Assessment of light impurity content at the WEST divertor during L-mode discharges based on an integrated multi-diagnostic workflow.

WEST is an actively cooled, long-pulse tokamak with nearly all plasma-facing components (PFC) made of tungsten (W). For long-pulse operation, the W impurity content and transport to the core plasma are critical concerns that require further investigation to improve plasma performance and PFC durability. Assessing the light impurity (B, C, N, O) content at the divertor is a key metric as it is the main contributor to tungsten sputtering at the divertor in L-mode.

This work details how the integrated multi-diagnostic workflow [Grosjean, IEEE TPS, 2022] is used to refine the low-Z impurity content near the divertor target using visible spectroscopy. The content of all impurity charge states is inferred with the use of a collisional-radiative model “ColRadPy”. The work is performed on a dedicated session of W source characterization performed in L-mode in WEST. A power scan was performed in a standard WEST magnetic equilibrium with lower hybrid (LH) injected power up to 4.5 MW. First results show that O, commonly used as the main light impurity proxy in WEST impurity transport modeling may not be the main W sputtering contributor: ~10 % for O and ~35% for C [Grosjean, NME, 2022]. This refined light impurity content is then used as a constraint for transport modeling (OEDGE code suite) to evaluate the impurity charge state distribution in the scrape-off layer (SOL). Synthetic data based on OEDGE modeling is then created using CHERAB and compared with visible spectroscopy to improve the refined impurity content.