Liquid lithium on the walls of the plasma fusion device helps it maintain a hot edge
Emerging research suggests that it may be easier to use fusion as a power source if liquid lithium is applied to the inner walls of the device containing the fusion plasma.
Plasma, the fourth state of matter, is a hot gas composed of electrically charged particles. Scientists at the Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) are working on solutions to efficiently harness the power of fusion to offer a cleaner alternative to fossil fuels, often using devices called tokamaxes, which confine plasma using magnetic fields.
“The purpose of these devices is to trap energy,” said Dennis Boyle, a research physicist at PPPL. “If you had better energy reservation, you could make the machines smaller and less expensive. That would make the whole thing more practical and cost-effective, so that governments and industry would want to invest more in it.”
The new findings, which were highlighted in a recent presentation called by Boyle at a meeting of the American Physical Society’s Plasma Physics Division, are part of a lithium tokamak beta (LTX-β) experiment at the laboratory. Related research is also published in the journal Nuclear materials and energy.
In recent experiments, a layer of liquid lithium was added to the inside of the tokamak’s wall, which helped the plasma stay hot at its edge. Preserving the hot edge is key to their unique approach, which the scientists hope will one day contribute to the design of a fusion power plant. Previous LTX-β experiments studied solid lithium layers and found that they could enhance plasma. The researchers were pleased that they were able to obtain similar results with liquid lithium, because it is more suitable for use in large-scale tokamaks.
One of the biggest challenges in developing fusion energy is building a viable wall for the device that confines the plasma, noted Richard Majeski, PPPL’s principal research physicist and head of LTX-β. PPPL is committed to finding solutions to this and other challenges to help fill the gaps in bringing fusion energy to the electricity grid.
“Although the LTX-β is a very modest-sized spherical tokamak, it is the first and only plasma capture device in the world to have a core plasma completely contained by a liquid lithium wall,” Majeski said. “The results for LTX-β have been very promising. Liquid lithium not only provides a wall that can withstand contact with 2 million degree plasma, it actually improves the performance of the plasma.”
Liquid lithium can reduce the need for repairs, as it acts as a shield for the device’s inner walls while they are exposed to the intense heat of the plasma.
Liquid lithium absorbs about 40% of the hydrogen ions exiting the plasma so that fewer of these molecules are recycled back into the plasma as a relatively cool neutral gas. Scientists refer to this environment as a low-recycling environment because many of the hydrogen ions expelled from the plasma are not recycled back in a way that would cool the edge of the plasma.
Ultimately, this lower recirculation environment meant that the temperature at the edge of the plasma was closer to the temperature at the plasma core. The temperature uniformity should allow the plasma to trap heat better than it would without liquid lithium by avoiding a variety of instabilities.
A. Maan et al., Improving neutral and plasma density control with increased lithium wall coating in the Lithium Tokamak-β (LTX-β) experiment, Nuclear materials and energy (2023). doi: 10.1016/j.nme.2023.101408
Provided by the Plasma Physics Laboratory at Princeton
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