Higher harmonic voltage analysis of magnetic-alloy cavity for CSNS/RCS upgrade project

Background A rapid cycling synchrotron (RCS) in China Spallation Neutron Source (CSNS) would be upgraded, and the proton beam power will reach 200–500 kW. Three RF cavities loaded by magnetic-alloy (MA) material will be added in RCS tunnel to form a dual-harmonic RF system with eight ferrite-loaded cavities in RCS. The bandwidth of the MA cavity is wide because of its intrinsic Q value of less than 1. Higher harmonics can be excited in MA cavities, because the RF power is directly fed into the MA cavity from the final stage of the RF power amplifier. The distortion of voltage in the cavity gap caused by higher harmonics may deteriorate the beam quality. The influence of the higher harmonics to beam needs to be evaluated and the amplitude of higher harmonics should to be suppressed. Methods We built a mathematical model with two functions based on the RF bucket acceptance and bunch factor to evaluate this influence. Groups of harmful amplitudes and phases of higher harmonics were found according to the mathematical model by using multi-objective particle swarm optimization (MOPSO). Conclusion The dual-harmonic RF system with higher harmonics in CSNS/RCS and a suitable harmonic suppression for MA cavities was discussed according to the results of MOPSO and the beam dynamics simulations.