Impedances and beam instability in RCS/CSNS

The rapid cycling synchrotron of the China Spallation Neutron Source is a high intensity proton accelerator, with an average beam power of 100 kW. The collective effects caused by the coupling impedances from the vacuum components may be the limit to beam power. The impedances from various kinds of components were investigated and the impedance budgets were summarized. According to the summarized impedances, the thresholds for instability were estimated. Because of the serious instability from the resistive wall impedance, a simple resistive-wall wake-field model was used to track the bunch oscillation, and the growth rate was derived from simulation which was consistent with analysis. When the natural chromaticity which helps to stabilize the beam is considered in the simulation, the instability of resistive wall will disappear. (authors)