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 (ñ) and long wavelength turbulent electron temperature fluctuations (T̃e) 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 ñ during NTM evolution: (1) small islands are characterized by a steep Te radial profile and turbulence levels comparable to those of the background; (2) large islands have a flat Te profile and reduced turbulence level at the O-point. Radially outside the large island, the Te 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 ñ2 across the island that is nearly in phase with the Te 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.