PPPL-5378
Energy Exchange Dynamics across L-H Transitions in NSTX
Authors: A. Diallo, S.J. Zweben
Abstract: We studied the energy exchange dynamics across the
low-to-high-con finement (L-H) transition in NSTX discharges using
the gas-pu ff 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 apply this study to
three type 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 s) resolutions. Analysis using
these velocity fields shows that the production term is
systematically negative just prior to the L-H transition indicating
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 turbulence depletion
mechanism may not be playing 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.