Evolution of elemental depth profiles on co-deposited layers at the divertor region of the WEST tokamak during its Phase 1 operations

We discuss detailed elemental, compositional, and structural properties of co-deposited layers formed on the special marker tiles at the divertor region of the WEST tokamak during its Phase 1 (2016–2021) plasma operations. The main new aspects are applying high depth and mass resolution, provided by a combination of state-of-the-art local analysis techniques, throughout the entire surface deposit and cross correlating the obtained results with published ones. We also show that proper comparison of the different data sets requires deconvoluting them with several experimental parameters, most importantly the lateral and depth resolutions as well as the analysis volume. The analyses reveal that the thickest deposits are formed towards the end of Phase 1, and the change is particularly noticeable after the C4 campaign with the longest exposure time and the largest number of boronizations carried out. Thin deposits (thickness up to 1–2 μm) are found on several poloidal regions of the analysed tiles, and they exhibit a clear sandwich-type of structure consisting of distinct B, C, O, and/or W-rich sublayers together with metallic impurities. Close to the inner strike point, thick deposits are measured (thickness several tens of micrometres) and they show complex, stratified structures, however, the same sublayer structure as for the thin deposits can still be recognized. On the thin deposits, the very surface is rich in B while for the thick deposits W dominates the topmost surface layers in the latter stages of Phase 1. For all the analysed samples, the superficial B and W concentrations are on average 10–30 at.% in addition to which high oxygen levels up to 15–40 at.% are measured. In the erosion-dominated regions, deposits can be observed but only in the microscopic scale inside recessed valleys, up to thicknesses of several micrometres. A dynamical erosion–deposition picture for the surface layers is confirmed, further contributing to their structure and composition.