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

• Published in: IEEE transactions on microwave theory and techniques Vol. 71; no. 6; pp. 2558 - 2566
• Main Authors: Wagner, Dietmar, Kasparek, Walter, Leuterer, Fritz, Monaco, Francesco, Plaum, Burkhard, Ruess, Tobias, Schutz, Harald, Stober, Jorg, Thumm, Manfred
• Format: Journal Article
• Language: English
• Published: New York IEEE 01-06-2023; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Cyclotron resonance devices; Cyclotrons; Diagnostic systems; Electromagnetic wave filters; Electron cyclotron heating; Electron cyclotron resonance heating; Frequency measurement; Geometry; Gyrotrons; Heating systems; Millimeter wave technology; Millimeter waves; Narrowband; notch filter; Notch filters; Passband; Plasma diagnostics; rectangular waveguide; Rejection; step-tunable gyrotron; stray radiation; Waveguides
• ISSN: 0018-9480; 1557-9670
• DOI: 10.1109/TMTT.2023.3234103

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.