REFERENCE

NBS A111D2 t.31SDb

PUBLICATIONS

NBSIR 86-3061

ELECTROMECHANICAL PROPERTIES

OF SUPERCONDUCTORS FOR HIGHENERGY PHYSICS APPLICATIONS

NAT’L INST OF STANDARDS & TECH R.I.C.

A1 11 02631 506

Ekin, J. W/Electromechanical properties QC100 .U56 NO.86-3061 1986 V19 C.1 NBS-P

J. W. Ekin

L. F. Goodrich

J. Moreland

E. S. Pittman

A. F. Clark

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

December 1986

-QC

100

.056

86-3061

1986

KBS

research

information CENTER NBSIR 86-3061

ELECTROMECHANICAL PROPERTIES ,, OF SUPERCONDUCTORS FOR HIGHENERGY PHYSICS APPLICATIONS

J. W. Ekin

L. F. Goodrich

J. Moreland

E. S. Pittnnan

A. R Clark

Electromagnetic Technology Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

December 1986

Prepared for Division of High-Energy Physics

U.S. Department of Energy

Washington, DC 20545

(DOE Interagency Agreement #DE-AI05-85ER40240)

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

CONTENTS

Page

I. REPORT SUMMARY 1

II. ELECTROMECHANICAL RELATIONSHIPS IN NbTi STARTING

STRAND MATERIALS

J. W. Ekin 3

III. STUDIES OF NbTi STRANDS EXTRACTED FROM CORELESS

RUTHERFORD CABLES

L. F. Goodrich, E. S. Pittman, and J. W . Ekin . . •44

IV. EFFECT OF FILAMENT IRREGULARITY ( " S A US A G IN G" ) ON

ELECTRIC-FIELD VS CURRENT CHARACTERISTICS OF NbTi

SUPERCONDUCTORS

J. W. Ekin 86

V. APPENDIX - ELECTRON TUNNELING INTO SUPERCONDUCTING

FILAMENTS: DEPTH PROFILING THE ENERGY GAP OF NbTi

FILAMENTS FROM MAGNET WIRES

John Moreland, J. W. Ekin, and L. F. Goodrich . . 99

ill
(

i
ELECTROMECHANICAL PROPERTIES OF SUPERCONDUCTORS FOR HIGH-ENERGY

PHYSICS APPLICATIONS

J. W. Ekin, L. F. Goodrich, J. Moreland, E. S. Pittman,

and A . F . Clark

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

This report covers the first year of a 33“month project

to establish a systematic base of experimental data on

electromechanical effects in superconducting wire and cables

for hi gh-ener gy- physi cs magnet applications. The research

is focused in four general research areas;

electromechanical relationships in NbTi, studies of NbTi

strands extracted from cables, sausaging effects in NbTi,

and studies of the NbTi energy gap.

Key Words: cables; electromechanical properties; highenergy physics; magnets; NbTi; stress effect;

superconductors

I. REPORT SUMMARY

This report covers the first year of a project to establish

a systematic base of experimental data on electromechanical

effects in superconducting wire and cables for high-energyphysics magnet applications. The overall objective is to

identify the main factors affecting cr i t i ca 1 - cur r en t degradation

in superconducting materials. These factors arise from

mechanical processing during conductor fabrication and Lorentzforce loading during operation in a magnet. A further objective

is to develop strategies leading to the most tolerant strain

damage limits in these conductors and, therefore, the least

overall performance degradation in magnet applications.

The research is focused on three general areas:

1) Electromechanical relationships in NbTi starting strand

materials

2) Studies of NbTi strands extracted from coreless cables

(Rutherf ord-type)

3) Other properties of NbTi superconductors.

a) Sausaging effects on V-I char ac t er i s t i cs of NbTi,

b) SET junction studies of NbTi energy gap.

1
Program highlights in each of these areas for the first year include the following:

1) A new apparatus to measure the effect of transverse

compressive stress on NbTi conductors has been

constructed, modified, and successfully tested.

Preliminary results indicate that the cr i t i cal- c ur r ent

degradation in NbTi from transverse compressive stress

is comparable to that from axial tension; also the

effect is highly reversible indicating that the major

effects will be exhibited only when the conductor is

under operational load stresses.

2) An initial series of tests were done on superconductor

strands extracted from partially keystoned Rutherford

cables fabricated at Lawrence Berkeley Lab. The tests

have shown that the principal source of critical-current

degradation occurs at the narrow keystone edge, not at

strand crossover points along the flat region of the

cable

3) Further reduction of data from the first study of

filament irregularity (sausaging) in NbTi was made,

yielding an approximate relationship between the shape

of the E-I characteristic and the distribution of

filament diameters in the composite. A new technique

for measuring the depth profile of the energy gap in

filaments extracted from commercial NbTi conductors was

developed and demonstrated.

2
II. ELECTROMECHANICAL RELATIONSHIPS IN NbTi STARTING STRAND

MATERIALS

by J. W. Ekin

A. SUMMARY

A series of experiments has been started to measure

intrinsic stress relationships in NbTi strand materials. The

results will form a data base for designing and setting stress

limits in the fabrication of NbTi Rutherford cable conductors.

The data are also expected to be useful in determining the

optimum combinations of conductor compaction and Lorentz-f or ce pressure in magnet design.

As part of the initial phase of this program, a new

apparatus for measuring the effect of transverse compressive

stress on the critical current of NbTi has been constructed,

modified, and successfully tested. This and other mechanical

test apparatus at NBS have been used in this initial study to

test the same NbTi starting strand material to investigate the

relative electrical effects of various types of stress. The

results are preliminary, but several conclusions can be made now:

1) The magnitude of critical-current degradation in NbTi from

transverse compression applied at 4 K is comparable in terms of

stress in the NbTi filaments.

2) Much more stress can be developed in NbTi filaments from

axial tension than from transverse compression, because the soft

copper matrix limits the buildup of transverse compression to low

levels. At such levels the I^^ degradation is only about at 8

T, for example. Thus, in many practical cases, axial tensile

stress (from tension or bending strain) is the more important

source of cr i t i cal -cur r ent degradation. For example, the I^

degradation is 2 '^% at 7 T when the NbTi filaments are strained

2 . 7 %.

3) The I^ degradation from both types of stress is about 98$

reve r sible , indicating that the effects of stress in NbTi will be

seen only when the conductor is under stress.

4) The data indicate that a primary origin of the criticalcurrent degradation in NbTi is a stress induced reversible

decrease in the upper critical field. This is true for

degradation due to both transverse-compression and a x i al - t ens i on

degradation. It is shown by the large relative increase in the

cr i t i ca 1 - cur r en t degradation from both types of stress as the

magnetic field approaches the upper critical field.

As a result of these preliminary findings, there are several

practical consequences for cable fabrication:

1) In Rutherford cables, not much transverse stress is generated

in the NbTi filaments along the cable faces because of yielding

3
of the soft copper matrix material surrounding the filaments.

Much more stress can be developed in the NbTi filaments from

axial tension at the cable edges where they are bent around the

cable. This may explain the relatively small degradation

measured along conductor strands extracted from the face of

Rutherford cables compared to the edge region, reported in Sec.

Ill of this NBSIR.

2) The potentially large effect of axial tension indicates that

tension on the cable strands should be set at the minimum level

needed for cable integrity.

3) Stress annealing the conductor may improve the critical

current under some circumstances.

Additional measurements will be needed to determine whether

the effect is predominantly a very low temperature (4-K) stress

phenomenon or also arises from stress applied at higher

temperatures. The consistency of the effects among different

NbTi superconductors needs to be tested and further bend testing

is needed to relate this to the axial tension results.

B. OVERVIEW OF EXPERIMENTAL PROGRAM

Six types of mechanical tests are planned, most of them at

two temperatures, 295 K and 4 K. A matrix of experimental tests

is shown in Table 1. Experiments 1 and 2 are transverse

compression tests, wherein critical current is measured either

during or after stress is applied perpendicular to the sample

axis. The first test involves compression applied by flat

anvils, the second test by anvil heads which have been machined

to match the cross section of the conductor (semi-hydrostatic

test). The third experiment measures the dependence of axial

tension on the conductor critical current. The last three

experiments involve the effect of bending strain on the critical

current: hairpin geometry with field perpendicular to the entire

length of conductor, hairpin geometry with field parallel to the

hairpin legs, and a long-coil bending-strain test.

The overall plan is to perform this matrix of tests on the

same starting strand material in order to make comparisons of the

relative degradation effects of each type of mechanical loading

in a consistent manner. Once the most important types of

mechanical stress have been indentified, these stress tests will

be performed on a variety of different NbTi materials.

Thus far, preliminary test results on one sample have been

obtained for the effects of transverse compressive stress applied

at 4 K, axial tensile stress at 4 K, and severe hairpin bending

strain applied at 295 K in two magnetic field orientations.

These are marked as "one sample" in Table 1. These were the

highest priority tests and preliminary results are presented

below.

4
Table 1. Experiment Test Matrix for Electromechanical

Relationships in NbTi

^ K 295 K

1

^transverse

uniaxial

One sample Next

2) ^transverse semi-hydrostatic

Next

3) ®axial’ ^axial One sample Next

^bend, Next One sample

5) ^bend,

r H

') ^11 to hair pin legs'^ 'iOne sample

6) ^bend, length (Hj_) Next

5
The next priority tests are marked "Next" in Table 1. In

the case of the room- temperature transverse compression and axial

tension tests, only a few points at high stress levels will be

measured initially since these tests are very time consuming and

expensive. The reason they are time consuming is that load has

to be applied at room temperature and maintained constant during

cooldown, requiring a separate experimental run for each data

point. This can be accomplished in spite of differential thermal

contraction between the various components of the test apparatus

by using a servo-hydraulic feedback system described below.

C. TRANSVERSE COMPRESSION TESTS

The effect of axial tension on the critical current of

practical mul t i f i lament ary NbTi superconductors was first studied

about ten years ago.^ Relatively few electromechanical studies

have been made on NbTi superconductors since then, and these have

been mostly axial stress studies.^ Less is known about the

effects of stress components other than the axial component.^ In

practical superconducting magnets, however, the superconductor is

subjected to three dimensional stresses. Typically the

transverse component of stress is large and compressive. For

example, in solenoidal magnets the transverse component arises

from hoop stress which compresses the magnet winding radially,

and in dipole magnets it arises from Lorentz force compression of

the magnet windings at the mid plane.

Apparatus

To obtain data on the intrinsic effects of this component of

stress on the critical current of practical superconductors, a transverse-stress apparatus has been designed and built. It

simultaneously applies mutually perpendicular components of

current, high magnetic field and transverse compressive stress to

a single strand superconductor in a 4-K liquid helium bath. The

apparatus has the following specifications:

Force limit ^.5 kN (-1000 lb)

Maximum magnetic field 10 T

Maximum sample current 600/3000 A

(two sets of vapor-cooled current

leads for different current ranges)

Test temperature ^ K

Gauge length of test section 1.27 cm

An overview of the test apparatus is shown in Fig. 1. The

cryostat in the right foreground houses a 1 0-T radial-access

6
Fig- 1. Transverse compression test system showing 10 T ri.i. : access magnet dewar, servohydraulic system, arid ^ i battery supply

7
magnet. Load is supplied by a servo-hydraulic system; the

actuator is positioned above the magnet cryostat and mechanically

attached to the sample apparatus after it is inserted into the

magnet Dewar. The ser vohydraul i c loading system is powered by an

oil pump capable of supplying oil at a pressure of 20 MPa (~3000

psi). It is con-trolled by the comparator circuits shown in the

middle of the picture which can operate to control either load,

stroke, or sample strain. Operating in load control enables a sample to be loaded at room temperature and load to be maintained

constant during cooldown until thermal equilibrium is reached at

4 K. Then the system is switched from load control to stroke

control for the actual measurement in both the loaded and

unloaded states. For single-strand testing, current is supplied

by a 900-A battery supply; for large-conductor and small-cable

testing, a 3000-A SCR supply is used.

Figure 2 shows the compression test probe (lying on its

side). At right is the head plate and mechanical linkage for

connecting the apparaus to the servohydraulic actuator. The

f or ce- transfer cyclinder transfers the load to the test section

shown at the left of the picture. The top 35 cm of the forcetransfer cylinder near the head plate is thinned to increase the

thermal resistance to heat flow into the liquid-helium bath.

Figures 3A and 3B show a closeup of the magnet- insert test

section and Fig. 4 gives a schematic view of its main

components. The sample is prepared in the shape of a flat sided

hairpin, or inverted U shape. The ends are soldered to two

copper blocks such that the central section of the U passes over

a stainless steel anvil head. Compressive load is applied to the

central 1.27-cm length of the test section by sandwiching the

sample between the upper and lower anvil heads.

The upper anvil head is designed to pivot so that it

conforms to the flat surface of the lower anvil head, thereby

applying stress uniformly along the sample test section. Also

the anvil heads are tapered at the edges where the sample enters

and exits the sandwich in order to avoid any stress concentration

at the anvil edges. As shown in Fig. 3B, voltage taps are

soldered to the sample section within the sandwich region so that

the electric field is measured only over the region where stress

is uniformly applied to the test specimen.

Magnetic field is applied mutually perpendicular to the

direction of the force and the sample current by a radial access

magnet. The field is also perpendicular to the hairpin legs so

that current transfer is not a problem. (Current transfer

voltages can be a problem when the magnet field is aligned

parallel to the legs of a hairpin-shaped sample because the

increased cr i t i cal- c ur r ent density of the filaments for this

orientation does not force current to transfer from the surface

to the interior filaments).

8
9

Compression

test

probe.
Fio. 3A. Closeup of magnet insert section of transversecompression test probe: closed anvil heads.

