Study of the ELM fluctuation characteristics during the mitigation of type-I ELMs

Transitions from type-I to small edge localized modes (ELMs) and back are studied using the electron cyclotron emission imaging (ECEI) diagnostic on the ASDEX Upgrade (AUG). ECEI measurements show that the average poloidal velocity of temperature fluctuations of both type-I ELM onsets and small ELMs is the same and is close to 5–6 km s−1. Radially, the temperature fluctuations are distributed in the same narrow 2 cm region between with associated poloidal mode numbers and toroidal mode numbers . The observed fluctuations related to both type-I ELMs and small ELMs vary over the transition simultaneously, however, showing slightly different behaviour. The similarities between type-I ELMs and small ELMs observed via AUG suggest that they have the same nature and evolve together. In the transition phase a temperature fluctuation mode ('inter-ELM mode') appears, which becomes continuous in the mitigated ELM phase and might cause the ELM mitigation. The mode characteristics (velocity, frequency and wave number) obtained in the analysis can be further used for direct comparison in various code simulations.