Three-dimensional simulations of tokamaks have been carried out, including self-consistent temperature evolution with a highly anisotropic thermal conductivity. The simulations extend over the transport time-scale and address the question of how disruptive current profiles arise at low-q or high-density operation. Sharply defined disruptive events are triggered by the m/n = 2/1 resistive tearing mode, which is mainly affected by local current gradients near the q = 2 surface. If the global current gradient between q = 2 and q = 1 is sufficiently steep, the m = 2 mode starts a shock which accelerates towards the q = 1 surface, leaving stochastic fields, a flattened temperature profile and turbulent plasma behind it. For slightly weaker global current gradients, a shock may form, but it will dissipate before reaching q = 1 and may lead to repetitive minidisruptions which flatten the temperature profile in a region inside the q = 2 surface.
MHD instabilities leading to disruptions in low beta JT-60U reversed shear plasmas