Tungsten in Sight: Visible Light Active Spectroscopy of High-Z Emission in W7-X

Spectroscopic measurements of impurity line emission provide insight into the time evolution of impurity densities in magnetically confined fusion plasmas, enabling the inference of diffusion and convection profiles that can be compared with theory predictions. This work presents new charge exchange observations of high-n tungsten line emissions in the Wendelstein 7-X stellarator. The observed lines, located in the visible wavelength range, are identified using the relativistic Rydberg formula and enable detailed monitoring of impurity charge states spanning from q = 21 to q = 47. A universal scaling law was employed to estimate the charge exchange effective emission coefficients for these high-n transitions, which are critical for reconstructing impurity density profiles from the measured light intensities. These reconstructed profiles can then be compared with synthetic profiles from the pySTRAHL transport solver, allowing for the inference of tungsten diffusion and convection coefficients that facilitate studies of high-Z impurity transport behavior in W7-X.

Acknowledgments. Work supported by US DOE Grant No. DE-SC0021972.