\

NISTIR 89-3912

ELECTROMECHANICAL PROPERTIES

OF SUPERCONDUCTORS FOR

HIGH-ENERGY PHYSICS APPLICATIONS

Part II

J. W. Ekin

L. F. Goodrich

S. L. Bray

N. F. Bergren

R. B. Goldfarb

National Institute of Standards and Technology

U.S. Department of Commerce

Boulder, Colorado 80303-3328

November 1989

NISTIR 89-3912

ELECTROMECHANICAL PROPERTIES

OF SUPERCONDUCTORS FOR

HIGH-ENERGY PHYSICS APPLICATIONS

Part II

J. W. Ekin

L. F. Goodrich

S. L. Bray

N. F. Bergren

R. B. Goldfarb

Electromagnetic Technology Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

November 1989

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, Robert A. Mosbacher, Secretary

National Institute of Standards and Technology, Raymond G. Kammer, Acting Director

CONTENTS

Page

Abstract 1

Summary 3

Effect of Room Temperature Stress on the Critical Current

of NbTi

S.L. Bray and J.W. Ekin 5

Electromagnetic Characteristics of NbTi Strands Extracted

From Rutherford Cables

L.F. Goodrich and S.L. Bray 23

The Effect of Local Area Ratio and Filament Diffusion-Barrier

Material on Cr it ical - Current Bend Degradation of Multifilamentary

NbTi Wires

J.W. Ekin, S.L. Bray, and N.F. Bergren 97

Measurement of Saturation Magnetization and Intrinsic

Coercivity of High-Permeability Steel for the SSC

R.B. Goldfarb 123

Appendix: Publications 129

Irregularity in Nb-Ti Filament Area and Electric

Field Versus Current Characteristics

J.W. Ekin, Cryogenics 27., 603 (1987) 131

Relationships Between Critical Current and Stress

in NbTi

J.W. Ekin, IEEE Trans. Mag., MAG- 23 . 1634 (1987) 137

Studies of NbTi Strands Extracted From Coreless

Rutherford Cables

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

IEEE Trans. Mag., MAG-23 . 1642 (1987) 141

Current Capacity Degradation in Superconducting

Cable Strands

L.F. Goodrich and S.L. Bray, Applied Superconductivity

Conference, ASC, San Francisco, California, Aug. 1988,

IEEE Trans. Mag., MAG-25 . 1949 (1989) 145

iii

ELECTROMECHANICAL PROPERTIES OF SUPERCONDUCTORS

FOR HIGH-ENERGY PHYSICS APPLICATIONS

J.W. Ekin, L.F. Goodrich, S.L. Bray, N.F. Bergren, R.B.Goldfarb

Electromagnetic Technology Division

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

This report presents data on superconductor performance

under mechanical load. The data are needed for setting

mechanical design constraints and measuring the electromechanical performance of NbTi superconductors for DOE

high-energy physics magnet applications. The effect of

axial tensile stress, applied at room temperature, on the

critical current of NbTi superconductor strands has been

measured. The data show a simple result that the effect

on the critical current is independent of the temperature

at which the stress is applied; this allows the existing

4 K data base to be used to determine critical current

degradation from room temperature fabrication stress,

from cool-down stress introduced by differential contraction, and from 4 K stress generated by the Lorentz force

when the magnet is energized. A study of the criticalcurrent variations along NbTi strands extracted from a Rutherford cable has been made also. The results show7 that

the principal mechanical degradation is extremely localized

at the regions w

There the NbTi strand is bent around the

edge of the cable. For example, only 3% of the total

strand length can contribute 92% of the total strand

voltage. A further study has been made of the effects of

bending strain on the critical current of NbTi conductors.

The degradation of the critical current from bending strain

is much greater at low values of electric field than at

high, suggesting that irregularity of the filament crosssectional area introduced by bending may be the source of

the I c degradation. The bend tolerance of a NbTi conductor

can be enhanced by increasing the local copper-to-superconductor area ratio. Measurements of the permeability,

saturation magnetization, and intrinsic coercivity of

several high-permeability steel alloys were made. The

overall differences of saturation magnetization and intrinsic coercivity between cold-rolled steel samples were

not significant.

Key words: bending strain; cable; coercivity; critical

current; electromechanical; magnetic properties; NbTi;

permeability; Rutherford cable; saturation magnetization;

SSC; Superconducting Super Collider.

1

Summary

This report presents data on superconductor performance under mechanical

load. The data are needed for setting mechanical design constraints and

measuring the electromechanical performance of NbTi superconductors for DOE

high-energy physics magnet applications. The main objective of the program 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.

In an earlier report from this program (NBS Internal Report # NBSIR 86-

3061), several basic aspects of the electromechanical degradation problem were

studied. Highlights of that report include the following:

*The critical current (I

c

) degradation in NbTi from transverse

compressive stress is comparable to that from axial tension.

^Results on superconductor strands extracted from keystoned Rutherford

cables indicate 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.

*An approximate quantitative relationship was established between

filament sausaging and the shape of the electric - field vs. current

(E vs. I) characteristic, given by n where E a I

n

)

.

Highlights of this report include the following:

The first measurements of the effect of axial tensile stress, applied at

room temperature, on the critical current of NbTi superconductor strands have

been measured. The data show a simple result that the effect on the critical

current is independent of the temperature at which the stress is applied; this

allows the existing 4 K data base to be used to determine critical current

degradation from room temperature fabrication stress, from cool-down stress

introduced by differential contraction, and from 4 K stress generated by the

Lorentz force when the magnet is energized.

A study of the cr it ical - current variations along NbTi strands extracted

from a Rutherford cable has been made. The results show that the principal

mechanical degradation is extremely localized at the regions where the NbTi

strand is bent around the edge of the cable. For example, only 3% of the

total strand length can contribute 92% of the total strand voltage. The

degree of variation depends on the orientation of the applied magnetic field.

However, the overall voltage is nearly independent of the field angle. All

conductors tested showed small resistive voltages associated with current

transfer in the regions of deformation.

A systematic study of the effects of bending strain on the critical

current of NbTi conductors has been performed. The I

c degradation decreases

with increasing magnetic field, suggesting that the source of degradation is

not an intrinsic degradation of H^, as with the elastic axial-strain effect.

The degradation of the critical current from bending strain is much greater at

low values of electric field than at high, suggesting that irregularity of the

3
filament cross-sectional area introduced by bending may be the source of the

I

c degradation. By testing a series of similarly fabricated NbTi wires, we

found that the bend tolerance of a NbTi conductor can be enhanced by

increasing the local copper- to - superconductor area ratio (LAR) . Improved bend

tolerance also correlates with using a filament diffusion barrier material

made of vanadium, instead of Nb or Nb/CuNi. The dependence of the bend

degradation on LAR represents a mechanical limit to reduction of the LAR when

controlling filament- sausaging effects.

Many high-energy-physics accelerators use high-permeability steel for

the return flux path of the superconducting magnets. Measurements of the

permeability, saturation magnetization, and intrinsic coercivity (H

c

) of

several high-permeability steel alloys were made. Values of Hc at 76 K ranged

from 110 A/m (1.4 Oe) for cold-rolled samples to 170 A/m (2.1 Oe) for hotrolled samples. The results were independent of whether the field was

parallel or perpendicular to the rolling direction. Hc was slightly larger at

liquid-nitrogen temperature than at room temperature. At liquid-helium

temperature, Hc is expected to be slightly higher still. The overall

differences of saturation magnetization and intrinsic coercivity between coldrolled steel samples were not significant.

4
Effect of room temperature stress on the critical current of NbTi

S. L. Bray and J. W. Ekin

National Institute of Standards and Technology

Electromagnetic Technology Division

Boulder, Colorado 80303

ABSTRACT

The effect of axial tensile stress, applied at room temperature, on the

critical current of NbTi superconducting wire was measured and compared with

the effect of tensile stress applied at liquid-helium temperature ("4 K) . The

results of these measurements indicate that the effect on the critical current

is independent of the temperature at which the stress is applied. Thus, the

existing 4 K data base can be used to determine I c degradation from room

temperature fabrication stress, from cool-down stress introduced by

differential contraction, and from 4 K stress generated by the Lorentz force

when the magnet is energized. To generalize these results for arbitrary

matrix- to- superconductor volume ratios, the data are presented in terms of the

stress on the NbTi portion of the composite conductor. Methods for

determining the stress on the NbTi from the total composite load are

presented

5
2

INTRODUCTION

Superconducting composites used in the construction of large-scale

magnets may be subjected to several sources of mechanical stress. These

stresses occur within three different temperature ranges: room temperature,

the transition between room and liquid-helium temperatures, and liquid-helium

temperature (~4 K) . At room temperature the sources of stress are associated

with the fabrication of the magnet; they are predominantly winding tension and

bending stress arising from cabling and conductor -winding operations. In the

transition between room temperature and 4 K, stress on the superconductor

arises from differential thermal contraction between the superconductor and

other materials that compose the magnet. Finally, at 4 K the source of stress

is the Lorentz force that is generated when the magnet is energized.

Until now, critical -current degradation in NbTi from uniaxial stress has

been measured only for stresses introduced at liquid-helium temperature.^-'^

This paper presents the results of the first measurements of the effect on

critical current of tensile stress applied at room temperature. The results

indicate a simple general relationship: the stress effect on the critical

current of NbTi is independent of the temperature at which the stress is

applied. The results should be useful in setting limits on the acceptable

tensile stress resulting from the combined effects of fabrication,

differential thermal contraction, and Lorentz force, particularly in large

magnet systems. This result has implications for the fundamental

understanding of the source of the stress effect on the upper critical field

(Hc 2 ) and critical-current density (J c )

6
3

EXPERIMENTAL APPARATUS

The superconductor used in this experiment is a multifilamentary NbTi

sample, containing 648 filaments in a copper matrix. The characteristics of

the sample are presented in Table I.

Tensile stress is applied to a 28 cm long straight sample of the

conductor by a servohydraulic feedback load system having the capabi lity of

load, stroke, or strain control. The background magnetic field is applied

perpendicular to the direction of stress by a 7 T radial - access solenoidal

magnet. Current is supplied to the sample by a 1000 A, controllable battery

supply. Details of the apparatus are described in Ref. 2.

EXPERIMENTAL PROCEDURE

The experimental procedure consists of two separate tests. The first

test (4 K test) is conducted entirely at liquid-helium temperature and serves

as a baseline for the room- temperature stress test. The sample is cooled to

liquid-helium temperature under no load, and then the critical current is

measured as a function of magnetic field and stress applied at liquid-helium

temperature. In the second test ( room- temperature test) another sample of the

same wire from the same spool as the first sample is stressed to a selected

level at room temperature and then lowered to liquid-helium temperature while

the load is maintained constant by the servohydraulic load system. Once

temperature equilibrium is reached at 4 K, the critical current is measured at

several applied magnetic fields.

In order to compare the room- temperature stress effect with the 4 K

stress effect in a generic way, the results are presented here in terms of the

stress on the NbTi portion of the composite. Comparisons in terms of overall

composite stress are specific to only one conductor because of the variations

7
4

in the copper - to - superconductor volume ratio for different conductors. The

relative portions of the total load carried by the Cu matrix and the NbTi

depend primarily on their volume ratio and their mechanical properties;

however, copper creep and loading history can also somewhat affect the

relative loading. For these reasons, the I c- degradation data are presented in

terms of NbTi stress. The method for applying these results to an arbitrary

conductor is discussed in the PRACTICAL APPLICATION section.

RESULTS

The results of these tests are presented graphically in Fig. 1 where

normalized critical current (I c /I cm ) i- s plotted versus the stress on the NbTi

component of the composite (NbTi Stress) . The normalized critical current is

calculated by dividing the critical current (I c ) measured at a particular load

and magnetic field by the maximum critical current (I cm ) measured at no load

and at the same magnetic field. The result (I c /I cm ) Is expressed as a

percent

I cni is measured after the sample is unloaded from its room- temperature

stress point. This procedure is justified by comparisons made between the

measured critical currents for the unloaded room- temperature samples and

virgin samples of the same wire that have not been loaded. These comparisons

show that, within the sample- to-sample variation measured for the virgin wire,

the critical-current recovery of the unloaded room- temperature samples is

complete

For comparison, the plot contains data points from a 4 K test and three

room- temperature tests at three magnetic fields and various stress levels.

The solid symbols correspond to the room- temperature tests and the open

symbols correspond to the 4 K test. The close agreement between the 4 K and

8
5

room- temperature data points indicate that the stress effect on NbTi is

independent of the temperature at which the stress is applied. The largest

disagreement between the 4 K and the room- temperature data occurs at the

maximum room- temperature stress level (~1.6 GPa) where the NbTi filaments are

just starting to yield (at room temperature). Here the room- temperature data

points are consistently slightly lower than the 4 K data points. The maximum

discrepancy occurs at 7 T where the critical current is approximately 7% lower

for stress applied at room temperature than for stress applied at 4 K. This

difference is less than the uncertainty of the room- temperature measurement.

METHOD OF DETERMINING NbTi STRESS

As stated above, the comparisons of I c degradation for stress applied at

room temperature versus 4 K shown in Fig. 1 are based upon the stress state of

just the NbTi portion of the conductor. The NbTi stress was used to make the

results more generic and not dependent on a specific copper- to - superconductor

ratio. Because direct measurement of the stress on the NbTi portion of the

composite is not practical, a method of extracting this information from the

available composite stress data is required. In the Appendix we discuss the

detailed breakdown of each component's load-strain relationship for relating

overall composite stress to the stress experienced by just the NbTi filaments.

An example of the use of the technique is shown in Fig. 2. The heavy lines in

Fig. 2 show the s tress - strain curve for the composite upon repeated loading

and unloading. The repeated loading portions of the curve between zero load

and the upper envelope has been omitted for clarity. [These portions of the

curve are equivalent to the A-to-C segment of the composite curve of Fig. 5 in

Appendix A and are not required for constructing the NbTi curve.]

9
6

The two other curves shown in Fig. 2, the "Cu" curve and the "NbTi"

curve, are derived from the composite curve by using the technique described

in the Appendix. This technique, however, is only applicable in the area

labeled "Linear Region." The lower nonlinear portion of the "Composite" curve

is due to yielding of the Cu, while the upper nonlinearity is due to yielding

of the NbTi. Consequently, extrapolation is necessary in order to determine

the NbTi load outside this linear region. This extrapolation is possible

based on two assumptions: one, that the "NbTi" load curve is linear below its

yield point and, two, that the "Cu" load curve is linear above its yield

point. Both of these assumptions are supported by independent s tress - strain

data on these materials.^

The Cu load points, indicated by triangular symbols in Fig. 2, are

determined by taking the difference between the composite and NbTi load

curves. The line defined by these symbols is then extended to the maximum

strain of the composite curve, again assuming linearity of the Cu curve above

the yield point. Finally, the upper portion of the NbTi curve is constructed

by taking the difference between the composite and Cu load curves. This

general technique was used to determine the NbTi stresses for both the room

temperature and the 4 K data that are presented in Fig. 1; (Fig. 2 shows only

the 4 K data)

.

PRACTICAL APPLICATION

Figure 3 is a plot of normalized I c versus NbTi stress for a number of

magnetic fields at 4 K . and it can be used to estimate the effect of stress at

room temperature. However, the stress on the NbTi portion of the conductor

must first be determined. Ideally, this should be achieved as described above

by measuring the room- temperature composite load-versus - strain characteristic

10
7

of the conductor and then constructing the associated NbTi load- versus - strain

curve. Due to the specialized equipment required for making these stress

measurements, this technique may, in some cases, be impractical.

Alternatively, a method for estimating the NbTi stress without the aid of

these measurements may be used.

Figure 4 is a plot of NbTi stress versus composite stress for several Cuto-NbTi volume ratios. The dashed curve (CuiNbTi = 1.77:1) is based on the

conductor used in these tests. The other curves were derived by using various

Cu-to-NbTi volume ratios. These curves are based on a linear model (equation

1 below) of the composite load- versus - strain characteristic and they are

sensitive to the material properties used in the model. The material

properties of the conductor tested here were used in generating these curves

and the values of these parameters are given below, following their

definitions. For NbTi superconductors in general, these curves may be used to

estimate, at least to first order, the NbTi stress and the resulting I c degradation due to room- temperature loading. If the material properties of a

conductor are known, they may be used in the following equation to more

accurately estimate the NbTi stress:

ENbTi [ L/Ac - e ym(l ' (^Cu’^Cu)

]

aNbTi = +

°V,

^

fENbTi + ( 3. - f )Mcu

where

aNbTi = the tensile stress on the NbTi

ENbTi = the modulus of elasticity for the NbTi (6X10^ N/m^)

L = the composite load

Ac = the cross-sectional area of the conductor (0.33 mnw)

11
8

f = the volume of NbTi in the conductor divided by the total conductor

volume

E£u = the modulus of elasticity for the Cu (7X10^ N/m2

)

MCu = the slope of the stress -versus - strain curve, above its yield point,

for the Cu matrix (3X10^ N/m2

)

o

p = the residual pre-stress on the NbTi due to the manufacturing process

(1X10 8 N/m2

)

tym = the composite strain at matrix yield (0.002)

This equation is applicable only beyond the Cu yield point, which is the

region where the effect of stress on the I c becomes large. For strain levels

that are below the Cu yield point the I c degradation is negligible.

CONCLUSIONS

The results of these tests indicate that the effect on critical current of

tensile stress applied to NbTi superconductors at room temperature is the same

as that of stress applied at 4 K. Thus, we have the simple result that

degradation from room- temperature loading can be determined from the existing

4 K stress - effect data base given in Fig. 3. To generalize the results for

arbitrary matrix- to - superconductor volume ratios, the data in Fig. 3 are

presented in terms of NbTi stress, which can be determined using the methods

outlined in the text. Because room- temperature and 4 K stress have an

equivalent effect on I c , we think that Fig. 3 can also be used to estimate the

effect of stress at intermediate temperatures generated by differential

thermal contraction between the different conductor and magnet materials

during cool-down. Presently, the typical level of tensile stress applied to

NbTi composites during magnet fabrication ranges from about 40 to 180 MPa. At

these stresses the resulting I c degradation is negligible. The data presented

12
9

here are useful in defining the acceptable limits of tensile stress in

forthcoming magnet designs where increased winding tension may be necessary

for restricting conductor motion at high Lorentz forces. For the conductor

measured in this study, a 5% degradation in I c occurs at a composite tensile

stress of approximately 550 MPa at 5 T and 390 MPa at 9 T.

13
10

APPENDIX: METHOD TO DETERMINE STRESS ON THE NbTi COMPONENT

Figure 5 is an idealized representation of the load- versus - strain

characteristic for a NbTi and Cu composite over a single load cycle. Three

separate load curves are shown in this figure: one for the NbTi filaments, one

for the Cu matrix, and one for the overall composite. The NbTi curve is a

straight line with a positive load intercept (point a) and no hysteresis

between loading and unloading (the arrows on the curves indicate the loading

sequence) . This reflects the assumption of perfect elastic behavior of the

NbTi for the strains measured here. The positive load intercept a represents

the small residual tensile stress on the NbTi; this stress is a result of the

manufacturing process. Similarly, the Cu curve shows a small negative

(compressive) load intercept (point a). The Cu curve is linear up to point b,

where the Cu begins to yield. There is a transition region between points b

and c followed by another linear region between point c and point d, the

maximum- strain point. This portion of the curve (c-d) is based on the

assumption that the yield strength of the Cu increases linearly with

increasing strain. As the load decreases to zero at point e the curve is

linear, again indicating elastic behavior. The load becomes compressive after

point e and, again, the Cu goes through another transition region followed by

a linear region. At point g the load on the Cu is equal to the load on the

NbTi and represents the unloaded state of the overall composite. The

composite curve is simply the algebraic summation of the NbTi and Cu curves.

The dashed lines in Fig. 5 are extensions of the linear portions of the

load curves and are needed for constructing the NbTi load curve from the

composite curve. A fundamental assumption made in drawing the Cu load curve

14
11

is that of symmetry between the compressive and tensile mechanical properties.

In particular, the curve indicates that the distances from point d to e and

from point e to f are equal. This equality shows that the distances from

point D to E and from point E to F on the composite curve are also equal. In

turn, this allows the determination of the load experienced by the NbTi (point

E) simply by bisecting the line between points D and F. Additional points can

be similarly determined by loading the conductor to higher levels and

unloading

.

15
12

References

:

1. J. W. Ekin, F. R. Fickett, and A. F. Clark, Proc . Int . Cryog. Mat. Conf.

(Aug. 1975), Adv. Cryog. Eng. 2_2, 449 (1977).

