Impurity Transport Studies Using High Speed Charge Exchange Spectroscopy of High-n Rydberg Transitions*

In fusion plasmas, core accumulation of impurities will cause poor reactor performance such that detailed understanding and control of impurity transport is imperative for future reactor operation. Presented here, is a new approach to investigate impurity convection and diffusion profiles by using charge exchange recombination spectroscopy (CXRS) to study radiation from high-n Rydberg transitions of impurities injected by laser blow off (LBO). A new CXRS diagnostic has been installed at the W7-X stellarator for the 2023 experimental campaign which uses five custom built dual channel spectrometers that provide sufficient light throughput for > 1kHz operation [1]. After LBO injections of Fe into plasmas with continuous NBI, active radiation from high-n (n ∼ 10 − 20) Rydberg transitions near 500 nm is observed from three charge states. Since high n levels cannot be reached by electron impact excitation, there is essentially no passive emission. Additionally, this new measurement technique proves to be a powerful diagnostic for studies of high Z impurities since wavelengths of the Rydberg lines are agnostic to the specific impurity. First results using this new diagnostic method at W7-X have been obtained via pySTRAHL, a 1D impurity transport code [2]. Impurity Diffusion and convection profiles are obtained for various plasma conditions and will be discussed.