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

The transitions from type-I to small edge localized modes (ELMs) and back are studied by the electron cyclotron emission imaging (ECEI) diagnostic on 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. Radially, the temperature fluctuations are distributed in the same narrow region of 2 cm in between 0.975 < rho-pol < 1.025 with associ- ated poloidal mode numbers m = 96 +/1 18 and toroidal mode numbers n = 16 +/- 4. 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 on AUG suggest that they both 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 (velocities, frequencies and wave-numbers) obtained in the analysis can be further used for the direct comparison in various code simulations.