Start-to-end simulation for a compact terahertz free electron laser with a beam chopper system
摘要
A thermionic gun is endowed with a long bunch tail, which presents challenges for the compact terahertz free electron laser (FEL) facility at the Huazhong University of Science and Technology. Owing to a large energy spread, the tail particles do not contribute to the radiation. In the original design, an x-direction slit is used in the dispersive section of the transport line to remove the tail particles. This paper presents an improved scheme to remove the tail by introducing an RF beam chopper system at the exit of the electron gun, to prevent a significant number of tail particles from entering the linac. The facility remains compact while effectively removing the tail of the bunch. The parameters of the beam chopper system are designed. Bunch parameters and radiation performance are analyzed via a start-to-end simulation. The findings indicate that 43% of the particles can pass through the beam chopper system for subsequent acceleration and transport, which saves the RF power, reduces beam loss in the linac, reduces background noise, and suppresses the sideband instability. Simultaneously, the beam chopper system causes an increase in beam emittance, energy spread, and an offset in the center of the bunch. These effects can be mitigated by a solenoid, linac, and steering coils. The simulation results for the FEL show that the micro-pulse energy is greater than 1.1