Graduate School in Nonlinear Science
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
PHYSICS OF ZONAL FLOWS IN DRIFT WAVE TURBULENCE IN TOKAMAK GEOMETRY
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
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
An externally applied flow shear, however, effectively suppresses the
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