We investigated the differences between type I and type II edge localized modes (ELMs) in ASDEX Upgrade using the MHD stability analysis. When plasma conditions are changed from typical type I ELMy conditions to type II ELMy conditions, the character of the edge instabilities change. With increased triangularity and edge safety factor, the low-n peeling–ballooning mode becomes more stable and more localized to the edge region. We find the same mode localization effect in almost double null configuration. When the plasma density is increased, the access to the second stability of the high-n ballooning modes is closed. The changes in the stability properties give a qualitative explanation to the observed small amplitude of the type II ELMs.
MHD stability analysis of ELMs in MAST