Pedestal characterization and stability of small-ELM regimes in NSTX

A. C. Sontag, J. M. Canik, R. Maingi, J. Manickam, P. B. Snyder, R. E. Bell, S. P. Gerhardt, S. Kubota, B. P. LeBlanc, D. Mueller, T. H. Osborne, K. L. Tritz

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Abstract

NSTX has observed transition to a desirable small-ELM regime (called type-V), in which the stored energy loss per ELM is less than 1%, by stabilizing type-I ELMs. This regime is accessed in a lower single null configuration with increased edge collisionality (ν* > 1). Coincident with the transition to this regime, a low-frequency (<10 kHz) n = 1 mode is observed at the plasma edge in magnetic and soft x-ray diagnostics, with harmonics up to n = 6 observed in some cases. Low-level density fluctuations associated with this mode are observed using microwave reflectometry, but there is no evidence that the mode is providing sufficient transport to stabilize the type-I ELMs. This mode rotates in the electron diamagnetic direction and has shown a phase inversion on USXR channels, indicating that it is resistive in nature. Discharges with type-V and type-I ELMs are both calculated to be on the peeling unstable side of the peeling-ballooning stability curve, with the type-V case at higher normalized pressure gradient and closer to the ballooning stability boundary.

Original languageEnglish
Article number103022
JournalNuclear Fusion
Volume51
Issue number10
DOIs
StatePublished - Oct 2011

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