Acoustic emission from superconducting magnets: the difference between layer wound coils and pancake wound coils

Acoustic emission from superconducting magnets: the difference between layer wound coils and pancake wound coils T. Ishikawa, S. Miura and Y. Nakagawa Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendal 980, Japan

Received 11 October 1985 The energy counting rate of acoustic emission (AE) is measured as a function of energizing current for several simple-shaped unimpregnated superconducting magnets. A simple theory based on the frictional motion of the conductor predicts that the AE rate increases monotonically with increasing current. This is confirmed by experimentation using test coils. The practical magnets, however, exhibit a different behaviour. For layer wound coils, the AE rate has a peak in the high current region when the current increases. For pancake wound coils, the AE rate has a peak in the low current region. On the basis of these results, the mechanism of the characteristic AE behaviour of each type of winding is discussed.

Keywords: superconducting magnets; windings; acoustic emission

Many studies have been carried out on acoustic emission (AE) in superconducting magnets. It has been found that major sources of AE are the frictional motion of the conductor ~, stress in the conductor 2 and crack or bond failure of impregnants in impregnated coils ~. The second and third sources indicate the Kaiser effect~,3 i.e. the magnet scarcely generates AE below the m a x i m u m current level it has experienced before. However, there are still many problems in the technique, such as propagation characteristics of acoustic waves in the magnets, the source identification of AE signals, etc. These problems arise as the superconducting magnets are composites of conductors, insulators and a coil bobbin. Several applications of the AE technique to practical magnets were proposed4-L For those applicatons, the technique has been most successfully applied to quench experiments ~. A cause and location of a coil quench are inferred from AE and coil voltage signals observed simultaneously just prior to the quench. The purpose of the present study is to investigate the possibility of monitoring hybrid magnets, which consist of an inner high-power water-cooled magnet and an outer superconducting magnet. Three hybrid magnets (HMI*, HM2 and HM3) have been installed in the High Field Laboratory for Superconducting Materials (HFLSM), The Research Institute for Iron, Steel and Other Metals, Tohoku University s. The structural variation of the outer superconducting magnets over a long term may be diagnosed using an AE technique. In this case, it is more convenient to deal with a number of acoustic signals statistically rather than to note each of them, as in the quench experiment mentioned above. Therefore, the AE rate during the current sweep is measured statistically. *The largest magnet, HM1, produced 30.7 T on 29 May, 1985; then a world record for a steady magnetic field s 0011-2275/86/050267-06 $03.00 © 1986 Butterworth Et Co (Publishers) Ltd

In the present experiment, all the superconducting magnets are unimpregnated and non-virgin magnet operation runs are dealt with. Thus the AE sources having the Kaiser effect are ignored, and only the frictional motion of the conductor is taken as the major source of acoustic signals in the magnets. The superconducting part of the hybrid magnets is named SMI, SM2 or SM3, corresponding to HM1, HM2 or HM3. They are further classified by the winding type, as a pancake wound coil (SM1 and SM2) and a layer wound coil (SM3). In addition to SM2 and SM3 other superconducting magnets of these winding types were examined in order to classify the relation between the AE rate and the winding types. The AE rate was also measured for some test coils, e.g. one double pancake and a stack of several double pancakes.

Experiments Superconducting magnets Table 1 lists various parameters of the superconducting magnets monitored with AE. There are three layer wound coils and three pancake wound coils. These are unimpregnated coils having a simple shape. Every magnet was recently installed in H F L S M except the 4 T superconducting magnet. Since power supplies of the outer coil and the inner coil of the 13 T superconducting magnet are separate, these coils were energized independently in the present experiment. In addition, the following two test coils were prepared in order to find the AE characteristics of the pancake wound coil: I, one double pancake and, II, a stack of eight double pancakes. Each double pancake (29 mm inner diameter, 103 m m outer diameter and 7 mm in

Cryogenics 1986 Vol 26 May

267

Acoustic emission from superconducting magnets: T. Ishikawa et al. Table 1

Specifications of superconducting magnets monitored with AE

Superconducting magnets (SM)

