The China Spallation Neutron Source (CSNS) is currently undergoing an upgrade to increase its beam power from 100 kW to 500 kW. As part of this enhancement, the linear accelerator's beam energy will be boosted from 80 MeV to 300 MeV, and the beam current will be raised from 10 mA to 40 mA. To achieve the higher beam energy, 52 superconducting cavities will be added following the drift tube linac. However, these new cavities are expected to increase both energy jitter and energy spread, potentially leading to significant beam loss during injection into the RCS ring. Therefore, it is crucial to carefully manage the energy jitter and energy spread. This paper first presents simulations of the beam energy jitter and energy spread at the end of the superconducting section with dynamic beam and cavities errors. Subsequently, it introduces a comparison of various compression schemes aimed at reducing energy jitter and energy spread.

For the China Spallation Neutron Source (CSNS), the rapid cycling synchrotron (RCS) accumulates and accelerates the injection beam to the design energy of 1.6 GeV and then extracts the high energy beam to the target. In this paper, firstly, the beam commissioning of the RCS have been comprehensively studied, including new injection system commissioning, longitudinal dynamics optimization, beam instability mitigation, tune optimization, closed orbit correction, beam loss optimization, bayesian optimization and so on. In order to meet the requirements of beam power increase and stable operation of the CSNS accelerator, the RCS beam losses from different sources are studied and optimized. With the aid of weekly radiation dose measurement, the hot spots of the RCS are studied in depth to explore the causes and find the solutions. Secondly, as the second phase of the CSNS, CSNS-II will achieve a beam power on the target of 500 kW. The injection energy of CSNS-II will be increased from 80 MeV to 300 MeV and the injection beam power will be increased about 20 times. In this paper, the challenges and solutions of the CSNS-II RCS will be introduced and the upgrade of the RCS will be studied. Based on the detailed simulation results and beam experimental results, the upgrade schemes of the critical systems for the CSNS RCS has been proven feasible.