Transport of tungsten to collector probes in DIII-D

Highlights: • Double-sided collector probes were inserted in the SOL of DIII-D to collect tungsten. • The asymmetric deposition profiles are explained in terms of a simple 2D picture. • The code 3DLIM is under development to model the inherent 3D nature of the problem. • Results are in indirect support of a long-hypothesized "impurity potential well". -- Abstract: Collector probes have long been used to measure impurity fluxes in the scrape off layer (SOL) of tokamaks. In this study, collector probes were inserted in the main SOL of DIII-D during the tungsten Metal Rings Campaign, and the W deposits on the probes were analyzed ex situ using Rutherford backscattering spectrometry analysis to obtain radial profiles of W deposition. A simple picture is hypothesized for how the W transports through the SOL from the target to the two sides of the probe, based on a long-theorized impurity accumulation at the crown (i.e. the top) of the plasma. The patterns observed in the deposition profiles along the probes support the hypothesized picture; however, 3D modeling and further experimental studies are needed to support more definitive conclusions.