Effects of negative magnetic shear in fusion plasma confinement. 1. A review of experimental studies

A negative magnetic shear configuration is a potentially attractive mode of operation which enables an economical steady-state tokamak fusion reactor with high beta, high confinement and large bootstrap current fraction. This paper provides a review of experimental studies on the negative shear. Though negative shear configurations are formed naturally with large bootstrap current fraction, they have also been formed by current ramp-up with neutral beam heating or off-axis current drive. Formation of internal transport barrier with remarkable reduction of particle and heat transport have been observed in negative shear plasmas. Neoclassical-level ion thermal diffusivity and particle diffusivity have been realized as a result of nearly perfect disappearance of anomalous transport. The thermal transport on electrons has also been reduced in some devices. High beta values have been attained in combination with the H-mode. High fusion performance has been demonstrated. (author). 84 refs