The proposed approach would still use deuterium and tritium, which are generally accepted as the most promising pair of fuels for fusion energy production. However, the quantum properties of the fuel would be adjusted for peak efficiency using an existing process known as spin polarization. In addition to spin polarizing half the fuels, the percentage of deuterium would be increased from the usual amount of roughly 60% or more.
Models created by scientists at the U.S. Department of Energy's Princeton Plasma Physics Laboratory (PPPL) showed the approach allowed tritium to burn more efficiently without sacrificing fusion power. This could dramatically reduce the amount of tritium needed to start up and maintain fusion reactions, leading to more compact and affordable fusion systems.
"Fusion is really, really hard, and nature doesn't do you many favors," said Jason Parisi, a staff research physicist at the Lab and first author on the research paper. "So, it was surprising how big the improvement was."
The paper, which was published in the journal Nuclear Fusion, suggests the approach could burn tritium as much as 10 times more efficiently. The research also underscores PPPL's role at the forefront of fusion innovation, particularly when it involves a system such as the one studied in Parisi's research, where gasses are superheated to create a plasma confined by magnetic fields into a shape similar to a cored apple. The Lab's primary fusion device, the National Spherical Torus Experiment - Upgrade (NSTX-U), has a similar shape to the one the researchers considered when they tested their approach.
"This is the first time researchers have looked at how spin-polarized fuel could improve tritium-burn efficiency," said staff research physicist and co-author Jacob Schwartz.
Minimizing tritium requirements by maximizing burn efficiency
PPPL principal research physicist and co-author of the paper Ahmed Diallo likens tritium-burn efficiency to the efficiency of a gas stove. "When gas comes out of a stove, you want to burn all the gas," Diallo said. "In a fusion device, typically, the tritium isn't fully burned, and it is hard to come by. So, we wanted to improve the tritium-burn efficiency."
The PPPL team consulted the fusion community and the broader community involved in spin polarization as a part of their work to find ways to enhance tritium-burn efficiency. "Fusion is one of the most multidisciplinary areas of science and engineering. It requires progress on so many fronts, but sometimes there are surprising results when you combine research from different disciplines and put it together," Parisi said.
A different kind of spin
Quantum spin is very different from the physical spin on a baseball. For example, a good pitcher can throw the ball with one of several different spins. There is a continuum of possibilities. However, there are only a few discrete options for the quantum spin on a particle - for example, up and down.
When two fusion fuel atoms have the same quantum spin, they are more likely to fuse. "By amplifying the fusion cross section, more power can be produced from the same amount of fuel," said Parisi.
While existing spin-polarization methods don't align every atom, the gains shown in the PPPL model don't require 100% spin alignment. In fact, the study demonstrates that modest levels of spin polarization can substantially improve the efficiency of the tritium burn, improving overall efficiency and reducing tritium consumption.
Improving efficiency to reduce tritium requirements
With less tritium required, the overall size of the fusion power plant can be reduced, making it easier to license, situate and construct. Collectively, this should lower the operating costs of the fusion system.
Tritium is also radioactive, and while that radiation is relatively short-lived compared to the spent fuel from nuclear fission reactors, reducing the amount required has safety benefits because it decreases the risk of tritium leakage or contamination.
"The less tritium you have flowing through your system, the less of it will get into the components," said Parisi. The storage and processing facilities required for the tritium can also be made much smaller and more efficient. This makes things like nuclear licensing easier. "People think that the site boundary size is somewhat proportional to how much tritium you have. So, if you can have a lot less tritium, your plant could be smaller, faster to get approved by regulators and cheaper."
New avenues to explore
The DOE's Office of Science has funded separate research about some of the technologies needed to inject the spin-polarized fuel into the fusion vessel. Further work is needed to investigate things needed to implement the proposed system but have yet to be fully explored. "Whether it's possible to have integrated scenarios that maintain a high-grade fusion plasma with these specific flows of excess fuel and ash from the plasma needs to be determined," Schwartz said.
Diallo said there are also potential issues related to polarization methods, but these create opportunities. "One challenge would be to demonstrate techniques to produce spin-polarized fuel in large quantities and then store them. There's a whole new technology area that would open up."
Research Report:Simultaneous enhancement of tritium burn efficiency and fusion power with low-tritium spin-polarized fuel
