Mechanism of self-excitation of an FEM oscillator based on coupling of propagating and trapped waves

In the paper FEM oscillator operation based on coupling of quasi cut-off mode with propagating wave which is amplified by the electron is considered. The coupling is realized by periodical corrugation of side wall of microwave system. The cut-off mode provides the feedback while the efficiency in steady-state regime of generation is almost completely determined by the propagating mode, synchronous to the beam. The main advantage of a discussed scheme is provision of higher selectivity over transverse index than traditional scheme of FEM with a Bragg resonator. The novel feedback scheme should be tested on a JINR- IAP FEM as a method of increasing operating frequency.