Non-perturbative measurement of cross-field thermal diffusivity reduction at the O-point of 2/1 neoclassical tearing mode islands in the DIII-D tokamak

Neoclassical tearing modes (NTMs) often lead to the decrease of plasma performance and can lead to disruptions, which makes them a major impediment in the development of operating scenarios in present toroidal fusion devices. Recent gyrokinetic simulations predict a decrease of plasma turbulence and cross-field transport at the O-point of the islands, which in turn affects the NTM dynamics. In this paper, a heat transport model of magnetic islands employing spatially non-uniform cross-field thermal diffusivity (chi(perpendicular to)) is presented. This model is used to derive chi(perpendicular to) at the O-point from electron temperature data measured across 2/1 NTM islands in DIII-D. It was found that chi(perpendicular to) at the O-point is 1 to 2 orders of magnitude smaller than the background plasma transport, in qualitative agreement with gyrokinetic predictions. As the anomalously large values of chi(perpendicular to) are often attributed to turbulence driven transport, the reduction of the O-point chi(perpendicular to) is consistent with turbulence reduction found in recent experiments. Finally, the implication of reduced chi(perpendicular to) at the O-point on NTM dynamics was investigated using the modified Rutherford equation that predicts a significant effect of reduced chi(perpendicular to) at the O-point on NTM saturation. Published by AIP Publishing.