Overview of plasma wall interactions in the new high particle fluence campaign of WEST operated with cold divertor plasma conditions

A new high fluence campaign with cold divertor plasma (T e  < 5 eV) was performed in WEST, in 2025, using the X-point radiator regime. The campaign consisted in the repetition of 175 long deuterium L-mode plasmas each of ∼70 s in duration, cumulating over 4.5 h of plasma exposure in full tungsten device without boronisation. A maximum deuterium fluence of approximately 5.6 × 10 26 m −2 was reached in the outer strike point region, representative of about two high performance ITER discharges. In comparison to the first high fluence campaign, performed in attached divertor conditions (T e ∼25 eV at the target) with equivalent cumulated fluence, a drastic reduction by 3 orders of magnitude of W gross erosion has been measured, as well as heat load reduction by a factor 4. Unlike the first high fluence campaign, no W ingress into the core plasma from flake ejection from thick high field side deposits was observed during the campaign. The first post-mortem characterization confirms the strong reduction of W erosion in high field side of the divertor with 4 times slower buildup of deposits that maintain good thermal contact with the target. This paper gives an overview of the first observations of this new high fluence campaign performed with low T e with a comparison to the first one in terms of operation, divertor conditions and evolution of the divertor ITER-grade components.