Potential Evidence of Helium Pumping by Lithium During HIDRA Steady-State Plasmas

The Hybrid Illinois Device for Research and Applications (HIDRA) at the University of Illinois Urbana-Champaign is a tokamak-stellarator hybrid dedicated to the study of plasma material interactions with an emphasis on liquid metal plasma facing component (PFC) research. HIDRA is capable of creating a steady-state plasma of which a 1000 s helium plasma has been demonstrated. The HIDRA Material Analysis Test-stand (HIDRA-MAT) enables in-vacuo liquid metal PFC characterization after exposure to HIDRA plasmas. LIBS and TDS results have shown no change in helium retention of the tungsten substrate with the introduction of liquid lithium to the surface after exposure to a 900 s helium plasma. However, some exposures resulted in the lithium completely evaporating from the sample surface, resulting in a significant change in plasma behavior. The introduction of lithium into the plasma in-situ resulted in a reduction of plasma impurities. Once the evaporated lithium was ionized, spectroscopy and pressure measurements indicated a drop in helium content in HIDRA even though there was a constant helium flow rate into the chamber. The scope of this work will cover an in-depth discussion on helium-lithium interactions, the resulting plasma behavior, and the potential mechanisms behind this perceived helium pumping.