3D SIMULATIONS ON OUTPUT POWER FLUCTUATION IN A SHORT BUNCH RF-LINAC FEL

A space-time dependent 3D simulation code has been developed in order to analyze the RF-linac FEL oscillator dynamics. Our simulation code e mployed both the transverse mode spectral method and the longitudinal finite difference method. The electron beam is modeled by a group of s uper particles which have a density profile in the time domain. In thi s model the electron beam is able to determine the energy spread and t he finite emittance. This simulation code enables us to describe the t ransverse mode competition and the slippage effects in the resonator c avity. In this paper, a high power infrared FEL with a short bunch ele ctron beam is investigated. The output power fluctuation with cavity d esynchronism is simulated with this code. Especially, we investigated the effects of the transverse mode competition, energy spread, and the finite emittance of the electron beam on the output fluctuation. Usin g FELIX parameters, the FEL oscillator is simulated for 300 passes. Th e output power oscillates periodically in the case of single transvers e mode and not in the case of multi-transverse modes. In a warm beam w ith multi-transverse modes, the emission is higher than that with a si ngle mode, and the optical pulse shape is almost the after 100 passes. Furthermore, the phase space motion of the laser field is periodic an d stable. As a result of the simulation, we recommend that high power infrared FEL operation should include multi-transverse modes in order to get higher emission and a more stable optical pulse.