Design of Surface Defects Loaded Selectively Notched W-Band Waveguide Filter for Millimeter Wave Diagnostic in Fusion Reactor

In the fusion experiment, sensitive radio frequency signal diagnostic instruments need to be protected against high power stray radiation coming from the Electron Cyclotron Resonance Heating. The design of a stopband notch filter in the transmission path of the diagnostic system is helpful to avoid the stray radiations. This work presents the study and design methodology of the notch filter based on a circular corrugated waveguide Bragg reflector that provides a single stopband between 75 and 105 GHz. The existence of the HE 11 mode, i.e. the only propagating mode in the W-band notch filter is the key aspect in the design of the filter. The proposed notch filter is compact and spans only 46.62 cm in length with an inner diameter of 16.4 mm. By optimizing the number of corrugation to 140, their period to λ through parametric analysis and unconventionally introducing uniformly spaced surface defects within the filter structure, a single stopband at 90 GHz is obtained corresponding to the LP 12 Bragg resonance. The notch filter may be useful in wideband diagnostics like Electron Cyclotron Emission and reflectometry in the fusion plasma experiments.