QUENCH-INDUCED BREAKDOWN CHARACTERISTICS OF LIQUID-HELIUM AND OPTICAL OBSERVATION OF THERMAL BUBBLES

This paper describes the breakdown characteristics of liquid helium (LHe) caused by quench of a superconducting wire exposed to a high electric field. The quench-induced breakdown of LHe took place at a considerably lower stress than the intrinsic level which could be withstood without the quench. A delay time of the order of milliseconds existed from quench onset to the breakdown. We also observed the behaviour of thermal bubbles in LHe generated by the quench using a fibrescope and a high speed video system. The optical observation clarified that the high electric field enhanced the propagation velocity of the bubbles, triggering the quench-induced dynamic breakdown of LHe.