Energy exchange dynamics across L–H transitions in NSTX

Diallo, A.; Banerjee, S.; Zweben, S. J.; Stoltzfus-Dueck, T.

doi:10.1088/1741-4326/aa6a24

Energy exchange dynamics across L–H transitions in NSTX

Journal Article · Wed May 10 00:00:00 EDT 2017 · Nuclear Fusion

DOI:https://doi.org/10.1088/1741-4326/aa6a24· OSTI ID:1360936

^[1]; Banerjee, S. ^[2]; ^[1]; ^[1]

Princeton Univ., Princeton, NJ (United States). Princeton Plasma Physics Lab.
Institute for Plasma Research, Gujarat (India)

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.

View Accepted Manuscript (DOE)

Research Organization:: Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)

Grant/Contract Number:: AC02-09CH11466

OSTI ID:: 1360936

Report Number(s):: PPPL--5378

Journal Information:: Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 6 Vol. 57; ISSN 0029-5515

Publisher:: IOP ScienceCopyright Statement

Country of Publication:: United States

Language:: English

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Energy Exchange Dynamics across L-H Transitions in NSTX Diallo, A.; Banerjee, S.; Zweben, S. Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States) https://doi.org/10.11578/1562017	dataset	January 2017
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Experimental observation of electron-scale turbulence evolution across the L–H transition in the National Spherical Torus Experiment Ren, Y.; Smith, D. R.; Zweben, S. J. Nuclear Fusion, Vol. 59, Issue 9 https://doi.org/10.1088/1741-4326/ab2f4f	journal	August 2019

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