Optimization of Mode Converters for Generating the Fundamental Te(06) Gyrotron Output at 140 GHz

摘要

Oversized high-power mode converters were designed to transform the TE06 output mode of a 140 GHz gyrotron into the fundamental circular symmetric TE01 mode for low-loss propagation and further conversion to the perfectly linearly polarized quasi-optical HE11 hybrid mode. The improved perturbation structures of the rippled-wall mode converters were optimized by numerically integrating the proper coupled-wave differential equations. Three types of mode transformation schemes were investigated: (1) step-by-step conversion, (2) direct conversion, and (3) two-step conversion. In all cases the predicted conversion efficiency is 98%. Type (1) transducer exhibits a relatively broad bandwidth but is very long (total length L = 4.2 m) whereas the type (2) transformer is short (L = 1.8 m) but has an extremely narrow bandwidth. The two-step converter (3) realizes a compromise with respect to bandwidth. The low-power experimental data for conversion scheme (1) at frequencies 70 and 150 GHz are in excellent agreement with computed values.