10
Fig. 3B. Closeup of magnet insert section of transversecompression test probe: open anvil heads showing

voltage tap placement.

11
FIXTURE

Fig. 4. Schematic view of main components of magnet insert

section of transverse-compression test probe.

12
The first test results were obtained on a NbTi starting

strand material that has been used to make many of the 30-strand

Rutherford cables in the LBL cable development program (cable

Sample B in Sec. Ill of this report). The NbTi strand

characteristics are given in Table 2. It was manufactured using

a single stack technique without a diffusion barrier around the

f i laments .

Results

Figure 5 shows the first measurements of the criticalcurrent degradation effect of transverse stress applied to a mul t i f i 1 amen tar y NbTi conductor at 4 K. The results are

tabulated in Table 3. The ordinate in Fig. 5 is the measured

critical current normalized by its starting (zero stress)

value. The abscissa is transverse force per unit sample length

(top scale). A second abscissa scale is also shown (lower scale)

which is an effective overall transverse stress obtained by

dividing the applied load by the projected area of the sample

along the axis of the applied load. This projected area is

simply taken as the length over which compressive load is applied

(9.5 mm) multiplied by the starting sample diameter (0.648 mm).

This effective stress is given for comparative purposes

only. In the experiment, the anvil heads were flat and the sample

round, so this simple method of determining stress does not take

into account the small initial contact area, nor the increase in

contact area between the anvil heads and the sample as the sample

is progressively deformed. However it represents reasonably well

the approximate average stress experienced by the NbTi filaments

within the composite.

The overall sample strain was quite great because of

deformation of the soft copper matrix. It amounted to about 24$

at the maximum f or ce-per-sample length (F/L) value of 458 kN/m.

This overall sample strain, however, is typically much greater

than the actual NbTi filament strain because the copper matrix

material yields at a relatively low stress. A picture of the

sample deformation after the maximum loading at 4 K (F/L = 458

kN/m) is shown in Fig. 6.

The separation of the voltage taps was relatively small, 8.7

mm. Consequently a relatively high critical-current criterion

was used, corresponding to an electric field of 5 pV/cm. The

cr 1 t i cal- c ur r ent values at more sensitive criteria are easily

calculated however using the measured values of the . exponent n given in Table 4 [I^^/I^2 = (^ 01 /^ 02

)^^’^^*

As seen from the results at 8 T in Fig. 5, little

degradation (£ 0 . 5 %) was observed for this sample up to a F/L

value of about 100 kN/m (corresponding to an approximate

transverse compressive stress, oJ_, of about 150 MPa). The

degradation effect then increased monotoni cally to about 4$ as

13
Table 2. Mul t i f i 1 amen t ary NbTi sample character 1st ics (billet

#5212, spool #3-5 ) . Strand diameter 0.648 mm

Filament diameter 1 6 pm

Number of filaments

(single stack construction)

648

Copper-to-super conductor volume ratio 1 .77

Twist pitch 1.27 cm

Superconductor area 0.1189 mm^

Total area 0.3295 mm^

14
Table 3. Critical current of NbTi Sample i^52^2 as a function of

transverse load per unit length applied at 4 K. Both

loaded and unloaded values of the critical current are

given at magnetic fields of 7, 5, and 3 T. A 5 pV/cm

cr i t i cal- cur rent criterion was used. The critical

current at other criteria may be determined using

values of n given in Table 4.

non-Cu area « 1.189E-007 Enax = 0.00% Ec = S.00uV/cm

Load/unit lenath Ic Field Jc JcB Ic/Icm

[N/n X 10“^ < Anoeres

)

(Tesla

)

(GA/n'-2 ) (6N/m"3 >

0 513.250 3 . 000 4.317 12.950 0.9952

432.250 4 . 000 3.635 14.542 0.9965

364.250 5.000 3.064 15.318 0.9950

301.100 6.000 2.532 15.194 0.9990

236.400 7.000 2.005 14.035 1 .0465

174.600 8.000 1.469 11.748 1 .0063

1 1 1 .200 9.000 0.935 8.417 0 . 0000

S 365.500 5.000 3.057 15.286 0.9940

t4 365 . 750 5.000 3.059 15.297 0.9947

20 364.000 5.000 3.061 15.307 0.9954

33 364.250 5.000 3.064 15.318 0.9950

51 365.000 5.000 3.070 15.349 0.9981

65 365.250 5 . 000 3.072 15.360 0.9989

89 3S5.500 5 . 000 3.074 15.370 0.9995

174.600 8 . 000 1 .469 11.748 1 .0063

4 174.300 8 . 000 1 .466 1 1 .727 1 .0046

1 !B 364.500 5.000 3.066 15.328 0.9967

137 364.250 5.000 3.064 15.318 0.9960

160 365.500 5.000 3.074 15.370 0.9995

171 .200 8 . 000 1 .440 11.519 0.9867

1 173.600 8.000 1 .460 11.661 1 .0006

179 362.500 5.000 3.049 15.244 0.9913

171 .800 8 . 000 1 .445 1 1 .559 0.9902

1 362.750 5.000 3.051 15.255 0.9919

173.200 8.000 1 .457 1 1 .654 0.9983

201 362.000 5.000 3.045 15.223 0.9899

171 .700 8 . 000 1 .444 1 1 .553 0.9695

1 364.000 5.000 3.061 15.307 0.9954

173.900 8 . 000 1 .463 11.701 1 .0023

237 512.500 3.000 4 , 3 1 C 12.951 0.9937

15
Table 3 cont’d

non-Cu area “ 1.1 89E

(_oad/unlt lenqth

N/n X 10‘3

237

e

225

1

2B5

0

0

290

2

334

1

39S

-007 Emax = 0.00% Ec - 5.00u'J/cn

Ic Field Jc JcB Ic/Icm

( Amperes

)

(Tesla > (8A/m''2 ) <GN/m"3)

169.900 8.000 1 .429 1 1 .431 0.9793

513.000 3.000 4.315 12.944 0.9947

362.750 5.000 3.051 15.255 0.9919

513.000 3 . 000 4.315 12.944 0.9947

513.000 3 . 000 4.315 12.944 0.9947

513.000 3 . 000 4.315 12.944 0.9947

363 . 000 5.000 3.053 15.265 0.9926

169.000 8 . 000 1 .421 11.371 0.9741

512.750 3 . 000 4.312 12.937 0,9942

362.750 5.000 3.051 15.255 0.9919

175. 100 8.000 1 .456 11.646 0,9977

512.250 3.000 4.308 12.925 0.9932

352.250 5.000 3.047 15.234 0.9906

172.600 8.000 1 .452 11.613 0.9948

512.500 3 . 000 4.310 12.931 0.9937

352.500 5.000 3.049 15.244 0.9913

169.900 8.000 1 .429 1 1 .431 0,9793

513.000 3.000 4.315 12.944* 0,9947

3S2.000 5.000 3.045 15.223 0.9899

173.200 8.000 1 .457 1 1 .654 0.9993

362.750 5.000 3.051 15.255 0.9919

29S.300 6 . 000 2.509 15.053 0.9897

234.800 7.000 1.975 13.824 1 .0307

169.000 8.000 1 .421 1 1 .371 0.8741

363.500 5.000 3.057 15.286 0.9940

299.900 6.000 2.522 15.134 0.9950

236,600 7.000 1 .990 13.929 1 .0586

172.700 8.000 1 .453 1 1 .620 0.9954

5 1 3 .500 3.000 4.319 12.956 0.9956

431 .750 4 . 000 3.631 14.525 0.9954

363.600 5.000 3.058 15.290 0.9943

298.800 6.000 2.513 15.078 0.9914

234.000 7.000 1 .968 1 3 . 776 1 .0272

167.800 8.000 1 .41 1 1 1 .290 0.9672

514.250 3 , 000 4.325 1 2 . 975 0.9971

16
Table 3 cont’d

non-Cu area = 1.189E-007 Emax = 0.00% Ec = 5.00uV/cn

Load/unit lenath Ic

N/n X 10*3 (Amperes)

Field

( Tesla >

Jc JcB

(GA/m*2) (6N/m*3)

Ic/Icm

1 432.250 4 . 000 3.635 14.542 0.9965

354.300 5.000 3.064 15.320 0.9962

300 . 400 6 . 000 2.527 15.159 0.9967

236.900 7.000 1 .992 13.947 1 .0399

172.900 8.000 1 .454 1 1 .634 0.9965

458 515.250 3.000 4.334 13.001 0.9990

435 . 000 4 . 000 3.642 14.567 0.9983

354.200 5 . 000 3.063 15.316 0.9959

299.300 6 . 000 2.517 15.103 0.9930

224.400 7.000 1 .887 13.21

1

0.9851

168.100 8.000 1 .414 11.310 0.9689

1 515.750 3 . 000 4.338 13.013 1 . 0000

433.750 4 . 000 3.648 14.592 1 . 0000

365.700 5.000 3.076 15.379 1 .0000

30! .400 6 . 000 2.535 15.209 1 .0000

227.800 7.000 1.916 13.411 1 .0000

173.500 8.000 1 .459 11.674 1 . 0000

17
Ic

/lc(0‘i=

0)

TRANSVERSE FORCE PER UNIT LENGTH, F/f(kN/m)

0 100 200 300 400

Fig. 5. Effect of transverse compressive force on the critical

current of NbTi Sample #5212 at 8 T.

18
1000X

Fig. 6. Cross sectional view of NbTi Sample #5212 after

application of 458 kN/mF transverse load at 4 K.

19
F/L was increased to 460 kN/m ( oJ[ = 700 MPa). Because the yield

stress of the copper matrix is so low, it is difficult to

generate much higher stress without severe sample deformation;

see Figure 6. Thus, the maximum cr i t i cal - c ur r ent degradation

from transverse stress is about at 7 T.

The effect was almost completely reversible. Upon unloading

from F/L = 460 kN/m, the critical current recovered to within

0.5^ of its original value. This recovery took place even though

the sample was permanently deformed by about 255^.

The transverse stress degradation was largest at high

magnetic field. This is shown in Figs. 7 and 8, which show that

the effect was less than 0.5 % at the maximum F/L value of 460

kN/ra at both 5 T and 3 T.

It is also apparent in Figs. 7 and 8 that there was a small

(0.5^) enhancement of the critical current at high values of F/L.

This is probably an aspect-ratio effect caused by an enhancement

of the pinning effectiveness for this field orientation, which

resulted from the deformation of the filaments at high stress

levels. A listing of these lower-field results is also given in

Table 3.

Values of the exponent n for this sample are given as a function of magnetic field in Fig. 9 and Table 4. For this

sample, which was fabricated using a single stack technique with

relatively large filaments (17 ym ) , n reached values of over 80

at low magnetic fields. This is relatively high for

mul t i f i 1 ament ar y NbTi superconductors.

D. AXIAL TENSION TESTS

Tests at Magnetic Fields Up to 7 T

To compare the relative magnitudes of the effects of

transverse compression with axial tension, another sample of the

same starting material (see Table 2) was tested in an axial

tensile mode. The experiments were carried out using an

apparatus described in Ref. 4. This apparatus utilizes a 7 T

radial-access magnet and the same servohydraul ic loading system

described above. The sample gauge length was 28.4 cm and the

voltage-tap separation was 3.3 cm, both of which are longer than

in the transverse compression tests. Consequently the critical

current could be directly measured at more sensitive electricfield criteria.

Results corresponding to a 5 yV/cm criterion (the same as that used for the transverse compression tests) are given in Fig.

10 as a function of stress and in Fig. 11 as a function of

strain. A tabulation of the results is also given in Table 5.

Stress was calculated using the total conductor cross sectional

area. Stress was calculated using the overall conductor cross

20
TRANSVERSE COMPRESSION, (t^ (MPa)

Fig. 7. Effect of transverse compressive force on the critical

current of NbTi Sample #5212 at 5 T.

21
TRANSVERSE COMPRESSION, Oj^ (MPa)

Fig. 8. Effect of transverse compressive force on the critical

current of NbTi Sample #5212 at 3 T.

22
Power

Magnetic field depende nee of the exponent n for NbTi

Sample #5212 measured i n the shor t-sample transverse

stress apparatus. The po we r n is defined ^c1 ^^c2

23
Table 4. Exponent n for NbTi Sample #5212 as a function of

magnetic field measured at zero load in the shortsample transverse-stress apparatus. The power n is

defined by E^,/E^2 *

n evaluated at 0.000 strain

Field ( T

)

3 . 000

4.000

5.000

6.000

7.000

8 . 000

9.000

nth DOwer

84.00

83.70

82.40

82.54

66,97

61.43

35.28

24
Log(Ic)

Fig. 10. Effect of axial tensile stress on the critical current

of NbTi Sample #5212 at 7, 5, and 3 T.

25
Log(Ic)

Fig. 11 . Effect of axial tensile strain on the critical current

of NbTi Sample #5212 at 7, 5, and 3 T.

26
Table 5. Critical current of NbTi Sample #5212 as a function of

axial tensile stress and strain applied at 4 K. Loaded

and unloaded values of the critical current are given

at magnetic fields of 7, 5, and 3 T. A 5 yV/cm

critical current criterion was used. The critical

current at other criteria may be determined using

values of n given in Table 6.

