Observation and analysis of fast-inner-target directed parallel flows during the DIII-D isotopic methane injection experiments

A set of highly diagnosed isotopic methane experiments were recently performed on DIII-D to study low-Z impurity transport in the scrape off layer (SOL). A toroidally symmetric injection of ¹³CD₄ was performed near the outer target, from which the ¹³C migrated through the SOL and deposited on collector probes inserted at the plasma crown and outboard midplane. The experimental conditions covered a wide range of L-mode operating parameters by varying the plasma density (n_e = 2.5-5.0 m³) and injected power (P_inj = 2.5-4.5 MW, P_SOL = 2.0-3.5 MW), and operating in both B_T directions. Reciprocating Mach probes provided radial profiles of the plasma density, temperature and parallel flow speeds. The expected flow patterns in the crown of fast inner-target-directed flow in the favorable B_T direction and relatively stagnant flow in the unfavorable B_T direction were measured. Recent studies of high-Z impurity SOL transport demonstrated the controlling role of parallel flows, and this study will investigate if the same conclusions hold for low-Z impurity transport. The range of Mach probe measurements paired with collector probe deposition profiles will enable study of the impact of parallel flows on near-SOL impurity accumulation, as well as the primary physics drivers behind the fast inner-target-directed parallel flows.