Pellet dropper device for ELM control on DIII-D

On several experimental tokamaks, pellet injection has been found to trigger edge localized modes (ELMs) in H-mode plasmas. This can provide a technique for ELM amelioration by reducing the ELM size with small high-frequency pellets. The key for success appears to be small pellets that penetrate just beyond the separatix, enough to trigger an ELM, but not enough to strongly fuel the plasma. To provide a source of small pellets, a pellet dropper device has been developed at the Oak Ridge National Laboratory and installed on the DIII-D tokamak. The pellet dropper consists of a batch extruder with an exit nozzle to provide a filament of solid deuterium (nominal 1-mm diameter), from which pellets are punched/dropped at rates of up to approximate to 50 Hz and at speeds of <10 m/s. The pellets are propelled directly downward and through a vertical injection port on DIII-D. In this paper, the design and the initial test results are presented, and the installation on DIII-D is described.