Type of winding

Inner diameter (mm)

Outer diameter (mm)

Height (mm)

Inner

Pancake

Outer

Layer

Magnetic field (T)

Current (A)

58.6

157.0

173.7

4.5

142

202.1a 249.4

249.3 a 334.0

299.3

8.5

458

306

330

16.5

135.6

Conductor

Conductor cross-section (mm)

mf-Nb3Sn

0.25 x 5

560

mf-NbTi

1.6 x 3.2 a 1.1 x2.2

660

Nb3Sn tape 0.11 x 5

Stored energy (kJ)

13TSM 9.2

16.5 T SM

Pancake

58

SM2(HM2)

Pancake

420

931.2

600

8.0

1470

61 O0

mf-NbTi

2.6 x 11.4

SM3(HM3)

Layer

290 a 338

338 a 426

444

8.0

780

1200

mf-NbTi

2 x4 a 1.6 x 3.5

4TSM

Layer

80

166

115

4.4

87

12.3

Cored NbTi 0.5~)

~lhe conductors are graded in two stages

Top flange

Centre pipe

P spacer

l.'

81

65

Venv(t) Poncoke

I

I

fl-

\

I I I I

I I 1 I

I

I

i i

I

/; fl

t

\VVV

Time

\t z

I

I I I I

I I I I

Bottom flong(

4

[, Figure 1

29~ -~

IO3+ 114~

Nut

Structure of the test coil (eight double pancakes)

Preamplifier Highposs filter

~

Bondpossfilter

Figure 3 AE signal voltage, V, as a function of time, t. Vth, threshold voltage of the discriminator; Ve,v, envelope voltage of the signal; t 1, t2, time when the envelope voltage crosses the threshold voltage

width) was a constituent of the 10 T superconducting magnet made by Intermagnetics General Corporation. The Nb3Sn tape (3 m m wide and 0.15 m m thick) was wound around a brass ring. An FRP (fibre reinforced plastic) plate, I m m thick, was inserted between two parts of each double pancake as a spacer. A construction of the test coil II is shown in Figure 1. The upper and lower flanges, the centre pipe and the nut were made of brass. For insulation between pancakes and the centre pipe, eight FRP strips (5 m m x 70 mm) were attached along the pipe. The eight double pancakes were clamped either tightly or loosely by adjusting the nut. The central field (flux density) of this coil is 5.4 T at 100 A, with stored energy of 3 kJ. The test coil I of one double pancake was supported by a similar structure with a shorter centre pipe.

AE signal processing Lt

Main amplifier ~

Discriminator ~ - ~

I SM current I

AE energy t processor

~"ltComputer ~

it I Memor I Figure 2

268

Block diagram for AE processing

Cryogenics 19 8 6 Vol 26 May

The block diagram forAE processing is shown inFigure2. Acoustic waves were transformed into voltage signals by a PZT type sensor attached to the external surface of the coil case (SM2 and SM3) or the top flange of the coil (for the other magnets). The cut-off frequency of the high-pass filter was 200 kHz and the band width of the band-pass filter was 200-400 kHz. Gains of the pre-amplifier and the main amplifier were chosen on each magnet. Figure 3 shows an output AE signal of the main amplifier schematically. In order to separate the AE signal from the

Acoustic emission from superconducting magnets: T. Ishikawa et al. 10

noise, the threshold voltage was set at 1 V by the discriminator. On each signal, the energy processor calculates the following E = ~

Venv(t) 2 dt

oL.

tl

whereE is the "energy' of the AE signal, and ~ is a constant, Venv is the envelope voltage of the signal, and t~ and t~ are the time when the envelope voltage crosses the threshold voltage. Since the values of h, t2 and Venv depend on the threshold voltage and the gains of the amplifiers, the absolute value of energy, E, is meaningless. However, the values are relatively comparable between a current increasing (sweep-up) mode and a current decreasing (sweep-down) mode on each magnet. In the present Paper, accumulated values of energy, E, on all AE signals detected during a unit interval of the current sweep are called the AE energy counting rate (or the AE rate). AE signals rarely overlap under usual experimental conditions. In general, the AE rate is independent of the sweep rate of the current. No AE is observed when the current sweep stops completely. Of the AE techniques the most popular is the 'ringdown" counting method in which the number of oscillations above a given threshold in an acoustic signal is counted. However, the energy count measurement adopted in the present experiment is more accurate than the ring-down count measurement to evaluate the AE intensity.

