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Inertial confinement fusion (ICF) target capsules are a key enabling component for the future of fusion energy. Energy gain through ICF requires target capsules with unimaginably smooth surfaces and ignition fuel suspended uniformly inside, making current target capsules at Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) an engineering miracle. However, fabricating and fielding these capsules is currently too difficult, costly, and time-consuming to advance future fusion technology such as an inertial fusion energy (IFE) power plant, which would require an estimated 800,000 capsules per day produced at less than $0.50 each.
Lawrence Livermore has begun exploring additive manufacturing (AM) as a potential path for mass producing ICF capsules faster, cheaper, and with previously impossible designs. A team of engineers, chemists, physicists, and technicians demonstrated the technology’s potential by conducting the first four NIF experiments with fully 3D-printed ICF capsules. The capsules feature leak-tight walls just a few micrometers thick and an internal lining of extremely low-density foam that wicks up and suspends liquid fuel for ignition—all made possible through the flexibility and ultrahigh resolution of two-photon polymerization (2PP) printing. “For a power plant or a facility with a high-repetition shot rate, the traditional capsule fabrication approach probably isn’t going to work,” says staff scientist James Oakdale. “Although the 2PP materials aren’t as perfect, our approach is potentially much faster because we can change parts on the fly, and we’re not locked into a specific manufacturing process or design.”
Foams and Flexibility
The 2PP technique uses light to print features as small as 100 nanometers (nm)—1,000 times thinner than a human hair. The printer shines a laser on a liquid resin. Once the irradiated energy is high enough, the resin absorbs photons and its monomer molecules link together to “cure” into a solid polymer. Other light-based AM techniques print by curing at the energy of one ultraviolet photon (wherever the light hits). 2PP cures at the energy of two infrared photons, which, statistically, can only happen over a small volume within a tightly focused beam, enabling high-resolution printing. “The beam is like a tiny laser pointer. Wherever it moves, the resin only polymerizes within that approximately 100-nanometer space,” says Oakdale.
Capsules printed using 2PP can be ready in 24 hours and filled within minutes, making them potentially much easier to fabricate and field than traditional targets. The process may enable rapid prototyping as well. “This is an exciting new territory,” says physicist Elijah Kemp, principal investigator on the first four experiments. “Having this level of control over properties is unheard of for capsule manufacturing.”
Kemp was an early champion of 2PP for printing ICF target capsules for direct-drive experiments, in which lasers hit targets directly (in contrast to indirect drive, with a hohlraum surrounding the target capsule). 2PP makes it possible to fabricate direct-drive concepts, which have unique design requirements that make them delicate and difficult to manufacture. “If we print a very thin shell with very low-density foam on the inside, these features support each other throughout the printing process so we can make the shell thinner and the density lower,” says staff scientist Xiaoxing Xia. “ICF targets are the perfect application for 2PP because these small, intricate parts need the precision only this technology can provide.”
The concept combines Oakdale’s work printing 2PP capsules for non-ICF experiments at NIF with precisely placed features, such as voids or doped elements, with wetted foam capsule design. (See S&TR, October 2017, Additive Manufacturing Helps Reinvent Nanoporous Materials.) Current high-yield ICF targets contain deuterium-tritium (DT) fuel vapor and a thin layer of solid DT ice that uniformly coats the inside surface, which is frozen manually in an arduous process that can take up to a week. Liquid fuel is just as effective and much easier to work with, but it needs to be uniformly suspended with a foam or other porous material.
Since 2016, the Laboratory has made wetted foam capsules by chemically growing aerogels—ultralow-density foams with nm-sized pores—to address this challenge, as the lower the foam density, the less detrimental it will be to the implosion. “Simply reducing the amount of foam mass is probably the most important lever to improve fusion performance,” says physicist Steve MacLaren. (See S&TR, January 2016, A Growing Family of Targets for the National Ignition Facility.)
Navigating a New Technology
The first 2PP target capsules fielded on NIF were both foam-lined capsules on direct-drive experiments performed in March and May 2024. The first target contained only fuel vapor, and the second contained only suspended liquid fuel. While the experiments were successful, fielding the unfamiliar target technology proved challenging and prompted innovation among the researchers, Livermore’s Target Fabrication team (TFab) and industry partner General Atomics. “Individually, 2PP and the printed wetted foam are different technology from other targets we’ve fielded at NIF, so using them both on the same experiment created novel situations for us,” says cryogenic process engineer Travis Briggs.
