Plasma flow measurements based on charge exchange recombination spectroscopy in the Wendelstein 7-X stellarator

The initial operation of neutral beams in Wendelstein 7-X enabled the first measurements of carbon (C6+) temperature and rotation with a multi-chord charge exchange recombination spectroscopy system. In this article we analyse the velocity measurements obtained with this system to infer the profiles of radial electric field, E (r), and net parallel velocity of fully stripped carbon ions, V (||). We include a general treatment of the flow geometry and the velocity corrections due to the energy dependence of the charge exchange cross section. The inverted profiles are compared with neoclassical calculations for various plasma conditions and two magnetic configurations. It is found that both the E (r) and V (||) experimental profiles agree in sign and order of magnitude with the neoclassical expectations and reproduce the expected dependencies on plasma parameters and magnetic configuration, within the reduced dataset presented. However, quantitative differences with respect to the neoclassical profiles are observed above the estimated errors in various conditions.