Characterization of the density profile collapse of type I ELMs in ASDEX Upgrade with high temporal and spatial resolution reflectometry

This paper characterizes the density profile dynamics during edge localized modes (ELMs), using high temporal and spatial resolution reflectometry, with main emphasis on the dynamics of type I ELMs. Qualitatively, the density profile dynamics of both type I and type III ELM ASDEX Upgrade tokamak discharges shows the same characteristic behaviour. The analysis of the behaviour of the density at the plasma edge allows the identification of three phases of the ELM event: precursor, collapse and recovery phases. For the three phases, the effect of ELMs is analysed both on the high-field side (HFS) and the low-field side (LFS) of the plasma at the ASDEX Upgrade. Asymmetries of the ELM event when comparing the HFS and LFS are found. The ELM event is associated with a sudden loss of particles from the main plasma (inside the last closed flux surface) to the scrape-off layer and then to the divertor. For type I ELMs the relative particle losses (to the pedestal particle content), assuming poloidal symmetry, was found to be of ∼10% using LFS measurements and of ∼5% using the HFS measurements. The type I ELM affected depths are about ∼25% and ∼15% of the plasma minor radius for the LFS and HFS profiles mapped to the outer midplane, respectively.