Humans have gotten pretty good at nuclear fission. But fusion, bringing two atomic nuclei together, has been much more tricky—at least for peaceful purposes. But In a new study out today in Nature, physicists from the National Ignition Facility report clearing a major hurdle toward harnessing the power of stars.
"For the first time anywhere, we've gotten more energy out of this fuel than was put into the fuel. That's a major turning point," said Omar Hurricane, lead author of the new paper and a physicist at Lawrence Livermore National Laboratory. To be clear, the NIF has not achieved its namesake goal of igniting a sustained fusion reaction. But these indications of a process called bootstrapping—in which particles created by the reaction feed energy back into the reaction—are a major step along the way.
Fusion reactions in the lab can work in a few ways, and the NIF uses a process called inertial confinement, which heats up fuel pellets made of deuterium-tritium until they implode and the fuel fuses. The reactions produce two particles, a neutron and an alpha particle. Alpha particles, which are helium nuclei, are the important ones: They must continue heating the fuel to keep more reactions coming as part of a process called alpha particle bootstrapping. In the NIF experimental "shots" reported today, this started to happen, Hurricane says.
Although this is a major milestone, it's hard to say whether or when this approach will lead to fusion as an energy source. Hurricane compared it to climbing an unfamiliar mountain whose summit is hidden in clouds. In Hurricane's metaphor, his team is at base camp right now, where they've found a safe and effective way forward.
"You can't see it. You don't have a map. You don't know what the terrain is … And someone calls you on your satellite phone and asks you, 'How long is it going to take you to climb the mountain?'" he says. "You don't know. You look down at your feet, you see what the terrain is, you try to choose the best path based on what you see. That's what we hear when someone says, 'How long is it going to take before you guys get to ignition?' It's anyone's guess."
Hohlraum geometry with a capsule inside through the cut-away of the hohlraum wall. Credit: Eddie Dewald.
The NIF works by directing 192 ultraviolet laser beams inside a tiny gold canister called a hohlraum (which sounds exotic but is just German for "hollow space.") It's about the size of a pencil eraser and holds a 2-millimeter plastic sphere. That sphere is coated with a thin layer of cryogenically frozen deuterium-tritium—the components of heavy water, essentially, in which the hydrogen atom in H2O has one or two extra neutrons. This DT ice is the fuel for fusion experiments.
The lasers bombard the walls of the hohlraum with 1.8 megajoules of energy, and the hohlraum heats up so much that it starts to emit X-rays. The X-rays rip apart the atomic structure of the plastic shell and implode it—"like a rocket driving the sphere inward," as Hurricane puts it. The fuel moves inward, too, and runs into itself, experiencing ferocious pressures, on the order of 150 billion atmospheres. That is 2.5 times the density at the center of the sun.
As atomic nuclei begin to fuse, the resulting alpha particles carry about 20 percent of the reaction's energy in the form of kinetic energy. This means they can escape. However, in the team's experiments this fall, some particles instead deposited their energy back into the fusion fuel. If more hot alpha particles drive more energy back into the fuel, this causes more nuclei to fuse together and ignite a runaway reaction.
Throughout this process, the fuel-holding sphere is compressed about 35 times, according to Debbie Callahan, a physicist at NIF. "If you started with a basketball, that would be like compressing it down to a pea," she says. And this has to happen completely symmetrically for the reaction to work. That is very hard to do, and previous experiments weren't as successful. The compression was unstable, and instead of reaching uniformly high pressures, the implosion tore itself apart.
This time, the physicists controlled the lasers in a different way, front-loading their collective energy towards the beginning of the 150-picosecond pulse. This is called a high-foot pulse. "In a way, we took a step back from what was being tried before, and it's given us a leap forward in what we've been able to accomplish," Hurricane says.
The results may be impressive, but according to Mark Herrmann, a physicist at Sandia National Laboratories who was not involved in this paper, by choosing this path to heat the fuel, the team won't be able to achieve ignition with these implosions. The fuel gain seen via the alpha particles is still less than the energy the researchers put into the process by a factor of 100, he says. The hohlraum absorbs some of the energy to cause the implosion, and the implosion deposits only a fraction of the energy into the fuel. So the biggest fusion yield was just 17 kilojoules, a far cry from the 1.8 mJ input.
"How far the high-foot implosions can be pushed remains an open question," Herrmann wrote in a commentary accompanying Hurricane's paper.
Hurricane acknowledged there's a tradeoff between energy input and high pressures. Even then, they have a long way to go: Bootstrapping is starting around 150 gigabars of pressure, but physicist and coauthor Tammy Ma said they might need to hit 300 gigabars, or 300 billion times the pressure at sea level, to ignite fusion.
The NIF team could use several tricks to improve the output. They're considering other ways of mediating the high-foot laser pulse and trying different capsule materials that might be easier to compress than plastic. They are also considering other hohlraum shapes. Since the September and November shots covered in this study, the NIF team has performed several other promising experiments and made more progress, Ma says.
The ultimate goal, Hurricane says, is a safe method of harnessing the furnace that powers the stars.
"Fusion is probably one of the most promising energy technologies that we could possibly develop. The resource, if we get it to work, is this deuterium we find in seawater—and that's essentially limitless, or more so than pretty much any other resource," he says. "And we're all scientists, and it's a very interesting puzzle to try to make this happen."