Effect of spatial distribution of impurity ions on the signal of ‘C/O monitor for Wendelstein 7-X’ - an indicator of plasma wall interactions

The ‘C/O monitor’ is a dedicated diagnostic system designed to monitor light impurities (B, C, N and O) in the Wendelstein 7-X (W7-X) stellarator. Its main goal is to provide fast (∼1 ms) information about the impurity level which is measured from a large plasma volume (high throughput). Its first subsystem dedicated to measure Lyman-α lines of H-like carbon (C5+ − 3.4 nm – used as PFC material) and oxygen (O7+ − 1.9 nm – common impurity absorbed by inner vessels’ walls) is going to be commissioned during the next Operational Phase of W7-X. Since the radiated photon intensity of a given impurity depends strongly on plasma kinetic parameters (Te, ne) and impurity transport, it is important to thoroughly understand the underlying correlation between these factors and registered signal. Because the dependences turn out to be non-trivial, a forward modelling was applied. This paper outlines the results of Lyman-α line intensities modelling performed for carbon and oxygen studies (including impurity transport), as only those two elements will be measured during the initial operational phase of the spectrometer. The obtained results indicates that the signals measured by the system will be weakly dependant on the plasma conditions.