Results from the Helium Retention Mechanism Experiment in a Stellarator (HeRMES) Campaign*

Lithium (Li) as a plasma facing material is of great interest due to its ability to remove recycled reactive atoms, decrease instabilities and increase plasma performance. Recent Li experiments on the University of Illinois HIDRA device have yielded surprising results with Helium (He). Evaporation of liquid Li from the HIDRA-MAT probe to the He plasma edge causes the background recycled He to decrease and allows higher performance of the plasma. Further experiments showed that He was pumped when Li⁺¹ ions appeared and not when only Li neutrals were present. The Helium Retention Mechanism Experiment in a Stellarator (HeRMES) is the first experimental campaign undertaken to investigate He retention during the low-recycling regime. Results show that the Li is deposited on the wall of HIDRA in defined stripes and He becomes trapped at the cold wall when the Li cools. A wall heating element was designed to have Li/He deposited on it during a 10 minute He plasma operation with Li, and 18 – 24 hours after operation, the element was heated up. The release of He was measured when Li reached its melting temperature, confirming He is being retained on the wall. This work potentially suggests if a flowing Li system was present, He atoms could be trapped long enough to be transported away. This would solve the He ash issue facing fusion. This work will introduce the results from the HeRMES campaign and show that He is trapped at the wall as Li is being co-deposited on PFC surfaces.