Current profile modification and MHD activity following pellet injection in Alcator C

Pellet fueling experiments on Alcator C focussed on the regime of improved energy and particle confinement, where it was observed that transport for the ionic species was reasonably well described by neo-classical theory. Ion energy transport dropped and particle confinement, particularly for impurity species increased dramatically. These effects have been ascribed to the suppression of ion temperature gradient driven modes (ηi) by the peaked density profile set up by pellet injection. Although radiation by high Z ions was unimportant in these discharges, the accumulation of low Z impurities, principally carbon, had a profound effect on the discharge by peaking the resistivity profile and reducing the current density near the plasma center. One result of this was a modification of sawtooth activity; a lengthening of the period between sawteeth and ultimately their complete suppression. After suppression of the sawteeth, very large amplitude m=1, n=1 oscillations were often observed. The appearance of this mode was often correlated with saturation of impurity confinement. We speculate that this mode is a form of the m=1 ideal mode that has been held responsible for the sawtooth crash. (author). 14 refs, 5 figs