Linear and a nonlinear axisymmetric motion of a pinch limited by a separatrix

The linear and nonlinear axisymmetric instability of a toroidal low-q pinch limited by a separatrix is studied numerically, and comparison is made with the analytical theory for weak noncircularity. The resistive one-fluid code TSC, where the plasma boundary is defined by the separatrix, is used to follow the nonlinear evolution. Contrary to the results of previous ideal MHD simulations, nonlinear stabilization is not observed for motions towards an x point. Apparently this is due to the absence of internal magnetic islands inside the plasma. For motions in the direction between two x points, however, nonlinear saturation is seen, consistent with the analytical theory.