Local modes of field-reversed configurations

Abstract

The local stability of field-reversed configurations (FRC) is analyzed using hydrodynamic stability theory. The equation of state includes both compressibility and double-adiabatic effects. For the first time, eigenmodes of the linearized equations of motion have been computed. The most unstable modes have fast growth rates, comparable to the Alfvén transit time across the FRC radius; i.e., somewhat faster than the frequency (or growth rate) of global modes. In realistic equilibria, the most unstable local modes concentrate, ballooning-mode style, in the high curvature region of magnetic flux lines. The familiar interchange stability criterion is irrelevant for FRCs, since the actual eigenmodes differ markedly from interchange, both in structure and stability. The appearance of fast local modes raises the possibility that they may regulate FRC equilibria. However, surprisingly, equilibria with realistic internal structure (i.e. resembling experiments) are more unstable to ideal local modes than less realistic equilibria, as have often been studied theoretically. Thus, a nonideal theory will be needed to explain the equilibria observed in experiments.