Diagnosing metastable populations in fusion edge plasmas using collisional-radiative modeling constrained by experimental observations*

The erosion of neutral tungsten at the plasma boundary is diagnosed in the Compact Toroidal Hybrid (CTH) experiment with a high-resolution spectrometer and a new Langmuirprobe allowing for simultaneous electron temperature, density and W spectral measurements. This data is used to constrain collisional-radiative modeling with ColRadPy, which suggests that neutral tungsten emission is dominated by non-steady-state metastable populations over a wide range of plasma conditions. Inferred non-steady-state metastable populations are presented using W I spectral lines near 260 nm. High-resolution tungsten spectra from CTH plasmas are compared to modeled emission using W I R-matrix excitation data and exchange classical impact parameter (ECIP) ionization constrained by Teand nefrom Langmuir probe measurements. The addition of a Chodura sheath for modeling fusion relevant divertor conditions and CTH plasmas suggest changes of the W relative metastable fraction by up to 50{\%} when compared to modeling with no sheath, underlining the importance of understanding metastable effects when diagnosing W emission in fusion edge plasmas.