Simulations of Competition of Fundamental and Second Cyclotron Harmonics in a Sub-Terahertz Gyrotron

The implementation of sub-terahertz gyrotrons operating at high cyclotron harmonics inevitably involves the use of oversized operating cavities having dense mode spectra. This leads to a problem of the mode selectivity. This article is based on our work aimed to creation of a sub-THz CW gyrotron operating at the second cyclotron harmonic. We give a detailed picture of competition between the operating wave and a parasitic fundamental-harmonic wave in the case when these two waves have similar starting currents and operating magnetic fields, so that the operating mode can be excited in a fairly narrow range of parameters. Two-mode simulations show that in such a situation the parasitic wave can be excited at currents below the starting threshold predicted by the single-wave theory.