Large scale inter-ELM fluctuations in the pedestal and the density limit in ASDEX Upgrade

Introduction In H mode plasmas in-between type-I edge localised modes (ELMs) [1] large scale fluctuations of the electron density and temperature were found recently in the pedestal of ASDEX Upgrade [2]. They appear in 2D poloidal snapshots of the electron density and temperature, measured by Thomson scattering, as ‘blobs’ and ‘dips’. For the type of plasma investigated in [2] the typical relative fluctuation amplitude of the electron density was around 21%. In this paper a set of type-I ELMy H mode plasmas with parameters covering a broad range are analysed: It is found that the relative fluctuation amplitudes in the pedestal increase with the plasma density normalised to the Greenwald density nGW [3]. There are experimental hints that the perpendicular transport of particles and heat in the pedestal increases with rising relative fluctuation amplitudes of electron density and temperature. This is similar to the findings on Alcator C-Mod, where increased convective transport in the scrape-off layer was found for ohmic L mode plasmas when approaching the density limit [4]. Experimental Setup