Energy exchange dynamics across L–H transitions in NSTX: Theory and experiment

ASDEX Upgrade seminar: ASDEX Upgrade Seminar

  • Datum: 22.05.2017
  • Uhrzeit: 15:00 - 16:30
  • Vortragende(r): Timothy Stoltzfus-Dueck
  • Ort: Garching
  • Raum: Seminarraum L6, 2.Stock Süd
  • Gastgeber: IPP

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

In the popular 'predator-prey' paradigm for the L-H transition, energy transfer to the mean flows directly depletes turbulence fluctuation energy, resulting in suppression of the turbulence and a corresponding transport bifurcation. To quantitatively evaluate this mechanism, one must remember that electron parallel force balance couples nonzonal velocity fluctuations with electron pressure fluctuations on rapid timescales, comparable with the electron transit time. For this reason, energy in the nonzonal velocity stays in a fairly fixed ratio to the free energy in electron density fluctuations, at least for frequency scales much slower than electron transit. In order for direct depletion of the energy in turbulent fluctuations to cause the L-H transition, energy transfer via Reynolds stress must therefore drain enough energy to significantly reduce the sum of the free energy in nonzonal velocities and electron pressure fluctuations. At low k, the electron thermal free energy is much larger than the energy in nonzonal velocities, posing a stark challenge for this model of the L-H transition. Indeed, orthogonal dynamic programming (ODP) velocimetry of gas-puff-imaging (GPI) movies of L-H transitions in NSTX indicates that the Reynolds work is about two orders of magnitude too small to explain the drop in turbulent free energy, when electron density fluctuations are included. As a complementary check, the energy in zonal flows is evaluated and found to be about two orders of magnitude smaller than the L-mode turbulent free energy. These results hold despite the fact that Reynolds stress is large enough to make a significant contribution to the zonal flows, simply because the fluctuating pressure holds so much more free energy than the turbulent ExB velocity.

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