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A powerful spatially extended planar Cherenkov maser project operating in ${W}$ -band is under development in collaboration between Budker Institute of Nuclear Physics of the Russian Academy of Sciences (BINP RAS) (Novosibirsk) and Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS) (Nizhny Novgorod). The ELMI accelerator (1 MeV/5–7 kA/ $3~\mu \text{s}$) forms a sheet electron beam with the transverse size (width) of up to 18 cm, which serves to drive the oscillator. The electrodynamic system of this Cherenkov maser is based on doubly periodical structure, which combines the properties of a slow wave system that realizes conditions for an effective Cherenkov interaction with a high-current rectilinear sheet electron beam and a high- ${Q}$ resonator that implements the mechanism of two-dimensional distributed feedback and provides selective excitation of the operating mode in the strongly oversized interaction space. In this article, the crucial elements and design parameters of the oscillator are discussed, and the results of simulations are presented to demonstrate the possibility of achieving a stable narrow-band generation regime for a transverse system size reaching about 50 wavelengths and a gigawatt output power level. To provide a single-directed output of radiation, a planar Bragg reflector is elaborated for installation at the cathode side of the interaction space. A mode converter formed by a slowly tapered waveguide section for transformation of the radiated wavebeam into a Gaussian-type wavebeam was designed to be installed at the oscillator output. In the electron-optical experiments, the formation of a large-width sheet electron beam with parameters acceptable to drive designed ${W}$ -band Cherenkov maser is demonstrated. [ABSTRACT FROM AUTHOR]

Two-dimensional distributed feedback is an effective method of producing ultrahigh-power spatially coherent radiation from an active medium, that is spatially extended along two coordinates, including relativistic electron beams with sheet and annular geometry. This paper describes the progress in the investigations of planar and coaxial free-electron masers (FEMs) based on a novel feedback mechanism. The theoretical analysis of these FEM schemes was conducted in the frame of the coupled-wave approach and 3-D simulations and agrees well with the experimental data obtained in "cold" and "hot" tests. As a result, the effective transverse (azimuthal) mode selection has been demonstrated under a transverse size of about 20-25 wavelengths, and narrow-frequency multimegawatt microwave pulses have been generated in the Kaand W-bands. [ABSTRACT FROM AUTHOR]