Demonstration of high-performance negative central magnetic shear discharges in the DIII-D tokamak

Reliable operation of discharges with negative central magnetic shear has led to significant increases in plasma performance and reactivity in both low confinement, L-mode, and high confinement, H-mode, regimes in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159]. Using neutral beam injection early in the initial current ramp, a large range of negative shear discharges have been produced with durations lasting up to 3.2 s. The total noninductive current (beam plus bootstrap) ranges from 50% to 80% in these discharges. In the region of shear reversal, significant peaking of the toroidal rotation [fφ(0)∼30–60 kHz] and ion temperature [Ti(0)∼15–22 keV] profiles are observed. In high-power discharges with an L-mode edge, peaked density profiles are also observed. Confinement enhancement factors up to H≡τE/τITER-89P∼2.5 with an L-mode edge, and H∼3.3 in an edge localized mode (ELM)-free H mode, are obtained. Transport analysis shows both ion thermal diffusivity and particle diffusivity to be near or below standard neoclassical values in the core. Large pressure peaking in the L mode leads to high disruptivity with βN≡βT/(I/aB)≤2.3, while broader pressure profiles in the H mode gives low disruptivity with βN≤4.2.