Generation of powerful narrow-band 75-GHz radiation in a free-electron maser with two-dimensional distributed feedback

A planar generator based on a free-electron maser (FEM generator) is developed employing a relativistic sheet electron beam (0.8 MeV/1 kA/4 $\mu$s) formed by an ELMI accelerator. A hybrid two-mirror resonator consisting of a two-dimensional upstream and a one-dimensional downstream Bragg reflectors is used as the electrodynamic system. The use of two-dimensional distributed feedback implemented in the upstream Bragg structure has made it possible to achieve stable well reproducible single mode generation regime with transverse dimensions of the system of $\sim$25 $\times$ 2.5 wavelengths. Experiments carried out at a frequency of 75 GHz have yielded narrow-band radiation with spectrum width of $\sim$20 MHz in pulses with a duration of $\sim$100–200 ns and power of 30–50 MW.