Mechanism of core confinement improvement in EAST I-mode detachment experiments with neon seeding

Abstract This work presents a dedicated study on improved core energy confinement observed during neon-seeded I-mode detachment experiments on the EAST. Experimental results demonstrate that neon seeding facilitates the divertor detachment, as evidenced by rollover in the ion saturation current and a reduction in target surface temperature, while simultaneously avoiding the I-L back-transition. Notably, an increase in plasma stored energy and the H_{98} confinement factor is achieved, primarily attributed to elevated core electron and ion temperatures, despite a slight degradation of the pedestal. Higher neon seeding rates are found to promote deeper detachment and greater core confinement improvement with modest neon concentration and radiative loss fraction. Power balance analysis using the ONETWO code reveals a significant reduction in ion and electron thermal diffusivities after neon seeding. The predictions of plasma profiles with TGYRO code confirm the increment of core temperature. Subsequent quasi-linear gyrokinetic analysis with the Trapped Gyro-Landau Fluid (TGLF) model identifies that core heat transport is predominantly driven by trapped electron modes (TEMs). The study demonstrates that neon seeding stabilizes TEMs, primarily through enhanced collisionality, with main ion dilution playing a minor role. This suppression of TEM turbulence leads to a reduction in heat fluxes, which outweighs the radiative cooling effect and thereby improves core energy confinement. These results demonstrate that neon impurity seeding can enable the simultaneous achievement of divertor detachment and enhanced core confinement in I-mode operation on EAST, offering a valuable reference for solving core-edge integration issue.