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 He2+, B5+, C6+ and Ne10+. 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 x 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.