Characterization of peeling–ballooning stability limits on the pedestal

The peeling–ballooning model for edge localized modes (ELMs) and pedestal constraints, based on ideal MHD instabilities driven by pressure gradients and current in the edge barrier region, has been broadly applied toward understanding ELM and pedestal behaviour in a number of tokamak experiments. Due in part to multiple driving terms, multiple wavelengths and second stability access physics, the peeling–ballooning stability limits which are proposed to constrain the pedestal and trigger ELMs depend sensitively on many details of the tokamak equilibrium. Here we present a technique for characterizing these stability constraints as a function of important parameters, using carefully constructed model equilibria. We discuss comparisons of calculated stability constraints to observed pedestal behaviour, in which an encouraging level of agreement is found. We then present results of an extensive series of calculations which characterize the peeling–ballooning stability constraints as a function of pedestal width, magnetic field, plasma current, density, and triangularity.