The filing shows that Acceleron Fusion has raised $15 million to fight cooler fusion

Fusion startups have been experiencing a fundraising slump lately, with young startup Acceleron Fusion joining the ranks, which has raised $15 million of its $23.7 million target, according to SEC filings.

The fusion sector has recently attracted interest from investors, undoubtedly encouraged by a groundbreaking experiment conducted two years ago at the National Ignition Facility, which proved that a controlled thermonuclear reaction can generate more power than is needed to trigger it.

The first company to build a power plant that can produce electricity that can be mass-sold to the grid could start making money in the multi-trillion-dollar global energy market. In particular, technology companies see fusion and nuclear power startups as possible pollution-free solutions to meet AI-driven energy demands.

Acceleron did not immediately respond to questions.

While most startups aim to recreate the superheated and negative-pressure conditions inside a star, Acceleron is taking a different approach, using subatomic particles called muons to lower the heat and pressure required for thermonuclear reactions to occur.

In nature, atoms tend to resist fusion, mainly because the electrons orbiting the atom repel other atoms. To get around this problem, most approaches to nuclear fusion are based on nature’s approach: they heat atoms hot enough and close enough that their electrons are released from their orbits, lowering the usual atomic inhibitions. As atomic nuclei move without electrons, some of them bump into each other, merging into a new nucleus and releasing enormous amounts of energy. This is what happens inside a star.

Muon-catalyzed fusion takes a different tack. Instead of heating and compressing hydrogen isotopes, it injects muons into the mixture. Muons are subatomic particles that resemble electrons – both have a negative charge – but are 207 times more massive. When muons bombard hydrogen isotopes, they replace electrons in some atoms. The muon orbits the nucleus of the atom much closer than the electron, which lowers the barrier necessary for atoms to fuse.

In muon-catalyzed fusion, the barrier is low enough that fusion can occur at room temperature and pressure. That’s why it’s sometimes called cold fusion, or at least colder fusion. (The term “cold fusion” has somewhat negative connotations after researchers announced in 1989 that they had evidence of thermonuclear reactions at room temperature, although no other lab was able to replicate the experiment.) Although muon catalysis has been demonstrated in the laboratory, the energy needed to produce muons so far exceeds the energy produced by thermonuclear reactions.

There are several reasons why muon-catalyzed fusion hasn’t worked yet. First, each muon only lives for about 2.2 microseconds before decaying into less useful subatomic particles. That’s long enough to enable about 100 thermonuclear reactions, but still too short for commercial purposes. Another problem is that in about 0.8% of cases, the muon sticks to another subatomic particle (the alpha particle) and no longer participates in any fusion reactions. This may not seem like much, but again it was high enough to thwart commercial plans.

Cambridge, Mass.-based Acceleron, which is spun out of NK Labs, hopes that by raising the pressure of its hydrogen isotope mixture and perhaps the temperature, it can reduce the rate at which muons adhere to alpha particles. The hope is that enough muons will remain in the mix to catalyze more fusion reactions, and ideally enough to offset the amount of energy needed to generate the muons.

NK Labs received a three-year, $2 million ARPA-E grant in 2020 to investigate whether higher pressure would improve the prospects for muon-catalyzed synthesis. The results, not all of which are currently public, appear to have piqued investor interest.

Update: Added details about the 1989 cold fusion experiment and updated the headline.