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 (∇p) and perturbed bootstrap current (δj_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 (ñ) 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 ñ inside the peaked island is dominantly driven by the Ion Temperature Gradient instability. These measurements suggest that ñ accelerates NTM recovery after an ELM crash via accelerating the relaxation of ∇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 ∇p and therefore restoring δj_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-β tokamak plasmas.