Abstract
In Madison Symmetric Torus Dexter et al., Fusion Technol. 19, 131 1991 discharges where
improved confinement is brought about by modification of the current profile, pellet injection has
quadrupled the density, reaching ne=41019 m−3. Without pellet injection, the achievable density
in improved confinement discharges had been limited by edge-resonant tearing instability. With
pellet injection, the total beta has been increased to 26%, and the energy confinement time is
comparable to that at low density. Pressure-driven local interchange and global tearing are predicted
to be linearly unstable. Interchange has not yet been observed experimentally, but there is possible
evidence of pressure-driven tearing, an instability usually driven by the current gradient in the
reversed-field pinch.