Single-and Two-Frequency Sub-THz Waveguide Notch Filters With Rejection Frequencies Within and Beyond the Passband

Notch filters are a key component in millimeter-wave plasma diagnostics systems for magnetically confined fusion plasmas. They are required to protect sensitive millimeter-wave receivers from stray radiation of electron cyclotron heating systems. These heating systems employ gyrotrons that emit strong millimeter-wave radiation (90 dBm) in one or, in the case of modern step-tunable gyrotrons, several narrowband frequency lines. In this article, we describe the design and performance of notch filters based on waveguide technology, operating in the F-band (90-140 GHz), that reject one or two selectable frequencies in the passband. We also present the solution for a filter where the rejected frequencies are outside of the passband of the plasma diagnostics. This is demonstrated for a filter operating in the W-band (75-110 GHz). To fully protect the diagnostic systems, typical stopbands with not less than 60-dB rejection and at least 500-MHz width are required.