Simulation study of the long range wakefield induced multi-bunch instability in the China X-ray free electron laser

Abstract

A 1.3GHz superconducting accelerator (SC) which can generate 32 bunches (0.2nC/bunch) with an interval of 2.3ns has been designed for the China free electron laser (CXFEL), the effects of the long range wakefield in the superconducting cavities are analyzed in this paper. In our simulation results, the fundamental mode of the longitudinal wakefield (beam loading) may result in an energy droop of 844eV/cavity to the bunch train, and may also disturb the bunch compression process by altering the energy chirp. The effects of the transverse wakefield are studied by injecting all bunches with an initial position error of 200µm, and also by applying a random misalignment of σΔx,y=100µm (in Gaussian distribution) to the cavities and the focusing quadrupoles, then the trajectory of the bunch train is corrected with horizontal and vertical correctors. In both cases the transverse wakefield induced bunch to bunch variation are negligible, whereas the misalignment may result in emittance degradation to all bunches.