In a gyrotron with quasioptical mode convertor, apart from the fundamental mode, excitation of a mode with opposite azimuthal rotation of the field occurs due to the wave reflected from the remote load. Interaction of the two oppositely rotating modes is investigated by using the quasilinear equation for slowly varying amplitudes. The developed theory allows explanation of the experimental results where periodic modulation of the transverse structure of gyrotron radiation partially reflected from a remote oscillating membrane was observed.
