Beam coupling impedance and collective effects in rapid cycling synchrotron of CSNS-II

Abstract The Rapid Cycling Synchrotron (RCS) functions as the principal accelerator for the China Spallation Neutron Source Phase II project (CSNS-II). Designed for a beam power of 500 kW, the RCS achieves the highest circulating beam current worldwide at this power level. The impedance has been upgraded in detail based on the findings outlined in the reference [1] during the design phase of CSNS, particularly regarding the new critical components in the RCS of CSNS-II, which are attributed to coupled impedance. This paper presents a systematic investigation of the impedances of vacuum components through analytical calculations, numerical simulations, and bench measurements, developing a comprehensive impedance model for the RCS. By utilizing this model, we conduct a thorough evaluation of potential beam instabilities within the ring, identifying impedance thresholds and growth rates for various instabilities, with special emphasis on critical instabilities, which are rigorously analyzed. The bunch length stretching induced by magnetic alloy cavities is extensively simulated, and a range of effective suppression techniques is proposed to mitigate transverse coupled-bunch instability, thereby facilitating the successful realization of the design objectives.