Enrichment of impurities seeded for exhaust control in a spherical tokamak power plant geometry

Through SOLPS-ITER simulations, we have investigated the ability of seeded argon and neon impurities to effectively control divertor power loading in a power-plant-class spherical tokamak geometry. We consider a connected double null equilibrium, with a well-baffled, extended outer divertor leg and a short, weakly baffled inner divertor leg. As the impurity can significantly impact fusion performance if it travels upstream to the main confined plasma, measures of the success in restricting power loads are given by the corresponding impurity compression and enrichment, quantifying the ratio of the amount of impurity found upstream to that in the divertor. In this study we work with a fixe input power of 100MW, constant transport coefficients, so no ballooning effects on transport are modelled and drifts are turned off, and weakly varied main ion fuelling from external gas puffs. The impurity seeding is varied to produce detachment, giving radiation losses from the closed field line region up to around 10MW. We find that argon reduces the target power loads effectively, with a high radiation efficiency, and remains well localized as the outer leg detaches. Argon compression and enrichment in the inner leg can be improved by seeding there directly, with the impurity concentration on the last closed flux surface reaching 1.5%. Neon, by comparison, is found to be a much less suitable impurity for use at this scale, with a low radiation efficiency and the impurity concentration on the last closed flux surface reaching up to 9%.