Simulation of MHD activity during density limit disruptions in tokamaks

The phenomenon of major disruptions in tokamaks is well known. Despite detailed experimental studies, the theoretical understanding has remained poor. In this paper, we present numerical simulations and a theoretical model for major disruptions triggered by exceeding the density limit. The model contains as its main elements nonlinear mode coupling induced by the presence of a large amplitude m=2/n=1 mode and nonlinear interaction involving primarily the 2/1, 1/1 and 3/2 modes. Our model is consistent with the experimental observation that major disruption takes place in two steps: first an internal relaxation flattens the temperature in the central region. In the second step, MHD turbulence develops, first in the q≤2 region, with a filamentation of the current and flattening of the q-profile. This leads to a final explosive growth of the m≥2/n=1 modes which flattens the entire current distribution and leads to a large drop in internal inductance. (author) 4 figs., 6 refs