Optical Diffraction in a Free-Electron Laser Oscillator with Two Electron Beams

The effects of the optical diffraction, the electron beam's emittance, energy spreads, and the higher-order modes in a free-electron laser (FEL) oscillator have been analyzed by using two electron beams of different energies based on the proposed FEL facility which is to be operated in the far-infrared and the infrared regions. The three-dimensional (3D) effects in a free-electron laser oscillator due to the diffraction, the electron beam's emittance, energy spreads, and higher-order modes were studied using an extended three-dimensional FEL code with two electron beams that we developed. The effects of the variation in the amplitude of the radiation on the electron beam's emittance and energy spreads were also calculated in the case with optical diffraction for the multiparticle and the multi-pass numbers by using a new 3D code. The mode construction was studied on the higher-order modes of the wiggler for various optical diffraction parameters for determining the FEL's performance, which is required for high-quality electron beam. The intensity of the radiation field for the higher order modes was highly sensitive to the diffraction parameter of B > 2.5, but was less sensitive to the emittance and the energy spread for B < 1.5 in the coupled two-beam oscillator.