Analysis on the Effect of Main Ion Density on High-Z Impurity Transport in the Scrape-off Layer of DIII-D Using Collector Probes*

Experimental results from the 2022 tungsten (W)-coated Small Angle Slot (SAS-VW) divertor campaign at DIII-D indicate that while core plasma density is increased approaching detachment in the unfavorable (ion B×∇B out of the divertor) toroidal magnetic field (B_T) direction, W concentration at the midplane far-Scrape-off Layer (SOL) is reduced while core W concentration increases. To better understand high-Z impurity transport and contamination of the core, an experiment was performed including a series of upper-single-null L-mode discharges in each B_T direction. The discharges incrementally increased the plasma density,  (line-averaged density = 3.15-4.35e19 m^-3), which approaches and slightly exceeds the divertor detachment threshold. W deposition measurements using double-sided, graphite Collector Probes (CPs) inserted at the outer midplane far-SOL revealed a 75% decrease in W areal density over the  scan when operating in the unfavorable B_T direction. Observations of flattened W deposition profiles allude to increased radial transport with increasing Soft X-ray (SXR) imaging data from the same discharges shows core W content that increases by 77% over the same scan. Both W core content and CP deposition changed marginally with the onset of (partial) divertor detachment.