Investigation of the dithering-like cycles induced by deuterium pellet injection during ELMy H-mode plasmas on EAST

By injecting cryogenic deuterium pellets into H-mode plasmas with edge localized modes (ELMs) from the low-field side (LFS) on EAST tokamak, the impact of these pellets on the edge plasma can be investigated. In some cases of pellet injection (PI), the intermediate phase (I-phase), i.e, the dithering-like cycles, have been observed. Experiments show that during one dithering-like cycle, three phases can be distinguished, which are closely related to the variations in the pedestal density profiles and density gradient after PI. Phase 1 is characterized by rapid particle loss and a density pedestal crash, phase 2 has a relatively low density gradient and enhanced edge density fluctuation and phase 3 corresponds to the relatively fast recovery of the density pedestal and reduced edge density fluctuation. Through an experimental analysis, it is shown that the plasma will enter the I-phase when the edge line-averaged density increases beyond a certain threshold value after PI. H-I-H transitions are more likely to be observed when the edge line-averaged density is greater than the core line-averaged density.