DYNAMIC CHARACTERISTICS OF LIQUID-HELIUM BREAKDOWN INDUCED BY QUENCH PHENOMENA OF AC SUPERCONDUCTING WIRE

We measured dynamic breakdown voltage V(D) of liquid helium (LHe) indu ced by quench phenomena of a superconducting wire under high electric field stress. V(D) was defined as the minimum value of the applied vol tage below which the breakdown was no longer induced with the quench. Experimental results revealed that V(D) fell down drastically compared with the static breakdown voltage V(S) without the quench. We also fo und that the delay time T(d) of millisecond order existed from quench onset to breakdown. V(D) and T(d) depended on the gap length. Moreover , we observed the behavior of thermal bubbles in LHe which were genera ted after the quench onset using a fiberscope monitoring system.