Graduate School in Nonlinear Science

Sponsored by The Danish Research Agency

MIDIT                              OFD                          CATS
Modelling, Nonlinear Dynamics      Optics and Fluid Dynamics    Chaos and Turbulence Studies
and Irreversible Thermodynamics    Risø National Laboratory     Niels Bohr Institute and 
Technical University of Denmark    Building 128                 Department of Chemistry
Building 321                       P.O. Box 49                  University of Copenhagen 
DK-2800 Lyngby                     DK-4000 Roskilde             DK-2100 Copenhagen Ø
Denmark                            Denmark                      Denmark


Bruce Scott
Max-Planck-Institut fuer Plasmaphysik
D-85748 Garching, Germany

Thursday, April 26, 2001, 14:30 h
at OFD Meeting Room, Building 130
Risų National Laboratory, 4000 Roskilde

Abstract: The interaction between sheared ExB flows and drift wave turbulence is investigated numerically with a variety of models from the simple 2D Hasegawa-Wakatani to the complete 3D warm-ion Landau fluid drift equations. Drift wave physics differs from other systems due to the strong dissipative coupling between ExB eddies and transported quantities, and so does its coupling to background flows. A background flow vorticity suppresses turbulence by exciting a Reynolds stress with the same sign as the vorticity, transferring free energy from eddies to flows. Zonal flow self generation works by the same mechanism as an instability: free energy is transferred preferentially from eddies to flows. In tokamak geometry, zonal flows are still generated but they are coupled to global kinetic shear Alfven waves via the geodesic curvature. This acts as a loss channel, keeping the zonal flow levels low enough that the turbulence does not self-suppress. Most of the difference between slab and toroidal drift Alfven turbulence and transport is actually due to this mechanism; the interchange forcing effects cause a difference of only 10 to 15 percent in the transport.

With the ion temperature present the interchange effects are more important but the turbulence still does not self suppress (this result is dependent on a vigorous electromagnetic electron response, so it is limited to tokamak edge scenarios). An externally applied flow shear, however, effectively suppresses the turbulence. The end result is that ExB flow shear generation by turbulence is weak or nonexistent for tokamak edge plasmas, so that confinement regime transitions would have to come from self-consistent properties of the neoclassical equilibrium.