Description
Future tokamak devices, will operate in high-confinement mode (H-mode) in order to achieve the plasma confinement required for sustained net energy production. A key parameter describing H-mode is the power threshold for the L-H transition ($P_{LH}$), which has been shown to depend on various parameters, such as the plasma density, main isotope and plasma shaping. Experiments on different tokamaks, including JET [E. Delabie, 2014], DIII-D [P. Gohil, 2011] and MAST [H. Meyer, 2011] showed that $P_{LH}$ can vary by up to a factor of two depending on the divertor configuration, indicating the importance of the edge and divertor conditions in the L–H transition. Despite this sensitivity, the widely used $P_{LH}$ scaling law [Y.R. Martin, 2008] does not include the divertor configuration as a parameter, which leads to a significant uncertainty in the predictions. In the ASDEX Upgrade tokamak, during the 2024 campaign, dedicated upper single null (USN) discharges with different divertor configurations were performed at low and high densities. Together with a set of lower single null (LSN) plasmas from a previous campaign, these discharges provide a dataset with similar operational parameters (plasma current, magnetic field and heating scheme) to study the divertor configuration dependence of $P_{LH}$ in ASDEX Upgrade. Significant variations in $P_{LH}$ were observed between the different divertor configurations in both the LSN and USN discharges and in both density branches. The analysis focuses on the divertor conditions and the evolution of the midplane kinetic profiles prior to the L–H transition. This study can help us to understand the underlying physics of the L-H transition, to clarify the role of the divertor configuration on the $P_{LH}$ and to improve our ability to optimize H-mode access in future fusion reactors.