Two-dimensional (2-D) distributed feedback gives the opportunity to construct generators of a coherent electromagnetic wave flux with very wide cross-section. We study such mechanism of spatial synchronization of electromagnetic oscillations for the case of a planar free electron maser (FEM) at the ELMI-device (INP, Novosibirsk). The FEM-oscillator consists of two channels driven by two sheet electron beams (1 MeV/ 1 kA/ 5 μs). Each channel contains a hybrid two-mirror cavity that is composed of upstream 2-D and downstream 1-D Bragg reflectors. The transverse sizes of the reflectors essentially exceed the radiation wavelength. Nevertheless, we have experimentally demonstrated well reproducible single-mode mm-wave generation regime of each channel due to the 2-D distributed feedback. In the first experiments at weak interchannel diffraction coupling the radiation pulses with the following parameters: frequency ∼75 GHz, close to one of the cavity eigenmodes, spectrum width ∼20 MHz, pulse duration ∼200 ns and power-few tens of MW were obtained.
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