Energy exchange dynamics across L–H transitions in NSTX

Abstract

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 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.

Overview of NSTX Upgrade initial results and modelling highlights

“…Theoretical work to understand the dynamics of the L-H transition [35] and the turbulence changes at the transition has recently been published [36]. The work addresses a popular model for the L-H transition, in which the energy in turbulent fluctuations is directly depleted via Reynoldsstress-induced energy transfer to the zonal flows.…”

Energy exchange dynamics across L–H transitions in NSTX

“…A. Diallo 1 , S. Banerjee 2 , S. J. Zweben1, and T. Stoltzfus-Dueck 1 1 Princeton Plasma Physics Laboratory, Princeton 2 Institute for Plasma Research, Bhat e-mail (speaker): adiallo@pppl.gov H-mode is planned for future devices such as ITER, and is preceded by a low (L) to high (H) transition. A key question remains.…”

“…Most theoretical descriptions of the L-H transition are based on the shear of the radial electric field and coincident ExB poloidal flow shear, which is thought to be responsible for the onset of the anomalous transport suppression that leads to the L-H transition. This talk will focus on the analysis of the flow dynamics across the L-H transition in NSTX [1].…”

Experimental validation of universal plasma blob formation mechanism

“…'Hot' colormap has been used where white color indicates the highest density and black color indicates the lowest density. The images have been superposed on quiver plots of the radial and poloidal velocities of the plasma obtained from velocimetry measurements [21,22]. It is to be noted that the visible images acquired by GPI are taken here as a proxy of the density contours as the intensity is mainly proportional to the density, and its T e dependence has been neglected.…”