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

The trajectories of ELMy H mode discharges in the pedestal temperature-pedestal density plane form cycles characterized by high pressure and low pressure boundaries. While at least for type-I ELMs the high pressure boundary seems to be associated with the ballooning limit in the pedestal, the low pressure boundary is poorly understood. The scaling of the low pressure boundary 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 suggest that the high pressure and low pressure boundaries are governed by distinct physics mechanisms. (author)