Evidence of near-SOL tungsten accumulation using a far-SOL collector probe array and OEDGE modelling in the DIII-D metal rings L-mode discharges. (May 2019)

• Record Type: Journal Article
• Authors: Elder, J.D.; Stangeby, P.C.; Unterberg, E.A.; Abrams, T.; Boedo, J.A.; Donovan, D.; McLean, A.G.; Rudakov, D.L.; Wampler, W.R.; Watkins, J.G.
• Is Part Of: Nuclear materials and energy. Volume 19(2019)
• Page Start: 287
• Page End: 294
• Publication Date: 2019-05
• Subjects: Nuclear energy -- Periodicals; Nuclear fuels -- Periodicals; Nuclear reactors -- Materials -- Periodicals; Radioactive substances -- Periodicals; 621.4833
• DOI: 10.1016/j.nme.2019.03.007
• Languages: English
• ISSNs: 2352-1791

Abstract

Highlights: OEDGE modelling of the DIII-D metal rings collector probe experiment is presented. Modelling indicates that tungsten must enter the peripheral SOL above the probe to reproduce experimentally observed deposition patterns. It is also found that a value for anomalous cross field transport of D ⊥ < 1 m 2 /s is supported by the experimental exponential decay length of deposition on the collector probe. OEDGE modelling of target sourced W indicates that W is transported upstream from the source at the outer divertor strike point toward the apex of the plasma. It then moves outward by cross field transport while also being transported in the direction of the outer target. The W enters the peripheral SOL and subsequently deposits on the probe. Experimental collector probe measurements show more W deposition on the inner target facing (ITF) side than the outer target facing (OTF) side for many discharges. In order for the W to reach the probe as seen experimentally, it is likely that tungsten accumulates at some point in the main SOL upstream of the collector probe location. Abstract: Experimental evidence is presented of near-scrape off layer (SOL) tungsten accumulation near the crown of lower single-null L-mode discharges in the DIII-D Metal Rings Campaign, based on a peripheral-SOL collector probe (CP) array and OEDGE modelling. Such accumulation has been long-theorized due to parallel force balance in the SOL dominated by the ion temperature gradient force [1, 2 ] but direct experimental evidence has been lacking. Impurity accumulation at this location is undesirable since it largely sets the boundary condition for impurity levels in the confined plasma. Toroidally symmetric rings of 5 cm wide tungsten-coated tiles were installed in the outer divertor of DIII-D. A CP array having multiple-diameter, dual-facing collector rods with axes in the radial direction, was inserted into the peripheral-SOL near the outer midplane to measure the plasma W content. In many cases, more W was deposited on the largest rod on the side facing toward the inner target along the field lines, despite the location of the W source being at the outer divertor strike point. Interpretation of the W deposition profiles on the CP rods support the long-theorized impurity accumulation hypothesis; however, more definitive conclusions will require further experimentation and modelling effort.