Design of the High-Q Approach Notch Filter for 60 GHz Collective Thomson Scattering System

This manuscript presents the design and implementation of a high-performance waveguide-based notch filter centered at 60 GHz, tailored for use in Collective Thomson Scattering (CTS) diagnostics systems on next generation burning plasma device in China. The filter features an ultra-narrow notch bandwidth (<0.4 GHz) and achieves over 90 dB rejection at the target frequency, significantly outperforming conventional designs with ~ 2 GHz bandwidth. By employing optimized cylindrical cavity resonators and precise waveguide aperture tuning, the filter maintains minimal insertion loss across the passbands (56-59.8 GHz and 60.2-64 GHz), ensuring full coverage of the ion temperature-dependent scattering spectrum. The design's high quality factor (Q) and robust performance, validated through electromagnetic simulations, make it ideal for protecting sensitive nanowatt-level receivers from multi-megawatt gyrotron interference. Practical implementation considerations, including compatibility with WR-15 waveguides and modular integration, highlight its suitability for next-generation fusion diagnostics. This work advances millimeter-wave filter technology, offering a reliable and scalable solution for high-precision plasma measurements in demanding fusion environments.