Speaker
Description
The transport of tungsten (W) impurities from W-coated plasma-facing components (PFCs) in the largely carbon (C) environment of the DIII-D tokamak was assessed leading to refinements of the existing modeling and theory framework that will be needed as the fusion community advances to high-Z PFC operations while maintaining a mixed-material environment in transition. Recent progress at DIII-D is presented - including the effects of mixed-material environments and surface morphology on impurity sourcing, as well as the role of kinetic effects on high-Z impurities transiting the Scrape-Off Layer (SOL) plasma and how that impacts divertor impurity leakage.
W impurity sourcing was measured from a W-coated divertor spectroscopically during H-mode discharges with Edge Localized Modes (ELMs) and compared with predicted erosion yields from the Free Stream Recycling Model (FSRM) [1]. The FSRM was found to over-predict W erosion yields, leading to the investigation of C-W mixed-material layer effects on sputtering yields and reflection coefficients and PFC surface roughness effects on C deposition. The effect of PFC surface morphology on re-deposition was further investigated in L-mode plasmas using the PYEAD-RustBCA-GITR modeling framework [2]. Net erosion can be significantly underestimated if the magnetic field B-to-PFC pitch angle (α) is larger than expected due to surface roughness or localized melting events. Sensitivity analysis suggests this geometric effect can compete with local plasma density and temperature in setting re-deposition rates [2].
W transport from a closed-divertor geometry was studied with regard to the ion Bx∇B drift direction while impurity leakage from the divertor region was assessed with impurity collector probes (CPs) to determine transport in the far-SOL [3] as well as soft x-ray tomography (SXR) and vacuum ultraviolet spectroscopy (SPRED) for W contamination of the core. Core W content was found to increase by up to ~3x when the ion Bx∇B drift was directed towards the divertor (favorable field direction for H-mode) relative to discharges with the unfavorable direction. However, CP measurements taken in the outer midplane far-SOL showed ~75% lower W deposition for discharges with favorable Bx∇B drift. A SOLPS-DIVIMP workflow was used to assess these results and determine the plasma conditions under which kinetic effects become dominant in high-Z parallel impurity transport in plasma regimes of both low impurity ion charge states and low collisionality.
Work supported by US DOE under DE-SC0023378.
[1] Cacheris et-al. PPCF, 65 (2023) 085010
[2] Easley et-al. PPCF, 67 (2025) 035023
[3] Messer et-al. NME, 38 (2024) 101566