Interpretation of Dα video diagnostics data as a contribution to plasma edge characterization

The spatial distribution of the neutral source profile is an important quantity for the understanding of the pedestal density profile and effects like its influence on rotational shear by momentum exchange. Analytic models as well as numerical predictive codes require experimental data on the magnitude and the poloidal distribution of the neutral flux density in order to be validated. This paper describes the measurement and interpretation of Dα radiance data obtained with a pair of tangentially viewing cameras at the ASDEX Upgrade tokamak. Reflection at the tungsten tile surfaces covering most of the tokamak wall is an issue to be addressed in the procedures for data analysis. In addition to a simple profile fit, a reflection-aware algorithm for tomographic reconstruction is adopted for radiance deconvolution. Results for the neutral flux density distribution at the high-field side (HFS) are presented for low and medium density scenarios, based on the comparison of experimental emission profiles and those obtained by the neutral penetration code KN1D. Absolute values for flux densities at the HFS separatrix are interpreted in terms of SOL parameters and the extension of the plasma-wall gap. A number of examples characterizes the Dα video diagnostics as a valuable tool to supply estimates on neutral parameters.