Pellet fueling experiments on the Advanced Toroidal Facility

The experimental results of pellet injection into electron cyclotron resonance heated (ECH) stellarator plasmas on the Advanced Toroidal Facility are presented. The research focused on two main topics: (1) pellet ablation and pellet particle deposition, and (2) plasma thermal collapse and recovery induced by pellet injection. The fueling efficiency of pellet injection, defined as the ratio of the number of atoms in the pellet before it enters the plasma to the number of electrons added to the plasma, was estimated to be 73%. The rate of D_α emission from the pellet ablation process was shown to be independent of the mass of the pellet, and the intensity of the emission coupled with the calculated position of the pellet in the plasma was not indicative of the measured particle deposition. The injection of a properly sized pellet was shown to lead to a reproducible response in which the plasma stored energy collapses. As it recovers, the plasma density remains higher than the pre-pellet density and the central electron temperature increases. This is followed by a relaxation of the density back to the pre-pellet value. The relaxation of the density back to the pre-pellet value occurs in two stages. First, the density profile broadens and then the temperature profile broadens. The evolution of the pressure profile in the plasma is consistent with the hypothesis that the broadening of the density profile is due to unfavorable pressure gradients that occur in the plasma as the stored energy recovers. The actual loss of particles is due to the establishment of a new equilibrium between the particle confinement time and the amount of edge recycling that occurs as the temperature increases in the edge region of the plasma.