2. J. W. Ekin "Mechanical properties and strain effects in superconductors"

Chapt. 7, p. 455 in Superconductor Materials Science , edited by S. Foner and

B. B. Schwartz, Plenum Press, New York (1981).

3. J. W. Ekin, IEEE Trans. Mag. 23, 1634 (1987).

4. R. P. Reed, R. P. Mikesell, A. F. Clark, Adv. in Cryog. Eng. 2_2 , 463

(1977)

.

16
13

Table I. Multifilamentarv NbTi sample

Strand diameter

Filament diameter

Number of filaments

(single stack construction)

Copper-to-superconductor volume ratio

Twist pitch

Superconductor area

Total area

characteristics

0.648 mm

1

6

/im

648

1.77

1.27 cm

0.119 mm^

0.330 mm^

17
(%)

UJ 0

I/°I

v.

:

c.

18

Fig.

1.

Normalized

critical

current

(Ic

/I

cm )

vs.

tensile

stress

on

NbT

i

portion

of

conductor

(NbTi

Stress)

at

magnetic

fields

from

7

to

7

T.

The

onpn

nod

solid

data

points

sliow

!

Ito

nonr

pquivnlonoo

of

Hip

of

foot"

of"

c=

r
Composite

19

FIft.

2.

Load

vs.

strain

characterist

Ics

at. UK

for

composite

conductor

and

derived

charac terits

irs

for

its

individual

components,

Nl>Ti

and

Cu
o

(%)

wo

ll°I

20

Fig.

3.

Normalized

critical

current

(Ic

/I

cm )

vs.

axial

tensile

stress

on

NhT i

portion

of

conductor

(NbTi

Stress)

at

magnetic

fields

from

3

to

10

.

j

for

generalized

use

in

estimat

trip,

the

effect

of

stre.ss

on

critical

curt

ent
(BdS) SS3U1S !iqN

21

NbTL

slross

vs.

composite

stress

cliarac

tor

Ist

ics

for

several

Cu.NbTl

o

approx

lin.i t

e1

y

determine

the

NbTl

stress

required

for

vise

ofH

.
Composite

03Q

22

Fi

p;.

5.

Lond

vs.

strain

rh.ir.irt

or i

rt i

or;

for

composite

conductor

nnd

it

individual

romnonontc:

Nl>Ti

ami

Cu
ELECTROMAGNETIC CHARACTERISTICS OF NbTi STRANDS EXTRACTED FROM RUTHERFORD

CABLES

by L. F. Goodrich and S. L. Bray

A . SUMMARY

The electromagnetic properties of NbTi strands extracted from Rutherford

cables were studied to clarify the effect of mechanical deformation, caused by

the cabling process, on the current capacity of the strands. The mechanical

abuse that these conductors are subjected to present a number of measurement

concerns that are not normally encountered in the testing of superconductors.

Consequently, a major portion of this study was aimed at determining how the

measurement and data - analys is techniques should be adjusted for these

conductors to allow a more complete and accurate determination of their

current capacities. Four different cables, all of which are prototypes for

the Superconducting Super Collider's dipole magnets were studied. The

extracted cable strands were instrumented to allow measurement of the voltage

across several key regions of mechanical deformation as a function of current

and the orientation of the applied magnetic field. The resulting data are

presented in several forms besides critical current in order to more

thoroughly characterize the conductors' electromagnetic properties. The cable

strands show very localized reductions in current capacity that are well

correlated with the regions of high mechanical deformation. For example, at a

particular field orientation, the voltage across a portion of the strand that

is only 3% of the total strand length contributes 92% of the total strand

voltage. Two applied magnetic - field orientations, parallel and perpendicular

23
to the cable's width, have pronounced effects on the electrical properties of

the strand. Both of these magnetic field orientations will arise in

application

.

Although there is a pronounced irregularity in the voltage along the

pitch length of these conductors and the degree of the irregularity depends on

the orientation of the applied magnetic field, the overall voltage of the

entire pitch length is comparatively constant. This is the result of a

statistical effect where a particular field angle that is detrimental to some

of the strand segments is equally beneficial to others. The measured critical

currents for these strands are only a weak function of the field angle, and

they are well correlated with the quench currents. Since certain field angles

result in large reductions in the critical current of individual strand

segments, this indicates that the local degradations do not act as weak links

in limiting the current capacity of the strand to the reduced level of the

most degraded segments. This tolerance for local cr it ical - current degradation

depends upon the strand's good longitudinal thermal conductivity, and any

reduction in this local cooling capacity might limit the strand's current

capacity to that of the weak link.

An important characteristic of all the conductors that were tested in

this study is an intrinsic current transfer that is apparently associated with

their mechanical deformation. In other words, at high-enough currents and

magnetic fields, the deformation of the cable strands results in a periodic

redistribution of the current among the superconducting filaments; as a

consequence, there are small resistive voltages associated with these
conductors. This characteristic definitely affects the measurement of these

conductors, and it may affect their performance in application.

The conductors tested in this study represent a continuing evolution in

design that is aimed at increasing the strand material's precabled current

capacity while decreasing its sensitivity to mechanical deformation. The

results of these tests indicate that considerable progress is being made

toward this goal.

B. INTRODUCTION

The measurements presented here are a continuation of earlier studies^- of

NbTi cable strands extracted from Rutherford- type cables (cables provided by

the Lawrence Berkeley Laboratory) . These are prototype cables for high-energy

physics applications. Specifically, they were designed for use in the inner

and outer windings of the Superconducting Super Collider's (SSC) dipole

magnets. The main emphasis of these studies is the effect of mechanical

deformation, during manufacture, on the cable's current capacity. In addition

to the bending deformation required to achieve the cable's twist pitch, the

cables are compacted into a keystone shape. These mechanical deformations

result in a very localized degradation in the electrical properties of the

cable strands. The deformation of the cable strands results in a periodic

geometry with the various segments of the strands defined by the degree and

type of deformation. The desired products of the current studies is

information that will lead to methods for reducing the current-capacity

degradation associated with the manufacture of superconducting cables, and the

25
determination of specific measurement considerations that will result in a

more complete and accurate determination of that current capacity.

For all the measurements, the orientation of the applied magnetic field

was perpendicular to the cable's longitudinal axis. The angle of the applied

field about this axis was varied from 0° to 360° with special emphasis given

to the 0° (parallel to the cables width) and 90° (perpendicular) orientations.

The reasons for emphasizing these field directions are twofold. First, there

is a pronounced effect on the strands^- electrical properties in these

orientations and, second, both of these field orientations arise in a dipole

magnet

.

The mechanical deformation of the cable strands during the cabling

process results in localized variations in the conductor's current capacity.

The sources of this variation fall into two categories, cross-sectional area

differences and angular effects. The cross-sectional variation of the

superconducting filaments is due to necking and serrated yielding of the NbTi

filaments. Under the second category, angular effects, the orientation of the

applied magnetic field with respect to the various segments of the strand

O

(pitch angle effect^) and their mechanically deformed cross sections (aspect

O ratio effect^) varies along the strand.

Several types of data are presented in order to emphasize different

characteristics of the cable strands. A number of different cables,

manufactured with different virgin wire (source wire for the cable strands)

materials and cabling techniques, were studied. These data allow comparisons

of the various cables. An overall critical current , defined as the critical

26
current (I c ) determined by adding the voltage contributions of each segment of

the strand and then dividing this voltage by the sum of the strand segment

lengths to arrive at an effective electric field, is presented as a function

of the applied field angle. This information is useful in examining the

cumulative effect of variations in the spatial orientation and the aspect

ratio of different strand segments at different field angles. The concept of

an overall I c is also useful for comparing the strand's I c with the I

c of the

virgin wire from which the cable was manufactured. These comparisons indicate

the I c degradation introduced by the cabling process. The variation of the

electric field along the strand's longitudinal axis, or the electric- field

profile, is presented as a measure of the wire's local current capacity. In

order to assess the relative contribution to the total voltage of different

strand segments, the voltage distribution at fixed magnetic field angles along

the conductor's length is also presented. In addition, plots of I c versus

position along the cable strand's length are presented to provide a comparison

of the different strand segments in a more conventional form than the

electric - field or differential -voltage plots do. Finally, the voltage of the

strand segments and of the overall strand, at a fixed current, are presented

as a function of the applied magnetic - field angle to exhibit which field

orientations are critical for different strand segments and for the overall

strand.

C . SAMPLES

Four different cables, denoted as W, X, Y, and Z, were studied. Detailed

specifications for these cables are given in Table 1. Cables W, X, and Y are

27
all outer -winding SSC conductors, whereas cable Z is an inner-winding

conductor. Cable W was re-rolled through a turkshead (for resizing) after it

had been cabled. The re-rolled cable is a worst-case situation that was

studied to more clearly identify the degradation features. Cable X is similar

to W; however, it was not subjected to as much mechanical deformation and is a

more typical example of late- 1985 to early- 1986 prototype outer cables. Cable

Y (1987) is an evolution of cable X where the cabling techniques were refined

in an attempt to reduce current - capacity degradation. Finally, cable Z is

similar to cable Y with respect to the cabling technique, but it is an inner,

rather than an outer, cable.

Single - strand samples of each cable were obtained by cutting an

appropriate length of the cable and then separating the strands. Initially,

the strands were difficult to remove from the cable without causing additional

mechanical damage. However, after several strands were removed, the cable

structure relaxes and additional strands may be removed without damage. These

undamaged strands were selected for the measurements.

28
Table 1 . Parameters for each of the four cables

Parameter W X Y Z

Cable No. SC297 SC305 SC361 SC362

Billet No. 5212-3 5914 5954-4 5955-2

Number of strands per cable 30 30 30 23

Pitch length, mm 69 69 69 79

Cable width, mm 9.7 9.7 9.7 9.3

Cable mid-thick, mm 1.17 1.17 1.17 1.46

Keystone angle, deg 1.24 1.24 1.24 1.6

Filament twist, cm'-'- 0.8 0.8 0.8 0.8

Filament dia., /um 16 5 6 6

Number of filaments 648 6006 4162 7249

Virgin wire dia, mm 0.65 0.65 0.65 0.808

Cu/superconductor ratio 1.77/1 1.64/1 1.78/1 1.40/1

29
D. EXPERIMENTAL DETAILS

Figure 1 is a cross-sectional view of a typical cable showing its key

regions. 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 segments had separations of 2 mm for the outer

cable and 2 . 5 mm for the inner cable. The voltage taps along the face of the

cable had voltage tap separations of about 10 mm for the outer cable (11 mm

for the inner cable) and did not include any of the corner region.

Figure 2 shows the instrumented portion of a cable strand and the location

of the voltage taps along the strand's length. The two key orientations of

the applied magnetic field relative to the strand's face, 0° and 90°, are also

shown. In this view, the cable's face is in the plane of the figure. Voltage

taps on the sample were made using three flat quads (miniature four-conductor

flat cable) of four #32-gauge wires. Within each quad, three adjacent

segments could be measured with a minimum of induced voltage. The end leads

of adjacent quads were connected to the same tap to allow complete coverage of

the sample. An additional voltage -tap pair was connected to the sample so

that it spanned an entire cable pitch including all of the other voltage -tap

pairs. In order to encompass an entire pitch length, the span of this overall

voltage -tap pair extended beyond the thick- and thin- edge taps by about onehalf the face length at each end. Data presented for these extreme strand

positions were derived by subtracting the sum of the measured segment voltages

from the overall voltage and then assuming that the voltages for the two end

30
regions were equal. The quad leads were soldered to twisted-pair leads near

the bottom of the sample test fixture. In this position, the soldered joints

were immersed in liquid helium during data acquisition. The twisted pairs

were continuous from the bottom of the test fixture to the voltmeter's input

cables. The junction between the voltage-tap leads and the nanovoltmeter's

input leads were maintained isothermal by small bags of lead shot placed under

and on top of the junction.

The sample holders are 1/2 in diameter 316 stainless steel rods with a

machined longitudinal channel that is slightly wider than the width of the

cable. The walls and bed of this channel are insulated with a thin layer of

epoxy formed with a TFE (polytetraf luoroethylene ) impressioner . The sample is

mounted in the holder with a thin layer of epoxy between its lower face and

the bed of the sample -holder groove. Small epoxy bridges are then applied

across the sample's upper face, leaving the majority of the upper face

directly exposed to the helium bath while still providing some mechanical

support. The ends of the sample extend past the ends of the stainless - steel

rod to allow attachment to the current contacts. The sample holder and

longitudinal axis of the sample strand are held perpendicular to the field of

a radial access, split-pair magnet. The sample is rotated about its

longitudinal axis resulting in various transverse orientations of the applied

field, as identified in Figures 1 and 2.

All the measurements were conducted below standard atmospheric pressure

(due to the low ambient pressure at the altitude of the test site) , resulting

in an equilibrium liquid-helium temperature of -4.02 K. The I c densities for

these samples at 4.22 K would be about 6% less at 5 T and 12% less at 8 T than

31
those reported here. The I c densities for these conductors were calculated

using the NbTi cross-sectional area, which was determined using the copper-tosuperconductor ratio.

Two four-channel, digital-processing, storage oscilloscopes were

simultaneously used for data acquisition. Five channels were used to monitor

the sample's voltage taps with analog nanovoltmeters, and a sixth channel was

used to monitor the sample current. These signals were measured and averaged

at a number of different current levels (point method) . The resulting data

points were then analyzed to construct the sample's voltage-current

characteristic. This procedure is different from the more typical procedure

where the current is ramped continuously with time while the voltage and

current are recorded. The point method has some advantages over the ramp

method. The point method allows for a longer time - averaged voltage to be

measured (this is beneficial if voltage noise is a problem) without greatly

increasing the time required for taking a complete V-I curve. This is

achieved by selecting a distribution of current-bias points that results in a

more precise measurement. This requires taking a few widely spaced points in

the low- current portion of the curve to establish the base line and then

taking a progressively closer- spaced point distribution as the current

increases toward I c

. The point method greatly reduces the effect of inductive

voltage and sample motion. The disadvantages are that it takes longer because

the voltage has to settle for each data point and the general shape of the

voltage -current curve has to be known a priori in order to determine the

positions of the data-acquisition points along the V-I curve.

During data analysis, the effects of voltage offsets, changes in

thermoelectric voltage, and current- transfer voltage were determined. The

32
voltage offsets and changes in thermoelectric voltage were determined using

the initial and final zero-current points of the waveforms. The

thermoelectric voltage corrections were made assuming a constant drift with

time. This correction was generally only a few nanovolts per centimeter. The

current- transfer voltages were determined using the low-current portion of the

waveforms. These current- transfer voltages may partially be an artifact of

the measurement method since only a 64 mm length of the sample was in full

magnetic field; however, the local voltage data indicate that there is also an

intrinsic component of the current- transfer voltage. There is a further

discussion of current transfer later in this report. Generally, the currenttransfer voltage was quite small because the current contacts were long (~4

cm) and were located more than 6 cm from the nearest voltage tap. A currenttransfer correction was made in the determination of the I c , but no correction

was made on the electric-field or differential -voltage data.

E. RESULTS

The first three conductors measured in this study, designated as cables W,

X, and Y, represent an evolution in the cable fabrication process aimed at

reducing cabling's detrimental effect on the critical current density (J c ) of

the finished cable as compared to that of the virgin wire.^ An evolution has

also occurred in the design and processing of the virgin wire . ^ This

evolution can be characterized by a decrease in filament size aimed at

reducing ac losses while attempting to maintain J c

. Initially, increased wire

drawing that was used to reduce the filament size of wire X in comparison with

wire W resulted in lower J c for the virgin wire. Later, improved
manufacturing processes that were used for wire Y resulted in a substantial

restoration of the J c

. Cable Z is an inner cable that was manufactured and

cabled using techniques similar to those used for cable Y.

Overall Critical Current

Critical - current measurements were made on the virgin wires from which

each cable was manufactured. Three pairs of voltage taps, each pair spanning

about 10 mm, were connected to the specimens. This allowed for a check of I c homogeneity with position. Cri tical - current measurements were also made as a

function of the applied magnetic - field angle. This allowed for a test of I c homogeneity with respect to field angle. The average I c of the virgin wire

was used to determine the relative degradation of each cable. The typical I c variation among voltage taps was ±0.3% at 5 T and ±0.5% at 8 T for cables W,

X, and Y. The I c variation among voltage taps for cable Z was about ±2.2% at

5 and 8 T. A reason for this large variation in the case of sample Z is not

evident. For cables X and Y, the I c variation of a given tap with respect to

field angle (0° to 360°) was measured to be about ±0.6% at 5 T.

Figures 3 and 4 show comparisons between the overall I c density of the

extracted cable strands and the virgin wires from which they were constructed.

The data presented in these plots is tabulated in table 2. Figure 3 shows

this comparison for each of the four conductors for applied magnetic-field

angles of 0° and 90° at 5 T. For cable strands W, X, and Y, there is a

consistent increase in J c at 0°, although the virgin wire's J c decreased

between wires W and X. Recal dangling that cable W is a worst-case situation

with respect to the cabling operation. The higher J c for strand X is due to

34
the greatly reduced mechanical deformation of the strand. Cable Y shows a

greater mechanical degradation of its J c than cable W, but, again, it shows an

increased strand J c due to its high virgin-wire J c

. For cable strand X, the

mechanical degradation of J c was somewhat higher at 90° than at 0° and,

consequently, its J c was less than strand W's. This increased degradation at

90° was not present in the case of strand Y and it showed a higher J c than its

two predecessors. Strand Z was very similar to strand Y.

The 8 T plot, Fig. 4, shows an increase in the strand's J c between W and Y

but there is a decrease between W and X. Comparison between the 5 and 8 T J c degradation for strands W, X, and Y shows a reduced sensitivity to mechanical

deformation at the higher magnetic field for all the strands. The virgin wire

W, which has larger diameter filaments, had a higher relative J c at 8 T than

did wires X and Y. This lower J c dependency (sensitivity) of wire W to

magnetic field results in a higher strand J c for W than for X. Comparing the

Jc degradation of strand Z at 5 and 8 T shows a reverse trend of higher

sensitivity to mechanical deformation at the higher magnetic field. At 8 T

,

strand Z has a considerably higher J c degradation than strand \

r

.

35
Table 2. Cable degradation and critical current density (A/mm2

) at 0.1 ^V/cm

Cable Critical

virgin

current

0°

5

density

T

90°

Cable

0°

degradation

90°

W 2827 2378 2469 -15.9% -12.7%

X 2464 2442 2383 -0.9% -3.3%

Y 3043 2835 2887 -6.8% -5.1%

Z 2933 2738 2713 -6.6% -7.5%

8 T

Cable Critical current density Cable degradation

virgin 0° 90° 0° 90°

W 1306 1183 1194 -9.4% -8.6%

X 1036 1051 1014 +1 .4% -2.1%

Y 1239 1218 1210 -1.7% -2.3%

Z 1257 1121 1092 -10.8% -13.1%

36
Elec trie - Fie Id Profile

Plots of the electric field versus position along the cable strand's

length, or the electric field profile, are presented for all four conductors

at two magnetic fields (5 and 8 T) and two orientations (0° and 90°) in

figures 5-18. At the top of each plot, located outside the axes, is an

outline of the cable strand's geometry positioned to correlate with the

horizontal or position axis. The cable outline is labeled to indicate the

position of the thin edge, the face, and the thick edge relative to the

horizontal axis. The local electric field was calculated by dividing the

measured voltage across a strand segment by its length. The discrete data

points, therefore, represent the average electric field over the strand

segment. The horizontal positions of the data points are at the centers of

each strand segment. The lines connecting the data points are intended as a

visual aid and not an interpolation between the data points. The actual

electr ic - f ie Id profile might have much greater extremes than those indicated

by the averaged data points shown in these plots.

There is an electric- field criterion associated with each of these plots.

This is an overall electric - field criterion arrived at by dividing the voltage

across one cable pitch by the sum of the segment lengths. Consequently, the

current associated with different plots or different curves within a plot are

not the same. This is necessary to compare different cables within the same

plot and to present data taken at different magnetic fields and orientations

in a comparable manner. In other words, each curve represents the electric

field profile of a cable strand at a selected I c criterion, and that criterion

is consistent for all curves within a particular plot. In examining these
plots, recognize the inverse relationship between the electric field and I c

:

a large local electric field indicates a small local I c.

