The Dimits Shift in More Realistic Gyrokinetic Plasma Turbulence Simulations
Authors: D.R. Mikkelsen and W. Dorland
Abstract:
In simulations of turbulent plasma transport due to long wavelength, (k⊥pi ≤ 1), electrostatic drift-type instabilities we find that a nonlinear upshift of the effective threshold persists. This `Dimits shift' represents the difference between the linear threshold, at the onset of instability, and the nonlinear threshold, where transport increases suddenly as the driving temperature gradient is increased. As the drive increases, the magnitudes of turbulent eddies and zonal ows grow until the zonal flows become nonlinearly unstable to 'tertiary' modes and their sheared ows no longer grow fast enough to strongly limit eddy size. The tertiary mode threshold sets the effective nonlinear threshold for the heat transport, and the Dimits shift arises when this occurs at a zonal flow magnitude greater than that needed to limit transport near the linear threshold. Nextgeneration tokamaks will likely benefit from the higher effective threshold for turbulent transport, and transport models should incorporate suitable corrections to linear thresholds. These gyrokinetic simulations are more realistic than previous reports of a Dimits shift because they include nonadiabatic electron dynamics, strong collisional damping of zonal flows, and finite electron and ion collisionality together with realistic shaped magnetic geometry. Reversing previously reported results based on idealized adiabatic electrons, we find that increasing collisionality reduces the heat flux because collisionality reduces the nonadiabatic electron drive.
__________________________________________________
Submitted to: Physical Review Letters (April 2008)
__________________________________________________
Download PPPL-4336 (pdf 325.59KB 11pp)
__________________________________________________