c

g a I 0 0 00 w

g t~

0

500 Current {A)

F i g u r e 5 AE energy counting rate (arbitrary units) v e r s u s current traces of SM3 (layer wound) for (a) 0 ~ 6 8 0 A and (b) 6 8 0 -- 0 A. Total gain of the amplifier is 98 dB

10

a

I

I

I

current for the 4 T magnet, SM3 and the outer coil of the 13 T magnet, respectively. In each Figure, the top trace is for a sweep-up mode, and the bottom trace is for a sweep-down mode. The characteristic AE pattern observed in these three coils is as follows. 1 In the sweep-up mode, the AE rate increases monotonically (Figure 4), or has a sharp peak in the high current region (Figures 5 and 6).

o ¢=

== o 0 I0 450-"0A ¢b laJ

5

0 I0

I0

b

l

I

I

A

(g

._=

o

g

-----F--

b

0 I0

I 0"*"350

w

~J

A

5

Layer wound coils Figures 4, 5 and 6 show the AE rate as a function of coil

c

I 0 -b'-450

Results

.c

I O00

I

-

T

A

I 350 "~'0 A

LU <[

14J

o

0

50

IOO

Current (A) F i g u r e 4 AE energy counting rate (arbitrary units) v e r s u s current traces of 4 T SM (layer wound) for {a) 0 ~ 8 0 A and, (b) 8 0 ~ 0 A. Total gain of the amplifier is 89 dB. - - - Calculated value using Equations (1) and (2), where/3 = 2.0 x 10 -5', /~=25Aandl m=80A

0

I

0

I00

200

300

400

500

Current (A) Figure 6 AE energy counting rate (arbitrary units) v e r s u s current traces of 13 T SM outer coil (layer wound). Total gain of the amplifier is 6 0 dB. (a) Maximum current is 4 5 0 A, (b) maximum current is 3 5 0 A

Cryogenics 1986 Vol 26 May 269

Acoustic emission from superconducting magnets: T. Ishikawa

,a,,i

10

I"

I'""

I

i

I

I

i

I

I

I

I

I'

1'

I

I

I

I

I

2

In the sweep-down mode, the AE rate has a broader peak at lower current than in the sweep-up mode, and it is asymptotic to zero as the current approaches zero. The reproducibility of this pattern was confirmed by measuring the AE rate repeatedly. When the AE rate has a peak in the sweep-up mode (Figures 5 and 6), it is important to examine whether the AE pattern in the sweep-down mode depends on the value of the maximum current, Ira, or not, (for discussion see below). In the experiment with the 13 T magnet outer coil, there is almost no difference between I m -- 450 A (Figure 6a) and lr, = 350 A (Figure 6b).

g ~

I

I

'

I

'

*

~

~'"'r~'

I

I

'

I

'

i

'

i

'

~

g ,<

e t al.

5

Pancake wound coils 0

I

I,

L

0

I 50

I

I

I

I

I I~1 100

!