Figures 5, 6, 7, and 8 are electric - field profiles for the outer cables W,

X, and Y at 5 T and 0°. The data in each of these four plots represents a

different electric - field criterion. The dominant elements in the structure of

these curves are the pronounced peaks located at the centers of the thin and

thick edges. In general, the major peak is at the thin edge. The imbalance

between the two peaks is greatest for cable W, the oldest of the three cables,

and progressively less for cables X and Y. This demonstrates a trend toward a

more uniform electric - field profile as these conductors have been refined.

The first cable, W, has the lowest electric field at the thick edge of the

three cables.

The negative electric fields shown in the plots indicate negative measured

voltages, not negative resistances. These negative voltages are projection

f) 7 voltages ’

associated with current redistribution among the filaments. The

strongest evidence of intrinsic (not associated with the location of the

current contact) transverse currents within the strand is a region of negative

voltage bounded by regions of large positive voltages. The curve for cable Y

is the best example of this structure. This type of structure is present in

the center of these plots. This location indicates that the current

redistribution is caused by the longitudinal variation in the relative

current-carrying capacity of the filaments rather than being caused by current

injection at the current contact. Thus, voltages associated with this type of

redistribution will occur periodically along the entire length of the cable

Q O JU

strand.
Although the negative voltages are the most obvious indication of current

redistribution, there are other indicators within the data curves. These

indicators have the same general character as the negative voltages with the

exception of polarity. The best example of this structure is the center

segment (at about 3.5 cm) of cable Y. At low currents, the voltage associated

with this hump in the electric field profile increases rapidly with increasing

current, and indicates an intrinsic I c

. At higher currents, this voltage

becomes a weak function of the current as indicated by the curves of Figs. 6,

7, and 8. This behavior is evidenced by the relative size of this central

hump in the electrical-field profile, for increasing current, in comparison

with the dominant edge peaks. This is further evidence of intrinsic current

redistribution among the conductor's filaments.

Figures 9, 10, 11, and 12 are the electric - field profiles for cables W, X,

and Y at a magnetic field of 5 T and an orientation of 90°. In comparison

with the 0° data, the extremes of the electric field profile are smaller in

these plots. This is due to the effects of pitch angle and aspect ratio when

the center of the strand edges are in a favorable position relative to the 90 °

orientation of the applied magnetic field. This effect is most evident for

cables X and Y where the peak in the electric field, observed at the central

edges in the 0° plots, is suppressed in the 90° case. Comparison of the 0°

and 90° curves for cable W also shows a reduction in the peak electric field

but to a lesser degree than for the other cables. Although the peak electric

field has switched from the center to the corners of the thin edge for cables

X and Y in this orientation, the peak still occurs near the region of greatest

mechanical deformation, the thin edge. Current redistribution and the

resulting current- transfer voltage are lower for this field orientation, as

39
evidenced by the reduced negative voltage and more uniform electric field

profile

.

Figures 13 and 14 are plots of the electric field for cables W, X, and Y

at an applied magnetic field of 8 T and orientations of 0° and 90° . These 8 T

plots are very similar to the 5 T plots. For the 0° orientation, the peaks of

the electric field profile are again at the edges of the strand, with the

larger peak occurring at the thin edge. At 90°, cables X and Y show peaks at

the corners of the thin edge and a strong suppression of the electric field at

the center of the thin edge. Cable W has a reduced peak at the center of its

thin edge at 8 T as it did at 5 T. The 8 T data is presented only at the 0.1

criterion, but it is typical of the similarities between the 5 and 8 T data

at all criteria.

The electric field profile for cable Z, the inner cable, is presented in

Figs. 15, 16, 17, and 18. Comparing these curves with those of the other

cables indicates a strong similarity to cables X and Y. At 0°, and both

magnetic fields, the peaks in the electric field profile are comparable, with

the thin edge showing a slightly higher electric field. Once again, at 90°

and at both fields, the peaks in the profile switch from the center of the

edges to the corners. If a general statement can be made it might be that

cable Z's profile is more uniform than that of X and Y.

Differential Voltage Profile

Figures 19-26 are similar to the electric- field profiles in that they

also exhibit the electrical properties of the cable strand as a function of

40
the position along its length. In this case, the differential voltage across

the various strand segments is plotted as a function of position. The

plateaus, or peak levels, in the curves represent the voltage across the

indicated strand segments at the indicated overall I c

. As was the case for

the electric - field plots, the lines connecting these data are only a visual

aid. Vertical lines, rather than smooth curves were selected because they

more accurately portray the differential nature of these data and more clearly

define the positions of the different strand segments.

The plots show that there is a considerable variation in the lengths of

the various strand segments. Herein lies the significance of these plots,

which show the relative contribution of the separate strand segments to the

overall voltage. In other words, because of the variation in segment length,

the electric field shows only the relative current capacity of the segments,

not each segment's contribution to the total voltage and the resulting overall

I c. For this reason, a comparison between the electric field and differential

voltage profiles shows that the voltage peaks at the strand edges is less than

that at the strand faces. This is due to the greater length of the face

segments. Moreover, the edges, although highly degraded, are a small portion

of the total pitch length of the strand and, therefore, have a limited effect

on the overall I c

.

This comparison between the two types of plots raises a question regarding

which of these parameters, local electric field or total voltage, has the

greatest practical importance. Several factors, including the sample's

longitudinal thermal conductivity, determine which of these parameters are

most indicative of the strand's current capacity. For the samples and

41
conditions existing in these measurements, the total voltage seems to

determine the current capacity of the conductor. This is evidenced by a close

agreement between the measured overall I c and the maximum achieved currents

(quench currents) . For a sample that has poor longitudinal thermal

conductivity, the short-edge segments might act as weak links, limiting the

current capacity at a total strand voltage below that required to reach the

overall I c criterion.

In general, the differential -voltage plots indicate many of the same

strand characteristics as do the electric - field plots. For all cables in the

0° orientation, the majority of the voltage is concentrated at the edges of

the cable with the central portions dominant. At 90°, the voltage

concentration increases at the corners of the edges in comparison with the

centers. The striking difference between these plots and the electric - field

plots is the contribution of the face segments at 90°. A comparison between

the 90°, 8 T, plots for cable Z (Figs. 18 and 26) illustrates this difference.

The electric field profile shows peaks at the corners of the strand edges with

a relatively small contribution from the faces. On the other hand, the

differential voltage profile shows that the majority of the voltage is

contributed by the face segments. This trend is evident for all but cable W

where the center of the thin edge remains the dominant segment.

Critical Current Profile

Figures 27-34 are plots of I c versus strand position. The horizontal

lines bounded by data symbols represent the average I c of the indicated strand

segment over the length of the segment. The vertical lines are, again,

42
intended only as a visual aid. Two data symbols are shown for each data point

to indicate the region over which each I c data point is averaged. The breaks

in the curves indicate missing I c data points for the corresponding strand

segments. Missing I c data occur for regions where I c was higher than the

quench current of the sample; thus, missing data implies relatively high I c

for those regions. Cable W has the most missing points because of its larger

range of local critical currents. A low electric - field criterion, 0.05 ^iV/cm,

was selected to reduce the number of missing points. A current - transfer

voltage correction was made in the determination of I c

.

The curves of I c versus position for the outer cables, X and Y, show a

smaller variation at 90° than at 0°. Because the combination of the pitch

angle and aspect ratio is advantageous to the more mechanically deformed edges

O at 90°, I c is more balanced. In a previous measurement of a W strand, the

thin edge was shunted with another superconductor in order to allow the I c of

the other segments to be measured. Those measurements showed that the

variation in I c was also smaller at 90° than at 0°. There are some problems

with local I c measurements because of the intrinsic current- transfer voltage

which was mentioned in the section of this report on electric-field profiles.

The voltage - current characteristic of this type of current transfer, where the

voltage rises abruptly and then plateaus, is difficult to separate from an

intrinsic I c characteristic. This type of current- transfer voltage caused the

relatively low measured I c for the center of the cable face in Fig. 29. The

reduction in the measured I c of this strand segment is artificial in the sense

that it was caused not by the strand segment itself, but rather was caused by

the mechanical deformation and resulting current redistribution associated

with an adjacent strand segment.

43
The I c -versus -position data for the inner cable, Z, also shows a smaller

variation at 90° than at 0° with the exception of one point on Fig. 34. This

point was a corner on the thin edge that had a negative transfer voltage at 0°

(missing, high I c ) and apparently an intrinsic, positive transfer voltage at

90°, which may have contributed to the lower measured I c

Differential Voltage Versus Angle

Plots of the differential voltage versus magnetic - field angle for various

positions along the cable strand's length are presented for all four

conductors at 5 T and a given current, Fig. 35-38. At the top of each plot,

located outside the axes, is an outline of the cable strand's geometry; in

this case it does not correlate with the horizontal axis. The cable outline

is labeled to indicate the position of the thin edge, the face, and the thick

edge. The data-point symbols are located, horizontally, at the center of each

segment on the cable outline. The discrete data points on the plot,

therefore, represent the differential voltage over each strand segment at a

given angle. The lines connecting the data points are intended as a visual

aid and not an interpolation between the data points. This is especially true

between the angles -180 and -105° because of the widely spaced data points. A

plus symbol was used for the total differential (overall) voltage of one cable

pitch length. The same data symbols were chosen for equivalent positions

along the thick and thin edges, with open symbols for the thin edge and filled

symbols for the thick edge. The differential-voltage curves for the end

segments of the strand, indicated by the circular data symbols, are equal for

the two segments because of the way in which they were derived. Separate data

were not available for these two segments; rather, the total voltage of the

44
two segments was available and, based on their equivalent positions, their

voltages were assumed to be equal.

It is difficult to follow all of the curves, but these plots reveal

significant features. In Fig. 35 (cable W) the differential voltage of the

center of the thin edge is where most of the total voltage drop occurs for

angles around 0°. Two of the positions along the face have a negative

differential voltage at all angles. The rest of the positions show little

structure with angle and have relatively small differential voltage. The peak

in the total differential voltage at 0 and -180° indicates the lowest I c , the

relative minimums at about ±45° are regions of larger I c , and the relative

peaks at ±90° are local minima in I c

.

Figure 36 (cable X) has some other features. The lowest I c was at ±90°

rather than 0°. The center of the thin edge still contributes the majori ty of

the total differential voltage around 0°. The corners of the thin edge

contribute the most to the total differential voltage near ±90° . The maximum

for each of these corners occurs at angles slightly shifted to one side or the

other of 90°. This is due to the difference in the average pitch angle of

each corner segment. Some positions have a negative differential voltage at

some angles and a positive differential voltage at other angles.

A comparison between Fig. 37 (cable Y) and Fig. 35 (cable W) shows a

strong similarity between the overall voltage curves. Both curves show a

large peak at 0°; however, unlike cable W, where almost all of the voltage is

due to the central thin edge, the peak in the overall curve for cable Y is the

product of contributions from several strand segments. Most notable, there is

45
a significant contribution to the overall voltage at 0° from the central thick

edge .

The overall curve of Fig. 38 (cable Z) is similar to that of Fig. 36

(cable X) with maximum differential voltages at ±90° , minimums at ±45° , and a

local maximum at 0°. In the case of cable X, the local maximum at 0° is

primarily due to the central thin edge. On the other hand, in the case of

cable Z, the 0° local maximum is due to approximately equal contributions from

both the thick and thin central edges.

Close examination of the overall voltage curves of Figs. 35-38 shows

several instances of 180° asymmetry. The most prominent example of this

asymmetry can be seen by comparing the overall voltages for cable Z at +90°

and -90°. The overall voltage at +90° is significantly higher than at -90°.

A number of the individual segment curves for cable Z show this same ±90°

asymmetry; therefore this effect is probably a characteristic of the conductor

and not a measurement error. Similar asymmetries exist in the individual

segment curves for the other three cables W, X, and Y.

This phenomenon may be due to a transverse asymmetry in the deformation of

the cable strand. This condition, in conjunction with the asymmetry of the

total magnetic field (the vector summation of the applied field and the

strand's self field) over the strand's cross section, might result in applied

field angles that are favorably or detrimentally oriented with respect to the

local deformation of the filaments. This may explain the asymmetry in the

differential -voltage curves.

46
F. DISCUSSION

The main objective of this work is to determine the current capacity of

several different cable strands with the ultimate goal an increased

understanding of the sources and degree of current-capacity degradation in

these conductors. Typically, I c of a conductor is an accepted practical

measure of its current capacity. However, for these conductors, there are

some difficulties in relating I c to the actual current capacity of the cable

strands. Owing to the electrical inhomogeneity of the strands caused by the

cabling operation, there is a large variation in I c along the strand's length.

The problem is determining what current level in this fairly broad range of

critical currents best reflects the strand's practical current capacity.

One approach is to assume a weak- link condition where the strand segment

that has the lowest I c determines the overall current capacity of the strand.

A comparison between the actual quenching currents (the maximum observed

current prior to thermal runaway) and the weak- link critical currents shows

that the current capacity of the strand is significantly higher than the weaklink I c

. This enhanced current capacity, above the weak- link I c , is due to

the strand's relatively good stability.

At the other extreme, the current capacity of the strands could be assumed

to be equal to the overall I c

. For the conductors tested here, this would not

be a bad assumption, as evidenced by the close agreement between the overall

critical currents and the quenching currents. However, there are two problems

associated with simply measuring the overall I c and assuming this to be the

strand's practical limit. First, the overall I c gives no information about

47
the voltage distribution along the strand's length and, consequently, is not

useful in understanding the sources of current degradation in the cable.

Also, for conductors that are less stable than those tested here, the actual

current limit might be close to the weak- link I c and the overall I c level

could not be achieved.

Another factor associated with the I c measurements for these cable strands

is the presence of intrinsic current- transfer voltages that are caused by

periodic current redistribution along the strand's length. This current

transfer is apparently caused by the periodic mechanical deformation of the

strand and, therefore, it is not isolated to the current contact area. In

this sense it is intrinsic to the conductor (not a product of the measurement

technique) and it presents a problem in data reduction and analysis. The

problem that this intrinsic current- transfer voltage poses is whether or not

it is appropriate to separate it from the flux-flow voltage for determining

the I c , as is the normal practice in the case of nondeformed conductors. In

other words, because of its intrinsic nature, these current - transfer voltages

play a role in determining the current capacity of the cable, not only in

testing but in application as well, and, therefore, their presence should be

reflected in the reported I c value. In contradiction to this precept, the

critical currents reported here were corrected for current- transfer voltages.

There are two reasons for this apparent inconsistency. First, for these

measurements it was impossible to attribute all of the observed current

transfer to intrinsic sources. Moreover, the relatively short length (64 mm)

of the sample that resided in the full magnetic field may have led to

nonintrinsic current transfer. This is why current- transfer corrections were

48
made to the overall-I c data. Second, as previously explained, the intrinsic

current- transfer voltages do not necessarily appear solely on the strand

segments that produced them. This does not present a problem for overall-I c

measurements (if the current transfer is purely intrinsic) where only the

magnitude of the voltage over one pitch length is important, not the specific

sources of the voltage. However, for the localizedI

c measurements presented

here, the comparison of the critical currents of the various strand segments

is intended as a measure of the damage caused by mechanical deformation at

different segments of the strand. Inclusion of current- transfer voltages,

resulting from damage to adjacent strand segments, in the I c criterion would

distort the results of this comparison.

I c data alone are insufficient for characterizing these conductors. For

this reason the electric - field and differential -voltage data were presented.

The electric - field data are the clearest indicator of the relative current

capacity of the different strand segments. However, the limitation of the

electric - field data is that they do not indicate the relative contribution of

the various strand segments to the total conductor voltage because the

segments are of different lengths. Consequently, the differential -voltage

data are a necessary supplement to the electric - field data to fully

characterize the conductors. Which of these data are the most indicative of

the actual current capacity depends on the stability of the particular

conductor. In the case of a stable conductor, the total voltage is an

important determining factor of the strand's current capacity and,

consequently, the voltage distribution along the strand shows which segments

are the most detrimental to the current capacity. On the other hand, for a

relatively unstable conductor, the local current capacity can be a limiting

factor and, in this case, the electric-field data are more relevant.

49
Because of the electrical inhomogeneity of these conductors, stability may

play a key role in determining their current capacity. This coupling between

stability and current capacity increases the importance of the matrix material

in the conductor design. For example, the use of CuNi for the cable's matrix

material, although beneficial in reducing proximity- effect coupling^ and

improving the wire's drawing characteristics, might result in a considerable

reduction in current capacity because of the relatively low thermal

conductivity of the CuNi and resulting reduction in the cable's stability.

Also, the increased electrical resistivity of a CuNi matrix would result in

larger current- transfer voltages at current-redistribution sites. Another

approach to reducing the proximity- effect coupling between filaments without

substantially lowering the stability of the wire is a magnetic approach where

Mn is added to the Cu matrix material. » ^ The Mn reduces the proximityeffect coupling by promoting magnetic scattering of the electron pairs.

To optimize the current capacity of a superconducting cable, three main

factors must be considered: the virgin wire's current capacity, its

sensitivity to mechanical deformation, and the degree of deformation

introduced by the cabling process. A truly optimal design would require the

simultaneous determination of the conductor and cabling parameters that, in

conjunction with each other, would result in the maximum current capacity of

the finished cable. Although ideal, this approach is impractical. Also, the

primary concern dictating the cables structural design is mechanical

stability, not current capacity. This limits the amount that the cabling

parameters can be varied to enhance the current capacity.

50
A second approach to this problem would be to simply optimize the virginwire parameters for maximum current capacity, optimize the cabling parameters

for minimum strand deformation, and then assume that the wire's sensitivity to

deformation will be acceptably low. It can be argued, however, that

optimizing the virgin current capacity of the wire is likely to make it

sensitive to mechanical deformation. In other words, if the virgin wire is

truly optimized, then any change in the wire (mechanical deformation for

example) must result in a degradation in its performance. For example, a

comparison of cables X and Y in Figs. 3 and 4 shows that the increase in

virgin-wire I c density from cable X to cable Y is accompanied by an increase

in mechanical degradation, although cabling had been improved for sample Y.

This apparent increase in sensitivity to deformation may be caused by changes

in the virgin-wire construction (matrix- filament configuration) that were

necessary to improve its J c

.

A third approach to the problem of cable optimization would be based on an

iterative design where a number of different virgin wires, all having

relatively high current capacities but different design parameters (filament

diameter, matrix- filament configuration, etc.), would be cabled using

different parameters (strand diameter, number of strands, strand twist, etc.)

and then measured for local degradation. The process would be repeated, with

the wire and cabling parameters adjusted according to the results of the

strand measurements, until an optimal combination of high virgin-wire current

capacity and low mechanical degradation was approached. Unlike the first

design method, this method might be practical and, unlike the second design

method, this method addresses the interrelationship between the virgin wire's

current capacity, its sensitivity to mechanical deformation, and the cabling

51
parameters. This iterative method might be a practical means of approaching

an optimal cable design.

Measuring extracted cable strands rather than an entire cable is partially

a matter of convenience; it does not require the high output current supply or

the large test fixture that are necessary for cable measurements. In

addition, for single - strand measurements, there is less uncertainty in the

current measurement because all of the supplied current is carried by the

single strand. For a cable measurement, however, the supplied current is

distributed among all of the cable strands and not necessarily equally. Also,

the lower output-current supplies that can be used for an extracted strand

measurement typically are less noisy than the larger supplies required for

cable measurements and, therefore, more sensitive voltage measurements can be

made at the lower current levels.

A limitation of the extracted- strand measurements is that they do not

indicate the effect, on an individual strand's current capacity, of localized

magnetic field variations produced by adjacent cable strands (self-field

effect)

.

The exact duplication of the magnetic field that the conductors will

encounter in application is beyond the scope of these measurements. The

intent of these localized measurements is to determine the degree and sources

of mechanical degradation of the strand, in general. Although the self- field

effect* will undoubtedly result in some variations in the local electrical

characteristics of the cable, the salient features of the mechanical

degradation, such as the locations of weak links and the critical magneticfield orientation, will remain the same. Another electrical characteristic of

the cable that is not duplicated by the extracted- strand measurements is

52
current transfer between adjacent cable strands. This interstrand current

transfer might slightly enhance the stability of the cable as compared to that

of an individual strand.

All the measurements reported here were made on single specimens of each

sample and, as a result, the measurements do not necessarily represent the

global characteristics of the cables. Other specimens of samples W and X were

measured at an earlier date and the results were reported elsewhere. ^ A

second specimen of sample Y was measured during the present tests. The

results for these other measurements were generally in good agreement with

those reported here. Additionally, the dominant electromagnetic

characteristics are consistent among the four samples. For example, the thin

edge of the cable strand contributes significantly to the overall strand

voltage for a magnetic - field orientation of 0° for all samples.

