Nonlinear Simulation of Edge-Localized Mode in Spherical Tokamak

A numerical modeling for the dynamics of an edge-localized mode(ELM) crash in the spherical toka- mak is proposed with a consecutive scenario which is initiated by the spontaneous growth of the ballooning mode instability by means of a three-dimensional nonlinear magnetohydrodynamic simulation. The simulation result shows a two-step relaxation process which is induced by the intermediate-n ballooning instability fol- lowed by the m/n=1/1 internal kink mode, where m and n represent the poloidal and toroidal mode numbers, re- spectively. By comparing with the experimental observations, we have found that the simulation result can re- produce several characteristic features of the so-called type-I ELM 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 loss of plasma. Furthermore, the model is verified by examining the effect of diamagnetic stabilization and comparing the nonlinear behavior 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 formed in contrast to the ballooning case.