Evaluation and validation of radial impurity density profiles from CXRS using neutral beam modelling in W7-X

Absolute radial impurity density profiles in the Wendelstein 7-X stellarator (W7-X) are derived from charge exchange recombination spectroscopy using modelling of the neutral beam. The approach is validated via cross comparisons of the neutral beam attenuation, the radial localization of ion temperature measurements, as well as a comparison of the effective plasma charge determined by other diagnostics. The latter implies the validity of the obtained absolute density levels. The simulation based approach novel to W7-X allows to assess the neutral beam halo population, introducing corrections to the shape and amplitude of determined impurity density profiles. To illustrate the capabilities of the derived impurity density profiles, the particle transport properties in a W7-X discharge heated by electron cyclotron resonance heating were
assessed. Density profiles for various impurities are found to be flat, consistent with dominant anomalous diffusion in the range 0.1–5.0 m2 s−1. This range of the anomalous transport in such plasmas is in line with other impurity diagnostics at W7-X.