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 n s and ne are the separatrix and pedestal densities, respectively. A semi-em pirical model for this ratio is proposed which puts particular emphasis on the role of edge physics. The empirical elements of the model and the detai led comparison with data rely on the extensive edge density database establ ished 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.