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Description
Power exhaust remains a key challenge for steady-state operation of future tokamak reactors. More than 90% of the heating power must be dissipated through radiation before reaching the divertor targets to ensure sufficient material lifetime and low sputtering. At the same time, the divertor must enable efficient pumping of fuel and helium neutrals, and prevent impurity accumulation in the plasma core, which would degrade the confinement. Whether the conventional single null (SN) divertor, as planned for ITER, can meet these combined requirements in a reactor environment with sufficient headroom remains uncertain. Alternative Divertor Configurations (ADCs), which employ advanced magnetic field shaping, have been proposed to improve exhaust performance compared to the SN divertor. These ADCs can affect a number of divertor processes. The modified magnetic geometry can result in enhanced cross-field transport due to increased connection lengths, while the larger divertor volume enhances neutral interactions, promoting power and momentum losses and facilitating detachment access.
Over the past years, ADCs have been investigated on several devices, including TCV and MAST-U [1, 2]. Following a major upgrade, ASDEX Upgrade (AUG) is the first tokamak to combine ADC capability with reactor-relevant heat fluxes, a full tungsten wall, and a cryopump, enabling studies under conditions approaching those in future reactors [3]. During the first operational campaign, all planned ADC configurations were successfully realized. Dedicated parameter scans in heating power, fuelling, and nitrogen seeding were carried out for several configurations, including the X-Divertor (XD) and the Low-Field-Side Snowflake Minus (LFS SF–). In both configurations, detachment of the near scrape-off layer was achieved and well characterized. However, to fully assess the potential advantages of ADCs and to understand the governing physical mechanisms, a detailed characterization of attached divertor regimes is required as well.
This contribution presents experimental analyses of attached divertor heat fluxes and plasma conditions in these configurations and compares them with the conventional SN. The study combines infrared thermography, Langmuir probe, divertor Thomson scattering, and bolometry measurements in deuterium and hydrogen plasmas. The results are interpreted using simplified models for heat flux profiles [4] and plasma–neutral transport simulations with EMC3-EIRENE. First findings on attached heat flux distributions in the XD and LFS SF– configurations are presented, offering new insights into transport processes in ADCs.
[1] Theiler, C., et al., NF, 2017
[2] Verhaegh, K., et al., NF, 2022
[3] Zammuto, I., et al., FED, 2025
[4] Lunt, T., et al, NME, 2017