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

For farinfrared FEL, it was found that the diffractive loss is so large that it is half of total loss due to the longer wavelength using holecoupled resonator. By changing the size of the hole, the diffractive loss can't be reduced efficiently. To improve the quality of resonator, the simulation and design of the CAEP FIR FEL with inserted scraper output coupler had been made with the help of our 3D FEL oscillator code. It is proved that the inserted scraper is effective and applicable for the FIR FEL. Using inserted scraper output coupler, the optical cavity Q can be tuned freely. 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.