Requirements for Q=10 in ITER at I_P < 15 MA, and relevant metrics for present-day tokamaks

Operation of ITER at plasma current (I_P) lower than 15 MA (q₉₅>3) could provide a safer path to the development of a scenario meeting the Q=10 goals, but will require enhanced energy confinement quality at density close to or above the Greenwald limit. DIII-D experiments in the high poloidal beta regime have recently yielded encouraging results in this direction [1]. This regime has the potential for providing Q ~ 10 operation at I_P ~ 7.5 MA with toroidal field B_T=5.3 T, that is at q₉₅>6 [2]. However, when the fusion performance of high poloidal beta plasmas in present day tokamaks is assessed via the figure of merit G=β_N*H/q₉₅² [3], the outcomes are usually of insufficient performance. In this study, it is shown that for q₉₅>3, some revisions to the formulation of G are needed to improve on the approximations regarding the plasma current dependence of H and q₉₅. In addition, requirements on the Greenwald density fraction cannot be neglected.

[1] S. Ding et al, Nature 629, 555 (2024).

[2] S. Ding, et al. Proc. 28th IAEA Fusion Energy Conference, France (2021).

[3] T.C. Luce, FS&T 48 1212 (2005).