F. Jenko

Gyrokinetic simulations reveal that microtearing mode (MTM) turbulence dominates transport in a Wendelstein 7-X (W7-X) discharge characterized by large density gradients, moderate temperature gradients, and low plasma beta. This conclusion is supported by the close agreement between simulated and experimentally measured heat and particle fluxes. The emergence of MTMs is attributed to the absence of competing instabilities -- such as ion temperature gradient modes and density-gradient-driven trapped-electron modes -- under these plasma conditions, together with the stabilizing influence of the W7-X max-J magnetic configuration. Furthermore, moderate collisionality and low magnetic shear are found to facilitate MTM onset. These findings advance the understanding of high-density-gradient regimes, which are essential for achieving high-performance operation in W7-X plasmas.

The theoretical investigation of relevant turbulent transport mechanisms in H-mode pedestals is a great scientific and numerical challenge. In this study, we address this challenge by global, nonlinear gyrokinetic simulations of a full pedestal up to the separatrix, supported by a detailed characterisation of gyrokinetic instabilities from just inside the pedestal top to pedestal centre and foot. We present ASDEX Upgrade pedestal simulations using an upgraded version of the gyrokinetic, Eulerian, delta-f code GENE (genecode.org) that enables stable global simulations at experimental plasma beta values. The turbulent transport is found to exhibit a multi-channel, multi-scale character throughout the pedestal with the dominant contribution transitioning from ion scale Trapped Electron Modes (TEMs)/Micro Tearing Modes (MTMs) at the pedestal top to electron scale Electron Temperature Gradient modes (ETG) in the steep gradient region. Consequently, the turbulent electron heat flux changes from ion to electron scales and the ion heat flux reduces to almost neoclassic values in the pedestal centre. ExB shear is found to strongly reduce heat flux levels in all channels (electron, ion, electrostatic, electromagnetic) and the interplay of magnetic shear and pressure gradient is found to locally stabilise ion scale instabilities.