Shrinking of core neoclassical tearing mode magnetic islands due to edge localized modes and the role of ion-scale turbulence in island recovery in DIII-D

Experimental signature of long-wavelength turbulence accelerating the recovery of Neoclassical Tearing Mode (NTM) magnetic islands after they have been transiently reduced in size due to interaction with Edge Localized Modes (ELMs) is reported for the first time. This work shows that perturbations associated with ELMs result in peaking of the electron temperature (T-e) in the O-point region of saturated core m/n = 2/1 islands (m/n being the poloidal/toroidal mode numbers). In synchronization with this T-e peak, the island width shrinks by as much as 30% suggesting a key role of the T-e peak in NTM stability due to modified pressure gradient (del p) and perturbed bootstrap current (delta j(BS)) at the O-point. Next, this T-e peak relaxes via anomalous transport (i.e., the diffusivity is 2 orders of magnitude larger than the neoclassical value) and the island recovers. Long-wavelength turbulent density fluctuations ((n) over tilde) are reduced at the O-point of flat islands but these fluctuations are increased when T-e is peaked which offers an explanation for the observed anomalous transport that is responsible for the relaxation of the T-e peak. Linear gyrokinetic simulations indicate that (n) over tilde inside the peaked island is dominantly driven by the Ion Temperature Gradient instability. These measurements suggest that (n) over tilde accelerates NTM recovery after an ELM crash via accelerating the relaxation of del p at the O-point. These observations are qualitatively replicated by coupled predator-prey equations and modified Rutherford equation. In this simple model, turbulence accelerates NTM recovery via relaxing del p and therefore restoring delta j(BS) at the O-point. The key physics of the relationship between the T-e peak and NTM stability has potentially far-reaching consequences, such as NTM control via pellet injection in high-beta tokamak plasmas. Published by AIP Publishing.