Fast-tracking fusion energy: Fusion could create the clean energy the world needs, but scientists are divided on the best approach

When the US National Ignition Facility (NIF) sparked a fusion reaction that generated 1.35 megajoules (MJ) of energy, enough to power the typical American household for around 20 minutes, the fusion community rejoiced. For years, researchers at the Lawrence Livermore National Laboratory facility had been firing intensely powerful laser pulses at peppercorn-sized fuel pellets to claw back only a few percent of the energy input. But in August 2021, they generated 70 percent of the laser energy injected to ignite the fusion fuel, smashing their 2018 record some 25 times over.

Further analyses have indicated that a burning plasma state was momentarily achieved in that pellet, meaning fusion reactions were the dominant source of heating in the plasma rather than the external laser energy that triggered the reaction. The world was closer to the green dream of self-sustaining fusion energy generation.

“For the first time, NIF had shown they could get a hotspot in the fuel pellet, and then get burn to propagate in the remaining fuel,” says Todd Ditmire of the University of Texas, Austin, who also founded National Energetics, which built the world’s most powerful laser for the European Extreme Light Infrastructure (ELI) Project in the Czech Republic. “From a science standpoint, [the NIF] shot was a huge deal.”

The huge deal didn’t stop with the science. NIF’s record shot not only ignited the plasma, it sparked the interest of myriad private investors, now supporting several laser-based fusion startups racing to build reactors. At the same time, the US Department of Energy has also pledged up to $50 million to help for-profit entities design their versions of a fusion pilot plant.

In yet another role, Ditmire is chief technology officer of Germany-based fusion energy start-up, Focused Energy, which raised $15 million in seed-round funds to pursue commercial fusion energy production, just one month after NIF’s achievement. Investors include baseball legend Alex Rodriguez, and ex-Walmart president Marc Lore. Similarly, other laser-powered fusion startups like Japan-based EX-Fusion, Germany’s Marvel Fusion, and HB11 Energy of Australia, have also won millions of dollars in private capital to advance their fusion technologies.

 “The laser fusion community has always been rooted in massive single-shot lasers,” Ditmire says, “but I now think a sea change is going to happen in this field.”

EX-Fusion Chief Revenue Officer Koichi Masuda, agrees: “We’re going to see further advancements in laser-powered nuclear fusion in the next five to 10 years that will really help drive commercialization.”

 NIF’s laser system is based on the principle of inertial confinement fusion (ICF) in which fuel targets are compressed and heated to create a fusion reaction. The facility comprises a 192-beam laser system that can deliver some 500 TW and up to 1.9 MJ of laser light, in a 20-nsec pulse, onto a centimeter-scale gold cavity holding a deuterium-tritium (DT) fuel pellet. The laser energy is focused onto the cavity, heating it to more than 3 million degrees Celsius with the ensuing X-rays imploding it, and compressing and heating the fuel to even more extreme temperatures and densities. Eventually, the DT hydrogen isotopes fuse, releasing high-energy neutrons and other forms of energy.

Still, NIF’s road to fusion will probably end at ignition. The experimental facility was designed to demonstrate the principle of ICF, and not to generate energy.

Photo shows the type of cryogenic target used to reach a burning plasma state for the first time in a laboratory experiment at NIF, the world's most energetic laser. Photo credit: Lawrence Livermore National Laboratory, Janson Laurea

 “From a commercial standpoint, one of the main problems with NIF’s approach is the loss of efficiency that takes place when you convert the lasers to X-rays,” says Ditmire. “You put in about two megajoules of energy and only get out about 280 kilojoules of energy on the pellet—you’re losing a factor of eight.”

What’s more, as a flashlamp-pumped laser, NIF can only fire a laser pulse every three hours. A commercial power-generating setup—that is economically competitive and can put power onto the grid—will need to crank that repetition rate up to several pulses per second. And tritium is scarce. Commercial DT fusion reactors will most likely need a chamber wrapped in lithium blankets—insulating layers packed with pebbles of a lithium-containing ceramic—to breed tritium.

Given these challenges, fusion startups are devising new reactor designs in a bid to reach commercial fusion based on ICF. Both Focused Energy and EX-Fusion are harnessing so-called fast ignition, in which the energy to heat the fuel comes from a second laser, rather than from the implosion process. The companies intend to use ultrashort-pulse petawatt lasers, driven by efficient diodes, to create the extreme conditions needed for fusion, and they are confident that the high intensity femtosecond-to-picosecond-scale pulses will outperform NIF.

