Numerical simulations of toroidal Alfven instabilities excited by trapped energetic ions

Extensive magnetohydrodynamic-gyrokinetic hybrid simulations have been carr ied out to study the excitations of Alfven instabilities by trapped energet ic ions in tokamak plasmas. Depending on the parameters, the most unstable mode can be either the toroidal Alfven eigenmode (TAE) or the energetic-par ticle mode (EPM). In both cases, the dominant instability driving mechanism is the resonance with the trapped-particle magnetic precessional drifts. T he mode frequencies of the most unstable modes, meanwhile, tend to be near the toroidal frequency gap in order to minimize the continuum damping. It i s also demonstrated that the instabilities can be quenched by reversing the magnetic precessional drifts via either negative magnetic shear or locatin g the magnetic turning points in the inner (strong B) side of the torus. (C ) 2000 American Institute of Physics. [S1070-664X(00)01305-7].