Quasi-optical output for a high gain, broadband W-band amplifier

The gyrotron-traveling wave amplifier (gyro-TWA), operating in W-band has previously been shown to provide an unsaturated output power of 3.4 kW over a 3-dB bandwidth of 9196.5 GHz in single pulse (350 ns) operation. The output section of the gyro-TWA was critical to maintaining the amplifier stability due to the 37 dB gain from a single gain section. This paper presents the re-design of the gyro-TWA output section suitable for several percent duty cycle and wideband operation over 90-100 GHz. We describe the design of the: wideband smooth output feedhorn, which couples to a fundamental Gaussian mode; the electron beam collector (forming part of the output horn), and the vacuum window. The feedhorn simulations predict a calculated >98% Gaussian efficiency and better than -30 dB return loss. The vacuum window is a multilayer design comprising three window discs: two quartz and one alumina. The predicted return loss is lower than -30 dB over the operating range.