Research Objective
A predictive model of turbulence and transport barrier formation in the tokamak edge.
Computational Approach
We model anomalous transport in the edge region of tokamaks with three-dimensional, electromagnetic simulations of plasma turbulence in a torus. The simulations are based on the Braginskii equations in a shifted-circle toroidal geometry, and are carried out in a poloidally and radially localized, flux-tube-like domain. Typical simulations are computationally intensive, requiring a grid resolution of roughly 128x128x128 and exceeding 10^5 time steps. We therefore rely heavily on the multiprocessing capabilities of J90s.
Accomplishments
In contrast to earlier electrostatic turbulence studies, we observe two distinct, new regimes in the electromagnetic system: an L-mode-like regime at low temperature (weak diamagnetic regime), in which the transport increases sharply as the plasma pressure approaches a small fraction of the ideal ballooning instability limit (see figure), and an H-mode-like regime at high temperature (high diamagnetic regime), in which complete suppression of the turbulence occurs as the ideal limit is approached. This suppression is due to local, sheared, self-generated ExB and diamagnetic flows. In the early stages of the formation of the barrier strong ExB flows develop. The ion pressure then rapidly evolves so that the local ion pressure gradient balances the radial electric field, the total ion poloidal rotation being small. The magnetic perturbations and finite ion temperature are essential for the the development of the barrier. These results suggest that a transport barrier in the edge region can form spontaneously when local diamagnetic effects are strong and the pressure gradient exceeds a threshold.
Significance
The study of tokamak edge transport and related phenomena, such as the L-H transition, the structure of the pedestal profiles, the density limit, and edge-localized modes, is among the most critical efforts in the controlled fusion community, and is vital to the accurate assessment of future tokamak designs.
Publications
J. Drake, Y. Lau, P. Guzdar, A. Hassam, S. Novakovski, B. Rogers, A. Zeiler. 1996. Local Negative Shear and the Formation of Transport Barriers, Phys. Rev. Lett. 3:494.
B. Rogers and J. Drake. 1997. Enhancement of Turbulence in Tokamaks by Magnetic Fluctuations, Phys. Rev. Lett. 79:229.
Turbulent density perturbations caused by the nonlinear development of drift-resistive
ballooning modes.