On the onset of type I edge localized modes

The onset of type I edge localized modes (ELMs) is investigated on the DIII-D tokamak. A fast imaging camera is used with an integration time of 1 µs and a time between frames of about 15 µs continuously recording for a period of 1.3 s. It is observed that type I ELMs onset starts with a precursor oscillation at the mid-plane caused by a toroidally rotating localized structure with a spatial scale increasing with time. This is confirmed by the toroidal set of magnetic probes and continues until the filamentary structure(s) strongly interacts with the first wall at the outer mid-plane. This triggers a strong plasma–wall interaction that later spreads to affect the whole scrape-off layer. The properties of the observed localized structure(s) are in good agreement with the ballooning finger structure occurring in the early non-linear phase of the peeling–ballooning instability.