Limiting Fuel Retention by Lithium in Future Fusion Devices

The Science

Tokamaks are donut-shaped fusion devices that use magnetic fields to confine plasma. They are a leading design for fusion power plants in the future. However, it is not clear what these devices will be made of. Materials that face the plasma must hold up under the extreme environment needed to sustain fusion reactions. Lithium is a leading candidate for regions in a tokamak that experience large changes in heat. This element melts to create a self-healing surface. It is also a light element, which limits how much lithium molecules released from the tokamak disturb the plasma. However, lithium is known to bond with hydrogen isotopes. This bonding can trap hydrogen fuel molecules in the walls of fusion devices. To address this concern, researchers at the DIII-D National Fusion Facility, a U.S. Department of Energy (DOE) Office of Science User Facility, evaluated how well lithium retains deuterium (a common fusion fuel) in an operating tokamak. The scientists compared how much deuterium was retained in pre-coated samples versus how much was retained during a process that injects lithium powder while the tokamak is operating. This research will help identify the optimal lithium conditioning technique for tokamaks.

The Impact

Identifying and improving plasma-facing materials are key steps toward commercializing fusion. Notably, future fusion power plants will not be able to use materials that trap large amounts of fuel. While lithium is a leading material, researchers are still studying how much fuel it retains. This study showed that pre-lithiated samples retained less fuel than injected lithium powder samples. This result suggests that pre-coating with lithium may be a preferred approach for future devices. Moreover, the amount of fuel retained in the samples indicates how important it is to avoid lithium collecting in cold wall regions, where fuel would be more likely to be trapped. These results provide key guidelines for how fusion device designers can incorporate lithium in future plans.

Summary

In this study, an international collaboration, involving 9 different institutions and led by a Ph.D. candidate, evaluated fuel retention in lithium added to the DIII-D tokamak via different approaches. While lithium is a leading candidate material for the walls of tokamaks, it is also known to bond hydrogen isotopes. Scientists must identify approaches that minimize retention before this material can be used in fusion.

The research team evaluated deuterium deposition in samples with lithium coatings of various thicknesses against samples with no pre-existing lithium that were exposed to lithium powder injection during tokamak operation. The work showed that the pre-lithiated samples retained similar amounts of deuterium regardless of the thickness of the lithium coating applied. In contrast, powder injection resulted in substantial co-deposition, removing more fuel from the reaction. In future studies, the team plans to evaluate what happens to the co-deposited lithium and fuel when the sample melts into liquid lithium, which is the preferred state for use in fusion devices. This will help them better understand what can be expected during lithium operation in future devices.

Contact

Maria Morbey
Dutch Institute for Fundamental Energy Research (DIFFER)

Florian Effenberg
Princeton Plasma Physics Laboratory
[email protected]

Funding

This work is part of the research programme "The Leidenfrost divertor: a lithium vapor shield for extreme heat loads to fusion reactor walls," which is (partly) financed by NWO. The research was funded by the European Union via the Euratom Research and Training Programme. It was also funded by DOE, Office of Science, Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science User Facility.

Publications

Morbey, M., et al. "Deuterium retention in pre-lithiated samples and Li-D co-deposits in the DIII-D tokamak." Nuclear Materials and Energy, 43, 101915 (2025). [DOI: 10.1016/j.nme.2025.101915]

Related Links

The complex relationship between fusion fuel and lithium walls, Princeton Plasma Physics Laboratory newsroom