Access to Sustained High-Beta With Internal Transport Barrier and Negative Central Shear in DIII-D

High values of normalized pressure ($\beta_N\sim\,$4) and safety factor ($q_{min}\sim\,$2) have been sustained simultaneously for $\sim$2~s in \hbox{DIII-D}, suggesting a possible path to high fusion performance, steady-state tokamak scenarios with a large fraction of bootstrap current. The combination of internal transport barrier and negative central magnetic shear results in high confinement ($H_{89p}>\,$2.5) and good bootstrap current alignment ($ƒ_{BS}\sim\,$60\%). Previously, stability limits in plasmas with core transport barriers have been observed at moderate values of $\beta_N$ ($<$3) [R.C.\ Wolf, Plasma Phys.\ Control.\ Fusion ${\bf 45}$, R1 (2003)] because of the pressure peaking which normally develops from improved core confinement. In recent \hbox{DIII-D} experiments the internal transport barrier is clearly observed in the ion temperature and rotation profiles at $\rho\sim\,$0.5 but not in the electron temperature profile, which is very broad. The misalignment of $T_i$ and $T_e$ gradients may help avoid a local pressure peaking. Furthermore, at low internal inductance $\sim$0.6, the current density gradients are close to the vessel and the ideal kink modes are strongly wall-coupled. Simultaneous feedback control of both external and internal sets of n=1 magnetic coils was used to maintain optimal error field correction and resistive wall mode stabilization, allowing operation above the free-boundary beta-limit. Large particle orbits at high safety factor in the core help to broaden both the pressure and the beam-driven current profiles, favorable for steady state operation. At plasma current flattop and $\beta \sim\,$5\%, a noninductive current fraction of $\sim$90\% has been observed. Stability modeling shows the possibility for operation up to the ideal-wall limit at $\beta \sim$6\%.