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For the measurement of the dynamics of fusion-born alpha particles $E_\alpha\leq 3.5$ MeV in ITER using collective Thomson scattering (CTS), safetransmission of a gyrotron beam at mm-wavelength (1 MW, 60 GHz) passing theelectron cyclotron resonance (ECR) in the in-vessel tokamak `port plug' vacuumis a prerequisite. Depending on neutral gas pressure and composition,ECR-assisted gas breakdown may occur at the location of the resonance, whichmust be mitigated for diagnostic performance and safety reasons. The concept ofa split electrically biased waveguide (SBWG) has been previously demonstratedin [C.P. Moeller, U.S. Patent 4,687,616 (1987)]. The waveguide islongitudinally split and a kV bias voltage applied between the two halves.Electrons are rapidly removed from the central region of high radio frequencyelectric field strength, mitigating breakdown. As a full scale experimentalinvestigation of gas and electromagnetic field conditions inside the ITERequatorial port plugs is currently unattainable, a corresponding Monte Carlosimulation study is presented. Validity of the Monte Carlo electron model isdemonstrated with a prediction of ECR breakdown and the mitigation pressurelimits for the above quoted reference case with $^1$H$_2$ (and pollutant high$Z$ elements). For the proposed ITER CTS design with a 88.9 mm inner diameterSBWG, ECR breakdown is predicted to occur down to a pure $^1$H$_2$ pressure of0.3 Pa, while mitigation is shown to be effective at least up to 10 Pa using abias voltage of 1 kV. The analysis is complemented by results for relevantelectric/magnetic field arrangements and limitations of the SBWG mitigationconcept are addressed.

physics of plasmas

Kinetic simulation of electron cyclotron resonance assisted gas  breakdown in split-biased waveguides for ITER collective Thomson scattering  diagnostic