First Alternative Divertor Configuration Experiments at ASDEX Upgrade

Power exhaust is a major challenge for future tokamak fusion reactors. Over 90% of the heating power must be radiated before reaching the divertor targets to ensure a sufficiently long material lifetime. The divertor must also pump fuel and helium neutrals efficiently while keeping core impurity levels low. It remains uncertain whether the conventional single null (SN) divertor planned for ITER can meet these requirements in a fusion reactor.

Alternative Divertor Configurations (ADCs), using strong magnetic shaping and additional X-points, may outperform SN divertors. ADCs have shown significant advantages at TCV, MAST-U, and other devices [1]. ASDEX Upgrade (AUG), following a major upgrade, is the first device combining ADC capability with reactor-relevant heat fluxes, a full tungsten wall, and a cryopump, enabling detailed studies under fusion-relevant conditions [2]. We present first ADC experiments at AUG, including H-modes with >20 MW heating and strong impurity radiation. As the flux expansion increases, electron temperatures close to the strike line and target particle fluxes are reduced. With the transition to configurations with multiple X-points, the radiation at the X-points is enhanced and the heat fluxes at the primary strike point reduced. The results are interpreted using analytic models and advanced transport simulations.

[1] A. Fil et al 2022 Nucl. Fusion 62 096026

[2] Zammuto, I., et al.,Fusion Eng. Des. 215 (2025): 115028.