Molten Tin Droplet Ejection in the Presence of a Hydrogen Plasma

As the fusion community works to find a way to mitigate the high heat and particle flux onto the first wall and divertor region, liquid metals have been gaining traction as a potential material for those two regions. Some of the biggest benefits of utilizing a liquid metal wall are its ability to self-repair any damage caused by transient events in the plasma, as well as the ability of a liquid metal wall to trap escaped fuel and waste material thereby promoting a lower recycling regime in the edge plasma regions. However, there are multiple liquid metals being considered for use as PFCs in future fusion devices: Tin, Lithium, Lead-Lithium, Tin-Lithium… etc. It is therefore important to investigate these liquid metal PFC candidates to determine the benefits and hazards of each individual liquid metal to the fusion device as well as the fusion plasma. This work builds on an observation made in the semiconductor industry by ASML, a company using molten tin as an EUV source, who noticed that if the tin is molten in a hydrogen plasma environment, tin droplets are ejected, or spit, from the tin surface. This was confirmed by some preliminary work which has shown that ejected tin droplets range in size from tens of nanometers to hundreds of microns. If tin is used in future fusion devices, the fusion plasma will cause tin to spit thereby contaminating the core fusion potentially causing increased power losses. This work characterizes the size and spatial distribution of the tin droplets that are ejected from the molten tin surface over a range of pressures and powers, thereby showing that tin should not be considered as a PFC for fusion devices.