Estimates of required impurity fraction for EAST divertor detachment

During the EAST radiative divertor experiments, one of the key challenges was how to avoid the occurrence of disruptive events caused by excessive impurity seeding. To estimate the required impurity fraction for divertor detachment, we introduce a reduced edge plasma radiation model. In the model, based on the momentum conservation along the magnetic field line, the upstream pressure is determined by the plasma density and temperature at the divertor target, and then the impurity radiation loss is obtained by the balance of the heat and particle fluxes. It is found that the required impurity fraction shows a non-monotonic variation with divertor electron temperature (${T}_{\mathrm{d}}$) when $0.1\;\mathrm{e}\mathrm{V} < {T}_{\mathrm{d}} < 10\;\mathrm{e}\mathrm{V}$. In the range of $0.1\;\mathrm{e}\mathrm{V} < {T}_{\mathrm{d}} < 1\;\mathrm{e}\mathrm{V}$, the position near the valley of required impurity fraction corresponds to strong plasma recombination. Due to the dependence of the volumetric momentum loss effect on the ${T}_{\mathrm{d}}$ in the range of $1\;\mathrm{e}\mathrm{V} < {T}_{\mathrm{d}} < 10\;\mathrm{e}\mathrm{V}$, the required impurity fraction peaks and then decreases as ${T}_{\mathrm{d}}$ is increased. Compared to 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 ${T}_{\mathrm{d}}$ always increases first and then decreases in the range of $1\;\mathrm{e}\mathrm{V} < {T}_{\mathrm{d}} < 10\;\mathrm{e}\mathrm{V}$, but the required impurity fraction decreases when the model that characterizes the strong loss in pressure momentum is used.