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 t…