An attempt to reduce training

Training, more or less tolerated in small superconducting magnets, is unacceptable for large coils. A simulation experiment was performed in which the influence of large conductor volume on training was substituted by high strain. Following this principle, the influence of warm cyclic prestrain on training was investigated. Optima/ pretreatment reduces training by a factor of two according to the defined weighting and training numbers.

An attempt to reduce training. C.H. Dustmann, and H. K6fler

Training, the effect that a superconducting magnet reaches its short sample current only after numerous quenches, has been more or less accepted for small magnets made of transient stabilized conductors. As the number of necessary training steps seems to grow with the conductor volume and the time needed for each training step increases with the energy stored, training is no longer tolerable for large coils. A possible cure is the stabilizing effect of copper. The amount of copper needed contradicts the requirement of low eddy current losses in magnets with pulsed field loads. This was the motivation for investigating another method to cure or at least reduce training. Edwards et all investigated the influence exerted by the coil stiffness and the resin on the training behaviour and found that certain resin types are better than others and that high stiffness of the coil is important. Helm ~ proposed adapting larger strain than the electromagnetic strain to the prefabricated coil by fitting into the magnetic bore a core consisting of low temperature coefficient material and cooling both the core and the magnet to a low temperature. The disadvantage of this method is obvious, especially for D-shaped torus coils. Furthermore, Schmidt 3 indicated that there is short sample training in bare Nb Ti-wires. Therefore, the origin of training seems to be in the superconductor itself and external influences like strain and structural materials affect this behaviour to a varying degree. As stress effects in metals are described by dislocations and their dynamics, it is conceivable that this also holds for superconductors and their stress induced training. If this is true, it should be possible to influence these dislocations and their dynamics simply by applying a cyclic prestrain on the finished conductor. In a first check a positive effect has been found in short sample training experiments by Pasztor. 4 The experiment described examines the cyclic prestrain influence on inserted coils to confirm the reduction of training for coils. The authors are with the Kernforschungszentrum und Universitat Karlsruhe Institut fur Experimentelle Kernphysik, 7500 Karlsruhe, Postfach 3640, Federal Republic of Germany. Received 8 July 1977.

CRYOGENICS. DECEMBER 1977

The experimental setup The leading thought in the experiment performed is the statistical background of training. The probability of quenching depends on 1 2 3 4

the applied strain e caused by conductor elongation or bending, the conductor volume V, the cooing conditions Q, the distance from the critical values AJ, AB, A T.

In a magnet the conductor volume is large, whereas the applied strain ranges from 0.05 - 0.2%. In order to get training conditions comparable with those in a magnet, the reduced volume in the test is compensated by higher strain. A conductor volume suitable for a great number of tests was chosen. The sample length was 3.4 m, which is comparable with a conventional short sample length. The strain of _~ 0.8% was applied in the form of Lorentz forces. The test insert was a single-layer race-track coil, energized in an external magnetic field (Fig. 1). The self field and the external field acted in the same direction so that the Lorentz force strained the sample. In addition, the test coil could be confined in a vacuumtight housing. So the influence of cooling conditions on training could be checked by changing the internal pressure. The strain was applied on the conductor sample by raising the current in a constant external field of 5 T (Fig. 2). The stress-strain behaviour of the conductor is given in Fig. 3.

Experimental results To characterize the results for a sample we define a weighting number Q. Q is the sum of the differences between the quench current and the short sample critical current for 100 quenches (shaded area in Fig. 4). Obviously, a good conductor has a low weighting number and a bad conductor a high weighting number. The reasons for restricting the evaluation of the weighting number to 100 quenches were that this number of quenches was thought to be the upper limit tolerable for large superconducting components, and it is also a compromise between the high

667

0.7 rrent ld

Potenti lead

0.6

ssel icuumtight)

Carbon resistor

0.5

Disploce~ recorde~

0.4

0.3 6T solenoid 0.2

Calculated Measured

0.1

I lO0

0

Fig. 2

Fig. 1

Experimental

I

I

I

I

I

I 1(300

Strain depending on current

set-up

number of samples tested and the experimental time necessary for one sample. Training and degradation are included in Q. In order to get these different types of behaviour separated, we had to choose a criterion after training had been finished. It was considered to be highly uncertain to refer to a peak performance current occurring only once in a number of 20 or even more quenches. The way we decided to go was the combination of currents of a large number of quenches into groups of ten quenches. This seems to be reasonable because the quench currents of the training steps are scattered round a trend function. Each group has a mean value and a standard deviation. We defined training to be finished when the mean value of a group is within the half standard deviation of the following group. Degradation was defined as the relative deviation of this mean value plus its standard deviation from the short sample critical current (Fig. 5). During the measuring programme no parameters were varied except for the strain applied and the number of strain cycles of warm pretreatment. The results are summarized in Table 2. Fig. 6 shows the dependence of the weighting number on pretreatment. The function Q = f(e, N) at low strain gives a broad curve with a fiat minimum at a rather high number of cycles. Increase of the strain shifts the minimum to lower cycle numbers. For the conductor

668

I

I I 500 /, A

T:4.2 K

5

4

A

,/

/i

E z

,¢/

_/./

,/

,,'./

.2- /

/

T = 293 K

,"

2

I

e" / ," , / /L'" / / r*##*

,

/ /

0

Fig. 3

I Q2

t 04

I 06

I 0.8

I Io

Stress-strain curve f o r Nb-Ti

CRYOGENICS.