In Focused Energy’s set-up, an initial nanosecond-long laser pulse will compress the fuel pellet with a second picosecond-scale pulse, igniting the fusion reaction. In their fast ignition scheme, the second intense laser pulse will strike a foil film, creating accelerated protons that then ballistically deposit huge amounts of energy into the compressed fuel.

Right now, Ditmire and colleagues are building a laser system for a single shot every three minutes using technology he helped develop for ELI. “We’re using the same liquid-cooling as ELI, which operated at up to one shot a minute. With our higher repetition rate, we’ll study more of the underlying physics of inertial confinement fusion.”

EX-Fusion also won funding in the wake of NIF’s success—a tidy $1 million from Tokyo-based venture capital firms ANRI and Osaka University’s in-house venture business. The company’s concept also features a nanosecond-pulsed laser system that will compress the fuel target, but then a second short-pulse laser will produce fast-moving electrons to be directly deposited into the pellet’s plasma to trigger fusion.

Cofounder and Chief Executive Kazuki Matsuo has worked on fast ignition at the University of California, San Diego, and has also been collaborating with researchers at the Institute of Laser Engineering (ILE), Osaka University, and the Graduate School for the Creation of New Photonics Industries (GPI) on a long-running fast-ignition experiment called FIREX. He believes his company is firmly on its way to laser-based fusion.

EX-Fusion already has access to a newly built 10-Hz higher energy diode-pumped laser called HELIA, Matsuo says. What’s more, cofounder and Chief Technology Officer Yoshitaka Mori, from GPI, has been working closely with Hamamatsu Photonics on a minireactor, trialing a 10-Hz fusion pellet injection system with laser target tracking that could form the basis of a commercial reactor. In recent research, they rapidly fired intense laser pulses at 3,500-mm-diameter polystyrene beads for five minutes, an incredible feat of synchronization.

“EX-Fusion should have this system finished in two years,” says Matsuo. “Nobody has worked on the technical aspects of a fusion reactor like this. We are doing this for researchers worldwide, including those related to NIF.”

And this is where EX-Fusion’s business plan becomes interesting. The company will focus on the technology that laser-based fusion businesses will need, assuming fusion continues to show potential. “Many start-ups are working on their own systems, but we’re saying ‘Look, we’re building a reactor that can work with different lasers.’ Our pre-seed funds will provide the financial stability we need to start developing technologies to get laser fusion commercialized,” Chief Revenue Officer Koici Masuda says.

Come 2027, Matsuo, Masuda, and colleagues hope to have the technology fully scaled for use in conventional applications such as materials processing and semiconductor manufacturing. “We anticipate selling our first target feeding or laser-tracking system in 2023,” says Masuda.

In the meantime, to demonstrate ignition in the coming decade, the Focused Energy team is building a fusion target lab in Darmstadt, Germany, and intends to build a facility in Austin, Texas. Ditmire admits the timeline is aggressive but highlights how he and his colleagues have been honing the efficiency of fast ignition for many years while working out how to compress DT fuel capsules to ever-higher densities. Assuming ignition success, the company plans to build its first pilot plant by around 2030.

The Texas Petawatt Laser's vacuum vessel includes two of the largest diffraction gratings in the world, which are used to spread the colors of the laser pulse out in such a way that they recombine in time and result in a 100-fs-long, 1 petawatt laser pulse. Photo credit: Todd Ditmire, University of Texas, Austin

But while Focused Energy, EX-Fusion, and other startups pursue DT fusion, an alternative would be to react hydrogen (protons) with the boron-11 isotope. On the plus side, hydrogen-boron fusion does not require tritium, nor does it produce high-energy neutrons. However, the temperatures required for fusion are more than 3 billion K, some 10 times greater than those in NIF’s experiment. As a result, many researchers have shied away from this flavor of fusion—until recently.

