Multi-field/-scale interactions of turbulence with neoclassical tearing mode magnetic islands in the DIII-D tokamak

Abstract

We present the first localized measurements of long and intermediate wavelength turbulent density fluctuations ((n) over tilde) and long wavelength turbulent electron temperature fluctuations ((T) over tilde (e)) modified by m/n = 2/1 Neoclassical Tearing Mode (NTM) islands (m and n are the poloidal and toroidal mode numbers, respectively). These long and intermediate wavelengths correspond to the expected Ion Temperature Gradient and Trapped Electron Mode scales, respectively. Two regimes have been observed when tracking (n) over tilde during NTM evolution: (1) small islands are characterized by a steep T-e radial profile and turbulence levels comparable to those of the background; (2) large islands have a flat T-e profile and reduced turbulence level at the O-point. Radially outside the large island, the T-e profile is steeper and the turbulence level increased compared to the no or small island case. Reduced turbulence at the O-point compared to the X-point leads to a 15% modulation of (n) over tilde (2) across the island that is nearly in phase with the T-e modulation. Qualitative comparisons to the GENE non-linear gyrokinetic code are promising with GENE replicating the observed scaling of turbulence modification with island size. These results are significant as they allow the validation of gyrokinetic simulations modeling the interaction of these multi-scale phenomena. Published by AIP Publishing.