Optimizing stability, transport, and divertor operation through plasma shaping for steady-state scenario development in DIII-D

Recent studies on the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] have elucidated key aspects of the dependence of stability, confinement, and density control on the plasma magnetic configuration, leading to the demonstration of nearly noninductive operation for >1 s with pressure 30% above the ideal no-wall stability limit. Achieving fully noninductive tokamak operation requires high pressure, good confinement, and density control through divertor pumping. Plasma geometry affects all of these. Ideal magnetohydrodynamics modeling of external kink stability suggests that it may be optimized by adjusting the shape parameter known as squareness (zeta). Optimizing kink stability leads to an increase in the maximum stable pressure. Experiments confirm that stability varies strongly with zeta, in agreement with the modeling. Optimization of kink stability via zeta is concurrent with an increase in the H-mode edge pressure pedestal stability. Global energy confinement is optimized at the lowest zeta tested, with increased pedestal pressure and lower core transport. Adjusting the magnetic divertor balance about a double-null configuration optimizes density control for improved noninductive auxiliary current drive. The best density control is obtained with a slight imbalance toward the divertor opposite the ion grad(B) drift direction, consistent with modeling of these effects. These optimizations have been combined to achieve noninductive current fractions near unity for over 1 s with normalized pressure of 3.5 <beta(N)< 3.9, bootstrap current fraction of >65%, and a normalized confinement factor of H-98(y,H-2)approximate to 1.5.