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, E r , using the radial force balance of impurity ions. E r has been derived from charge exchange (CX) spectra measured on different impurity species, such as He 2+ , B 5+ , C 6+ and Ne 10+ . The resulting E r profiles are found to be identical within the uncertainties regardless of the impurity species used, thus, demonstrating the validity of the diagnostic technique. The E r 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 E r well is created by the main ion species. The E r profile has been measured in different confinement regimes including L-, I- and H-mode. The depth of the E r well and the magnitude of the E r shear are correlated with the ion pressure at the pedestal top. The temporal evolution of the measured CX profiles and the resulting E r have been studied during an edge-localized mode (ELM) cycle. At the ELM crash, the E r 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 E r 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 E r due to the MPs has been observed.