Figures 7, 8 and 9 show the AE rate as a function of coil

I 150

Current (A)

Figure 7

AE energy counting rate (arbitrary units) v e r s u s current traces of 16.5 T S M (pancake wound) for (a) 0 ~ 1 2 0 A and, (b) 1 2 0 ~ 0 A. Total gain of the amplifier is 7 8 dB I0

a

I

I

I

I

I

I

I

I

I

I

I

I

I

I

'

I

5-

-,o

i

g

-g_ C

°

"-

,,

current for the 16.5 T magnet, the inner coil of the 13 T magnet and SM2, respectively. In each Figure, the top trace is for a sweep-up mode, and the bottom trace is for a sweep-down mode. The characteristic AE pattern observed in these pancake wound coils is as follows: in both the sweep-up and the sweep-down mode, the AE rate has a peak in the low current region, and decreases in the high current region. Reproducibility of these patterns was also confirmed. The results of the experiment using test coil I with a single double pancake is shown in Figure 10. The test coil II with eight double pancakes was examined in two states: with the eight double pancakes tightly clamped by the nut, and, with them loosely clamped. The results are shown in Figure lla and b, respectively. The latter is very similar to the patterns of the pancake wound coils shown inFigures 7, 8 and 9, while the former resembles that of one double pancake shown in Figure 10.

W

'<

,5

-

0 0

I I 100

50

I

I

I 150

Current (A)

Discussion

AE generated by the frictional motion of conductor I n Figures 4, 10 a n d ]]a, the A E rate increases m o n o t o n i c a l l y w i t h i n c r e a s i n g current. T h e s i m i l a r result f o r a l a y e r w o u n d c o i l w a s r e p o r t e d in an e a r l i e r p a p e r ' ° i

Figure 8 AE energy counting rate (arbitrary units) v e r s u s current traces of 13 T S M inner coil (pancake wound) for (a) 0 ~ 1 4 0 A and, (b) 1 4 0 ~ 0 A. Total gain of the amplifier is 92 dB |OlaF--

I

I

I '1

I

i

I

I

I

I

I

i

10

ID

5

I

I

I

I

I

I

I

t

I

-

i

g

,g

50 0

II ~

I

I

I

I

500

I I000

I

I

I

,,I.

AE energy counting rate (arbitrary units) v e r s u s current traces of S M 2 (pancake wound) for (a) 0 ~ 1 4 0 0 A and, (b) 1 4 0 0 ~ 0 A. Total gain of the amplifier is 8 6 dB

Cryogenics 1 9 8 6

Vol 2 6 M a y

0

-

I-I

m I -

I I

-I

i

air

I00

I

I

200

Current (A)

1500

Current(A)

Figure 9

I. I

b

••

UJ

270

I

t

C

0

1

I

/

~1 I

a

Figure 1 0

AE energy counting rate (arbitrary units) v e r s u s current traces of the test coil of 1 double pancake for (a) 0 ~ 1 5 0 A and, Ib) 1 5 0 ~ 0 A. Total gain of the amplifier is 8 3 dB. , Calculated value using Equations (1) and (2), wherefl=3.8x 10 - s , I s = 8 0 A a n d / m = 150A

Acoustic emission from superconducting magnets: T. Ishikawa et al. Moreover, a theory agreeing with the experiment was presented by Tsukamoto and Iwasa n. This is the theory based on the frictional sliding of the conductor on which the electromagnetic force, the elastic force and the frictional force act. According to their theory, the AE ring-down counting rate R (where R = dN/d/with N = accumulated ring-down counts and, I = coil current) for the non-virgin runs is expressed as follows. For increasing current 0

forO ~
/I/(/= - I s 2)

for/s ~ I ~ /m

R =

(1)

and decreasing current R={

for I m >1 I > (I2m- ~ ) ~

0

(2)

[Y[(I=m- I s 2 ) - I 2 1

for (I2m-Is2) ~ /> I i> 0

where/3 is a constant, I s the current at which AE starts in the sweep-up mode and I m the maximum current. Tsukamoto and Iwasa derived Equations (l) and (2) by assuming that the displacement of the conductor

IO r

a

,

,

,

,

,

,

,

.]

