The input coupler is an important component for a microwave amplifier. In this paper, a side-wall single-hole input coupler for a W-band gyrotron travelling wave amplifier (gyro-TWA) that operates at the frequency range of 90-100 GHz was designed and measured. Instead of using a cutoff waveguide, a broadband Bragg-type reflector with a small spread in phase was optimized for use as part of the input coupler. The minimum radius of the reflector was two times the size of a cutoff waveguide which reduced the possibility for some of the beam electrons being collected in this section and lost to the amplifier interaction region.
The design of two fundamental mode rectangularto-circular waveguide input couplers for a low-terahertz gyrotron-traveling wave amplifier (gyro-TWA) is presented. A T-junction input coupler with a Bragg reflector and a multiplehole directional coupler were optimized for operation between 360 and 384 GHz, the proposed gyro-TWA bandwidth. The T-junction coupler and the multiple-hole coupler achieved the respective bandwidths of 10% and 35%. The benefits and potential limitations of the low-terahertz wave coupler topologies are discussed alongside the challenging manufacturing methods of the submillimeter-wave components.Index Terms-Gyrotron-traveling wave amplifier (gyro-TWA), mode coupling, terahertz input coupler, waveguide coupler. Jason R. Garner received the B.Sc. (Hons.) degree in physics and the M.Sc. degree in high-power radio frequency science and engineering from the University of Strathclyde, Glasgow, U.K., in 2010 and 2012, respectively, where he is currently pursuing the Ph.D. degree with the Scottish University Physics Alliance.His current research interests include the design of passive components for millimeter-wave gyro-amplifiers.
The design of a higher order mode (HOM) input coupler for a low-terahertz gyrotron travelling wave amplifier is presented. A two-branch waveguide coupler based on the even distribution of incident power is designed to couple the rectangular TE 10 mode to the circular TE 61 mode. The optimised tapered waveguide input coupler achieved an operating frequency range of 359-385 GHz, equating to a bandwidth of 7%. A prototype waveguide coupler scaled to W -band (75-110 GHz) was manufactured. The vector network analyser measured return loss of the HOM coupler is shown to be <10 dB at 90-96 GHz. A transmission and phase measurement is presented to analyse TE 61 mode conversion in the coupler.Index Terms-Gyrotron travelling wave amplifier (gyro-TWA), mode coupling, terahertz input coupler, waveguide coupler.
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