As changes are made in the cable design and manufacture, the degree and

character of the mechanical degradation might change considerably. For

example, these prototype cables were sized using a nonpowered turkshead, but

the cables might be sized using a powered turkshead in mass production. A

change of this type in the manufacturing process could significantly affect

the degradation of the cable. By including periodic measurements of extracted

strands as part of the cable development process, the effect of new design and

manufacturing parameters on current capacity can be thoroughly evaluated.

An extremely large body of information is contained within the data plots

presented here, and only a small portion of that information was specifically

addressed. The presentation of this data serves two purposes; it demonstrates

53
the intricate and complex nature of these conductors and it documents the

evolution and present status of the cable design.

G. CONCLUSIONS

Based on these measurements, some general conclusions can be stated

regarding the electromagnetic character of these cables and its effect on

their current capacities. Also, the measurements have implications regarding

short sample testing of these cables.

The mechanical deformation of the cable strands associated with cabling

can lead to very localized reductions in their current capacity. The widest

spread in local current capacity occurs with the applied magnetic field

parallel to the cable's width. The deformation geometry of the cable is such

that the interaction between the applied magnetic field and a strand segment's

aspect ratio or its pitch angle have the same tendency. In other words, a

field angle that is either detrimental or beneficial to a strand segment's

current capacity due to its aspect ratio is also detrimental or beneficial,

respectively, due to its pitch angle. However, strand segments have different

critical orientations, so the detrimental and beneficial effects of the field

angle on those segments tend to cancel one another over the total length of

the strand. Consequently, variations of the overall I c and the quenching

current, with the magnetic - field angle, are significantly less than the I c variation for individual segments of the strand. For these conductors, the

relevant I c criterion may be a spatial average (the overall I c ) because the

degradation in current capacity is concentrated over short regions of the

54
cable strand and local temperature rises are limited by strong end cooling.

Any compromise in sample stability may lower I c to that of the weak- link

segment of the cable strand.

These results have several implications for short-sample I c testing of

cables. First, the data indicate that both magnetic-field orientations,

parallel to the cable's width and perpendicular to its width, need to be

tested in order to determine the limiting case for I c

. As shown above, the

limiting magnetic - field orientation can be parallel or perpendicular to the

cable's width and this is not known beforehand. Second, a large difference in

current-capacity degradation can exist between the thick and thin cable edges;

therefore, changing the direction of the test current (for each field

orientation) can affect the measured I c. This is due to a self-field effect

where the location of the peak magnetic field depends on the current

direction. Consequently, the peak- field location should be part of the data

recorded for short-sample measurements of these conductors. These

measurements that address the orientation of the applied field and the

location of the peak field should be made periodically as part of the cable

development. Also, because of their intrinsic nature, the current- transfer

voltages that are associated with mechanical damage of the cable strands

should be included in their characterization.

The wire from which these cables are made has evolved to the point where

the virgin wire has a high current capacity and a relatively high tolerance to

mechanical deformation. Considering the amount of mechanical deformation of

the strands in the cabling process, the resulting degradation in current

55
capacity is surprisingly low. The maximum observed degradation of the overall

I c of the latest outer -winding cable, sample Y, was only 6.8%.

ACKNOWLEDGMENTS

The authors extend their thanks to T. C. Stauffer for sample preparation;

to A. N. Srivastava for data plotting; and to R. M. Scanlan and J. M. Royet

(Lawrence Berkeley Laboratory) for samples and discussions.

REFERENCES

1. L. F. Goodrich, E. S. Pittman, J. W. Ekin, and R. M. Scanlan, "Studies of

NbTi Strands Extracted from Coreless Rutherford Cables," IEEE Trans, on

Magnetics, MAG 23, 1642, 1987.

2. 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) .

3. 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), p. 833-840.

56
4. J. Royet and R. M. Scanlan, "Manufacture of Keys toned Flat Superconducting

Cables for Use in SSC dipoles," IEEE Trans, on Magnetics, MAG 23, 480, 1987.

5. 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.

6. L. F. Goodrich, J. W. Ekin, F. R. Fickett, "Effect of Twist Pitch on

Short- Sample V-I Characteristics of Multifilamentary Superconductors," Adv.

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

7. L. F. Goodrich, "The Effect of Field Orientation on Current Transfer in

Multifilamentary Superconductors," IEEE Trans, on Magnetics, MAG 19, 244,

1983.

8. J. W. Ekin, L. F. Goodrich, J. Moreland, E. S. Pittman, and A. F. Clark,

"Electro-Mechanical Properties of Superconductors for High Energy Physics

Applications," NBSIR 86-3061, National Bureau of Standards, Boulder, Colorado

(December 1986) .

9. E. W. Collings, "Stabilizer Design Considerations in Fine-Filament Cu/NbTi

Composites," Adv. Cryog. Eng. - Materials, Vol. 34, Plenum Press, New York

(1988), p. 867-877.

57
10. T. S. Kreilick, E. Gregory, J. Wong, R. M. Scanlan, A. K. Ghosh, W. B.

Sampson, and E. W. Collings, "Reduction of Coupling in Fine Filamentary CuNbTi Composites by the Addition of Manganese to the Matrix," Adv. Cryog. Eng.

- Materials, Vol. 34, Plenum Press, New York (1988), p. 895-900.

58
59

Figure

1.

Cross

section

of

cable

W

showing

the

orientation

of

the

applied

magnetic

field

and

the

approximate

locations

of

the

voltage

taps,

indicated

by

arrows

in

the

figure

(full

cable

pitch

tap

positions

omitted

for

clarity).
Figure 2. Instrumented portion of an extracted cable strand showing the

voltage tap placement and the orientation of the applied magnetic

field

.

60
CRITICAL

CURRENT

DENSITY,

A/

E

W X , Y Z

ANGLE/CABLE

Figure 3. Critical - current density of cable strands and their respective

virgin wires for 0 and 90° at 5 T. The virgin wire J c is

represented by the continuous line segment spanning 0 to 90° for

each cable. The strand J c at 0 and 90° are represented by the

shaded bars for each cable.

61
CRITICAL

CURRENT

DENSITY,

A/m

CM

1350

1250 -

1150

1050 -

950

0

° 90° 0

° 90° 0

° 90° 0

° 90° W X , Y Z

ANGLE/CABLE

Critical-current density of cable strands and their respective

virgin wires for 0 and 90° at 8 T. The virgin wire J c is

represented by the continuous line segment spanning 0 to 90° for

each cable. The strand J c at 0 and 90° are represented by the

shaded bars for each cable.

Figure 4.
ELECTRIC

FIELD,

^V/c

POSITION, cm

Figure 5. Electric - field profiles at 5 T and an applied magnetic field

angle of 0° for cables W, X, and Y at currents of 267 A, 283 A,

and 317 A, respectively, which corresponds to an overall

electric field of 0.01 /iV/cm.
ELECTRIC

FIELD,

uV/c

POSITION, cm

Figure 6. Electric - field profiles at 5 T and an applied magnetic field

angle of 0° for cables W, X, and Y at currents of 279 A, 299 A,

and 332 A, respectively, which corresponds to an overall

electric field 0.05 /iV/cm.
POSITION, cm

Figure 7. Electric - field profiles at 5 T and an applied magnetic field

angle of 0° for cables W, X, and Y at currents of 285 A, 307 A,

and 339 A, respectively, which corresponds to an overall

electric field of 0.1 /iV/cm.
ELECTRIC

FIELD,

jiV/cm

POSITION, cm

Figure 8. Electric - field profiles at 5 T and an applied magnetic field

angle of 0° for cables W, X, and Y at currents of 300 A, 326 A,

and 351 A, respectively, which corresponds to an overall

electric field of 0.5 jiV/cm.
ELECTRIC

FIELD,

^V/c

POSITION, cm

Figure 9. Electric - field profiles at 5 T and an applied magnetic field

angle of 90° for cables W, X, and Y at currents of 275 A, 277 A,

and 327 A, respectively, which corresponds to an overall

electric field of 0.01 /iV/cm.

67
ELECTRIC

FIELD,

^V/c

POSITION, cm

Figure 10. Electric - field profiles at 5 T and an applied magnetic field

angle of 90° for cables W, X, and Y at currents of 289 A, 292 A,

and 339 A, respectively, which corresponds to an overall

electric field of 0.05 /xV/cm.

68
ELECTRIC

FIELD,

fjS

/

c

POSITION, cm

Figure 11. Electric - field profiles at 5 T and an applied magnetic field

angle of 90° for cables W, X, and Y at currents of 296 A, 300 A,

and 345 A, respectively, which corresponds to an overall

electric field of 0.1 /iV/cm.

69
POSITION, cm

Figure 12. Electric - field profiles at 5 T and an applied magnetic field

angle of 90° for cables W, X, and Y at currents of 307 A, 318 A,

and 358 A, respectively, which corresponds to an overall

electric field of 0.5 ^V/cm.

70
•*\ o

_l

Ld

o

I

o

Ld

I Ld

POSITION, cm

Figure 13. Electric-field profiles at 8 T and an applied magnetic field

angle of 0° for cables W, X, and Y at currents of 142 A, 132 A,

and 145 A, respectively, which corresponds to an overall

electric field of 0.1 ^V/cm.

71
ELECTRIC

FIELD,

uV/c

POSITION, cm

Figure 14. Electric - field profiles at 8 T and an applied magnetic field

angle of 90° for cables W, X, and Y at currents of 143 A, 128 A,

and 144 A, respectively, which corresponds to an overall

electric field of 0.1 /iV/cm.

72
POSITION, cm

Figure 15. Electric - field profiles at 5 T and an applied magnetic field

angle of 0° for cable Z at a current of 585 A, which corresponds

to an overall electric field of 0.1 /iV/cm.

73
ELECTRIC

FIELD,

uV/c

POSITION, cm

Figure 16. Electric - field profiles at 5 T and an applied magnetic field

angle of 90° for cable Z at a current of 579 A, which

corresponds to an overall electric field of 0.1 /xV/cm.
ELECTRIC

FIELD,

/jV/c

POSITION, cm

Figure 17. Electric - field profiles at 8 T and an applied magnetic field

angle of 0° for cable Z at a current of 239 A, which corresponds

to an overall electric field of 0.1 /iV/cm.
ELECTRIC

FIELD,

uS

/

c

POSITION, cm

Figure 18. Electric - field profiles at 8 T and an applied magnetic field

angle of 90° for cable Z at a current of 233 A, which

corresponds to an overall electric field of 0.1 /iV/cm.
DIFFERENTIAL

VOLTAGE,

Figure 19. Differential -voltage profiles at 5 T and an applied magnetic

field angle of 0° for cables W, X, and Y at currents of 285 A,

307 A, and 339 A, respectively, which corresponds to an overall

electric field of 0.1 /iV/cm.
DIFFERENTIAL

VOLTAGE,

Figure 20. Differential -voltage profiles at 5 T and an applied magnetic

field angle of 90° for cables W, X, and Y at currents of 296 A,

300 A, and 345 A, respectively, which corresponds to an overall

electric field of 0.1 /IV/cm.

73
DIFFERENTIAL

VOLTAGE,

POSITION, cm

Figure 21. Differential -voltage profiles at 8 T and an applied magnetic

field angle of 0° for cables W, X, and Y at currents of 142 A,

132 A, and 145 A, respectively, which corresponds to an overall

electric field of 0.1 /iV/cm.

79
DIFFERENTIAL

VOLTAGE,

>

=i

Figure 22. Differential -voltage profiles at 8 T and an applied magnetic

field angle of 90° for cables W, X, and Y at currents of 143 A,

128 A, and 144 A, respectively, which corresponds to an overall

electric field of 0.1 ^V/cm.

80
DIFFERENTIAL

VOLTAGE,

POSITION, cm

Figure 23. Differential -voltage profiles at 5 T and an applied magnetic

field angle of 0° for cable Z at a current of 585 A, which

corresponds to an electric field of 0.1 /jV/cm.
DIFFERENTIAL

VOLTAGE,

0.30

>

^

0.25

0.20

0.15

0.10

0.05

0.00

- 0.05 01 2345678

POSITION, cm

Figure 24. Differential -voltage profiles at 5 T and an applied magnetic

field angle of 90° for cable Z at a current of 579 A, which

corresponds to an electric field of 0.1 /xV/cm.

82

X~

i

—

i
DIFFERENTIAL

VOLTAGE,

THIN

EDGE

rAc)

f H at 0°

THICK \EDGE

0.6 T T TTT I f H'T

T

[' ! T'T 'I fI TTT| I I M "] I I I I

| HT'I j > Cable Z

1

0.4

- 0.2 LJ—I—1—I 1 1 I 1—1—i LJ lJ L-L-L-L-Lj LJ 1 1—1 i I I 1 1

1 2 3 4 5 6 7 8

POSITION, cm

Figure 25. Differential -voltage profiles at 8 T and an applied magnetic

field angle of 0° for cable Z at a current of 239 A, which

which corresponds to an electric field of 0.1 ^V/cm.

83
DIFFERENTIAL

VOLTAGE,

POSITION, cm

Figure 26. Differential -voltage profiles at 8 T and an applied magnetic

field angle of 90° for cable Z at a current of 233 A, which

which corresponds to an electric field of 0.1 /xV/cm.
POSITION, cm

Figure 27. Critical-current profiles at 5 T and an applied magnetic field

angle of 0° for cables W, X, and Y at an electric field

of 0.05 /iV/cm.
CRITICAL

CURRENT,

Figure 28. Critical-current profiles at 5 T and an applied magnetic field

angle of 90° for cables W, X, and Y at an electric field

of 0.05 /iV/cm.

86
__l

<

o

O

POSITION, cm

Figure 29. Critical - current profiles at 8 T and an applied magnetic field

angle of 0° for cables W, X, and Y at an electric field

of 0.05 /iV/cm.

87
POSITION, cm

Figure 30. Critical-current profiles at 8 T and an applied magnetic field

angle of 90° for cables W, X, and Y at an electric field

of 0.05 /iV/cm.

o o
CRITICAL

CURRENT,

01 2345678

POSITION, cm

Figure 31. Critical-current profile at 5 T and an applied magnetic field

angle of 0° for cable Z at an electric field of 0.05 ^V/cm.

89
POSITION, cm

Figure 32. Critical-current profile at 5 T and an applied magnetic field

angle of 90° for cable Z at an electric field of 0.05 jiV/cm.

90
POSITION, cm

Figure 33. Critical-current profile at 8 T and an applied magnetic field

angle of 0° for cable Z at an electric field of 0.05 ^iV/cm.

91
POSITION, cm

Figure 34. Critical-current profile at 8 T and an applied magnetic field

angle of 90° for cable Z at an electric field of 0.05 /iV/cm.

92
DIFFERENTIAL

VOLTAGE,

FIELD ANGLE, DEGREES

Figure 35. Differential-voltage plotted as a function of the applied field

angle for all of cable W's taps at 5 T and a current of 285 A,

which corresponds to an overall electric field of 0.1 jzV/cm at 0°
DIFFERENTIAL

VOLTAGE,

FIELD ANGLE, DEGREES

Figure 36. Differential-voltage plotted as a function of the applied field

angle for all of cable X's taps at 5 T and a current of 307 A,

which corresponds to an overall electric field of 0.1 /iV/cm at 0

94
DIFFERENTIAL

VOLTAGE,

>

=i

Figure

FIELD ANGLE, DEGREES

37. Differential-voltage plotted as a function of the applied field

angle for all of cable Y's taps at 5 T and a current of 339 A,

which corresponds to an overall electric field of 0.1 /xV/cm at 0
DIFFERENTIAL

VOLTAGE,

FIELD ANGLE, DEGREES

Figure 38. Differential-voltage plotted as a function of the applied field

angle for all of cable Z's taps at 5 T and a current of 585 A,

which corresponds to an overall electric field of 0.1 /iV/cm at 0°
THE EFFECT OF LOCAL AREA RATIO AND DIFFUSION- BARRIER MATERIAL

ON CRITICAL- CURRENT BEND DEGRADATION OF MULT I FI LAMENTARY NbTi WIRES

J. W. Ekin, S. L. Bray, and N. F. Bergren

Electromagnetic Technology Division

National Institute of Standards and Technology

Boulder, Colorado 80303

Abstract

A systematic study of the effects of bending strain on the critical current of

NbTi conductors has been performed. The critical-current (I c ) degradation

caused by bending decreases with increasing magnetic field, suggesting that

the source of degradation is not an intrinsic degradation of Hc 2 , as with the

elastic axial-strain effect. The degradation of the I c from bending strain is

much greater at low electric field criteria Ec than at high (for example, 12%

degradation at E c = 1 ^V/cm versus over 22 % at Ec = 0.1 /j,V /cm for a

conductor- core - to-bend-diameter ratio of 0.43). The effect of bending on the

critical current may be characterized simply by a decrease in the value of n

toward the same low value for all conductors, typically 10 to 15. Here n

represents the sharpness of the electric fie Id- vs .- current relationship for

the NbTi wire (E a I n

)

.

The degradation of n suggests that irregularity of

the filament's cross-sectional area, introduced by bending, may be the source

of the I c degradation. By testing a controlled series of similarly fabricated

NbTi wires in which the local copper- to- superconductor area ratio (LAR) was

systematically varied, we found that as the LAR increased from 0.4 to 0.7, the

I c bend degradation decreased by about 1/7. The increasing susceptibi lity of

a superconductor to bend degradation with decreasing LAR thus places a
2

mechanical limit on the reduction of the LAR for controlling filamentsausaging effects. In another controlled series of NbTi wires in which the

diffusion barrier material that surrounds each NbTi filament was

systematically changed, the I c bend degradation was less by about 1/4 to 1/5

for diffusion barriers made of vanadium, than for diffusion barriers made of

niobium or niobium with a copper-nickel matrix.

Introduction

In the manufacture of superconducting cables for many magnet

applications, compaction of the cable is necessary to achieve high filling

factors and, consequently, high overall critical - current densities. However,

compaction can also introduce strain concentrations in the cable strands;

these may act as weak links in limiting the cable's critical current (I c )

.

In

both rope cables and braided cables these strain concentrations occur

periodically at the points where the cable strands cross one another. In

flattened hollow cables (sometimes referred to as Rutherford cables) the

strain is concentrated at the cable edges where each strand is subjected to

severe bend strain and flattening.

Rutherford cables were developed for use in high-energy physics

applications, the Fermilab Tevatron accelerator and the Superconducting Super

Collider (SSC)

,

for example. The Rutherford design allows for higher

compaction then rope or braided cables with minimal strain concentration and

strand deformation. Rutherford cables are typically rectangular in cross

section, or trapezoidal in the case of a keystone cable, and each strand

follows a path of full transposition about the cable's cross section. In
3

traversing from one cable face to the other, the strands must periodically

bend around the cable edges. The severity of the bend depends primarily on

the strand diameter, the cross-sectional dimensions of the cable, and its

pitch length. This paper presents measurements on a controlled series of NbTi

conductors to determine strand- design parameters that could be changed to

minimize the effect of bending on the critical current.

Localized I c measurements of NbTi strands that were extracted from

prototype SSC cables indicate that significant I c degradation can occur at the

edges of a Rutherford cable (See Ref 1-4). Microscopy studies of these

specimens have shown that the severity of filament damage and the resulting I c degradation varies among strands that are extracted from different cables. In

an extreme case, the damage is characterized by serrated yielding and breakage

of the NbTi filaments, which is accompanied by a reduction of more than 30% in

the measured I c

. This level of degradation is not typical of most cables, nor

is it typical of the specimens that were studied here. A reduction of less

than 20% in the I c and virtually no visible filament damage is typical for

these specimens. The results presented here are a systematic study of the

effect of severe bending on the I c of NbTi wires that are candidate strands

for SSC cables. The filament damage for the specimens tested in this study is

similar to what is commonly found in cable strands.

The measurements were conducted in two stages. First, the effect of

bending on I c was measured in a number of different deformation geometries in

order to determine a standard test method that would yield a significant and

reproducible test of the effect. Once this test method was determined, it
4

was then used to evaluate a number of different conductors to obtain a

relative measure of the effect of local area ratio diffusion-barrier material

on critical - current bend degradation.

Rather than attempt to reproduce or even mimic the complex strand

deformation found in Rutherford cables, a much simpler bend geometry was

selected for the standard test method. There are several reasons for this

approach. To begin, the cost and time required to manufacture Rutherford

cables from the broad range of strand materials that were tested would be

prohibitive. Furthermore, this approach would also introduce a great number

of variables that would diminish the level of experimental control. Finally,

the results of measurements using the simple bend geometry are more general

and, therefore, they begin to address the issue of understanding the basic

mechanism underlying bend degradation of I c in NbTi superconductors.

