

and self field effects can be observed in I measurements on a cable, which

would account for some of the degradation o? I from strand to cable.

Large variations in filament deformation, critical current, and aspect

ratio can be observed in rectangular conductors along the length of a wire.

ACKNOWLEDGMENTS

The authors extend their thanks to M. S. Allen for drafting and cryostat

construction; F. R. Fickett for being an always available sounding board;

J. W. Ekin for discussions of flux pinning; R. G. Benson for making sample

holders; R. A. Shenko for preparing this paper; and to the rest of the

Superconductors and Magnetic Materials Group.

REFERENCES

1. Standard Test Method for D-C Critical Current of Composite Superconductors, Annual Book of ASTM Standards, ASTM B7 14-82, Part 02.03, pp.

595-98, American Society for Testing and Materials, Philadelphia,

Pennsylvania (1983).

2. S. L. Wipf, Los Alamos National Laboratory, Los Alamos, New Mexico,

private communication.

3. L. F. Goodrich and F. R. Fickett, "Critical current measurements: a compendium of experimental results," Cryogenics 22: 222-241, 1982.

4. C. King, Intermagnetics General Corporation, Guilderland, New York,

private communication.

5. A. M. Campbell and J. E. Evetts, "Flux vortices and transport currents

in type II superconductors," Adv. Phys. 21: 199-429 (1972).

6. J. W. Ekin, "Critical currents in granular superconductors," Phys. Rev.

B 12: 2676-2681 (1975).

7. M. Suenaga, Brookhaven National Lab., Upton, New York, private

communication

50
APPENDIX B

Quench detector circuit for superconductor testing

W. P. Dub6 and L. F. Goodrich

Electromagnetic Technology Division, National Bureau ofStandards, Boulder, Colorado 80303

(Received 13 September 1985; accepted for publication 17 October 1985)

A quench detector is a device that interrupts the flow of current through a superconductor in the

event the superconductor reverts to the normal, resistive state. This new design has adjustable

filtering and sensitivity. The input is well isolated from the output, eliminating any possible

ground loop through the detector. It also has excellent noise immunity. A detector has operated

with no false trips for more than two years, detecting hundreds of quenches.

INTRODUCTION

When testing superconducting wire with high current densities, it is usually necessary to interrupt sample current in the event of a quench. 1 A quench occurs when the sample leaves the superconducting state and begins a thermal runaway

caused by resistive heating. Sample damage due to overheating may occur within a fraction of a second after the onset of

a quench, depending on factors such as sample geometry,

cooling, and thermal mass. In order to prevent such damage,

an automatic device is necessary to detect the quench and

reset the power supply to zero. The block diagram of a criti- cal current measurement apparatus, shown in Fig. 1, illus- trates a typical use of a quench detector in conjunction with

other instruments in a critical current experiment. 2 A good quench detector should have the following qualities: ( 1 ) Be capable of resetting the power supply rapidly

enough to prevent sample damage. ( 2 ) Be able to detect both

slow and rapid quenches. (3) Introduce no ground loops or extraneous signals into the system. (4) Be insensitive to noise and only reset the power supply in the event of an

actual quench. (5) Be relatively simple, cheap, and easy to

build.

1. CIRCUIT DESCRIPTION

Experiments have shown that all of the above qualities were incorporated in the design of the detector. The resistors are 5% tolerance, except where noted on the schematic, Fig

2. Low-noise, high slew-rate, FET-input operational amplifiers were used in the prototype, but lower quality devices

would have sufficed. The first stage of the device is a differential amplifier

with a 20-kfl input impedance and a gain of ten. The differ- ential input tends to eliminate false tripping due to common- mode voltages, dc coupling was chosen to allow the detection of a gradual quench. The input isolation from ground is more than a gigaohm if a high-quality, high-isolation power

supply or a battery power supply is used. If a medium-quality commercial power module is used, the low-voltage com- mon or center tap should be tied to line ground to prevent

common-mode voltages caused by capacitive coupling of the

primary and secondary windings of the power transformer.

These common-mode voltages may appear on the input ter- minals if the low voltage center tap is not grounded. In this configuration, the input isolation would be reduced to about

50 kfl to ground.

The second stage of the device is a variable, third-order,

low-pass, active filter.

