Improved core fueling with high field side pellet injection in the DIII-D tokamak

The capability to inject deuterium pellets from the magnetic high field side (HFS) has been added to the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)]. It is observed that pellets injected from the HFS lead to deeper mass deposition than identical pellets injected from the outside midplane, in spite of a factor of 4 lower pellet speed. HFS injected pellets have been used to generate peaked density profile plasmas [peaking factor ${\mathrm{(n}}_e{\text{(0)/〈n}}_e\mathrm{〉)}$ in excess of 3] that develop internal transport barriers when centrally heated with neutral beam injection. The transport barriers are formed in conditions where $T_e{\mathrm{\sim T}}_i$ and $\text{q(0)}$ is above unity. The peaked density profiles, characteristic of the internal transport barrier, persist for several energy confinement times. The pellets are also used to investigate transport barrier physics and modify plasma edge conditions. Transitions from L- to H-mode have been triggered by pellets, effectively lowering the H-mode threshold power by 2.4 MW. Pellets injected into H-mode plasmas are found to trigger edge localized modes (ELMs). ELMs triggered from the low field side (LFS) outside midplane injected pellets are of significantly longer duration than from HFS injected pellets.