Broadband Nonuniform Terahertz Multimode Conversion Series with Compactness and Pure Pattern

In our 0.22 THz gyrotron system, the overmoded conversion sequence is the critical component to convert the TE 03 output mode of the gyrotron oscillator to the TE 10 input mode of the gyrotron traveling wave amplifier (gyro-TWA). However, with the increase of the gyrotron’s operating frequency and output power, the output waveguide generally adopts highly overmoded structures, increasing the longitudinal size and narrowing the bandwidth of the traditional mode converter with uniform period and radial perturbation. Due to a much more significant coupling strength factor, a faster power increment in target modes TE 02 and TE 01 causes the axial length to decrease significantly in the proposed converters with nonuniform double perturbations. In our numerical and simulation results, compared to the traditional uniform-ripple-wall converters, the proposed nonuniform circular TE 03 -TE 02 -TE 01 converter shortens 44.18% of the total length and increases the bandwidth over 95% conversion efficiency by 150%. Meanwhile, we employed a sidewall converter other than the inline converter to complete the CWG TE 01 -RWG TE 10 conversion for its high efficiency and compactness. Based on the operation principle of the monostatic radar imaging system, the mode pattern measurement corroborates the desired electric field distributions near the waveguide port. The experimental results indicate the proposed mode conversion sequence has advantages of high conversion efficiency over 80%, low loss, and compact structure.