Design considerations for powerful continuous-wave second-Cyclotron-harmonic coaxial-cavity gyrotrons

The mode-selection ability of coaxial gyrotron cavities with a longitudinally corrugated inner conductor is systematically examined, as regards powerful, continuous-wave (CW) operation at the second harmonic of the electron-cyclotron frequency. Suitable cavity geometries for the excitation of a high-order mode of eigenvalue around 100 at the second harmonic have been identified, employing coaxial inserts both of high and of low conductivity. Focusing on submillimeter-wave radiation, we investigate frequency and CW limitations on efficiency, output power, and cavity machining and we present pertinent design considerations. The feasibility of powerful and efficient, CW, submillimeter-wave operation of coaxial cavities at the second harmonic is illustrated by two realistic designs of a second-harmonic gyrotron delivering a power over 100 kW CW at 340 GHz with electronic efficiency around 29%. The performance of the designs is confirmed by independent numerical simulations, which in addition show weak sensitivity to a spread in the electron velocity ratio.