EX/7-4Rb Nonlinear Simulations of Fishbone Instability and Sawteeth in Tokamaks and Spherical Torus

We report new results of self-consistent nonlinear simulations of shbone instability and sawtooth oscillations obtained using the extended MHD code M3D. Hybrid simulations of energetic particle-driven shbone instability in a circular tokamak show dynamic mode saturation as the particle distribution is attened and mode frequency is reduced strongly. MHD nonlinearity reduces the mode saturation level. Resistive MHD simulations of the CDX-U spherical torus show repeated sawtooth cycles with calculated periodicity consistent with the experimental observation. The two- uids simulations show that the induced sheared plasma rotation suppresses the island growth so that the magnetic eld no longer passes through a stochastic state as observed in the MHD simulations. Resistive MHD simulations of sawtooth crashes in the TEXTOR tokamak show that the localization eect of reconnection is rather small. Even when the pressure in the core is attened due to stochasticity in a nonlinear state, the complete reconnection process still tends to proceed with q(0) rises above one.