DECEMBER

1977

-

Critical current

Critical current

IO00

~

Oe0 o,,oo o lit

I100

x

ref currenl ~

-

-

~_

xXX

2

/X X

t~r

T

IOO0

500

9(30

__1

I

I

I

0

l 50

I

I

l

I

I I O0

Number of quenches Fig. 4

800

T r a i n i n g curve (weighting n u m b e r illustration)

I

~ _.1__ JJ 50

investigated (Table 1) an optimal treatment was found with 0.5% strain cycled 50 times. Fig. 5

For control short sample measurements of the pretreated samples were performed before and after the investigation of training. The measured values were in all cases higher than or equal to the critical current of the untreated wire. These measurements will be reported elsewhere. The strain and degradation values of the untreated wires corresponding to one another (Table 2) indicate that strains of the level applied already cause remarkable degradation, which is more than the strain dependent degradation of short samples, s Before we decided to do all measurements in liquid helium, tests had been performed which proved the influence of cooling conditions on training and degradation. The temperature of the sample was recorded during current rise for various cooling conditions. In vacuum and He gas a temperature rise was found due to losses. Calculations and tests with various current rise times indicated that beside losses due to hysteresis and eddy currents the mechanical energy deposited due to material damping was remarkable. 6 Under these poor cooling conditions the measured degradation was 37%, whereas the same sample degraded afterwards by only 9% in liquid helium.

IOO Number of quenches

T r a i n i n g curve

3xiO4

d I

I I I ! ! I I

e

/ '

ZxtO"

!

, \

E ==

/ \

,,

%..

.~ I xlO 4

/

I !

•\

I

!

I I I

/

I

I I

%%

'\

~=

/,

/

/

/

iI

/" Strain , %

Table 1. Parameters of the wire investigated diameter f i l a m e n t number f i l a m e n t diameter Cu: NbTi, ratio I at 5T, 4 . 2 K C • insulation

• o

x []

1.45 m m 1700 20/am 2:1 1280A

O

I tOO

I 150

I 200

Cycles Fig. 6

C R Y O G E N I C S . DECEMBER 1977

I 50

0 0.25 0.5 0.75 1.0

Weighting n u m b e r -- dependence on strain and cycles

669

Table 2. Experimental results Pretreatment (warm)

Sample

No.

emax, %

10 20 27 14 12 19 15 16 17 23 24 21 18 25 26

. . . . 0,25 0.25 0.25 0.25 0.25 0.5 0.5 0,5 0.5 0,75 0.75 0.75 1

eres, %

. . 0.084 0.13 0.098 0.11 0.096 0.2 0.193 0.178 0.185 0.206 0.218 0.214 0.26

Ornax , MN

cycles

%

m -=

. . 186.0 175.8 178.5 184.2 175.8 180.0 274.0 275.0 275.0 362 359 368 457

ema x, calc,

10 55 100 150 200 10 55 100 150 5 10 30 5

Experiment (cold) ernax, meas, eros, % %

0.82 0,83 0.77 0,84 0,87 0.88 0.66 0,85 0,87 0.79 0.68 0,87 0.79 0.67 0.85

Summary The training behaviour of superconducting coils was simulated with a single-layer race-track-coil operated in an external field. The results show a significant reduction of training if the conductor investigated was pretreated with a cyclic prestrain at room temperature. For this conductor an optimal prestrain and cycle number were found. This effect has been confirmed by Schmidt for small coils in the self field] An explanation by metallurgists of this behaviour of NbTi-superconductors is still to be found. The authors would like to thank Mr E. S~J/3and Mr G. Hedan for their aid in preparing and performing the experiment. Thanks are also due to Dr E. Seibt and Mr J. Pytlik for having performed the short sample measurements. Last but not least we appreciate many fruitful discussions with colleagues. This work was based on a contract between the GfK Karlsruhe, and IPP Garching concerning the cooperation

670

0.40 0.33 0.32 0.37 0.27 0.21 0.26 0.29 0.34 0.28 0.19 0.13 0.21 0.27 0.27

-

0.75 --

--

0.83 0.7 0.79 -

0.7 0.79

Omax, calc, MN m -2 626.0 630.0 600.0 638.0 650.0 654.0 534.0 641.0 650.0 612.0 558.0 650.9 612.0 554.0 641.0

Weighting number

2.15 X 1.55 X 2.01 × 1.43 × 1.17 X 1.1 × 2.97 X 1.3 × 0.83 × 1.41 × 2.56 × 1.24 × 2.19 × 3.07 × 2.01 ×

104 10 4

104 10 4

104 104 104 104 104 104 104 104 104 104 104

Training number

Degradation

90 50 40 40 60 30 80 50 40 30 40 80 40 30 30

%

9 10 16 11 6 3 16 7.8 6.1 8.1 18.7 1 18.9 27 18.3

in the field of superconducting technology and was supported by Euratom.

References

1 2 3 4 5 6 7

Edwards,V.W., Scott, C.A., Wilson, M.N. IEEE Trans Vol MAG-I 1 (1975) No 2 Heim, J.1L FRG Pat. DT 2607329 A1 Schmidt, C., Pasztor, G. IEEE Trans Vol MAG--I3 (1977) No 1 Pasztor, G., Schmidt, C. private communication GESSS-Meeting Karlsruhe May 1977 Ekin, J.W., Fiekett, F.IL, Clark, A.F. Advances in Cryogenic •ngineering Vol 22 p 449 Kr/Jger,D.M., Easton, D.S., Moazed, A. IEEE Trans Vol MAG-13 (1977) No 1 Schmidt, C. private communication, GESSS-Meeting Karlsruhc. May 1977

CRYOGENICS.

D E C E M B E R 1977