Optimization of mode converters for generating the fundamental TE01mode from TE06Gyrotron output at 140 GHz

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Abstract
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 HE11hybrid mode. The improved perturbation structures of the rippled-wall mode converters (with input- and output diameter D = 27.8 mm) were optimized by numerically integrating the proper coupled-wave differential equations. Three types of mode transformation schemes have been investigated: (a) step-by-step TE06 → TE05 → TE04 → TE03 → TE02 → TE01 conversion, (b) direct TE06 → TE01 conversion, and (c) two-step TE06 → TE04 → TE01 conversion. In all three cases the predicted conversion efficiency is approximately 98%. The type (a) transducer exhibits a relatively broad bandwidth but is very long (total length L = 4.2 m) whereas the type (b) transformer is short (L = 1.8 m) but has an extremely narrow bandwidth. The two-step converter (c) realizes a compromise with respect to bandwidth. The low-power experimental data for conversion scheme (a) at frequencies around 70 and 150 GHz are in excellent agreement with corresponding computed values.

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