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

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.