Fusion “Breakthrough” Won’t Lead to Practical Fusion Energy
It’s not about to solve climate change, but what happened at Lawrence Livermore National Lab’s National Ignition Facility (NIF) early in the morning on 5 December is still noteworthy. In a press conference on the morning of 13 December, U.S. Secretary of Energy Jennifer Granholm announced that one of NIF’s experiments had, for the first time, resulted in a fusion reaction that produced more energy than required to kickstart it—a result called ignition. The achievement is a significant one for NIF, which uses lasers to kick-start the reaction, a process called inertial confinement fusion, rather than the magnetic confinement approaches that are more technologically mature.
The road from this achievement to green, safe, and abundant fusion energy is still murky at best. But, as Lawrence Livermore director Kim Budil said during the announcement, this result is a necessary first step.
NIF officials acknowledged during the announcement that the test’s results would support Lawrence Livermore’s work in nuclear weapons development and testing, as well as the facility’s stewardship role in maintaining the United States’ stockpile. Unlike previous milestone announcements from NIF in years past, officials were more candid this time around about that more central aspect of the laboratory’s work (Clean energy research at the lab is generally seen to be an off-shoot of that work).
At NIF, located in Livermore, Calif., 192 ultraviolet laser beams were aimed at a peppercorn-sized capsule filled with partially frozen deuterium and tritium—two isotopes of hydrogen. The lasers caused the capsule to implode, and the hydrogen atoms collided with one another at roughly 400 kilometers per second. In a timeframe measured in billionths of a second, some of the hydrogen isotopes fused into helium atoms. In the process, they released neutrons and extra heat, which briefly drove further collisions—about 4 percent of the total hydrogen by the time the reaction petered out.
During NIF’s 5 December laser shot, the amount of energy produced by the fusion reaction (3.15 megajoules) exceeded the laser energy (2.05 MJ) aimed at the capsule. Put another way, for the briefest blink of a moment, the deuterium and tritium within the capsule underwent a self-sustained reaction. From a scientific point of view, this is a tremendous step.
“The fusion reaction is heating the fusion reaction, which is making more fusions happen,” says Steven Cowley, director of the Princeton Plasma Physics Laboratory. “It’s like the fire has been lit. This is the first controlled fusion ignition that we’ve ever seen, and that’s spectacular.”
From a practical standpoint, this is just one tiny step on the long and unclear road to commercial fusion power. “The extrapolations are enormous,” says Martin Greenwald, deputy director of the Plasma Science and Fusion Center at the Massachusetts Institute of Technology. In particular, there are five major extrapolations—and they are major—that experts IEEE Spectrum spoke with highlighted.
First, the reaction needs to be at least one hundred times more effective than NIF’s laser shot on 5 December. Although the energy that NIF produced within the capsule exceeded the laser energy aimed at it, the amount falls well short of the energy it took to fire the lasers, which draw about 300 MJ of power from the grid for every shot. Tammy Ma, the lead for Lawrence Livermore’s institutional initiative in inertial fusion energy, acknowledged during the announcement that this so-called “wall plug” efficiency of the lasers isn’t a factor for NIF’s scientific experiments, but would need to be much better for any kind of working reactor.
(NIF has previously pulled some energy accounting sleight of hand. In 2014, the facility heralded a fusion breakthrough called fuel gain, an important step toward ignition. However, it turned out that NIF’s researchers had made the assessment using only the laser energy that hit the capsule—discounting the vast majority of the lasers’ energy in the process.)
Second, the rate at which any reactor fires its lasers at the hydrogen capsules needs to be orders of magnitude faster. Ideally, the lasers would be able to fire several times a second—NIF currently fires its lasers just several times a year, in part because the researchers are tweaking and redesigning their experiments after every shot.
Third, even if the inertial fusion process is more efficient, the produced energy still needs to be converted into usable electricity. For this, most proposals rely on a design that would wrap the entire setup in a “blanket,” absorbing the ejected neutrons in order to produce heat and drive a steam turbine. “We’ve made mockups, but we’ve never made a working blanket,” Cowley says. Magnetic confinement reactors including ITER, the international project based in Europe, are also counting on blanket technology.
The peppercorn-sized fuel capsule is held within a type of cavity called a hohlraum at the center of NIF’s experimental setup.Lawrence Livermore National Laboratory
Fourth, the cost to run and maintain an inertial fusion reactor needs to decrease dramatically. Each of NIF’s hydrogen capsules costs hundreds of thousands of dollars and takes several months to develop. The capsules need to be made just right—imperfections the size of a single bacterium can impact performance. During the announcement, Alex Zylstra, the principal experimentalist for the 5 December shot, said that the research team would sweat laser alignment discrepancies on the order of trillionths of a meter and time discrepancies of 25 trillionths of a second. And currently, NIF’s lasers are so powerful that they damage their own guiding optics every time they fire.
Lastly and most crucially, even after all of those other concerns, it’s still not obvious whether inertial fusion can be commercially competitive. The technology is so early in its development that no large-scale estimates of commercial viability have been made.
To say there’s still plenty of work to be done is a bit of an understatement. But the project isn’t entirely hopeless. For one thing, the energy output may improve rapidly with small increases in laser power. “It’s almost a cliff once you start to self-heat. So [the energy output] rises very rapidly. For every inch you can give, it gives you another mile,” says Cowley. How much energy bang NIF will get for each additional laser power buck is something to watch closely in the coming years.
During the announcement, NIF officials made that case that at the end of the day, the goal of clean abundant energy is important enough to invest in every possible solution. Their most recent result finally puts inertial fusion on the map as a contender.
“There’s an awful lot of work to be done to make commercial fusion energy via magnetic [fusion] but it’s pretty certain that it can be done. But this result says that [inertial fusion] could also be a route to commercial fusion. And we didn’t know that two weeks ago,” Cowley says.