Liquid Metal Droplet Ejection Through Bubble Formation Under Hydrogen Plasma and Radical Exposure

Liquid tin constrained in a capillary porous structure could be an alternative plasma-facing component to tungsten for the divertor of a future magnetic confinement fusion reactor. However, due to the hydrogen–tin interaction droplets can be ejected, which is a potential showstopper due to an increased radiation in the plasma core. This has been recently observed in experiments in the ASDEX Upgrade tokamak. In this work, the theory of droplet ejection is reviewed, both theoretically and experimentally and potential solutions are tested in nano-PSI, a low flux unmagnetized plasma device. Droplet ejection was demonstrated via shadowgraphy observations to be driven by bubble formation and bursting followed by jetting. The generality of droplet ejection was verified by exposing liquid lithium, sodium, potassium, gallium, indium, tin, lead, and bismuth to hydrogen plasma in nano-PSI. Furthermore, the influence of the capillary structure was tested, by exposing multiple CPS targets. Ejection of droplets was observed for all post-transition metals and with all targets. Moreover, it was shown that free radicals alone are sufficient for droplet ejection, rather than plasma ions. Further, we predict and observe that the droplet ejection is suppressed by increasing the temperature above a critical value for a given radical flux. Our analysis shows that droplet production is highly challenging to prevent under expected fusion reactor conditions. Since droplet ejection cannot be prevented, the approach of using tin as a liquid metal plasma-facing material requires revision.