Experimental Details

The detailed specifications of the superconducting samples are given in

Table I. The conductors are similar in the sense that they are all prototype

materials for SSC cables. These are round NbTi wires with a 0.648 mm

diameter. The samples have different local copper- to- superconductor area

ratios (LAR, the local area ratio of the cross-sectional area of the filament

to that of the copper surrounding each filament going into the billet)

,

different diffusion barrier materials locally surrounding each filament, and,

in some cases, different filament configurations.
5

For this study, the background magnetic field is provided by a 9 T, 3.8

cm bore superconducting solenoid. A voltage-controlled 1000 A battery-powered

constant-current supply is used as the sample-current source.

The basic testing geometry is that of a coil, shown in Fig. 1, where the

diameter of the coil determines the severity of the bending strain, . The

tested bend diameters range from a maximum of 31.7 mm (minimum €]-,) to a

minimum of 0.38 mm (maximum e-^) . The I c degradation for the 31.7 mm coil

measurements is negligible, as substantiated by measuring the I c of a straight

sample. Consequently, the 31.7 mm coil measurements serve as a baseline for

determining the bend degradation of the small diameter coil samples. The

sample coils are wound on cylindrical measurement mandrels that are made from

either nonmagnetic stainless steel (#316) or, in the cases of the 31.7 mm and

the 3.18 mm mandrels, fiberglass - epoxy composite (G-10) tubes. A 1.28 mm

stainless - steel mandrel (approximately twice the wire diameter) is the

standard selected for the second stage of testing where various samples were

tested at a common level of to determine whether there were conductor

parameters that could be changed to maximize the bending strain tolerance.

This standard bend diameter yields significant and repeatable degradations in

the I c of a given wire sample.

A significantly different coil geometry was also tried in an attempt to

simulate the current- transfer effects that might occur in actual cable

strands. In these tests the sample coil was wound around a thin rectangular

plate with rounded edges. This resulted in a series of hairpin bends at the

edges of the plate, separated by straight nondeformed sections of the wire

101
6

across the faces of the plate to simulate the deformation found in actual

Rutherford cable strands. This plate geometry was abandoned in favor of the

cylindrical geometry because it was difficult to control and it yielded

inconsistent bending at the plate edges.

To address the effect of the magnetic field generated by the sample coil

on the measured I c , the I c measurements were made with two different

orientations between the background field and the solenoidal sample field,

parallel and antiparallel. The reported I c is the average of these two I c measurements. The difference between the two measurements was, in all cases,

less than 10% of their average value. The sample coils were bonded to the

mandrels with a light coating of varnish. The varnish coating is necessary

for containing the outward Lorentz force acting on the sample in the parallel

magnetic - field case.

With the exception of the 31.7 mm diameter mandrels, the sample mandrels

are held in the test fixture by the current contacts; see Fig. 1. Round

copper rod of 12.7 mm diameter is used for the current contacts. The ends of

the contacts that hold the mandrel in place are tapered to allow a gradual

transition from the smaller diameter of the sample mandrel to the larger

diameter of the current contact.

A pair of voltage -tap leads are soldered to the sample with a separation

of about 12 coil -turns. One of the voltage leads is counter-wound around the

sample coil to minimize induced voltage noise from variations in the magnetic

field. To measure the length of the sample that is spanned by the voltage
7

taps, first a known length of the sample wire is weighed using an analytic

balance and then the actual sample coil is cut at its voltage taps and the

resulting coil segment is weighed. The length of the instrumented portion of

the sample coil is calculated from these weight values.

An automated, computer controlled, data acquisition and analysis system

that was developed at NIST was used in this study. This system measures the

voltage - current (V-I) relationship at discrete points along the V-I curve by

holding the current constant, at preselected levels, and monitoring the

resulting voltage. This technique allows averaging of the voltage data at

each current level and, therefore, reduces the uncertainty associated with the

voltage signal noise. Furthermore, the system uses an optimization process

that automatically adjusts the locations of the discrete data points along the

nonlinear V-I curve using voltage feedback. This process was designed to

enhance the precision and accuracy of the I c measurement while minimizing the

data- acquisition time. The overall accuracy of the I c measurement is within

±3%, the precision within ±0.5%.

Results

The effect of severe bending on the electrical characteristics of NbTi

wires is shown in Figures 2 and 3 where the logarithm of the current (LOG I)

is plotted against the logarithm of the electric field (LOG E) for several

different bend diameters. The data presented in Fig. 2 is for sample A of

Table 1 and the data in Fig. 3 is for sample B.

101
8

In Fig. 2 each curve is labeled with the sample coil's mandrel bend

diameter with the exception of the curve that is labeled "STRAIGHT." A

straight specimen of sample A was measured to determine what degradation, if

any, is present in the 31.7 mm coil samples that serve as a baseline for

comparison of the small-diameter-coil measurements. The close agreement

between the "straight" curve and the "31.7 mm" curve, in their overlap region,

indicates that the 31.7 mm coil data adequately represent the nondegraded

state of the conductor.

Several effects of bending are evident in both of these plots. First,

bend degradation is much more severe at low electric-field criteria (Ec ) than

at high Ec

. In other words, the slope of the log E-log I curves vary with e^.

For these logarithmic plots, the slope of the E-I curve is equal to the

characteristic value n, where n is given by the approximate relationship

between the electric field and the current E « l n . Thus, the effect of

bending is characterized by a decrease in the value of n.

Figures 2 and 3 show that the overall magnitude of the bend effect is

nontrivial; it amounts, for example, to over 29% at Ec = 1 /iV/cm for Sample A

at the most severe (bend diameter = 0.38 mm) and increasing to 35% at Ec =

0 . 1 /iV/cm

.

In Figures 4 and 5, the normalized critical currents of various specimens

of samples A and B are plotted against the reciprocal of the normalized bend

diameters of the specimens. The data shown in Fig. 4 were taken at 5 T while

the data in Fig. 5 were taken at 7 T. The critical currents are normalized
9

with respect to the I c of a 31.7 mm coil specimen of each of the sample wires,

A and B. The samples have a core region formed by the NbTi filaments. The

bend diameter of each specimen is normalized with respect to the diameter of

this NbTi core region. This method of bend-diameter normalization is

motivated by the fact that the level of axial strain applied to the NbTi

filaments, as a result of bending the conductor, depends on the distance

between the filaments and the wire's center, or its neutral axis.

Consequently, the reciprocal of the normalized bend diameter is a measure of

the maximum axial strain in the outermost NbTi filaments of a specimen. Both

of these plots show a substantial degradation in the measured I c with

decreasing bend diameter. The degradation for sample A is significantly

greater than for sample B at both 5 T and 7 T, and the degradation for both

samples decreases as the magnetic field is increased.

In Fig. 6 , n is plotted as a function of the normalized reciprocal of the

bend diameter for Samples A and B at three magnetic fields. At the largest

bend diameters, n is considerably higher for sample A; however, at the

smallest bend diameters, n is reduced to a comparable level for both samples.

The conspicuous feature of this plot is that both samples show a degradation

in n that seems to approach a comparable limiting value. The degradation of n

suggests that irregularity in the filament cross-sectional area introduced by

bending may be the source of the I c degradation.^7

As part of the second stage of testing, the results of a systematic

comparison of three samples (C, D, and E) are presented in Fig. 7. As

indicated by the specifications of Table 1, these three conductors are
10

comparable to one another with one exception, their local area ratios. The

local area ratios are 0.70:1, 0.55:1, and 0.40:1, respectively, for samples C,

D, and E. The normalized I c at the standard bend diameter (1.28 mm) is

plotted as a function of the applied magnetic field for each of the three

conductors. The data indicate a correlation between the LAR of the sample and

the level of I c degradation that results from bending. As the LAR of these

conductors increases, their sensitivity to bending decreases. All of these

samples show a decreasing level of I c degradation with increasing magnetic

field. This indicates that the source of the degradation is not an intrinsic

degradation of Hc 2> as with the elastic strain effect.-*

Figure 8 is a similar plot for three samples that have the same local

area ratios (0.55:1), but different diffusion barrier materials locally

surrounding each filament (samples F, G, and D) . These data indicate that the

diffusion barrier material may be at least as important as the LAR in

determining the sensitivity of a conductor to e ]-> . The vanadium diffusionbarrier material correlated with the least I c degradation from bending.

Differences in the shear strength of the filament-matrix- filament interface

for these different diffusion barrier materials may account for this

dependence

.

Figure 9 is a plot of the normalized I c at the standard bend diameter as

a function of the LAR at several magnetic fields. The difference between the

I c degradation for the maximum and minimum local area ratios is about 1/7 the

total I c degradation. Once again, the tendency for the degradation to

increase with decreasing LAR is evident.
11

Discussion

Two possible sources of the degradation in I c are residual elastic strain

in the NbTi filaments^ > ^ and local reductions in their cross-sectional

areas.

^ '9 The data presented in Figs. 7, 8, and 9, where the I c degradation

decreases with increasing magnetic field, refute the former explanation of the

I c degradation. The elastic - strain effect results in an intrinsic degradation

of Hc 2 , which, in turn, results in greatly increasing I c degradation at fields

approaching Hc 2- This is just the opposite of what is observed. The data

presented in Fig. 6, where the n value is shown to decrease with increasing

ej-j , supports the latter explanation, that I c and n degrade from an increase of

the filaments' cross-sectional irregularity with bending.

The correlation between the LAR and I c degradation that is shown in

Fig. 9 indicates that in order to increase a conductor's tolerance to bending

its LAR should be large. However, if the LAR is made to large, the increase

in the wires' tolerance to bending may be negated by a decrease in its

tolerance to drawing. A large LAR is conducive to the development of filament

irregularities, or sausaging, during drawing, which, in turn, results in

degradation of I c and n. Consequently, the optimum LAR may be a compromise

between the wire's tolerance to bending and its tolerance to drawing.

A large LAR apparently reduces the strain that is transmitted to the

filaments as a result of bending. A speculative explanation for this behavior

is that the low yield stress and resulting compliance of the copper matrix

material, between the filaments, in a large LAR conductor facilitates

independent motion of the filaments. Furthermore, different diffusion-barrier
12

materials may affect the filament- to- filament shear strength of the composite.

This increased independence of motion may allow the filament bundle to flatten

during bending, with the outer filaments moving toward the wire's neutral

axis, thereby reducing the axial strain of individual filaments introduced by

bending

.

Conclusions

Severe bending strain can cause a large reduction in the I c of

multifilamentary NbTi wires. This degradation is not associated with an

intrinsic Hc 2 effect, as evidenced by a reduction in degradation with

increasing magnetic field. Bending strain results, however, in a significant

degradation in the n values of these conductors . This suggests that the

degradation is associated with filament cross-sectional area irregulari ty

introduced by bending.

By using a series of samples where the local area ratio (LAR) of copper

to superconductor is systematically varied, it is shown that the wire's

tolerance to bending can be increased by increasing its LAR. However,

conductors that have a large LAR are more susceptible to the development of

filament irregularities, or sausaging, during drawing. A compromise between

these two conflicting effects (sausaging and bending degradation) is required

in order to optimize the I c of these conductors.

For the conductors tested in this study, bending degradation of I c is

also correlated with the diffusion-barrier material locally surrounding each

filament. The effect is at least as great as that of the LAR. Of the
13

diffusion-barrier materials that were tested, the vanadium barrier material

resulted in less sensitivity to bending strain than niobium barriers or

niobium barriers with a copper-nickel matrix.

This systematic study of bending degradation in NbTi superconductors has

thus shown that the critical-current degradation from bending can be reduced

through an appropriate choice of conductor -design factors, including local

area ratio and diffusion-barrier material. The overriding factor in the

cables' I c degradation is nevertheless the care used in cabling; no amount of

bending tolerance engineered into the conductor design can compensate for the

damage that can be introduced during cabling.

Acknowledgements

We gratefully acknowledge the assistance of R. Scanlan, who supplied the

NbTi samples for this study, and Intermagnetic General Corp. for fabricating

them as part of a Department of Energy SBIR program. We also extend our

appreciation to C. Thompson for designing, machining, and performing the

initial testing of the bend test fixtures; R. Scanlan, H. Kanithi, L. F,

Goodrich, T. M. Larson, G. Reinacker, and W. Bahn for useful discussions; L.

F. Goodrich for his help in constructing the current supply; and J. Brauch and

T. M. Larson for their assistance in writing the program for the critical

current automatic data acquisition system. This research was supported by the

U. S. Dept, of Energy, Div. of High Energy Physics under Interagency Agreement

No. DE-AI05 - 85ER40240

.

109
14

References

1. J. W. Ekin, L. F. Goodrich, J. Moreland, E. S. Pittman, and A. F. Clark,

"Electromechanical Properties of Superconductors for High Energy Physics

Applications," National Bureau of Standards Internal Report No. NBSIR 86-

3061, December 1986.

2. L. F. Goodrich, E. S. Pittman, J. W. Ekin, and R. M. Scanlan, "Studies of

NbTi Strands Extracted from Coreless Rutherford Cables," IEEE Trans.

Magn. , MAG-23 . 1642 (1987).

3. J. W. Ekin, "Relationships Between Critical Current and Stress in NbTi,"

IEEE Trans. Magn. MAG-23 . 1634 (1987).

4. L. F. Goodrich and S. L. Bray, "Current Capacity Degradation in

Superconducting Cable Strands," IEEE Trans. Magn. (1989), to be

published

.

5. J. W. Ekin, F. R. Fickett, and A. F. Clark, "Effect of Stress on the

Critical Current of NbTi Multifilamentary Composite Wire," Proc . Int'l.

Cryog. Mat. Conf. (August 1975), Adv. Cryo . Eng. 22 , 449 (1977).

6. S. L. Bray and J. W. Ekin, "Effect of Room Temperature Stress on the

Critical Current of NbTi," to be published in Jour. Appl. Phys . (1989).

7. J. W. Ekin, Quarterly Progress Report No. SR-724-13-82 , National Bureau

of Standards (1982) and Quarterly Progress Report No. SR-724-26-82

,

National Bureau of Standards (1982). J. W. Ekin, "Irregularity in Nb-Ti

Filament Area and Electric Field versus Current Characteristics,"

Cryogenics 2_7, 603 (1987).

8. M. Garber, M. Suenaga, W. B. Sampson, and R. L. Sabatini, Adv. Cryog.

Eng . 32, 707 (1986)

.

110
15

9. W. H. Warner and D. C. Larbalestier , Proc . Int'l. Svmp . on Flux Pinning

and Electromagnetic Properties in Superconductors (Ed. T. Matsushita, K.

Yamafuj i , and F. Irie) p. 156, Matsukuma Press, Japan (1986).
Table

1.

Sample

Characteristics

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VOLTAGE TAP

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BEND MANDREL

SAMPLE

SOLDER

1 cm

CONICAL

COPPER

CURRENT

LEADS

Fig. 1. Long bend test coil configuration.
(wo/A) 3 001

Fig.

2.

Logarithmic

plot

of

the

electric

field

(

E)

-vs

.

-

current

(I)

characteristic

of

different

coils

of

Sample

A

bent

around

mandrels

with

progressively

smaller

diameters,

showing

the

large

degradation

of

n

at

severe

bending

strain.
<

(

D

3

m

tZ

•r1

(uuo/a) 3 001

Log

E-vs.-Log

I

characteristic

of

different

coils

of

Sample

B

bent

around

mandrels

with

progressively

smaller

diameters,

showing

less
(0=9?) °| / (-Bia pueg) °|

03

• m*

Q

"D

C

CD

DO

dams Q

<r> oO

b0

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PE-I

Relative

degradation

ofI

c

of

Samples

A

and

B

as

a

function

of

bending

strain

at

5

T.
(0=93) °i/(-B!a pusg) °i

in

Fig.

5.

Relative

degradation

of

I

c

of

Samples

A

and

B

at

7

T,

showing

a

decrease

in

degradation

at:

higher

magnetic

field

(compare

with
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c6

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Fig.

6.

Bending

strain

degradation

of

rt

of

Samples A

and

B,

showing

the

approach

to

the

same

degraded

value

ofn

at

severe

bending

strain.
=

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7.

Effect

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of
=

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Effect

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05

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121

Fig.

9.

Dependence

of

bending

degradation

of

I

c

on

local

area

ratio,

constant

magnetic

field.

Measurement of Saturation Magnetization and Intrinsic Coercivity

of High-Permeability Steel for the SSC

R. B. Goldfarb

Electromagnetic Technology Division

National Institute of Standards and Technology

Boulder, Colorado 80303

Accelerators such as the proposed Superconducting Super Collider (SSC)

and the Tevatron at Fermilab use high-permeability steel for the return flux

path of their superconducting magnets. It is the same type of steel that is

commonly used for electrical transformers. In addition to high permeabi lity

( fi ) , desirable magnetic properties include large saturation magnetization (Ms ) and small intrinsic coercivity (Hc )

.

It is useful to define Ms and Hc with reference to an idealized curve of

magnetization (M) vs. magnetic field strength (H) . Figure 1 represents a

hysteresis loop for a sample with zero demagnetization factor, such as a long,

thin needle. The parameters are indicated. If required, magnetic flux

density (B) may be derived as

B - MO (H+M) (1)

(SI units, where m 0i- s t^ie permeability of free space) and plotted as a

function of H [1,2].

In actual measurements with instruments such as a vibrating- sample

magnetometer, a sample geometry with finite demagnetization factor is used

[3,4]. Magnetic field (H) in the above equations and plot refers to the

internal field (H^nt ) , equal to the applied field (Hext ) corrected for the

proper geometric demagnetization factor (D) [5],

Hint = Hext ' DM. (2)

As a result of a sample's demagnetization factor, its plot of M vs . Hext

is "sheared" along the H-axis [6]. Generally, to deduce parameters

characteristic of the material of which the sample is made, the data have to

be replotted as M vs . H^nt: . Several points should be noted, however.

1. Hc is independent of demagnetization factor; when M is zero, H^nt is

identical to Hext .

2. Ms is independent of demagnetization factor. At high fields, DM is

only a small correction to Hext in Eq . 2.

3. It is difficult to extract the exact value of m from magnetometer

data when m is large. The most successful methods use samples with exactly

known demagnetization factors, such as spheres, or with zero demagnetization

factors, such as toroids with azimuthal fields [7].

/ ,
In this brief note, I report on the measurement of Ms and Hc for several

steels suitable for accelerator applications. We used a conventional

vibrating- sample magnetometer [8]. A high-field superconducting solenoid was

used for the Ms determination and a low-field copper-coil Helmholtz transverse

pair was used for the Hc measurements.

The samples consisted of rectangular prisms, about 1 . 5 mm thick, 6 mm

wide, and 12 mm long. All magnetic measurements were made with the field in

the longest axis. This minimized the demagnetization factor. Brief sample

descriptions are given in Table I.

Magnetization was measured in liquid helium (4 K) for maximum applied

fields of ±5.6 MA/m (±70 kOe , or ±7 T in free space). There were no

significant differences between the samples; they all saturated at Ms - 1.75

MA/m (1750 emu/cm^ , or 4ttM s - 22 kG) . A typical hysteresis loop is shown in

Fig. 2. The field was cycled from zero to +7 T to -7 T to +7 T. There was no

measurable hysteresis on this coarse scale.

Intrinsic coercivity was determined from the field required for zero

magnetization. Measurements were made at room temperature (296 K) and in

liquid nitrogen (76 K) . Maximum applied fields were ±800 kA/m (±10 Oe , or ±1

mT in free space). The samples were initially demagnetized [9].

The field was increased to 1 mT while magnetization was recorded. At 1 mT , the samples were briefly exposed to field of 0.1 T in an attempt to

approach saturation. (A larger field would have been preferable, but was not

available. Generally, Hc is a weak function of the maximum applied field.)

Thereupon data were recorded as the field was reduced to -1 mT. At - 1 mT , the

sample was exposed to a field of -0.1 T. The cycle continued to 1 mT. A

typical trace is shown in Fig. 3. Note that the loop is sheared in the sense

discussed above.

Table II summarizes the results for the five samples. Values of Hc ranged from 110 A/m (1.4 Oe) for cold-rolled samples to 170 A/m (2.1 Oe) for

hot-rolled samples. The results were independent of whether the field was

parallel or perpendicular to the rolling direction. Liquid-nitrogen Hc was

slightly larger than room- temperature Hc

. At liquid-helium temperature, Hc is

expected to be slightly higher still.

