Collector probe analysis of tungsten transport to the far-SOL from the DIII-D SAS-VW divertor experiment

• Published in: Nuclear materials and energy 2023-12, Vol.38
• Main Authors: Messer, Seth H., Mateja, Jeremy D., Nichols, Jacob H., Cacheris, Alec, Duran, Jonah D., Zamperini, Shawn A., Sinclair, Gregory, Donovan, David, Abrams, Tyler, Odstrcil, Tomas, Ren, Jun, Garcia, W., Unterberg, Ezekial A., Stangeby, Peter C., Rudakov, Dmitry L., Elder, John D., Watkins, J.
• Format: Article
• Language: English
• Subjects: 70 PLASMA PHYSICS AND FUSION TECHNOLOGY
• ISSN: 2352-1791; 2352-1791

Experimental results from the 2022 tungsten (W)-coated Small Angle Slot (SAS-VW) divertor campaign at DIII-D coupled with interpretive 3DLIM modelling show opposing trends for core impurity content when compared to impurity deposition on far-Scrape Off Layer (SOL) Collector Probes (CPs) with increasing main ion density. SAS-VW is a closed, W-coated divertor designed to more easily facilitate divertor detachment while reducing impurity leakage. An experiment performed a series of upper-single-null L-mode discharges in each toroidal magnetic field (BT) direction, with increasing main ion density (line-averaged density = 3.15–4.35e19 m–3) that approaches and slightly exceeds the divertor detachment threshold. The results indicate: a) increased radial W transport with decreasing peak $T^{LP}_{e,t}$; and b) negligible change in W content in the far-SOL at the outer mid-plane with the onset of divertor detachment. Preliminary W deposition measurements using double-sided, graphite CPs inserted at the Midplane Materials Evaluation System (MiMES) reveal a 75% decrease over the density scan when operating in the unfavorable (ion B×∇B out of the divertor) BT direction. In contrast, soft X-ray (SXR) radiation from the same discharges is used as a proxy for W core contamination, showing core W content that increases by 77% with increasing line-averaged density. Similar L-mode discharges conducted in the favorable BT direction result in significantly less deposition on CPs. Using an interpretive modeling workflow following Zamperini 2022 [1] for assessing the transport of W sputtered from the SAS-VW divertor, the analysis suggests that W migration to the main chamber surfaces during the campaign may also contribute to far-SOL deposition.