5ii^.Vnon-Cu area - 1.183E-007 Emax = 0.00% Ec = B.00uU/cm Efract = 4.0B1

E Stress Ic Field Jc JcB Ic/Icm

(%) MPa ( Arperes ) (Tesla) (GA/m"2

)

(GN/m'-3 )

0 . 00 499.310 3 . 000 4.199 12.598 1 . 0000

418.810 4 . 000 3.522 14.090 1 .0000

353. B30 5.000 2.974 14.871 1 . 0000

292.230 5.000 2.458 14.747 1 . 0000

230.720 7.000 1.941 13.584 1 . 0000

0.2E 73 499. G30 3.000 4.202 12.505 1 .0005

353.070 5.000 2.970 14.848 0.9934

229.900 7.000 1 .934 13.535 0.9955

0.55 143 495.940 3 . 000 4.171 12.513 0.9933

350.800 5-000 2.950 14.752 0.9920

228.330 7.000 1.920 13.443 0.9895

0. 18 353 . 450 5.000 2.973 14.854 0.9995

0.79 222 493.810 3 . 000 4.153 12.450 0.9890

347.980 5.000 2 . 927 14.534 0.9840

225.950 7.000 1 . 900 13.302 0.9793

0.24 352.530 5.000 2.955 14.829 0.9972

1 .05 297 4B7.380 3 . 000 4.099 12.297 0.9751

343 . 450 5.000 2.889 14.443 0.9712

222.350 7.000 1 .870 13.091 0. 9537

0.3! 352.200 5.000 2.952 14.811 0.9950

1 .32 373 479.630 3 . 000 4.034 12.102 0 . 96 0b

337.480 5.000 2.838 14.192 0.9543

217.700 7.000 1.831 12.817 0.9435

0.37 352.080 5.000 2.961 14.805 0.9955

1.58 441 471 .000 3 . 000 3.951 11.884 0.9433

330.750 5.000 2 . 782 13.909 0.9353

211.980 7.000 1 .783 12.480 0.9188

0.39 351 .780 5.000 2.959 14.793 0.9948

1.85 50B 450.440 3 . 000 3.873 11.518 0.9222

322.000 5.000 2.708 13.541 0.9105

205.080 7.000 1 .725 12.074 0,8889

0.44 351 .250 5.000 2.954 14.771 0.9933

2.21 587 442.440 3 . 000 3.721 11.153 0.&9E

1

27
Table 5 cont’d

non-Cu area * 1.189E-007 Emax • 0.00% Ec = 5.00u'w>/cm Efract * 4.0G1

E Stress Ic Field Jc JcB Ic/Icn

(% ) MPa ( Amperes

)

(Tesla) (6A/m'-2 ) (6N/n"3

)

2.21 587 308.250 5.000 2.593 12.963 0.8717

194.400 7.000 1 .635 11.445 0.8426

0.49 351 .080 5.000 2.953 14.764 0.9928

2.63 680 418.310 3 . 000 3.518 10.555 0.8379

290.380 5.000 2.442 12.21

1

0.821

1

179.600 7.000 1 .51 1 10.574 0.7784

0.B5 350.600 5.000 2.949 14.744 0.9914

3.08 765 394.940 3 . 000 3.322 9.965 0.7910

272.300 5.000 2.290 1 1 .451 0.7700

165.350 7.000 1 .391 9.735 0.7167

0.68 349.580 5.000 2.940 14.701 0.9886

3.52 842 370.690 3.000 3.118 9.353 0.7424

252.900 5.000 2.127 10.635 0.7152

149.530 7.000 1 .258 8.803 0.6491

1 .20 347.980 5.000 2.927 14.634 0.9840

3.52 713 396.190 3 . 000 3.332 9.996 0.7935

271 .530 5.000 2.284 11.419 0.7678

162.830 7.000 1 .370 9.587 0.7058

28
sectional area. The results of Fig. 10 compared with Figs. 5, 7,

and 8 show that the effect of axial tensile stress applied at 4 K

is significantly greater than the effect of transverse

compression at 4 K, in terms of the overall stress on the

conductor . For example, the degradation of the critical current

at 7 T is about 24/S at an average tensile stress of 700 MPa

(tensile strain of 2 . 1 %). This compares with an 8 T degradation

of about 4/t at 700 MPa transverse stress, about one sixth.

However, because the copper matrix has a relatively low

yield stress (about 140 MPa), the axial stress in the NbTi

filaments is actually higher (about 2.3 times higher) than the

overall stress on the conductor. Accounting for this, the

degradation from axial tension at 700 MPa in the NbTi filaments

corresponds to about 340 MPa overall conductor stress, or a degradation of only about 1 % at 8 T. Thus, the two effects

(axial and transverse) are nearly comparable in terms of actual

NbTi filament stress . The slight difference in cr i t i cal-curr ent

degradation between the two components of stress may arise from

texturing of the filaments introduced during fabrication.

Much more stress can be developed in the NbTi filaments from

axial tension than from transverse compression because the soft

copper matrix limits the buildup of transverse compression.

Thus, in many practical cases, axial tensile stress is the more

important source of cr i t i cal- cur r ent degradation.

In other respects the two types of stress effects (axial and

transverse) are similar. Both stress effects increase with

increasing magnetic field. For example, in Table 5 the

degradation at an axial stress of 700 MPa { 2 . 1 % strain) was 1 6/J

at 3 T, increased to 1 85K at 5 T, and then to 22 % at 7 T.

Both types of stress effects are highly reversible also.

For example, upon unloading from an applied axial stress of about

700 MPa (strain of 2 . 1 %) the critical current at 5 T recovered to

within ^% of its original value. This extreme reversibility

persisted up to the fracture point of the sample, which occurred

at a strain of 4.065^. Just before the sample fractured, the

critical current still recovered to within 1.5^ of its original

value upon unloading the sample; see Table 5.

A significant serrated yield event (load drop of 10?) took

place at a tensile strain of 3.0?. Figures 10 and 11 show that

this did not significantly affect the smooth monotonic decrease

of the critical current with stress and strain.

However, a quench event took place while the sample was

under high stress, 842 MPa (3.5? strain), which significantly

affected the degradation. As shown in Fig. 11, the critical

current rose significantly after the transient heating that

accompanied the thermal runaway and curr ent-supply shutdown.

During this quench event the strain in the sample was maintained

constant, but the stress decreased about 15? due to annealing.

29
Table 6. Exponent n for NbTi Sample it52'\2 as a function of

magnetic field measured at zero load in the long sample

axial stress apparatus.

n evaluated at 0.000 7, strain

Field (T) nth oower

3 . 000

4.000

5.000

6 . 000

7.000

7G.09

75.78

75.20

72.89

71.92

30
Power

Fig. 12. Magnetic field dependence of the exponent n for NbTi

Sample #5212 measured in the long sample axial stress

apparatus

.

31
Thus the event indicates that critical-current degradation

in NbTi is predominantly stress controlled rather than strain

controlled, since strain was constant while both the stress and

critical current recovered. The event suggests that strain and

deformation alone are not the prime causes of degradation in

NbTi, but rather the stress that accompanies deformation is the

major cause, at least at 4 K. Thus, stress annealing of the

conductor might be a method for improving the critical current.

Values of n determined in the axial stress apparatus are

given in Table 6 and plotted in Fig. 12. As seen from the data,

n was nearly constant between 3 T and 7 T and had a value of

about 75. This is slightly smaller than the value of n determined above using the short-sample transverse stress

apparatus. The difference is not great, 83 vs. 75, but it may be

significant. Specifically, it may be due to the longer length of

the sample measured in the axial stress apparatus, which would

include long as well as short filament irregularities along the

sample test length. It thus would be more sensitive to

degradation of n arising from longer-f ilament irregularities.

Tests at Magnetic Fields Above 7 T

To measure the effect at magnetic fields approaching the

upper critical field, another series of tests were made at fields

above 7 T. These were made using a short sample test apparatus

(described in Ref. 5) that was designed for use in the Bitter

magnets at the National Magnet Laboratory. The results are shown

in Fig. 13 and tabulated in Table 7. The overlap in critical

current results at 7 T is good if the results are corrected to

the same temperature. These measurements were carried out at sea

level in boiling helium at an approximate temperature of 4.2 K.

The results below 7 T were made about 1600 m above sea level at an approximate temperature of 4 K.

The results show quite clearly that the relative magnitude

of the stress effect becomes much larger as the magnetic field

approaches the upper critical field. At 2.14/S strain, for

example, the cr i t i cal- c ur r ent degradation is 19/S at 8 T, 24$ at 9

T, 42$ at 10 T, and rises to 60$ at 10.5 T.

The dependence of n on magnetic field is given in Fig. 14

and Table 8. Note the rapid decrease in this high-n sample at

fields above 7 T

.

E. BENDING STRAIN TESTS AT 295 K

To compare these 4 K stress effects with room-temper atur

e

ben d i ng- s t r a i n effects, the same superconductor strand material

was subjected to a severe bending strain test. The conductor was

bent into a hairpin shape at 295 K and placed transversely to the

bore of a 9 T solenoidal magnet. This field orientation was

chosen to eliminate current- transfer effects. These would

32
Lo^Ic)

Fig. 13. High field dependence of the axial strain effect on the

critical current of NbTi Sample #5212.

33
Table 7. High-field critical current of NbTi Sample #5212 as a function of axial tensile strain applied at 4 K.

S",C, area = 1 . 1S9L-007 Emax = 0.00% Ec “ 2 . 00uV/cm

E Eo Ic Field Jc JcB I c/ Icr7 % ) ( % ) < Anneres ) (Tesla) ( GA/m'’2 ) ( GN/n-'3 )

0,.00 0,.00 20G..070 7

,

.000 1 ,, 733 12. 132 1 .. 0000

145..900 8

,

. 000 ! . T-iy 9. 817 1 .. 0000

86,. IIG 9

,

. 000 0.,724 6. 5 1 8 1 ..0000

33..235 10,. 000 0,.280 7 795 1 ,. 0000

14,. 0 1 ,8 10., 500 0..118 1 238 1 ..0000

I ,.075 1 1 ..000 0..009 0. 099 1 ,. 0000

0,. 15 0,. 16 145,. 820 8

,

. 000 1 ..226 9. 81 1 0. 9995

85,.936 9,. 000 0.,723 6 . 505 0., 9979

7,2 . .85.5 10,. 000 0,,276 2 . 763 0.. 9895

13,,621 !0,.500 0.,115 1 . 203 0., 9717

0. ^ p 0.. 39 144, 1 00 8.. 000 ] .212 9. 695 0,, 9877

84 ,. gts g

,

.000 0 ,714 5. 429 0.. 9861

32

.

,687 10,.000 0 ., 275 2

.

74 9 0., 9835

13,.42 9 1 0 .,500 0.,113 ] 1 96 0.. 9580

0, 1 9 0,. 1 9 143..650 Q ^. 000 j .208 9. 656 0.. 9846

0..58 0...58 Ml , 970 a..000 1 .. 1 94 q

,

552 0.. 9731

S3

.

.4 77 9,.000 0

.

. 702 6. 31 9 0 ,. 9694

7 ~- .017 10,.000 0..269 7 6 93 0..9634

\ 2.. 943 10..500 0

,

. 109 1 . 1 43 0,. 9233

0.. 28 0.. 28 1 42

.

. 870 8

,

, 000 1 ,.202 9. 613 0,. 9792

1 .05 1

. 05 138,. 600 8.. 000 1 ,, 166 9

.

C w/4. U 0.. 9500

80.. 90S 9

.

.000 0,. 6,80 6 . 124 0 ,, 9395

30.,009 10..000 0,, 252 2

.

524 0,. 9029

1 1 .,.864 10,. 500 0.. 100 1 . 049 0.. 8463

0. — T 0..37 143., 800 Q ,, 000 1 .,209 D

J . 67.5 0 .. 9856

1 ,44 t . 44 131.,220 8 _.000 1 ., 104 8. 825 0., 3954

7-5

.

,003 9

.

.000 0.. E3

!

5

,

6'77 0. 87 1 0

26. 10..000 0. 22 1 7 209 0.. 7899

g ,, 398 10., 500 0..079 0. 830 0,, 5704

0..45 0. 45 142.,620 9

.

, 000 1 . 200 9. 596 0. 9775

! . 84 1 .. 84 125., 030 8 .,000 1 .,052 8. 4 1 3 0.. 9570

69. ct;7 9

,

,000 0.,585 5

.

265 0.,8077

22

.

, 557 1 0 ,, 000 0. 1 90 1 . 897 0.,6787

7. 732 10.. 500 0.,•065 0. 6.83 0., 55! 6

0

.

55 0.,55 142.,560 6

.

000 1 1 99 9

.

592 0. 977 1

34
Table 7 cont ’d

SC “1.-1 89E-007 Erna.x “ 0.00% Ec = 2.00uV/cm

E Ed Ic Field Jc JcB Ic/Ic<n

(%) (%) ( A^Dcres ) < Tesla

)

<6.A/n"2

)

(BN/mM)

2c14 2.14 118.430 8.000 0.996 7.968 0.81 17

64.679 9.000 0.544 4.896 0.7558

19.369 10.000 0.163 1 =629 0.5828

5.684 10.500 0.0A8 0.502 0.4055

e>.67 0.6? 141 .B30 8.000 1.191 9.530 0. 9707

35
Power

Fig. 14. Magnetic

for NbT i field dependence of the exponent n measured

Sample #5212 at high fields.

36
Table 8. High-field dependence of the exponent n on magnetic

field, measured for NbTi Sample #5212 at zero load in

the short-sample high-field test apparatus.

n evaluated at 0,000 % strain

Field (T) nth power

6.000

9.000

1 0.000

10.500

11.000

47.09

24.75

12.23

9.95

3.08

37
38
400X

Fig. 16. Cross section view of the bend section of the sample

used in the severe hair pin bend test. The apparently

elongated filaments in the upper section of the picture

are an artifact due to the cross section not being cut

exactly at the middle of the bend section.

39
BEND STRAIN IN OUTER FILAMENTS, £g (%)

Fig. 17. Effect of room temperature bend strain on the critical

current of NbTi Sample #5212 at 8 T. The abscissa is

the calculated bending strain in the outermost

f ilaments .

