Design and experiment of a far-infrared FEL

Abstract To design a FEL system, we have developed a three-dimensional simulation code including nonlinear effects such as interaction of electrons with laser fields. The code can calculate the motion of electrons and the variation of laser fields in a wiggler. It can simulate processes of bunching of electrons, and amplification and saturation of laser fields. This code, however, cannot include the effects of inhomogeneity of the wiggler field and beam misalignment. To estimate the gain reduction due to the above effects, we have also developed a linear simulation code based on Madey's theorem including the following effects: actual wiggler field, beam envelope, energy spread, and electron beam misalignment to the wiggler axis. The FEL system we have designed consists of a 20 MeV, 100 mA rf linac and a wiggler with 6 cm pitch, 28 periods and 2.5 kG peak field. The simulation results show that the FEL system has about 30% gain and several MW output power at 37 μm laser wavelength. Our FEL system has been completed and experiments have been started.