Results from the Hydrogen Absorption and Desorption Experiment in a Stellarator (HADES) Campaign in HIDRA

Lithium (Li) as a plasma facing material is of great interest due of its ability to retain and pump recycled reactive atoms, decrease instabilities and increase plasma performance. The most recent Li experimental campaign in the HIDRA device at the University of Illinois, Urbana-Champaign, named the Hydrogen Absorption and Desorption Experiment in a Stellarator (HADES) campaign, has shown increased hydrogen retention during in-operando Li evaporation into a hydrogen plasma. This results in a low recycling regime operation, supported by spectroscopy, Residual Gas Analyzer (RGA), and pressure data. Spectroscopy data has shown that Li ions, not just excited neutrals, are needed for retention effect to be observed. A wall heating element next to the HIDRA wall, placed in the path of high Li deposition on the walls, was used to test the location of the retention mechanism, serving as a controlled heating source. Wall heater desorption tests showed increased hydrogen desorption, measured by RGAs, post Li evaporation into hydrogen plasma shots. The desorption increased as the wall heater surface reached 300 C, signifying hydrogen alpha desorption from the Li. The LiH formation could not be investigated due to the heater’s maximum temperature capability, limiting the temperature to about 350 C, too low for LiH decomposition; investigation of this is part of future work. The hydrogen retention at HIDRA’s stainless steel walls by Li is hypothesized to be due to the chemical bond formation between the two species.