Stephen Jardin, W. W. Lee, Z. Lin, D. Stotler, D. Mikkelsen,
W. Park, M. A. Beer, and G. W. Hammett, Princeton Plasma Physics Laboratory

Results from large-scale full torus gyrokinetic particle simulations of plasma microturbulence in tokamak with device-size scans for realistic parameters show that Bohm-like transport can be driven by microscopic-scale fluctuations with isotropic spectra. These simulation results resolve some apparent physics contradictions between experimental observations and turbulent transport theories.

Research Objectives
(1) Improving the existing 3D gyrokinetic particle code to study neoclassical and turbulent transport in tokamaks and stellarators, as well as to investigate hot-particle physics, toroidal Alfvén modes, and neoclassical tearing modes. (2) Enhancing the existing 3D beam equilibrium, stability, and transport (BEST) code, used for two-stream and filamentation instability studies for space-charge dominated beams in accelerators and chamber transport in heavy ion fusion research. (3) Developing a new simulation model with application to studies of laser-plasma interaction physics in the areas of turbulence and transport. (4) Study of tokamak and stellarator plasma turbulence with a 3D flux-tube gyrofluid code (GRYFFIN), which has recently been extended to include equilibrium-scale sheared E X B flows. (5) Coupling the DEGAS 2 Monte Carlo neutral transport code and the UEDGE fluid plasma transport code to analyze experimental results and predict scrape-off layer conditions in future devices.

Computational Approach
(1 and 2) The particle-in-cell method is used for transport studies. (3) A 2D ion + fluid electron code has been developed to study laser-driven acoustic turbulence. A 1D Monte-Carlo code for studying nonclassical drive and transport of electrons in laser-plasma instabilities is being developed. (4) The gyrofluid code is a 3D nonlinear pseudo-spectral code, using both spectral and grid representations with fast Fourier transforms between the two representations. (5) The DEGAS 2 code has been parallelized and demonstrates excellent scaling. While UEDGE has been parallelized, some testing and possible improvements remain. Since the two codes parallelize differently, schemes for parallelizing the coupled code system must be established and evaluated.

Accomplishments
Using the GTC code, we demonstrated the importance of nonlinearly generated zonal flow for the reduction of ion thermal transport, as well as the role played by ion-ion collisions for the bursting behavior observed in the tokamak experiments. We used the BEST code to study two-stream instabilities in space-charge-dominated beams in accelerators, helping to explain the beam loss observed in various machines. We demonstrated numerically the existence of non-KV equilibria in a periodic focusing lattice. We devised an efficient numerical particle scheme for treating electrons; it enables us to circumvent the parallel Courant condition.

We extended the nonlinear gyrofluid code to include equilibrium-scale-sheared E X B flows, with a coordinate system that shears in time to follow the flow. The nonlinear effects of this equilibrium-sheared flow can be significantly different than the linear effects. Our gyrokinetic continuum simulations show that electron-temperature-gradient turbulence can be much larger than suggested by naive scaling from ion-temperature-gradient turbulence because of the different adiabiatic species response. We used the DEGAS 2 code to interpret CMOD data and have completed the initial coupling of the UEDGE and DEGAS 2 codes.

Significance
Three-dimensional modeling helps build a fundamental understanding of plasma turbulence processes and turbulence suppression methods, improves the analysis of experimental results, and suggests new ways to improve magnetic and heavy ion fusion reactor designs.

Publications
Z. Lin, T. S. Hahm, W. W. Lee, W. M. Tang, and R. B. White, "Gyrokinetic simulations in general geometry and applications to collisional damping of zonal flows," Phys. Plasmas 7, 1857 (2000).

Z. Lin, M. S. Chance, T. S. Hahm, J. A. Krommes, W. W. Lee, I. Manuilskiy, H. E. Mynick, H. Qin, G. Rewoldt, W. M. Tang, and R. B. White, "Numerical and theoretical studies of turbulence and transport with E X B shear flows," Nuclear Fusion 40, 737 (2000).

I. Manuilskiy and W. W. Lee, "The split-weight particle simulation scheme for plasmas," Phys. Plasmas 7, 1381 (2000).

http://w3.pppl.gov/theory/