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

Diallo, A.  ^[1]; Banerjee, S. ^[2]; Zweben, S. J.  ^[1]; Stoltzfus-Dueck, T.  ^[1]

---

1. Princeton Univ., Princeton, NJ (United States). Princeton Plasma Physics Lab.
2. 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.

• Research Organization: Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Fusion Energy Sciences (FES)
• Grant/Contract Number: AC02-09CH11466
• OSTI ID: 1360936
• Report Number(s): PPPL-5378
• Journal Information: Nuclear Fusion, Vol. 57, Issue 6; Related Information: Note that the digital data for this paper can be found at http://arks.princeton.edu/ark:/88435/dsp01f1881p40x.; ISSN 0029-5515
• Publisher: IOP Science
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
confinement
L–H transition dynamics
gas-puff-imaging
zonal flows
Reynolds stress