Refining light impurity content estimates at the lower divertor based on experimental data in WEST

WEST is an actively cooled, long-pulse tokamak with nearly all plasma-facing components (PFC) made of tungsten (W). One of the aims of WEST is to study plasma operation with W PFCs in preparation for long-pulse operation on W divertor devices like ITER (Bucalossi et al., 2014) [1]. For long-pulse operation, the W impurity content and transport to the core plasma are critical concerns that require further measurement and interpretation in order to improve plasma performance and PFC durability. In low confinement mode (L-mode) operation, the sputtering of tungsten is not expected to be driven by the plasma itself (D), but by the main light impurities (C, B, O, N) observed at trace levels in WEST. Identifying the main W sources and understanding W transport in the SOL are key metrics for plasma performance, which are simulated with Scrape-Off Layer (SOL) impurity transport codes (SOLEDGE-ERO, OEDGE-DIVIMP). Previous studies have used O as proxy for light impurity with a global content varying from 3 to 5% in SOL transport simulations (Genova et al., 2021; Rooij et al., 2020; Klepper et al., 2022) [2-4]. A power scan was performed in a standard WEST magnetic equilibrium with LH injected power up to 5 MW. Density and temperature were measured with flush-mounted Langmuir probes (Dejarnac et al., 2021) [5]. The impurity fluxes are estimated by applying the number of ionizations per photons (S/XB) coefficients calculated with the collisional-radiative model ColRadPy (Johnson et al., 2019) [6] to the spectral radiance measured with the WEST visible spectroscopy system (Meyer et al., 2016) [7]. This work details how the low-Z impurity content (C, O) near the divertor target was refined using the visible spectroscopy for several impurity charge states, coupled with ColRadPy to infer the content of all impurity charge states at the divertor. First results shows that the main light impurity is C and not O as originally assumed. This inferred impurity content is then used to evaluate the W sputtering at the divertor, considering the sheath at the divertor. A quantitative comparison is performed with the W flux at the divertor measured with visible spectroscopy that shows a qualitative agreement, but not fully consistent at the strike point position. This, in turn, suggests that the impurity content is not complete with O and C only.

Automated summary

WEST is an actively cooled, long-pulse tokamak that aims to study plasma operation with tungsten plasma-facing components (PFCs) in preparation for long-pulse operation on tungsten divertor devices like ITER. The sputtering of tungsten in low confinement mode (L-mode) operation is driven by light impurities (C, B, O, N) observed in trace levels in WEST. Identifying the main sources of tungsten and understanding its transport in the Scrape-Off Layer (SOL) are important for plasma performance and durability of PFCs.