Description
High confinement mode (H-mode) access in tokamaks is based on the formation of a radially narrow steep gradient region in the plasma edge and a concomitant radial electric field ($E_r$).
There is growing evidence that the profiles of temperature and density in the H-mode edge are partly determined by turbulent transport connected with microinstabilities driven by the steep gradients and its interplay with the $E\times B$ shear flow related to the $E_r$-well.
Through a gyrokinetic investigation with the code GKW it is shown that the slab branch of the ion temperature gradient (ITG) driven instability is dominantly unstable under edge conditions of edge localized mode (ELM) suppressed H-modes in ASDEX Upgrade (AUG).
This work explicitly takes into account the gyroaverage in the background $E \times B$ shear flow model.
It is shown that this has significant impact on linear stability and turbulent transport:
The slab ITG is then hardly stabilized and turbulent transport is not reduced when a gyroaveraged $E\times B$ shear flow is added.
This lack of stabilization renders the slab ITG a strong candidate for setting the ion temperature profile in the steep gradient edge region.
Furthermore, a sign sensitive $E \times B$ shear flow induced radial symmetry breaking of slab ITG Eigenmodes is discussed.
In the inhomogeneous $E_r$-well this symmetry breaking mechanism determines the most unstable Eigenmode and renders linear stability and turbulent transport highly sensitive to the sign of the $E \times B$ shear flow.
Hence, it is not only the magnitude of the $E \times B$ shear rate, but also its sign which determines microturbulence in the steep gradient edge region.
Finally, three-dimensional (3d) magnetic perturbations (MPs), applied in the considered AUG scenario to suppress ELMs, are shown to impact slab ITG turbulence:
Single helicity 3d MPs with high mode number parallel to the ambient magnetic field destabilize the slab ITG and thereby augment turbulent transport.
The 3d MP aided increase of slab ITG turbulence and connected particle transport might relate to the density pump-out responsible for ELM suppression.