Optimizing Stability, Transport, and Divertor Operation Through Plasma Shaping for Steady-state Scenario Development in DIII-D

Recent studies on DIII-D have elucidated key aspects of the dependence of stability, confinement, and density control on the plasma magnetic configuration, leading to noninductive operation (i.e., total inductive flux change $\approx\,$0) for $>$1$\,$s with pressure 30\% above the free boundary limit. Achieving fully noninductive operation requires high $\beta$, good confinement, and density control through divertor operation. Plasma geometry affects all of these. Ideal MHD modeling of the $n=1,2,$ and 3 external kink stability suggests it may be optimized by adjusting the shape parameter known as squareness ($\zeta$). Experiments confirm stability varies strongly with $\zeta$ in qualitative agreement with the modeling. Optimization of $n=1$ stability also seems to raise pedestal stability. Adjusting $\zeta$ above and below the midplane independently allows for small changes in the magnetic divertor balance about a double-null (DN) configuration. Energy confinement is found to be sensitive to this balance, with 20\% higher confinement observed in a balanced DN compared to a slightly unbalanced case. However, adequate density control requires a small imbalance to achieve densities necessary for efficient external noninductive current drive. The best density control (20\%-30\% below the balanced DN case) is obtained with a slight imbalance toward the divertor opposite the ion grad(B) drift direction. Consistency of modeling with these observed effects requires inclusion of $E\times B$ drifts in the divertor. Simultaneous optimization has been applied to achieve noninductive current fractions near 1 for over 1$\,$s with $\beta_N \sim\,$3.5-3.7, bootstrap fraction $>$65\%, and good confinement.