Shadowing effects in simulated Alcator C-Mod gas puff imaging data

Abstract

A synthetic gas puff imaging (GPI) diagnostic is applied to a gyrokinetic turbulence simulation of Alcator C-Mod with the objective of assessing the impact of turbulent plasma fluctuations on the GPI neutral density field. We find that these effects are significant for much of the run, resulting in qualitatively different light emission patterns relative to the idealized limit of a fixed neutral profile. Spatial and temporal variations in the local neutral density within the GPI gas cloud are shown to be comparable in magnitude to those in the plasma, which would affect the interpretation of plasma fluctuations made using GPI. Assessing the experimental implications of these results will require additional study using validated turbulence simulations. Our findings are compatible with previous theoretical investigations concluding that GPI may unreliably diagnose plasma turbulence more than one neutral mean free path away from the gas source. Although we used a comprehensive, time dependent simulation to arrive at these results, our analysis shows that a frozen-plasma treatment of the neutrals is adequate in this case. That is, we can assume that the neutral transport is too fast to be affected by temporal fluctuations in the turbulent plasma structures. Keywords: Synthetic diagnostic, Gas puff imaging, Neutral transport simulation, Plasma turbulence simulation, Alcator C-Mod