In summary, there appear to be no significant differences in these steel

samples with regard to saturation magnetization and intrinsic coercivity.

ACKNOWLEDGMENTS

R. L. Spomer assisted with the measurements. The steel samples were

provided by M. V. Bala, Inland Steel Company. The work was supported by the

Department of Energy, Division of High Energy Physics.

Certain commercial materials are identified to adequately specify the

experimental study. Recommendations or endorsements by NBS are not implied.
REFERENCES

"Intrinsic coercivity" is the H at which M goes to zero. The terms

"coercive force" and "coercivity" refer to the value of H at which B goe

to zero. The symbol Hc is commonly used for both meanings.

Permeability ^ is defined as dB/dH. A related quantity, susceptibility

X, is dM/dH. In SI units, /i = 1+xR. M. Bozorth and D. M. Chapin, J. Appl. Phys . .13., 320 (1942).

T. L. Templeton and A. S. Arrott, IEEE Trans. Magn. MAG- 23 . 2650 (1987).

R. B. Goldfarb, Cryogenics 2J7 , 621 (1986).

If a sample has a large M but unknown D, the value of D can be estimated

as that required to "unshear" the hysteresis loop, using Eq. 2. For

example, see: J. C. Mallinson, The Foundations of Magnetic Recording . (Academic Press, San Diego, 1987) p.23.

R. B. Goldfarb and H. E. Bussey, Rev. Sci. Instrum. 58., 624 (1987).

S. Foner, Rev. Sci. Instrum. 30, 548 (1959).

The ac demagnetization process consists of exposing the sample to an

alternating field that is decreasing in amplitude. In practice, a simpl

wire-wound solenoid is energized with a variable transformer ("Variac")

at line frequency. The purpose is to remove any remanent magnetization

in the sample.
Table I. Samples for Ms and Hc measurement.

Sample Description

A Inland, plate steel, hot rolled, ultra-low carbon

B Inland, cold rolled, H parallel to rolling direction

C Inland, cold rolled, H perpendicular to rolling direction

D Armco, cold rolled, ZnP coating, H parallel to rolling direction

E Armco, cold rolled, ZnP coating, H perpendicular to rolling direction

Table II. Intrinsic coercivity at 296 and 76 K.

Sample

296 K 76 K

Hc (A/m) Hc (Oe) Hc (A/m) Hc (Oe)

A 160 2.0 170 2.1

B 110 1.4 130 1.6

C 100 1.3 110 1.4

D 100 1.3 110 1.4

E 100 1.3 110 1.4

M

Figure 1. Idealized hysteresis loop of magnetization (M) vs. magnetic field

(H) , showing saturation magnetization (Ms ) and intrinsic coercivity (Hc )

.
External Field (kOe)

-80 -40 0 40 80

2000

1000

n

E

D

E

.0)c

0

13

s

<L> -1000 g

CO

-2000

External Field (MA/m)

Figure 2. Hysteresis loop of magnetization vs.

temperature for sample E. Curves for the other

applied field at liquid-helium,

samples are similar.

External Field (kOe)

-0010 -0.005 0000 0.005 0010

External Field (kA/m)

c

o

ccN

c

or

to

Figure 3. Plot of magnetization vs. applied field (±1 mT maximum field) at

room temperature for sample A. Curves for the other samples at room

temperature and liquid-nitrogen temperature are similar. Upon reaching ±1 ml,

the sample was exposed to a field of ±0.1 T.
'
Appendix: Publications

Irregularity in Nb-Ti Filament Area and Electric

Field Versus Current Characteristics

J.W. Ekin, Cryogenics 27, 603 (1987)

Relationships Between Critical Current and Stress

in NbTi

J.W. Ekin, IEEE Trans. Mag., MAG-23, 1634 (1987)

Studies of NbTi Strands Extracted From Coreless

Rutherford Cables

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

Trans. Mag., MAG-23, 1642 (1987)

Current Capacity Degradation in Superconducting

Cable Strands

L.F. Goodrich and S.L. Bray, Applied Superconductivity Conference, ASC, San Francisco, California,

Aug. 1988, IEEE Trans. Mag., MAG-25, 1949 (1989)

129

Irregularity in Nb-Ti filament area and electric

field versus current characteristics

J.W. Ekin

Electromagnetic Technology Division, Center for Electronics and Electrical Engineering,

National Bureau of Standards, Boulder, CO 80303, USA

Received 23 December 1986

There is a correlation between irregularity in filament area ('sausaging') and the shape of a superconductor's electric field (£) versus current (/) relationship. The shape of the E-l

characteristic is quantified in terms of the resistive transition parameter, n, defined by E x fn

. Low values of n less than about 20 correlate with a wide filament diameter distribution, while n values over 50 correspond to a distribution more than 2.5 times smaller. It is proposed that

the low-field (constant) value of n be used as an index of filament quality in evaluating

different superconductors for practical applications. A model is also suggested to explain

this effect in terms of a locally depressed filament critical current, which forces current to transfer across the normal matrix material into neighbouring filaments. The relationship

between n and the statistical distribution of filament diameters may be useful as an easy

method of estimating the extent of sausaging in practical multifilamentary Nb-Ti superconductors from measurements of n.

Keywords: superconductors; Nb-Ti; sausaging; multifilamentary superconductors

In a multifilamentary superconductor a correlation exists

between irregularity in filament cross-sectional area (sometimes called ‘necking’ or ‘sausaging’) and the shape

of a superconductor's electric field versus current characteristic 1

. Measurements show that the greater the extent of irregularity in filament cross-sectional area, the more

gradual is the rise in the electric field versus current ( E-I ) characteristic. The shape of the E-l curve is quantified in terms of the resistive transition parameter, n, defined by

TfocT". Low values of n indicate significant filament

irregularity. The first part of this article proposes that the

parameter n be used as an index of filament quality in evaluating different superconductors for practical

applications.

A model is also suggested to explain this effect : filament

necking depresses the critical current, 7C , of a filament

locally; this forces current to transfer across the normal

matrix material into (unnecked) neighbouring filaments.

The current transfer is intrinsic to the wire and occurs continuously along its length wherever the filaments are necked down to a smaller cross-sectional 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. (According to

this model, filaments that are uniform along their length

but with different cross-sectional areas do not cause

intrinsic current transfer. It is only when filaments neck

that significant current transfer occurs and the value of n is degraded.)

The second part of this article presents a quantitative

relationship between n and the statistical distribution of

filament diameters 2 measured for the Nb Ti superconductors studied in the first part. The relationship ma\

be useful as a quick method for estimating the extent of

sausaging in practical multifilamentary Nb-Ti superconductors from measurements of n. This article is based on two internal reports of the

Electromagnetic Technology Division of the National

Bureau of Standards 12

. The two main features of these

reports - the correlation of n with filament sausaging and

the filament-necking model - have been confirmed b\

subsequent research at several laboratories 3-9

. The

sausaging model and data should aid in present efforts to produce high critical current density multifilamentar\

conductors and to develop a theory treating 7C inhomogeneity. They also help explain and quantify some

observations of a preliminary nature, which noted a concurrence of filament sausaging and a lowering of

/ 10-12

* c The transition parameter, n, was introduced some time

ago by Voelker 13 in another connection relating to filament breakage rather than sausaging. The n value (n ^ 3) he measured was significantly lower than the n values

reported here; in a sense, his work represents an extension

of filament sausaging to the extreme case of filament

breakage. We suspect that filament necking may have also

contributed to the relatively low n values (7 ^ n ^ 12) in some of the ‘good ’ multifilamentary samples he measured,

since n-values are typically much greater than 30 in multifilamentary superconductors with little filament

necking.

The model of Jones et a/. 14

, which has formed the basis

Cryogenics 1987 Vol 27 November 603

131
Irregularity in Nb Ti filament area: J. W Ekin

for much 7C inhomogeneity analysis 814 " 17

, does not

include the effects of current transfer between sausaged

filaments, as described in the model 1

. The data and model

presented here show that filament sausaging. rather than

chemical or intrinsic material inhomogeneity, is the

dominant source of 7C inhomogeneity that needs to be

considered at low and medium fields in many practical

multifilamentary Nb-Ti superconductors. The effective

sausaging length is several filament diameters, which

makes filament-to-filament current transfer an important

consideration in 7C inhomogeneity theory. Recently,

Evetts and Plummer8 have incorporated current transfer

effects into a unified analysis of sausaging effects in multifilamentary conductors. A preliminary comparison

indicates that these data are in reasonable agreement with

their model.

The following two sections are based, respectively, on

References 1 and 2; the third section contains an addendum that summarizes the statistical data in convenient

form. The identities of the superconductors have been

relabelled, but otherwise the internal reports appear in their original form.

Effect of superconductor filament

irregularity

During critical current testing of several Nb-Ti conductors. it was noticed that the electric field versus current

(E-I) characteristic for one conductor did not show the

sharp take-off behaviour normally observed in other

Nb-Ti: copper superconductors we have tested. Instead,

the rise in voltage in this conductor started well below the

critical current, 7C

, showing a gradual rise as 7 approached

7C rather than a sharp rise right at 7C (see sample A in Figure I). This limits the usefulness of the conductor in magnet applications, since a significant electric field in the

conductor below 7C leads to heating and decreased

stability. Several tests were consequently undertaken to determine the source of this electric field The effect may be quantified by describing the superconductor £-/ characteristic in terms of the equation

Figure 1 Normalized electric field versus current plots for three multifilamentary Nb-Ti superconductors, showing the more gradual

resistive transition in sample A compared with samples B and C

Figure 2 Logarithmic plot of electric field versus current for the curves in Figure 1. showing the low value of n in sample A compared

with samples B and C (

n

is defined by the relationship VxT)

where: 7C = critical current; and £c = electric field crite- rion at which /c is determined The larger the exponent n, the sharper the £-/ take-off at /c

. Thus, large n is desirable

for magnet applications.

In Figure 2, log £/£c is plotted versus ///c for sample A

as well as another previously tested conductor, sample B. to determine comparative values of n. As seen from the curves in Figure 2, sample B has an n-value of * 35,

whereas sample A has an n- value of only * 18. One possible source of the low n in sample A is filament

irregularity. If there is sausaging in the filaments, current

will transfer out of a filament where it necks dow n, into a neighbouring filament which can carry the extra current. When the current in the neighbouring 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 Ic

. resulting in a low value of n. 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 chemical!)

removed and the filaments photographed longitudinally

The results are shown in Figures 3a and 4a. Significantly

greater filament irregularities were found in sample A

than in sample B. As shown in Table 1, the average

diameter fluctuations, AZ), for sample A amounted to *9 gm, or *33% of its average filament diameter <£>>. In sample A, in contrast, A D was less than 7 /im, or * 20% of its average filament diameter. The relative

amount of filament irregularity, A £>/</>>, in these con- ductors thus correlates well with degradation in the value

of n characterizing 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 Figure 5 and Table 1, AD for this conductor was only *6% of its <Z)>. The E-I characteristic 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

604 Cryogenics 1987 Vol 27 November

132
Figure 3 Longitudinal (a) and cross-sectional (b) views of the filaments in sample A

characteristic of conductors containing high percentages

of copper stabilizer. In summary, these results show that the degradation in the value of n for these multifilamentary Nb-Ti conductors correlates well with the relative 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 Nb-Ti conductors and these data w'ould 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-l characteristics, complementing the /c value usually supplied with commercial

conductors.

Irregularity in Nb-Ti filament area: J.W. Ekin

Relationship between n and the statistical

distribution of filament sizes

In the preceding section, it was reported that the electric

field versus current characteristics for sample A did not

show the sharp take-off normally found in other

Nb-Ti :copper conductors we have tested. The effect was

quantified in terms of a parameter, n, which for sample A

was only *18, compared with values of n = 35 for sample B and n = 53 for sample C (another high copper

to Nb-Ti ratio superconductor). The low value of n w as ascribed to irregularities (sausaging) in the filament

diameter of sample A as observed in SEM longitudinal

micrographs of isolated filaments.

To quantify these preliminary observations, a series of

SEM micrographs of the complete cross-section of the

conductor has been obtained (shown in Figures 3b-5b ).

Figure 4 Longitudinal (a) and cross-sectional (b) views of the

filaments in sample B

Table 1 Filament diameter variations for three Nb-Ti conductors

Conductor

Size (mm)Number of filaments

Twist

pitch

(cm) Cu: Nb-Ti

Average

filament diameter.

<£>> (/im)

Dmax

^min tsD

l/im)

Deformation

length

«0»* n

Sample A 0.51 x 1.02 187 1.8 4.77 27 1.33 8.9 4 18

Sample B 0.56 x 0.71 180 1.3 1.8 33 1.20 6.5 8 35

Sample C 1 .00 diameter 114 2.5 5.5 37 1.06 2.2 6 53

133

Cryogenics 1 987 Vol 27 November 605
Irregularity in Nb-Ti filament area: J. W. Ekin

Figure 5 Longitudinal (a) and cross-sectional (b) views of the filaments in sample C

Figure 6 Distribution of filament diameters in samples A, B and C

Horizontal bars indioete the sampling interval for each histogram

Irregularities 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 at the same radial position

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, £>, about the average filament diameter of each

ring of filaments, <Z)>. 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 irregularities. 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 report as the explanation for the relatively large

electric field appearing in sample A at currents below the

critical current. In the SEM micrographs in Figures 3b-5b note the

presence of the large centre island of copper in conductor

samples B and C, compared with the lack of this featu re in sample A. It is believed this centre copper island is a

significant factor in minimizing the filament irregularity

problem (particularly in conductors having a high copper

to superconductor volume ratio).

Addendum

The n values reported above were nearly independent of

magnetic field from 3 to 7 T. Over this field range they

varied by less than ^5%. A graph of the standard

deviation of the filament diameter distributions show n in Figure 6 is plotted as a function of n in Figure 7. The

Figure 7 Approximate relationship between n and the standard

deviation of the filament diameter distribution for several multifila- mentary Nb-Ti superconductors (from Figure 6)

606 Cryogenics 1987 Vol 27 November

134
approximate relationship shown in Figure 7 may be useful

for quickly estimating the extent of filament irregularity in practical multifilamentary Nb-Ti superconductors from

measurements of n.

Acknowledgements

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. This work was sponsored by

the Naval Ship Research and Development Center and

by the US Department of Energy. Division of High

Energv Physics under Interagency Agreement No. DEA105-85ER40240.

References

1 Ekin, J.W. Superconducting Coil Characterization Quarter!) Progress Report No. SR-724-I3-82, Di\ 724.05. National Bureau of Standards. Boulder. Colorado. USA (March 1982) unpublished.

Distributed at the First Workshop on Nb-Ti Superconductors,

Universit) of Wisconsin. Madison. Wisconsin. USA (July 1983)

2 Ekin. J.W. Superconducting Coil Characterization Quarterly Progress Report No. SR-724-26-82, Div 724.05, National Bureau of Standards. Boulder, Colorado. USA (June 1982) unpublished.

Irregularity in Nb-Ti filament area: J.W. Ekin

Distributed at the First Workshop on Nb-Ti Supercon Juctors, University of Wisconsin. Madison. Wisconsin. USA (July 1983)

3 Garber. M„ Mclnturff, A. and Larbalestier. D. presentations made

at the Second Workshop on Nb-Ti Superconductors, University of Wisconsin, Madison, Wisconsin, USA (July 1984) 4 Gregory, E., Hemachalam, Hong, M., Garber, M.. Sampson,

W „ Suenaga, M., Mclnturff, A., W arnes, W.H., Larbalestier, D. and

Scanlon, R. presentations made at the Third Workshop on Nb-Ti

Superconductors. Lawrence Berkeley Laboratory, Berkeley.

California. USA (January 1985)

5 Garber, M., Suenaga, M„ Sampson, W.B. and Sabatini, R.L. Ad\

Cryog Eng Plenum Press. New York. USA (1986) 32 707

6 Larbalestier, D.C. and West, A.W. Ada Met (1984) 32 187]

7 Warnes, W.H. and Larbalestier, D.C. Proc Ini Symp Flux Pinning

and Electromagnetic Properties in Superconductors ( Ed Matsushita.

T., Yamafuji, K. and Irie, F.) Matsukuma Press, Japan (1986) 156

8 Evetts, J.E. and Plummer, CJ.G. Proc Ini Symp Flux Pinning and

Electromagnetic Properties in Superconductors (Ed Matsushita. T.. Yamafuji, K.. and Ine. F.) Matsukuma Press, Japan (1986) 146

9 Plummer, CJ.G. and Evetts, J.E. IEEE Trans Magn (1987) MAG23 1179

10 Smathers, D.B., Marken, K.R., Larbalestier, D.C. and Scanlan. R.M. IEEE Trans Magn (1983) MAG-19 1417

1 1 West, A.W., Warnes, W.H_ Moffat, D.L and Larbalestier, D.C. IEEE Trans Magn (1983) MAG-19 749

12 Evetts, J.E. IEEE Trans Magn (1983) MAG-19 1109

13 Voelker, F. Panicle Accelerators (1970) 1 205

1 4 Jones. R.G., Rhoderick. E.H. and Rose-lnnes, A.C. Phvs Leu ( 1 967

)

24A 318

1

5

W arnes, W.H. and Larbalestier, D.C. Appl Phys Lett ( 1 9861 48 1 403

16 Warnes, W .H. and Larbalestier, D.C. Cryogenics ( 1 986 1 26 643

17 Warnes. W.H. and Larbalestier, D.C. IEEE Trans Magn (1987)

MAG-23 1183

Cryogenics 1987 Vol 27 November 607

135

I6.U IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-23, NO. 2, MARCH 1987

RELATIONSHIPS BETWEEN CRITICAL CURRENT AND STRESS IN NbTi

J. W. Ekin

Electromagnetic Technology Division

National Bureau of Standards

Boulder, Colorado 80303

ABSTRACT

The effects of various types of stress on the

critical current of a multifilamentary NbTi superconductor are reported. Degradation of critical

current due to axial tension applied at 4 K, trans- verse compression applied at 4 K, and hairpin bending

strain applied at room temperature has been measured.

The degradation from axial tension is much greater

than from transverse compression in many practical cases because the soft copper matrix limits the buildup of transverse compression. The degradation from

typical levels of transverse compression is only about

4J at 8 T, for example. For axial tension, on the

other hand, higher stresses can occur that will degrade the critical current by 24J, for example, at 7 T

and 2.7J strain. Both the axial-tensile and the

transverse-compressive stress effects are about 98J

reversible; thus the degradation will be seen only

when the conductor is under operational stress. The

results indicate that a primary origin of the critical

current degradation in NbTi is a stress-induced

reversible decrease in the upper critical field.

INTRODUCTION

In the initial phase of this study, reported

here, several types of mechanical tests were made on

the same conductor material to evaluate the criticalcurrent degradation from different types of stress.

Once identified, the most important stress tests will

be performed on many different conductors in a later

phase of testing to obtain a data base useful for

minimizing cr itical-current degradation in magnet

design.

In this paper, measurements of the effect of 4-K

transverse compressive stress on the critical current

of NbTi are reported. These results are compared with

the effects of axial tension applied at 4 K and severe hairpin bending strain applied at 295 K, all measured

on samples of the same NbTi conductor. The conductor

chosen for tnese first tests was the strand material

used to make many of the 30-strand Rutherford cables

in the Lawrence Berkeley Laboratory (LBL) cabledevelopment program.

TRANSVERSE COMPRESSION TESTS

1

The effect of axial tension on the critical

current of practical multifilamentary NbTi superconductors was first studied about ten years ago.

Little electromechanical research has been made on NbTi superconductors since then, and this has 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 threedimensional stress. Typically the transverse com- ponent of stress is compressive. For example, in

solenoldal magnets the transverse component arises

from hoop stress which compresses the magnet winding

radially. In dipole magnets it arises from Lorentz

force compression of the magnet windings at the

midplane.

Manuscript received September 30. 1986.

Contribution of NBS, not subject to copyright.

To obtain data on the effects of this component

of stress on the critical current of practical superconductors, a transverse-stress apparatus was 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 sample is prepared in the shape of a flat

sided hairpin. Compressive load is applied by sandwiching the central flat section of the hairpin

between two stainless-steel anvil heads. Magnetic

field is applied mutually perpendicular to both the

directions of the force and the sample current with a radial access magnet. The field is also perpendicular

to the hairpin legs of the sample to minimize currenttransfer effects. (Otherwise, if the hairpin legs were parallel to the applied magnetic field, a large

difference in critical current would arise between the

legs and the central section.) Further details and

pictures of the apparatus are given in Ref. 4.

