Enhancement of Edge Stability with Lithium Wall Coatings in NSTX

ELM reduction or elimination while maintaining high confinement is essential for ITER, which has been designed for H-mode operation. Large ELMs are thought to be triggered by exceeding either edge current density and/or pressure gradient limits (peeling, ballooning modes). Stability calculations show that spherical tori should have access to higher pressure gradients and pedestal heights than higher R/a tokamaks, owing to access to second stability regimes[...1]. An ELM-free regime was recently observed in the NSTX following the application of lithium onto the graphite plasma facing components[......2]. ELMs were eliminated in phases[.....3], with the resulting pressure gradients and pedestal widths increasing substantially. Calculations with TRANSP have shown that the edge bootstrap current increased substantially, consistent with second stability access. These ELM-free discharges have a substantial improvement in energy confinement, up to the global $\beta _{N} \quad \sim $ 5.5 limit. * Supported by US DOE DE-FG02-04ER54520, DE-AC-76CH03073, and DE-FC02-04ER54698. [.1] P. B. Snyder, et. al., \textit{Plasma Phys. Contr. Fusion} \textbf{46} (2004) A131. [2] H. W. Kugel, et. al., \textit{Phys. Plasma} \textbf{15} (2008) {\#}056118. [3] D. M. Mansfield, et. al., \textit{J. Nucl. Materials} (2009) submitted.