Elena V. Belova Abstract Narrative for APS-DPP Invited Paper

“Kinetic Effects on the Linear and Nonlinear Stability Properties of Field-Reversed Configurations”

Elena Belova, Plasma Physics Laboratory, Princeton University, Princeton, NJ 08543

The Field-Reversed Configuration (FRC) offers a unique fusion reactor potential because of its compact and simple geometry, translational properties, and high plasma beta. One of the most important issues is FRC stability with respect to low-n (toroidal mode number) MHD modes. This paper presents the results of hybrid and two-fluid simulations of prolate FRCs aimed at resolving the long-standing controversy between the theoretical predictions of ideal MHD instabilities in FRCs and their experimentally observed macroscopic stability. The n=1 tilt instability mechanism and various stabilizing effects are investigated in detail. It is shown that the Hall effect determines the mode rotation and the change in the linear mode structure in the kinetic regime; however, the reduction in the growth rate is mostly due to finite Larmor radius effects. The experimentally observed scaling of the growth rate of the n = 1 tilt mode with the S*/E parameter is demonstrated for a class of long elliptically-shaped configurations, where S* is the ratio of the separatrix radius to the ion Larmor radius, and E is the separatrix elongation. It is shown that, contrary to the usually assumed stochasticity of the ion orbits in FRCs, a large fraction of the orbits are regular in long configurations when S* is small. A regularity condition has been obtained, and the number of regular orbits has been found to scale approximately linearly with 1/S*. Resonant particle effects are shown to maintain the instability in the large gyroradius regime. However, a nonlinear saturation of the instability is observed in these cases. Additional stabilizing factors, such as finite electron pressure and velocity shear, are also investigated. The simulation results clearly demonstrate that a combination of kinetic and nonlinear effects is key to understanding the experimentally observed FRC stability properties.

*Research supported by the DOE under Contract No. DE-AC02 76CH03073