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.
Issue date: 2020
Creative Commons Attribution 4.0 International (CC BY)
Cite as:
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. Enhanced Pedestal H-mode at low edge ion collisionality on NSTX [Data set]. Princeton Plasma Physics Laboratory, Princeton University. https://doi.org/10.11578/1661168
  author      = {Battaglia, D. J. and
                Guttenfelder, W. and
                Bell, R. E. and
                Diallo, A. and
                Ferraro, N. and
                Fredrickson, E. and
                Gerhardt, S. P. and
                Kaye, S. M. and
                Maingi, R. and
                Smith, D. R.},
  title       = {{Enhanced Pedestal H-mode at low edge ion
                 collisionality on NSTX}},
  publisher   = {{Princeton Plasma Physics Laboratory, Pri
                nceton University}},
  url         = {https://doi.org/10.11578/1661168}

The Enhanced Pedestal (EP) H-mode regime is an attractive wide-pedestal ELM-free high-betap scenario for NSTX-U and next-step devices as it achieves enhanced energy confinement (H98y,2 > 1.5), large normalized pressure (betaN > 5) and significant bootstrap fraction (f_BS > 0.6) at I_p/B_T = 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. EP H-mode was most often observed as a transition following a large ELM 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 elevated ion temperature gradient that triggers the transition to EP H-mode. Linear CGYRO and M3D-C1 calculations are compared to beam emission spectroscopy (BES) 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 KBM and kink-peeling, can be responsible for the increased particle transport in EP H-mode.

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