G. Klipping

Use of narrow-pore filters in He II transfer devices - new results

With the aim of finding simple methods for the fabrication of He II refilling devices, He II flow has been studied through filters made from various fine powders (oxides and metals, grain sizes in the range 0.05–2 μm) by compacting them under pressure. The results obtained for the different states of He II flow, especially in the “breakthrough” and “easy flow” range, are explained by the fountain effect, He II hydrodynamics and the choking effect. According to the results, pressedpowder filters can be classified into three groups with different flow characteristics, of which the “good transfer filters” with a behaviour neatly described by simple theory are suitable for use in He II refilling devices.

Effects of transient heat transfer to liquid helium on steady propagation velocity of normal zones in superconducting wires

Abstract The transient cooling mechanism of liquid helium which affects the propagation velocity of normal zones in bere superconducting composite wires is discussed in relation to the transit time of a zone front region. Previously the propagation velocity was calculated using the transient cooling mechanism either due to metastable nucleation on the heated wire or due to thermal diffusion in vapour films covering the wire. In the present paper, both of the above-mentioned cooling mechanisms are considered for evaluating the propagation velocity in the manner that one mechanism switches to the other at a certain time. The propagation velocity is obtained by solving numerically a one-dimenional heat balance equation on the basis of the present mechanisms. Comparison of the present results with our experimental data shows good agreement.

Stability of He II phase separation-investigations with the active phase separator

Abstract External influences or internal disturbances may result in phase separation instabilities which could lead to operational failure of He II space cooling systems. Possible instabilities are defined and, using the active phase separator, suitable measures for prevention and control are discussed. Experimental results with a superleak arranged in parallel to the active phase separator indicate an additional possibility for adaptation of the latter to specific demands.

Heat Transfer to Helium in the Near-Critical Region

Critical temperatures for today’s applied superconducting materials range from 5 to 20 K. Therefore, single-phase cooling by supercritical helium (T > 5.2014 K and p > 2.274 bar [1]) can be used for superconducting systems. However, the necessity of providing a sufficiently large current-carrying capacity restricts the practical operation of superconducting systems with critical temperatures near 10 K and cooled by supercritical helium to a temperature range of 5.2 to 7 K.

ac magnetic instability in non-ideal type-II superconductors

Abstract The theoretical approach is carried out with the combined thermal and magnetic equations which are based on the critical state model for non-ideal type-II superconductors and include controllable physical parameters. Also, the behaviour of flux jumps is studied on Nb-Ti-Zr ternary alloy wires in the temperature region 1.8–9.0 K under various heat transfer conditions. The ac field method applied here has the advantage of determining flux jump fields more exactly than the linear sweep method.

Zero-g experiments with a He II active phase separator for space application

The Active Phase Separator (APS), which has previously been described in detail 1,2 is a system comprising an annular gap (width d < 10 µm) of variable length, a downstream heat exchanger and an upstream ball closure (Fig.1). It is intended for temperature control in He II space cooling systems and must provide effective liquid/ gas separation, which means that it must under all circumstances prevent liquid coolant from reaching warm parts of the vent line behind the heat exchanger. In addition to the laboratory tests 1,2 these functions of the APS also had to be verified in a zero-g environment .

Mechanism of an Active Phase Separator for Space Applications

Two methods have proved to be suitable for achieving phase separation in helium-II space cooling systems: (1) porous plugs [1–5] and (2) active phase separators with a micron-sized flow channel of adjustable length and well-defined geometry [6,7]. The first measurements with an active phase separator have shown that it is suitable for such applications, and some mass flow characteristics for the helium-II and helium-I region have been obtained [7]. The measurements presented here are aimed at a better understanding of the flow of helium through such phase separators and of the cooling mechanism.

INVESTIGATION OF He II PHASE SEPARATOR OPERATION INSTABILITIES

Regardless wether a porous plug or an active phase separator is used for liquid/vapour separation and temperature control in He II space cooling systems, operation instabilities may occur resulting from outside influences or from disturbances in the system itself. It is specifically the question wether the inverse fountain effect can inadvertently be established. Tests suitable for recognizing such instabilities and for finding ways to overcome them are discussed. An experimental set-up for ground experiments and for zero-g flight experiments is described.

RELATION BETWEEN AC FLUX JUMPS AND QUENCHES OF SUPERCONDING NbTi WIRE WITHOUT COPPER CLADDING

Measurements of the relation between flux jump fields and quench currents Iq were carried out for a Nb-50at.%Ti wire without Cu cladding. A coil simulation method was used applying inphase magnetic fields and currents up to 0.4 T and 65 A, respectively. At 4.2 K two regions of quench behaviour can be distinguished. Whereas in the low current region Iq is constant, it depends almost only on the total surface field in the high current region indicating that the quenches in this case are induced directly by flux jumps. At lower temperatures flux jump and quench fields are not directly related.

Low temperature skin treatment

Abstract Although freezing has been a successful method of curing various kinds of skin lesions for at least 80 years, little progress has been made regarding the techniques and instruments available to the dermatoligist for applying cold. The attempts to improve this technique are reviewed, and the requirements is for successful cryotreatment are discussed taking warts as an example. With these requirements in mind, a simple and effective cryoprobe has been developed by the authors. Its design is described, and the experiences from a years routine application of the probe to the treatment of warts are discussed.