40
otherwise arise from the large difference in critical current

between the legs of the hairpin and the bent section when the

hairpin legs are parallel to applied magnetic field.

The sample was subjected to bending strain, starting at

about 15^ and increasing to a little over 100/6. This was done

progr essi vely in the same sample by bending it at room temperature, cooling the sample to 4 K, measuring I , warming the sample

to room temperture, bending the sample into a tighter radius, and

repeating the whole process. The critical current was measured

at each radius until the radius was reduced to nearly zero. A

picture of the sample after being bent to a nearly zero radius is

shown in Fig. 15 and a cross sectional view of the sample is

shown in Fig. 16.

The results corresponding to a 5yV/cm criterion are shown

in Fig. 17. In this figure, the relative critical-current

degradation is plotted as a function of bending strain in the

outer filaments, eg. Critical current was measured for current

flowing in both directions in the hairpin sample, one direction

where the self field of the sample added to the background field

(marked +) and the other where the self field subtracted from the

background field (marked -). The average of the two criticalcurrent values is plotted with open circles in Fig. 17.

The magnitude of the degradation at the high bending strains

shown in Fig. 17 is small. The effect of room temperature

bending strain on the critical current was about at bending

strains of over 100^, using a 5iiV/cm criterion. All bending was

done at room temperature, and there was no stress applied to the

sample when it was cooled for the critical current measurement.

Further bend tests need to be made, especially to measure the

effect at lower electric fields.

CONCLUSION

Summarized below are the main results of these preliminary

measurements of the critical-current degradation arising from

different types of stress applied to the same NbTi starting

material;

1. The magnitude of cr i t i cal- cur r en t degradation from axial

tensile stress applied at 4 K is about twice that from

transverse compression at 4 K, when the results are compared

in terms of stress applied to the NbTi filaments (excluding

copper ) .

2. Both effects are about 9856 reversible, indicating that

the effects of stress in NbTi will be seen only when the

conductor is under stress.

3. The effect appears to be a stress controlled, rather

than a strain-controlled phenomenon. Even very high stress

41
accompanied by severe deformation (from transverse

compression or axial tension at 4 K, or from bending strain

at room temperature) produced little cr i t i cal - cur r en

t

degradation 1 ) when the stress was removed. Annealing

of NbTi, which reduces stress at constant strain also

appears to increase the critical current.

4. The data indicate that a primary origin of the criticalcurrent degradation in NbTi is a stress induced reversible

decrease in the upper critical field. This is true for both

transverse-compression and axial-tension degradation, as shown by the alrge relative increase in the magnitude of the

critical-current degradation from both types of stress as the magnetic field approaches the upper- cr i t i cal field.

These data are only preliminary. Additonal measurements

will be needed to determine whether the effect is predominantly a very low temperature K) stress phenomenon or also arises from

stress applied at higher temperatures. The consistency of the

effects among different NbTi superconductors needs to be tested

and further J^-bend data is needed at lower electric fields.

ACKNOWLEDGEMENTS

The author gratefully acknowledges J. C. Brauch, C. A.

Thompson, and D. L. Rule for their help with apparatus design and

construction as well as data acquisition and reduction.

Appreciation is also expressed to R. M. Scanlan, J. M. Royet, and

C. Taylor (Lawrence Berkeley Laboratory) for discussions and

supplying the samples. L. F. Goodrich and the other members of

the Superconductor and Magnetic Measurements Group at NBS

contributed a number of helpful discussions to this research.

The manuscript was typed by R. A. Shenko and R. B. Goldfarb

contributed many useful comments on the text. The high magnetic

fields needed for part of this study were obtained using the

magnet facilities of the Francis Bitter National Magnet

Laboratory

.

42
References

1. J. W. Ekin, F. R. Fickett, and A. F. Clark, "Effect of

Stress on the Critical Current of NbTi Mul t i f i lament ar

y

Composite Wire," Proc. Int*l. Cryog. Mat. Conf. (Aug. 1975),

Adv. Cryog. Eng. 22, 449 (1977).

2- For a review, see J. W. Ekin, "Mechanical Properties and

Strain Effects in Superconductors," in Superconducting

Materials Science; Metallurgy, Fabrication, and

Appl i cat i ons , edited by S. Foner and B. B. Schwartz, Plenum

Press, New York, p. 455-510 (1981).

3. The effect of cr i t i cal- cur r en t anisotropy introduced by

room-temper atur e flattening has been studied by Garber,

unpublished, and by L. F. Goodrich, W. P. Dube, E. S.

Pittman, and A. F. Clark, Adv. Cryog. Eng. 32 , 833 (1986).

4. J. W. Ekin, "Superconductors," Chapt. 13 in Materials at Low

Temperatures , edited by R. P. Reed and A. F. Clark, American

Soc. for Metals, p. 465-513 (1983).

5. J. W. Ekin, "Strain Scaling Law for Flux Pinning in

Practical Superconductors, Part I: Basic Relationship and

Application to NboSn Conductors," Cryogenics _2_0, 611-624

( 1980 ).

43
III. STUDIES OF NbTi STRANDS EXTRACTED FROM CORELESS RUTHERFORD CABLES

by L. F. Goodrich, E. S. Pittman, and J. W. Ekin

A. SUMMARY

The electromechanical properties of NbTi strands extracted from coreless

cables (Rutherford) were studied to clarify the relative effects of strand

location and field angle on current degradation that occurs in cables that

have been compacted into a keystone shape. Detailed critical-current measurements were made on two samples. One sample was from a cable that was commonly

used for cable studies late in 1985 and early 1986. The other sample was from

a cable that had been compacted more than usual. Measurements on the second

sample were done in order to more clearly Identify the degradation features.

The two cable samples, which had been fabricated under controlled conditions,

were provided by Lawrence Berkeley Laboratory. These are prototype cables for

high energy physics applications. Specific factors that are addressed are the

nature, location, and amount of degradation. This information is intended to

lead to methods for reducing the amount of critical-current degradation in

cable manufacture.

The cr itical-current measurements on both cables showed that the same region of the cable had the lowest critical current for the limiting case in a dipole magnet. The orientation of current and magnetic field that is the

limiting case for a dipole magnet is with the magnetic field nearly parallel

to the wide face of the cable and current orientation. The weak link of the

cable strand occurred at the thin edge. For a typical cable, this portion,

which only represents 5^ of the total length of the cable, contributed about

55)S of the total voltage. For the more degraded cable, this same 5? of the

total length contributed about 95/5 of the total voltage. Decreasing amounts

of I(^ degradation were measured in the portion of the strand from, respectively, the corners of the thin edge, the thick edge of the cable, the corners

of the thick edge, and the faces of the cable. Cabling can lead to very

localized reductions in critical current and any improvements have to focus on the thin edge of the keystone. Conversely, any additional lowering of the

thin-edge critical current will be directly reflected in the overall performance of the cable.

B. INTRODUCTION

These measurements of electromechanical properties of NbTi strands

extracted from coreless cable were made to clarify the nature of criticalcurrent degradation that occurs in cables that are compacted into a keystone

shape. All the measurements reported here were made on two samples, denoted

as Sample A (billet 591^, cable SC 305) and Sample B (billet 5212, cable SC

297). Sample A was a strand from a cable that was commonly used for cable

studies late in 1985 and early 1986. Sample B was a strand from a cable that

was re-rolled through the turkshead (to re-size it) after it had been cabled.

This re-roll may have been done in the wrong direction, opposite the original

cabling and roll direction. The re-rolled cable Sample B was a worst-case

situation that was studied to more clearly identify the degradation features.

The comparison of the results of measurements on these two cables indicated

significant similarities and differences.

44
The critical current, 1^, of strands extracted from cables was measured as functions of position along the cable and magnetic field orientation. The two

cables, which had been fabricated under controlled conditions, were pro-vided

by Lawrence Berkeley Laboratory. These were prototype cables for high energy

physics applications. Specific factors that were addressed were the location

and field angle dependencies of the critical-current degradation.

Some of the conductor parameters are given in Table 1 . Sample A had a Nb

diffusion barrier on each of the 6006, 5 ym diameter filaments. Sample B did

not have a diffusion barrier. The Sample A billet was double stacked; Sample

B was single stack. The starting-wire critical currents (before the cabling

process) were determined at 5 T and 4.02 K using a 0.1 yV/cm criterion. The

cr itical-current density was calculated using the NbTi cross-sectional area,

which was determined using the copper-to-superconductor volume ratio. The

critical-current density at 4.22 K would be about less than the values

listed at 4.02 K. The starting wire I^’s were measured in another magnet as another part of this program; thus their values might be shifted by a few

percent relative to the measurements on the cable strands. For Sample B, see Table 5 in section II D for starting strand material critical current.

Some of this work was originally presented at the 5th Workshop on NbTi

Superconductor (March 25-27, 1986, Menlo Park, CA). Discussions at this workshop are acknowledged, especially with R. M. Scanlan and J. M. Royet (Lawrence

Berkeley Laboratory). Information on the cable fabrication and current density using highly homogeneous NbTi alloy can be found in References 1 and 2.

C. EXPERIMENTAL DETAILS

This is a discussion of sample preparation, notation, data acquisition,

and data analysis. The samples were from 30 strand cables and started as 0.65

mm diameter round wires with a right hand filament twist of about 0.8 twist/

cm. The cable had a left hand lay with a 69 mm pitch and a cable width of 9.7

mm, mid-thickness of 1.17 mm, and a keystone angle of about 1.24°.

The critical current measurements reported here were made using a straight sample geometry and a radial access, split pair magnet. The orientation of the sample was changed by rotating the sample cryostat relative to

the background magnet. The zero angle was defined as the orientation with the

magnetic field parallel to the wide face of the conductor (see Fig. 1). The

critical current was defined as the current at which the electric field

strength was 0.1 yV/cm. This corresponds to a voltage of only 20 nV between

the taps with the smallest separation. A number of repeated determinations of

I^ were made at 0° to give an indication of measurement precision. The precision was about ±15^ and the accuracy was about ±2 %.

A number of voltage taps were placed along the length of the strand to

determine its local electrical properties after it had been deformed by the

cabling process. Three regions of the cable were studied: the thin edge , the

face (top or bottom), and the thick edge . Each of these regions was divided

with voltage taps into three approximately equal-length, adjacent segments.

The three segments of each cable edge were denoted as the central flat edge

and the two corners . These edge voltage taps had separations of 2 to 2.5 mm

(thin edge) and 2.5 to 3 mm (thick edge). The voltage taps along the face of

45
Table 1 . Conductor Parameters

Sample

Billet

No

Cable

No

#

Filaments

Filament

Diameter Cu/sc

Starting

Wire I^^(5 T)

at 0.1 yV/cm

and 4.0 K

Starting

Wire J^(5 T)

Jq using NbTi area

A 5914 SC305 6,006 5 pm 1 .64 320A 2500 A/mm^

B 5212 SC297 648 1 6 )jm 1 .77 347A 2900 A/mm^

46
47

Figure

1

.

Photograph

of

Sample

A

cable

cross

section

with

voltage

taps

and

magnetic

field

angle

identified.
the cable had separations of about 9 mm and they did not include any of the

cable corner regions.

The voltage tap notation used in this report was a combination of numbers

and letters to denote the various portions of the cable strand (see Fig. 1).

The numbers 1, 2, and 3 refer, respectively, to the three regions of the

cable, starting with the thin edge (1), then the face (2), and finally the

thick edge (3). Within each of these regions, the three segments were denoted

with the letters a, b, and c. Thus, as shown in Fig. 1, the sequence along

the strand was la, lb, 1c, 2a, 2b, 2c, 3a, 3b, and 3c.

Voltage taps on the sample were made using three flat quads (miniature

four-conductor flat cable) of y/32 gauge wires. Within each quad, three adjacent segments could be measured with a minimum of induced voltage. The end

taps of adjacent quads were common to allow complete coverage of the sample.

The quads of voltage taps were soldered to twisted pairs near the bottom of

the sample cryostat and immersed in liquid helium during data acquisition.

The twisted pairs were continuous from the bottom of the cryostat to the

voltmeter input cables to avoid thermally reduced voltages. The junctions at room temperature between the voltage tap leads and the nanovoltmeter input

leads were maintained isothermal by putting small bags of lead shot under and

on top of the junction.

The sample holder was a 1/2" diameter 316 stainless steel rod with a groove machined along its length that was wider than the cable. The walls of

this groove were insulated with a thin layer of epoxy formed with a TFE impressioner. After the voltage taps were soldered to the sample, a second

layer of epoxy was used to hold the sample in the rod. The ends of the sample

extended past the end of the rod and were used for the current contacts.

The field-angle rotations were done slightly above atmospheric pressure

(about 4.02 K). Measurements were made at 5 and 8 T. Some data at 0 and 90° were taken at other magnetic fields from 4 to 9 T. Additional data were taken

at a higher temperature (about 4.24 K) for magnetic fields from 4 to 9 T. This allowed comparisons with other measurements and this was used to determine the temperature dependence of different segments of the strand.

Three voltage signals and current-versus-time were simultaneously ac- quired on a digital processing and storage oscilloscope. The acquisition time

for each set of curves was about 51 seconds. The current waveform consisted

of an initial zero current baseline, a fast current ramp up, a slow ramp

toward and then (if the sample did not quench) a slow ramp down, followed

by a fast ramp down and a final zero-current baseline. If the sample quenched

before the current ramp was reversed, the rest of the waveform was at zero current after the quench detector had reset the current supply. For most of

the measurements the current was increased until the sample quenched. This was done in order to get as high a current and voltage as possible, since some portions of the strand had a higher than others and the sample could quench

before very much voltage could be developed on some voltage taps. The raw waveforms were stored for later analysis.

