Simulations and Measurements of Selective Properties of Rectangular Grooves in Open Cavities

The mode selective properties of open cavity can be improved by a rectangular groove with specially selected sizes made in resonator wall parallel to electric field. This method is attractive for use in gyrotrons to obtain single-mode operation at high cyclotron harmonics. The influence of a resonant scattering element in the form of a rectangular groove on the mode Q-factor of open resonator is theoretically and experimentally studied. The analytical expressions for groove optimal dimensions are obtained for both Fabry-Perot two-mirror cavity and cylindrical open cavity of the gyrotron. The groove can provide mode selection when its width is between 1/2 and 3/2 of operating mode wavelength. The analytical formulas are confirmed by numerical simulations based on FDTD method. Both the theoretical formula and the simplified calculation method allow the fast design of resonators with sparse mode spectrum. The calculations are verified experimentally. The mirrors with grooves with various widths with design frequency of 135GHz were irradiated by Gaussian beams formed by corrgugated horn and 3D-printed plastic lens, and the reflection coefficients were measured. The obtained experimental data is in good agreement with theoretical and numerical models. The method promises the capability of selective excitation of high cyclotron harmonics in sub-terahertz gyrotrons with oversized cavities.