Study of the recovery of energy confinement and density pump-out following impurity injection in EAST high-density plasmas

This work presents an investigation conducted on the Experimental Advanced Superconducting Tokamak (EAST) high-density H-mode discharges with low-Z impurity (argon) injection, aiming to explore potential approaches and mechanisms for the recovery of core energy confinement. A clear increase in \(H₉₈\) and \(T_i\) was observed following impurity injection, accompanied by a simultaneous decrease in density. The quasi-linear Trapped Gyro–Landau Fluid (TGLF) model is employed for analysis. Modeling results show that core low-k ion temperature gradient (ITG) modes are stabilized, which corresponds to the observed reduction in ion heat flux. It is mainly attributed to the dilution of main ions, which raises the ITG instability threshold. Moreover, the increase in Tᵢ/Tₑ during this process further elevates the ITG threshold, creating a positive feedback effect. Impurity injection also leads to an increase in the collisionality, which enhances the outward pure convective pinch of particle flux in the outer region, thereby triggering a reduction in electron density. Additionally, TGYRO simulation results unambiguously eliminate weakly enhanced toroidal rotation as the causal mechanism for the observed confinement improvement. Instead, the simulations indicate that a deeply deposited fueling source can both compensate for density reduction and enhance density peaking.