,

d w

o g

5

0-"80

A

-4

J ¢/

c

g

o

.n,,

I0

I

4

I 8o-~oA

i. m c o Lg


0

50

I0

I

I

I

I

I

I00 I

I

I

b

I

o-so A

o°

Although the AE pattern in Figure 4 is well explained by the simple theory of frictional motion, the AE rate of the other layer wound coils has a peak in the sweep-up mode as shown inFigures5 and6. For the decrease o f t h e A E rate in the high current region, the following two different interpretations are possible:

g c :3

~"

I

I

80--~-OA

o= t.. (g c W

'~

There are two kinds of frictional motion caused by the electromagnetic force in the pancake wound coils: the frictional motion between neighbouring conductors (or, between a conductor and an insulator in a pancake), and the frictional motion between the pancake and its supporting structure, such as the centre pipe. This latter motion is caused by the electromagnetic force which compresses each pancake along the coil axis when a stack of pancakes is energized. It is considered that the AE pattern of test coil I, one double pancake, shown in Figure 10, is generated by the former frictional motion. A similar pattern is obtained for test coil II, eight double pancakes clamped tightly, as shown in Figure lla. These patterns are also similar to that of the layer wound 4 T magnet, shown in Figure 4. However, in the case of the eight double pancakes clamped loosely, the AE pattern shown in Figure llb is probably generated by the latter frictional motion, as the double pancakes can move along the centre pipe. The AE rate inFigure 11b tends to decrease with increasing current, since the axial displacement of the pancakes must be saturated. It would be expected that the AE pattern due to the former frictional motion, shown in Figure lla, would be superposed on the pattern of Figure llb, but this was not observed in this experiment. This is probably because the path of the acoustic wave due to the former frictional motion is cut off at the boundary between the flange and the winding where a small gap is generated as a result of the pancake compression. The AE patterns of the pancake wound magnets, shown inFigures 7, 8 and 9, are similar to that of Figure llb. Therefore, it is assumed that most of AE observed in these magnets is generated by the axial motion of the pancakes. It was thus concluded that the pancakes of the practical magnets were clamped loosely, as compared with the enormous electromagnetic forces.

AE pattern of layer wound coils

o t.

u

AE pattern of pancake wound coils

5

-

o

frictional motion is proportional to the ring-down counts. It seems more probable, however, that the displacement is proportional to the energy counts measured in the present experiment. The dashed lines in Figures 4, 10 and lla are the theoretical traces obtained by substituting suitable values into r, I s and Im of Equations (l) and (2). It is to be noted thatFigure 4 shows the result of the layer wound coil while Figures I0 and lla show the results of the double pancakes. In both cases, however, agreement between the theoretical and experimental traces is very good.

5

0

O

50

"-=-"

~

'

IOO

Current (A) AE energy counting rate (arbitrary units) v e r s u s o f the test coil of 8 double pancakes. Total gain o f the amplifier is 83 dB. (a) Tight clamping; (b) loose clamping. • Calculated value using Equations (1) and (2), w h e r e /~ = 2.1 x lO-S, Is = 30 A a n d I m = 8 0 A

Figure current

11

traces

l

the frictional motion of the conductor does not occur above a threshold value of the current; and

2

the path of the acoustic wave is cut off above a current threshold value, while the frictional motion of the conductor continues to occur. A similar effect is actually observed in the pancake wound test coil experiment (see the previous section).

The difference between points l and 2 is that, above a current I0, either the frictional motion disappears or the acoustic wave cannot be propagated. These two interpretations can be distinguished by the dependence of the AE