During data analysis, the effects of voltage offsets, changes in thermoelectric voltage, inductive voltage, and current transfer voltage were determined and the waveforms corrected. The voltage offsets and changes in

48
thermoelectric voltage were determined using the initial and final zero current baselines of the waveforms. The thermoelectric voltage correction was

made assuming a constant drift with time. This correction was generally only

a few nV/cm. The inductive voltage (voltage proportional to dl/dt) and

current-transfer voltages were determined using the fast and slow ramp portions of the waveforms well below the critical current. The current-transfer

voltage may or may not be intrinsic to the wire since only a 64 mm length of

the sample was in full magnetic field. There is a further discussion of current tranfer later in this report. Generally the amount of current transfer

voltage was quite small.

It was useful to consider what the effective critical current of one strand pitch length would be. The effective critical current was defined as

the critical current determined by adding up the voltage contributions of each

strand segment, taking into account that the portions along the face need to

be counted twice for one pitch length. This total voltage was then converted

to an effective electric field by dividing by one pitch length. This was useful because it related more directly to measurements made on the whole cable

where several pitch lengths were included between the voltage taps.

D. RESULTS

The data presented here are preliminary in that they may not represent

averages of a number of specimens. There is no reason to believe, however,

that these two specimens were not typical. Independent measurements on the

cables [ 3 ] indicated that Sample A was degraded by about 9? and had a field

anisotropy of about 1 1 and Sample B was degraded by 225^ and had an anisotropy

of -11?. See Reference 4 for definitions of degradation and anisotropy.

These results are consistent with those presented here. The additional data

presented here on the nature, location, and amount of degradation is intended

to lead to methods for reducing the degradation.

Photographs of the two cable samples with voltage tap pairs indicated are shown in Figs. 1 and 2. The voltage taps along a strand are represented here

on the cable cross section. Along its length, the individual strand goes

through all positions shown in the cable cross section. The tap notation was defined in the Experimental section. Notice the strand cross section for each

position and the tap notation.

In the first run of Sample B, the thin edge had the lowest critical curent, and this region was limiting the current. For the second run a short

section of the thin edge was shunted with a 0.7 cm length of another cable

strand. This allowed the current to get slightly higher and enabled testing

of the other regions of the strand before the sample quenched. The data

presented for Sample B are a combination of these two runs, not-shunted and

shunted.

Position dependence

The critical current as a function of position along the strand at 0° and

90° field angle is given in Figs 3 and 4 for Sample A. The critical current

values varied by as much as 27? from the highest at 0° and 1 1 ? at 90°. At 0°

the central flat portion of the thin edge had the lowest I^. The central flat

49
50

Figure

2.

Photograph

of

Sample

B

cable

cross

section

with

voltage

taps

and

magnetic

field

angle

identified.
SAMPLE A AT 0 DE6REES

350

325

-300

i_u

3 275

C_J)

T=^250

cn

225

200

0 1 2 3 4 5

POSITION . cm

Figure 3. Sample A, critical current versus position at 0°.

51
SAMPLE A AT 90 DEGREES

nc

cn

ZZ)

c_z

C_J

cn

c_z»

POSITION , cm

Figure 4. Sample A, critical current versus position at 90°.

52
portion of the thick edge was less degraded. The corners of both edges had a higher and had some asymmetry in the same cable lay direction. This may be

due to mass flow away from the cable edge during the cable fabrication. The

difference between 0 and 90° is due to the effect of the conductor aspect

ratio [5] and the fact that the applied magnetic field is not exactly perpendicular to the current for all portions of the strand at all angles. The

cable pitch is about 17°, which would increase at 0° by about 8? [6] for

the portions across the cable face. At 90° the applied magnetic field is

perpendicular to the cable face, but it is not perpendicular to portions of

the cable edges. The major deformation direction for the cable edge is 90°

from that for the cable face. The edges have their lowest at 0°. The

central region of the cable face does not have very much change in between

0 and 90° because the strand is almost round there.

The critical current as a function of position at 0° and 90° for the

Sample B is given in Figs 5 and 6. The critical current values varied by as much as 35% from the highest at 0° and 33% at 90°. At 0° the central flat of

the thin edge had the lowest 1^, and unlike Sample A, it did not increase very

much at 90°. For both samples the difference between the thin and thick edges

were similiar, as were the corner values. The central flat of the thin edge

is the weak link and would appear to have a degraded rather than a large

aspect ratio effect.

Field angle dependence

The biggest difference in I^’s along the strand occurs with the magnetic

field at 0° (parallel to the wide face of the cable). This is unfortunately

near the orientation that is most critical for a dipole magnet. The orientation. at the midplane of the magnet is 90° but the magnetic field is lower

there than at the poles. Therefore, the limiting case is expected to be at

the poles for both the inner and outer windings. The actual orientation of

the cable and magnetic field at the pole is between 0° and 15° (for both

windings) but, as shown below, the values at 0 and 15° are about the same.

Critical current as a function of angle for each tap on Sample A is shown

in Figs 7, 8, and 9. These Iq's were determined using an electric field

strength criterion of 0.1 yV/cm. These curves are smooth to within the precision of the measurement. There is some asymmetry between the positive and

negative angles that may be due to which side of the strand the self field and

applied field are adding or a bias put in by the cable lay direction. There

is some structure in the plots at 45° which represents a cross over or compromise between the 0 and 90° character. The values at 0 and 90° show most of

the information and are concentrated on in what follows.

The critical current as a function of angle for each tap on Sample B is

shown in Figs 10, 11, and 12. The biggest difference between the two samples

is that the 1^^ of Sample B at the thin edge was lower and did not increase at

90°. The higher values of in these plots have a larger uncertainty because

they were at or near the quench current.

Electric field profiles

The electric field profile along Sample A near the quench current is

shown in Figs. 13 (at 0°) and 14 (at 90°). The value of maximum E was defined

53
CRITICAL

CURRENT

SAMPLE B AT 0 BE6PEES

POSITION , cm

Figure 5. Sample B, critical current versus position at 0°.

54
SAMPLE B AT 90 DEGREES

<c

I-LUC

<r

C_J)

cn

C_3

350

325

300

275

250

225

200

0 1 2 3 4 5

POSITION , cm

Figure 6. Sample B, critical current versus position at 90°,

55
SAMPLE A

IQ ! I I I I I I I I I i I I I I I I I I I ^ I ^ 1 -120 -90 -60 -30 0 30 60 90 120

ANGLE , degrees

Figure 7. Sample A, critical current versus angle for thin edge taps.

56
SAMPLE A

I_L C

CT

C_3 275

cn

CJ)

250 -

225 -

200

tap symbol

2a o

2b ^

2c 0

J I I I I I

1

I

-120 -90 -60 -30 0 30 60 90 120

ANGLE , degrees

Figure 8. Sample A, critical current versus angle for cable face taps.

57
SAMPLE A

120 -90 -60 -30 0 30 60 90 120

ANGLE , degrees

Figure 9. Sample A, critical current versus angle for thick edge taps,

58
SAMPLE B

ANGLE , degrees

Figure 10. Sample B, critical current versus angle for thin edge taps.

59
SAMPLE B

-120 -90 -60 -30 0 30 60 90 120

ANGLE , degrees

Figure 11. Sample B, critical current versus angle for cable face taps.

60
SAMPLE B

ANGLE , degrees

Figure 12. Sample B, critical current versus angle for thick edge taps.

61
as the electric field a few tenths of a second before thermal runaway started.

Slight changes in quench current for the different observations causes large

changes in the electric field even in logarithmic space. At 0°, the flat

portion of the thin edge is clearly the weak link and the asymmetry of the corners is still present. The profile at 90° is more uniform.

The electric field profile along Sample B just before quench is shown in

Figs. 15 (at 0°) and 16 (at 90°). The profile for this sample is much more nonuniform than for Sample A. The center of the thin edge is more clearly the

weak link. The points with values of E below 0.01 pV/cm in Fig. 15 were assigned a value of 0.01 pV/cm in order to keep the plot scale relevant.

Effective critical current

A relevant question at this point is: What is the effective critical

current of these strands as a function of angle? One possibility is the

current at which the average (see the Experimental section) electric field

strength is equal to some criterion. The effective critical current and

quench current for Sample A are plotted as a function of angle in Fig. 17.

The effective critical current at various electric field criteria had an angular dependence very similar to the quench current. This was not expected

considering that the quench current should be strongly dominated by the degraded section (or weak link) of the strand. This was not the case, which

suggests that the average E does give a relevant critical current. The end

cooling around the weak link must have been sufficient that the local heating

did not have an adverse effect on 1^. The weak link at 0°, tap 1b, contributed about 55% of the total voltage even though it only represented 5% of the

total length.

The effective critical currents and quench current for Sample B are plotted as a function of angle in Fig. 18. There are two curves without

symbols. The upper was the quench current after a shunt was placed on the

thin edge. The lower was the quench current before the shunt was placed on the thin edge. The lowest curve is the effective at 0.1 pV/cm. The weak

link at 0°, tap 1b, contributed about 95% of the total voltage even though it

represented only 5% of the total length.

Table 2 shows the percentage differences of the Iq's from the starting

wire (before cabling) values. The starting wire I^'s are approximate because

they were measured as another part of this program in another magnet. Notice

that the additional lowering of the weak-link for Sample B resulted in a much lower effective 1^. In fact, the difference in effective Iq’s was greater than the difference in weak link I^’s at 0.1 pV/cm. At 0°, of

Sample B was an additional 12% lower than that of Sample A but the effective

was 20% lower. The difference in I^'s at higher electric fields, however,

was more than 12%. This increased effect may show that the local heating had

an adverse effect on for Sample B.

Current-transfer voltages

The current-transfer correction as a function of position for Sample A at

0 and 90° respectively are plotted in Figs. 19 and 20. These electric fields

correspond to very small voltages. The small negative values for some positions may be a real, but slight, voltage projection phenomenon [7]. This

62
SAMPLE A AT 0 DE6REES

63
SAMPLE A AT 90 DE6REES

POSITION . cm

Figure 14. Sample A, maximum electric field versus position at 90°.

64
SAMPLE B AT 0 DEGREES

Figure 15. Sample B, maximum electric field versus position at 0°.

65
SAMPLE B AT 90 DEGREES

POSITION , cm

Figure 16. Sample B, maximum electric field versus position at 90°.

66
SAMPLE A

ANGLE , degrees

Figure 17. Sample A, effective critical current and quench current versus ang
C_J

i_u

LU

1_L_

L±_

U_J

SAMPLE B

2QQ ‘

' ' * ' ' I ' ' ' ' ' • ' ' III! L__l I 1 I -120 -90 -60 -30 0 30 60 90 120

ANGLE . degrees

Figure 18. Sample B, effective critical current and quench current versus angle.

68
Table 2. Percentage differences in from starting wire I^'s

at 5 T, 4.0 K, and 0.1 jaV/cm

Sample A Sample B

Starting Wire 320 A 347 A

Field Angle 0° 90° 0° 90°

highest segment + 2.8^ - 4.7$ - 4.6$ - 4.0$

lowest segment - 25 . 3 % -15.3$ -37.5$ -34.9$

effective - 6 . 9 % - 7.5$ -27.7$ -22.8$

69
amount of current transfer voltage indicates that there is not very much cu- rrent redistribution taking place at about 10 % of Current redistribution

that takes place closer to will show as a lowering of n, the exponent in

the empirical relationship V a

The current transfer correction as a function of position for Sample B at

0 and 90°, respectively, are plotted in Figs. 21 and 22. The voltage on the

center of the thin edge is significant, at both 0° and 90°. If this is intrinsic to the cable (i.e., occurs at every thin edge), it may start to have

implications on the performance of the cable. The difference between 0 and

90° may be due to the relative improvement of for the center of the thin

edge at 90°. The fact that it is larger on this strand than on Sample A,

measured in the same way, may indicate that it is intrinsic. However, more current-transfer voltage will appear in a short sample geometry. If the implications of this are significant, an experiment can be designed to determine how much is intrinsic.

The shape of the voltage-current curve near can give insight into

state of the filaments in a conductor [8]. A figure of merit is the exponent

in the empirical relationship V a A plot of the exponent n as a function

of angle for each segment of Sample A is shown in Figs. 23, 24, and 25. The

net value is 25 to 30, which is typical for cables. The highest values are an artifact of a fit to a limited number of points at low voltage with a high

percentage of noise. The dip in the exponent for the center of the cable face

around 0° is not now understood.

Other parameters

A plot of the electric field at 330 A as a function of angle for each

segment of Sample A is shown in Figs. 26, 27, and 28. These plots display

another asymmetry of the corners which was expected. For the corners of each

edge, the peaks in the electric fields (lowest 1^) occur on opposite sides of

90°. This is not the mass flow asymmetry discussed above but an aspect ratio

effect. It occurs because the net deformation direction for these two corners

are shifted on either side of 90° (see the cable cross sections).

The critical temperature for a given magnetic field, T*, is a parameter

in the linear approximation of the temperature dependence of 1^^. Table 3

shows values of T* that were determined for some angles and segments of both

samples at 5 and 8 T. Because of the small tap separations, the uncertainty

of these values was about ±0.3 K at 5 T and ±0.1 K at 8 T. These data at 8 T

indicate that T* was a little higher for Sample B (larger filaments) and that

there may not be much angular and positional dependence.

The results of the data at 8 T for both samples were very similar to

those presented at 5 T. The only difference observed was a slight decrease in

the range or distribution of critical currents at 8 T. All of the curves had

the same general character.

