Enhanced Confinement and Stability in DIII-D Discharges with Reversed Magnetic Shear

Peaked pressure profiles and central ion temperatures up to 20 keV are observed in DIII-D $H$-mode discharges with a central region of strongly reversed magnetic shear. Short wavelength ballooning modes are stabilized by access to the second stable regime, longer wavelength modes by a conducting wall and possibly by rotational shear. Values of normalized beta, ${\ensuremath{\beta}}_{N}=\frac{\ensuremath{\beta}\mathrm{aB}}{I}$, up to 4 (%-$m$-T/MA), energy confinement are a factor of 3 better than $L$ mode, and at least 50% of the noninductive plasma current in this regime demonstrates compatibility with requirements for a steady-state high-beta tokamak power plant.