The cavitation phenomenon in He II flow through a two-dimensional Venturi channel was investigated by visualization and pressure measurement. Liquid helium flow was made by shrinkage of bellows pump, and the cavitation was induced in the flow at the throat of the channel. The overview of cavitation can be observed through the optical windows of the cryostat, and was recorded by a high-speed video camera and a still camera. The pressure loss due to the flow through the channel was measured by a differential pressure transducer. By analyzing the pressure loss data, the static pressure at the throat of the channel could be estimated. Through this experiment, it was found that cavitation in He II flow has some unique characteristics different from ordinary fluids. Once cavitation is induced in an He II flow, it jumps suddenly to fully developed scale without the weak cavitation stage seen in other fluids.
We have proposed a thermoacoustic Stirling refrigerator which has a potentially long life and can be produced low cost. The refrigerator consists of a looped tube of about one wave length including a regenerator that connects via a branch tube to an oscillating device at the opposite end. To investigate the proper geometry for an efficient refrigerator, we have analyzed the acoustic wave in the simplified refrigerator model under uniform temperature conditions, obtaining useful data on the effects of regenerator position and branch tube length. The maximal value of the acoustic intensity in the regenerator is taken when the regenerator is located 40 to 45% of the looped tube from the branch. The minimal value of the acoustic intensity in the regenerator is taken when the regenerator is located at around 5% of the looped tube from the branch. Through our experiments, we have confirmed that the refrigeration effect is deeply dependent on the acoustic intensity in the regenerator and the data on the proper regenerator position. The experimental data on the effect of branch length agree with the analytical data qualitatively, but not quantitatively. It is suggested from the analytical data on the velocity in the regenerator that the cause of the discrepancy be due to the regenerator loss caused by incomplete heat exchange between regenerator and fluid.
Long flexible tapes with high-transport critical current density (Jc) are needed for the application of HTS to electric power technology. YBCO retains high Jc in high magnetic fields at 77K because of its strong pinning property. Recently, YBCO tapes are considered to be the second generation of HTS wire, and several groups have succeeded in the fabrication of YBCO tapes on metallic substrates of about 1m in length. We also continuously fabricated long and high Jc YBCO film tapes on flexible Ni-based alloy (width: 10mm, thickness: 0.1mm). Biaxially aligned YBCO films were deposited by pulsed laser deposition (PLD) on textured YSZ buffer layers formed using ion-beam-assisted deposition (IBAD). Uniform textured YSZ buffer layers were demonstrated at the length of 3m. Consumption of the YBCO target was controlled by scanning a YBCO target in the PLD process. The reduction ratio of the film thickness after 9h was controlled within about 10%. Y2O3 second-buffer layers were deposited for the purpose of preventing the interface reaction and lattice mismatch between YBCO/YSZ. A Jc value of 2.4×105A/cm2 and Ic value of 25.3A were obtained at 77K, 0T over the whole length of 2.1m.