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In a future fusion reactor, the power crossing the separatrix must be at least 100$\,$MW [1], with a power fall-off length $\lambda_q$ in the order of millimeters [2]. This presents a significant challenge to the divertor target plates, which cannot be solved without strong radiative losses in the divertor volume, leading to detachment. Thus, high impurity concentrations are required in the divertor region, which could be detrimental to the core.
One possible solution to reduce the impurity concentrations needed for detachment are alternative divertor configurations (ADCs). They augment the classic single null (SN) divertor through an increase in the connection length $L_c$ from the outboard midplane to the divertor target, e.g. by the introduction of another X-point in the divertor plasma.
The longer $L_c$ results in an effective increase in radial transport, as well as a greater volume for plasma neutral interactions. The combined effects were shown previously to reduce the required neutral pressures and impurity concentrations for reaching similar detachment depths as in the SN configuration, both experimentally [3] and in simulations [4].
At the tokamak ASDEX Upgrade, the upper divertor was recently equipped with additional coils and a cryopump to study a variety of ADCs, for the first time in a machine with fusion-relevant heating power and a tungsten wall [5]. First experiments explored, among others, the X-divertor (XD) with increased flux expansion at the target, and the low-field-side snowflake minus (LFS SF$^-$) with a secondary X-point in the LFS scrape-off layer.
This contribution presents the first L-Mode experiments with ADCs in ASDEX Upgrade, where feedback-controlled density ramps were performed in the SN, XD and LFS SF$^-$ configuration. The detachment onset could be determined from Langmuir probe data.
Close to the primary strikepoint, the plasma was detached throughout XD and LFS SF$^-$ discharges, in contrast to the attached SN. Further away from the separatrix, however, no difference in detachment behavior depending on edge density or divertor neutral pressure was found between the configurations. These results are interpreted using the plasma and neutral transport code SOLPS-ITER. In addition, a comparison to the typically observed detachment behavior in H-mode is presented.
[1] R. Wenninger et al., NF 2016
[2] T. Eich et al., NF 2013
[3] C. Theiler et al., NF 2017
[4] T. Lunt et al., NME 2019
[5] I. Zammuto et al., FED 2025