Erosion and redeposition patterns on divertor tiles after exposure in the first operational phase of WEST

Erosion and deposition patterns were obtained by ion beam analyses (Rutherford backscattering (RBS) and nuclear reaction analysis (NRA)) and scanning electron microscopy (SEM assisted by FIB cutting and EDX) on entire W-coated graphite divertor tiles exposed during the 1st operational phase of WEST (2017–2021) covering the campaigns C1 to C5.The thicknesses of layers in regions with thick and thin deposition show a progression with plasma exposure duration in a non-linear manner. Very thick deposits exceeding 50 µm after C5 are observed on the high field side of the divertor after 7.3 h of plasma operation. These deposits peel off on sub-millimetre length scales, most probably triggered by arcing. The areal fraction altered by delamination exceeds 20%. The stratified layers are composed of a mixture of boron, carbon, oxygen and tungsten with minor additions of nitrogen, copper, iron (together with nickel) and silver. The deuterium concentration in the thick deposits is at most several atomic percent.The tungsten net erosion at the inner and outer strike point areas exceeded already after the C3 campaign the 1–2 µm thickness of the W layer above the marker layer on the special plasma-facing units installed before phase 1 in 2016. The two-dimensional (poloidal and toroidal) net erosion patterns of 5 pairs of inner and outer W-coated standard graphite tiles were analysed to determine the effect of the magnetic ripple on the erosion. These data allow assessing the total net erosion volume around the torus for the complete phase 1 of WEST, resulting in an erosion volume of 1.4 and 1.1 cm3 for the inner and outer strike point areas, respectively. The maximal net erosion is found at the ripple maximum at the inner strike point area (8 µm erosion averaged over several mm2, while erosion reaches microscopically up to 15 µm due to the microscopically inhomogeneous erosion pattern).