Pellet injection into PDX diverted plasmas

Abstract

Initial hydrogen pellet injection experiments have been performed in plasmas characterized by a low particle re-cycle fraction resulting from the action of a poloidal magnetic divertor. As in past experiments, the interaction of the fuel with the plasma is observed to be adiabatic. Moreover, the pellet mass is accounted for in the core plasma density increase, indicating only a small loss of fuel while the pellet transits the divertor scrape-off plasma. The effect of edge re-cycling on the density was studied by comparing divertor (low-re-cycle) and limiter (high-re-cycle) plasmas; the distinction between the two cases is clearest in the edge plasma region where the density decay rates differ most. Particle transport subsequent to pellet injection is less ambiguous for divertor cases, and, by comparing the density profile relaxation and the electron temperature recovery with an empirical transport model that closely approximates the pre-injection plasma conditions, it is concluded that the plasma confinement properties do not deteriorate as a result of pellet injection. The principal difference between central and edge fuelling is demonstrated by a peaking of the density profile and an extended decay time for the density perturbation.