Ex situ investigation of boron, hydrogen, and oxygen distributions on W7-X divertor surfaces using picosecond-LIBS under vacuum

Quantitative, spatially resolved measurements of near-surface material composition on
plasma-facing components (PFCs) are critical for interpreting plasma–wall interaction (PWI)
driven impurity sources, assessing wall-conditioning performance, and providing model
constraints for impurity migration and co-deposition in present-day magnetic confinement
devices and future reactors. In this work, picosecond laser-induced breakdown spectroscopy
(ps-LIBS) is applied to post-mortem graphite divertor tiles from the stellarator Wendelstein 7-X
(W7-X) under device-relevant high vacuum (∼10−7 mbar) to obtain poloidally resolved
two-dimensional maps and depth profiles of carbon (C), boron (B), hydrogen (H), and oxygen
(O). The ps-LIBS results are cross-validated against isotope-sensitive diagnostics (nuclear
reaction analysis, NRA; laser ablation molecular isotopic spectroscopy, LAMIS) and
complemented by profilometry, focused ion beam scanning electron microscopy
(FIB-SEM)/energy-dispersive x-ray spectroscopy (EDS) and x-ray photoelectron spectroscopy
(XPS) to relate lateral patterns to film thickness and near-surface chemistry. Pronounced
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poloidal heterogeneity is observed: the thickness of mixed co-deposited layers range from
∼0.3 to 0.4 μm in erosion-dominated regions, to ∼0.9 μm in B-rich deposition zones, and
exceeds 12 μm in 13C-rich deposits near the outer strike line after 2.5 h of H plasma operation.
Depth profiling reveals that late-phase 13CH4 injection forms a C–H–O enriched top layer that