RF design of a compact 648 MHz single spoke cavity at CSNS

Purpose In order to increase CSNS beam power from 100 to 500 kW, the linac injection energy needs to be increased from 80 to 300 MeV. The combined layout of superconducting spoke cavities and elliptical cavities will be adopted to accelerate H− beam to 300 MeV. In this paper, a compact single spoke cavity with Φ50-mm beam aperture was proposed, with the RF performance of the spoke cavity optimized to Ep/Eacc < 5, Bp/Eacc < 9, as well as the good multipacting behavior.