Characteristics of type I and type III elm precursors in asdex upgrade

摘要

The temporal evolution of the edge electron pressure gradient during the development of a type I ELM shows that proximity of p_edge to the ideal ballooning limit is not sufficient to trigger a type I ELM. Thus, the MHD structure of ELMs is investigated further. The present discussion focuses on the phenomenology of type I and typeⅢ ELM precursors. The ELM precursor types are well distinguished by their frequency behaviour and mode structure. The type I ELM precursor oscillation originates from a thin layer close to the plasma edge. For typeⅢ ELMs, on the contrary, p_edge has a much stronger influence as indicated by their occurrence during L mode.