Compensation of plasma fluctuation induced shadowing effects in gas puff imaging data

For gas puff imaging (GPI) studies of turbulent processes in the plasma edge, it is often assumed that the emitted light is mainly proportional to the local electron density. This, however, only holds for low electron densities in the far scrape-off layer. As GPI typically only detects one emission line, data interpretation further inside the plasma (i.e. around the separatrix) is complicated by two effects. First, there is the strongly non-linear connection between plasma temperature and density and the measured line emission. This can lead to incorrect data interpretation such as an artificial doubling of the frequency of periodic plasma perturbations. Second, a shadowing effect in the neutral distribution can hamper the localization of turbulent structures and modes. For this study thermal helium beam data of an ASDEX Upgrade EDA H-mode scenario are analyzed. By comparing these measurements to a synthetic diagnostic, we demonstrate the presence of diagnostic effects and illustrate their origins. It is shown that when line intensity ratios are used for the analysis, instead of emission from individual lines, the underlying plasma dynamics can be well recovered.