Analysis and simulation of optical cavity in CAEP far-infrared FEL

The waveguide is used in the whole optical cavity of CAEP far-infrared FEL. The influence of waveguide on the net gain of the optical cavity is estimated by calculating the transverse coupling factor between the optical beam and electron beam. The influence of the waveguide is simulated using 3D code. The simulation experiments have been done by reducing the wave guide gap and enhancing field intensity to increase the optical gain. The results show that the gain can be increased by 7%~15% when the wave-guide gap is reduced by 2 mm, and the net gain can be increased almost by 5%~10% when the magnetic field intensity is enhanced by 0.02 T.