The investigation of the cooling process in the vicinity of λ-transition in liquid helium is of crucial importance for cryogenic engineering. In spite of the 2nd order transition, when the temperature crosses downward over the λ-point, a He I-He II interface appears. In this study, the dynamic behavior of a He I-He II phase boundary was experimentally investigated. When He I under the condition of saturated vapor pressure is suddenly cooled from an initial state at a temperature slightly higher than Tλ by vapor evacuation, an interface appears and propagates downward from the liquid helium-free surface in a cryostat. We investigated the interface behavior by measuring the temperature with a superconductive thin-film temperature sensor. It was confirmed that the He I-He II interface propagates downward at a speed of several centimeters per second in direct proportion to the cooling rate and that the interface speed decreases as the distance from the free surface increases. A preliminary theoretical approach for the phenomena is also discussed briefly. It was confirmed that a modified Stefan problem can be a fundamental model for the construction of the theory.
The inductive heating method is often used to originate initial normalcy in a stability experiment of a cable-in-conduit conductor (CICC). However, its magnitude cannot be evaluated easily. The calibration method to determine the CICC's inductive heating energy using a calorimetric technique was studied, and the inductive heating energies of two CICCs whose geometries were different from one another were successively evaluated applying this method. In addition, the experimental results show that the eddy currents in the strands and conduit electrically couple separately from one another and that this phenomenon affects the heating energy in the strands and conduit. The inductive heating energy in the strands and conduit was evaluated taking into account this effect.
The correlation between the microstructure and critical current density of NbTi/Nb/Cu multilayer superconducting sheet has been studied. Two types of Ti precipitates have been found in the NbTi layers of the sheet. One type is elongated in the rolling direction with a major axis of about 240nm and a minor axis of about 160nm, and is precipitated at the grain boundaries. The other is fine, 20 to 30nm in length, a few nm in thickness and is precipitated in the crystal grains and at the grain boundaries. The sum of the critical current densities calculated from the size and volume fraction of the two types of Ti precipitates and the critical current density attributed to the dislocation pinning agree well with the measured critical current density of the specimens. The critical current density of transverse specimens subjected to a parallel 5T magnetic field was about 1.2×103A/mm2, and the critical current density of longitudinal specimens subjected to the same 5T parallel magnetic field was 40% lower. The critical current density of specimens subjected to a perpendicular magnetic field was about 40% of that of the specimens subjected to the parallel magnetic field. The dependence of critical current density on the rolling and magnetic-field directions can be qualitatively explained by the shape of Ti precipitates elongated in the rolling direction.