This paper describes a highly selective electromagnetic cavity for a 30 GHz free electron maser (FEM). The cavity consists of two Bragg reflectors with overlapping reflection bands. Reflectors have been synthesized with the help of coupled waves method. For the sake of mode selectivity reflectors are separated by a gap which is wide in the wavelength scale. The operating mode in the interspace between the reflectors represents a Gaussian wavebeam of small diameter, so that its field is detached from the cavity walls, and the loss of radiation through the gap is negligible. At the same time, spurious modes undergo strong diffraction losses through the gap due to different transverse structure of the field. In the calculations, the Q-factor of the operating mode was shown to be at least three times as big as the Q-factors of all spurious modes within the Bragg band. The tested stainless still cavity contained a cylindrical middle-section covered with the absorbing material alsifer to emulate the gap. In low-power tests, the only high-Q mode was observed in the Bragg band, with the frequency of that mode corresponding to the calculated frequency of the operating mode. High-power testing of the cavity is planned in the nearest future
High-power free-electron maser operated in a two-mode frequency-multiplying regime
