Enhanced pedestal H-mode at low edge ion collisionality on NSTX

Battaglia, D. J.; Guttenfelder, W.; Bell, R. E.; Diallo, A.; Ferraro, N.; Fredrickson, E.; Gerhardt, S. P.; Kaye, S. M.; Maingi, R.; Smith, D. R.

doi:10.1063/5.0011614

Enhanced pedestal H-mode at low edge ion collisionality on NSTX

Journal Article · Fri Jul 24 00:00:00 EDT 2020 · Physics of Plasmas

DOI:https://doi.org/10.1063/5.0011614· OSTI ID:1643137

; Guttenfelder, W.; ; Diallo, A.; ; Fredrickson, E.; Gerhardt, S. P.; Kaye, S. M.; Maingi, R.; Smith, D. R.

The Enhanced Pedestal (EP) H-mode regime is an attractive wide-pedestal high-βp scenario for the National Spherical Torus Experiment Upgrade (NSTX-U) and next-step devices as it achieves enhanced energy confinement (H98y,2 > 1.5), large normalized pressure (βN > 5), and significant bootstrap fraction (fBS > 0.6) at Ip/BT = 2 MA/T. This regime is realized when the edge ion collisionality becomes sufficiently small that a positive feedback interaction occurs between a reduction in the ion neoclassical energy transport and an increase in the particle transport from pressure-driven edge instabilities. The EP H-mode was most often observed as a transition following a large edge-localized mode in conditions with low edge neutral recycling. It is hypothesized that the onset of pressure-driven instabilities prior to the full recovery of the neutral density leads to a temporary period with an elevated ion temperature gradient that triggers the transition to EP H-mode. Linear CGYRO and M3D-C1 calculations are compared to beam emission spectroscopy and magnetic spectroscopy in order to describe the evolution of the edge particle transport mechanisms during the ELM recovery and the saturated EP H-mode state. The observations are consistent with the hypothesis that the onset of pressure-driven edge instabilities, such as the kinetic ballooning mode and kink-peeling mode, can be responsible for the increased particle transport in EP H-mode.

View Accepted Manuscript (Publisher)

Sponsoring Organization:: USDOE

Grant/Contract Number:: AC02-09CH11466

OSTI ID:: 1643137

Journal Information:: Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 7 Vol. 27; ISSN 1070-664X

Publisher:: American Institute of PhysicsCopyright Statement

Country of Publication:: United States

Language:: English

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