3 This stage enables the user to. choose

high noise rejection, fast reset speed, or a compromise

between these two extremes. Roll-off frequency selection ranging from 10 Hz to 1 kHz is adequate for typical uses. The third stage provides a variable gain from 0. 1 to 200. The gain directly determines the minimum dc trip level. Reset speed is also a function of gain. Higher gain settings in- crease reset speed but tend to decrease noise rejection.

An arm/stand-by switch connects the third and fourth

stages. This switch enables or disables the detector without

causing the power supply to reset. Some current supplies

produce a large transient when initially turned on. If the

detector were not disabled during this transient, it might

reset such a power supply whenever an attempt was made to turn on the supply.

The fourth and final stage of the detector is an optocoupler that isolates the internal detector circuitry from any

voltage appearing on the output terminals. This section has

multiple purposes: first, it virtually eliminates any possibility

of a ground loop through the detector; second, it prevents

any noise, emanating from the controller or elsewhere, from

entering the detector via the output terminals; and third, the diodes in the opto-isolators have a threshold voltage that provides a good transition between output logic states The output of the quench detector is connected to the

reset input of the current supply controller. The controller

reset input shown in Fig. 2 is typical, but other configurations may be used as long as the input/output isolation is not degraded. This degradation may occur if any part of the in- ternal circuitry of the detector becomes electrically coupled

with the controller circuitry.

It should be noted that it is best to use a separate set of

sample voltage taps for the quench detector input These

taps should not be used for data acquisition This reduces the

possibility of one system interfering with the other Ideall)

the quench detector voltage taps should be placed at the extreme ends of the sample. In this way, a quench occurring

in any portion of the sample will trigger the detector

M0 R*v. Set. Inatrum. 57 (4), April 1M6 51
II. DISCUSSION

Fig. 1. Block diagram showing a typical use of a quench detector.

This section describes why a quench detector must be

adjustable for different experimental conditions and how to go about determining the actual gain and filter settings.

Many factors must be considered in choosing the optimum quench detector gain and filter settings. The primary

goal is to limit the temperature of the sample under test. The

superconducting properties of the sample will be affected if the temperature is allowed to stay too high for too long. The

difficulty arises in that there is no known way to calculate the minimum temperature ( Td ) at which damage or change

will occur. If the sample is subjected to mechanical stress, Td can be suprisingly low, perhaps 77 K or lower. This mechanical stress can be caused by Lorentz force, differential ther- mal contraction, or deliberate loading. Under these conditions, sample damage is due to plastic deformation which

causes a change in the superconducting properties of the

sample. Since yield strength and creep rate are both strong

functions of temperature, Tdis a strong function of mechanical stress. Large nonlinearities in sample heat capacity, resistivity,

normal-zone propagation velocity, heat transfer to the liq- uid-helium bath, etc. make rigorous mathematical analysis

of the time-and-temperature-dependent sample voltage an

arduous, if not impossible, task. This, in turn, makes calculation ofthe true optimum setting of the gain and filter imprac120 k& in I! steps

Fig. 2. Schematic diagram of the quench detector. The use of manufacturers’ part numbers does not imply an endorsement by the National Bureau of Standards, nor

does it imply that the devices identified are necessarily the best avail- able for the purpose.

68 1 Rev. Sel. Instrum., Vol. 57, No. 4, April 1 986 Quench detector 52

681
tical The simplest and perhaps the most conservative ap- proach to this problem is to estimate the low-temperature (4

K ) normal-state resistance of the sample. The gain setting can be approximated as follows: GAIN as 1.0/ (RI)

using the estimated resistance R and test current I. The filter setting is determined experimentally by start- ing at the highest ( 1 kHz ) setting and attempting to energize

the sample. The filter setting is then reduced as necessary to avoid false trip events. The distinction between true and false events can be made using an oscilloscope. If an actual quench has occurred, normal-state voltage appears along the

sample before the current interruption and an inductive voltage, due to the rapid decrease in current, appears after the current interruption. If a quench has not occurred, only the

voltage induced by the rapid decrease in current will be evi- dent. If the filter setting derived by this method results in an

excessively long time delay between quench event and current interruption, the filter setting selection process must be

performed again at lower gain setting. The objective is to keep the sample temperature rise to a minimum, but this ideal must be balanced against the time lost due to false trip events. The filter setting greatly affects the time delay between

quench event and current shut down. With the filter section

bypassed, and the sensitivity set to 5 mV, the elapsed time

between the input of a 10-mV step and controller shut down

is less than 30 /xs. Using the same input and sensitivity set- tings, the delay varies from 0. 38 ms at a filter setting of 1 kHz

to 31 ms at a filter setting of 10 Hz. Time delay through the

detector is a function of filter setting, gain setting, and input

signal magnitude and shape. Time delays due to the power

supply and the controller must be considered when estimating the total system time delay.

