First observations of the impact of plasma shaping on impurity sources in WEST*

WEST is an actively cooled, long-pulse tokamak with nearly all plasma-facing components (PFC) made of tungsten (W). This peculiarity makes WEST even more relevant in light of the recent decision to use W PFCs for the ITER main wall and divertor, where W erosion/redeposition and contamination will be a major challenge. In 2023, a dedicated L-Mode experiment was carried out on WEST (I_P = 500 kA, P_LH = 4.2 MW, P_SEP = 2.3 MW, n_e = 3.7.10¹⁹ m^-2) to characterize the impurity sources (W and light impurities), where the separatrix was driven away from the upper divertor (4 – 8 mm) during the discharges to see its impact on impurity sourcing from multiple regions of the upper and lower divertor as well as the antenna. The edge conditions were measured with a set of flush mounted Langmuir probes in the lower and upper divertors as well as in the baffle (peaks of 20 – 25 eV and 2 – 8 – 2 10¹⁸ m^-3 respectively), coupled with reciprocating Langmuir probe measurements upstream and in the antenna protection limiter (APL). The sources were characterized with visible spectroscopy, while the overall contamination is assessed with bolometry measurements and VUV spectroscopy. The first observations show that the shape changes impact the electron temperature and density as well as the sourcing in the areas monitored, while the overall radiated power experienced minimal change.

Work supported by US DOE under DE-SC0020414.