Abstract
For the measurement of the dynamics of fusion-born alpha particles Eα≤3.5 MeV in ITER using collective Thomson scattering (CTS), safe transmission of a gyrotron beam at mm-wavelength (1 MW, 60 GHz) passing the electron cyclotron resonance (ECR) in the in-vessel tokamak “port plug” vacuum is a prerequisite. Depending on neutral gas pressure and composition, ECR-assisted gas breakdown may occur at the location of the resonance, which must be mitigated for diagnostic performance and safety reasons. The concept of a split electrically biased waveguide (SBWG) has been previously demonstrated in C.P. Moeller, U.S. patent 4,687,616 (1987). The waveguide is longitudinally split and a kV bias voltage is applied between the two halves. Electrons are rapidly removed from the central region of high radio frequency electric field strength, mitigating breakdown. As a full scale experimental investigation of gas and electromagnetic field conditions inside the ITER equatorial port plugs is currently unattainable, a corresponding Monte Carlo simulation study is presented. Validity of the Monte Carlo electron model is demonstrated with a prediction of ECR breakdown and the mitigation pressure limits for the above-quoted reference case with 1H2 (and pollutant high Z elements). For the proposed ITER CTS design with a 88.9 mm inner diameter SBWG, ECR breakdown is predicted to occur down to a pure 1H2 pressure of 0.3 Pa, while mitigation is shown to be effective at least up to 10 Pa using a bias voltage of 1 kV. The analysis is complemented by results for relevant electric/magnetic field arrangements and limitations of the SBWG mitigation concept are addressed.
Highlights
ITER will be the first fusion reactor to achieve a fusion power gain of Q ! 1
Depending on neutral gas pressure and composition, electron cyclotron resonance (ECR)-assisted gas breakdown may occur at the location of the resonance, which must be mitigated for diagnostic performance and safety reasons
It was shown that ECR-assisted gas breakdown dynamics reported from experiments could be reproduced with our model for breakdown and the Monte Carlo electron simulations
Summary
In the frame of RF diagnostics for fusion devices or heating schemes, such as ITER CTS or ECR heating, no theoretical study has been concerned with the analysis of mitigation schemes to prevent undesired ECR-assisted breakdown within the diagnostic’s waveguide. This aspect has been addressed experimentally in previous works by Moeller et al.,[14–18] who proposed a longitudinally split electrically biased waveguide (SBWG) design to avoid in-waveguide ECR breakdown by promoting the removal of electrons from the central region of high RF electric field strength and amplifying wall loss (detailed later). A homogeneous magnetic field magnitude B 1⁄4 2.14 T is imposed in both cases (note the different magnetic field directions for the Moeller and ITER CTS cases)
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