xB shear mechanism*

H-mode access with external heating only is a critical issue for ITER. Experiments on JET-ILW highlight strong dependencies of the L-H threshold power, such as a factor 2 variation in threshold depending on strike point positions, that cannot be extrapolated without understanding of the underlying physics principles. We will show that most of the observed trends in the JET L-H database can be reconciled with the paradigm of turbulence suppression by sheared E_rxB flows but all terms contributing to the E_r shear as well as parametric dependencies of the turbulence drive are required to explain all the observed trends. An explanation of the divertor configuration effect will be presented based on a near-SOL contribution to the shear in the outer part of the E_r well. Observations of a stronger peaked sheath voltage at the outer strike point in the pulses with lowest threshold are in qualitative agreement with EDGE2D/EIRENE modelling, pointing to the effect of the recycling pattern on broadening the target temperature profiles. The influence of the near-SOL on the transition could also explain some of the dynamics of dithering transitions as the inner divertor plasma attaches and detaches during each cycle, pointing to a breakdown of the sheath and the associated poloidal E_rxB flow in the SOL at the transition into H-mode. An interpretation of the isotope effect in the context of E_rxB shear stabilization remains elusive. Both the stored energy as well as the edge kinetic profiles before the transition are well matched between hydrogen and deuterium. This implies a similar E_r well structure as there are no explicit mass dependencies in the radial force balance equation. The requirement of a similar critical E_rxB stabilization would imply a similar turbulence drive, which is at odds with the observation of higher transport in hydrogen.