The recent report of a near-ignition fusion reaction at Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) is bolstering the aspirations of companies hoping to bring to market laser-driven fusion energy systems.
Even before the NIF announcement, at least four startups around the world had attracted multimillion-dollar investments to their proposals for commercializing laser-driven fusion, also known as inertial fusion energy (IFE). Today the companies are either currently seeking new investments or will soon do so. The IFE hopefuls are years behind the half dozen or more private ventures in magnetic fusion—involving searing plasmas confined in reactors called tokamaks—that have built one or more experimental devices. So far, the IFE research has consisted of numerical simulations and laser experiments.
Three of the hopefuls base their optimism on the advent of short-pulse petawatt-scale lasers. Their founders believe these lasers’ high-intensity femtosecond- to picosecond-long pulses can let them leapfrog NIF performance to ignite fusion reactions that yield many multiples of the energy needed to spark them.
Riccardo Betti, chief scientist at the University of Rochester’s Laboratory for Laser Energetics (LLE), says the unconventional IFE approaches should be pursued because they could permit fusion energy production without having to attain the almost perfectly symmetrical implosions that are required at NIF, which took 11 years and billions of dollars to come close to ignition. But he cautions that the physics of the short-pulse method has yet to be proved. That, Betti says, will require building billion-dollar-scale lasers that can produce 100 times the energy of LLE’s OMEGA EP, currently the most powerful short-pulse laser in the US.
Two of the companies seek to push the fusion frontier even further by designing machines that will create fusion between protons and boron-11. That is a feat far more difficult than the deuterium–tritium (D–T) reaction created by NIF and pursued in most commercial magnetic fusion concepts.
NIF wasn’t designed with a power plant in mind. It has been supported by the Department of Energy’s National Nuclear Security Administration as a tool for maintaining the US’s nuclear weapons stockpile in the absence of underground testing. Most experts say that NIF’s indirect drive approach for coupling laser to fuel—first converting laser beams to x rays that implode fuel capsules—is far too inefficient to be commercially relevant. Michael Campbell, director of the LLE in Rochester, says NIF’s “wall-plug” efficiency—the amount of energy drawn from the grid that is deposited on the fusion fuel—is about 0.5%.
But laser technology has advanced since NIF was designed in the 1990s, and electrical-to-optical efficiencies greater than 20% are now possible for solid-state petawatt-class lasers driven by efficient diodes, says Campbell. Those would be attractive for direct drive, where lasers deposit their energy straight onto the fuel, he says. Congress has shown interest in further pursuing IFE beyond NIF (see sidebar).
Modern, petawatt-scale lasers are enabled by chirped pulse amplification, for which the Nobel Prize in Physics was awarded to Donna Strickland and Gérard Mourou in 2018. They include two component lasers of the European Union’s Extreme Light Infrastructure; China’s Shanghai Superintense Ultrafast Laser Facility; lasers at the Institute of Laser Engineering in Osaka, Japan, and at the University of Texas at Austin; Lawrence Berkeley National Laboratory’s BELLA; Lawrence Livermore National Laboratory’s (LLNL’s) ARC; and LLE’s OMEGA EP.
The petawatt lasers produce peak intensities of 1019–1021 watts/cm2 in picosecond pulses. For comparison, 1015 watts/cm2 is emitted by NIF, OMEGA, and France’s Laser Megajoule, which produce tens of kilojoules to megajoules in nanosecond durations. The recent record-breaking shot at NIF, for example, had a nominal pulse length of 8 nanoseconds, yielding 1.9 megajoules.
NIF, OMEGA, and Laser Megajoule are flashlamp-pumped lasers and are capable of just a few shots per day at most. That’s a far cry from the multiple pulses per second that could be needed for an economically viable IFE energy source. Most reactor concepts envision lasers firing at fuel pellets that drop at a steady pace into the target chamber, with the thermal energy drawn off to generate steam. Some of the current petawatt lasers can fire at 1 Hz or greater.
Georg Korn, chief technology officer of the Munich-based startup Marvel Fusion, believes that ultrashort petawatt-scale shots can overcome the hydrodynamic instabilities that have prevented the fuel at NIF from imploding symmetrically and reaching the conditions for ignition. Rather than building its own laser, Marvel is “fine-tuning petawatt systems all over the world” for its 18- to 24-month experimental program, Korn says.
Marvel plans to fire those powerful lasers on p11B fuel. Fusion with protons and 11B would have the advantages of requiring no tritium and producing no neutrons. The fuel is cheap and abundant. And the alpha particles that are produced may be sufficient to produce electricity directly by magnetic induction, its proponents say, which would eliminate the need for steam production and turbines. But the temperatures required for a thermal p11B reaction are more than 3 billion kelvin, about 10 times that of the D–T reaction produced at NIF and at tokamaks around the world. Moreover, p11B fusion yields only half the energy of D–T, which critics say will make it much more difficult to create viable fusion power.
Korn says that p11B is a superior fuel to D–T for Marvel, which hopes to achieve a nonthermal ignition process that relies on proprietary nanostructured targets. Marvel has not published any research about its process. Korn says that will happen “in due time.”
Bedros Afeyan, who is president of a company that conducts research on laser–plasma interactions and has worked in laser fusion programs at LLNL, Los Alamos, and Sandia National Laboratories, says that p11B efforts like Marvel’s are doomed. “There is no exothermic reaction for p11B except at ion temperatures over 100 keV, and we don’t even know how to make plasmas of a few keV burn.”
