The world of fusion energy is a world of extremes. For instance, the center of the ultrahot plasma contained within the walls of doughnut-shaped fusion machines known as tokamaks can reach temperatures well above the 15 million degrees Celsius core of the sun. And even though the portion of the plasma closer to the tokamak's inner walls is 10 to 20 times cooler, it still has enough energy to erode the layer of liquid lithium that may be used to coat components that face the plasma in future tokamaks. Scientists thus seek to know how to prevent hot plasma particles from eroding the protective lithium coating.
Physicist Tyler Abrams has led experiments on a facility in the Netherlands called Magnum-PSI that could provide an answer. The research, published in Nuclear Fusion in December 2015, found that combining lithium with the hydrogen isotope deuterium substantially reduced the erosion. Abrams conducted the research as a doctoral student in the Princeton Program in Plasma Physics substantially based at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL). He currently is a postdoctoral research fellow at General Atomics. The research was funded by the DOE Office of Science.
"One potential issue with lithium is that it tends to erode off the chamber walls surfaces very quickly when it gets hot," said Abrams. "In my research I was trying to determine exactly how much lithium actually comes off the wall under the conditions expected for fusion reactors."
Physicists have long known that in fusion devices with low levels of plasma flux, meaning that the flow of charged particles within them is relatively small, the rate at which lithium eroded depends on the plasma's temperature. Physicists had not, however, studied what would happen to lithium coatings in high-flux plasmas with a greater flow of particles. That increased flow will occur in future tokamaks. Scientists had thought that erosion would be greater in such machines.
But Abrams and the team found that the opposite was true while performing experiments at the Dutch Institute for Fundamental Energy Research. They found that the amount of lithium erosion in high-flux plasmas was much less than that in low-flux plasmas. The team conjectured that the difference stemmed from the chemical properties of lithium deuteride (LiD), a molecule created when deuterium atoms from plasma bond with the liquid lithium coating.
To test the conjecture, Abrams and his colleague Dr. Mohan Chen, of Princeton University, created a computer program that modeled how deuterium combined with lithium. The new computer program indicated that the observed low rate of lithium erosion could stem from two factors. First, lithium deuteride molecules have a strong binding energy, meaning that incoming deuterium ions from the plasma have a hard time knocking lithium atoms loose from their bonds. Second, when deuterium ions in a plasma hit lithium deuteride molecules, they tend to knock the deuterium atoms out of the molecules and leave the lithium atoms in place.
Once the computer program had been completed, Abrams and the other scientists performed experiments on Magnum-PSI. They shot streams of plasma at samples of lithium that were placed inside the machine and recorded how much lithium came off. The amount of lithium that was eroded was similar to the amount predicted by Abrams' model. In addition, the simulations showed that a layer composed of lithium deuteride would erode 20 times more slowly than would a layer of pure lithium.
"My results suggest that lithium is able to handle significantly higher amounts of plasma exposure and higher temperatures than others had previously expected," said Abrams. "This suggests that liquid lithium will not erode too quickly if it is used on the walls of fusion reactors and will not contaminate the core plasma too much, making lithium coating a much more attractive alternative to solid metals walls."
PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas -- ultra-hot, charged gases -- and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy's Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.
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