| Summary: | The input coupler is an important component for a microwave amplifier. A sidewall single-hole input coupler for a W-band gyrotron travelling-wave amplifier 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, simulation, construction and measurement of a W-band waveguide input coupler that used a backstop Bragg reflector that allowed for high electron beam transmission from a cusp electron beam source into a 3-fold helically corrugated interaction waveguide (HCIW) of a gyrotron travelling wave amplifier is presented. Experimental results are reported of a broadband, high power, gyrotron traveling wave amplifier (gyro-TWA) operating in the 75-110 GHz frequency band. The second harmonic cyclotron mode of a 55 keV, 1.5 A, axis-encircling electron beam was used to resonantly interact with a traveling TE21-like eigenwave, achieving broadband amplification. The gyro-TWA demonstrated a 3 dB gain bandwidth of at least 5.5 GHz in the experimental measurement with 9 GHz predicted for a wideband drive source with a measured unsaturated output power of 3.4 kW and gain of 36-38 dB. The inclusion of the Bragg reflector in the W-band waveguide input coupler enabled efficient (4.4%) beam-wave interaction to be achieved within the gyro-TWA. The design of 1) a T-junction fundamental mode rectangular to- circular waveguide input coupler, 2) a multiple-hole input coupler and 3) a higher order mode (HOM) input coupler for a low-terahertz gyrotron travelling wave amplifier were studied. The T-junction input coupler with a Bragg reflector and a multiple-hole directional coupler were optimized for operation between 360 and 384 GHz, the proposed bandwidth of a low THz gyro TWA. The T-junction coupler and the multiple-hole coupler achieved the respective bandwidths of 10% and 35%.A higher order mode (HOM) two-branch waveguide coupler based on the even distribution of incident power was designed to couple the rectangular TE10 mode to the circular TE61 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 was shown to be < 10 dB at 90–96 GHz. A transmission and phase measurement is presented to analyse TE61 mode conversion in the HOM coupler.
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