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 E x B 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 v E x B minimum, an approximation of the E x B shear, has been found. This identifies v E x B 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 v E x B of (6.7 ± 1.0) 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 Bt, density and surface dependence of Pthr.