Connecting the global H-mode power threshold to the local radial electric field at ASDEX Upgrade

The relation between the macroscopic input power required at ASDEX Upgrade to access the H-mode \pthr\ and the microscopic \exb\ shear has been investigated via fast charge-exchange recombination spectroscopy (CXRS) measurements at various toroidal magnetic fields, different electron densities, and in both hydrogen and deuterium plasmas. For the H-mode onset, a threshold in the \vexb\ minimum, an approximation of the \exb\ shear, has been found. This identifies \vexb\ and not \er\ as the important player for the L-H transition. A database of measurements including CXRS, Doppler reflectometry measurements and comparison to neoclassical approximations shows a threshold \vexb\ of $(6.7 \pm 1.0)\, \SI{}{km/s}$ ranging over a factor of three in \pthr. Using these findings, a simple derivation of the \pthr\ scaling is proposed giving a physics interpretation of the $B_t$, density and surface dependence of $P_{\rm thr}$.