Numerical estimate of multi-species ion sound speed of Langmuir probe interpretations in the edge plasmas of Wendelstein 7-X
The recently modified EMC3-EIRENE code package has been widely applied as an edge-plasma analysis tool and resulted in successful validation against various measured trends seen in stellarator and tokamak plasma boundaries. It has been shown that the code package applied for Wendelstein 7-X (W7-X) discharges in the interpretive mode can assess the impact of impurity effects on the electron density, measured by a set of Langmuir probes. In particular the spatial quantification of impurities and effects from the effective charge state Zeff and effective mass meff, which are non-trivial to record by diagnostics, were examined. The results showed that earlier assumptions of the effective charge-state distribution and effective mass for reported Langmuir probe measurements must be revised. Subsequently, reprocessing these measurements with code-interpreted spatial profiles of the effective charge state and effective mass led to an overall improved physical consistency.