Nonlinear evolution of resistive modes for q<1 in tokamaks

The nonlinear interaction of tearing instabilities resonant inside the q=1 surface can lead to stochastization of the plasma core without involving the n=1 mode. For certain profiles, the q=1 surface can survive this disruption, so that q<1 remains valid inside. These results are consistent with some experimental sawtooth observations in which q<1 occurs in the core even after the disruption. The competition between this process and the usual resistive reconnection of the (m=1)/(n=1) magnetic island can be sensitive to the initial perturbation as well as to the plasma configuration and profiles.