Access to sustained high-beta with internal transport barrier and negative central magnetic shear in DIII-D

High values of normalized beta (beta(N)similar to 4) and safety factor (q(min)similar to 2) have been sustained simultaneously for similar to 2 s in DIII-D [J.L. Luxon, Nucl. Fusion 42, 64 (2002)], 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 at high beta results in high confinement (H-89P > 2.5) and large bootstrap current fraction (f(BS)> 60%) with good alignment. Previously, stability limits in plasmas with core transport barriers have been observed at moderate values of beta(N) (< 3) because of the pressure peaking which normally develops from improved core confinement. In recent DIII-D experiments, the internal transport barrier is clearly observed in the electron density and in the ion temperature and rotation profiles at rho similar to 0.5 but not in the electron temperature profile, which is very broad. The misalignment of T-i and T-e gradients may help to avoid a large local pressure gradient. Furthermore, at low internal inductance similar to 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 flat top and beta similar to 5%, a noninductive current fraction of similar to 100% has been observed. Stability modeling shows the possibility for operation up to the ideal-wall limit at beta similar to 6%.