Scaling of the separatrix to pedestal density ratio in ASDEX Upgrade H modes

Recently, there has been increased interest in the ratio n(S)/n(C), where ns and ne are the separatrix and pedestal densities, respectively. A semi-empirical model for this ratio is proposed which puts particular emphasis on the role of edge physics. The empirical elements of the model and the detailed comparison with data rely on the extensive edge density database established at ASDEX Upgrade. The derived scaling can be cast into a form where n(S)/n(C) is essentially determined by the proximity to the Greenwald limit.