The role of fast ion redistribution leading to loss of differential rotation and concomitant onset of disruptive m/n=2/1s

In this work, we present integrated TRANSP-Kick analyses quantitatively characterizing how n>1 islands redistribute the fast ions in the plasma, modify torque profiles, and flatten the rotation profile leading to 2,1 NTM (neoclassical tearing mode) onset. Magnetic islands driven by the NTM present a major concern for the operation of present-day fusion devices as they can significantly decrease particle confinement and can lead to plasma termination. In low-torque DIII-D plasmas characterized by the ITER normalized parameter set and shape, disruptive NTMs are most commonly seeded by non-linear 3-wave coupling when the differential rotation between the q=1 and q=2 surfaces (Δf_1,2) approaches zero. The rotation profile flattening is correlated with the growth of n>1 non-disruptive core islands. The simulations presented here elucidate the experimentally observed non-linear relationship between Δf_1,2 and the amplitudes of the n>1 modes, showing that the non-linearity arises from phase-space resonance overlaps between fast ions and magnetic islands. Interestingly, the observed non-linear coupling between the core islands and the fast ion population, caused by the phase-space island overlaps, also provides insights into the origins of the previously reported exponential 2,1 NTM onset time distribution, suggesting the possibility of a chaotic system of core islands and fast ions governing the statistics of disruptions from 2,1 NTMs.