The first test results were obtained on an unused

segment of NbTi strand material similar to that used

to make many of the 30-strand Rutherford cables in tne

LBL cable development programs. The NbTi strand

character istics are given in Table 1. The sample was manufactured using a single stack technique without a diffusion barrier around the filaments.

Figure 1 shows the degradation of criticalcurrent by transverse stress applied to the sample at

4 K. The ordinate in Fig. 1 is the measured critical

current normalized by its starting (zero-stress)

value. The abscissa is transverse force per unit

sample length, F/L (top scale). A second abscissa

scale is also shown (bottom scale) which is an effective overall transverse stress obtained by dividing

the applied load by the projected area of the sample

in the direction of the applied load. This projected area is simply taken as the length over which com- pressive load is applied (9.5 mm) multiplied by the

starting sample diameter (0.648 mm).

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

0 100 200 300 400

Figure 1. Effect of transverse compr essive force on

the critical current a NbTi 'conductor at 8 T.

137
1635

Table 1. Characteristics of the multifilamentary NbTi

sample studied in this paper.

Strand diameter 0.648 mm

Filament diameter 1 6 ym

Number of filaments 648

Copper- to-superconduc tor

volume ratio 1 .77

Twist pitch 1.27 cm Superconductor area 0.1189 mnr Total area 0.3295 mm

2

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 in- crease 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 2^% at the maximum value of forceper-saraple-length (F/L) of 458 kN/m. This overall

sample strain, however, is typically much greater than

the actual NbTi filament strain because the copper ma- trix material yields at a relatively low stress. A picture of the deformed sample after the maximum loading (F/L *58 kN/m) at 4 K is shown in Fig. 2.

The separation of the voltage taps was relatively small, 8.7 mm. Consequently a relatively high

critical-current voltage criterion was used, corre- sponding to an electric field of 5 yV/cm. The critical-current values at more sensitive criteria are easily calculated, however, using the measured values

of the exponent n given in Fig. 3 C

I

c1

/

I

C 2

“ (E c1 /E c2

)

/n

j. Note that for this sample, which was fabricated using a single stack technique with relatively large filaments (16 ym), n reached values of

about 80 at low magnetic fields. This is relatively

high for multifilamentary NbTi superconductors.

As seen from the results at 8 T in Fig. 1 , little degradation (£ 0.5*) was observed for this

particular sample up to a F/L value of about 100 kN/m

(corresponding to an approximate transverse compressive stress, o]_, of about 150 MPa). The degradation

effect then increased monotonically to about 4* as F/L was increased to 460 kN/m ( o_|_ - 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 2. Thus, the

maximum critical-current degradation from transverse

stress is about 4} at 7 T.

Figure 2. Cross section view of the NbTi conductor

after application of 458 kN/m transverse load at 4 K.

The effect was almost completely reversible.

Upon unloading from »VL - 460 kN/ro, 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 25*.

The transverse stress degradation was largest at high magnetic field. At lower fields (5 T and

3 T) the effect was less than 0.5 * at a F/L value of

1*60 kN/m.

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 1) was tested in an axial tensile mode. The

experiments were carried out using an apparatus

described in Ref. 6.

Results corresponding to a 5 uV/cm criterion

(the same as that used for the transverse compression

tests) are given in Fig. 4 as a function of stress and

strain. Stress was calculated using the overall conductor cross sectional area. The results of Fig. 4 compared with Fig. 1 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* at an average tensile stress of 700 MPa

(tensile strain of 2.7*). This compares with an 8 T degradation of about 4* at 700 MPa transverse stress,

about one sixth.

However, because the copper matrix has a relatively low yield stress (about 130 MPa), the axial

stress in the NbTi filaments is actually higher (about

2.1 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 72 at 8 T. Thus, the two

effects (axial and transverse) are nearly comparable

in terms of actual NbTi filament stress . The slight

difference in critical-current 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

Figure 3. Magnetic field dependence of the exponent n.

138
iO.'O

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 itical-current 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, the degradation at an axial

stress of 700 MPa (2.7% strain) was 1 9% at 3 T» and

increased to 265 at 7 T. For transverse stress, the

degradation was negligible (SO. 5?) below 5 T, and

increased to ^5 at 8T.

not the prime causes of degradation in NbTi, but

rather the stress that accompanies deformation is the

major cause, at least at ^ K. Thus, stress annealing

the conductor night be a method for improving

the critical current.

Tests at Magnetic Fields Above 7 T

To measure the effect at magnetic fields approaching the upper critical field, another series of

tests was made at fields above 7 T using an apparatus

described in Ref. 7. The results are given in Fig. 4 along with the low-field results.

Both types of stress effects are also highly

reversible. For example, upon unloading from an applied axial stress of about 700 MPa (strain of 2.75),

the critical current at 5 T recovered to within 15 of

its original value. This extreme reversibility persisted up to the fracture point of the sample, which

occurred at a strain of H.065. Just before the sample

fractured, the critical current still recovered to

within 1.55 of its original value upon unloading.

A significant serrated yield event (load

drop of 105) took place at a tensile strain of 3*05.

Figure iJ shows that this did not significantly affect

the smooth monotonic decrease of the critical current

with stress and strain.

However, a thermal runaway event, a "quench"

took place while the sample was under high stress, 8^2

MPa (3*55 strain), which did significantly affect the

critical current degradation. As shown in Fig. U, the

critical current rose significantly after the transient heating that accompanied the thermal runaway and

current-supply shutdown. During this quench the

strain in the sample was maintained constant, but the

stress decreased about 155 due to annealing. 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 STRESS (MPa)

These data show that the relative magnitude

of the stress effect becomes ouch larger as the magnetic field approaches the upper critical field. At

2.H5 strain, for example, the critical-current degradation is 195 at 8 T, 2^5 at 9 T, U25 at 10 T, and

rises to 605 at 10.5 T.

HAIRPIN BEND TEST AT 295 K

To compare these H K stress effects with

room-temperature bending-strain effects, the same su- perconductor 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 reduce current-transf er effects.

The sample was subjected to bending strain,

starting at about 155 and increasing to a little over

1005. This was done progressively in the same sample

by bending it at room temperature, cooling the sample

to 4 K, measuring I c

, warming the sample to room tenperture, bending the sample into a tighter radius, ar.d repeating the whole process. The critical current was measured at each radius until the radius was reduced

to nearly zero.

The I c results corresponding to a 5yV/cm

criterion are shown in Fig. 5. In this figure, the

relative critical-current degradation is plotted as a function of bending strain in the outer filaments,

eB

. Critical current was measured for current flowing

BEND STRAIN IN OUTER FILAMENTS, (%)

Figure 4. Effect of axial tens ile strain and stress

on the critical current of a NbTi conductor at magnetic fields from 3 T to 10.5 T. A much coarser scale

was used here than in Figs. 1 and 5.

Figure 5. Effect of room-temperature bend strain on the critical current of a NbTi conductor £ t 8 T. The

abscissa is the calculated bending strain in the

outermost filaments.

139
16

in two 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 cr itical-current

values is plotted with open circles in Fig. 5.

The magnitude of the degradation at the high

bending strains shown in Fig. 5 is small. The effect

of room temperature bending strain on the critical

current is less than ^ % at bending strains of over

100J. 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 to better simulate cabling

conditions. Note that the conductor strand was not

under tension when it was bent, nor was it bent around

a mandrel or severely pressed. The critical current

under bending strain also needs to be measured at

lower electric fields.

CONCLUSION

The main results of these measurements are summarized below:

1. The magnitude of critical-current degradation in NbTi from transverse compression applied at

4 K is much less than from axial tension in terms of

overall conductor stress, but the two are 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 4J at 8 T, for example. Thus, in many practical cases, axial

tensile stress (from tension or bending strain) is the more important source of critical-current degradation.

For example, the I c degradation is 24? at 7T when the

NbTi filaments are axially strained 2.72.

3. The I c degradation from both types of

stress is about 98% reversible , 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 critical-current 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 axial-tension degradation.

It is shown by the large relative increase in the

critical-current degradation from both types of stress

as the magnetic field approaches the upper critical

field.

There are several practical consequences of

these preliminary findings for cable fabrication:

1. In Rutherford cables, not much trans- verse stress is generated in the NbTi filaments along

the cable faces because of yielding 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.

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

nay 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 con- sistency 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.

ACKNOWLEDGEMENTS

J. C. Brauch, C. A. Thompson, and D. L. Rule

helped with apparatus design and construction as well

as data acquisition and reduction. R. M. Scanlan, J. M. Royet, C. Taylor, L. F. Goodrich, A. F. Clark, and

members of the NBS Superconductor and Magnetic

Measurements Group contributed a number of helpful

discussions to this research. Thanks are also ex- tended to E. S. Pittman for preparing the photomicrograph in Fig. 2, R. A. Shenko for typing the manu- script and R. B. Goldfarb for a detailed reading of

the text. This research was supported by the U.S.

Department of Energy, Div. of High Energy Physics

under Interagency Agreement No. DE-AI05-65ER40240.

The high magnetic fields needed for part of this study

were obtained using the magnet facilities of the

Francis Bitter National Magnet Laboratory.

REFERENCES

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

"Effect of Stress on the Critical Current of NbTi

Multifilamentary Composite Wire," Proc. Int'l.

Cryog. Mat. Conf. (Aug. 1975), Adv. Cryo. 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 Applications , edited by S. Foner

and B. B. Schwartz, Plenum Press, New York, p. 455-51 0 (1981).

3. The effect of critical-current anisotropy introduced by room-temperature 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. ^2, 833 (1986).

4. J. W. Ekin, L. F. Goodrich, J. Moreland, E. S. Pittman, and A. E. Clark, "Electromechanical

Properties of Superconductors for High Energy

Physics Applications," NBS Internal Report, NBSIR

#86-3061 (1986), in press.

5. J. Royet and R. M. Scanlan, "Manufacture of

Keystoned Flat Superconducting Cables for Use in

SEC Dipoles," Paper LB-1 of these conference

proceedings.

6. 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.)

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

in Practical Superconductors, Part I: Basic Relationship and Application to Nb,Sn Conductors,"

Cryogenics 20, 611-624 (1980).

8. L. F. Goodrich, E. S. Pittman, J. W. Ekin, and R. M. Scanlan, "Electromechanical Properties of NbTi

Strands Extracted From Cables," Paper MH-S of

these conference proceedings.

140
1642 IEEE TRANSACTIONS ON MAGNETICS. VOL. MAG-23. NO. 2, MARCH 1987 STUDIES OF NbTi STRANDS EXTRACTED FROM CORELESS RUTHERFORD CABLES

L. F. Goodricn, E. S. Pittman, and J. W. Ekin

National Bureau of Standards

Boulder, Colorado 80303

and

R. M. Scanlan

Lawrence Berkeley L-aooratory

Berkeley, California 94720

Abstract

The electromechanical properties of NbTi strands

extracted from coreiess Rutherford cables were studied

to clarify the relative effects of strand location and

fieid angle on current degradation that occurs in

cables that have been compacted into a keystone snape.

Detailed critical-current measurements were made on

two samples wnicn were fabricated under controlled

conditions. These are prototype cables for high

energy pnysics 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 caole manufacture.

Introduction

The measurements reported here were made on two

samples, denoted as Sample A (billet 5914, caole SC

305) and Sample B (billet 5212, cable SC 297). Sample

A was a strand from a caole that was commonly useo for

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

the turxshead (to re-size it) after it had been

cabled. This re-rolling may have been done in the

direction opposite the original cabling and roll direction. The re-rolled caole Sample B was a worstcase 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. The critical current, I c

, of strands extracted from cables was measured as a function of position along the cable and

magnetic field orientation. A more complete report on

this work can be found in Ref. 1 . Information on the

caole fabrication and current density with hign homogeneous NbTi alloy can be found in Refs. 2, 3 and 4.

Experimental Details

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°. Sample A had a Nb diffusion barrier on each of the 6006, 5-um-diameter filaments. Sample B had 648, 1 6 -urn-diameter filaments and

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 (Sample A, 320 A; Sample B, 347 A).

The critical-current density was calculated using the

NbTi cross-sectional area (Sample A, 2500 A/mm^;

Sample B, 2900 A/mm^) p which was determined using the

copper-to-superconductor volume ratio (Sample A, 1.64;

Manuscript received September 30, 1986.

Sample B, 1.77). The critical-current density at 4.22

K would be about 61 less than tne values listed at

4.02 K.

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 tne 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 caole eage were denoted as the ce ntral flat edge and the two corners

ibent portion). These edge voltage taps had separations of 2 to 2.5 mm (thin edge) and 2.5 to 3 mm (thick edgej. The voltage taps along the face of the

caole had separations of about 9 mm and tney did not

include any of the cable corner regions. The precision and accuracy of the critical current measurements were limited by the low voltage for these

closely spaced voltage taps and the fact that the

sample would quench before very much voltage drop

could be developed between some voltage taps. The

precision was about ±1% and the accuracy was about

± 21 .

The effective critical current was defined as the

critical current determined by adding the voltage con- tributions of each strand segment, taking into account

that the positions 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.

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^ indicated that Sample A was degraded by about 9% and had a field anisotropy of

about 111 and Sample B was degraded by 221 and had an anisotropy of -111. Anisotropy refers to 1 minus the

ratio of perpendicular I c (90°) to parellel 1 Q

(0 8

) and degradation refers to 1 minus the ratio of perpendicular I c to starting^wlre I c (see Ref. 6). These

results are consistent with those presented here.

These additional data presented here on the nature,

location, and amount of degradation are Intended to

lead to methods for reducing the degradation.

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

Contribution of NBS, not subject to copyright.
1643 another cable stranc. This allowed the current to get

slightly higher and enabled testing of the other re- gions of the strand before tne sample quenched. The

data presented for Sample B are a combination of these

two runs, not shunted and shunted.

The critical current as a function of position

along the strand at 0° and 90° field angle is given in

Figs. 1 and 2 for Sample A. The critical current was defined as the current at which the electric field

strengtn was 0.1 yV/cm. The zero angle was defined as the orientation with the magnetic field parallel to the wide face of the cable. The critical current

values varied by as much as 27? from tne maximum at 0°

and 11% at 90°. At 0° the central flat portion of the

thin edge had the lowest I c. The central flat portion

of the thick edge was less degraded. The corners of

both edges had a higher I 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

7 to the effect of the conductor aspect ratio 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 I c at 0° Dy ‘about 8% for tne

portions across the cable face. 3 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 I at 0°. The central region of the

cable face does not have very much change in I c 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. 3 and 4. The critical current values varied by as raucn as 35?

from the maximum at 0° ana 33? at 90°. At 0° the

POSITION , cm

Figure 1. Sample A, I c

V s. position at 0°.

central flat of the thin edge had the lowest l n , anc unlike Sample A, it did not increase very mucn a 90°. For both samples the difference between the thin

and thick edges were similar, as were the corner

values. The central flat region of the thin edge is

the weak link and would appear to have a degraded I rather than a large aspect ratio effect.^

A relevant question at this point is: What is

the effective critical current of these stranas as a function of orientation? One possibility is the

current at whicn the average electric field strength

is equal to some value (see experimental section).

The effective critical current and quencn current for

Sample A are plotted as a function of angle in Fig. 5. The effective critical current at various electric

field criteria had an angular dependence very similar

to the quench current. This was not expectea con- sidering that the quench current should be strongly

dominated by the degraded section (or weak link) cf

the strand and consequently lower at 0°. This

suggests that the average E does give a relevant

critical current. The end cooling around the weaK

link must have been sufficient such that the local

heating did not have an adverse effect on I The

weak link at 0° contributed about 55? of the total

voltage even thougn it only represented 5? of the

total lengtn. For the effective critical current, tne

weak link region is at a high electric field wnere tne

effect of a single bend is not as large (See Ref. 9 for effect of bending).

The effective critical currents and quench cur- rent for Sample B are plotted as a function of angle

in Fig. 6. There are two curves without symbols. The

upper was the quench current after a shunt was placec

on the thin edge. The lower was the quencn current

before the shunt was placed on the thin edge. The

lowest curve is the effective I at 0.1 yV/ cm. The

Figure 2. Sample A, I c vs. position at 90°.

142
1644

350 1—1—i—!—l—l—i—i—|—!—

r

325 h

-00 h

i' l I 275 r

|

t

|

cI5 I- thin

eage I 225 hi I

f $

t 200

-rr

face

-i—i—i—i—

|—i—i—:—

r

-Gihick J

eage I

o 2 3 4

POSITION , cm

i i i ‘ 1 L—L—1—1—1 1 I 1 1 1 I 1 ! L J

Figure 5. Sample B, I vs. position at 0°.

weak link at 0° contributed aDout 951 of the total

voltage even thougn it represented only 51 of the

total lengtn.

Table l snows the percent differences of the

I ’s from the starting wire (before cabling) values.

The starting wire I c 's are approximate because they

Figure 5. Sample A, effective I c and quench I vs. angle.

were measured as another part of this program in

another magnet. ^ The additional lowering of the weaklink I for Sample B resulted in a much lower

effective I . In fact, the difference in effective

I ’s was greater than the difference in weak link I„'s

at 0.1 uV/cm. At 0°, I of Sample B was an additional

Figure 6. Sample B, effective I c and quench I vs. angle.

143
1645

Table 1 . Percent differences in I c from starting

wire I 's at 5 T, 4.0 K, and 0.1 uV/cm

Starting Wire I

_

Sample A 320 A Sample B 347 A

Field Angle 0° 90° 0° 90°

highest segment

lowest segment

effective l c

2.8% - 4.7%

-25.3% -15.3%

- 6.9% - 7.5%

- 4.6% - 4.0%

-37.5% -34.9%

-27.7% -22.8%

12% lower than that of Sample A but the effective I c was 201 lower. The difference in I c 's at higher

electric fields, however, was more than 12%. This

increased effect may show that the local heating had

an adverse effect on I c for Sample B.

Discussion

Understanding the relationship between the

mechanical deformation in the cabling process and the

electrical performance may lead to improvements in the

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 7% improvement. On the

other hand, any compromise in end cooling could have a significant effect.

These results have two implications for shortsample cr itical-current testing of cables. First, the

data indicate that both magnetic-field 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.

Second, the difference between thick and thin

edge degradation shown in these data indicates tnat

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

Ref. 6). 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.

Conclusions

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. The widest spread

in local critical current values along the cable

strands occurs with the magnetic field perpendicular

to the cable edge. Unfortunately in the dipole magnet

application, this orientation is near the critical

orientation. The relevant critical -current criteria

for local measurements may be based on a spatial

average. This is because of the extreme localization

of damage, limited extent of the voltage, and strong

end cooling effect which all limit the local temperature rise. Any compromise in sample stability may

lower the critical current to that of the weak link.

Acknowledgements

The authors extend their thanks to S. L. Brav anc

W. E. Look for sample holder construction, data ana.- ysis, and plotting; J. M. Royet for samples and

discussions; and R. A. Shenko for typing this paper.

Research at NBS was supported by the U.S. Department

of Energy, Division of High Energy Physics under

Interagency Agreement No. DE-AI05-85ER40240 and

research at LBL was supported by the U.S. Department

of Energy, Office of Energy Sciences, Office of Energy

Research under Contract No. DE-AC03-765F00098

References

1. J. W. Ekin, L. F. Goodrich, J. Moreland, E. S. Pittman, and A. F. Clark, "Electro-Mechanical

Properties of Superconductors for High Energy

Physics Applications," NBSIR 86-3061 , National

Bureau of Standards, Boulder, Colorado.

2. J. Royet and R. M. Scanlan, "Manufacture of

Keystoned Flat Superconducting Cables for Use in

SSC Dipoles," these proceedings, paper LB-1.

3. R. Scanlan, J. Royet, and C. E. Taylor, "Superconducting Materials for the SSC," Adv. Cr yog. Eng. - Materials, Vol. 32, 697, Plenum Press, New fork

( 1986 ).

4. D. C. Larbalestier , A. W. West, W. Starch, W. Warnes, P. Lee, W. K. McDonald, P. O'Larey, K. Hemacnalam, 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.

5. W. B. Sampson, Brookhaven National Laboratory.,

Upton, New York, private communications (1986).

6. M. Garber and W. B. Sampson, "Critical Current

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

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

F. Clark, "The Effect of Aspect Ratio on Critical

Current in Multif llamentary Superconductors, " Adv.

Cryog. Eng. - Materials, Vol. 32, 833. Plenum

Press, New York (1986).

8. A. F. Clark, L. F. Goodrich, F. R. Fickett, and J. V. Minervini, "Development of Standards for Superconductors, Interim Report Oct. 1980 to Jan.

