Thomson scattering analysis of large scale fluctuations in the ASDEX Upgrade edge

Abstract

Large scale fluctuations in between edge localized modes (ELMs) are the main source for the scatter in plasma edge H mode profiles of electron density and temperature, as measured by high precision, high resolution Thomson scattering. These large scale fluctuations are also observed with electron cyclotron emission. They are quantitatively analysed by 2D poloidal snapshots of electron density and temperature, based on a 5 × 10 matrix of scattering volumes provided by the Thomson scattering system. Fluctuations with a quasi-periodic structure are found in a 2D snapshot with a frequency of about 61%. When interpreted as field-aligned helical structures toroidal quasi-mode numbers of 6–48 are found. The amplitudes of the fluctuations decrease with increasing quasi-mode number and edge profile gradient lengths. The amplitudes of the large scale structures in the steep gradient region are anti-correlated with the divertor Dα-intensity. The particle loss during an ELM is at least to a significant fraction due to the electron density ‘blobs’ observed in the scrape-off layer. The large scale fluctuations also perturb the measurement of 1D radial profiles. In the middle of the steep gradient region the perturbations are symmetric, but asymmetric both further inside (more minima) and further outside (more maxima).