Injector optimization for the IR-FEL operation at the compact ERL

KEK's Compact (cERL) is a test accelerator for developing ERL technology and operating with high average beam current and high beam quality. The cERL consists of an injector using a photocathode electron gun, a superconducting accelerating cavity (main linac) for energy recovery, a recirculation loop, and a beam dump. As one of the industrial applications of cERL, the upgrade to cERL mid-infrared free electron laser (IR-FEL) was carried out, but new conditions required for FEL oscillation were imposed on the injector. The beam conditions of the injector required for FEL oscillation are to minimize the longitudinal RMS emittance for a beam with a bunch charge of 60 pC, and to have an RMS bunch length of 2 ps or less and a transverse RMS emittance of 3 π mm mrad. The beam transport conditions from the electron gun to the main cavity were optimized using the particle tracking code GPT including the space charge effect. In this paper, we report on the strategies and results of injector optimization to improve the performance of cERL-FEL. (author)