Two-dimensional reconstruction of filament temperatures and densities with the thermal helium beam at ASDEX Upgrade

In the scrape-off layer (SOL) of fusion plasmas, radial particle and energy transport is mainly carried by plasma filaments. The plasma parameters of such filaments can be measured by the thermal helium beam diagnostic (THB). By means of an extended collisional radiative model (CRM) used for the evaluation of the diagnostic data, the light intensity response of neutral helium transitions from filaments is studied, which depends on the parameters of the filament and the background. The electron density of the filament is found to be the dominant parameter impacting the line intensities. By applying a numerical reconstruction algorithm based on the CRM, electron temperatures and densities as well as the sizes of experimentally measured filaments are obtained. This method allows for the first time measurements of the temporal decay rates of filament density and temperature in the co-moving frame of the filament and its radial propagation velocity.