The Field-Reversed Configuration (FRC) is a compact toroid with little or no toroidal field. It offers a unique fusion reactor potential because of its compact and simple geometry, translation properties, and high plasma beta. These attractive features have motivated extensive experimental and theoretical research on FRCs. At the present time there are at least six active FRC and ion ring experiments in the United States, including Los Alamos National Laboratory, Princeton Plasma Physics Laboratory, the University of Washington, Cornell University, UC Irvine, and Swarthmore College. One of the most important scientific issues in FRC research is the stability of the FRC configuration with respect to low-n (toroidal mode number) MHD modes.
According to an empirical scaling based on the experimental data for prolate FRCs, stability with respect to global MHD modes is observed for S*/E < 3-4, where E is the separatrix elongation, and S* is a kinetic parameter which measures the number of thermal ion gyro-radii in the configuration. The FRC parameters are defined as follows: The separatrix elongation E is the ratio of the separatrix half-length to its radius (Fig. 1); the kinetic parameter, S*, is the ratio of the separatrix radius to the ion skin depth (S* is also approximately the ratio of the separatrix radius to the ion gyroradius in the external field). There is a clear discrepancy between the observed macroscopic resilience of FRC plasmas in low-S* experiments, and the predictions of standard MHD theory that many modes should be unstable on an MHD time scale. Resolution of this discrepancy requires advanced theoretical and numerical studies including kinetic effects. On the other hand, in order to achieve good plasma confinement, larger devices with S*>100-300 are needed, and conditions for large-S* FRC stability are yet to be investigated both experimentally and theoretically.
FIGURE 1. Contour plot of poloidal flux for a prolate FRC configuration.
Under the auspices of the present DOE contract, most of the theoretical work on FRCs at the Princeton Plasma Physics Laboratory has focused on investigating a variety of non-ideal MHD effects, including plasma flow and kinetic effects on FRC stability properties, particularly with respect to the n=1 tilt mode. A new 3D nonlinear hybrid and MHD simulation code (HYM) has been developed at PPPL and this code has been used to study the relative roles of various competing kinetic effects on linear stability behavior. These studies have demonstrated that the n=1 tilt instability can saturate nonlinearly in highly kinetic FRCs. Although considerable progress has been made, particularly in describing kinetic stability properties, a complete understanding of the observed FRC stability properties is still lacking. In order to assess the potential of FRCs as an innovative confinement concept for fusion energy, it remains essential to develop an improved understanding of FRC macroscopic stability properties both in the low-S* (kinetic) and high-S* (MHD-like) regimes.