Impurity studies with Charge Exchange Spectroscopy on W7-X*

Charge Exchange Recombination Spectroscopy (CXRS) is a diagnostic suitable for impurity studies as it measures the emission of low-Z ions. This presentation will give an overview of the passive and active CXRS systems on the Wendelstein 7-X (W7-X) stellarator. The understanding of the behavior of impurities and the control of their concentrations is necessary to adjust the desired radiation level.

During the 2017 experiment campaign of W7-X, a passive CXRS system was employed to observe impurity line emission from the plasma edge. The analysis focuses on inverted spectral radiance, temperature and rotational velocity profiles of carbon-VI. In the analyzed plasmas, the carbon-VI emission seems to be localized inside the last closed flux surface. The experimental observations will be supported by the first comparison with transport modeling using the STRAHL-code.

The active CXRS system requires an external neutral source to give an electron to the fully stripped low-Z ions in the plasma. Neutral Beam Injection (NBI) will be in operation for the first time during the 2018 W7-X campaign. A dedicated spectroscopic observation system has been set-up and the first active measurements will be carried out.