Interaction of neoclassical tearing modes with trapped fast ions

Resonance between rotating neoclassical tearing mode (NTM) and precessing t rapped energetic ions has been considered. Resonant ions experience large r adial excursions of their banana guiding centers leading to the loss of a c onsiderable fraction of fast ions deposited in the resonance region. The re sulting toroidal torque accelerates the magnetic island rotating in the dir ection of the ion diamagnetic drift. The torque balance equation has been c oupled with the modified Rutherford equation, retaining the contributions f rom the bootstrap current and the ion polarization current. It has been sho wn that, due to the resonant interaction with energetic ions, this autonomo us system of equations exhibits a Hopf bifurcation for some critical ratio of the momentum damping rate to the resistive diffusion rate. The resulting stable limit cycle resembles the frequency jump events observed during NTM activity in the Axisymmetric Divertor Experiment Upgrade (ASDEX Upgrade) t okamak [S. Sesnic , Phys. Plasmas 7, 935 (2000)]. (C) 2001 American Institu te of Physics.