Studies of the regime of improved particle and energy confinement following pellet injection into Alcator C

Pellet fueling in Alcator C can result in a plasma regime in which both energy and particle confinement are dramatically improved. In this regime, ion thermal losses (highly anomalous in gas fueled plasmas) are reduced to approximately neoclassical values, resulting in increased global energy confinement for high density discharges where ion losses are important. There is little or no change in electron thermal conductivity. The pellet fueled discharges exhibit peaked density profiles, which tends to confirm the importance of the ηi parameter in determining ion thermal diffusivity. At the same time, impurity particle transport is reduced to neoclassical levels, leading to accumulation of impurities and strong peaking of impurity profiles. Confinement of the hydrogenic species in the discharge core is also increased. The modifications of ion thermal transport, central particle transport, and impurity transport are well correlated; there are no discharges where only one or two of these parameters change. In the favourable transport regime, sawteeth are partially or completely suppressed. This is probably due to the strong peaking of low Z impurities which flatten the central current profile by increasing the plasma resistivity. Radiation does not play an important role in the power balance of these discharges. For plasmas in which the sawteeth are suppressed, the improved confinement is sustained for the remainder of the discharge. (author)