Estimates of required impurity fraction for EAST divertor detachment

During the EAST radiative divertor experiments, one of the key challenges is how to avoid the occurrence of disruption events caused by the excessive impurity seeding. To estimate the required impurity fraction for divertor detachment, we introduce a reduced edge plasma radiation model in which impurity radiation loss obtained by the balances of the heat and particle fluxes and upstream pressure determined by the plasma density and temperature at the divertor target based on the momentum conservation along the magnetic field line. It is found that the required impurity fraction shows a non-monotonic variation with divertor electron temperature (T_d) when 0.1<T_d<10eV. In the range of the 0.1<T_d<1eV, the position near the valley of required impurity fraction corresponds to the strong plasma recombination. Due to the dependence of the volumetric momentum loss effect on the T_d in the range of 1<T_d<10eV, the required impurity fraction peaks and then decreases as the T_d is increased. Compared to the neon, the usage of argon reduces the impurity fraction by about twice. In addition, for the various fitting parameters in the pressure-momentum loss model, it is shown that the tendency of required impurity fraction with T_d always increases first and then decreases in the range of 1<T_d<10eV, but the required impurity fraction decreases when the model that characterizes the strong loss in pressure -momentum is used.