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Weakness of the electron-wave coupling in compact low-power terahertz-frequency-range gyrotrons due to relatively low electron beam currents and operation at high cyclotron harmonics results in the use of long-length cavities with high diffraction ${Q}$ -factors, and, therefore, with a high share of the ohmic loss of the radiated wave power. In this work, we demonstrate a possibility for stable operation at higher axial modes possessing relatively low diffraction ${Q}$ -factors. Enhanced efficiency of the electron-wave interaction is provided due to the excitation of higher axial modes in the traveling-wave-tube regime.

A method of improving the selectivity of open cavities of electron cyclotron masers (gyrotrons) operating at high cyclotron harmonics in the terahertz frequency range is proposed and described. This method is purely electrodynamical and based on a simple deformation of the cavity wall. The proposed approach paves the way for the increase in the gyrotron frequencies and for enhancing the selectivity of open cavities in other applications.

Recently, 30-keV/0.7-A continuous wave (CW) subterahertz large-orbit gyrotron was experimentally realized. In this experiment, two different modes at different (second and third) cyclotron harmonics were excited in the same cavity separately at slightly different operating magnetic fields. This work is dedicated to description and theoretical explanation of various regimes of competition of these two modes observed in the experiment.