E. DISCUSSION

Understanding the relationship between the mechanical deformation in the

cabling process and the electrical performance may lead to improvements in the

70
SAMPLE A AT 0 DE6PEES

C_J

cn

i_u

L_l_

CD

cn

POSITION , cm

Figure 19. Sample A, transfer electric field versus position at 0°.

71
CJ

cm

L_U

I_l_

CO

cm

SAMPLE A AT 90 DEGREES

POSITION , cm

Figure 20. Sample A, transfer electric field versus position at 90°.

72
SAMPLE B AT 0 DEGREES

Figure 21. Sample B, transfer electric field versus position at 0°.

73
SAMPLE B AT 90 DEBPEES

C_J

cm

L_U

Ll_

cn

cm

POSITION , cm

Figure 22. Sample B, transfer electric field versus position at 90°.

74
EXPONENT

SAMPLE A

Q

I I 1 I I I I I I I I I I I I I I ^ 1 i \ I I -120 -90 -60 -30 0 30 60 90 120

ANGLE , degrees

Figure 23. Sample A, exponent versus angle for thin edge taps.

75
LU

CZD

CLX

LU

SAMPLE A

Q

I I 1 I I I I I \ I 1 I I I I \ ^ I I I I I I -120 -90 -60 -30 0 30 60 90 120

ANGLE , degrees

Figure 24. Sample A, exponent versus angle for cable face taps.

76
EXPONENT

SAMPLE A

Q

\ I I 1 I I I I 1 I \ 1 L_i I I I 1 1 \ 1 ^ 1 -120 -90 -60 -30 0 30 60 90 120

ANGLE . degrees

Figure 25. Sample A, exponent versus angle for thick edge taps.

77
SAMPLE A AT 330

ANGLE , degrees

Figure 26. Sample A, electric field at 330A versus angle for thin edge taps.

78
SAMPLE A AT 330

ANGLE , degrees

Figure 27. Sample A, electric field at 330A versus angle for cable face taps.
SAMPLE A AJ 330

ANGLE , degrees

Figure 28. Sample A, electric field at 330A versus angle for thick edge taps.

80
Table 3. K

Sample A Sample

5T Tap 0° 90° Tap

1b 7.39 7.78 1 a

2b 7.57 7.68 1b

3b 7.88 7.46 3b

8T Tap 0° 90°

• Tap

1b 5 . 68 5.67 la

2b 5.62 5.58 1b

3b 5 . 66 5.57 3b

0

°

7.81

7.98

7.68

0

°

6.00

5.95

5.77

81
critical current of a cable. Comparisons of these two samples should be tempered by the fact that the starting wires were not the same but were similar

enough to support the conclusions drawn from each sample.

Additional current sharing that can take place in the cable will help

reduce the effects of local changes in critical current. However, the experiment of shunting Sample B by soldering another strand to it (with no transport

current of its own) may indicate that the best case would only be a 1 % improvement. On the other hand, any compromise in end cooling could have a significant effect. One proposed [9] change that would compromise stability

is use of a Cu-Ni matrix to possibly improve the drawing to finer filaments.

This could decrease the strand to that of the local weak link, a significant decrease.

An open question in this work is: What does this imply about cable mea- surements? The answer may be beyond the scope of the program. The aspect

ratio effects are opposite for different parts of the conductor and the thin

edge is clearly the weak link. If the local were homogeneous along a strand, the solution would be simpler [4]. The fortunate observation was that

the effective criterion corresponds to an average electric field criterion

(or resistivity criterion) if end cooling is adequate. In the bifilar, short

cable measurements, the critical current may have to be measured in both cur- rent directions with the magnetic field perpendicular to the cable face in

order to attempt to cancel the asymmetry in the cable edges. This may not be

a problem for the better cables because of the thin edge at 90° is not as depressed as for more degraded cables. For a more degraded cable, the peak

field on the thin edge may further lower to the point where local heating

becomes a problem.

A comparison of these results on extracted strands with independent

measurements [3] on the same cable is instructive. The degradation in Sample

A measured here was very close to its cable measurements using the same material. Using their notation [4] for degradation, the strand value measured

here was 7? (see effective in Table 2), its cable value was 9/S. The anisotropy of Sample A was 1 1 /S for the cable measurement and \ % for the strand

measurement. The difference in anisotropy may have to do with the details of

self-field effects and no conclusions are drawn from this difference. The

values of degradation in Sample B were also very close to the cable measurement; the strand value was 235S, the cable value was 22^. The Sample B anisotropy was -11/S for the cable (I^^ at 0° lower than at 90°) and -5/S for the

strand. Thus, the limiting case for Sample B was 32/S less than starting wire

according to the cable measurements and 285S according to strand measurements.

The agreement between these two measurements was good but further improvements

may be possible.

These results have two implications for short-sample critical-current

testing of cables for SSC. First, these data indicate that both magneticfield orientations need to be tested in order to determine the limiting case

for the critical current. As shown above, for more degraded cables, the

parallel-field case may limit. In the less degraded cables, the perpendicular

case limits. The amount of degradation is not known beforehand, so both

orientations need to be measured.

82
Second, the difference between thick and thin edge degradation shown in

these data indicates that changing the direction of the test current (for each

field orientation) will affect the critical current. This is because the

location of the peak magnetic field changes with current direction (see Fig. 3

in [4]). Thus, the peak field location relative to the thin edge should be

recorded. The conservative approach in short-sample cable tests would be to

make the measurement for both directions of the test current and use the

critical current for the worst case. The additional measurements will help

increase the understanding of cable degradation.

There might be differences in cable degradation for cables made with a powered turkshead. The cables tested here were made without power to the

turkshead. Future tests will be made when samples and time permit.

Other future tests that have been considered are: inner 23 strand cable,

cable with strand twist and cable lay in the same direction, and outer cable

with 29 strands instead of 30. In the present dipole magnet design, the cable

used for the inner windings has 23 strands with a copper-to-superconductor

ratio of 1 .3/1 • A lower ratio may cause more anisotropy in filament deformation and result in a larger aspect ratio effect. A lower ratio may also com- promise the end cooling around the locally depressed and could introduce

more degradation to the local 1^. Making the outer cable with 29 strands

instead of 30 may decrease the amount of degradation that occurs on the cable

edges because of less edge compaction. The compaction at the edge will be

less due to the lower number of strands and also possibly due to the odd number of strands. The odd number staggers the position on each side of the

cable where the strands round the cable edge, thus avoiding having a strand

centered on each edge at the same time as the cable passes through the turkshead. There is a possibility that the 29 strand cable may have a higher

critical current than the 30 strand cable with the same cross-sectional area

because the edge degradation dominates the 30 strand cable. The 29 strand

cable would also result in a saving in material. The strand diameter might

have to be increased in order to make the cable mechanically stable. This may

also further increase the critical current of the cable.

F. CONCLUSIONS

1 . Cabling can lead to very localized reduction in critical current within a single strand. The lowest critical current was observed for the central flat

part of the thin edge and this segment contributes most of the overall voltage drop.

2. The widest spread in local critical current values along the cable strands

occurs with the magnetic field perpendicular to the cable edge. Unfortunately, this is near the orientation that is most critical for a dipole magnet.

3. The cable corners have a critical-current asymmetry in the same cable

pitch direction. This may be due to mass flow during the cable fabrication.

4. The relevant critical-current criteria for local measurements may be a spatial average, because of the extreme localization of damage, limited extent

of the voltage, and strong end cooling effect which all limit the local tem83
perature rise. Any compromise in sample stability may lower the critical

current to that of the weak link.

5. Despite large local variations in critical current, the quench current and

effective critical current do not vary much with angle. The effects of the

conductor aspect ratio and cable pitch angle are opposite for different segments of the cable strand, causing them to tend to cancel.

6. Any additional lowering of the thin edge critical current is directly

reflected in the quench current and effective critical current of the cable

strand. For critical-current degradation greater than observed in Sample A,

the critical current of the weak link seems to dominate.

ACKNOWLEDGMENTS

The authors extend their thanks to S. L. Bray and W. E. Look for sample

holder construction, data analysis, and plotting; R. M. Scanlan and J. M.

Royet (Lawrence Berkeley Laboratory) for samples and discussions; R. A. Shenko

for typing this report; and to the rest of the Superconductor and Magnetic

Measurements Group.

REFERENCES

1 . R. Scanlan, J. Royet, and C. E. Taylor, "Superconducting Materials for the

SSC" Adv. Cryog. Eng. - Materials, Vol. 32, Plenum Press, New York (1986) in

press.

2. D. C. Larbalestier , A. W. West, W. Starch, W. Warnes, P. Lee, W. K. McDonald, P. O'Larey, K. Hemachalam, B. Zeitlin, R. Scanlan, and C. Taylor,

"High Critical Current Densities in Industrial Scale Composites Made From High

Homogeneity Nb 46.5 Ti", IEEE Trans, on Magnetics, MAG 21, 269, 1985.

3. W. B. Sampson, Brookhaven National Laboratory., Upton, New York, and R. M.

Scanlan, Lawrence Berkeley Laboratory, Berkeley, California, private

communications (1986).

4. M. Garber and W. B. Sampson, "Critical Current Anisotropy in NbTi cables,"

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

5. 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.

Cryog. Eng. - Materials, Vol. 32, Plenum Press, New York (1986) in press.

6. 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).

7. L. F. Goodrich, J. W. Ekin, F. R. Fickett, "Effect of Twist Pitch on Short-Sample V-I Charater istics of Multifilamentary Superconductors," Adv.

Cryog. Eng. - Materials, Vol. 28, Plenum Press, New York (1981), p. 571~580.

84
8. J. W. Ekin, "Superconductor Coil Characterization, Quarterly Progress

Report", NBS Report # SR-724-3-82, March 1982. J. W.Ekin, "Superconductor

Coil Characterization, Quarterly Progress Report", NBS Report SR-724-6-82,

June 1982.

9. 5th Workshop on NbTi Superconductor, March 25~27, 1986, Menlo Park, CA

85
IV. EFFECT OF FILAMENT IRREGULARITY ( " SAUSAGING" ) ON ELECTRICFIELD VS CURRENT CHARACTERISTICS OF NbTi SUPERCONDUCTORS

by J. W. Ekin

SUMMARY

This article contains two internal reports of the Superconductor

and Magnetic Measurements Group of the National Bureau of

Standards, issued consecutively in March and June, 1982.^’^

Report I contains the first measurements of a correlation between

filament Irregularity (sometimes called "necking" or "sausaging") and the shape of a superconductor’s electric-field vs. current characteristic. Data are presented which show that the

greater the extent of irregularity in filament crossect ional area

the flatter the take-off in the electric field vs. current (E-I)

characteristic. The shape of the E-I characteristic is quantified in terms of the resistive transition parameter n, defined by

E a I*^. Low values of n indicate significant filament irregularity. It is proposed in Report I that the parameter n could be

used as a valuable index of filament quality in evaluating different superconductors for practical applications.

A model is also suggested in Report I to explain this effect.

The necking depresses the critical current of a filament

locally, which forces current to transfer across the normal

matrix material into (unnecked) neighboring filaments. The

current transfer is intrinsic to the wire and occurs continually

along its length wherever the filaments are necked down to a smaller crossect ional area. These currents crossing the matrix

generate significant voltage at transport current levels well

below the critical current, which causes the reduction in the

value of n. Report II presents a quantitative relationship between n and the

statistical distribution of filament diameters measured for the

NbTl superconductors studied in Report I. The relationship may

be useful as a quick method of estimating the extent of sausaging

in practical mult i f i lamentary NbTi superconductors from measurements of n.

1 R In the years since these reports were first publicized, ’ the

correlation of n with filament sausaging and the f i lament-nec k i ng

model originally proposed here have been confirmed by further^^ ^ research at several of

Brookhaven National Lab

S 7 Wisconsin.-^’ '

The transition parameter n was introduced some time ago by

Voelker° in another connection relating to filament breakage

rather than sausaging. The n value (n=3) was significantly lower

than the n values reported here; in a sense it represents a logical extension of filament sausaging to the extreme case of

the U. S. superconductor manf acturers

,

Fermi Lab,^ and the University of

86
filament breakage. We suspect that filament sausaging may have

also contributed to the relatively low n values (7 ^ n ^ 12) in

some of the "good" mu It i f i lament ary samples he measured.

The identities of the superconductors have been relabeled, but

otherwise the reports appear in their original form.

I. EFFECT OF SUPERCONDUCTOR FILAMENT IRREGULARITY (Report No.

SR-724- 1 3-82 )

During critical-current testing of several NbTi conductors,

it was noticed that the electric field versus current (E-I) characteristic for one conductor did not show the sharp take off behavior normally observed in other NbTi:copper superconductors we

have tested. Instead, the rise in voltage in this conductor

started well below the critical current, 1^^, showing a gradual

rise as I approached I rather than a sharp rise right at 1^^ (see

Sample I in Fig. 1). This limits the usefulness of the conductor

in magnet applications, since a significant electric field in the

conductor below leads to heating and decreased stability.

Several tests were consequently undertaken to deter-mine the

source of this electric field.

The effect may be quantified by describing the superconductor E-I

characteristic in terras of the equation.

where is the critical current and E^ is the elec tr i c- f i e 1

d

criterion at which is determined. The larger the exponent n is, the sharper the E-I takeoff at I^. Thus, large n is desirable for magnet applications.

In Fig. 2, log E/Eq versus I/Iq is plotted for Sample A as

well as another previously tested conductor, sample B, in order

to determine comparative values of n. As seen from the curves in

Fig. 2, sample B has an n value of about 35, whereas sample A has

an n value of only about 18.

