Measurements and 2-D modeling of recycling and edge transport in discharges with lithium-coated PFCs in NSTX

The application of lithium coatings on plasma facing components has been shown to profoundly affect plasma performance in the National Spherical Torus Experiment, improving energy confinement and eliminating edge-localized modes. The edge particle balance during these ELM-free discharges has been studied through 2-D plasma-neutrals modeling, constrained by measurements of the upstream plasma density and temperature profiles and the divertor heat flux and Dα emission. The calculations indicate that the reduction in divertor Dα emission with lithium coatings applied is consistent with a drop in recycling coefficient from R ∼ 0.98 to R ∼ 0.9. The change in recycling is not sufficient to account for the change in edge density profiles: interpretive modeling indicates similar transport coefficients within the edge transport barrier (D/χe ∼ 0.2/1.0 m2/s), but a widening of the barrier with lithium.