Transport and performance in DIII-D discharges with weak or negative central magnetic shear

Discharges exhibiting the highest plasma energy and fusion reactivity yet realized in the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] have been produced by combining the benefits of a hollow or weakly sheared central current profile [Phys. Plasmas 3, 1983 (1996)] with a high confinement (H mode) edge. In these discharges, low-power neutral beam injection heats the electrons during the initial current ramp, and “freezes in” a hollow or flat central current profile. When the neutral beam power is increased, formation of a region of reduced transport and highly peaked profiles in the core often results. Shortly before these plasmas would otherwise disrupt, a transition is triggered from the low (L mode) to high (H mode) confinement regimes, thereby broadening the pressure profile and avoiding the disruption. These plasmas continue to evolve until the high-performance phase is terminated nondisruptively at much higher β T (ratio of plasma pressure to toroidal magnetic field pressure) than would be attainable with peaked profiles and an L-mode edge. Transport analysis indicates that in this phase, the ion diffusivity is equivalent to that predicted by Chang–Hinton neoclassical theory over the entire plasma volume. This result is consistent with suppression of turbulence by locally enhanced E×B flow shear, and is supported by observations of reduced fluctuations in the plasma. Calculations of performance in these discharges extrapolated to a deuterium–tritium (DT) fuel mixture indicates that such plasmas could produce a DT fusion gain Q DT =0.32.