Impact of nitrogen seeding on L–H transitions and their role for detachment in ASDEX Upgrade

The required heating power, , to access the high confinement regime (H-mode) in nitrogen seeded deuterium plasmas is investigated at the ASDEX Upgrade tokamak. It is shown that a stepwise increase of the nitrogen seeding level increases monotonically by up to a factor of two with respect to the unseeded reference. At the highest seeding rates a stable plasma discharge from a detached low confinement mode (L-mode) to a detached H-mode without interrupting the divertor detachment during the L–H transition is achieved. The results suggest that the impurity seeding level required for continuous divertor detachment, as it is mandatory for a reactor-scale fusion plasma, inevitably leads to a substantial increase in . Therefore, the impact of required impurity seeding for detachment purposes on must be taken into account for future reactor-scale plasma scenarios.