Interaction of neoclassical tearing modes with trapped fast ions

Abstract

Resonance between rotating neoclassical tearing mode (NTM) and precessing trapped energetic ions has been considered. Resonant ions experience large radial excursions of their banana guiding centers leading to the loss of a considerable fraction of fast ions deposited in the resonance region. The resulting toroidal torque accelerates the magnetic island rotating in the direction of the ion diamagnetic drift. The torque balance equation has been coupled with the modified Rutherford equation, retaining the contributions from the bootstrap current and the ion polarization current. It has been shown that, due to the resonant interaction with energetic ions, this autonomous 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) tokamak [S. Sesnic et al., Phys. Plasmas 7, 935 (2000)].