Characterizing Tungsten Sourcing and SOL Transport during the Metal Rings Campaign

The Metal Rings Campaign on DIII-D utilized two isotopically and poloidally distinct toroidal arrays of tungsten coated inserts in the lower divertor to study W divertor erosion near the outer strike point (OSP) and divertor entrance and subsequent migration in a mixed-material (C-W) environment. In AT hybrid discharges (P$_{\mathrm{AUX}}=$14 MW, H$_{\mathrm{98}}=$1.6, $\beta_{\mathrm{N}}=$3.7) with rapid ELMs (f$_{\mathrm{ELM}}$\textasciitilde 200 Hz, $\delta $W/W\textasciitilde 0.7{\%}) W impurities are seen to reach the midplane predominantly from the OSP region rather than the divertor entrance (far-SOL). Conversely, in scenarios with less frequent larger ELMs (f$_{\mathrm{ELM}}$\textasciitilde 60 Hz, $\delta $W/W\textasciitilde 3.6{\%}) , the W impurities are found to transport equally from the OSP and entrance region. ELM-resolved spectroscopic measurements of W sourcing indicate that large ELMs can source W at many times the inter ELM rate. The peak W erosion rate can shift radially outwards consistent with the ELM energy flux, thereby shifting the balance between strikepoint and far-SOL sources. Changes in the peak erosion locations between forward and reversed Bt discharges are consistent with ExB ion drift effects. Evidence for a near-SOL impurity buildup between the divertors driven by the parallel grad-Ti force is also seen.