Upgrade of SLiC spherical tokamak with dynamic liquid lithium free surface: Li purity control and measurement

The SLiC spherical tokamak experiment has been upgraded to handle a larger total Li inventory (from 3 L to now 15 L) with a recirculating 2-chamber loop that adds the ability to drain and fill the plasma vessel from two separate points. In previous experiments with a single drain/fill port and smaller capacity storage tanks we observed a thin slag layer develop on the surface of the Li puddle due to interaction with plasma hydrogen and other impurity gases. This slag layer persisted even during vigorous Li nonlinear wave motion and its presence may degrade plasma performance. The upgrade provides a way to maintain a pure Li surface through incremental recirculation, as well as collect Li samples for composition analysis. A first round of XPS and IGF analysis showed that the slag was mostly LiOH with lesser amounts of LiH, various carbon compounds, and some NH₃. Trace elements were characterized by ICP mass spectroscopy. The slag layer may also be partially responsible for some Li droplet ejection mechanisms, which are being studied within SLiC. The upgrade is also enabling SLiC to operate as a diagnostic testbed for the upcoming FDP device (Fusion Demonstration Plant) where we will need AXUV, Thomson Scattering, optical Zeff measurements to operate in the presence of a dynamic free surface of liquid Li.  Lastly, the increased Li inventory will allow deeper Li in the plasma vessel; observing the wave motion with high speed video will benchmark corresponding MHD-CFD simulations over a wider range of parameters.