Dynamic breakdown characteristics of liquid helium caused by quench phenomena of ac superconducting wire

We investigated the dynamic breakdown characteristics of liquid helium (LHe) caused by quench phenomena of superconducting wire. Dynamic breakdown voltage V_D caused by quench was measured as well as static breakdown voltage V_s for LHe. Experimental results revealed that VD fell down drastically to at most one-tenth of V_s , and increased exponentially with the gap length. V_D also proved to be independent of the polarity of the applied dc and ac voltages. It was also shown that the delay time T_d of the order of ms existed from the onset of the quench to breakdown. T_d decreased exponentially with the applied voltage and increased with the gap length. Scattering of T_d was large at larger gap length. T_d was also insensitive to the polarity difference of the applied voltage. We also observed development of thermal bubbles in LHe generated by the quench using a fiberscope inserted in an FRP dewar. The fiberscope observation clarified that bubbles emerged and propagated in the gap space after the onset of the quench wire. The bubble behavior under the high electric field stress proved to be differ in the bubble propagation from that without stress, and could trigger dynamic breakdown