The Relationships Between ELM Suppression, Pedestal Profiles, and Lithium Wall Coatings in NSTX
Authors: D.P. Boyle, R. Maingi, P.B. Snyder, J. Manickam, T.H. Osborne, R.E. Bell, B.P. LeBlanc, and the NSTX Team
Abstract:
Recently in the National Spherical Torus Experiment (NSTX), increasing lithium wall coatings
suppressed edge localized modes (ELMs), gradually but not quite monotonically. This work details
profile and stability analysis as ELMs disappeared throughout the lithium scan. While the quantity of
lithium deposited between discharges did not uniquely determine the presence of ELMs, profile analysis
demonstrated that lithium was correlated to wider density and pressure pedestals with peak gradients
farther from the separatrix. Moreover, the ELMy and ELM-free discharges were cleanly separated by
their density and pedestal widths and peak gradient locations. Ultimately, ELMs were only suppressed
when lithium caused the density pedestal to widen and shift inward. These changes in the density gradient
were directly reflected in the pressure gradient and calculated bootstrap current. This supports the theory
that ELMs in NSTX are caused by peeling and/or ballooning modes, as kink/peeling modes are stabilized
when the edge current and pressure gradient shift away from the separatrix. Edge stability analysis using
ELITE corroborated this picture, as reconstructed equilibria from ELM-free discharges were generally
farther from their kink/peeling stability boundaries than ELMy discharges. We conclude that density
profile control provided by lithium is the key first step to ELM suppression in NSTX.
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Submitted to: Plasma Physics and Controlled Fusion (May 2011)
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Download PPPL-4659 (pdf 744 KB 27 pp)
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