3D BOUT edge plasma simulations show large density fluctuations on the outboard side for L-mode |n/n|~0.2 and for H-mode |n/n|~0.05. H-mode structures are broken up and their amplitudes are suppressed by flow shear.

Bruce Cohen, Maria Caturla, Harry McClean, Andris Dimits, Thomas Rognlien, Xueqiao Xu, Alice Koniges, Gary Kerbel, Dan Shumaker, Tomas Diaz Delarubia, Lynda LoDestro, Marvin Rensink, Brian Wirth, Babak Sadigh, Simon Woodruff, and Susanne Ramsey, Lawrence Livermore National Laboratory

Research Objectives
(1) Edge plasmas: Continued development of simulation tools to model the boundary region of magnetic fusion energy (MFE) devices between the hot core plasma and material walls. (2) Core transport: Gyrokinetic simulation of drift-wave driven transport of energy and plasma across magnetic field lines. (3) Neutron interactive materials: Simulations to determine the effect of irradiation on materials properties. (4) Simulation of spheromak plasmas: Extend previous simulations of spheromak plasmas to study formation, stability, and field line closure.

Computational Approach
(1) A 2D fluid transport code (UEDGE) includes multispecies impurities to calculate edge-plasma profiles, and a 3D fluid turbulence code (BOUT) predicts the anomalous turbulence-induced transport of particles and energy across the confining magnetic field in the edge region. (2) In five-phase-space dimensional particle simulations of plasma, the Vlasov-Maxwell or Vlasov-Poisson systems of equations are solved on a grid in configuration space and with a Monte Carlo sampling technique in velocity space. (3) A molecular dynamics (MD) code (MDCASK) uses empirical, alloy interatomic potentials to simulate the evolution of displacement cascades, fundamental kinetics properties of defects in metals, and the interactions between defects.

Accomplishments
(1) Substantial progress has been made in understanding edge turbulence and its role in formation of the edge H-mode transport barrier. BOUT shows that X-point damping, coupled with the change in boundary flow direction, plays a crucial role in determination of the fluctuation levels.

(2) Gyrokinetic results are described in the NTTP abstract.

(3) Simulations of the evolution of recoils in Cu and W for energies from a few eV to 100 keV show damage in the form of vacancy and interstitial clusters in Cu, but few vacancy clusters in W. Large-scale MD simulations to study irradiation-produced defects in face-centered cubic metals show that a key mechanism in the formation of defect-free dislocation channels is the absorption of stacking fault tetrahedra by moving dislocations.

(4) A series of simulations indicate how the gun potential, the plasma resistivity, and the dimensions of the gun and flux conserver influence formation and magnetic flux buildup in the spheromak plasma.

Significance
Edge plasmas and core turbulence are both critical issues for improved performance of MFE devices. The application of fusion as a viable energy source also depends on developing structural materials that can withstand the harsh radiation conditions of the fusion environment without experiencing severe degradation.

Publications
A. M. Dimits et al., "Comparisons and physics basis of tokamak transport models and turbulence simulations," Phys. Plasmas 7, 969 (1999).

M.-J. Caturla, N. Soneda, E. Alonso, B. D. Wirth, T. Diaz de la Rubia, and M. Perlado, "Comparative study of radiation damage accumulation in Cu and Fe," J. Nucl. Mat. 276, 13 (2000).

X. Q. Xu, R. H. Cohen, T. D. Rognlien, and J. R. Myra, "L-H transitions simulations in divertor geometry," Phys. Plasmas 7, 1951 (2000).