Betti notes that in addition to the extreme temperature requirements, p11B fusion reactions have a much smaller probability of occurring than do D–T fusion reactions. “D–T is already very difficult, and p11B is significantly more difficult in the requirements to make useful energy,” he says.
Experiments performed on petawatt lasers, including at the Institute of Laser Engineering in Osaka, have produced significant numbers of p11B fusions and alpha particles but fall well short of the numbers that would be needed for ignition and useful energy production. Indeed, Korn and the other researchers who reported the Osaka results last year stated that “fusion reactors based on [the] pB reaction are clearly not for the near future.”
Marvel chose the p11B fusion route in part because avoiding the need for tritium and the production of neutrons could satisfy the antinuclear sentiment in Germany, which is phasing out nuclear power, says Korn. Marvel has raised €30 million ($35.5 million) to date from Blue Yard Capital and unnamed angel investors. But the company lost a major backer, SKion, the investment company of German billionaire Susanne Klatten, when it chose p11B.
Another hopeful that is banking on p11B is HB11, an Australian company that grew out of the research of fusion physicist Heinrich Hora at the University of New South Wales. The Sydney-based startup plans to use short-pulse petawatt lasers, though its exact technological approach is still being worked out. “We are unapologetically a research project which is being funded by alternative means: private investors,” says CEO Warren McKenzie.
Australia has no petawatt lasers, and HB11 doesn’t plan to build one. Its research program is carried out mostly by sponsored researchers at facilities on other continents. HB11’s largest contributor is German tech entrepreneur Lukasz Gadowski. The company has raised AU$4.8 million ($3.5 million), McKenzie says.
Sticking with D–T
Focused Energy, another IFE hopeful, shares common origins with Marvel. But their paths diverged when Marvel decided to pursue p11B fusion. Markus Roth, who had been Marvel’s chief scientific officer, left last year and in July cofounded Focused Energy in Darmstadt, Germany.
Focused Energy’s planned two-laser approach would compress D–T fuel using a nanosecond-long pulse from a laser like the one at NIF. The compressed blob would then be ignited by a picosecond-long pulse from a second petawatt laser, which would hit a thin (µm-sized) spherical foil. Accelerated protons from the back side of the foil would ballistically focus energy onto the fuel.
The two-pulse approach would make imperfections in fuel capsules less of a problem than it is for NIF. And the total laser energy required is planned to be 500–750 kJ, about one-quarter of NIF’s energy output, Roth says. The aim is to produce about 200 times the energy gain of NIF’s record shot.
Focused Energy has raised “double-digit millions” from US investors, says company CEO Thomas Forner. Backers include technology entrepreneur Marc Lore and retired New York Yankees slugger Alex Rodriguez, who are partners in a venture capital fund.
The krypton fluoride (KrF) gas laser technology that is the basis of San Diego–based Innoven Energy’s planned scheme was developed for the Strategic Defense Initiative to incapacitate enemy satellites. Though it was never deployed, the technology has been pursued for its fusion potential at Los Alamos National Laboratory and by scientists at the Naval Research Laboratory, with funding from DOE’s National Nuclear Security Administration. Robert Hunter Jr, the KrF laser’s inventor, is Innoven’s CEO.
Conner Galloway, Innoven cofounder and chief technology officer, says the company’s goal is to shorten the microsecond-long KrF pulses to nanoseconds. In the planned two-laser indirect–direct drive approach, beams from opposing lasers would be directed to a chamber holding targets of D–T fuel. Innoven has raised $8–10 million from angel investors and will be approaching venture capital firms this fall.
Afeyan says Innoven’s KrF lasers would require “wide-aperture, behemoth” optics that “defy credibility.” Betti agrees that building them at a megajoule scale will be difficult, but he says the light from KrF lasers has inherently better properties for implosions than that of NIF-type solid-state lasers. “I think we should look more carefully at KrF,” he says.
Even if the physics works out for one or all of the approaches, significant financial and engineering challenges will have to be overcome. The mass production of the fuel targets will ultimately determine the cost of IFE, says Roth, and the millions of targets per day that could be required means that individual polishing is impossible. Engineers may have to turn to self-organizing schemes found in nature, he says. A process might be found to mimic how a bubble of water in zero gravity forms a perfect sphere, for example.
D–T fusion energy requires a steady supply of tritium. Roth says most of the 17 kg on the planet outside of nuclear weapons is being reserved for ITER, the $25 billion–plus, seven-party international project to build a giant fusion test reactor in France. Fusion reactors of any kind will have to breed their tritium from lithium and neutrons inside the target chamber. The tritium would then be stripped out and loaded into fuel capsules in combination with deuterium.
The most common breeding proposals envision liquid metal breeding “blankets” containing lithium. Innoven plans to use a fluorine, lithium, beryllium eutectic salt, or FLiBE. The material would be circulated to carry heat out of the chamber to be used for electricity generation or other industrial processes. FLiBE also would shield reactor components from damage caused by high-energy fusion neutrons, which can shorten reactor lifetimes.
Editor’s note, 27 October: The last sentence of the sidebar about congressional funding has been updated to remove the mention of IFE funding in DOE’s fiscal year 2023 budget request.
Editor’s note, 21 October: This article has been corrected to indicate that the Naval Research Laboratory’s krypton fluoride laser research is ongoing. The lab also pursues research on argon fluoride lasers.