Contour plots of density perturbations in the poloidal plane for ion temperature gradient mode turbulence, comparing (a) r* = 0.0025 (240 ion gyroradii) and (b) r* = 0.0075 (80 ion gyroradii) simulations with moderate profile variation and an adaptive source to maintain equilibrium profiles. The diffusion is in a gyroBohm scaled regime. Cases closer to threshold with more rotational velocity shear show Bohm, scaled diffusion.

A. D. Turnbull, J. Candy, M. S. Chu, J. R. Ferron, L. L. Lao, P. B. Snyder, G. Staebler, R. E. Waltz, D. Brennan, and the DIII-D team, General Atomics, Inc.
A. M. Garofalo, Columbia University
E. J. Kinsey, Lehigh University

Research Objectives
The aim of this research is fourfold: (1) Provide support calculations for the DIII-D National Fusion Facility, including experimental predictions and analysis and interpretation of data. (2) Establish an improved theoretical and computational scientific basis for the physics of fusion plasmas. (3) Optimize presently known Advanced Tokamak configurations for high performance and identify potential new Advanced Tokamak configurations. (4) Explore and optimize alternative magnetic confinement configurations, and elucidate the relationships between these and tokamak configurations.

Computational Approach
The principal codes used are EFIT and TOQ (equilibrium); GATO, TWIST-R, MARS, BALOO, DCON, NIMROD, and BOUT (MHD stability); GLF23, GYRO, BALDUR, TRANSP, ONETWO, CORSICA, MCGO, and P2D (transport and fuelling); CQL3D, CURRAY, and TORAY (current drive); and UEDGE and DEGAS (edge physics). New computational tools are also being developed and tested, especially linearized MHD stability codes (ELITE, TWIST-R) and the new highly parallelized simulation codes GYRO, GRYFFIN, and FORTEC.

Accomplishments
Considerable progress was achieved in understanding the physics of rotating wall-stabilized plasmas and in increasing DIII-D performance. Stability calculations were used in conjunction with improvements in DIII-D experiments to show that rotational stabilization can be maintained with values up to twice the limit attainable without wall stabilization. Calculations were also used to model the intelligent shell realization for active control used in the DIII-D experiments. These calculations are being incorporated into designs for an extension of the active control system on DIII-D.

New insight into how the fundamental processes determining the size and field strength scaling of confinement can be obtained from dimensionally similar tokamak discharges was gained through analysis of numerical results and experiments. The transport bifurcation from L- to H-mode and internal transport barriers, as well as the edge confinement improvement in the DIII-D VH-mode, can be explained by a theory based on E x B rotational shear driven by changes in the diamagnetic flows at the plasma edge.

Theory-based transport modeling is providing new understanding of the fundamental transport processes in high- plasmas. Gyrofluid nonlinear ballooning mode flux tube methods developed for numerically simulating 3D homogeneous turbulence in toroidal geometry were applied to determine the dependence of transport on shear, safety factor, toroidicity, and sheared E x B rotation. Parallel electromagnetic gyrofluid simulations using the GRYFFIN code showed that microturbulence takes on an electromagnetic character even at low values of ; significant electromagnetic effects on turbulent transport were found, such as a reduction in heat transport at low and a significant increase in heat transport as the MHD limit is approached.

Significance
Recent progress in fusion has been accelerated as a result of a renewed emphasis on scientific understanding of tokamak plasmas, which has been brought about by a strong coupling between theory, computation, and experiments. It is therefore important to pursue more theoretical investigations in areas such as macroscopic stability, microinstabilities, and turbulence.

Publications
M. R. Wade et al., "Progress toward long-pulse high-performance Advanced Tokamak discharges on the DIII-D tokamak," Phys. Plasmas 8, 2208 (2001).

R. E. Waltz et al., "Animation of drift ballooning modes and zonal flow turbulence," Phys. Plasmas (in press).

A. D. Turnbull et al., "Predictive capability of MHD stability limits in high performance DIII-D discharges," Nucl. Fusion (in press).

http://fusion.gat.com