FUSION IS THE ENGINE OF THE UNIVERSE. The smallest microbe to the most complex life found on alien worlds exist because of the cataclysmic physics happening at the heart of every star. Every pinprick of light in the night sky is a mixture of immense gravity and heat creating a self-sustaining thermonuclear explosion that makes life possible.
For nearly a century, scientists have known that this explosive dance of ionized plasma at the heart of our sun is what bathes our planet in light and heat. And since that discovery, universities, laboratories, government agencies, and international coalitions have invested billions into finding some way to bottle the sun and generate clean, limitless energy.
Dozens of private companies funded by some of the richest people in the world are forging ahead with varying ways to create commercial fusion. One of the newest firms in the race for limitless energy wants to harness the laser-blasting tech behind America’s greatest fusion breakthrough to date, while other firms are taking a different route that involves magnets, superconductors, and super-hot plasma.
No matter which approach wins out, if we hope to make commercial fusion a reality within the coming decades, it’ll take a level of scientific dedication (and funding) that would make the Apollo program look like a high school science project.
This is the story of nuclear fusion—how it works, where it’s headed, and what kind of human society it’ll leave in its wake.
FOR ALL THE GRANDIOSE PROMISES OF NUCLEAR FUSION, the science at its heart happens on an extremely small atomic scale. At its most basic level, nuclear fusion occurs when two light nuclei (i.e. hydrogen) combine or fuse together to form a heavier isotope called helium-4. Meanwhile, fission, which is the science that powers all nuclear reactors today, is sort of the opposite: a neutron particle slams into a larger atom, such as Uranium-235, and splits it into two smaller ones, like barium and krypton.
“Chemistry is basically the story of atoms trying to be more stable, and they’re going to find partners to react with to be more stable,” says Vincent Tang, Ph.D., principal deputy director at the National Ignition Facility and Lawrence Livermore National Laboratory. “When they find a partner that makes them more stable, they release energy as heat or light—the same analogy holds for nuclear reactions. If they partner up and they fuse, or fiss, they want to become more stable—they want to become Iron-56.”
Easier said than done, because these two hydrogen protons desperately don’t want to fuse. Under normal conditions, two positively charged protons would simply repel due to electrostatic repulsion, basically the atomic version of trying to force two negative poles of a bar magnet together; it’s possible, but it’s not easy.
To force two nuclei together (a single proton is the nucleus of a hydrogen atom), they must overcome this repulsion so that the strong nuclear force—one of the four fundamental forces in the universe that holds together an atom’s nucleus—takes over and fuses the two hydrogen atoms together.
To do that, things need to get hot, like plasma hot, because heat excites atoms, breaks them apart, and speeds them up so that they overcome this mutual electrostatic resistance. Even the heat at the center of the sun isn’t hot enough to normally fuse these elements, but a concept known as quantum tunneling allows a fraction of hydrogen protons to fuse.
Here’s where these unseen atomic and quantum processes can create something as incredible as our sun. When two hydrogen atoms fuse in the core of the sun, the resulting helium atom weighs less than the two original hydrogen atoms. As Albert Einstein and his famous equation E = MC² tells us, mass doesn’t simply disappear, but is instead converted into energy.
To achieve a more efficient reaction at lower temperatures, scientists instead use two hydrogen isotopes called deuterium and tritium to fuse into helium (while also producing one spare neutron).
Creating this initial fusion reaction—and, most crucially, sustaining it—has been the work of nuclear physicists around the world for nearly a century. Although fusion reactors can come in many shapes and sizes, they essentially fall into three categories, defined by how each machine confines the super-hot plasma needed to create fusion reactions: gravitational, inertial, and magnetic confinement fusion reactors.
A GRAVITATIONAL CONFINEMENT FUSION REACTOR is a fancy way of saying “the sun.” Because of its immense size, the sun is essentially a burning cocktail of heat and pressure, which provides the perfect environment for fusion. Fusion in the sun follows a multi-step thermonuclear reaction process (or nucleosynthesis), known as the proton-proton chain.
In this process, four hydrogen protons and two electrons fuse in a multi-step process and eventually create Helium-4, two electron neutrinos, and six gamma rays (which we eventually experience as sunlight on Earth). Helium-4, an isotope of helium, has a remarkably stable nucleus; it’s also an outlier on the nuclear binding energy curve, a graph that shows how tightly atomic nuclei are held together across different elements (see below). So, when atoms fuse to form helium-4, even more energy is produced, and though these reactions don’t have a high probability of fusing, the sun’s “too big to fail” philosophy makes up for these energy inefficiencies.
“The sun works in a very different fusion regime,” Tang explains. “The confinement time is really large in the sun … [there’s also] a much lower probability for each fusion event, but that’s ok because it’s so huge and there’s so many nuclei in the sun.”
