2D ECE measurements of type-I edge localized modes at ASDEX Upgrade

Abstract

The installation of a 2D electron cyclotron emission imaging (ECEI) diagnostic on ASDEX Upgrade has provided a new means to observe the nature of edge localized modes (ELMs). For a series of ELMs in a typical type-I ELMy H-mode (with q95 = 4.7), the 2D dynamics have been characterized. Firstly, a clear distinction between so-called ‘fast’ and ‘slow’ ELMs was found to be the occurrence of an off-mid-plane fluctuation in case of the latter. This mode has its amplitude strongest off-mid-plane and its poloidal and toroidal mode numbers are m ∼ 110 and n ∼ 30. Secondly, prior to the onset of the ELM's temperature collapse, a mode is observed that covers the whole ECEI-observation window. Here, the estimated poloidal and toroidal mode numbers are m ∼ 75 and n ∼ 20. These have been seen to increase towards the ELM crash, simultaneously with a velocity increase of the mode (in poloidal direction). Finally, filaments have been identified during the temperature collapse phase and their motion could be followed in the vertical direction. In contrast to both the off-mid-plane fluctuation and the ELM-onset mode, which only have been seen rotating in the electron diamagnetic drift direction, the first few filaments have sometimes been observed to move in the opposite direction as well.