Interplay between central ECRH and saturated (m, n) = (1, 1) MHD activity in mitigating tungsten accumulation at ASDEX Upgrade

Abstract

The interplay between central electron cyclotron resonance heating (ECRH) and saturated (m, n) = (1, 1) magnetohydrodynamic (MHD) instabilities in mitigating central W accumulation at ASDEX Upgrade is analysed. The evolution of the intrinsic tungsten density in a typical ASDEX Upgrade H-mode discharge with central ECRH and saturated (1, 1) modes in-between sawtooth crashes is presented. The W density profile evaluated averaging over mode rotation and assuming axisymmetry are deeply hollow inside the q = 1 surface during mode saturation. In order to provide a mode-resolved picture, a new technique for the determination of 2D mode-resolved intrinsic W density maps in the presence of saturated MHD instabilities is developed. For the first time the full decoupling of the impurity density from electron density and temperature contributions to experimental 2D SXR tomographic reconstructions in the presence of saturated MHD activity can be performed. These mode-resolved 2D W density maps reveal that the impurity hole is located inside the displaced core of the saturated (1, 1) mode, while the W density inside the magnetic island is flat. Modelling the W density using the combined neoclassical and gyrokinetic codes NEO and GKW assuming axisymmetry and including the effects of toroidal rotation reproduces the poloidal asymmetries outside of the q = 1 surface correctly, but predicts centrally peaked W density profiles in the centre. This suggests that the hollowness inside the q = 1 surface is tied to the presence of the saturated (1, 1) mode.