Discharges with negative central magnetic shear (NCS) hold the promise
of enhanced fusion performance in advanced tokamaks. However, stabili
ty to long wavelength magnetohydrodynamic modes is needed to take adva
ntage of the improved confinement found in NCS discharges. The stabili
ty limits seen in DIII-D [J. L. Luxon and L. G. Davis, Fusion Technol.
8, 441 (1985)] experiments depend on the pressure and current density
profiles and are in good agreement with stability calculations. Disch
arges with a strongly peaked pressure profile reach a disruptive limit
at low beta, beta(N) = beta(I/aB)(-1) less than or equal to 2.5 (% m
T/MA), caused by an n = 1 ideal internal kink mode or a global resisti
ve instability close to the ideal stability Limit. Discharges with a b
road pressure profile reach a soft beta limit at significantly higher
beta, beta(N) = 4 to 5, usually caused by instabilities with n > 1 and
usually driven near the edge of the plasma. With broad pressure profi
les, the experimental stability limit is independent of the magnitude
of negative shear but improves with the internal inductance, correspon
ding to lower current density near the edge of the plasma. Understandi
ng of the stability limits in NCS discharges has led to record DIII-D
fusion performance in discharges with a broad pressure profile and low
edge current density. (C) 1997 American Institute of Physics.