Spectroscopic observations of carbon transport in the near scrape-off layer following controlled axisymmetric injection of methane in DIII-D

Recent experiments in DIII-D have provided charge-resolved spectroscopic measurements of long-range transport pathways for impurities sourced in the outer divertor. Isotopically-enriched methane (13CD4) was injected from axisymmetric gas baffles into the outer divertor of attached L-mode plasmas, as a proxy for intrinsic impurities sputtered from the divertor target. Experiments were repeated at injected power levels ranging from 2.4-4.5 MW, which led to moderate changes in temperature in the near scrape-off layer (SOL). Results are presented from an array of spectroscopic diagnostics, tracking changes in C-II, C-III, C-IV, C-V, and C-VI emission in both the divertor and upstream SOL. A key finding was a strong increase in C-V emission in the upstream SOL during the puff, indicating long-range transport of the injected impurities. A collisional-radiative model is used to decouple changes in photon emission coefficients from changes in C densities, and the derived tracer carbon densities are compared to models that predict enhanced SOL impurity buildup in the presence of larger parallel temperature gradients. This experimental database will be utilized to benchmark models for SOL impurity transport and divertor leakage.