Confinement Optimization by Controlling q-shear in DIII-D Steady-State Discharges*

Recent experiment in DIII-D to study effects of varying shear of safety factor (q) on transport and fluctuations showed that negative shear (NS) discharges formed an internal transport barrier inside $\rho$ = 0.5, resulting in substantial improvement in ion confinement in the core, and reduced amplitudes of broad turbulent (low- to mid-k) spectra compared with those for the positive shear (PS) discharges. Previous DIII-D experiments under similar conditions but with higher on-axis NBI powers and reversed-B$_T$ direction ($\bigtriangledown$B drift toward upper single null divertor rather than lower single null divertor), showed longer confinement improvement periods with broad NS q-profile. Measured transport characteristics are analyzed in both experiments and compared with predictions of theory-based transport models using the integrated plasma simulation frameworks (OMFIT/IPS). Based on such validated transport models, predictions of prolonged confinement improvement with better sustained ITB using higher powers of off-axis NBI and ECCD available in DIII-D will be discussed.