Commercial 2PP printers, such as TFab’s, print in small, square sections that are stitched together similar to patches of a quilt to form structures, but the stitching was a major source of capsule defects. “Stitching that is too big can cause instability and impact implosion,” says Abbas Nikroo, NIF’s deputy director for physics integration. Xia and Oakdale sought to improve capsule quality by designing a custom, dual-wavelength 2PP printer optimized for capsule fabrication. The printer’s motion stage—the path of the print head—moves in a ring shape, and a galvo scanner directs the beams in three dimensions so the stitching better complements the spherical shape of the capsules.
The printer also seeks to improve precision and design flexibility with a dual-wavelength printing (two-color) configuration. Oakdale says this configuration eases simultaneous printing of different features and compositions, such as using one color of light to print porous foam and another to print a solid shell, or adding doping elements to improve performance. “The two wavelengths give us spatial control over density of the material and chemical composition, which gives us more tools to respond to designs that physicists dream up,” he says. To complement the printer, Oakdale and colleagues Magi Yassa and Johanna Schwartz are developing a dual-wavelength photo resin. “We’re targeting a single formulation where we can turn on one wavelength and the material comes out nanoporous, and we can turn on the other wavelength and the material comes out solid,” says Oakdale. “That way, we could print the shell and inner foam all at once by turning the lasers on and off or choosing one laser over the other.”
TFab is instrumental in every NIF shot, but the team has been especially crucial for fielding 2PP target capsules. “One of the biggest challenges was delivering a leak-tight capsule that would retain the fuel,” said TFab engineer Montu Sharma, whose team solved the problem by improving the printed shell quality and adding a new target coating. Another obstacle was the “ice plug”—a tiny piece of ice that seals the fill tube to keep liquid fuel inside the capsule and prevent the foam layer from overfilling. The team addressed this challenge by developing a dual-heat switch target assembly that controls the temperatures of the capsule and fill tube independently so the ice plug forms farther away from the capsule.
TFab achieved a leak-tight, liquid-filled capsule in November 2024, giving the team the confidence to perform the first layered (with both a central D2 vapor region and a liquid-wetted foam layer) direct-drive experiment in March 2025. “I’m proud of our flexibility and the work we’ve done so far to develop a process to fill the capsule and maintain that fill,” says Briggs.
Beginning a Journey
Targets printed with 2PP are promising, but they face a long road toward viability. Even the best printers cannot yet match the quality of traditional capsules or achieve the fabrication speed needed for an IFE plant. Xia’s team is working to improve production speed through parallelization—essentially an assembly line of print heads working simultaneously. One method they explored uses metalenses, a microfabricated chip with tiny nanostructured pillars that focus an ultrafast laser to create 100,000 focal spots in parallel. (See S&TR, April/May 2024, A New Dimension of Glass.) With a metalens array, the team’s 2PP printer could print sample structures across a 3-centimeter wafer 1,000 times faster while retaining 100-nm resolution. Another project, inspired by colleagues from the Laboratory’s IFE STARFIRE initiative, explores scrolling lightsheet AM in which a photo resin moves past a series of parallel print heads on a conveyer belt.
Other potential options for rapid target fabrication at Livermore include microfluidics and volumetric AM, which prints objects all at once instead of layer by layer. With every approach, the challenge is finding the right balance of speed and precision. “This could be the way of the future, but we have to be conscious of the fact that ICF needs perfection, and we need time to achieve our goals for printed ICF targets,” says Nikroo.
All experimental data for ICF is difficult to come by, given the demands for shot time at NIF, but the team aims to use every chance it gets to learn as much as possible about the 2PP targets. MacLaren is leading a series of tests with 2PP wetted foam intended for capsules for indirect-drive ICF to measure the impact of four distinct foam geometries on implosion performance as his team prepares for proposed shots on NIF in 2026. “Our goal is to field an implosion with wetted foam that is nearly identical to ice-layered implosions except for the fuel layer,” says MacLaren. “This test gives us the chance to benchmark our models of a liquid layered indirect-drive implosion.”
Accurate models can help researchers design around 2PP’s limitations or leverage its capabilities to design future targets with unprecedented performance. “In theory, 3D printing has the capability to optimize designs, so I’m interested in finding out how, given any configuration constraints, we can find a design solution that gives us the best result,” says Xia. He acknowledges that the team is on a long journey, but he remains optimistic. Says Xia, “A dedicated team required decades to even get close to fusion ignition, and going from ignition to an IFE power plant is another decades-long effort that will require a lot of work. But I feel good about Livermore’s team, and I’m very excited to be a part of it.”
—Noah Pflueger-Peters
For further information contact Xiaoxing Xia (925) 423-6489 (xia7 [at] llnl.gov (xia7[at]llnl[dot]gov)).
