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.