Non-linear simulations of internal reconnection events in spherical Ttokamaks

Three dimensional MHD simulations are executed in a full toroidal geometry to clarify the physical mechanisms of the internal reconnection event (IRE) which is observed in spherical tokamak experiments. The simulation results reproduce several main properties of IREs. A notable property in the linear growth of modes is the simultaneous excitation of multiple low n modes, which grow together with similar growth rates. A spontaneous phase alignment mechanism among these excited modes is found in the non-linear stage of the development, which causes a bulge-like deformation of the torus and subsequent expulsion of heat energy to the external region through the occurrence of external reconnection.