Some difficulties encountered with superconducting niobium zirconium coil magnets

Goedemoed,

S. H.

Physica

Kes, P. H. Jacobs,

32

1978-1984

F. Th. A.

de Klerk,

D.

1966

SOME DIFFICULTIES ENCOUNTERED WITH SUPERCONDUCTING NIOBIUM ZIRCONIUM COIL MAGNETS by S. H. GOEDEMOED, P. H. KES, F. TH. and D. DE KLERK

A. JACOBS

Communication No. 349~ from the Kamerlingh Onnes Laboratorium, Leiden, Nederland

Synopsis During irregular vestigations that

they

experiments field

with

fluctuations

were disappear

carried below

a superconducting were

out

noticed

niobium

during

on the occurrence

the lambda

zirconium

the variation of these

coil magnet

of the field.

fluctuations.

small

Some

in-

It was found

point.

1. Introduction. During the last years experiments have been performed in the Kamerlingh Onnes Laboratory on the flux penetration in superconducting niobium in the mixed statel) 2) 3). In the earlier experiments use was made of an iron core electromagnet. Later, when the influence of the speed of variation of the magnetic field was studied a superconducting coil magnet was used in which the field was varied at constant dH/dt. During these investigations we observed a large number of small sudden inhomogeneous field fluctuations in the coil, of the order of some tens of oersteds, which seriously spoiled the results of our experiments. For this reason we performed some experiments on the magnet coil itself. 2. Experimental details. The magnet coil was wound from commercial “Supercon” 3Nb-Zr wire with copper coating and formvar insulation. The coil was 10 cm long, the inner diameter was 2 cm and the outer diameter 4 cm. The homogeneity of the field was improved by means of extra turns at the ends as shown in fig. 1. The field was 944 Oe/A. The field measurement was carried out by means of a coil F with a total winding area of 50.000 cm2 which was connected to the input of an integrating circuit. The output of the integrator could be connected to an oscilloscope. The magnet coil was energized from storage cells. The current was varied practically linearly with time by means of a transistor circuit which could be operated at various speeds with a motor driven helipot. Occasionally -

1978

-

DIFFICULTIES

small

oscillations

in the transistor

WITH

occurred circuit.

SUPERCONDUCTING

between

They

COIL MAGNETS

the coil magnet

were damped

1979

and the condensers

by means

of a resistance

of

a few tenths of an ohm parallel to the coil. This, however, spoiled somewhat the linearity of the current verszls time in the coil. This is shown in fig. 2, which was obtained by displaying the field strength on the vertical deflection plates against

Fig. 1. Niobium

the time base.

zirconium

coil (M) with built-in coils. For further

details

magnet

pick-up

Fig. 2. Field strength three different

Versus time for field rates.

see text.

Two small coils of 4000 turns each (Sr and Ss of fig. 1) were wound in opposite directions and connected in series. Their unbalance could be compensated by connecting another small coil (B in fig. 1) in series with them with an adjustable potentiometer parallel to it. This combination as a whole was connected to the input of a second integrating circuit. During the original experiments on the flux penetration in niobium (see section 1) the sample was mounted in Sr and the magnetization curve was displayed on the oscilloscope screen by connecting the output of Sr, Ss and B (through the intermediate of the integrator) to the vertical oscilloscope plates and the field strength (as derived from coil F and its integrator) to the horizontal plates. During the investigations on the magnet

1980

S. H. GOEDEMOED,

P. H. KES,

F. TH.

A. JACOBS

AND

coil itself the experiments were carried out in exactly with the niobium sample removed from coil Sr.

D. DE KLERK

the same way but

3. Results. Some experimental results are shown in fig. 3 and fig. 4. The field was first increased from zero to about 12 kOe (so-called “first quadrant”), then decreased to zero (“second quadrant”), subsequently it was increased to 12 kOe in the opposite direction (“third quadrant”) and finally decreased to zero again (“fourth quadrant”). The experiments of fig. 3 were carried out at various temperatures and a field rate of 1.1 kOe/s. If the field increase were smooth or if only field discontinuities would occur which were homogeneous over the coil volume the curves would be horizontal straight lines, due to the balance of the coils Sr, Ss and B. It follows from the figures that very sharp and noticeably inhomogeneous field fluctuations occur.

