A Multifrequency Notch Filter for Millimeter Wave Plasma Diagnostics based on Photonic Bandgaps in Corrugated Circular Waveguides

¹ Max-Planck-Institut für Plasmaphysik, EURATOM-IPP, Boltzmansstr.2, D-85748, Garching
² Institut für Grenzflächenverfahrenstechnik und Plasmatechnologie, Univ. Stuttgart, D-70569 Stuttgart, Germany
³ Karlsruhe Institute of Technology, EURATOM-KIT, Institut für Hochleistungsimpuls- und Mikrowellentechnik, D-76021, Karlsruhe, Germany
⁴ Dutch Institute for Fundamental Energy Research, EURATOM-DIFFER, Edisonbaan 14, NL-3439 MN Nieuwegein, The Netherlands

^a Corresponding author: dietmar.wagner@ipp.mpg.de

Published online: 12 March 2015

Abstract

Sensitive millimeter wave diagnostics need often to be protected against unwanted radiation like, for example, stray radiation from high power Electron Cyclotron Heating applied in nuclear fusion plasmas. A notch filter based on a waveguide Bragg reflector (photonic band-gap) may provide several stop bands of defined width within up to two standard waveguide frequency bands. A Bragg reflector that reflects an incident fundamental TE₁₁ into a TM_1n mode close to cutoff is combined with two waveguide tapers to fundamental waveguide diameter. Here the fundamental TE₁₁ mode is the only propagating mode at both ends of the reflector. The incident TE₁₁ mode couples through the taper and is converted to the high order TM_1n mode by the Bragg structure at the specific Bragg resonances. The TM_1n mode is trapped in the oversized waveguide section by the tapers. Once reflected at the input taper it will be converted back into the TE₁₁ mode which then can pass through the taper. Therefore at higher order Bragg resonances, the filter acts as a reflector for the incoming TE₁₁ mode. Outside of the Bragg resonances the TE₁₁ mode can propagate through the oversized waveguide structure with only very small Ohmic attenuation compared to propagating in a fundamental waveguide. Coupling to other modes is negligible in the non-resonant case due to the small corrugation amplitude (typically 0.05·λ₀, where λ₀ is the free space wavelength). A Bragg reflector for 105 and 140 GHz was optimized by mode matching (scattering matrix) simulations and manufactured by SWISSto12 SA, where the required mechanical accuracy of ± 5 μm could be achieved by stacking stainless steel rings, manufactured by micro-machining, in a high precision guiding pipe. The two smooth-wall tapers were fabricated by electroforming. Several measurements were performed using vector network analyzers from Agilent (E8362B), ABmm (MVNA 8-350) and Rohde&Schwarz (ZVA24) together with frequency multipliers. The stop bands around 105 GHz (- 55dB) and 140 GHz (-60dB) correspond to the TE₁₁-TM₁₂ and TE₁₁-TM₁₃ Bragg resonances. Experiments are in good agreement with theory.