A prebunched FEL based on coherent transition radiation in the far-infrared region

A short-bunch beam of electrons of 150 MeV is led to an open resonator of semi-confocal type. The wavepackets of coherent transition radiation (TR), emitted from bunched electrons passing through mirrors of the resonator, go back and forth in the resonator and superpose on the subsequent bunches to stimulate coherent TR. The main mode is assigned as TEM01, and the intensity depends on the square of the beam current. The output intensity of the resonator is compared with the coherent spontaneous TR generated within the resonator to show amplification of radiation due to the stimulated emission.