Andrew Shone, Ph.D. Candidate
Dr. Daniel Andruczyk, Director of Research
March 25, 2024 | 11:00am - 1:00pm CST
This final examination will be held in 101A Talbot Laboratory.
Helium Retention Induced by In-Operando Lithium Evaporation
ABSTRACT: Removal of helium ash is critical to the operation of fusion power plants (FPPs). High energy helium ions (3.5 MeV), called alpha particles, are created from the fusion of deuterium and tritium atoms in fusion devices. Alpha particles redistribute their energy back into the plasma to help heat the plasma and sustain fusion conditions. When alpha particles transfer their energy to the plasma, they thermalize, resulting in the formation of low-energy helium ions known as helium ash. A build-up of helium ash in the plasma (>5% helium concentration) can deteriorate plasma performance and prevent fusion reactions from occurring. Current methods for removal of helium ash employ intricate divertor designs and large cryopump stations which add complexity and cost to the construction and operation of FPPs. Finding lower cost, higher efficiency methods for helium ash removal is necessary to create economically viable fusion energy.
The Center for Plasma Material Interactions (CPMI) at the University of Illinois Urbana-Champaign (UIUC) focuses on the design, development, and testing of plasma facing components (PFCs) for fusion devices. The Hybrid Illinois Device for Research and Applications (HIDRA) at CPMI is a tokamak-stellarator fusion research device used as testbed for PFC research. HIDRA has the ability to emulate some of the plasma conditions experienced in an FPP. In particular, HIDRA’s plasma temperature (5-25 eV), particle flux (Γ=1022 m-2s-1), and long pulse lengths (≤10000s) can match the fluence and some of the PMI behavior observed in the FPP divertor, the region of a fusion device’s wall which experiences the highest heat and particle fluxes, during steady state operation. Steady state heat fluxes are expected to be in the range of 5-10 MW/m2 and traditional solid PFCs (tungsten) will need to be replaced frequently because of damage induced by the plasma and neutron fluxes. Due to the numerous issues with solid PFCs, flowing liquid-metal divertor concepts using lithium have emerged as potential candidates for next-generation PFCs. Understanding lithium’s interaction with plasmas is vital to developing lithium PFCs and this topic is the focus of fusion research efforts at UIUC.
In-operando lithium evaporations into HIDRA helium plasmas were carried out utilizing the HIDRA Material Analysis Test-stand (HIDRA-MAT) and resulted in an 85% reduction of helium recycling from the walls. The reduction of helium in HIDRA, despite constant helium gas flow, suggested the helium was being actively retained. Helium retention was studied across three experimental campaigns in HIDRA (Zeus shots, Lithium Evaporation EXperiment, and Helium Retention Mechanism Experiment in a Stellarator). International research collaborations with the Dutch Institute for Fundamental Energy Research (DIFFER) in the Netherlands and the Experimental Advanced Superconducting Tokamak (EAST) in China have been conducted and also observed helium retention behavior.
The work presented in this thesis will introduce data and analysis from experimental campaigns that culminate in results showing an unprecedented reduction of low-energy (<25 eV) helium particles in a toroidal device through in-operando lithium evaporation. The helium retention phenomenon has been repeated several times, measured by multiple types of independent diagnostics, and observed in three fusion research devices (HIDRA, Magnum-PSI, and EAST) around the world. The leading hypothesis for helium retention is the formation of helium nanobubbles in a lithium layer during co-deposition of helium and lithium atoms at the wall. The helium becomes trapped on the cold wall (<80 oC) when the evaporated lithium cools. 24 hours after in-operando evaporation, a section of the HIDRA wall was heated, and desorption of helium was observed at the melting point of lithium (Tmelt = 180.5 oC) confirming helium was being retained on the wall.
The helium retention results are novel and impactful on FPP PFC design regarding the removal of helium ash in the divertor. The data provides a foundation for future research to investigate the interactions between helium and lithium while providing evidence for further utilizations of lithium in fusion devices. Future research will focus on investigating the existence, size, and stability of the helium nanobubbles as well as performing helium plasma exposures on the lithium loop experiment at UIUC. Helium pumping in a lithium divertor would further justify the use case for lithium PFCs in FPPs and bring the world one step closer to fusion power.
