Nonlinear simulation of edge-localized mode in a spherical tokamak

A numerical modelling of the dynamics of an edge-localized mode (ELM) crash in the spherical tokamak is proposed by means of a three-dimensional nonlinear magnetohydrodynamic simulation. The simulation result shows a crash of the edge pressure profile due to the spontaneous growth of the ballooning mode instability. The simulation result shows good agreement in several characteristic features of the experimental observation of large scale ELMs in an appropriate time scale: (1) relation to the ballooning instability, (2) intermediate-n precursors, (3) low-n structure on the crash, (4) formation and separation of the filament and (5) considerable amount of convective loss of plasma, where n is the toroidal mode number. Furthermore, the model is verified by examining the effect of diamagnetic stabilization and by comparing the nonlinear behaviour with that of the peeling modes. The ion diamagnetic drift terms are found to stabilize some specific components linearly; nevertheless they are not so effective in the nonlinear dynamics such as the filament formation and the amount of loss. For the peeling mode case, no prominent filament structure is found to appear in contrast to the ballooning mode case.