Influence of reflections on mode-competition processes in a high-power multimode gyrotron

Abstract

Reflection of radiation from an output window may strongly affect operation of a gyrotron. In recent years, the control of gyrotron radiation by reflection has become a subject of active study. In this paper, we study the effect of reflection on mode competition in a multimode gyrotron. We consider the interaction of five modes with neighboring azimuthal indexes in a gyrotron with a fixed Gaussian axial field profile. We assume the mode spectrum to be nearly equidistant and set the spectral density parameter value as in high-power gyrotrons used for plasma heating. We analyze the domain of stability of the operating mode on the cyclotron frequency mismatch–normalized beam current parameter plane with and without the reflections. The possibility of suppressing spurious sideband modes and expanding the single-mode oscillation zone of the operating mode by injecting the reflected signal is demonstrated by numerical simulation. The most advantageous situation is when the designed mode is matched with the output window while the reflections of the parasitic modes are rather strong and the reflected waves return into the gyrotron resonator in the adverse phase reducing the Q-factors of the parasitic modes.