One possible source of the low n in Sample I is filament

irregularity. If there is sausaging in the filaments, current

will transfer out of a filament where it necks down, into a neighboring filament which can carry the extra current. When the

current in the neighboring filament encounters a necked region,

it too will be forced out of that filament through the matrix

material and into another filament (perhaps back into the

original filament if its diameter is back to average size or

greater). In this manner, filament irregularity forces current

transfer through the resistive matrix material continuously,

causing significant electric fields along the conductor well

below I^, resulting in a low value of n.

87
o

m

UJ

88

Figure

1

.

Normalized

electric

field

vs

current

plots

for

three

raultif

ilamentary

NbTi

superconductor

,

showing

the

more

gradual

resistive

transition

in

sample

A

compared

with

samples

B

and

C.
E/E

I/Ic

Figure 2. Logarithmic plot of electric field versus current for

the curves in Fig. 1 , showing the low value of n in

sample A compared with samples B and C (n is defined

by the relationship V a I^).

89
To test this hypothesis a series of scanning electron

micrographs were made of the filaments in each conductor. The

copper matrix of each conductor was chemically removed and the

filaments photographed longitudinally along their length. The

results are shown in Figs. 3A and 4A. Significantly greater

filament irregularities were found in sample A compared with

sample B. As shown in Table 1, the average diameter fluctuations

AD for sample A amounted to about 9 ym, or about 33/S of its

average filament diameter <D>. In sample A, on the other hand,

AD was less than 7 ym, or about 20/S of its average filament

diameter. The amount of filament irregularity in these conductors thus correlates well with degradation in the value of n charac t er i z i ng their E-I curves.

To see if this large filament irregularity is a characteristic of conductors containing a large volume fraction of copper,

a third conductor. Sample C, was also tested. The conductor contained a large amount of copper similar to sample A but had a somewhat larger filament diameter. As shown in Fig. 5 and Table

1 , AD for this conductor was only about 6^S of its <D>. The E-I

character ist i c of this conductor was correspondingly sharper,

with a measured n value of 46, which is more than twice that of

sample A. Thus, large values of AD need not be an inherent

characteristic of conductors containing high percentages of

copper stabilizer.

In summary, these results show that the degradation in the value

of n for these mu 1 1 i f i 1 amen t ar y NbTl conductors correlates well

with the amount of filament irregularity observed in each and

that such irregularities need not be a characteristic of conductors containing a large fraction of copper. The value of n measured for sample A is significantly lower than that measured

for other NbTi conductors and these data would suggest it could

be substantially improved by reducing the large filament diameter

irregularities observed in this conductor. Furthermore, it would

seem that the parameter n could be used as a valuable index of

filament quality in evaluating different superconductors. Certainly, n should be an extremely useful parameter in characterizing the quality of a superconductor's E-I characteristics,

complementing the I^ value usually supplied with commercial

conductors.

II, RELATIONSHIP BETWEEN n AND THE STATISTICAL DISTRIBUTION OF

FILAMENT SIZES (Report No. SR-724-26-82 )

Report I reported that the electric f i e Id - ver su s- c ur r en

t

characteristics for sample A did not show the sharp take-off

normally found in other NbTi:copper conductor we have tested.

The effect was quantified in terms of a parameter n, which for

sample A was only about 18, compared with values of n=35 for

sample B and n=53 for sample C (another high copper -to-NbTi ratio

superconductor). The low value of n was ascribed to irregularities ( " sausag i ng" ) in the filament diameter of sample A as

90
SAMPLE A

Figure 3. Longitudinal (a) and cross-sectional (b) views of the filaments In

Sample A.

91
SftMPLE 8

Figure 4. Longitudinal

Sample B.

(a) and cross-sectional (b) views of the filaments in

92
SAMPLE C

Figure 5. Longitudinal (a) and cross-sectional (b) views of the filaments I Sample C.
observed in SEM longitudinal micrographs of isolated filaments.

In order to quantify these preliminary observations, a series of

SEM micrographs of the complete cross section of the conductor

have been obtained (shown in Figs. 3B-5B). From many measurements of the individual filament diameter in these micrographs, a statistical distribution of the filament diameters in each conductor has been obtained. Figure 6 shows the distribution of

filament diameters D about the average filament diameter of each

ring of filaments <D>. The spread in the distribution for sample

A is much broader than for samples B or C, confirming the earlier

qualitative treatment given the filament irr egular it ies . The

half width of the filament diameter distribution correlates very

nicely with the variations in the parameter n. This evidence is

consistent with the current-transfer hypothesis advanced in the

previous quarterly report as the explanation for the relatively

large electric field appearing in sample A at currents below the

critical current.

From SEM micrographs in Fig. 3-5 note the presence of the

large center island of copper in conductor Samples B and C,

compared with the lack of this feature in Sample A. It is

believed this center copper island is a significant factor in

minimizing the filament irregularity problem (particularly in

conductors having a high copper- to-superconductor volume ratio).

Note added in proof:

The n values reported above were nearly independent of

magnetic field from 3 T to 7 T; over this field range they varied

by less than about A graph of the standard deviation of the

filament diameter distributions shown in Fig. 6 is plotted as a function of n in Fig. 7. The approximate relationship shown in

Fig. 7 may be useful for quickly estimating the extent of

filament irregularity in practical mul t i f i lame n tar y NbTi

superconductors from measurements of n.

ACKNOWLEDGMENTS

The author extends his appreciation to E. S. Pittman and D.

Rule for their assistance in reducing the filament-diameter

distribution data, and to L. F. Goodrich for several helpful

discussions. The Initial work was sponsored by the Naval Ship

Research and Development Center. Further data reduction and the

publication of these reports were supported by the U. S.

Department of Energy, Division of High Energy Physics under

Interagency Agreement No. DE - A 1 0

5

- 8 5 ER 40 2 M 0

.

Ref er ences

1) J. W. Ekin, Superconducting Coil Characterization, Quarterly

Progress Report No. SR-724- 1 3"82 , March 1982, unpublished.

94
2) J. W. Ekin, Superconducting Coil Characterization, Quarterly

Progress Report No. SR - 7 2

4 - 2

6 -8 2 , June 1 982, unpublished.

3) First Workshop on NbTi Superconductor, University of

Wisconsin, Madison, WI , July 1983.

4) Second Workshop on NbTi Superconductor, University of

Wisconsin, Madison, Wl, July 1984.

5) Third Workshop on NbTi Superconductor, Lawrence Berkeley

Laboratory, Berkeley, CA, January 1985.

6) M. Garber, M. Suenaga, W. B. Sampson, R. L. Sabatini, Adv. in

Cryo. Eng. (1986) (to be published).

7) W. H. Warnes and D. C. Larbales t ier , p. 156 in Proc . Int '

1

. Symp. on Flux Pinning and Electromagnetic Properties in

Superconductors , Matsukuma Press (1986).

8) F. Voelker, Particle Accelerators 1, 205 (1970).

95
Table

1.

Filament

Diameter

Variations

for

Three

NbTi

Conductors

CO LTi on

c: r— on LO

co

-p x:

03 4-> A S tJO Q =T oo CO L C V

O (U —

1

^ 1—

1

(D Q

Q s LH CM

<3 a • . .

' ^ oo CO CM

X c oo O cO

03 •rH oo CM o

S E • • • Q a '

• A ^ Q

•rH V

• E CO lo-

• s 3. CM m on

bO o3 '

>< Q

E-< JD to- z to- CO un

• • • • • lO O

-p x;

OT o

•r^ X> E oo on LTi 5 -p o • • . E- Ou ' CM

1 * s- o

O -P OO CO

s: Du '

CMO s- •

• • E

0) '—

o 03

SI E P

•rH E X X •o

00 —

»— CO o

in LTi o

• • • o o

X

o < CQ o

-po 03 CD (D 3 1—

1

1 1

1—

1

•o cx CX CX c E E E

o 03 03 CCS o OO CO CO
0.75 0.80 0.85 0.90 0.95 1.0 1.05 1.10 1.15 1.20

D/<D>

Figure 6. Distribution of filament diameters in samples A, B,

and C. Horizontal bars indicate the sampling interval

for each histogram. Irregular i ties in the cross sectional area of each filament mainly determine n,

rather than variations in the average filament

diameter from filament to filament. In these samples,

the average filament diameter varied with radial

position in the composite. The radial effect was

eliminated from the plotted distribution by determining the variation in diameter of each filament

relative to the average diameter of other filaments

occupying the same relative radial position in the

compos i te

.

97
EXPONENT

n

(E

a

T)

STANDARD DEVIATION OF DIAMETER DISTRIBUTION (%)

Figure 7. Appr6xiraate relationship between n and the standard

deviation of the filament-diameter distribution for

several mul t i f i lamentary NbTi superconductors.

98
V APPENDIX - ELECTRON TUNNELING INTO SUPERCONDUCTING

FILAMENTS: DEPTH PROFILING THE ENERGY GAP OF NbTi

FILAMENTS FROM MAGNET WIRES

by John Moreland, J. W. Ekin, and L. F. Goodrich

(To be published in Adv. in Cryo. Eng. 3

2

, 1986 .)

SUMMARY

A new technique for measuring the depth profile of the energy

gap in filaments extracted from commercial NbTi conductors was

developed. A demonstration of the technique was made on NbTi conductor typical of that manufactured in the early 1970 ’s. It was

found that the energy gap in a filament surface layer several micrometers thick was reduced to approximately one third of that in the

filament interior. The energy-gap degradation was found to correlate

with the presence of very small amounts of copper ( 3 wt%) in the

filament surface layer. Further study of the energy gap in NbTi alloy

with controlled amounts of copper contamination are needed to clarify

the observation and its implications for the manufacture of fine

filament NbTi superconductors.

99
ELECTRON TUNNELING INTO SUPERCONDUCTING FILAMENTS: DEPTH PROFILING THE

ENERGY GAP OF NbTi FILAMENTS FROM MAGNET WIRES.*

John Moreland, J. W. Ekin, and L. F. Goodrich

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado

ABSTRACT

Squeezable electron tunneling (SET) junctions consisting of superconducting NbTi filaments (extracted from magnet wires) and sputtered Nb thinfilm counter electrodes were used to determine the energy gap at the

surface of the filaments. The current versus voltage curves of junctions

immersed in liquid helium at 4 K were measured for a series of filaments

taken from the same wire. Each filament had been etched to remove a surface

layer of varying thickness so that the energy gap could be determined as a function of depth into the surface of an "average" filament. It was found

that some manufacturing processes yield filaments having surface layers

with reduced energy gaps of 0.4 meV compared to measured interior bulk

values ranging from 1.2 to 1.3 meV.

INTRODUCTION

Multifilamentary-NbTi superconductors are used extensively in the

construction of large-scale magnets. This is primarily due to the relative

ease of manufacture and the strain insensitivity of the superconducting

properties of NbTi filaments. With the proper heat treatment, NbTi alloys

precipitate Ti rich inclusions within the material. It is generally

accepted that these precipitates act as vortex pinning sites that give rise

to the large critical currents observed in NbTi filaments. A good understanding of the alloy metallurgy is therefore vital to developing better

high-current superconductors.^’^ Perhaps equally important are the effects

of various steps in manufacturing processes on microscopic intrinsic

superconducting parameters. In particular, the magnitude of the superconducting energy gap. A, directly affects T H and along with alloy

metallurgy, determines

An electron tunneling experiment is an excellent probe of the fundamental parameters that govern the superconducting state, providing a direct

measurement of the energy gap. This is because the conductance of a superconducting tunnel junction as a function of applied bias (often

*Contribution of the National Bureau of Standards, not subject to copyright.

100
referred to as a tunneling spectrum) is directly related to the density of

states of thermal excitations in a superconductor as a function of energy.

Recently it has been shown that electron tunneling experiments can be

performed using ultrastable electromechanical transducers to space two

electrodes about 1 nanometer apart, stable to within a few picometers.® In

this way the oxide barrier previously required for tunneling measurements

has been eliminated, providing an alternative method for obtaining tunneling

spectra. The concept of mechanically adjustable barriers is attractive

because it is relatively straightforward to make a tunneling contact to any

conductor including bulk materials. In particular, we have shown in an earlier paper that a superconducting squeezable-electron tunneling (SET)

junction can be constructed between a filament and a thln-film counter

electrode in liquid helium at 4 K.^

Tunneling electrons probe a superconducting surface to a depth of

about one BCS coherence length. It should therefore be possible to delinlate the superconducting properties of a material as a function of depth to

within submicron distances. Presented here, for example, are results

obtained when the SET-junction technique mentioned above was applied to

NbTi superconductors. Individual filaments were removed from multifilamentary NbTi superconductors by chemically etching away the surrounding copper

stabilizer. The surfaces of many such filaments were then etched different

amounts to obtain the "depth profile" of the energy gap of the "average"

filament within a given conductor.

APPARATUS

The experimental apparatus used to "squeeze" filament SET junctions is

described in earlier papers.^* ^ Essentially the filaments were positioned

above a 0.2 pm -thick Nb counter electrode sputtered onto a flexible Si substrate with 1 pm-thlck evaporated SiO spacers. An electromagnetic squeezer

adjusted the bow in the substrate and thus the distance between the filament

and the Nb counter electrode. In this way tunneling barriers a few nanometers thick could be obtained with stability sufficient for tunneling

spectroscopy of energy gaps. The squeezer was mounted on a probe designed

for rapid measurements at 4 R in a liquid-helium storage Dewar. It was possible to cool, perform a tunneling measurement, and warm within a 30-

mlnute time span.

The I-V curve was monitored over the range from -5 to +5 mV as the

junction was mechanically adjusted in liquid helium to obtain a tunneling

characteristic. Four-terminal contact was made to the junction. Superconducting counter electrodes were used because they gave rise to distinct

structure in the I-V curves that provided a quality check for the tunneling

barrier even if the sample electrodes were normal. Also, it was easier to

detect small energy gaps for the sample electrodes as shifts in the I-V

characteristic using a superconducting counter electrode, compared to

shifts of a broad zero-bias inflection in a relatively featureless curve using a normal counter electrode.

