Current profile flattening and hot core shift due to non-linear development of resistive kink mode

A new and interesting phenomenon that the toroidal current profile, which at first exhibits peaking due to ohmic heating, flattens around the magnetic axis is obtained through a full compressible resistive MHD simulation for a 3-D toroidal geometry. It is concluded that the current flattening is caused by non-linear excitation of the unstable m=1/n=1 resistive kink mode, specifically by the quasi-linear effect of the coupling of the plasma velocity and the magnetic field of the n=+or-1 mode. The time-scale of the current profile flattening is of the order of the MHD time-scale. When the current profile is largely flattened, the plasma dynamic pressure due to the strongly excited kink mode pushes the hot core plasma in the radial kink flow direction and the hot core starts deviating from the magnetic surface owing to the dynamic pressure and the plasma compressibility. Because of the reduction of the magnetic shear due to the current flattening in the q<1 region, magnetic field reconnection is driven by the strong kink flow and leads to the destruction of the magnetic field structure within the q=1 rational surface. Subsequently, the magnetic field configuration recovers to an axisymmetric profile. This can explain the fast crash phase of sawteeth in tokamaks. The effect of the thermal conduction on the evolution of such a process is also discussed