2.46 OKill

J 35

I

-10

I

-5

2.46OK:

2.0kOe/s

2.46”K:

2.6kOe/s I 1

I

0

5

1OkOe

H

Fig. 3. Field fluctuations

ent temperatures.

1

’

at five differ-

Fig. 4. Influence

of the field rate on the

The arrows indicate

field fluctuations

at 2.46’K.

the sequence of the experiments.

After

each trace the oscilloscope beam was moved somewhat down in order to prevent the traces from falling on top of each other. The vertical sensitivity is indicated

kOe/s

at the left of the figure.

details see subscript

For further

of fig. 3.

DIFFICULTIES

At 4.2”K

WITH

SUPERCONDUCTING

COIL MAGNETS

the field increase in the first quadrant

1981

takes place relatively

smoothly. The decrease in the second quadrant begins smoothly as well, but a few sharp discontinuities occur just before the field is zero. The third quadrant is very bad, a large number of irregular discontinuities being observed. The fourth quadrant is very similar to the second one. At 3.05”K the effects are similar to those at 4.2”K, but they are more pronounced. At 2.46”K the irregularities have penetrated into large parts of the first, second and fourth quadrants as well. Just above the lambda point the regions where the irregularities occur are approximately the same as at 2.46”K, but the amplitudes seem to have decreased somewhat (all the pictures were taken at the same oscilloscope sensitivity). The most remarkable observation is that all the irregularities vanish suddenly as soon as the temperature is decreased slightly below the lambda point. The last two pictures of fig. 3 were taken at 2.17”K and 2.15”K respectively. Fig. 4 shows another set of pictures, all taken at 2.46”K, but at three different field rates. It follows that the influence of dH/dt is small with a tendency for smaller irregularities at the higher speeds. We found that if the field was increased and decreased twice in the same direction the results for the second time were very similar to those of the first and second quadrants of the curves of figs. 3 and 4. If, after the fourth quadrant, the current direction was reversed again the results were similar to those of the third and fourth quadrants. 4. Discussion. The observation that all the discontinuities vanish immediately below the lambda point indicates that thermal effects play a preponderant role in these phenomena. We suppose that small amounts of flux are trapped inside the niobium zirconium windings of the magnet coil, or maybe between them, causing field differences at both sides of a wire. When flux starts moving the wires are locally heated, giving rise to local avalanches. The effects are the most pronounced in the third quadrant, where flux annihilation in the wires causes extra heating. Below the lambda point most of the local heating is suppressed due to the large heat conductivity of the liquid helium, preventing the occurrence of avalanches. Our results seem to be in agreement with the unpublished observations of Das e.a. of this laboratory that it is impossible to operate a niobium zirconium coil in vacuum unless special precautions are taken for the heat removal. The influence of the coil fluctuations on the experiments on the magnetization of niobium is demonstrated in fig. 5. These measurements were also taken at a field rate of 1.1 kOe/s. At 4.2”K the first quadrant of the hysteresis loop shows no irregularities, whereas a few flux jumps are observed in the second and fourth quadrants and manyin the thirdquadrant. The influence of the coil jumps on the experiments is so drastic that it is quite

1982

S. H. GOEDEMOED,

P. H. KES,

F. TH.

A. JACOBS

AND

D. DE KLERK

impossible, for instance, to derive the area of the hysteresis niobium from the oscillogram with any precision.

loop of the

At 3.35”K two flux jumps of the niobium sample are observed in the first quadrant and a few flux jumps of the coil occur in the second quadrant. It follows from these pictures that the flux jumps of the sample and those of the coil are quite different in character, so that it is hardly a problem to recognize them from each other. The niobium jumps go very steeply

4.2"K;llkOe/s

335"K;llkOe/s

1.30"K:l.lkOe/s -8

-6

-4

-2

0

2 H

Fig. 5. Influence curve

of niobium.

