Effects of divertor geometry on detachment and core plasma performance with impurity seeding in EAST

Divertor detachment operation compatible with the core plasma is an effective method to alleviate the steady-state heat flux approaching the divertor target; this scheme will be adopted by high-performance tokamaks in the future, such as International Thermonuclear Experimental Reactor (ITER). Currently, two ITER-like tungsten divertors with different geometries have been installed at the top and bottom of Experimental Advanced Superconducting Tokamak (EAST), providing conditions for the present study to investigate the effect of divertor geometry on the compatibility between detachment and core plasma. Recent H-mode experiments show that the electron temperature and heat flux of the outer target of the lower divertor decrease lower than those of the upper divertor after similar amounts of impurity are truly injected into the plasma. SOLPS simulations further prove that the closed lower divertor with a 'corner slot' structure is beneficial for trapping impurity and deuterium particles, thereby increasing momentum and energy losses. For the upper single-null discharges, the divertor electron temperature can be relatively reduced by increasing impurity seeding, but it is also easy to decrease the plasma stored energy and even lead to an H-L back transition. In addition, statistical data on discharges with impurity seeding show that the proportion of detachment discharges achieved by using the closed divertor is higher than that achieved by using the open divertor. The discharges with the closed divertor after detachment have less damage to the core plasma performance than those with the open divertor, irrespective of whether neon or argon seeding is employed. This research will support long-pulse detachment operation in EAST and provide a reference for other tokamaks in divertor design.