Two modes of shock waves propagating in He II, that is, a compression and a thermal shock waves, were investigated by a measurement of temperature and pressure with superconductive temperature sensors and pressure transducers and by visualization. We adopted the experimental method in which a gas dynamic shock wave impinged onto a He II-free surface by a newly developed superfluid shock tube facility. It was found that the temperature sharply drops as a result of compression by a compression shock wave. However, the temperature drop turns to a rise when the phase of liquid helium changes from He II to He I by shock compression. It was also found that in the immediate vicinity of the vapor-He II interface, no thermal shock wave is excited, where a thermal boundary layer with a large temperature gradient in which the physical state changes from supercritical helium to He II via He I is formed with a thickness of several mm. The temperature profile of a thermal shock wave was found to be a single triangular waveform similar to that of a thermal shock wave generated by large heat flux by a planar heater with a long heating time.
A multilayer coaxial superconductor has been investigated theoretically and experimentally for realizing homogeneous current distribution. The theory, based on the magnetic conservation law defining net magnetic flux enclosed by the electric center lines of the two filaments in the two strands corresponding to adjacent layers to be zero, gives a generalized formula governing the current distribution. It was shown from the formulation that the current distribution can be homogenized by adjusting the twist pitches of a multilayer superconductor with particular layer radii and twist direction. To verify the theory, we have fabricated a three-layer coaxial superconductor 1m long and thereby carried out its test. The conductor was composed of Ag-sheathed Bi2223 tapes wound spirally onto three tubular FRP (Fiber Reinforced Plastic) formers with pitches of 333.33mm, 166.67mm, and 111.11mm corresponding to the first (innermost), second and third layers, respectively. To minimize contact resistance between the HTS tapes and terminals, a ladder-shaped pretreated Cu block was employed for the end connection. The test was performed at 77K by imposing AC transport current in the range of 0-100 ampere maximum value with a frequency of 100Hz. The measured currents of the first, second, and third layers were 30.25%, 36.82% and 32.935% of total transport current, respectively. This result evidently is in good agreement with that of the theory. This paper describes the design, construction and experimental results of a test superconducting multilayer cable based on the homogeneous current theory.
The purpose of this paper is to show the effectiveness of the two-dimensional analysis of low temperature helium using a numerical model proposed in the previous paper. The model is applicable for a wide range of temperatures from He II to the vapor He I. Two-dimensional numerical analysis for a horizontal channel heated at the bottom was performed. The results were compared with experimental results. For low heat flux, the temperature distributions in the channel in a steady state obtained from the numerical analysis showed quantitative agreements with those from the experiment. For higher heat flux, numerical results could simulate experimental temperature distributions reasonably but not as accurately. The changes in velocity fields caused by the phase transition were observed in the transient analysis. It was verified that the proposed model using the two-dimensional system is valid even in the temperature range including the phase transition.
The authors have carried out a series of tests on extruded ethylene-propylene rubber (EPR) insulated low temperature superconducting power cables. This paper focuses on tanδ, a dielectric dissipation factor, of EPR insulation in the form of cable configuration at cryogenic temperatures. The problem of electric contact between EPR insulation and a measuring electrode was solved by adopting SZ wire shielding, which enabled us to obtain reliable data even in the cryogenic temperature region. Although it is reported that a film sample gives fairly high tanδ at the liquid helium temperature, the cable samples used in this study showed very stable tanδ, as low as in the area of 10-4. The estimated dielectric loss of the cable was comparable with the AC loss of superconductors.