Experimental basis in support of an alternative low plasma current path for the Q=10 goal in ITER

Experiments on DIII-D have achieved line-averaged density Greenwald fraction (f_Gr) above 1 simultaneously with H_98y2 around 1.5, providing an experimental basis that addresses the need for an alternative low plasma current path to the Q=10 goal in ITER. As stated in a recent ITER report [Loarte, ITR-24-004 (2024)], ITER is seeking an alternative higher q₉₅ (≥6) plan for its Q=10 mission. The target I_p is around 7.5 MA at B_T=5.3 T. A dedicated 0-D calculation in this study shows a requirement of f_Gr≥1.0 and H_98y2>1.2 at I_p≤12 MA while keeping β_T and density constant as the original values in the 15 MA case. Further reduced I_p requires higher f_Gr and higher H_98y2, simultaneously. There was no experimental support for such operating space due to insufficient H_98y2 at f_Gr≥1.0, until the recent high poloidal-beta (β_P) experiments on DIII-D [Ding, Nature (2024)]. A synergy between increased H_98y2 and increased f_Gr is observed in the experiment, due to the build-up of an internal transport barrier at large radius in the temperature and density channels. Turbulence suppression at higher density gradient has been observed in both transport modeling and turbulence measurements in the experiment. This result not only supports the previous ITER integrated modeling work [Ding, 28^th IAEA FEC (2021)] for Q=10 using a scenario with low plasma current (~8 MA), high β_P, and f_Gr above 1, but also lays a physics foundation for the latest ITER experimental plan.