Mode-Converting Corrugations for Cavities of Second-Harmonic Gyrotrons with Improved Performance
A new method of improving mode selection in cavities of sub-terahertz second-harmonic gyrotrons is investigated. As an example, a second-harmonic gyrotron with frequency of 0.3 THz is considered. The gyrotron is designed for collective Thomson scattering (CTS) diagnostics of fusion plasmas and has a limited output power due to competition between the operating TE13,2 mode and first-harmonic modes. For suppression of the first-harmonic competing modes periodic longitudinal corrugations are used in the gyrotron cavity. Such corrugations can induce coupling of the normal cavity modes known as azimuthal Bloch harmonics. The corrugation depth is set close to the half- and quarter-wavelength of the operating second-harmonic mode and competing first-harmonic modes, respectively. Under this condition, longitudinal corrugations of the cavity generally have only a slight effect on the operating mode, but can initiate strong conversion of the competing modes to high-order Bloch harmonics. The full-wave method of coupled azimuthal harmonics is applied to investigate the influence of dimensions of the corrugated gyrotron cavity on eigenvalues, ohmic losses and beam-wave coupling coefficients for the operating TE13,2 mode and the most dangerous competing modes. Using the self-consistent theory of beam interaction with the operating and competing modes, the most optimal parameters are found for a gyrotron cavity with mode-converting corrugations, which ensure the widest range of a single mode operation for the 0.3-THz second-harmonic gyrotron. It is shown that, in this range, the gyrotron output power can be increased from 100 kW to 180 kW, as required by CTS plasma diagnostics. It is found that output mode purity of the 0.3-THz second-harmonic gyrotron falls off due to mode-converting corrugations, which induce undesirable coupling of the operating TE13,2 mode with neighboring Bloch harmonics in the output section of the gyrotron cavity.
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