Fusion power has experienced a renaissance in recent years, with billions of dollars in private investment flowing into the field. Acceleron Fusionis the latest startup to take a swing at this challenging nuclear-energy technology, banking on a novel approach that uses beams of heavy subatomic particles called muons to achieve fusion at much lower temperatures.
The company, based in Cambridge, Mass., is designing a plant that will rely on muon-catalyzed fusion, a phenomenon first observed in the 1950s. Its reactor will work by firing a beam of muons at a pellet of nuclear fuel kept under extremely high pressure. Using this approach, Acceleron’s plant could operate below 1,000 °C—not exactly “cold” fusion, but not nearly as hot as other strategies such as magnetic confinement or inertial confinement.
These other fusion approaches require temperatures in the millions of degrees to heat fuel until it becomes a plasma. The plasma must be contained using extremely powerful magnets or lasers, which are complex and power hungry, so being able to do without them is a significant benefit for Acceleron’s lukewarm approach to fusion. “It adds a great amount of technical simplicity and engineering flexibility,” says Ara Knaian, electrical engineer and CEO and cofounder of Acceleron.
Today, Acceleron announced it has closed a US $24 million funding round to help develop prototypes of key reactor components and has now completed 100 hoursof continuous fusion at its test facility at the Paul Scherrer Institute in Villigen, Switzerland. The experiments are aimed at gathering data rather than producing useful amounts of energy.
Muon-Catalyzed Fusion Power
The muons at the heart of Acceleron’s approach are from the same family of subatomic particles as electrons, but roughly 200 times their mass. They are created when protons and neutrons collide, which generates particles known as pions that then decay into muons. In nature, they occur when cosmic rays hit the upper atmosphere of Earth, but they can also be generated artificially by firing an ion beam from a particle accelerator into a target, typically made of carbon or metal.
The resulting muons can help kick-start a fusion reaction by being directed at a mixture of the hydrogen isotopes deuterium and tritium—popular fuel for fusion reactors. Muons displace electrons in the hydrogen atoms, resulting in hydrogen molecules made up of a deuterium and a tritium atom chemically bonded by a muon rather than an electron. Because muons are much heavier, the length of this bond is reduced to roughly 0.5 percent of the original. This brings the two atoms close enough that the strong nuclear force—the force that holds nuclei together—takes over and pulls the atoms together until they fuse, releasing massive amounts of energy. Crucially, this reaction doesn’t require particularly high temperatures, says Knaian.
Of course, getting all of this to work in practice, and to produce more energy than what’s put in, is quite challenging, says Spencer Kelly, a teaching fellow at Imperial College London, who is not involved with Acceleron. The particle accelerators used to generate muons are very energy intensive, so many fusion reactions must be generated per muon to break even in energy output.
Plus, muons last for just 2.2 microseconds before decaying into other particles, and roughly 1 percent of the time they stick to other particles generated by the fusion reaction and can no longer combine with hydrogen atoms. This means that typically each muon participates in only about 100 fusion reactions before it is taken out of circulation, which generates considerably less energy than it took to create the particle in the first place.
The most fusions per muon achieved stands at about 150, a record set by researchers at the Los Alamos National Laboratory in 1986, which is still insufficient. Research largely petered out in the early 1990s because significantly improving on the numbers seemed daunting, says Kelly. “It’s frustrating, because muon-catalyzed fusion is really not far from viability,” he adds. “It’s just that finding the path between where we are and viability is is not obvious.”
Acceleron Fusion’s Innovative Reactor
But there’s hope. The calculations that pointed to a dismal future for muon-catalyzed fusion were based on 1980s hardware, says Knaian. Accelerator and reactor component design have greatly improved since then. Prompted by these advancements, Knaian and Acceleron cofounder Seth Newburg applied for a $2 million Advanced Research Projects Agency–Energy grant to investigate ways to boost the efficiency of the approach, which they received in 2020. They won another $500,000 in grant money from ARPA-E in 2023.
Acceleron’s approach is to first try to slash the energy required to produce muons, in part by piggybacking on improvement in accelerator efficiency. This has jumped from around 20 percent in the 1980s to 50 percent today, Knaian says, and the U.S. Department of Energy targets 75 percent for next-generation accelerators. Acceleron is also designing a novel muon source that should produce the particles for considerably less energy. The company’s computer simulations suggest that inducing electrical and magnetic fields inside the target could help collect and focus the particles much more efficiently. “That’s the area where we think we could have the biggest contribution,” says Knaian.
The researchers are also trying to boost the number of fusion reactions per muon by compressing the fuel in a diamond anvil to between 10,000 and 100,000 PSI—much higher than previous experiments. For the past four years, the company has been running experiments testing fusion yields at different temperatures, pressures, and ratios of deuterium and tritium at the Paul Scherrer Institute.
Overcoming Hurdles to Colder Fusion Energy
Muon-catalyzed fusion faces unique challenges, says Dennis Whyte, a professor of engineering at MIT who specializes in fusion power. Unlike other approaches to fusion where the key barriers are technological, the biggest impediment to muon-catalyzed fusion is the fundamental physics of the reaction rates, he says. There were hints in some earlier muon-catalyzed fusion experiments that at certain temperatures and pressures one could achieve significant improvements, he adds, but they were far from conclusive. “It’s still at the point where, on paper, it’s energy balance negative,” he says.
And reaching break-even energy is just the first step, Whyte adds. To be practical, a commercial power plant would need to produce roughly five times as much energy as what’s put in. Getting such significant energy gains will be considerably harder for muon-catalyzed fusion than approaches like magnetic confinement, where the plasma become self-heating and self-sustaining after a certain point. “If you get to that condition, you can go to infinite gain,” he says.
Nonetheless, fusion research is rarely a wasted effort, Whyte says. “It tends to push you to the frontiers of your scientific and technical capabilities,” he says. “You do great science on the way.”
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