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Description
Ion cyclotron resonance heating (ICRH) is one of the essential heating systems in fusion devices, and the only mean of depositing energy directly on ions to boost fusion reactions. To maximize the power coupled to the plasma, ICRH antennas, surrounded by protection limiters, must be placed as close as possible to the plasma. If the antenna is too far, little power can be injected, but if it is too close, plasma surface interactions (PSI) kick in and deteriorate the performance. Optimal operation therefore often boils down to a trade-off between good wave coupling efficiency and well-mitigated interactions (either heat loads or erosion).
Three tiles of an ASDEX Upgrade (AUG) 3-strap ICRH antenna tungsten limiter were coated with molybdenum, to use it as trace material [1] allowing dedicated investigations by means of spectroscopy. Due to pre and post-exposure analysis of these tiles, the erosion during a whole AUG campaign with a mixture of discharge scenarios with and without ICRH can be determined.
This contribution focuses on the net erosion measurement of the 5 µm thick Mo-coated tiles after exposing them during the campaign 2024/25 in AUG. The two Mo-coated tiles from the left limiter, one in the upper half and the other one in the lower half, were analyzed by ion beam techniques (IBA) and scanning electron microscopy (SEM) before and after exposure in AUG. The erosion was determined by layer thickness change obtained with IBA and direct observation of FIB-prepared µm-rulers with SEM. The maximal Mo net erosion is below 1 um. Along the toroidal direction, the tile of the lower half (L3) shows two erosion maxima, one as expected at the point with the smallest distance to the plasma. A second maximum is about 1 cm farer away from the plasma and about 2 cm closer to the antenna strips. The Mo-coated tile from the upper half (L10) shows only marginal erosion. On both tiles deposition composed of boron and tungsten was observed in grooves even in the erosion areas. These results allow to strengthen the assessment of the life time of the ICRH limiter tiles.
[1] Urbanczyk G et al., 2020 Nucl. Fusion 60, 126003