HeRMES: Helium Retention Mechanism Experiment in a Stellarator*

The issue of helium ash build-up poses a significant obstacle to achieving commercial fusion from both scientific and economic perspectives. The inability to effectively remove thermalized alphas from the plasma decreases the operational regime to realize ignition. Current plans to remove helium ash involve the use of large pump ducts that increase reactor size. This increases construction costs and thus hinders fusion reactors from being economically competitive. Previous experiments at the University of Illinois Urbana-Champaign (UIUC) showed helium recycling being reduced up to 85% during lithium evaporations into helium plasmas. The Helium Retention Mechanism Experiment in a Stellarator (HeRMES) is the next experimental campaign to be conducted at UIUC to study helium retention effects in the Hybrid Illinois Device for Research and Applications (HIDRA). HeRMES aims to identify the mechanism associated with helium retention observed from in-operando lithium evaporations in HIDRA helium plasmas. HeRMES consists of several plasma shots where lithium induces a low-recycling regime in HIDRA and conditions such as gas flow and ECRH power are adjusted during this regime.The HIDRA Materials Analysis Test-stand will be utilized to perform thermal desorption spectroscopy post-evaporation to investigate the potential trapping of helium in lithium. HeRMES will provide critical information for addressing the issue of helium ash and will advance the development of future plasma-facing components.