Nonlinear MHD Dynamics of Tokamak Plasmas on Multiple Time Scales

Long wavelength instabilities in fusion plasmas are often described within the framework of MHD theory. The various types of instabilities have different time scales associated with their growth. In real experiments, an understanding of how the growth rate varies with the free energy source is important for fully understanding the stability limits and the subsequent growth of the mode.

	Figure 3   Plot of tearing mode energies in DIII-D discharge 86166, where a series of nearly identical sawteeth led to the emergence of a confinement-degrading 3/2 mode. Despite similar equilibria, the m/n = 3/2 island grows in the later discharge after the sawtooth.

Kruger et al. have used the NIMROD code to explore the onset mechanisms of instabilities in the DIII-D tokamak experiment. In their first simulations, they modeled the disruption occurring in DIII-D discharge 87009 as an ideal MHD instability driven by neutral-beam heating. The mode grows faster than exponential, but on a time scale that is a hybrid of the heating rate and the ideal MHD growth rate as predicted by analytic theory.

The second series of simulations, which occur on a much longer time scale, focus on the seeding of tearing modes by sawteeth, where pressure effects play a role both in the exterior region solutions and in the neoclassical drive terms (Figure 3). The ability to routinely run simulations of tokamak discharges at realistic values of diffusivities (kinetic, electrical, and thermal) has greatly increased the power of these calculations to make quantitative comparisons with experimental results.

INVESTIGATORS
D. D. Schnack and S. E. Kruger, Science Applications International Corporation; D. P. Brennan, General Atomics; T. A. Gianakon, Los Alamos National Laboratory; C. R. Sovinec, University of Wisconsin, Madison.

PUBLICATION
S. E. Kruger, D. D. Schnack, D. P. Brennan, T. A. Gianakon, and C. R. Sovinec, “Nonlinear MHD dynamics of tokamak plasmas on multiple time scales,” Nucl. Fusion (in press).

URL
http://www.nimrodteam.org/