Description
In order to prevent damage to plasma facing components caused by excessive heat loads, a tokamak reactor plasma must be operated with a detached divertor, typically achieved by seeding of impurities into the divertor region. It is imperative that the detached state is sustained throughout all phases of the discharge, including L-mode, fully developed H-mode, and transitory phases such as the transition from L-mode to H-mode (L-H transition) and the transition from H-mode back to L-mode (H-L transition). Systematic experiments at ASDEX Upgrade have demonstrated that nitrogen seeding for divertor detachment increases the L-H power threshold by up to a factor of two in comparison with the unseeded reference [1]. This finding is mainly due to heightened levels of nitrogen-related radiation. However, alterations in the shape of radial profiles of temperature and the radial electric field suggest an additional effect of the seeding on transport. At highest seeding levels, a plasma with a continuously sustained divertor detachment can be achieved including the L-H transition into a stable H-mode without type-I edge localized modes (ELMs) [1] controlled by active feedback on the vertical position of the X-point radiator [2]. This demonstrates the feasibility of achieving a continuously detached and stable plasma without ELMs and decent confinement. However, this appears to be inevitably accompanied by a substantial increase in the L-H power threshold.
[1] G. Birkenmeier et al., Nucl. Fusion 66 (2026) 026024
[2] M. Bernert et al., Nucl. Mater. Energy 43 (2025) 101916