Harmonic emission from high-power high-frequency gyrotrons

Abstract

Abstract The results of a study of second-harmonic emission from a gyrotron designed for high-power high-frequency operation at the fundamental of the cyclotron frequency are presented. Stable, very narrow bandwidth second-harmonic cavity emission from 209 GHz to 302 GHz has been observed. An output power of 25 kW and efficiency of 6·5% in the TE11, 2, 1, mode at 241 GHz is reported; this represents the highest power obtained to date from a high-frequency (> 100 GHz) harmonic gyrotron. These experiments have been carried out in a cavity for which the mode density is very high; the cavity diameter is approximately six free-space wavelengths for emission at the second harmonic. Mode competition between fundamental and second-harmonic modes is discussed. It is also shown that, in general, gyrotrons designed for high-power low-Q operation in overmoded cavities at the fundamental will also have high efficiencies and strong emission in second-harmonic modes. Prospects for high-frequency harmonic gyrotrons for plasma diagnostics and other applications are described