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ICRH coupling optimization and impurity behavior in EAST and WEST

Abstract : This study compares experimental observations on the two challenges of Ion Cyclotron Resonance Heating (ICRH), namely the coupling of waves to the plasma and the enhanced plasma surface interactions in EAST and WEST medium size tokamaks. In WEST, Lower Hybrid (LH) power helps improving ICRH coupling. In both machines experiments reveal that fueling from the midplane not only helps to couple waves from nearby antennas like in other devices, but also has an impact on the scrape-off layer (SOL) density in regions that are not magnetically connected to the valves. Localized midplane nozzle valves allow similar (in WEST) or better (in EAST) coupling compared to poloidally distributed valves. Core density control requirements for long-pulse operation, in particular in L-mode regime, however limit the amount of gas that can be injected. During ICRH impurities can contaminate the plasma to a level detrimental for the operation, e.g. 100% of ICRH power radiated on WEST. In WEST, tungsten (W) production measured by visible spectroscopy increases on all the observed objects during ICRH compared to a reference phase without ICRH. On some components (antenna side limiters, baffle, divertor) the rise is larger than with a similar LH power. The relative contribution of each object and physical process (RF-sheaths, fast ion ripple losses) to core contamination yet remains poorly known. Comparing antenna limiters with W-coating vs low-Z materials would help quantifying the role of these components. In EAST, the core W content, measured by EUV spectroscopy in presence of divertor sources only, is correlated with the total injected power, either from ICRH or LH. Since 2018 the LH guard limiter tiles were W-coated. Their contribution to the core W content appears more important than divertor sources, when the magnetically connected ICRH antenna is powered. Two-strap ICRH antennas magnetically connected to W components at the midplane already compromise high performance operations.
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Submitted on : Friday, February 14, 2020 - 1:12:10 PM
Last modification on : Saturday, February 15, 2020 - 1:28:38 AM
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  • HAL Id : cea-02479171, version 1

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G. Urbanczyk, L. Colas, X. Zhang, W. Helou, Y. Zhao, et al.. ICRH coupling optimization and impurity behavior in EAST and WEST. 23rd topical conference on radiofrequency power in plasmas, May 2020, Hefei, China. ⟨cea-02479171⟩

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