Design study of a far-infrared free electron laser with a 20 MeV RF linear accelerator

A FEL in the far-infrared region has been designed using a low energy RF linear accelerator. First we estimate a small signal gain from spontaneous emission using the Madey's theorem. In the calculation following effects are included: an actual field distribution (using a measured magnetic field), beam envelope in the phase space through the undulator, energy spread, and electron beam mis-alignment to the undulator axis. We have developed a code which can simulate three dimensional processes of the electron interaction with multi-mode laser fields in the undulator. From this code we could obtain the time dependent bunching process of electrons and amplification of the laser field. During the calculation we assume an electron beam of 20 MeV, 100 mA with a pulse length of 3 μs, and an undulator of 28 periods, 6 cm periodic length and 2.5 kG peak field. The results from these calculations show that the small-signal gain over 40 % can be obtained, but mis-alignment of the beam severely degrades the gain. The results also show that the output power of several MW can be obtained under the above conditions. Considering the simulation results, a FEL beam line was constructed and the beam size at the undulator was measured. And electrons were focused enough for the FEL experiment. (author)