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.

A 1.5-dimensional model of Compton/Raman free-electron laser (FEL) amplifiers with helical wigglers has been composed by using the helical-trajectory approach. This model has been used to analyze results of earlier experimental investigations of Compton/Raman millimeter-wavelength FEL amplifiers. The results of the 1.5D simulation are in reasonable agreement with 3D simulation and experimental results obtained in the regimes with reversed and zero guide magnetic fields. Thus a convenient tool has been developed to determine basic amplification parameters for possible experiments in the future.

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