Nonlinear excitation of parasitic modes in harmonic gyrotrons

The investigation of interaction between modes resonant with electrons at different cyclotron harmonics is presented. The study is focused on mode interaction in the region of currents lower than the start current of the parasitic mode at the fundamental. In this region, the excitation of such parasitic modes is possible due to nonlinear excitation of them by the harmonic modes only. It is shown that, when the beam current increases, various oscillation regimes replace each other: when the current exceeds the start current of the harmonic mode only slightly, the single-frequency oscillations of this mode are stable. At higher currents, due to nonlinear excitation, the oscillations of both modes with constant amplitudes are possible. Then, this regime is replaced by the auto-modulation of two modes with periodically varying mode amplitudes. Finally, at even higher currents, the parasitic mode at the fundamental suppresses the parental second harmonic mode. The role of two saturation mechanisms (the orbital phase bunching and the depletion of the free rotational energy in the process of beam-wave interaction) in the nonlinear excitation of parasitic modes at the fundamental is analyzed. The parallels between this set of events and events that take place in the prey-predator theory are discussed.