Edge plasma diagnostics on W7-AS and ASDEX-Upgrade using fast Li beams

Abstract

Knowledge of the impurity concentration and temperature in the core plasma gradient region as well as the SOL is a vital element on the road to documenting and understanding the physics of L- and H-mode transport and the transport barrier itself. To this end, the Li-beam diagnostic capabilities on W7-AS and ASDEX–Upgrade have been expanded to include the measurement of radial profiles of impurity ion density and temperature via charge-exchange-spectroscopy (Li-CXS). After presenting the method of Li-CXS this paper describes the experimental setup on W7-AS as well as ASDEX–Upgrade and presents first results on both experiments. Measurements of C 6+ and Ne 10+ spectral lines prove the viability of Li-CXS. In the plasma edge region of W7-AS ( r eff > 8 cm) a C 6+ concentration of about 0.5% could be measured. Temperature values found for C 6+ are similar to proton/deuteron temperatures. The intensities of several LiI spectral lines (2p→2s, 3d→2p, 4s→2p, 4d→2p) have been measured and are used to critically check the underlying cross section database employed within the collisional-excitation Li-beam model, especially for collision processes involving higher Li-states ( n ⩾ 3). It was found that the ratio of the spectral lines corresponding to 3d→2p and 2p→2s transitions is overestimated by the model. These deviations could be eliminated by using improved cross sections in the database.