High-accuracy characterization of the edge radial electric field at ASDEX Upgrade

The installation of a new poloidal charge exchange recombination spectroscopy (CXRS) diagnostic at ASDEX Upgrade (AUG) has enabled the determination of the radial electric field, Er, using the radial force balance of impurity ions. Er has been derived from charge exchange (CX) spectra measured on different impurity species, such as He2+, B5+, C6+ and Ne10+. The resulting Er profiles are found to be identical within the uncertainties regardless of the impurity species used, thus, demonstrating the validity of the diagnostic technique. The Er profile has been compared to the main ion pressure gradient term, which is found to be the dominant contribution at the plasma edge, thus, supporting that the Er well is created by the main ion species. The Er profile has been measured in different confinement regimes including L-, I- and H-mode. The depth of the Er well and the magnitude of the Er shear are correlated with the ion pressure at the pedestal top. The temporal evolution of the measured CX profiles and the resulting Er have been studied during an edge-localized mode (ELM) cycle. At the ELM crash, the Er minimum is less deep resulting in a reduction of the E × B shear. Within 2 ms after the ELM crash, the edge kinetic profiles have nearly recovered and the Er well is observed to recover simultaneously. In high density type-I ELM mitigated H-mode plasmas, obtained via externally applied magnetic perturbations (MPs) with toroidal mode number n = 2, no clear effect on Er due to the MPs has been observed.