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

Abstract

During recent years a pellet-based system for feedback controlled core density fuelling has been developed at the ASDEX Upgrade tokamak, making the device well suited for the investigation of the reactor relevant high-density regime in an appropriate configuration. Efforts have proven that 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 that the beneficial increase of confinement with density saturates when approaching the Greenwald density. When taking this into account properly, it has emerged that for standard configurations the energy confinement can be sustained when the density is increased. However, the surplus confinement by different means, such impurity seeding or strong shaping, cannot be fully preserved. This is presumed to be attributed to a separatrix density which is not yet adequately controlled.