Improving selectivity of free electron maser with 1D Bragg resonator using coupling of propagating and trapped waves

A novel scheme of the Bragg free-electron maser (free-electron laser) [FEM (FEL)] based on the coupling of an amplified propagating wave with a quasicutoff mode trapped inside the cavity is considered. The cutoff mode is essential for FEM self-excitation while energy extraction from the electron beam in the steady-state oscillation regime is almost completely determined by the propagating mode, synchronous to the beam. The main advantage of the discussed scheme over the traditional scheme of Bragg FEM is improving selectivity over the transverse index. Based on the proposed feedback scheme the advance of the JINR-IAP FEM to the shorter wavelength is discussed.