I. Introduction
The gyrotron traveling wave amplifier (gyro-TWA) has attracted huge research interest in the past decades for a variety of applications including high resolution radar, high frequency communications, plasma diagnostics, materials processing etc. [1], [2]. A W-band gyro-TWA based on a helically corrugated waveguide [3], [4] and a cusp electron gun is currently being studied to achieve an output power of 5 kW in the frequency band of 90–100 GHz. To successfully operate such a wide bandwidth amplifier, an output microwave window of sufficient low reflection in the operating frequency range is required.