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

Abstract

Abstract 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 SOLPS-ITER simulations with full drifts and currents. It is found that the n e 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 × 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,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 ∼ T e t , p e a k − ( 1.4 ∼ 2.1 ) n e , s e p 1.6 ∼ 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.