Transport Barrier Dynamics
W. Horton, T. Tajima, P. Zhu, and L. Leonard, University of Texas at Austin
Research Objectives
This project develops the theoretical and simulation tools for describing the radial transport in tokamaks under conditions where local radial regions develop fast time-scale dynamics. This occurs in transport models that allow for the bifurcations between different transport mechanisms, as is the general case in both neutral-fluid and plasma-turbulent transport.
Computational Approach
The parallel processors of the Cray T3E are distributed over the radius of the tokamak plasma to build a high-radial-resolution transport code capable of following the very steep gradients found in the internal transport barriers of high performance tokamaks. The time dynamic in each radial zone is adaptive to the appropriate fast or slow evolution in that particular radial zone.
Accomplishments
We have developed a model on the Cray T3E that describes the bifurcations occurring in the radial temperature and density profiles in tokamak plasmas. The bifurcations arise from the interaction of the turbulence with the transport profiles through the mechanisms associated with shearing in the radial electric field and the velocity flows. High spatial and temporal resolution algorithms are used to capture the physics of the bifurcations. The space and time scales of the bifurcations are hybrids between the long global transport scales and the mircoscales of the drift-wave turbulence. The present model is built on the ion-temperature gradient driven turbulence; generalizations to include other forms of microturbulence are in the planning stages.
Significance
The problem of bifurcations in the turbulent transport of fluids and plasmas is of wide scientific interest. Observed bifurcations to states with sheared flows in the problem of Bernard convection are relevant to understanding the more complex problem in plasmas. Gaining such an understanding is of key importance for plasma confinement, since several of the large mega-ampere systems find their record fusion performance shots only after making the bifurcation to a high confinement state (as shown in the figure). The signature for the high confinement state is the formation of an internal transport barrier (ITB). The project will allow several hypotheses for the improved confinement modes to be tested quantitatively. The presence or absence of an internal transport barrier could make the difference in an acceptable fusion power gain for a magnetic fusion reactor.
Vorticity levels in a 6 x 6 cm box perpendicular to the magnetic field and centered on the minimum q-surface. The ion temperature gradient convection forms a transport barrier with tilted vortices on each side arising from self-consistent E x B flows.
Publications
G. Hu and W. Horton, "Minimal model for transport barrier dynamics based on ion-temperature gradient turbulence," Physics of Plasmas 4, 3262-3272 (1997).
W. Horton, T. Tajima, J.-Q. Dong, J.-Y. Kim and Y. Kishimoto, "Ion transport analysis in a high beta-poloidal JT-60U discharge," Plasma Phys. Control. Fusion 39, 83-104 (1997).
H. Sugama and W. Horton, "Neoclassical electron and ion transport in toriodal rotating plasmas," Physics of Plasmas 4, 2215-2228 (1997).