Transport and Stability Characteristics of High $q_{min}$ Steady-State Scenarios with Off-Axis NBI

DIII-D experiments show that off-axis neutral beam injection (NBI) improves access to high $\beta$ and fully noninductive current sustainment at elevated minimum $q$ ($q_{min}$) by broadening the plasma current and pressure profiles. Off-axis NBI modifies the underlying characteristics of transport and stability, as expected from theory. For on-axis NBI, the electron (ion) confinement time normalized to ITER H-mode scaling, $\tau_{th,H_{98}}$ increases (decreases) with $q_{min}$. For off-axis NBI, both power balance analysis and TGLF modeling indicate better ion but worse electron confinement, when compared with on-axis NBI at the same $q_{min}$. There appears to be a strong confinement scaling with Shafranov shift, the ratio of electron to ion heating, and the location of heat deposition. The broader pressure profile obtained from off-axis NBI helps to avoid low-order tearing modes and increases the calculated ideal-wall $\beta_N$ limit. Nonlinear optimization of confinement and stability to achieve higher $\beta_N$ ($>$4) with fully noninductive current drive will be discussed using theory-based scenario modeling validated against high $q_{min}$ discharges with on- and off-axis NBI.