Abstract
Lithium wall coatings have been shown to reduce recycling, suppress edge localized
modes (ELMs), and improve energy confinement in the National Spherical Torus
Experiment (NSTX). Here we document the effect of gradually increasing lithium wall
coatings on the discharge characteristics, with the reference ELMy discharges obtained in
boronized, i.e. non-lithiated, conditions. We observed a continuous but not quite
monotonic reduction in recycling and improvement in energy confinement, a gradual
alteration of edge plasma profiles, and slowly increasing periods of ELM quiescence. The
measured edge plasma profiles during the lithium coating scan were simulated with the
SOLPS code, which quantified the reduction in divertor recycling coefficient from ~98%
to ~90%. The reduction in recycling and core fueling, coupled with a drop in the edge
particle transport rate, reduced the average edge density profile gradient, and shifted it
radially inward from the separatrix location. In contrast, the edge electron temperature
(Te) profile was unaffected in the H-mode pedestal steep gradient region within the last
5% of normalized poloidal flux, ψN; however, the region of steep Te gradients extended
radially inward from the top of the H-mode pedestal for 0.8< ψN <0.94 with lithium
coatings. The peak pressure gradients were comparable during ELMy and ELM-free
phases, but were shifted away from the separatrix in the ELM-free discharges, which is
stabilizing to the current driven instabilities thought to be responsible for ELMs in
NSTX.
