Startup Methods for Single-Mode Gyrotron Operation

Experimental start-up results of a 118 GHz ${\mathrm{TE}}_{22,6}$ gyrotron are presented and compared with theory. The theoretical excitation regimes of competing modes are computed in the energy--velocity--pitch-angle plane. The start-up paths through the plane are determined by the time evolution of the beam parameters during the start-up phase and are seen to determine the cavity oscillating mode. Experimental results show that competition between the ${\mathrm{TE}}_{22,6}$ and ${\mathrm{TE}}_{\ensuremath{-}19,7}$ modes can be completely eliminated by using the proper start-up method in a case where a typical triode start-up results in oscillation in the competing ${\mathrm{TE}}_{\ensuremath{-}19,7}$ mode. These new results, applicable to any arbitrary gyrotron cavity mode, are shown to be in excellent agreement with theory.