Characterization of the H-mode edge barrier at ASDEX Upgrade

The scaling of the edge transport barrier (ETB) that sustains H-mode plasmas is crucial for the performance of next step tokamaks. At ASDEX Upgrade, the suite of edge diagnostics has been significantly improved to address this issue. High spatial resolution profiles of most of the key edge plasma parameters necessary to determine the magneto hydrodynamic (MHD) stability are now available. New high temporal resolution measurements give clear indications of the nonlinear evolution of the ELM crash. The correlation lengths of edge turbulence have been shown to be correlated with the edge radial electric field shear using a new correlation Doppler reflectometer system. The measured pressure gradient in the ETB is found to be consistent with ideal MHD stability limits, both for Type I and II ELMs. In addition, the edge electron temperature and density gradient lengths are found to be strongly correlated, leaving only the ETB width as a free parameter. In ASDEX Upgrade, the ETB width does not vary significantly over the entire H-mode edge database. Modelling of the transport of comparison discharges in hydrogen and deuterium shows that the expected mass effect on neutral penetration is largely compensated by more efficient heating of deuterium neutrals but requires a transport barrier in both the energy and particle channels in order to reproduce the measured edge temperature and density profiles.