Experimental conditions determine the amount of time

delay that may be tolerated. The following conditions, listed in descending order of importance, will allow an increase in the length of time that the sample may remain in the normal,

resistive state without damage: ( 1 ) low sample current density; (2) low mechanical stress; (3) high sample heat capacity; (4) good thermal anchoring of the sample to a sample

holder with high thermal diffusivity and heat capacity; and

(5) sample and sample holder that are capable of good convective heat transfer to the surrounding liquid-helium bath. This quench detector has prevented sample damage

during many critical current and transient loss measurements. The detector appears to have all the qualities neces- sary for safe, reliable detection of quench and shut down of

the sample power supply.

Because of the differential input, the high input impedance, and the good output isolation, no noticeable undesirable signals or ground loops were introduced into the system.

Noise produced by nearby electrical equipment (drills, sol- dering guns, etc. ), or ripple and SCR spikes produced by the

sample current supply never caused false tripping when an

appropriate filter setting was selected. Repeatable steadystate dc trip levels from 0.0006 to 0.500 V were possible be- cause dc coupling was used throughout the detector. ACKNOWLEDGMENT

This work was supported in part by the Department of

Energy through the Office of Fusion Energy and the Division of High Energy Physics.

'Annual Book ofASTM Standards. Standard Test Method for dc Critical Current of Composite Superconductors (American Society for Testing and Materials, Philadelphia, 1983), B714-82, Part 2.03, pp. 595-98

J L. F. Goodrich and F. R. Fickett, Cryogenics 22, 225 ( 1982).

3A. B. Williams, Active Filter Design (Artech House, Dedham, MA, 1975)

682 Rev. Sd. Instrum., Vol. 57, No. 4, April 1986 53

Quench detector M2

APPENDIX C

PUBLICATIONS AND PRESENTATIONS

1. A. F. Clark and J. W. Ekin, ’'Defining Critical Current," IEEE Trans. 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 J_8, 137-141 (1978).

4. J. W. Ekin, "Current Transfer in Multif ilamentary Superconductors.

I. Theory," J. Appl. Phys. 4J^, 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. A. F. Clark, J. W. Ekin, R. Radebaugh, and D. T. Read, "The Development

of Standards for Practical Superconductors," IEEE Trans. Mag. MAG- 1

5

, 224-227 (1979).

7. 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).

8. F. R. Fickett and A. F. Clark, "Standards for Superconductors," Prof.

1979 Mech. and Mag. Energy Storage Contractors' Review Meeting, Conf-790854,

U.S. Dept, of Energy, Washington, DC (Dec. 1979).

9. 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 J^9, 327-332 (1979).

10. F. R. Fickett and A. F. Clark, "Development of Standards for Superconductors," Prof. 8th Int’l. Cryo. Eng. Conf. (ICEC8) , IPC Science and

Technology Press (1980) pp. 494-498.

11. F. R. Fickett and L. F. Goodrich, "NBD Superconductor Standardization

Program," Prof. 1980 Superconducting MHD Magnet Design Conference A. M.

Dawson, ed., (Report published by Bitter National Magnet Laboratory, MIT,

Oct. 1981) p. 87-89.

12. 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 319-325 (1980).

13. 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. 589-598.

55
14. 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).

15. G. Fujii, J. W. Ekin, R. Radebaugh, and A. F. Clark, "Effect of Thermal

Contraction of Sample-Holder Material on Critical Current," Tech. Report

ISSP Univ. of Tokyo, Ser. A, No. 1074 (Aug. 1980).

16. A. F. Clark, "Development of Standards for Practical Superconductors,"

Proc. Superconductivity Technical Exchange Meeting, IAPG, PIC-ELE-SC

209/1 (1980), p. 103 (presentation only).

17. 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. Mag. MAG-17, 69-72

(1981)

.

18. R. Radebaugh, G. Fujii, D. T. Read, and A. F. Clark, "A Standards Program

for ac Losses in Superconductors," Proc. IVth Int'l. Congress on Refrigeration (1980), Comm. A 1/2-10 (1980)., pp. 1-4.

