W deposition patterns on a collector probe in the far SOL provide insight into near SOL phenomena

Collector probes inserted into DIII-D during the Metal Rings Campaign in June 2016 collected measurable amounts of tungsten via Rutherford Backscattering Analysis (RBS) and Laser Ablation Mass Spectrometry (LAMS). The operating conditions varied greatly over the campaign, yet, consistent patterns and trends were observed for the probes. Radial decay lengths of W deposits on opposite-facing sides of the probes support a simple model describing transport to each probe face. Comparing the degree of asymmetric W deposition to near SOL parameters indicates that W reached the probe from the direction of the crown of the plasma, even though the only W-coated tiles were in the lower outer divertor with all other tiles being graphite. 2D surface profiles of the isotopic W probe deposits via LAMS show increased W content toward the edges of the probe faces, and development of a 3D collector probe model called 3D LIM is underway to model these effects. The 2D profiles also show random spots of increased W content. These spots are tentatively attributed to W particulates originating from some location in DIII-D where W accumulated over the campaign.