Impact of Magnetic Island - Turbulence Multi-Scale Interaction on Plasma Confinement and Magnetic Island Stability

Recent measurements$^{1}$ and gyrokinetic simulations$^{2}$ reported the reduction of turbulent density fluctuations (ñ) inside magnetic islands, and ñ increase outside magnetic islands, when the island width (W) exceeds a threshold (W$_{T}$). As the cross-field transport is dominantly driven by ñ, this calls into question the conventional understanding of confinement ($\tau$$_{e}$) degradation and Neoclassical Tearing Mode (NTM) stability physics. We report that the increase in ion-scale ñ outside the island correlates with higher heat and particle fluxes, i.e., ñ increases temporarily when $\tau$$_{e}$ is decreasing, while in the following stationary state ñ is comparable to before NTM onset. This indicates that the decrease of the plasma stored energy results from ñ-NTM interaction. On the other hand, simultaneous ñ reduction at the O-point has a destabilizing effect on NTMs. These observations suggest that driving ñ at the O-point could prevent small islands from growing large, allowing better plasma confinement and safer tokamak operation. [1] Bardóczi, L. et al. Phys. Plasmas 24, 056106 (2017) [2] Navarro, A. B. et al. Plasma Phys. Control. Fusion 59, 034004 (2017)