Description
Using MAST-U as a testbed and employing predict-first workflows for coil current optimization to obtain stable plasma shapes, this work examines how trade-offs between neutral beam timing, density control, divertor closure, and plasma shaping enable access to high-q, high-βe scenarios compatible with detached Super-X divertor (SXD) operation.
Early application of NBIs enables rapid access to H-mode which helps to suppress IREs and maintain a higher q profile for longer, delaying the onset of the performance-limiting 2/1 mode. However, early heating before full divertor formation enhances carbon influx and can drive strong reverse magnetic shear. The impurity influx can be mitigated by early divertor formation or sufficient ELM activity to flush impurities from the edge.
Density control is strongly linked to pedestal temperature through its impact on ELM frequency. Pedestal temperatures near 200 eV are associated with frequent Type-I ELMs and effective density control. When pedestal temperatures rise above 250–300 eV, ELM frequency drops sharply as the pedestal enters an ELM-stable regime, leading to uncontrolled density rises. Elevated pedestal temperatures arise either from improved divertor closure, which can increase the baseline pedestal temperature by up to 100 eV, or during MHD activity, where pedestal temperatures can reach around 400 eV while core temperatures decrease.
Optimal divertor closure occurs when the last limiting flux surface intersects the divertor baffle near ψN ≈ 1.15. For ψN < 1.15, baffle erosion increases main-chamber neutral density and core impurity density, while for ψN > 1.15 insufficient plasma plugging allows neutrals to escape the divertor and degrade the pedestal. The latter sensitivity is reduced in SXD configurations, even in detached operation, compared to conventional divertors.
Both medium and high elongation scenarios achieve βe ≈ 5 %. High elongation, κ ≈ 2.5, enables more sustained high-βe operation, whereas at medium elongation, κ ≈ 2, βe is reduced due to the earlier onset of the 2/1 mode. High κ scenarios also featured smaller, higher frequency ELMs which aid density control. Conversely, medium elongation typically provides better confinement overall, with H98 approaching unity. Increased triangularity improves confinement but can limit density control through enhanced ELM stability.