A Circular TE02 Mode Generator With High Purity for Gyro-TWT Study

A new topology to excite circular TE ₀₂ mode for studying of gyrotron traveling-wave tube (gyro-TWT) is presented. The mode generator consists of a rectangular TE ₁₀ to cross waveguide TE ₄₄ mode converter and a cross waveguide TE ₄₄ to circular TE ₀₂ mode converter. To obtain a high-purity TE ₀₂ mode output and make the generator compact, a cross waveguide with sidewall coupling apertures and a novel mode transition section with metal diaphragm are introduced. The proposed mode generator is fabricated and measured. A good agreement between simulation and measurement is obtained, and the 1-dB bandwidth from 28 to 34 GHz is obtained in back-to-back test. The angle-independent transmission demonstrates high mode purity and the presence of TE ₀₂ mode confirmed by the field pattern measurement. The mode exciter features high mode purity, broad bandwidth, and is easy to be fabricated, which can be applied to cold-test experiments of gyro-TWT components and the high power transmission line devices.