Response of superconducting cavities to high peak power

A technique to find the transient cavity Q from transmitted power is presented. This technique can facilitate finding the Q as a function of accelerating electric field for low power pulsed measurements, but it has a special application to analyze the thermal breakdown behavior during high peak power pulsing. With high power, in short time scales, the fields in a superconducting cavity can be driven well past the CW breakdown limit. With knowledge of the Q during breakdown, one can show that a large fraction of the surface was still superconducting as the cavity reached high fields. A lower bound to the critical RF magnetic field can then be determined. Results of pulsing a 1.3 GHz Nb cavity with 340 kW for 150 μs are presented. The Q extraction technique is used to measure a lower limit of H_c^RF over the range of 2 K to 8.3 K despite the presence of a thermal defect