WEST: Impact of wall conditions on impurity sources and core contamination for various plasma shapes*

WEST’s unique characteristics with nearly all tungsten (W) PFCs offer an ideal platform to study plasma operations and plasma scenario development for long-pulsed, actively cooled, nearly all W-PFC tokamaks [1] . W erosion/redeposition of the PFCs and the resulting contamination of the plasma will be a major challenge for next step devices, such as ITER and SPARC. In WEST, the radiated fraction does not correlate with the measured impurity sources ( [2] , [3] ). A dedicated plasma shape scan was developed to investigate the upper divertor impurity source contribution to the core in lower single null (LSN) during which the crown of the primary separatrix was driven away from the upper divertor (5 / 35 / 110 / 165 mm) with a constant primary and secondary X-point position in the same pulse. From 2024 to 2025 during the C9 to C11 experimental campaigns, 6 reproducible plasma pulses were performed at constant plasma current (420 kA), LH injected power (2 MW) and central line integrated densities (n l = 3.3 10 19 m −2 ) in the flat top phase. The effects of changing PFCs across multiple campaigns and the evolution of wall conditions with increasing cumulative injected energy are observed on these various shapes. The impact of the wall conditions on the plasma performances is monitored by evaluating different relevant parameters as a function of the cumulated energy (E cum ) from the previous glow discharge boronization (GDB). These parameters are: the impurity sources intensity (i.e., B, C, N, O, W fluxes), the radiated power (P rad ), central electron temperature (T ECE ) and the confined plasma W concentration estimation (n W ). Each plasma shape of the same pulse are impacted similarly by the wall conditions. Pulses with BN tiles used in inner bumpers in C9 show a higher amount of N in the upper and lower divertor sources, but also C and W, while T ECE is significantly lower in these pulses. In the lower divertor, B and O levels are within other campaigns trends. The upper divertor, which experienced much lower plasma flux in C9, shows increased levels of B and O. B fades away quickly (E cum of < 1 GJ) as other impurities increase (C, O, W) and radiated power increase with E cum . T ECE and n W do not demonstrate a clear correlation to the upper and lower divertor impurity sources evolution with E cum .