Comparison of deuterium pellet injection from different locations on the DIII-D tokamak

Abstract

Deuterium pellets have been injected into plasmas in the DIII-D tokamak from the inner wall, top, and outer midplane port locations to investigate fuelling efficiency, mass deposition and interaction with edge localized modes (ELMs). Pellets injected from the outer midplane port (low field side (LFS)) show a large discrepancy in the mass deposition profile and fuelling efficiency from conventional pellet ablation theory, while the penetration depth compares favourably with theory. The mass deposition from pellets injected from inner wall and top locations is deeper than expected from ablation theory. The profile measurements indicate that pellet mass is deposited inside the measured penetration radius, thus verifying that a drift of the pellet ablatant is occurring in the major radius direction during the toroidal symmetrization process. The scaling of the measured drift magnitude in DIII-D is found to depend strongly on the pellet size and plasma pedestal temperature. Extrapolation to a burning plasma configuration on ITER is favourable for inner wall pellet fuel deposition depth well beyond the separatrix. Pellets injected into H-mode plasmas from all locations trigger ELMs with much larger ELM events induced by the outside midplane injected pellets. This suggests that the LFS is more sensitive to ELM triggering and may be the preferred location to inject very small pellets to trigger frequent small ELMs and thus minimize ELM induced damage to the divertor material surfaces.