Collisional and Turbulent Transport in Fusion Plasmas
Z. Lin, T. S. Hahm, W. W. Lee, W. M. Tang, and R. B. White, Princeton Plasma Physics Laboratory
Research Objectives
The primary purpose of this project is to develop a gyrokinetic particle code capable of simulating transport processes in realistic tokamak fusion plasmas, to use the code to enhance our physics understanding of particle and energy confinement, and to compare the simulation results with the measurements from magnetic fusion experiments.
Computational Approach
The Gyrokinetic Toroidal Code (GTC) was implemented as a platform-independent program based on a domain decomposition algorithm using Message Passing Interface (MPI). GTC achieved nearly perfect scalability on two massively parallel processing (MPP) systems, the Cray T3E and the SGI Origin 2000.
Accomplishments
We have developed a general geometry parallel gyrokinetic toroidal code to study both collisional and turbulent transport processes in magnetically confined toroidal plasmas. In the area of collisional transport, we have resolved the apparent contradiction that ion thermal transport levels in enhanced confinement tokamak plasmas have been observed to fall below the "irreducible minimum level" predicted by standard neoclassical theory. In turbulence simulations, linear poloidal flow damping simulations exhibit an asymptotic residual flow in agreement with recent analytic calculations. Nonlinear global simulations of ion-temperature-gradient instabilities in toroidal magnetized plasmas provide key first principles results supporting the physics picture that turbulence-driven fluctuating E x B flows can significantly reduce turbulent transport. Finally, the outstanding differences in the flow dynamics observed in global and local simulations are found to result from profile variations.
Significance
Our simulation effort has finally enabled us to make contact with existing machine experiments. With the increase in computing power and addition of new physics into the code, we expect gyrokinetic particle simulation to have a significant impact on fusion research in the future.
Poloidal contour plots of fluctuation potential 
in the steady state of nonlinear global simulation with E x B flows included (a) and with the flows suppressed (b). This comparison shows the key mechanism of transport reduction by turbulence-generated E x B flows through the breaking of turbulent eddies, and consequently, the reduction of the radial decorrelation length and associated fluctuation level.
Publications
Z. Lin, T. S. Hahm, W. W. Lee, W. M. Tang, and R. B. White, "Turbulent transport reduction by zonal flows: Massively parallel simulations," Science 281, 1835 (1998).
Z. Lin, W. M. Tang, and W. W. Lee, "Neoclassical transport in enhanced confinement toroidal plasmas," Phys. Rev. Lett. 78, 456 (1997).
W. W. Lee and R. A. Santoro, "Gyrokinetic simulation of isotope effects in tokamak plasmas," Phys. Plasmas 4, 169 (1997).