Impact of Current Profile on Transport and Stability in High Noninductive Fraction DIII-D Discharges*

Experiments addressing the issue of $J_{BS}$ and $J_{EC}$ alignment and the optimum $q$ profile for stable noninductive operation show the $J_{NI}$ and $J$ profiles are best aligned at $q_{min}\sim 1.5$, $q_{95}\sim 6.8$. The kinetic profiles vary systematically with $q_{min}$ and $q_{95}$. Transport analysis shows that electrons dominate losses at low $q_{min}$, while at high $q_{min}$ ions dominate. Drift wave stability analysis with the TGLF model shows trends in the linear growth rates that contradict these observations. Systematic scans of EC deposition indicate that a broad ECCD profile at $\rho\sim 0.3-0.55$ yields a $J$ profile that is more stable to the tearing modes that limit the duration of the discharges. Optimal alignment of $J_{EC}$ for tearing stability coincides with the region where additional NI current is needed for $f_{NI}=1$.