Carbon concentration in the Wendelstein 7-AS stellarator measured with the high-energy Li beam

Abstract

The carbon concentration in the plasma of the stellarator experiment Wendelstein 7-AS is measured in the framework of the first island divertor campaign to examine divertor impurity control. Dedicated plasma density and electron cyclotron resonance heating power scans are performed and the influence of both the magnetic configuration and boronization of the vessel are investigated. A new diagnostic system is employed to measure C6+ densities via charge-exchange collisions with the neutral high-energy Li beam in the plasma edge and the core region. Carbon concentrations with respect to electron density are derived by using the density evaluation of the Li(2p) light profile. The results obtained during a period of approximately 2500 shots demonstrate that boronization reduces the carbon core concentration by a factor of three to a base level of 0.5–1%, which is not affected by further boronizations. The carbon concentration in island divertor configurations is found to be far below those obtained in a limiter configuration, which is attributed to a more than a factor two better impurity screening. The analysis of the carbon influx by spectroscopic measurements of C1+ radiation suggests that the carbon core concentration in discharges of different densities and heating power is adequately described using the electron density at the last closed flux surface normalized to the line-averaged electron density as a scaling parameter. This simple global approach underlines the important role of the balance of impurity sources and impurity dilution. The effect of changes in impurity confinement on the measured carbon concentrations is also discussed.