H-mode confinement in the pellet-enforced high-density regime of the all-metal-wall tokamak ASDEX Upgrade

During the last years a pellet based system for a feedback controlled core density fuelling has been developed at the ASDEX Upgrade tokamak, making it a device most suited for the investigation of the reactor relevant high-density regime in an appropriate configuration. Efforts have proven an adequately applied pellet actuator enables safe and reversible access to core plasma densities as intended in current reactor designs and even beyond. From the analysis of a wide range of different plasma scenarios it becomes apparent the beneficial increase of confinement with density saturates when approaching the Greenwald density. When taking this into account properly, it emerged for standard configurations the energy confinement can be sustained when increasing the density. However, the surplus confinement by different means like impurity seeding or strong shaping cannot be fully preserved. Presumably this can be attributed to a yet not adequately controlled separatrix density.