Calculations done at NERSC explain the observed large improvement in the confinement of tokamak discharges seeded with neon impurities. The figure shows a fully kinetic linear growth rate calculation for ITG modes over a range of wavenumbers, with reduced growth rates at the high end of the wavenumber spectrum for the discharge with neon. The inset shows the evolution of the measured electron density fluctuations; the discharge with injected neon impurities shows a marked drop in fluctuation level, indicating reduced turbulence.

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

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 (AT) configurations for high performance and identify potential new 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, and BALOO (MHD stability); GLF23, BALDUR, TRANSP, ONETWO, CORSICA, MCGO, and P2D (transport and fueling); CQL3D, CURRAY, and TORAY (current drive); and UEDGE and DEGAS (edge physics). New computational tools being developed and tested include linearized MHD stability codes (ELITE, TWIST-R) and the highly parallelized simulation codes GYRO, GRYFFIN, and FORTEC.

Accomplishments
Gyrokinetic growth rate calculations analyzing the drift-wave stability of a variety of tokamak plasmas found that discharges with neon injection had improved energy confinement due to the suppression of ion temperature gradient (ITG) mode turbulence. The growth rates were reduced both directly by the neon and by E X B shear, which was synergistically enhanced. These neon-injection discharges have significant potential as a new option for improved confinement in ATs.

New electromagnetic gyrofluid simulations of tokamak plasmas, which quantified the transition from electrostatic to electromagnetic turbulence with increasing , call into question the validity of the electrostatic approximation commonly employed in turbulent transport studies. The new simulations found that microturbulence takes on an electromagnetic character even at low values of , and that significant electromagnetic effects on turbulent transport occur.

A new working model of edge localized modes (ELMs), which have been observed but poorly understood for two decades, was developed and shown to describe the DIII-D ELM behavior well. Analysis of results from resistive wall mode (RWM) closed loop feedback experiments in DIII-D showed the first clear evidence that the n = 1 RWM can be controlled by an applied external magnetic field.

Significance
Recent progress in fusion has been accelerated as a result of a strong coupling between theory, computation, and experiments. Previous calculations over the past decade identified several extremely promising AT configurations that are now the focus of the U.S. tokamak program. AT and alternative configurations need to be optimized further to guide future experiments.

Publications
G. M. Staebler, G. L. Jackson, W. P. West, S. L. Allen, R. J. Groebner, M. J. Schaffer, and D. G. Whyte, "Improved high-mode with neon injection in the DIII-D Tokamak," Phys. Rev. Lett. 82, 1692 (1999).

J. Kinsey, R. E. Waltz, and J. C. DeBoo, "Perturbative tests of theoretical transport models using cold pulse and modulated electron cyclotron heating experiments," Phys. Plasmas 6, 1865, (1999).

R. E. Waltz and R. L. Miller, "Ion temperature gradient turbulence and plasma shape," Phys. Plasmas 6, 4265, (1999).

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