Achievement of a high-density, high-confinement, and high beta tokamak plasma regime for ITER and FPP

The high poloidal beta regime, through alpha stabilization (α ~ dβ_P/dr), provides a practical way to improve the energy confinement in a fusion reactor to levels required for a compact FPP, as well as for Q=10 in ITER at reduced currents (I_P,ITER < 10 MA). A recent DIII-D experiment serves as an existence proof of this high-performance regime for sustained duration (~2τ_R). We demonstrated sustainment of high performance in a previously unattained parameter regime of simultaneous very high energy confinement quality (H_98y2 ≥1.5), very high density Greenwald fraction (ƒ_Gr = π a² <n> / I_P ≥ 1.4), and high toroidal beta (β_T ≥ 3%).

Improvements on MHD stability were made by a plasma shape with increased triangularity, made possible by the Shape and Volume Rise (SVR) divertor, which is predicted to enable MHD stability at higher β_N, as well as by a decrease in the outer gap to increase the β_N limit.

Reproducibility was shown with 4 discharges at near-record performance metrics, at varying duration in the high-beta phase. A slow decay in confinement quality was observed (H₉₈ from 1.8 down to 1.4) due to high-Z impurity accumulation in the core during the high-beta phase. The high-performance duration is ultimately limited by the growth of an MHD mode destabilized as the current profile slowly continued to evolve. Analysis is underway to determine if future additions of RF off-axis current drive might alleviate this issue.