Curves of dynamic conductance (dl/dV) versus voltage were measured

with a tunneling spectrometer that utilized a feedback circuit to keep a constant modulating voltage (250 jiV, 100 Hz) across the junction. This

signal was superposed onto the slowly swept dc voltage (± 5 mV, sweep rate

® 100 pV/s). The conductance was proportional to the first-harmonic signal

measured with a lock-in amplifier. For two superconducting electrodes the sum of the energy gaps, and ^

2

, can be estimated from the location of

the peaks in the conductance spectrum of the junction using the formula

eV - 2A^ + 2L^. (1)

101
V is the voltage separating the peaks in the conductance spectrum that occur near the current inflections in the 1-V curve and e is the electron

charge. This procedure is used instead of measuring + A

2 from an apparent zero bias so that small preamplifier offsets need not be considered

in the energy gap measurements.

The energy gap of the Nb thin-film counter electrodes was determined

by measuring the tunneling energy gap of a SET junction with two identical

thin films and found to be l.A meV. One of these calibrated counter

electrodes was then transferred to the f ilament-tunneling apparatus. The

contribution from the counter electrode = 2.8 meV) was subtracted

from the sum of the energy gaps to obtain the energy gap of the filaments

Typically the data lead to measurements precise to within ±

Tunneling contacts were nondestructive in that the magnitude of the

tunneling energy gap was independent of the resistance for a given junction

setting as long as the junction was not shorted. Since a nondestructive

tunneling contact was made between the filament and the film and care was taken to avoid moisture condensation between runs, the counter-electrode

films could be reused many times.

FILAMENT SAMPLES

Individual NbTi filaments from 20 to 30 ym in diameter (filaments with

diameters less than 20 ym were difficult to mount in the apparatus) were

removed from multif ilamentary-NbTi superconductors by etching away the

surrounding Cu matrix in a 50/50 solution of nitric acid and water at room

temperature. Subsequent etches to remove varying-thickness surface layers

from a single filament were performed in freshly prepared solutions consisting of A parts hydrofluoric acid, 36 parts nitric acid, and 60 parts water

at room temperature. The etch rate was determined for several filaments

etched different amounts using a scanning electron microscope (SEM) to

compare adjacent etched and unetched portions of the filaments. The etch

rate was constant over the measured etching range from 0 to 10 ym. It had

a value of 50 ± 5 nm/s, which was about the same as the etch rate for pure

Nb.

Typical SEM micrographs of etched NbTi filaments are shown in figure 1. The filaments in figures la, lb, and Ic were taken from a wire that was made using a conventional manufacturing process that included high temperature extrusion followed by cold working and a final Cu annealing step.

This wire was made in the early 1970 's and may not be representative of

conductors fabricated using current technology. The small nodules a few

microns in size on the surface of the unetched filament (Fig. la) have been

attributed to a Cu-NbTi surface layer that forms during the

2hgt extrusion

and subsequently breaks up into debris during cold working. ’ After a short etch (20 s) these nodules are removed from the filament surface (Fig.

lb) . Further etching serves not only to reduce the filament diameter but

also polishes the filament surface, removing striations acquired during the

wire-drawing process (Fig. Ic)

.

Other filaments tested include those extracted from "cold-process”

wire previously developed for experiments aimed at understanding mixture

and alloy formation and their effect on NbTi superconductors.® This wire

was not optimized for a high J . It was processed without any hightemperature extrusion, was drawn at low temperatures, and was given no heat

treatments during processing. SEM micrographs of unetched filaments of this

kind were devoid of Cu-NbTi nodules (fig. Id).

102
ISytim

Fig. 1 Electron micrographs of various NbTi filaments.

a) Unetched conventionally processed filament with Cu-NbTi nodules.

b) Conventionally processed filament after 1 pm etch.

c) Conventionally processed filament after 6 ym etch.

d) Unetched ’’cold-process” developmental filament.

DEPTH-PROFILE RESULTS

Figure 2 shows the tunneling spectra (conductance versus voltage) of a series of NbTi-f ilament SET junctions with Nb thin-film counter electrodes

immersed in liquid helium at 4 K. Each filament had been etched for a different time. The etching depths are based on the measured etch rate of

50 nm/s mentioned above. The peaks defining the gap edges in each of the curves in Fig. 2 are used to determine the energy gap of the filaments.

Data derived from curves like those shown in figure 2 are presented In

profile format in figure 3. Here we plot the filament energy gaps derived

from the conductance curves as a function of estimated etching depth for

two different kinds of wire. The dashed lines are included to aid the

viewer’s eye. Each data point represents a local measurement of the energy

gap. The variability of the data taken at a given etch depth may be an indication of the variability between filaments. It may also, however, be

due to changes that occur along the surface of a filament.

103
NbTi-Nb SET for

conventional wire

(0-1 /im etch, Fig. la, 1b)

NbTi-Nb SET for

conventional wire

(5 etch. Fig. 1c)

j*—5.4mV—

NbTi-Nb SET for

cold-process wire

(unetched. Fig. 1d)

Nb-Nb SET for

counter electrode

calibration

Fig. 2 Tunneling spectra (conductance versus voltage) of various

NbTi-f ilament/Nb-f ilm SET junctions in liquid helium at 4 K.

The results for wire manufactured using the conventional method are shown in figure 3a. In general, eV increases with increasing etching times

from a value as low as 3.2 meV to about 5.4 meV, corresponding to filament

energy gaps that range from 0.4 meV at the filament surface to 1.3 meV in

the filament interior (see equation 1). There appears to be a 3- to 4-ym

thick layer at the surface of an unetched filament with a substantially

reduced energy gap compared to the highest interior bulk value.

Figure 3b, on the other hand, shows the profile of a wire that had

undergone cold drawing without any heat treatments or extrusion, thus

avoiding the formation of Cu-NbTi mixtures. The energy gap is relatively

constant as a function of depth having a value of about 1.2 to 1.3 meV even

at the unetched-f ilament surface.

Also included in figure 3 are the corresponding energy-dispersive

X-ray microanalysis results that show the weight percent Cu as a function

of filament etch depth for both types of wire. A discernable difference

(dashed line) from the instrumental background signal (solid line) is

apparent in Fig. 3c for conventionally processed filaments that had undergone an etch of 4 ym or less. [The dashed line is shown for reference

only; the signal to noise ratio is about 2, which does not permit the

detailed shape of the Cu profile to be determined.] In contrast, very

little Cu above background levels could be detected in the cold-process

filaments as shown in figure 3d.

104
1.4

1.2

I

1.0

I

0.6 «S 0.4

<

0.2

1 1 1 1

1 1 1 • •

• ;

1.4

1.2

1 r—

• i - • .

• .

• ' /

.

% 1.0

T— • J E

0.8

/ /

••1

0.6 -

CO £ 0.4 -

a)

1 1

1 1 1—1—

<

0.2 b)

1 1 012345 678

012345678

Etch Depth (^m)

0 12 3

Etch Depth (^m)

7

3

o

o> I

0 12 3

Etch Depth (^m)

Fig. 3 Depth profiling results for NbTi filaments from magnet wires:

^filame t ^ function of estimated etch depth for an average

fila^n? extracted from (a) conventionally processed and

(b) developmental cold-processed wires (T = 4 K) . Weight percent

Cu determined by X-ray microanalysis as a function of estimated

etch depth for an average filament extracted from (c) conventionally processed and (d) developmental cold-processed wires. The

instrumental Cu background signal was approximately 2%.

The experiment was repeated with both the conventionally-processed and

the cold-processed wires etched simultaneously in the same hydrofluoricnitric-water bath. The same X-ray microanalytic Cu concentration results

were obtained as before. This indicates that the difference between the

two types of samples was not simply an artifact of surface contamination

introduced when the filaments were etched in different baths of the same

solution.

Energy-dispersive X-ray microanalysis is a semi-quantitative technique

that yields average weight percent of an element over the area illuminated

by the SEM beam (in our case about 10 ym^) . Further, the penetration depth

of the 25 keV beam used in these studies is about 1 ym. In comparison,

tunneling electrons probe only to a depth of about one BCS coherence length

(about 10 nm for NbTi) over a very small SET junction area (probably less

than 1 ym*)

.

CONCLUSION

We have demonstrated a new depth profiling technique for studying the

energy gap in practical superconductors. The results show that a NbTi

conductor made by conventional manufacturing methods of the early 1970's

105
had filament surface layers with energy gaps as low as O.A meV at A K. The

low gap region extended 3-5 pm into a 30 ym filament. NbTi wires that had

not undergone high-temperature processing showed very little energy-gap

reduction at the filament surface (A 1.3 meV at A K)

.

The tunneling data combined with the X-ray microanalysis on the

conventionally-processed filaments suggests that there is a correlation

between the presence of Cu and the observed low energy gaps near the

filament surface. The amount of Cu detected is very small, less than about

3 wt % above the instrumentational background signal. It is unlikely that

it is simply an artifact of SEM electron penetration through a thin (< 1 ym) high-Cu content intermetallic layer. This is because these small Cu

concentrations are measured at etching times well beyond the point where a thin intermetallic layer should have been chemically removed. We conclude,

therefore, that a low-concentration Cu diffusion layer is introduced during

processing. It is unclear at this time if this layer is particular to the

early 1970 's conductor which we tested, or whether it occurs more generally.

Further studies will be needed to accurately clarify its nature and origin.

filament

Other phenomena, including surface mechanical damage or oxygen contamination introduced during conventional processing may also be important. In

addition, the etching process used to remove filament surface layers may

affect the outcome of a given profile measurement. The fact that A,

does not decrease for the cold-process wire and Increases in the

conventionalprocess wire with further etching shows that at least the etch

is not detrimental to the energy gap. It may, however, change the Cu

concentration from the original diffusion profile. A more conclusive and

more quantitative study might therefore include tunneling into bulk alloys

purposely contaminated with known amounts of Cu.

Depending on the current flowing through and the magnetic field within

the filaments of a magnet wire, the effective area of each filament may be

substantially reduced by the presence of a surface layer with a degraded

energy gap. Considering that many applications require superconducting

magnets to be operated near their critical parameters, the observed energygap degradation could have significant effects (especially at high fields near H

2

) • It is therefore important to isolate the manufacturing steps

that lead to the formation of a degraded-superconductor layer.

ACKNOWLEDGMENTS

Partial support was provided by the Department of Energy. Support was

also provided to one of us (J.M.) as a National Research Council Postdoctoral

Associate. Helpful discussions with R. Scanlan, A. West, D. Larbalestier

,

M. Suenaga, M. Garber, and P. Purtscher are gratefully acknowledged.

REFERENCES

1. D. C. Larbalestier and A. W. West, New perspectives on flux pinning

in niobium-titanium composite superconductors, Acta Metall . 32:1871-1881 (198A).

2. M. Garber, M. Suenaga, W. B. Sampson and R. L. Sabatini, Effect

of Cu^Ti compound formation on the characteristics of NbTi

accelerator magnet wire, to be published in IEEE Trans, on Nuclear

Science, 1985; D. C. Larbalestier, Li Chengren, W. Starch, and P. J.

Lee, "Limitation of critical current density by intermetallic

formation in fine filament Nb-Ti superconductors," to be published

in IEEE Trans, on Nuclear Science, 1985; see also papers CZ-2 and

CZ-3 of this conference proceedings and the references cited

therein.

106
3. J. Bardeen, L. N. Cooper and J. R. Schrleffer, "Theory

of Superconductivity”, Phys. Rev. 108:1175-1204 (1957).

4. A. A. Abrikosov, On the magnetic properties of superconductors of

the second type, Sov. Phys. JETP 5:1442-1452 (1957).

5. R. Hampshire, J. Sutton and M. T. Taylor, Effect of temperature on

the critical current density of Nb-44 wt. percent Ti alloy, in:

"British Cryogenics Council Conference on Low Temperatures and

Electrical Power”, Program preprint HR, Royal Society, London,

England (1969), pp. 69-76.

6. J. Moreland and P. R. Hansma, Electromagnetic squeezer for

compressing squeezable electron tunneling junctions. Rev. Sci.

Instrum. 55:399-403 (1984).

7. J. Moreland and J. W. Ekin, Electron tunneling into

superconducting filaments using mechanically adjustable

barriers, Appl. Phys. Lett. 47:175-177 (1985).

8. A. W. West, Rensselaer Polytechnic Institute, Troy, New York, private

communication.

107
NBS-1 14A (REV. 2-80

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See; nstructions)

1. PUBLICATION OR

REPORT NO. NBSIR 86-3061

2. Performing Organ. Report No. 3. Publication Date

December 1986

4. TITLE AND SUBTITLE

Electromechanical Properties of Superconductors for High-Energy Physics Applications

5. AUTHOR(S)

J. W. Ekin, L. F. Goodrich, J. Moreland, E. S. Pittman, and A. F. Clark

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP)

Division of High-Energy Physics

U.S. Department of Energy

Washington, DC 20545

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)

This report covers the first year of a 33-month project to establish a systematic base of experimental data on electromechanical effects in superconducting wire and cables for high-energy-physics magnet applications. The

research is focused in four general research areas: electromechanical

relationships in NbTi, studies of NbTi strands extracted from cables, sausaging

effects in NbTi, and studies of the NbTi energy gap.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) cables; electromechanical properties; high-energy physics; magnets; NbTi; stress

effect; superconductors

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[1^ Uni imited PRINTED PAGES

1 1 For Official Distribution, Do Not Release to NTIS

1 1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

112

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