Mode competition in free-electron masers with oversized planar Bragg resonators

Abstract

The nonlinear dynamics of free-electron masers with planar Bragg resonators is studied. Here, in contrast to previous studies devoted to this class of self-oscillators, the transverse field structure is not fixed and consists of a set of modes of a regular planar waveguide. A criterion for the admissible oversize (the ratio of the gap between the plates forming the resonator to the wavelength) such that the stationary oscillation regime is stable under the variation of the electron beam parameters is obtained. It is demonstrated that, in the case of a large oversize, the synchronism mismatch variation due to the instability of the electron beam parameters should result in jumping of the oscillation frequency. It is found that different oscillation frequencies correspond to excitation of mode bunches with different transverse indices of partial waves. The region of synchronism mismatch corresponding to the excitation of the surface mode whose field localizes close to the corrugated surface is found. It is shown that, unlike regions of excitation of volume modes, this region is weakly sensitive to the gap between the plates.