A global collaboration involving nine institutions has yielded new findings for designing fusion power plants with lithium walls, helping to maximize use of the rare fusion fuel tritium.
Lithium is a key element in future commercial tokamak fusion power plants, but its impact on fuel retention in tokamak walls was previously unclear. This study reveals that the primary driver of fuel retention is co-deposition—where fuel is trapped along with lithium—potentially from lithium added directly during plasma operation or from previously deposited, eroded, and redeposited lithium.
The research also shows that adding lithium during operation, rather than pre-coating walls, results in a more uniform temperature from plasma core to edge, favoring the stable plasma conditions needed for commercial fusion. Going beyond prior work, the study examines lithium wall behavior in a tokamak, providing insights more relevant to the complex environments of commercial fusion systems and aiding future tokamaks in better managing tritium.
Published in Nuclear Materials and Energy, the study is the first to directly compare the amount of fuel captured by lithium added in different ways before and during fusion operations. Lithium powder injected during operation primarily forms a protective coating, improving plasma-facing surfaces, reducing harmful impurities entering the plasma, and promoting co-deposition. The results show that pre-coated lithium layer thickness has little effect on fuel capture, with most retention occurring when lithium is added during plasma discharges.
Florian Effenberg from the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL) noted that since graphite walls erode quickly and produce dust, tokamak wall materials are shifting to tungsten and similar metals, requiring regulation methods—lithium is the leading candidate. Powder injection technology offers a viable path toward fully liquid lithium walls. A plan is being developed to include lithium injectors in PPPL's National Spherical Torus Experiment-Upgrade (NSTX-U), eventually incorporating components facing liquid lithium plasma. The lab is also developing a tokamak called the Spherical Tokamak for Advanced Reactors (STAR).
In addition to PPPL, the team included researchers from the Dutch Institute for Fundamental Energy Research (DIFFER), Eindhoven University of Technology, General Atomics, Sandia National Laboratories, Auburn University, University of Tennessee Knoxville, University of California San Diego, and Lawrence Livermore National Laboratory.
Lithium can melt to form a self-healing layer, protecting components directly facing the plasma in fusion vessels. At sufficiently high temperatures, it can also form a gas or vapor shield to protect vessel walls. Lithium wall designs help stabilize the plasma edge, improve confinement, and increase power density, but they also cause fuel retention—especially of radioactive, scarce, and tightly regulated tritium. Excessive tritium retention reduces fuel availability, complicates the tritium fuel cycle, and raises safety and operational concerns.
The study emphasizes that tokamak designs must avoid cold wall regions where lithium and fuel easily accumulate. Techniques such as flowing liquid lithium or maintaining higher wall temperatures can prevent unwanted co-deposition and direct tritium to areas where it can be more easily managed and recovered.
During the study, researchers used material samples embedded in DIII-D wall tiles to evaluate two lithium addition methods: pre-coating before plasma exposure and adding lithium to material samples inserted into wall tiles using an impurity powder dropper system during plasma exposure. Results showed that solid lithium and deuterium co-deposition captures more fuel than existing lithium coatings. Maria Moberg plans to heat tiles to liquefy lithium for further comparison.
Effenberg said this step brings the research closer to how lithium would operate in a fusion power plant, where liquid lithium could ultimately provide thermal protection and flow paths to recover and recycle tritium fuel. The research also helps identify key areas in tokamaks where tritium may accumulate, and understanding how fuel becomes embedded is crucial for making fusion a safe and economical energy source.
Strong magnetic fields confine most plasma in a donut-shaped tokamak, but some particles escape and strike inner wall components. Whether particles are trapped or bounce back has trade-offs: trapped tritium atoms cannot be recovered for energy production, while bouncing particles lower overall plasma temperature, affecting fusion reactions.
Also contributing from PPPL were Shota Abe, Alessandro Bortolon, and Alexander Nagy; from General Atomics, Tyler Abrams; from Sandia National Laboratories, Ryan Hood; from Auburn University, Ulises Losada; from University of Tennessee Knoxville, Jun Ren; from University of California San Diego, Dmitry Rudakov; from Lawrence Livermore National Laboratory, Michael Simmonds and Dinh Truong; and from DIFFER and Eindhoven University of Technology, Thomas Morgan.
The research was funded by the U.S. Department of Energy Office of Science, Fusion Energy Sciences, and used the DIII-D National Fusion Facility operated by General Atomics, with additional funding from the European Union through the Euratom Research and Training Programme.
