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 lliptical cavities will be adopted to accelerate H-beam to 300 MeV.In this paper,a compact single spoke cavity withφ50-mm beam aperture was proposed,with the RF performance of the spoke cavity optimized to E_(p)/E_(acc).<5,E_(p)/E_(acc)<9,as well as the good multipacting behavior.Methods CST Microwave Studio was used to optimize the RF performance(E_(peak)/E_(acc),B_(peak)/E_(acc) R/Q)with cavity geometry parameters.After electromagnetic design was finished,multipacting behavior was studied with CST Particle Studio module,and cavity shape modification was made to get a reasonable<SEY>value.Results The simulation got optimum results of E_(peak)/E_(acc)=4.97 and B_(peak)/E_((acc)=8.42 mT/(MV/m),and the<SEY>max is 1.52at E_(acc)=7.74 MV/m.Conclusion A 648 MHzβ=0.4 single spoke cavity with beam apertureФ50 mm was proposed in the paper.This compact cavity may be a very promising option for CSNS linac upgrading.In addition to the optimization of the RF parameters,the multipacting behavior of the cavity is also studied,and cavity shape is optimized to reduce<SEY>value.