Dielectric loss of liquid helium

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form/Pro, where a is much greater than 2. One of the authors (Irie) suggests that the superconductor distorts the magnetic field in the copper layer and that the eddy current loss in the composite conductor is modified from that in the free space. The calculation on the basis of a theory shows ~ (t~ > 2) dependence of the eddy current loss) The authors wish to thank Drs K. Kuroda and H. Kimura for their discussions and encouragements. We are also indebted to Dr G. Kamoshita for his support of this work.

Hint 0 ¢ Fig.4 Square frequency dependence component of loss in the composites. The solid lines show the calculated values of the eddy current loss in the copper layer

References

1 Rhodes, R. G., Rogers, R. C., Seebold, P~ T. A. Cryogenics 4 (1964) 206 2 Pech,T., Duflot, J. E, Fournet, G. Phys Lett 16 (1965) 201 3 Irie, F. (Forthcoming)

Dielectric loss of liquid helium R. LI. Nelson

The loss tangent of a capacitor A with liquid helium dielectric and stainless steel electrodes was investigated by the author 1 : a sample of polytetrafluoroethylene (PTFE), 'Fluon', was found to have a value of approximately +2.5/arad relative to the capacitor, at 60 Hz, 4.2 K and low electric stress. The absolute loss tangent of 'Fluon' is about 1 to 2 microradians at the same frequency and temperature.2, 3 Therefore, it seems reasonable to assume that the loss tangent of capacitor A, and hence the helium dielectric, was < 1/arad at low electric stress. At values greater than 4 to 5 MV m "1 (rms), however, the loss of the capacitor appeared to increase. 1 In order to investigate the variation of the loss tangent of liquid helium with electric stress further, and to obtain a reliable loss reference for measurements at higher voltages, a low-stressed, co-axial cylindrical capacitor, B, was built for use with a liquid helium dielectric. (A PTFE sample, reference, at 4.2 Kwas considered. It was thought, how-

The author is with the Central Electricity Research Laboratories, Kelvin Avenue, Leatherhead, Surrey, UK. Received 1 March 1974.

C R Y O G E N I C S . JUNE 1974

ever, that deterioration of the vacuum-deposited contact electrodes 1 with repeated thermal cycling could result in inaccuracies. 2,4) The inner cylinder served as the highvoltage (hv) electrode. The outer cylinder was in three sections; the middle section was the measuring electrode and the two end sections were earthed, guard electrodes. An electrode separation of ~2 mm was obtained using two annular spacers. To prevent errors due to leakage currents the spacers were positioned between the guard and hv cylinders. The electrode material was stainless steel with a finely machined surface. The difference between the loss tangents of the capacitor B and capacitor A was measured at 4.2 K and 60 Hz, using the method described in a previous paper. 1 The electrode separation of capacitor A was *180/am and both capacitances were %130 pF. The measured difference was 'x,O.0/arad for voltages up to 700 volts (rms). The detector resolution was about 0.3/arad. The loss of capacitor A increased (relative to B) for higher voltages; that is when the stress exceeded 4 MV m "1 in A. This increase was a property of the stressed helium dielectric as confirmed by the following experiment with another capacitor: the loss

345

and (16) were obtained immediately after a discharge in the stressed helium capacitor, and that points (17) and (18) were measured 10 minutes later - other measurements were made before the discharge. The interval between the first and final measurement was 95 mins.

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Hence it was concluded that the loss tangent of liquid helium was < 1/arad at stresses ,~ 4 MV m "1 . Therefore, the capacitor B could be used as a loss reference for voltages ~ 8 kV.

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The loss in liquid helium may be due to conduction by charged particles, such as impurities, as has been previously argued. 1 The higher loss observed immediately after the discharge occurred could then be accounted for by assuming a sudden increase in the concentration of charged particles. Therefore, the stress value, •4 MV m "l , at which the helium exhibited a measureable loss may not be an intrinsic property of helium; helium of impurity content ~<20 vpm was used. (The inside of the cryostat was carefully cleaned before an experiment was carried out.) A loss mechanism due to discharges is unlikely as pressurization of the cryostat to 17 x 104 N m "2 ( 2 10 lb in "2 g) did not give a discernible change in the measured loss tangent at 5.5 MV m "1 .

Stress variation of liquid helium loss (4.2 K, 60 Hz)

tangent of a circular, three-electrode, brass, parallel-plate capacitor, C, of electrode separation 'x,230/am and also employing a liquid helium dielectric, was measured relative to capacitor B. Fig.l shows the experimental results. The difference was 'x,0 grad for various voltages up to 900 V (corresponding to an internal stress in C ~< 4 MV m "1). For higher voltages, as expected, the loss of capacitor C was seen to increase rapidly with stress. The numbers refer to the chronological order of the measurements. It should be noted that data points (I 5)

The electrodes were constructed in the Electrical Engineering Division workshop of the Central Electricity Research Laboratories (UK). The work reported was carried out at CERL, and is published by permission of the Central Electricity Generating Board. References

1 2 3 4

Nelson,R. LI. Proc IEE (forthcoming) Vineett, P. S. BritJApplPhys(JPhysD) 2 Seres 2 (1969) 699 Carson, R. A.J. PhD thesis, Univ of Cambridge (1972) Mathes,K. N., MeGowan, E. J. Philadelphia: ASTM(STP 420) p3

A simple aid to the efficient filling and re-filling of liquid helium cryostats J. J. R o w l a n d

There is a problem associated with the positioning of a siphon used to transfer liquid helium into a cryostat.

must not be allowed to impinge upon the surface of the liquid helium already in the cryostat.

When initially cooling a cryostat from 77 K to 4.2 K the outlet of the liquid helium siphon should be below any apparatus (such as a superconducting magnet) which is in the cryostat. In this way not only the latent heat of the liquid but also the enthalpy of the helium gas evolved is used to cool the apparatus.

The problem may be overcome by changing the position of the siphon between the initial filling and 'topping-up' operations. This is often inconvenient and this paper describes an extremely simple device which enables the required conditions to be achieved without moving the siphon.

Consider, however, the refilling of a cryostat already containing liquid helium. If the siphon extends below the level of this liquid, the hot gas passing through the siphon at the start of the transfer boils away part of the liquid already present. Therefore, for 'topping-up' a cryostat, the exit of the siphon tube must be above the lowest acceptable liquid level and moreover the jet of gas emerging from the siphon The author is with the Physics D e p a r t m e n t , University of Manchester, I n s t i t u t e of Science and T e c h n o l o g y , Manchester, U K . Received 20 February 1974.

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T h e device

This is a simplification and extension of a device described by Hegland. 1 A cylindrical funnel 'F' (see Fig.l) is placed around the end of the liquid helium siphon. Its operation is as follows. Consider a cryostat which, having been pre-cooled to liquid nitrogen temperature, is about to be filled with liquid helium. Initially helium gas emerges from the siphon and the temperature of this gas quickly falls below that to which

CRYOGENICS . JUNE 1974