Abstract
This paper describes the breakdown characteristics of liquid helium (LHe) caused by quench of a superconducting wire exposed to a high electric field. As the thermal energy injected into the superconducting wire after the quench-onset increased, the quench-induced dynamic breakdown of LHe took place at lower stress than the static breakdown limit without the quench. There existed a delay time Td of the order of ms from the quench-onset to the dynamic breakdown. At small gap lengths, Td decreased under the higher stress irrespective of the thermal energy, while at large gap lengths, Td reduced with increasing the thermal energy and the electric field strength as well. Thermal bubble behavior was observed using a fiberscope inserted into the FRP dewar and a high speed video system, and it was also analyzed numerically. By these investigations, it was found that the dynamic breakdown of LHe have two different mechanisms: (1) small gap breakdown and (2) large gap breakdown. The small gap breakdown occurs when thermal bubbles reach the plane electrode above the superconducting wire. The large gap breakdown takes place after thermal bubbles rebound from the plane electrode into the gap space.
