Mechanism of free electron maser self-excitation using coupled propagating and trapped modes

A new scheme of feedback in a free-electron maser (FEM), which is based on the mutual scattering of propagating and quasi-cutoff (trapped) modes on a periodic Bragg structure, is considered. The trapped mode makes possible self-excitation of the system, while the energy exchange in a stationary generation regime is almost entirely determined by the interaction with a synchronous propagating wave. Using the proposed method, it is possible to improve mode selectivity with respect to the transverse index and accordingly decrease the working wavelength.