Historical research guide · 1095 · 2010
Nonlinear Flow Generation By Electrostatic Turbulence In Tokamaks
W. X. Wang · P. H. Diamond · T. S. Hahm · S. Ethier · G. Rewoldt · W. M. Tang
In one minute
Six plasma physicists at Princeton and the University of California San Diego — W. X. Wang, P. H. Diamond, T. S. Hahm, S. Ethier, G. Rewoldt and W. M. Tang — put the Gyrokinetic Tokamak Simulation code on national supercomputers to chase a puzzle that turns up in almost every tokamak: the plasma starts spinning by itself, with nothing pushing it. Their simulations locate the push. The plasma's own small-scale turbulence builds large, slow, banded flows — zonal flows, the same self-organising banding you see in Jupiter's clouds — and the shear in those bands tilts the turbulence so that waves travelling one way along the magnetic field outnumber waves travelling the other way. That imbalance is a real internal torque, and it spins a plasma up from rest. The team report the effect for the first time in trapped-electron turbulence, find this residual stress carrying more than half the total momentum flux, and watch flow perturbations march radially inward in what they name a flow pinch.
Connection
Chapter 9 is about plasma that organises itself — structures that build and sustain their own fields rather than being held in place from outside. This is that behaviour measured in the most heavily instrumented plasma humans operate: turbulence generating its own large-scale flow, and that flow feeding back to steer the turbulence. Chapter 12's case for a compact high-density energy source depends on exactly this loop, because the spin the plasma gives itself is what stabilises it and holds the heat in.
From the source
In collisionless trapped-electron-mode turbulence, nonlinear residual stress is generated both by the fluctuation intensity and by the gradient of that intensity, once the symmetry of the parallel wavenumber spectrum is broken — a result identified here for the first time.Abstract; Section V, Figures 13 and 14; Conclusions, item i
Turbulence self-generated low-frequency zonal flow shear is a key, universal mechanism for breaking the parallel wavenumber symmetry across turbulence regimes, and that symmetry breaking is the critical ingredient that lets turbulence accelerate a net parallel and toroidal flow.Section IV, Figure 7; Conclusions, item i
In a simulated DIII-D plasma started with zero rotation and no momentum source at all, the residual stress acts as an internal torque and spins up a net toroidal rotation in the co-current direction across the whole turbulent region, the parallel flow reaching about 5 percent of the local ion thermal velocity.Section V, Figure 15; Conclusions, item ii
For typical tokamak parameters the nonlinearly generated residual stress contributes more than 50 percent of the total momentum flux driven by ion-temperature-gradient turbulence, and the intrinsic Prandtl number rises from about 0.4 to about 0.7 as the normalised ion temperature gradient goes from 5.5 to 8.5.Section III, Figure 6; Conclusions, item v
A flow pinch appears in collisionless trapped-electron-mode turbulence: parallel flow perturbations generated locally near the plasma centre propagate radially at about seven thousandths of the sound speed, which may explain the inward penetration of modulated flows seen in the JT-60U perturbation experiments.Section V, Figure 15; Conclusions, item iii
With kinetic electrons retained, the correlation between zonal flow shear and the parallel wavenumber weakens, showing that mechanisms beyond E cross B shear also break the symmetry; magnetic shear, nonlinear mode coupling and turbulent radial current are the named candidates for the next measurement.Section V, Figure 10; Conclusions, item vi
The way in
https://doi.org/10.2172/984349How to cite it
W. X. Wang, P. H. Diamond, T. S. Hahm, S. Ethier, G. Rewoldt, W. M. Tang (2010) Nonlinear Flow Generation By Electrostatic Turbulence In Tokamaks doi:10.2172/984349