Properties of the new divertor IIb in ASDEX Upgrade

A new divertor configuration (DIV IIb) has been implemented in ASDEX Upgrade. In order to accommodate a large variety of plasma shapes with bottom triangularities (δbot) up to 0.48, the outer strike point region was modified and the roof baffle was lowered and diminished at its outer part in comparison with the previous divertor (DIV II). The inner part of the divertor strike point module remains unchanged, but at the divertor entrance a smooth transition to the central column is provided to minimize local hydrogen recycling. According to experiences with power handling in DIV II, ordinary fine grain graphite has been chosen for the outer strike point and, as before the tiles are slightly tilted in toroidal direction to hide the leading edges. A first characterization of DIV IIb reveals that the beneficial behaviour of DIV II is essentially maintained. There is an increase of the power density due to geometrical reasons at the outer target, whereas the divertor radiation for similar magnetic configurations is unchanged. The pumping characteristics for D and He are almost retained, suggesting a large influence of the inner divertor leg, the configuration of which remains unchanged. A significant reduction (20%) of the L-H threshold is observed consistent with larger temperature gradients inside the separatrix just before the transition. (author)