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 Nucl ear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] have been produced by combining the benefits o f a hollow or weakly sheared central current profile [Phys. Plasmas 3, 1983 (1996)] with a high confinement (H mode) edge. In these discharg es, low-power neutral beam injection heats the electrons during the in itial current ramp, and ''freezes in'' a hollow or flat central curren t profile. When the neutral beam power is increased, formation of a re gion of reduced transport and highly peaked profiles in the core often results. Shortly before these plasmas would otherwise disrupt, a tran sition is triggered from the low (L mode) to high (H mode) confinement regimes, thereby broadening the pressure profile and avoiding the dis ruption. These plasmas continue to evolve until the high-performance p hase is terminated nondisruptively at much higher beta(T) (ratio of pl asma pressure to toroidal magnetic field pressure) than would be attai nable with peaked profiles and an L-mode edge. Transport analysis indi cates that in this phase, the ion diffusivity is equivalent to that pr edicted by Chang-Hinton neoclassical theory over the entire plasma vol ume. This result is consistent with suppression of turbulence by local ly enhanced ExB flow shear, and is supported by observations of reduce d fluctuations in the plasma. Calculations of performance in these dis charges extrapolated to a deuterium-tritium (DT) fuel mixture indicate s that such plasmas could produce a DT fusion gain Q(DT)=0.32. (C) 199 7 American Institute of Physics.