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Title: Energy exchange dynamics across L–H transitions in NSTX

Abstract

Here, we studied the energy exchange dynamics across the low-to-high-confinement (L–H) transition in NSTX discharges using the gas-puff imaging (GPI) diagnostic. The investigation focused on the energy exchange between flows and turbulence to help clarify the mechanism of the L–H transition. We applied this study to three types of heating schemes, including a total of 17 shots from the NSTX 2010 campaign run. Results show that the edge fluctuation characteristics (fluctuation levels, radial and poloidal correlation lengths) measured using GPI do not vary just prior to the H-mode transition, but change after the transition. Using a velocimetry approach (orthogonal-dynamics programming), velocity fields of a $$24\times 30$$ cm GPI view during the L–H transition were obtained with good spatial (~1 cm) and temporal (~2.5 μs) resolutions. Analysis using these velocity fields shows that the production term is systematically negative just prior to the L–H transition, indicating a transfer from mean flows to turbulence, which is inconsistent with the predator–prey paradigm. Moreover, the inferred absolute value of the production term is two orders of magnitude too small to explain the observed rapid L–H transition. These discrepancies are further reinforced by consideration of the ratio between the kinetic energy in the mean flow to the thermal free energy, which is estimated to be much less than 1, suggesting again that the turbulence depletion mechanism may not play an important role in the transition to the H-mode. Although the Reynolds work therefore appears to be too small to directly deplete the turbulent free energy reservoir, order-of-magnitude analysis shows that the Reynolds stress may still make a non-negligible contribution to the observed poloidal flows.

Authors:
ORCiD logo [1];  [2]; ORCiD logo [1]; ORCiD logo [1]
  1. Princeton Univ., Princeton, NJ (United States). Princeton Plasma Physics Lab.
  2. Institute for Plasma Research, Gujarat (India)
Publication Date:
Research Org.:
Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)
OSTI Identifier:
1360936
Report Number(s):
PPPL-5378
Journal ID: ISSN 0029-5515
Grant/Contract Number:
AC02-09CH11466
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Nuclear Fusion
Additional Journal Information:
Journal Volume: 57; Journal Issue: 6; Related Information: Note that the digital data for this paper can be found at http://arks.princeton.edu/ark:/88435/dsp01f1881p40x.; Journal ID: ISSN 0029-5515
Publisher:
IOP Science
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; confinement; L–H transition dynamics; gas-puff-imaging; zonal flows; Reynolds stress

Citation Formats

Diallo, A., Banerjee, S., Zweben, S. J., and Stoltzfus-Dueck, T. Energy exchange dynamics across L–H transitions in NSTX. United States: N. p., 2017. Web. doi:10.1088/1741-4326/aa6a24.
Diallo, A., Banerjee, S., Zweben, S. J., & Stoltzfus-Dueck, T. Energy exchange dynamics across L–H transitions in NSTX. United States. doi:10.1088/1741-4326/aa6a24.
Diallo, A., Banerjee, S., Zweben, S. J., and Stoltzfus-Dueck, T. Wed . "Energy exchange dynamics across L–H transitions in NSTX". United States. doi:10.1088/1741-4326/aa6a24. https://www.osti.gov/servlets/purl/1360936.
@article{osti_1360936,
title = {Energy exchange dynamics across L–H transitions in NSTX},
author = {Diallo, A. and Banerjee, S. and Zweben, S. J. and Stoltzfus-Dueck, T.},
abstractNote = {Here, we studied the energy exchange dynamics across the low-to-high-confinement (L–H) transition in NSTX discharges using the gas-puff imaging (GPI) diagnostic. The investigation focused on the energy exchange between flows and turbulence to help clarify the mechanism of the L–H transition. We applied this study to three types of heating schemes, including a total of 17 shots from the NSTX 2010 campaign run. Results show that the edge fluctuation characteristics (fluctuation levels, radial and poloidal correlation lengths) measured using GPI do not vary just prior to the H-mode transition, but change after the transition. Using a velocimetry approach (orthogonal-dynamics programming), velocity fields of a $24\times 30$ cm GPI view during the L–H transition were obtained with good spatial (~1 cm) and temporal (~2.5 μs) resolutions. Analysis using these velocity fields shows that the production term is systematically negative just prior to the L–H transition, indicating a transfer from mean flows to turbulence, which is inconsistent with the predator–prey paradigm. Moreover, the inferred absolute value of the production term is two orders of magnitude too small to explain the observed rapid L–H transition. These discrepancies are further reinforced by consideration of the ratio between the kinetic energy in the mean flow to the thermal free energy, which is estimated to be much less than 1, suggesting again that the turbulence depletion mechanism may not play an important role in the transition to the H-mode. Although the Reynolds work therefore appears to be too small to directly deplete the turbulent free energy reservoir, order-of-magnitude analysis shows that the Reynolds stress may still make a non-negligible contribution to the observed poloidal flows.},
doi = {10.1088/1741-4326/aa6a24},
journal = {Nuclear Fusion},
number = 6,
volume = 57,
place = {United States},
year = {Wed May 10 00:00:00 EDT 2017},
month = {Wed May 10 00:00:00 EDT 2017}
}

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