Simulation of the resonator with inserted scraper output coupler for the CAEP far-infrared FEL

To improve the quality of the resonator, the simulation and design of the CAEP FIR FEL with inserted scraper output coupler has been made using our 3D FEL oscillator code. Using inserted scraper output coupler, the optical cavity Q can be tuned conveniently. The transverse modes of the optical cavity are calculated. The saturated power, the net gain and the coupling efficiency are evaluated as a function of the scraper radius and the distance between scraper and the center of mirror.