Investigation of the influence of toroidal field direction on impurity radiation efficiency by SOLPS-ITER simulations with neon seeding for a typical EAST upper single-null configuration

Impurity radiation plays an essential role in mitigating heat loads onto divertor targets in future fusion reactors. The impurity transport and radiation distribution can be dramatically affected by the direction of the toroidal magnetic field due to its influence on plasma drifts. A recent experiment on ASDEX Upgrade Chen et al (2024 Nucl. Fusion64 126001) reveals the enhanced impurity retention in the divertor due to the formation of the X-point potential well in the private flux region (PFR) under unfavorable , which indicates the potential in improving the performance of radiative divertor. Considering both requirements of the low effective ion charge number in core plasma and high radiation fraction for the future fusion reactor, in this work the influence of the direction on the impurity radiation efficiency (defined as , where is the poloidally averaged at the core-edge interface (CEI)) is investigated using SOLPS-ITER simulations based on a typical EAST upper single-null configuration with neon (Ne) seeding. A systematic parameter scan is performed by varying the Ne puffing rate ( from the outer divertor) as well as the particle and heat fluxes across the CEI under both favorable and unfavorable directions. It is found that, within the range of the parameter scan, the radiation efficiency is generally improved under unfavorable due to the formation of a potential well near the X-point in the PFR, which implies the feasibility of improving the radiative divertor performance under unfavorable . The effect of the potential well on the impurity transport includes: (1) reversing the electric drift in the near-scrape-off layer region of the outer divertor and leading to a vortex of the impurity flow along the separatrix in the inner divertor, both of which enhance the impurity retention in the divertor region; (2) creating a fast-in and fast-out channel through the core region near the X-point, which leads to being insensitive to upstream parameters. It should be noted that the scan of is restricted due to the collapse of the simulation cases under unfavorable when the radiation front moves upstream of the X-point. Further exploring the influence of direction under higher radiation fraction is required in the future work, which should be conducted by dedicated simulations with higher .