Developing high poloidal beta scenarios with internal transport barriers at large radius on KSTAR

Recent high-β_P experiments on KSTAR show promising results with ~25% improvement in β_N and enhanced T_i gradient at mid-radius at constant power and gas puffing rate with both carbon divertor and tungsten divertor. The high-β_P scenario with internal transport barriers (ITBs) at large radius is a candidate operating scenario for steady-state tokamak reactors [Ding, Rev. Mod. Plasma Phys. 2023; Ding, Nature 2024]. A joint high-β_P experiment on DIII-D with KSTAR-like constraints achieved excellent confinement quality due to successful establishment of such ITBs [Jeon, 66^th APS-DPP], and was used as the basis for the KSTAR experiment. Besides achieving higher β_N and stronger T_i gradient, the new experiment also shows lower l­_i than the values in the previous KSTAR long-pulse high-β_P operation, indicating progress in broadening the current profile. However, it is important to note that the achieved confinement in the recent KSTAR experiment is lower than its counterpart in DIII-D. The T_i gradient is not as strong in the KSTAR case, and the location of the high gradient is inside ρ~0.4, instead of ρ>0.6. There is no sign of ITB in the T_e and n_e channels. Based on the experimental data, TGLF modeling shows that the magnetic shear is not low enough at mid-radius, leaving the plasma on the low α_MHD (normalized pressure) side of an instability mountain. Further optimization on the early phase of the discharge is planned for the coming KSTAR experiment.