Self-modulation instability of gyrotron emission

A description is given of a method for analyzing the conditions of self-modulation instability (SMI) in a wide class of electron masers with resonators having a nearly equidistant spectrum of natural frequencies. It is shown that stable high-efficiency single-mode oscillation can be achieved in a gyrotron with an arbitrarily dense spectrum of competing modes. However, in the case of an arbitrary frequency distribution of competing modes, SMI arises if the magnitude of the beam current is more than twice that of the efficiency-optimal current under conditions of high-efficiency single-mode oscillation.