The influence of passive edge emission on the charge exchange (CX) spectra of carbon (C5+, n = 8 → 7 at 529 nm) measured in fusion plasmas at the ASDEX Upgrade tokamak is investigated. The spectra are obtained viewing the plasma edge tangentially with eight lines of sight while the plasma is swept to enhance the spatial density of the measurements. A forward model to deconvolute the measured line-integrals is employed. The local emissions are then compared with the simulated radiation obtained with the 1D impurity transport code STRAHL using transport coefficients which are determined independently. Depending on the background neutral deuterium densities the simulation predicts the absolute line intensities and the relative contributions of electron impact excitation and thermal CX to the measured signals. Therefore, a background neutral deuterium density profile has been determined. For the passive emission line of C5+, the comparison between forward model and simulations yields that electron impact excitation and thermal CX are both important for understanding the passive line. Indeed, thermal CX proves to be affecting the passive emission line considerably via two mechanisms, i.e. change in the ionization equilibrium through CX recombination and radiation due to local CX reactions.
Influence of CX-reactions on the radiation in the pedestal region at ASDEX Upgrade