LASING WITHOUT INVERSION IN CHERENKOV FREE-ELECTRON LASERS

Quantum theory is applied to interference of emission and absorption i n a two-section Cerenkov free-electron laser (FEL). A scheme for absor ption cancellation with enhanced gain, which resembles ''lasing withou t inversion'' in atomic systems, is proposed. This scheme is based on the adjustment of the phases of electrons and light by appropriate dis persion between the two sections. It enables FEL operation even with a broad electron momentum spread. Such a scheme is interesting in the c ontext of EEL operation in the ultraviolet and X-ray regimes.