of the field

fluctuations

I

I

I

4

6

BkOc

’

on the measurement

of the magnetization

Full hysteresis loops at 4.2”K and 1.30”K (top and bottom); loops at 3.35”K, 2.69”K, 2.29”K and 1.57”K.

half-

down and are followed by a region with approximately the initial slope of the magnetization curve. The coil jumps behave much more irregularly. At 2.69”K, in the first quadrant, we observe a number of flux jumps of the sample between 2.5 and 5.0 kOe, and coil jumps above 5.5 kOe. At 2.29”K above 4.5 kOe the flux jumps of the niobium sample are completely lost in the irregularities of the coil. At 1.57”K, below the lambda point, the flux jumps of the coil have

DIFFICULTIES

WITH

SUPERCONDUCTING

vanished,

so that those of the niobium

hysteresis

loop at 1.3”K is shown at the bottom

COIL MAGNETS

sample are clearly

observed.

1983 A full

of fig. 5.

In these experiments the niobium sample was not mounted in a helium gas atmosphere, as in our earlier experimentsl) s), but it wasdirectlyimmersed in the liquid helium. This demonstrates that the flux jumps in the niobium zirconium coil do not have the same origin as those in the niobium sample. The flux jumps in the niobium sample below the lambda point are somewhat smaller in these experiments than when the sample was in a helium gas atmosphere (especially between 2.5 and 4 kOe in the first quadrant at 1.3”K, and in the region from 2 kOe in the second quadrant to 4 kOe in the third quadrant) but they distinctly do not disappear. It is plausible that there should be a difference in character between the sample jumps and the coil jumps. For instance, the wires of the niobium sample are parallel to the magnetic field, whereas the windings of the coil are perpendicular to it. Further the windings of the coil are under current during the experiment and the niobium sample is not. At the end of the third quadrant at 1.3”K, above 7 kOe, we observed very tiny fluctuations (in fig. 5 they are somewhat exagerated) which we could not detect in other regions of the loop at the same temperature, nor could we find them in the curves at 1.57”K. Maybe they are of the same origin as those in the third quadrant above the lambda point, but they are so small that they don’t influence our experiments in the niobium sample. An estimate of the sizes of the field fluctuations in the coil could be made in the following way. The vertical scale of fig. 5 was derived from the assumption that the slope of the (- 4nM) versus H curve in the first quadrant below Hc, is equal to one. From this the vertical scales of figs. 3 and 4 were computed by multiplying with the ratio of the areas of cross section of coil Si (38.5 mms) and of the niobium sample (0.785 mms). It turns out that, under the assumption that a fluctuation occurs in coil Si or Ss alone (fig. l), the biggest ones are of the order of 20 Oe, though most of them are appreciably smaller. If a fluctuation occurs in a region extended over both coils we can only say that the inhomogeneity over that region is of the order of 20 Oe or less. It is obvious that a fluctuation of 20 Oe could not be noticed in the field curves of fig. 2. The conclusion of the above is that niobium zirconium coils are very suitable for experiments in constant magnetic fields, but some difficulties may be encountered if dynamic experiments must be carried out in varying fields. The problem could be solved by using two concentric liquid helium dewars. The outer one, which contains the coil magnet, is kept at a temperature of roughly 2°K. The actual experiments must be performed in the inner dewar. Received 25-S-66

i 984

DIFFICULTIES

WITH

SUPERCONDUCTING

COIL

MAGNETS

REFERENCES

1)

Goedemoed, (1964)

2)

Goedemoed, Kamerlingh

3)

S. H., Van

Kolmeschate,

C., De

Klerk,

D. and Gorter,

C. J., Physica

30

1225.

Goedemoed,

S. H., Van Onnes Lab., S. H., Van

den No. 34%~; Physica

Kolmeschate, Leiden

No. 342b;

Kolmeschate,

32 (1966)

1183.

C., Metselaar, Physica

J. W.and

De Klerk,

D., Commun.

31 (1965) 573.

C., Kes,

P. H. and De

Klerk,

D., Commun.

Lei-