Interplay between turbulence, neoclassical and zonal flows during the L-H transition at ASDEX Upgrade

It is widely accepted that the $E \times B$ velocity shear is responsible for the suppression of the edge turbulence, thus leading to the transition from L- to H-mode. However, the origin and the evolution of the edge radial electric field ($E_r$) profile and the accompanying $E \times B$ flow is still debated. The $E \times B$ flow may be generated by turbulence stresses or by collisional (neoclassical) processes via the main ion pressure gradient. A recent upgrade of the charge exchange recombination spectroscopy diagnostic at ASDEX Upgrade provides a full reconstruction of the impurity density, temperature and $E_r$ profiles at 100$\,\mu$s time resolution and allows the evaluation of the fast dynamics of these quantities during the L-H transition. The behaviour of $E_r$ and the ion profiles during the L-H transition will be presented for discharges with different L-H power thresholds obtained via different electron densities, a $B_t$-scan and a change of isotope (deuterium and hydrogen). A comparison of neoclassical and of measured $E_r$ profiles to the evolution of the turbulent fluctuation points to a leading role of neoclassical flow in the L-H transition.