U. Ruppert

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.

Influence of porous coatings on heat transfer in superfluid helium

Abstract Different types of porous materials as surface coatings on samples (RhFe wire)are investigated at HeIIs and HeIIp in respect to their influences on the heat transfer. Experimental results of measurements on samples with and without coatings are presented. Wires covered with capillary filter plates as well as a mixture of gypsum and alumina abrasive show higher (up to 3.5 times) peak and recovery heat flux densities than those of the comparable bare wire at HeIIs. They show an increasing tendency of heat flux density with decreasing bath temperature at HeIIs similar to those at HeIIp. A discussion of the results is also given in the paper.

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.

Transient Heat Transfer to Superfluid Liquid Helium

Transient heat transfer from thin superconducting and normal wires to saturated Hell and to pressurized Hell was investigated. Rectangular current pulses of 0.5 milliseconds duration and longer were applied to the sample while voltage and current were detected with a resolution in the microsecond range. For the determination of the sample temperature the temperature characteristic of the wire resistivity was used in the case of normal conductors and the temperature characteristic of the critical current in the case of superconductors. Data are reported for bath temperatures of 2 K and, in the case of the superconducting samples, transverse magnetic fields of 1.5 T.

Liquid helium vibration cryostat for space qualification tests

Abstract A continuous flow, liquid helium cooled vibration cryostat with a useful cold space of 200 mm diameter and 160 mm height is described. The cryostat (weight 9 kg) is attached to the armature of a vibration magnet. Cool-down to 4.2 K takes ∼ 15 min and warm-up ∼ 1 h. The vacuum jacket and radiation shields can be removed so that samples can easily be exchanged. The vibration transfer of the cryostat is quite flat up to 600 Hz. The performance of the cryostat has been verified up to a level of 25 g (r.m.s.) in the random mode and up to 30 g (0-p) in the sinusoidal mode. A number of infrared filters and polarizers for the ISOPHOT experiment, an active phase separator and certain cryomotors have already been successfully tested.

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.

Heat Transfer From Superconductor Wire to Superfluid Helium

Publisher Summary This chapter conducts a systematic research to find and compare the heat transfer patterns of superconductor in different coolant baths. It focuses on the influence of the insulation layer of superconductor on heat transfer. A systematic research on NbTi/Cu wires (single core and multi-filaments) has been conducted in HeI, HeIIs, and (pressurized superfluid) HeIIp, separately. The results measured in three baths show a large difference in current sharing zone of a same sample, but a similar wire with insulation layer in HeII baths gives the results like those of bare wire in HeI. It means the insulation of superconductor is the main obstacle of heat transfer. In order to complete the measurement in a small cryostat avoiding destroying sample, a prediction based on the pre-test at room temperature is performed to provide the maximum of resistance and voltage of sample, which makes the measurement high efficient and more reliable.

Heat flux investigations on wires coated with porous insulation in superfluid helium

The influence of different porous insulating coating materials on the heat flux in superfluid helium is studied. Measurements of peak and recovery heat fluxes are performed for coated as well as for bare RhFe and Cu wires. The temperature dependence of these quantities is measured for saturated as well as for pressurized He II. For all coatings the heat flux increases monotonically with decreasing temperature. It is found that the He IIs values can be increased considerably in the case of coated wires so that no significant difference between He IIs and He IIp exists. This might be explained by a pressure zone inside the porous medium due to the thermomechanical effect. The influence of different materials and surface conditions is more complicated.

Cryogenic valve actuator

Abstract The design of a cryogenic power drive for actuating cold valves in the liquid hydrogen and liquid oxygen supply lines of space carriers is described. In room temperature and 80 K tests the required performance data are verified.

Normal zone propagation along superconducting wires in superfluid liquid helium

Abstract The propagation velocity of normal zones along bare technical NbTi-superconductors, cooled with saturated and pressurized HeII was measured at bath tempertures of 2.0 K and magnetic fields of 1.77 and 2.5 T. Although in saturated HeII propagation velocities up to 18 m/s were obtained, in pressurized HeII no propagation was observed up to transport currents close to I c . Furthermore transient heat transfer from thin bare wires to HeII was investigated with a time resolution in the μs-range. The results are discussed with regards to a theoretical model of quench dynamics.