High Azimuthal Mode Selectivity of a Cavity with Mode-Joining Corrugations for High-Harmonic Gyrotrons

Mode-joining longitudinal corrugations are studied as a means of high azimuthal mode selectivity for cavities of high-harmonic terahertz gyrotrons. Their number dictates the choice of the jointed operating mode, which has a form of strongly coupled co- and counter-rotating azimuthal harmonics. It is found that the distinctive feature of this mode is a weak dependence of eigenvalue and ohmic losses on corrugation size. First, this favors the use of mode-joining corrugations with variable depth for efficient suppression of all competing modes by both diffractive and ohmic losses in the gyrotron cavity. Second, this provides a good robustness of gyrotron performance against manufacturing errors in the size of corrugations and only a minor conversion of the operating mode to spurious modes at junctions of the corrugated cavity with smooth-walled waveguides. The beneficial properties of mode-joining corrugations are demonstrated by a cavity design for a gyrotron operated in the second-harmonic TE_±9,4 and third-harmonic TE_±18,4 modes at 398 GHz and 593 GHz, respectively.