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ION CYCLOTRON RANGE OF FREQUENCY POWER: Progress in Operation and Understanding for Experiments with Metallic Walls

Abstract : Significant progress in applying ICRF power to ASDEX Upgrade (AUG) through improved power coupling and reduced impurity production has been associated with progress in understanding and modelling. First, the coupling of the fast wave was improved using outer mid-plane gas injection. The local edge density increase in front of the antenna shifts the fast wave cutoff position closer to the antenna by ~2cm, a result confirmed with new density measurements in front of the antenna. The ICRF coupling increases by 120% (only 25% for top gas puffing). The increased density and resulting improved coupling. was also numerically modelled. Second, the new 3-strap antennas in AUG demonstrated clearly that a proper antenna design can successfully mitigate ICRF-specific tungsten (W) sputtering. The reduction of the W sputtering was achieved by minimizing the RF currents on the antenna surfaces exposed to the scrape-off-layer (SOL) plasma. The local RF currents, rectified DC currents and the W sputtering yield at the antenna side limiters experience a clear minimum close to a phasing between the central and the outer straps of 180° and a power balance ratio Pcen/Pout of ~ 2. At this optimum, the local source of sputtered W at the limiters is reduced by a factor between 1.5 and 6, depending on the location. The experiments and modelling confirm the hypothesis of sheath rectification as the source of the sputtered W. Furthermore, the new 3-ion ICRF heating scenario, which can produce very energetic particles, has been successfully reproduced in AUG. The progress in operation, in understanding and modelling is strongly supported by improved ICRF diagnostic coverage including density measurements directly in front of the antenna by reflectometry, advanced RF coupling characterization, measurements of antenna limiter currents, B-dot probes, Ion Cyclotron Emission (ICE) measurements and by dedicated tests on the experimental device IShTAR (Ion cyclotron Sheath Test ARrangement).
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J.-M Noterdaeme, V Bobkov, Ye Kazakov, A Kostic, R Ochoukov, et al.. ION CYCLOTRON RANGE OF FREQUENCY POWER: Progress in Operation and Understanding for Experiments with Metallic Walls. 27th IAEA Fusion Energy Conference - IAEA Gandhinagar India (22-27 October 2018), Oct 2018, Gandhinagar, India. ⟨hal-02970366⟩

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