Edge transport and turbulence reduction with lithium coated plasma facing components in the National Spherical Torus Experiment

Abstract

The coating of plasma facing components (PFCs) with lithium improves energy confinement and eliminates ELMs in the National Spherical Torus Experiment, the latter due to a relaxation of the density and pressure profiles that reduces the drive for peeling-ballooning modes. 2-D interpretive transport modeling of discharges without and with lithium shows that a reduction in the PFC recycling coefficient from R ∼ 0.98 to R ∼ 0.90 is required to match the drop in Dα emission with lithium coatings. A broadening of the edge barrier region showing reduced transport coefficients is observed, with a ∼75% drop of the D and χe from 0.8 < ψN < 0.93 needed to match the profile relaxation with lithium coatings. Turbulence measurements using an edge reflectometry system as well as high-k microwave scattering show a decrease in density fluctuations with lithium coatings. These transport changes allow the realization of very wide pedestals, with a ∼100% width increase relative to the reference discharges.