­­­Interpretative analysis of SOL carbon transport using isotopic tracers and multiple, far-SOL collector probes on DIII-D

Experiments with outer strike point injection of isotopically enriched methane (¹³CD_4­) in DIII-D L-mode discharges have demonstrated the ability to infer near scrape-off-layer (SOL) impurity density profiles based on: far-SOL collector probe (CP) measurements; a stable isotopic mixing model; and SOL impurity transport modelling. This work enables one of the first in-depth investigations on the source and transport of SOL impurities which could hinder performance of future fusion devices. Modelling by DIVIMP and 3DLIM of ¹³C SOL evolution is consistent with diagnostic observations and indicates that the buildup of injected impurities on plasma-facing surfaces must be considered while inferring representative impurity distributions. Namely, ¹³C deposits on the inner and outer targets are shown to contribute 50% at a minimum of the enriched ¹³C deposition on CPs and to cause poloidal shifting of the impurity density peaks in the near-SOL. This analysis elucidates the importance of source location, connection length, poloidal diffusion, and radial convective velocity of impurities to accurately model and interpret SOL impurity behavior.