Radial Electric Field Measurements In Reversed Shear Plasmas

Measurements of the radial electric field have been obtained on the Tokamak Fusion Test Reactor utilizing the motional Stark effect diagnostic. A large negative excursion in the radial electric field occurs before the transition to the enhanced reversed magnetic shear mode. The electric field is localized to a narrow spatial region and is not observed in discharges without the transition to improved confinement. Concomitant with the radial electric field excursion is a change in the measured impurity poloidal rotation velocity and bursts of magnetic fluctuations.