Confinement and stability of DIII-D negative central shear discharges

Negative central magnetic shear (NCS) discharges with beta(N) less than or equal to 4, H less than or equal to 3 and up to 80% of the current non-inductively driven are reproducibly produced in the DIII-D tokamak. Strong peaking of T-i, plasma rotation and, in some cases, n(e) are observed inside the NCS region. Transport analysis shows that the core ion thermal diffusivity is substantially reduced and near the neoclassical value after the formation of the internal transport barrier. The negative central shear is necessary but not sufficient for the formation of this transport barrier. The power required for the formation appears to increase with the toroidal magnetic field. The high performance phase of H-mode NCS discharges often ends with an ELM-like collapse initiated from the edge whereas the L-mode discharges which have a more peaked pressure profile tend to end with a more global n = 1 MHD event.