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Net erosion of tungsten (W) in the divertor region has been investigated by exposing small platinum (Pt) marker samples, as well as bulk molybdenum (Mo) and W samples, to a series of plasma discharges in the outer strike point (OSP) region during dedicated L- and H-mode experiments in ASDEX Upgrade (AUG). Pt was chosen as a proxy for W due to its comparable erosion characteristics, addressing the previously assessed limitations [1] of Au in earlier experiments [2]. Net erosion of the Pt marker samples was quantified using Rutherford Backscattering Spectrometry (RBS), while the material composition and spectral peak identifications were performed using Nuclear Reaction Analysis (NRA) for low-Z elements and Particle-Induced X-ray Emission (PIXE) for high-Z elements. Measurements were performed on both the sample surfaces and the regions between the Pt markers, allowing characterisation of poloidal variations in erosion/deposition patterns on the markers and deposition of W and other materials between the marker areas. PIXE and NRA analyses were performed at the same points, providing a detailed picture of the material composition along the poloidal coordinate with additional microbeam measurements and microscopy studies to characterise the shape of the re-deposition patterns with high spatial resolution. On the bulk Mo and W samples, Focused Ion Beam (FIB) cross-sectional analysis was performed to investigate net erosion rate as well as the structure and thickness of deposited layers.
Analysis of RBS data shows pronounced net erosion at the OSP, gradually decreasing toward the far scrape-off layer. In H-mode, measurements indicate nearly an order of magnitude stronger net erosion compared to L-mode, potentially reflecting the contribution of ELM-induced erosion. Both experiments reveal, alongside Pt re-deposition, accumulation of W and low-Z elements on the surfaces near the OSP. Additionally, indications of local deposition were observed directly adjacent to the Pt markers near the OSP. If confirmed as deposited Pt, these observations would represent clear experimental evidence of prompt re-deposition in marker erosion experiments, extending earlier studies based on campaign-integrated W marker erosion [3]. This offers valuable insight into material deposition processes in the OSP area and the transport of W and other species in plasma discharges.
[1] S. Saari et al., Nuclear Materials and Energy 45 (2025) 102032
[2] A. Hakola et al., Nuclear Fusion 61 (2021) 116006
[3] D. Naujoks et al., Journal of Nuclear Materials 210 (1–2) (1994) 43–50