Fusion’s alpha particles may aid reactors by suppressing turbulence

There may be good news for fusion supporters: a phenomenon that might have stood in the way of generating energy from nuclear fusion could turn out to be helpful.

In simulations of two planned fusion reactors, energetic fusion products known as alpha particles helped break up small-scale turbulence—small eddies of particles that would otherwise draw heat from the reactor’s core and reduce performance. The finding, described in a paper submitted May 11 to arXiv.org, adds to growing research indicating that alpha particles will influence turbulence in ways that improve reactor performance rather than make it worse, as had been feared.

Fusion is the process that powers the sun: two atomic nuclei merge into one, releasing energy. If harnessed on Earth, fusion could generate 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 trying to build commercially viable fusion reactors. Interest in the technology is rising: on June 9, the U.S. Department of Energy released a roadmap for fusion power in the coming decade. But no reactor has yet produced the conditions needed for fusion to flourish, and the physics remains uncertain. One uncertainty involves alpha particles. In fusion reactors, magnetic fields confine a cloud of charged particles, called plasma, into a tight, superhot bundle. Inside that plasma, hydrogen nuclei fuse and produce alpha particles—positively charged helium nuclei. Keeping the plasma confined is essential for fusion, but it was unclear whether the alpha particles would help or hurt. The idea that alpha particles could regulate turbulence and improve fusion performance has been envisioned for a long time but without clear evidence. Now experiments and simulations are making the process clearer, said plasma physicist William Heidbrink of the University of California, Irvine, who was not involved in the research. “Maybe this thing, which seems sort of magical and fanciful, could really work positively.”

Alpha particles are central players in fusion reactors. They carry energy that is deposited into the surrounding plasma, heating it. Once a reactor is really operating, it should become self-sustaining: alpha particles produced by fusion reactions heat the plasma, keeping conditions favorable for more fusion.

“If you don’t know how the alphas will behave, there is no way to make an economically viable reactor,” said plasma physicist Jacobo Varela of the University of Texas at Austin, who was not involved with the research. “In a reactor, everything is about the alphas and how they behave.”

For the new study, plasma physicist Alessandro Di Siena and colleagues simulated two reactors currently under construction: ITER, an international research project in southern France, and SPARC in Devens, Mass., designed by Commonwealth Fusion Systems, which partly funded the study. Both are doughnut-shaped devices called tokamaks that confine plasma with strong magnetic fields.

In the simulations, alpha particles launched plasma flows that disrupted small-scale turbulence, keeping the plasma hotter and better confined. That led to more fusion and more alpha particles. “What we see is that you can enter in a type of positive feedback loop,” said Di Siena, of the Max Planck Institute for Plasma Physics in Garching, Germany. When this effect was included, alpha particle heating rose by up to 25 percent in SPARC and up to 18 percent in ITER.

Experimental evidence has pointed in the same direction. Although existing tokamaks cannot create the exact conditions relevant to a commercially useful fusion reactor, experiments have suggested that energetic charged particles such as alpha particles could aid confinement—including a 2024 study at the Joint European Torus in England, which is now being decommissioned. A 2025 study at the DIII-D tokamak in San Diego found turbulence effects similar to those in the new simulation.

There are still uncertainties in simulations of this kind, including around the predicted heating boost of up to 25 percent. So, as far as specific numbers go, “I would take it with something of a grain of salt,” said Phil Snyder, vice president of plasma physics at Commonwealth Fusion Systems. But the overall trend is what matters, he said. When the alphas’ effect on turbulence is included, “you can end up producing significantly more fusion power than you would have predicted if you did not include this effect.”