Simulation study of the influence of upstream density and power in the scrape-off layer on the double-peaked density profile at the divertor target

The double-peaked distribution of particle deposition at divertor targets has been observed in various tokamaks, and is considered a potential approach for mitigating divertor particles and heat load in future fusion reactors. Recently, the systematical analysis of the double-peaked distribution behavior during EAST experiments shows that the appearance of the double-peaked profile is related to the line-average density and heating power. In order to understand general trends and related mechanisms, the influences of the upstream density (n(e,sep)) and power in the scrape-off layer (P-SOL) on the double-peaked density profile are investigated by SOILS-ITER simulations with full drifts and currents. It is found that the ne peak near the strike point is mainly contributed by the strong ionization source close to the target, and the n(e) peak in the far-SOL region is caused by the synergetic effects of poloidal and radial E x B drifts along the SOL. The double-peaked distribution is affected by the P-SOL, and impurity seeding by increasing or decreasing the whole profile of the electron temperature at the target (T-et). When the peak value of T-et (T-et,T-peak) is fixed, the density peak in the far-SOL is increased for higher n(e,sep) by reducing the T-et in the far-SOL region on the lower-field side under unfavorable B-T and by the upstream-extended ionization source due to the geometry effect on the high-field side under favorable B-T. Statistical analysis of the simulated results shows that the scaling expression of the peak ratio is similar to T(et,peak)(-(1.4 similar to 2.1))n(e,sep)(1.6 similar to 1.8). In addition to the upper boundary found in the analysis of EAST experiments, a lower boundary of the region where the double-peaked feature appears on the P-SOL-n(e,sep) plane is identified by simulations and preliminarily confirmed according to the measurements in several EAST discharges.