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.