The complex relationship between fusion fuel and lithium walls

(Press-News.org) Lithium is considered a key ingredient in the future commercial fusion power plants known as tokamaks, and there are several ways to use this metal to enhance the process. But a key question remained: How much does it impact the amount of fuel trapped in the walls of tokamaks?

According to new research from a global collaboration spanning nine institutions, the dominant driver of fuel retention is co-deposition: a process where fuel is trapped alongside lithium. Co-deposition can happen with lithium that is directly added during plasma operations, or lithium that has been previously deposited on the walls, only to wear away and be redeposited.

The research also showed that adding lithium during operation is more effective than pre-coating the walls with lithium in terms of creating an even temperature from the core of the plasma to its edge, which can help create the stable plasma conditions needed for commercial fusion.

This new study goes beyond earlier work by examining lithium wall behavior in a tokamak, offering insights that are more reflective of the complex environment in commercial fusion systems. The insights can help future tokamaks better manage tritium, a rare and essential fusion fuel.

Published in Nuclear Materials and Energy, the study is the first to directly compare the amount of fuel trapped by lithium coatings applied inside a tokamak before fusion operation begins with lithium powder injected over the plasma during a fusion reaction. This mid-operation lithium powder injection is primarily used as a protective coating to improve plasma-facing surfaces and reduce the amount of unwanted material coming off the tokamak’s walls and into the plasma. It also inherently stimulates co-deposition.

The study also found that the thickness of the lithium coating applied before a plasma shot did not significantly affect how much fuel was trapped. “It turns out there’s little impact in making these coatings extra thick,” said Maria Morbey, lead author on the study and a doctoral degree candidate with the Dutch Institute for Fundamental Energy Research (DIFFER) and the Eindhoven University of Technology. “Most of the fuel retention happens when lithium is added during the plasma shot — not beforehand.”

“As we transition tokamaks away from graphite walls because of their high rate of erosion and the dust produced and toward wall materials such as tungsten, we need to find a way to condition these walls so that the hot core of the plasmas better tolerates them,” said Florian Effenberg, a staff research scientist with the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) who supervised the research.

Lithium is a leading candidate for the job, Effenberg said, noting that powder injection offers a practical bridge toward fully liquid lithium walls. A plan is in development to potentially include a lithium injector and, ultimately, liquid lithium plasma-facing components in PPPL’s National Spherical Torus Experiment-Upgrade (NSTX-U). The Lab is also working on a tokamak based on NSTX-U’s design, called the Spherical Tokamak Advanced Reactor (STAR).

In addition to other researchers from PPPL, a leader in lithium research, the team also included people from DIFFER, Eindhoven University of Technology, General Atomics, Sandia National Laboratories, Auburn University, University of Tennessee-Knoxville, University of California-San Diego, and the DOE’s Lawrence Livermore National Laboratory (LLNL). 

A liquid metal heat shield
Lithium can melt, creating a self-repairing layer over the inner components of a fusion vessel. This protective layer can help shield some of the parts that directly face the plasma from its intense and potentially damaging heat, as it reaches temperatures hotter than the core of the sun. If the fusion vessel wall temperatures are high enough, lithium could also protect the vessel wall by forming a gas or vapor shield.

“Lithium walls are intentionally used to create an environment where fuel atoms are absorbed rather than reflected, helping to stabilize the plasma edge, enhancing plasma confinement and enabling operation at higher power densities. These are key advantages for compact, more efficient tokamak designs,” said Effenberg.  

However, this same property leads to significant fuel retention, particularly of tritium, which is radioactive, scarce and tightly regulated. Excessive tritium trapping reduces fuel availability, complicates the tritium fuel cycle and poses safety and operational concerns, especially in colder and inaccessible areas where tritium may accumulate over time.

The study findings highlight that in tokamak designs, it will be critical to avoid cold wall regions where lithium and fuel can accumulate. Using flowing liquid lithium, maintaining higher wall temperatures and implementing additional techniques to prevent unwanted co‑deposits will help direct tritium into areas where it can be more effectively managed and recovered.

Solid results comparing two application techniques
During the study, two approaches for using lithium were assessed using material samples embedded in wall tiles in DIII-D, a tokamak operated by General Atomics in California. In the first approach, the samples were pre-coated with lithium before they were exposed to a fusion plasma. In the second scenario, lithium was added to the material samples inserted into the wall tiles. At the same time, they were exposed to a fusion plasma using a system known as an impurity powder dropper, which sprinkles the lithium on top of the plasma. 

Morbey said the findings indicate that the co-deposition of lithium and deuterium results in more trapped fuel than in a preexisting lithium coating — at least when the lithium is solid. Morbey plans to run similar experiments with the tiles heated to liquefy the lithium and then compare the results.

“This step would get us close to how we want to operate lithium in a fusion power plant: as a liquid. Once it can flow, it will finally also provide thermal protection and a flow path to locally purify the lithium stream so that tritium fuel can be recovered and reused,” Effenberg said.

The research is also important because it can help to identify key areas in the tokamak where tritium might build up. “We have to find a way of preventing that fuel retention in these cold spots,” Morbey said, such as between tiles or on certain parts of the tokamak’s exhaust system. 

Minimizing fuel requirements
Understanding the details of how fuel embeds in the innards of a tokamak is critical to making the systems needed for fusion power to become a safe and economical energy source. The way tokamaks are designed today, an ongoing supply of tritium is essential for fueling the reaction. But tritium is rare, radioactive and produced inside the tokamak at a limited rate through nuclear reactions, so it’s not ideal if significant amounts are trapped in lithium coatings on the inner walls of the tokamak. The study used deuterium as a stand-in for tritium, as is typical in such research, because the two would have the same behavior in the context of the experiment, Effenberg said.

Strong magnetic fields hold the bulk of the plasma in a doughnut shape inside a tokamak, but some plasma particles escape. Many of these particles will hit the inner walls or other components inside the vessel that surrounds the plasma. When a particle hits a wall, for example, it can bounce back into the plasma or get stuck in whatever it hits.

Each scenario has advantages and disadvantages. A tritium atom that is stuck in the wall will not naturally be recycled back into the plasma and used to make more energy. Alternatively, a trapped particle can’t thwart the fusion reaction. Particles reemitted from the wall have lost their energy and are significantly cooler than the particles that never left the hot core. When these colder recycling particles mix in with the core plasma, the overall temperature can drop. If the plasma cools too much, fusion stops.

Shota Abe, Alessandro Bortolon and Alexander Nagy of PPPL also contributed to this research, as well as Tyler Abrams (General Atomics), Ryan Hood (Sandia National Laboratories), Ulises Losada (Auburn University), Jun Ren (University of Tennessee-Knoxville), Dmitry Rudakov (University of California-San Diego), Michael Simmonds (LLNL), Dinh Truong (LLNL) and Thomas Morgan (DIFFER and Eindhoven University of Technology). 

The research was 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 operated by General Atomics, under awards DE-AC02-09CH11466, DE-FC02-04ER54698, DE-NA0003525, DE-SC0019256, DE-SC0023378, DE-SC0015877 and DE-AC52-07NA27344. It was also funded by the European Union via the Euratom Research and Training Programme under grant agreement number 101052200 EUROfusion.

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