Laser-fusion great, Heinrich Hora, put forward the idea of nonthermal hydrogen-boron fusion in the 1970s, but predicted the reaction would only become viable once laser pulses had become powerful and fast enough. Petawatt lasers followed, and by 2005, scientists at the Research Institute of Pulse Technology had initiated fusion reactions in boron-hydrogen plasmas using intense picosecond laser pulses. Ensuing experiments have consistently raised hydrogen-boron fusion reaction rates. And just last year, a pan-European team of physicists broke all proton-boron fusion records.

The team of Dimitri Batani from the Centre Lasers Intenses et Applications at the University of Bordeaux, France, and colleagues from fusion research institutions including ELI and ILE, fired a petawatt laser directly at a proton-boron target, creating more fusion reactions than ever before. Their results not only raised hopes of using this type of fusion to generate clean electricity, but piqued interest in harnessing the process to make radioisotopes for medicine.

Following this achievement, Marvel Fusion won nearly $35 million in funds to develop its novel technology with Siemens Energy, TRUMPF, and Thales. Similarly, HB11 has been awarded $42 million to develop key components for a petawatt laser suited to hydrogen-boron fusion as well as fusion fuels. Hora is the company’s scientific director, while Batani has joined as lead scientist. They hope to have a detailed engineering design of a prototype reactor by 2028.

According to HB11 Managing Director Warren McKenzie, the company’s planned hydrogen-boron fusion setup will most likely be a merger of concepts. For example, the company is investigating a hybrid approach in which a hydrogen-boron target is compressed, as in DT fusion, and then a proton beam is generated by a picosecond laser to trigger ignition. “The exciting thing about what we’re doing is we’ve already got these great results... and we’re now explaining the science,” McKenzie says.

Batani adds, “Over the years we’ve seen an unexpected production of the charged alpha particles, which has re-opened interest in proton-boron fusion, but we still have many, many questions to answer.”

Indeed, Batani and colleagues are investigating exactly what happens to boron under the extreme pressures demanded by proton-boron fusion. “We know very little about the state of boron at fusion’s high pressures—this really is a completely new domain for us.”

The unknowns underline researchers’ reservations about hydrogen-boron fusion. Ditmire’s view echoes many who are gunning for DT fusion and wonder if the even more extreme conditions, including higher temperatures, necessary for this flavor of fusion can be reached. “To me, right now, the physics looks insurmountable,” he says. “For example, with energy losses…it isn’t even known whether a hydrogen-boron plasma will ever ignite, even if one could somehow create the very high plasma temperatures.”

“That doesn’t mean the world shouldn’t work on it as a science project. But I don’t see how it can be viable commercially on a 2030 timescale,” he adds.

Batani and McKenzie are fully aware of the hydrogen-boron criticism and acknowledge fusion’s “split” community. “You can say there’s so little known about our science, but if you look at deuterium-tritium, there’s so little known about the engineering,” McKenzie says.

Both Batani and McKenzie point out how boron, unlike tritium, is not radioactive, making fuel storage easier. They also highlight how proton-boron fusion produces fewer high-energy neutrons than DT fusion reducing radioactive waste. And, as they also explain, boron is solid under standard conditions, so fuel pellets do need to be held under cryogenic conditions, prior to implosion, unlike DT targets.

“If we get through the [proton-boron fusion] science, then our engineering will be orders of magnitude easier,” claims McKenzie. “I suspect whatever the approach taken for DT engineering, it’s going to be very difficult to economically achieve.”

Batani agrees, adding, “I don’t think that DT and proton-boron fusion are enemies. Myself and many other scientists are working on both.

“DT fusion has recently got extremely important results, making a lab demonstration of fusion very close,” he continues. “But it does have engineering issues, including tritium breeding, so it’s wise to investigate other approaches.”

Despite their differences, researchers agree that the future of fusion energy hinges on collaboration. Ditmire emphasizes that he and his colleagues are building on the efforts of many in the DT fusion community. Indeed, when the analysis of NIF‘s record fusion results was published, more than 150 researchers from the US and Europe had put their names to the work.

For Batani, collaboration can only speed success. “We need to make many more experiments, and HB11, for one, cannot do all of this research on its own,” he says. “We all rely on collaborations, and you know, in an ideal world where we could all do all of the experiments that we want, we would probably answer many of our scientific questions in the next five years.”

Rebecca Pool is a UK-based freelance writer.

For more from SPIE on fusion energy, see the SPIE Conference on High Power Lasers for Fusion Research VII.