Separating the roles of magnetic topology and neutral trapping in modifying the detachment threshold for TCV

Total flux expansion, a divertor magnetic topology design choice embodied in the Super-X divertor, is predicted through simple analytic models and SOLPS calculations to reduce the plasma and impurity density detachment thresholds as the outer divertor separatrix leg position and the strike-point major radius, R-t, are increased. However, those predictions are contradicted by recent TCV experimental results. In this study, utilizing the SOLPS-ITER code, we are able to both match TCV results and demonstrate that the effect of total flux expansion is counteracted by two other divertor geometry design characteristics that affect neutrals: (a) the strike-point angle to the outer target; and (b) the effect of physical baffles that reduce the amount of neutrals escaping from the divertor. We quantify the role of those neutral effects through developing and applying a quantitative definition of neutral trapping. The results of this study indicate that improved divertor design, properly utilizing the three design characteristics discussed should lead all effects to be additive in reducing the detachment threshold. A second implication of this study is that any assessment of alternative topologies must separate out the effects of magnetic topology from neutral design characteristics.