The radial localization of the transition from low to high confinement mode in the ASDEX Upgrade tokamak

A novel experimental method is applied to localize the initial suppression of turbulence, in the form of density fluctuations, at the transition from the low (L-) to the high (H-) confinement mode in toroidal magnetic fusion plasmas. The high radial and temporal resolution, combined with the unprecedented statistical significance, provided the awaited information on a possible dominant E × B shear layer in L-H transition physics. We show, for the first time, that the H-mode turbulence suppression is initiated at the inner E × B shear layer in the ASDEX Upgrade tokamak possibly shedding light on the causality behind the L-H transition process.