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Description
Tungsten as high-Z material has a relatively low physical sputtering. Chemical erosion – if occurring at all – is negligibly small. In addition, tungsten has a very large melting point of about 3400°C. However, the core plasma tungsten concentration in a fusion device has to be kept at extremely low values around 3E-5 to minimize plasma dilution and in particular plasma cooling. Therefore, it is of great importance to control and minimize the net source of tungsten from the various wall elements. In this context prompt redeposition, which primarily happens for high-Z elements of large mass like tungsten, is an important process, which significantly can reduce the gross erosion to much smaller net erosion.
The prompt redeposition of sputtered tungsten at the inner wall of ITER during current ramp-up in limiter configuration has been simulated with the ERO code. Plasma parameters from SOLPS-ITER simulations in deuterium for a medium-density (with a peak electron density of 4E12 $cm^{-3}$ at the inner wall) and a high-density (1E13 $cm^{-3}$) case have been used as input for ERO. CX neutrals and impurities like oxygen or seed species hitting the wall will change the tungsten gross erosion source but not the fraction of prompt redeposition and are therefore not of relevance for the present study. Simulations without anomalous cross-field diffusion for sputtered tungsten ions reveal peaked prompt redeposition profiles in poloidal direction. At the tangency point with largest electron temperature ($T_{e}$ = 65 eV for medium and 48 eV for high-density case) and density, maximum prompt redeposition fractions of about 60% for the medium-density and 80% for the high- density case occur. At a distance of 50 cm away from the tangency point, prompt redeposition decreases to 10% (medium-density) and 20% (high-density case). The simulations without anomalous cross-field diffusion show that the overall redeposition is the same as the prompt redeposition thus the overall redeposition is only due to prompt redeposition. An anomalous cross-field diffusion of 1 $m^{2}/s$ leads to slightly increased prompt redeposition, however, for both medium and high-density case there is now also a significant amount of non-prompt redeposition. The modelled profiles of prompt redeposition can be used as input for plasma simulation codes like SOLPS-ITER to improve the assumptions of net tungsten wall sources.