A new type of high-selective Bragg resonator having a step of corrugation inside the interaction region was used as a microwave system for a free-electron maser (FEM). Using a LINAC LIU-3000 (1 MeV/200 A/200 ns) to drive the FEM oscillator, a single-mode single-frequency operation was achieved at a frequency of 30.74 GHz with an output power of about 50 MW, which corresponded to a record efficiency of 35% for a millimeter wavelength FEM.

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A tunable planar narrow-band Bragg reflector based on coupling of the two propagating modes and a cutoff mode is considered. Coupled-wave analysis together with direct numerical simulations demonstrate operation of the proposed scheme up to the terahertz frequency band. Compatibility with the transportation of an intense electron beam encourages the use of a novel Bragg reflector in powerful long-pulse free electron lasers.

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