19. G. Fujii, "Present Practices in Japan for the Measurement and Definition

of Various Superconducting Parameters," Cryogenics 2 \_, 21-38 (1981).

20. 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

, 73-76 (1981).

21. G. Fujii, M. A. Ranney, and A. F. Clark, "Thermal Expansion of Nb.Sn and

V„Ga Multifilamentary Superconducting Cables, Fiberglass-Epoxy ana Cotton-Phenolic Composite Materials," Jap. J. Appl. Phys . _20, L267-L270

(1981).

22. 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. 571-580.

23. A. F. Clark, G. Fujii, and M. A. Ranney, "The Thermal Expansion of

Several Materials for Superconducting Magnets," IEEE Trans. Mag. MAG- 1

7

, 2316-2319 (1981).

24. 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).

25. L. F. Goodrich and F. R. Fickett, "Critical Current Measurements: A

Compendium of Experimental Results," Cryogenics 2_2, 225-241 (1982).

26. A. F. Clark, L. F. Goodrich, F. R. Fickett, and J. V. Minervini, "Development of Standards for Superconductors, Interim Report Oct. 80 to Jan. 82,"

NBSIR 82-1678, National Bureau of Standards, Boulder, Colorado (July 1982).

56
27. Standard Definitions of Terms Relating to Superconductors, Annual Book of

ASTM Standards, ASTM B713-82, Part 2.03, pp . 591-594, American Society

for Testing and Materials, Philadelphia, PA (1983).

28. Standard Test Method for D-C Critical Current of Composite Superconductors,

Annual Book of ASTM Standards, ASTM B714-82, Part 2.03, pp. 595-98,

American Society for Testing and Materials, Philadelphia, PA (1983).

29. L. F. Goodrich, "The Effect of Field Orientation on Current Transfer in

Multifilamentary Superconductors," IEEE Trans. Mag. MAG- 1

9

, 244-247

(1983).

30. L. F. Goodrich, E. S. Pittman, and A. F. Clark, "Critical Current Measurements on Standard Reference Material," Adv. Cryo . Eng . - Materials , Vol . 30 , Plenum Press, New York (1984), pp. 953-960.

31. A. F. Clark, L. F. Goodrich, and F. R. Fickett, "Experience in Standardizing Superconductor Measurements," J. Physique 45 (C 1) , 379-382 (1984).

32. L. F. Goodrich, D. F. Vecchia, E. S. Pittman, J. W. Ekin, and A. F.

Clark, "Critical Current Measurements on an NbTi Superconducting Wire

Standard Reference Material," NBS Special Publication 260-91 (1984).

33. A. F. Clark and L. F. Goodrich, "Charadterization of a Standard Reference

Superconductor for Critical Current and a Summary of Other Standard

Research at NBS," Proc . Tenth International Cryogenic Engineering

Conference (1984), Butterworth & Co. Ltd., p. 433-437 (1984).

34. L. F. Goodrich, J. V. Minervini, A. F. Clark, F. R. Fickett, J. W. Ekin,

and E. S. Pittman, "Development of Standards for Superconductors, Interim

Report Jan. 82 to Dec. 83," NBSIR 85-3027, National Bureau of Standards,

Boulder, Colorado (Jan. 1985).

35. L. F. Goodrich, W. P. Dube, E. S. Pittman, and A. F. Clark, "The Effect

of Aspect Ratio on Critical Current in Multifilamentary Superconductors,"

Adv . Cryo . Eng . - Materials , Vol. 32 , Plenum Press, New York (1986).

36. W. P. Dube and L. F. Goodrich, "Quench Detector Circuit for Superconductor Testing," Rev . Sci . Instrum. , 57 , 680-682 (1986).

57
NBS-1 14A (rev. 2 -ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1 . PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR 87-3066 April 1987

4. TITLE AND SUBTITLE

DEVELOPMENT OF STANDARDS FOR SUPERCONDUCTORS

Interim Report

January - December 1985

5. AUTHOR(S)

L.F. Goodrich, S.L. Bray, W.P. Dube, E.S. Pittman, A.F. Clark

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

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

bibliography 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

January through December 1985 is reported. The major effort was the measurement

of large conductor critical current. Other work reported here includes stability

and a discussion of possible future Standard Reference Materials.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) angle; aspect ratio; cable; critical current; critical parameters; measurement;

standards; superconductor

13. AVAILABILITY 14. NO. OF

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