Radiation characteristics near the X-point region on EAST during nitrogen seeding

Impurity seeding is typically used to achieve plasma detachment. In this study, nitrogen (N₂) seeding detachment experiments are performed on EAST in high-confinement mode (H-mode) plasmas. The experiments are conducted under full-metal-wall conditions, featuring a tungsten divertor and boronized first wall surfaces. The radiation characteristics near the lower X-point region during plasma detachment are systematically investigated. The formation of the X-point radiation region is observed, primarily contributed by the radiation from seeded impurities. The X-point radiation intensity is related to the N₂ seeding rates and the degree of detachment. During the partial detachment with low N₂ seeding rates, the X-point radiation remains weak. However, as the N₂ seeding rate increases, a strong X-point radiation region emerges along with the deep detachment process. The strong X-point radiation can be a direct consequence of deep divertor detachment. When the amount of N₂ injected is excessive, the strong X-point radiation region can expand upwards toward the bulk plasma region, leading to modifications of the plasma parameter profiles. Nevertheless, the upward expansion of the strong X-point radiation region does not adversely affect the sustained deep detachment state and remains compatible with H-mode confinement, with only a slight confinement degradation of no more than 10%.