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.