By reducing turbulence, alpha particles from fusion could improve reactor performance
Good news for fusion fans: A phenomenon that could have hampered efforts to produce energy through nuclear fusion could actually be beneficial.
In computer simulations of two planned fusion reactors, fusion energy products called alpha particles helped dissipate small-scale turbulencetiny swirls of particles that would otherwise have sapped heat from the center of the reactor, thereby degrading its performance. The result, reported in a paper submitted May 11 to arXiv.org, adds to a growing body of research suggesting that alpha particles will affect turbulence in ways that improve reactor performance rather than deteriorating itas previously feared.
Fusion is the process that powers the sun: two atomic nuclei fuse into one, releasing energy. If it could be harnessed on Earth, fusion could produce energy without the carbon emissions of fossil fuels or the long-lived radioactive waste produced by nuclear reactors based on fission, the splitting of atomic nuclei.
Several companies are working to build commercially viable fusion reactors. Interest in this technology is booming: on June 9, the US Department of Energy published a roadmap for fusion energy in the coming decade. But no reactor has yet created the conditions in which fusion can thrive, and uncertainties swirl around the physics.
One of these uncertainties concerns the alpha particles themselves. In fusion reactors, magnetic fields keep a cloud of charged particles, called plasma, confined in a tight, very hot beam. In this plasma, hydrogen nuclei fuse to produce alpha particles – positively charged helium nuclei. Maintaining plasma confinement is crucial for fusion, but it was unclear whether alpha particles would be helpful or harmful.
The idea that alpha particles could regulate turbulence and improve fusion performance has long been considered, but without clear proof. Now experiments and simulations are making this process clearer, says plasma physicist William Heidbrink of the University of California, Irvine, who was not involved in the research. “Maybe this thing, which seems magical and fanciful, could really work in a positive way.”
Alpha particles are key players in fusion reactors. They carry energy which is poured into the surrounding plasma and heats it. Once a reactor actually starts up, it should become self-sustaining: alpha particles produced by fusion reactions heat the plasma, maintaining conditions for increased fusion.
“If you don’t know how the alphas will behave, there’s no way to make an economically viable reactor,” says Jacobo Varela, a plasma physicist at the University of Texas at Austin who was not involved in the research. “In a reactor, everything revolves around the alphas and their behavior.”
For the new study, plasma physicist Alessandro Di Siena and his colleagues simulated two reactors currently under construction: ITER, an international research project in the south of France, and SPARC in Devens, Massachusetts, designed by Commonwealth Fusion Systems, which partly funded the study. Both are donut-shaped devices called tokamaks that confine plasma with powerful magnetic fields.
In the simulations, the alpha particles triggered plasma flows that broke up small-scale turbulence, keeping the plasma hotter and better confined. This produced more fusion and even more alpha particles. “What we find is that you can get into a kind of positive feedback loop,” says Di Siena, of the Max Planck Institute for Plasma Physics in Garching, Germany. When this effect was included, alpha particle heating increased by up to 25% in SPARC and up to 18% in ITER.
The experimental evidence points in the same direction. Although existing tokamaks cannot produce the exact conditions needed for a commercially useful fusion reactor, experiments have suggested that energetically charged particles such as alpha particles could be beneficial for confinement – including a 2024 study at the Joint European Torus in England, currently decommissioned. And one 2025 study at DIII-D the San Diego tokamak found turbulence effects similar to those in the new simulation.
There are still uncertainties in these types of simulations, particularly in the predicted increase in heating of up to 25 percent. So when it comes to hard numbers, “I would take them with a grain of salt,” says Phil Snyder, vice president of plasma physics at Commonwealth Fusion Systems. But it’s the general trend that’s important, he says. When the effect of alphas on turbulence is included, “you can end up producing a lot more fusion power than you would have predicted if you hadn’t included this effect.”
