Fueling efficiency of pellet injection on DIII-D

Pellet injection has been used on the DIII-D tokamak to study density limits and particle transport in II-mode and inner wall limited L-mode plasmas. These experiments have provided a variety of conditions to examine the fueling efficiency of pellets injected into DIII-D plasmas. The fueling efficiency, defined as the total increase in number of plasma electrons divided by the number of pellet fuel atoms, is determined by measurements of density profiles before and just after pellet injection. We have found that there is a decrease in the pellet fueling efficiency with increased neutral beam injection power. The pellet penetration depth also decreases with increased neutral beam injection power so that, in general, fueling efficiency increases with penetration depth. The fueling efficiency is generally 25% lower in ELMing H-mode discharges than in I-mode due to an expulsion of particles with a pellet triggered ELM. A comparison with fueling efficiency data from other tokamaks shows similar behavior. (C) 1999 Elsevier Science B.V. All rights reserved.