Impact of neoclassical tearing mode-turbulence multi-scale interaction in global confinement degradation and magnetic island stability

Recent measurements of turbulent density (ñ) and electron-temperature (Te) fluctuations have reported turbulence modifications by Neoclassical Tearing Mode (NTM) islands: turbulence decreases (increases) inside (outside) the island region when the island width (W) exceeds a threshold (W_T), in qualitative agreement with gyrokinetic simulations. As the cross-field transport in tokamaks is dominantly driven by turbulence, these observations call into question the conventional understanding of confinement degradation by NTMs and magnetic island stability physics. The experimental data presented here support the following points: (i) When profiles flatten at the O-point and gradients increase outside of the island, ñ decreases (increases) inside (outside) the island. Along with the parallel transport resulting in increased fluxes inside the island, the increase of ñ outside of the island offers an explanation for the temporal increase of fluxes in that region. As the plasma stored energy (W_MHD) gradually decreases in synchronization with the island growth and saturation, gradients, ñ and fluxes also decrease outside the island until they become about the same as before NTM onset. These fluxes balance the constant sources, and the plasma comes to a steady state at lower W_MHD. (ii) Turbulence reduction in the O-point region has a destabilizing effect on the island. This effect is, however, nearly compensated by the reduced confinement. These observations suggest that driving turbulence in the island region could lead to smaller saturated islands offering a path toward better confinement and safer operation of reactor-scale fusion devices.