Circular TE₀ₙMode Content Analysis Technique for High-Power Gyro-TWT System

Abstract

A circular TE_0<i>n</i> mode content analysis technique based on the multi-mode coupler with high mode selectivity is proposed. To improve the mode selectivity, a deformed waveguide, entitled the quad-polar waveguide, which discriminates the azimuthally symmetric TE-mode on the waveguide wall, is introduced. The influence of deformation on the TE₀₁ and TE₀₂ modes&#x2019; field pattern, the corresponding transmission loss, and the power capacity in the quad-polar waveguide are analyzed. Based on the analysis, a dual-mode coupler of the circular TE₀₁ and TE₀₂ modes, consisting of an oversized deformed circular waveguide coupler and a pair of circular-to-quad-polar waveguide transitions, is designed for a high-power <i>Ka</i>-band gyrotron traveling-wave tube system. To verify the high mode selectivity of a quad-polar waveguide, a simple coupling with equal-radius and equal-interval holes are used. A prototype is fabricated and measured, and the results show that the coupling coefficient is &#x2212;40 dB with the directivity of 27 dB for the TE₀₁ mode, suppressing the unwanted mode TE₀₂ mode with 53 dB. For the TE₀₂ mode, the coupling coefficient is &#x2212;40 dB with the directivity of 25 dB and suppresses the unwanted mode TE₀₁ mode with 75 dB from 30 to 40 GHz.