Dependence of ELM instability on separatrix density in EAST long-pulse H-mode plasmas

The transition from small edge-localized modes (ELMs) to large ELMs has been repetitively observed in minute-scale long-pulse high-confinement mode (H-mode) discharges during the 2017 EAST campaign. The appearance of large ELMs is found to be strongly correlated with the decrease in separatrix density due to the gradual decrease in fuel recycling during long-pulse H-mode operations (LPHOs). By the numerical scanning of separatrix density with a fixed temperature profile, it has been found that the dependence of ELM instability on separatrix density is related to the competition between the ion diamagnetic stabilizing effect and destabilizing effect of pressure gradient and current density in the pedestal region. This sheds light on a comprehensive understanding of the different roles of separatrix density in ELM instability observed during EAST experiments. With a high separatrix density, the ideal ballooning mode can be destabilized near the separatrix, which is thought to help achieve small ELMs in EAST LPHOs. During the 2021 EAST campaign, an experiment of large ELM control was performed through actively changing fuel recycling by moving the strike point location on the lower tungsten divertor target plate. It has been demonstrated that the mitigation of large ELMs is strongly correlated with the significant increase in separatrix density, which is thought to be attributed to a higher ionization source in the scrape-off layer (SOL) region by SOLPS-ITER simulation. The high ionization source in the SOL region is believed to provide a strong fueling effect near the separatrix and thus raise the local density, which is considered an important reason for triggering ballooning instabilities near the separatrix and achieving small ELMs.