Design of Quasi-Optical TE17,6 Mode Converter for High-Power 170-GHz Gyrotrons

Abstract

A compact quasi-optical (QO) mode converter for a high-performance gyrotron operating in $\text {TE}_{17,6}$ mode at 170GHz has been developed. This converter features a dimpled wall launcher and employs a two-stage perturbation method based on coupled mode theory, complemented by a specialized mirror system. MATLAB code was developed to analyze the electromagnetic field in the launcher wall. The radiation field emitted by the launcher is evaluated in free space, and the mirror system consists of a quasi-elliptical mirror, an elliptical mirror, and a parabolic mirror to ensure effective phase correction Through optimization of the radius, perturbation amplitude, and perturbation length, we successfully achieved efficient mode conversion and beam shaping within a compact 95-mm design. Comprehensive simulations using FEKO confirmed that this shortened launcher maintains high mode purity and low loss. Remarkably, its length is significantly shorter than other existing models and typical theoretical estimates, highlighting the effectiveness of parameter optimization in developing compact, high-performance launchers. The results indicate that the total length of the launcher is only 95 mm, with a cut length of 17 mm, comprising 17.8% of the total launcher length. Simulation findings reveal that, following phase correction, the scalar Gaussian mode content and vector Gaussian mode content at the output window reach 99.3% and 98.4%, respectively.