Multipacting study for fundamental power coupler of a superconducting cavity

The research work of Hefei Advanced Light Facility (HALF) is in progress. A 499.8 MHz superconducting cavity will be used in the HALF RF system. The preliminary RF design and thermal analysis for the fundamental power coupler (FPC) of HALF 499.8 MHz superconducting cavity have been completed. Multipacting (MP) is one of the major causes of the breakdown in many high power input couplers and RF cavities, thus, MP simulation for our FPC is necessary. In this paper, the MP simulation for the FPC was performed by using the CST. The MP performance of the FPC was studied. The formation process of the MP in the FPC was investigated and the physical mechanism of the delay of the MP establishment was analyzed. The simulation results show that MP will occur when the power level is higher than 175 kW, and the MP location is near the ceramic. Therefore, MP suppression is required in the FPC. In order to suppress the MP, the effect of the biasing DC electric voltage to MP in the FPC was analyzed in this paper. The simulation results indicates that the biasing DC electric voltage not only can suppress the MP, but also can excites new MP in the coupler. Thus, it is crucial to select the appropriate biasing DC electric voltage for the MP suppression in the coupler. The results show that the MP in the FPC can be suppressed very well by using 2 kV or 3 kV biasing DC electric voltage.