Splitting instability of a high beta tokamak with noncircular cross section.

The splitting of a tokamak with elliptical and bean-shaped cross sections is studied for finite beta plasmas. When the plasma beta exceeds a critical value, an elliptical tokamak is subject to an ideal pressure-driven instability, which deforms the ellipse in such a way that a thinned plasma current sheet is formed around the magnetic axis. As a result, magnetic reconnection is nonlinearly driven and the ellipse is split. The bean-shaped tokamak, however, is stable against a splitting perturbation for limited equilibria that could be numerically constructed. An interesting similarity to the energy relaxation process in a force-free plasma, namely, a two-step evolution (initial occurrence of an ideal magnetohydrodynamic instability and subsequent occurrence of driven reconnection), is discussed. [ABSTRACT FROM AUTHOR]