Post-mortem analysis of the erosion/deposition pattern on ITER-like actively cooled W-monoblock from the WEST divertor (Phase I)

Abstract During Phase I of WEST operation, one of the lower divertor sectors, known as the test sector, was partially equipped with actively cooled ITER-like plasma-facing units (PFUs) made of 35 monoblocks (MBs). A visual inspection of the test sector after the C4 campaign revealed changes on the surface condition of several MBs located in the outer strike area. In this work, one of the MBs of interest, MB 28 from PFU#13, was extensively investigated focusing on fuel retention, material migration (erosion, transport, redeposition), and surface morphology. The plasma exposed surface of MB 28 exhibited four distinct zones along the toroidal direction. In the first zone, extending up to 4 mm from the leading edge (LE), the surface showed signs of erosion, with periodically spaced cracks oriented perpendicular to the LE. The second zone exhibited the highest material deposition (B, N, Fe and Cu) and fuel (D and He) retention, with the maximum deposit thickness reaching approximately 1.5 μ m. In the third zone, the deposition gradually decreased and accumulated in linear areas. The fourth zone, near the trailing edge, was marked by the maximum hydrogen ( H ) and carbon ( C ) deposition. In this zone, linear deposition areas rich in O and B were detected, running parallel to the edge. A clear correlation was observed between the deposition of B, N, Fe and Cu and the retention of D and He.