Cryogenics 1986 Vol 26 May

271

Acoustic emission from superconducting magnets: T. Ishikawa et al. pattern on the m a x i m u m current, Ira. According to Tsukamoto and Iwasa's theory, the AE rate in the sweepdown mode must depend o n I m. Even if the path is cut off above I0, as expected in the case of point 2, the AE rate below I0 depends on Ira. In the case of point 1, above, however, the current sweep between I0 and Ira does not affect the AE rate in the sweep-down mode, since the displacement of the conductor above I0 is due to elastic deformation. Thus the AE pattern is independent of the value oflm as long as I m/> Io. In the experiment with the outer coil of the 13 T magnet, the AE pattern in the sweepdown mode [Figures 6a and b] is almost independent of the value of Ira (see Layer wound coils section). Therefore, the interpretation of point 1 above is consistent with the experiment, i.e. it is considered that the frictional motion of the conductor disappears above the current I 0. The decrease of the AE rate at I0 is, in fact, not so sharp, probably because the value of I0 is distributed. The same interpretation may also be applied to SM3. However, the AE rate of the 4 T magnet continues to increase in the sweep-up mode, followinKTsukamoto and lwasa's theory. It is possible that the conductor of a rectangular cross-section (outer coil of the 13 T magnet and SM3) has better fixation than that of a circular one (4 T magnet), resulting in the appearance of the threshold current, Io. According to the theory, the total energy in the sweep-up mode should be equal to that in the sweep-down mode. This is confirmed for the 4 T magnet, as shown in Figure 4. However, the latter is 500 times as much as the former in the case of Figure 5 and 5 times as much as for Figure 6. This suggests a different mechanism of friction between the sweep-up mode and the sweep-down mode, but it has not yet been fully clarified.

Conclusions The AE energy counting rate for the non-virgin runs for unimpregnated superconducting magnets, having a simple shape, has a characteristic pattern depending on the type of winding, i.e. layer wound coils and pancake wound coils exhibit different behaviour. It was found, by the experiments using test coils, that the AE rate from a 'pancake' follows a simple theory which was presented by Tsukamoto and Iwasa. This theory was formulated to interpret an earlier result for a layer wound coil. However, the AE rate of practical pancake wound magnets has a peak in the low current region and decreases in the high current region. This seems to reflect the axial motion of

272

Cryogenics 1986 Vol 26 May

the pancakes. Deviations from Tsukamoto and Iwasa's theory are also found in some layer wound coils; the AE rate in the sweep-up mode decreases above a threshold current, reflecting the disappearance of frictional motion. It is difficult to determine whether the AE technique is of use in monitoring the practical superconducting magnet. However, it should be noted that a secular change of the AE pattern was observed for SM3: the peak position in the sweep-up mode was 200 A higher after one year. Moreover, for another magnet, SMI, (pancake wound coil; 12 T, 22.5 MJ) it was found that a characteristic peak of the AE rate in the low current region disappeared after a slight modification of the coil structure. (The details will be reported at a later date). These observations indicate that the variation of the coil structure can be reflected sensitively by the AE rate measurement.

Acknowledgements The authors would like to express their thanks to Professor Y. Muto, Dr K. Noto, Mr. A. Hoshi and Mr K. Watanabe of the High Field Laboratory for Superconducting Materials for their helpful encouragement. This work was partially supported by the Grant in Aid for Scientific Research from the Ministry of Education in Japan.

References 1

Tsukamoto, O., Sinclair, M.W., Steinhoff, M.F. and lwasa, Y. Appl Phys Lett (1981) 38 718 2 Pasztor, G. and Schmidt, C.J. Mater Sci (1981 ) 16 2154 3 Maedn, H. and lwasa, Y. Cryogenics (1982) 22 473 4 Tsukamoto, O., Steinhoff, M.F. and lwasa, Y. Proc 9th Syrup Eng Problems Fusion Research (1981) 309 5 Kensley, R.S., Yoshida, IC, Tsuji, H. and Shimnmoto, S. Cryogenics (1983) 23 17 6 Maeda, H., Koizumi, M. and Murase, S. Cryogenics (1983) 23

444 7 8 9 10 11

Lore, J., Tamada, N., Tsukamoto, O. and lwasa, Y. Cryogenics (1984) 24 201 Miura, S., Hoshi, A., Nakagawa, Y., Noto, K., Watanabe, K. and Muto, Y. High Field Magnetism (Ed Date, M.) North Holland (1983) 331 Nakagawa, Y., Noto, K., Hoshi, A., Miura, S., Watanabe, K., Kido, G, and Muto, Y. in Proc 9th lnt ConfMagnet TechnolZurich, (1985) to be published Sinclair, M.W., Tsukamoto, O. and lwasa, Y. IEEE TransMagn (1981) MAG-17 1064 Tsukamoto, O. and lwasa, Y. J Appl Phys (1983) 54 997