Rotating RMP stabilization of ``Neoclassical Tearing Modes''

In DIII-D, a slowly rotating nonaxisymmetric magnetic field (RMP) with toroidal harmonic n=1 can, using feedback, lock onto the rotation of an unstable m=2/n=1 neoclassical tearing mode (NTM) and prevent it from growing to a disruption[1]. Nonlinear simulation with the extended MHD code M3D shows a complex picture. The MHD evolves much faster than the RMP rotation. The background RMP-stabilized plasma has relatively high central beta. Without the RMP, it is unstable to a new type of instability over q=1, a nonlinear ideal-MHD unstable mode with coupled n=1 and 2 harmonics, that grows to an internal-kink-like sawtooth crash. A perturbation with n=1 and m=2/n=2 remains over q=1⁺ and couples to other harmonics across the entire plasma radius, consistent with observations of δT_i and δT_e. No large 2/1 magnetic island is seen in simulation or the experiment. The vacuum rotating field from the I-coil with 2.5 kA current is larger than the MHD mode on the large-R side of the plasma and comparable over q<1, suggesting that the RMP couples to, and influences, the full n=1 perturbation. These processes will be further studied.
[1] M. Okabayashi et al., Nuc. Fusion 57, 016035 (2017).