The ASDEX Upgrade divertor IIb - A closed divertor for strongly shaped plasmas

The closed ASDEX Upgrade divertor (DIV II) has been modified in order to accommodate a large variety of strongly shaped plasmas, allowing the access to improved confinement at high plasma density and to the type-II ELM regime. Since then DIV IIb has been successfully operated and discharges with additional heating powers up to 18 MW have been performed. As expected, the target power load at the outer strike point is increased due to geometrical reasons, but the integrated divertor radiation is almost unchanged. This is also true for high triangularity discharges, which were previously run with the outer leg on the roof baffle and now are performed on the vertical target. Consistent with recent modelling results, no distinct differences are found in the neutral flux densities and in the He-compression for the two versions of the divertor. The bypasses of DIV IIb have only a minor influence on the exhaust rate and on the main chamber recycling, suggesting the presence of radial ion fluxes to the wall which cause the major part of the recycling. A significant reduction of the L-H threshold is observed which may be connected to a larger electron temperature gradient inside the separatrix. (author)