A model for the low pressure boundary of type-I H mode cycles

The trajectories of ELMY H mode discharges in the pedestal temperature-pede stal density plane form cycles characterized by high pressure and low press ure boundaries. While at least for type-I ELMs the high pressure boundary s eems to be associated with the ballooning limit in the pedestal, the low pr essure boundary is poorly understood. The scaling of the low pressure bound ary is assessed for JET type-I H modes. The discussion is guided by a semi- empirical model which emphasizes the role of SOL physics. Our results sugge st that the high pressure and low pressure boundaries are governed by disti nct physics mechanisms.