1982," NBSIR 82-1678, National Bureau of

Standards, Boulder, Colorado (July 1982), pp IT36.

9. J. W. Ekln, "Critical Current vs. Stress

Relationships in NbTi Superconductors," these

proceedings, paper MH-6.

144
IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 2, MARCH 1989 1949

CURRENT CA?AC I TV DEGRADATION IN SUPERCONDUCTING CABLE STRANDS

L. F. Goodrich and S. L. Bray

National Bureau or Standards

Bouider, Colorado 80303

Abstract

The electromagnetic properties of NbTi strands

extracted from Rutherford caoies were studied to clarifv the effect of mechanical deformation, caused by

me cabling process, on the current capacity of the

strands. Three different cables were studied, ail of

which are prototypes for the Superconducting Super

Collider's dipoie magnets. The extracted cable strands

were instrumented to allow measurement of the voitage

across several key regions of mechanical deformation as

a function of current and the orientation of the

applied magnetic field. The resulting data are presented in terms of the strand's voitage profile as veil as its critical current in order to more thoroughly characterize the conductor's

electromagnetic properties. The cable strands show verv localized reductions in current capacity that are well correlated with the regions of high mechanical

deformation. For example, at a particular field

orientation, the voitage across a portion of the strand

that is oniv 3% of the totai strana length contributes

?2% of the total strand voitage. Two applied magnetic

field orientations, parallel and perpendicular to the

caole's width, have pronounced effects on the

electrical properties of the strand. Both of these

magnetic field orientations will arise in applications.

Introduction

The measurements presented here are a continuation

of earlier studies* of NbTi cable strands extracted

from Rutherford cables (cables provided by the Lawrence

Berkeley Laboratory) . These are prototype cables for

high energy physics applications. Specifically, they

were designed for use in the outer windings of the

Superconducting Super Collider's (SSC) dipole magnets.

The main emphasis of these studies is the effect of

mechanical deformation, during manufacture, on the

cable's current capacity. In addition to the bending

deformation required to achieve the cable's twist

pitch, the cables are compacted into a keystone shape.

These mechanical deformations resuit in a very

localized degradation in the electrical properties of

the cable strands. The deformation of the cable

strands resuits in a periodic geometry with the various

segments of the strands defined by the degree and type

of deformation. The desired product of this work is

information that will lead to methods for reducing the

current-capacity degradation associated with the

manufacture of superconducting cables.

For all the measurements, the orientation of the

applied magnetic field was perpendicular to the cable's

longitudinal axis. The angle of the applied field

about this axis was set at either the 0* (parallel to

the cable width) or 90* (perpendicular) orientation.

The reasons for emphasizing these field directions are twofold. First, there is a pronounced effect on the

strand's electrical properties in these orientations

and, second, both of these field orientations arise in

a dipole magnet. The experimental details of this work

will be reported in a future publication.

The mechanical deformation of the cable strands

during the cabling process results in localized

variations in the conductor's current capacity. The

sources of this variation fall into two categories

.

cross - sectional area differences and angular effects

The cross - sectional variation of the superconducting

filaments is due to necking and serrated yielding of

the NbTi filaments. Under the second category, angular

effects, the orientation of the applied magnetic field

with respect to the conductor's longitudinal axis

(pitch angie effect) and its mechanically deformed

cross section (aspect ratio effect -5 ) vary along the

strand.

Three different cables, manufactured with different

virgin wire (source wire for the cable strands) and

cabling techniques, were studied. These data allow

comparisons among the cables. The strand critical

current (strand I c ) is defined as the current at which

the sum of the voltages from each of the strand

segments divided by the sum of the segment lengths is

equal to a selected electric field criterion. The

strand I c is presented for both magnetic field

orientations. 0° and 90°. This information is useful

in examining the cumulative effect of variations in

spatiai orientation anc aspect ratio of different

strana segments at different field angles. The concept

of a strana I c is also useful for comparing the

strand's I c with the I c of the virgin wire from which

the cable was manufactured. These comparisons indicate

the amount of I c degradation introduced by the cabling

process. In order to assess the relative contribution

to the total voitage of different strand segments, the

voltage distribution along the conductor's length, at both magnetic field angles, is also presented.

Three different cables, denoted as W, X, and Y, were studied. Cable W was re -rolled through a turksheac

(for resizing) after it had been cabled. The re-rolled

cable is a worst-case situation that was studied to more clearly identify the degradation features. Cabie

X is similar to W; however, it was not subjected to as much mechanical deformation and is a more tvpicai

example of late 1985 to early 1986 prototype outer

cables. Cable Y (1987) is an evolution of cable X

where the cabling techniques were refined in an attempt

to reduce current-capacity degradation.

Results

The conductors measured in this study represent an evolution in the cable fabrication process aimed at

reducing its detrimental effect on the critical

currant density (J c ) of the finished cable in

comparison to that of the virgin wire. 4 An evolution

has also occurred in the design and processing of the

virgin wire. 5 This evolution can be characterized by a decrease in filament size aimed at reducing ac losses

while attempting to maintain the J c

. Initially, the

increased wire drawing that was used to reduce the

filament size of wire X in comparison with wire V

resulted in a lower J c

. Later, improved manufacturing

processes that were used for wire Y resulted in a substantial restoration of the J c of the virgin wire.

Strand Critical Current

I c measurements were made on the virgin wires from

which each respective cable was manufactured. Three

pairs of voltage taps, with each pair spanning about

10 mm, were connected to the specimens. This allowed

Manuscript received August 22, 1988. Contribution of NBS, not subject to copyright.

U.S. Government work not protected by U.S. copyright.
1950

for a check of I c homogeneity with position. I c measurements were aiso maae as a function of the

applied magnetic field angle. This allowed for a test

of I c homogeneity with respect to field angle. The

average I c of the virgin wire was used to determine the

relative degradation of each cable. The typical I c variation among voltage tats was ±0.3% at 5 T and ±0.5%

at 8 T for ail of the virgin wire samples. For samples

X anc Y, the I- variation of a given tap with respect

to field angie (0° to 360°) was measured to be about

±0.6% at 5 I.

Figures 1 and 2 show comparisons between the J c of

the extracted cable strands (strand J c ) and the virgin

wires from which they were constructed. Figure 1 shows

this comparison for each of the three conductors for

applied magnetic field angles of 0” and 90° at 5 T. There is a consistent increase in the J c at 0° from

cable W to cable Y' although the virgin wire's J c decreased between wires W and X. Since cable U is a worst-case situation with respect to the cabling

operation, tne higher J c for strand X is due to the

reduced mechanical deformation of the strand. Cable Y

shows a greater mechanical degradation of its J c than

cable X. but. again, it shows an increased strand J c due to its high virgin wire J c

. For cable strand X,

the mecnanicai degradation of the J c was somewhat

hiener at 90° than at 0° ano. conseauentiv , there was a decrease in the J c of stranc X as compared with the J c of strand w. This increased degradation at 90° was

not present in strand Y and strand Y showed an increased J c in comparison with its two predecessors.

The 6 T plot. Fig. 2, shows an increase in the

strand's J c between V and Y but there is a decrease

between W and X. Comparison between the 5 and 8 T J c degradation for ail of the strands shows a reduced

sensitivity to mechanical deformation at the higher

magnetic field for ail the strands. The virgin wire W,

which has larger diameter filaments, had a higher

relative J c at 8 T than do wires X and Y. This lower

sensitivity of wire V to magnetic field results in a higher strand J c for W than for X.

Differential Voltage Profile

Figures 3 and U show the electrical properties of

the caoie strand as a function of the position along

its length. At the too of each plot, located outsioe

the axes, is an outline of the cade strand's geometry

positioned so as to correlate with the horizontal or position axis. The cable outline is labeled to indicate the position of the thin edge, the face, and

the thick edge relative to the horizontal axis. The

differential voltage across the various strand segments

is plotted as a function of position. The plateaus,

or peak levels, in the curves represent the voltage

across the indicated strand segments at the indicated

strand I c. The lines connecting the data are only a visual aid and they are not Intended as an interpolation between the data points. The current

levels for each of the different samples is not the same in these plots (current levels for each of the

strands is given in the figure captions); however, the

overall voltage level is the same for each of the

samples. In other words, the data in these plots

represents the same strand I c criterion (0.1 jiV/cm) for

each of the samples.

The plots show that there is a considerable

variation in the lengths of the various strand

segments. Herein lies the significance of these plots,

which show the relative contribution of the separate

strand segments to the overall voltage.

For the samples and conditions existing in these

measurements , the total voltage seems to determine the

CN

o CABLE

Figure 1. Critical current density of cable strancs

and their respective virgin wires for 0 and 90 c at 5 T The virgin wire J c is represented by the continuous

line segment spanning 0 to 90° for each cade. The

strand J c values at 0 and 90° are represented by the

shaded bars for each cable.

o CABLE

Figure 2. Critical current density of cable strands

and their respective virgin wires for 0 and 90* at 8 T The virgin wire J c is represented by the continuous

line segment spanning 0 to 90* for each cable. The

strand J c values at 0 and 90* are represented by the

shaded bars for each cable.

146
1951

>

Ld

o

<

GBBBe Coble X OS &***£ Coble Y

0.5 44

current capacity cf the conauctor. This is evidenced

by a ciose agreement between tne measured strand

critical currents anc the maximum acnieved currents

(quench currents;. For a sampie that has poor

longitudinal thermal conductivity, the short edge

segments might act as weak links, limiting tne current

capacity to a value below that required to reach the

strand I c criterion. For all caDies and the 0°

orientation, the majority of the voltage is

concentrated at the edges of the caole with the central

portions being dominant. At 90°. the voltage

concentration increases at the comers of the edges it. comparison with the centers.

o

—05 1 : : - J— j 1 1 - - - : 1 1 ;

1 '012 3 4 5 6 7

POSITION, cm

Figure 3. Differential voltage profiles at 5 T and an applied magnetic field angie of 0° for cables W, X, and

Y at currents of 285 A, 307 A, and 339 A respectively.

POSITION, cm

Figure 4. Differential voltage profiles at 5 T and an applied magnetic field angie of 90* for cables W, X,

and Y at currents of 296 A, 300 A, and 345 A

respectively.

The negative voltages shown in the plots are projection voltages^ associated with current

redistribution among the filaments. The strongest

evidence of intrinsic (not associated with tne location

-of the current contact) transverse currents within tne

strand is a region of negative voltage bounded by

regions of large positive voltages. The curve for

cable Y is the best example of this structure. This

type of structure is present in the center of these

plots. This indicates that the current redistribution

is caused bv the longitudinal variation in the relative

current carrying capacity of the filaments rather than

being caused by current injection at the current

contact. Thus, voltages associated with this tvpe of

redistribution will occur periodically aiong the entire

length of the cable strand.

Although the negative voltages are the most obvious

indication of current redistribution, there are other

indicators. These indicators have the same general

character as the negative voltages with the exception

of polarity. The positive current transfer voltages

are evident when observing a series of differential

voltage plots in which the sample current is

increasing. There are portions of the differential

voltage profile that rise abruptly, like a flux flow

voltage, and then level off with increasing current

until the actual flux flow region is reached. These

transitory positive voltages, like the negative

voltages, are indicative of intrinsic current

redistribution within the cable strand.

Discussion

The main objective of this work is to determine the

current capacity of several different cabie stranas

with the ultimate goal being an increased understand."

of the sources and degree of current capacity

degradation in these conductors. Typically, the I c of

a conductor is an accepted practical measure of its

current capacity. However, for these particular

conductors, there are some difficulties in relating I c to the actual current capacity of the cable strands.

Owing to the electrical inhomogeneity of the strands

caused by the cabling operation, there is a large

variation in the I c along the strand's length. The

problem is determining what current level in this

fairly broad range of critical currents best reflects

the strand's practical current capacity.

One approach is to assume a weak link situation

where the strand segment that has the lowest I c determines the overall current capacity of the strand.

A comparison between the actual quench currents (the

maximum observed current prior to thermal runaway) and

the weak link I c shows that the current capacity of the

strand is significantly higher than the weak link I c

. This enhanced current capacity, above the weak link

l c , is due to the strand's relatively good stability.

At the other extreme, the current capacity of the

strands could be assumed to be equal to the strand I c

. For the conductors tested here, this would not be a bad

14 7
1952

assume :ion. as eviaenced by the close agreement

between the strana I c and the ouench currents.

However, there are two proolems associated with simply

measuring tr.e strand I c and assuming this to be the

strand's practical limit. First, the strand I c gives

no information aDout the voltage distribution aiong the

strand's lengrn and. conseauentiy , it is not useful in

understanding the sources of current degradation in the

caoie. Also, for conductors that are less stable than

those tested here, the actual current limit might be

ciose to the weatc link I c and. in this case, the

strand I c level could not be achieved.

Another factor associated with the I c measurements

for these caoie strands is the presence of intrinsic

current- transfer voltages that are caused by periodic

current redistribution aiong the strand's length. This

current transfer is apparently caused by the periodic

mecnanical deformation of the strand and, thus, it is

not limited to the current contact area. Due to its

intrinsic nature, these current transfer voltages play

a role in determining the current capacity of the

cable, not only in testing but in application as well.

It is clear that I c data alone are insufficient for

characterizing these conductors For this reason the

differential vo;ltage data were *al‘so presented.

Cone ius ior.s

3ased on these measurements, some general

conclusions can be stated regarding the electromagnetic

character of these cables and its effect on their

current capacities. Also, the measurements have

implications regarding short sample testing of these

caoles

.

The mechanical deformation of the cable strands

associated with the cabling process can lead to very

localized reductions in their current capacity. The

widest spread in local current capacity occurs with the

applied magnetic field parallel to the cable's width.

Despite large local variations in the cable strand's

current capacity, the quench current and the strand I c do not vary much with the angle of the applied magnetic

field. The deformation geometry of the cable is such

that the interaction between the applied magnetic field

and a strand segment's aspect ratio or its pitch angle

have the same tendency. In other words, a field angle

that is detrimental (beneficial) to a strand segment's

current capacity due to its aspect ratio is also

detrimental (beneficial) due to its pitch angle.

However, the various strand segments have different

critical orientations such that the detrimental and

beneficial effects of the field angle on the various

segments tend to cancel one another over the total

length of the strand. For these particular conductors,

the relevant I c criteria may be a spatial average (the

strand I c ) because the degradation in current capacity

is concentrated over short regions of the cable strand

and local temperature rises are limited by strong end

cooling. Any compromise in sample stability may lower

the I c to that of the weak link segment of the cable

strand

.

These results have two implications for short-sample

I c testing of cables. First, the data indicate that

both magnetic field orientations, parallel to the

cable's width and perpendicular to its width, need to

be tested to determine the limiting orientation for

the l c

. The limiting magnetic -field orientation can be

parallel or perpendicular to the cable's width and the

limiting orientation is not known beforehand. Second,

a large difference in current capacity degradation can

exist between the thick and thin cable edges; thus,

changing the direction of the test current (for each

field orientation) can affect the measured I c

. This is

due to a self field effect where the location of the

peak magnetic field depends on the current direction.

Thus, the peak field location snouic be part of the

data recoraed for snort sample measurements of these

conductors. Consequently, these more detailed cable

measurements should be made periodically as part of the

cable development process.

The wire from which these cables are made has

evolved to the point where thev have a hi eh virgin

current capacity and a relatively high tolerance to mechanical deformation. Considering the amount of

mechanical deformation of the strands caused bv the

cabling process, the resulting degradation in current

capacity is surprisingly low. In the case of the

latest outer winding cable, sample Y, the maximum

observed degradation in its strand I c was onlv 6.8%.

Acknowledgements

The authors extend their thanks to T. C. Stauffer

for sample preparation, to A. N. Srivastava for data

plotting; and to R. M. Scanlan and J. M. Rovet

(Lawrence Berkeley Laboratory) for samples and

discussions

.

This work was supported bv the Department of Energy,

Division of High Energy Physics.

An effort was made to avoid the identification of

commercial products by the manufacturer's name or label, but in some cases these products might be

indirectly identified by their particular properties.

In no instance does this identification imply

endorsement by the National Bureau of Standards, nor

does it imply that the particular products are necessarily the best available for that purpose.

Rgf?penpes

[1] L. F. Goodrich, E. S. Pittman, J. U. Ekin, and

R. M. Scanlan, "Studies of NbTi Strands

Extracted from Coreless Rutherford Cables," IEFF

Trans, on Magnetics . MAG 23. 1642, 1987.

[2] J. W. Ekin, L. F. Goodrich, and S. L. Bray,

NBSIR, National Bureau of Standards, Boulder,

Colorado (to be published).

[3] 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. Crvoe. Eng. -Materials . Vol. 32, 833, Plenum Press, New Yorx (1986).

[4] J. Royet and R. M. Scanlan, "Manufacture of

Keystoned Flat Superconducting Cables for Use ir. SSC dipoles," IEEE.J.ggTIS , 9n Mggn? tics , MAG 2 3

,

480. 1987.

[5] D. C. Larbalestier ec ml., "High Critical

Current Densities in Industrial Scale Composites

Made From High Homogeneity Nb 46.5 Ti", IEEE

Trans, on Magnetics. MAG 21. 269, 1985.

[6] L. F. Goodrich. J. W. Ekin. F. R. Fickett,

"Effect of Twist Pitch on Short-Sample V-I

Characteristics of Multifilamentary

Superconductors , " Adv. Crvog. Eng. - Materials , Vol. 28, Plenum Press, New York (1981), p. 571-580.

[7] L. F. Goodrich, "The Effect of Field Orientation on Current Transfer in Multifilamentary

Superconductors," IEEE Trans. Mag. MAG - 1

9

. 244-

247 (1983).

1 /. Q
NIST-1 14A

(REV. 3-89)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

1. PUBLICATION OR REPORT NUMBER

NISTIR 89-3912

4. TITLE AND SUBTITLE

ELECTROMECHANICAL PROPERTIES OF SUPERCONDUCTORS FOR HIGH EF

PART II

fERGY PHYSICS APPLICATIONS

5. AUTHOR(SJ

J. W. Ekin , L. F. Goodrich, S. L. Bray, N. F. Bergren, and R. B. Goldfarb

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

GAITHERSBURG, MD 20899

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

2. PERFORMING ORGANIZATION REPORT NUMBER

BIBLIOGRAPHIC DATA SHEET 3. PUBLICATION DATE

November 1989

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP)

Division of High-Energy Physics

U.S. Department of Energy

Washington D.C. 20545

(DOE Interagency Agreement //DE-A105-85ER40240)

10. SUPPLEMENTARY NOTES

See NBSIR 86-3061 dated December 1986, title as above.

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 OF

LITERATURE SURVEY, MENTION IT HERE.)

This report presents data on superconductor performance under mechanical load. The data are

needed for setting mechanical design constraints and measuring the electromechanical performance of NbTi superconductors for DOE high-energy physics magnet applications. The effect of

axial tensile stress, applied at room temperature, on the critical current of NbTi supercondu.

tor strands has been measured. . The data show a simple result that the effect on the critical

current is independent of the temperature at which the stress is applied; this allows the exi;

ting 4 K data base to be used to determine critical current degradation from room temperature

fabrication stress, from cool-down stress introduced by differential contraction, and from 4 F

stress generated by the Lorentz force when the magnet is energized. A study of the critical

current variations along NbTi strands extracted from a Rutherford cable has been made also.

The results show that the principal mechanical degradation is extremely localized at the regions where the NbTi strand is_bent around the edge of the cable. For example, only 3% of tee- total strand length can contribute 92% of the total strand voltage. A further study has been

made of the effects of bending strain on the critical current of NbTi conductors. The degradation of the critical current from bending strain is much greater at low values of electrical

field than at high, suggesting that irregularity of the filament cross-sectional area introduced by bending may be the source of the I c degradation. The bend tolerance of a NbTi conduc

tor can be enhanced by increasing the local copper-to-superconductor area ratio. Measurements

of the permeability, saturation magnetization, and intrinsic coercivity of several high-permeability steel alloys were made. The overall differences of saturation magnetization and intrinsic coercivitv between cold-rolled steel samples were not significant

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITAUZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

bending strain; cable; coercivity; critical current; electromechanical; magnetic properties;

mechanical properties; NbTi; permeability; Rutherford cable; saturation magnetization; SSC;

stress effect; Superconducting Super Collider

13. AVAILABILITY 14. NUMBER OF PRINTED PAGES

X UNUMITED

154

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS).

ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE,

WASHINGTON, DC 20402.

15. PRICE

X ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161. ELECTRONIC FORM

U.S GOVERNMENT PRINTING OFFICE: 1989-773-753