Simulation study of the influence of the outer target geometry on tungsten contamination: Horizontal vs. vertical target

The core tungsten (W) concentration must be limited to an extremely low level for future fusion reactors. It was found that the effects of E × B drift can lead to a dramatic increase in the core W concentration in our recent work [J. Guo et al., Nucl. Fusion 63 (2023) 126033], in which the simulation study is performed for the divertor geometry with both inner and outer vertical targets. In this work, the influence of the outer target geometry is further investigated by numerical simulations under the direction of toroidal field B_T favorable for H-mode access based on a typical EAST upper single-null configuration with two different outer target geometries, i.e. horizontal target (HT) and vertical target (VT). The background plasmas are simulated using SOLPS-ITER, and the W transport is simulated separately using DIVIMP. For the HT case, it is found that detachment in the outer divertor is promoted relative to the VT case, which leads to the suppression of W sputtering flux from the outer target. In contrast, the W sputtering flux from the outer target dominates for the VT case. The plasma profiles are similar for both cases at the inner vertical target, where detachment is achieved due to the drift effects under favorable B_T except for the region near the inner strike point. Because the plasma detaches across most of the inner and outer target, the upstream W flux is significantly reduced for the HT case, and the core W concentration is about 2 orders of magnitude lower compared to the VT case. According to the analysis of the W transport for the HT case, two leakage pathways through the inner and outer scrap-off layer near the separatrix are indicated for the W impurities sputtered near the inner strike point.