The sun experiences 100 million quadrillion quadrillion fusion reactions every second, and has been undergoing this form of nucleosynthesis for five billion years. In another five billion years, the sun will exhaust the hydrogen responsible for those initial reactions in the proton-proton chain, and the star will start burning helium, thus transitioning to a Red Giant (and having the unfortunate side effect of destroying Earth).
The sun provides an awe-inspiring blueprint for how to achieve nuclear fusion, but there are few problems that make mimicking the sun an impossibility. For one, Earth is nowhere near massive enough to rely on gravity for confinement, and because of this gravitational deficiency, any terrestrial fusion reactor would also need to be many times hotter than the sun to be effective.
Nuclear physicists and engineers have worked for nearly a century trying to overcome these physical limitations, and they’ve come up with basically two possible solutions. One uses magnets, the other, inertia.
CONTAINING PLASMA IS A TRICKY BUSINESS. This soup of electrons, protons, and neutrons needs to push the mercury to at least 100 million degrees Celsius (for deuterium-tritium reactions), but getting plasma that hot isn’t necessarily the hard part. Making sure that plasma stays confined and doesn’t touch anything else—well, that’s another story.
“For fusion, you have to do three things—you have to get enough particles together, you have to get them hot enough, and you need to hold them long enough for the reaction to take place,” says Phil Ferguson, Ph.D., director of the Material Plasma Exposure eXperiment (MPEX) Project at Oak Ridge National Laboratory. “You need a material solution. Give me the materials that can hold this thing together, at temperature, to be efficient.”
Enter the tokamak, a fusion machine of unimaginable sophistication. Designed by scientists in the Soviet Union in the late 1950s, the name “tokamak” is a Russian acronym for “toroidal chamber with magnetic coils.” Toroidal is just a wonky physics way of saying “donut-shaped,” but it’s the “magnetic coils” part that is the most important. At their most basic, tokamaks use a meticulously designed array of electromagnets, as well as an electromagnetic pulse in the plasma itself, to create a contained fusion reaction—emphasis on the word “contained.”
“You don’t really want that really hot plasma touching the metal walls because it will damage the walls, but it’ll also damage the plasma,” says Wayne Solomon, Ph.D., vice president of General Atomics’ Vice Magnetic Fusion Energy Division. “You won’t be able to keep it hot if you’re touching something cold.” (General Atomics operates the largest fusion tokamak in the U.S., known as D-III D.)
Speaking of something cold, these magnets are superconducting, meaning they experience no electrical resistance whatsoever; theoretically, an electric current could exist within a superconductor forever. However, many superconducting materials can only operate at temperatures approaching absolute zero, or -459.67 degrees Fahrenheit. So magnetic fusion reactors effectively need to contain some of the most extreme temperatures in the universe at the same time.
“At the center of the device, you’ve got something that can be ten times hotter than the center of the sun,” Solomon says. “If you continue toward the wall, you’re around room temperature, and then you get to the actual magnets, which are around absolute zero.”
Tokamaks and another magnetic confinement ideas, known as “stellarators,” mainly differ in how the magnets contain the plasma, but both work on the underlying principle of using superconducting magnets to bottle a star.
So once you have this suspended, contained plasma inside a vacuum chamber, how exactly does it power the lights in your home? Between the plasma and magnets is a layer of complicated technology with a surprisingly low-tech name: a blanket. But these “blankets” are not for snuggling. For example, the International Thermonuclear Experimental Reactor (ITER) in southern France, one of the most advanced tokamaks on the planet, will use 440 blanket modules (weighing in at 4.6 tonnes each) to transfer fusion energy into usable electricity.
Covered in beryllium, these blankets collect the kinetic energy of neutrons and convert that energy into heat (and they can also breed tritium, more on that later). That heat is transferred to water coolant, which is then used in turbines powered by electromagnetic induction, which is how every power plant—whether coal or nuclear—works today. This is one of the most technically challenging aspects of magnetic confinement fusion because it directly interacts with plasma.
“The plasma is 100 million degrees and you want to contain it in this bottle, but if you’ve ever seen pictures of the sun, you get instabilities—little flares come out,” says Ferguson, who is currently designing plasma-rated materials for ORNL’s MPeX project. “At 100 million degrees, those flares touch this wall. You only have to handle it for an instant, but even for an instant, 100 million degrees is pretty intense.”
More than 60 years after their inception, tokamaks are still considered by many fusion scientists, as well as the U.S. Department of Energy, as the leading magnetic fusion concept, as they’re currently the most adept at confining plasma and keeping it hot—a must-have feature for any future fusion power plant.
But there’s another fusion idea that takes a radically different approach, and this machine has been able to achieve something completely unprecedented in the century-long history of fusion science.
ON DECEMBER 5, 2022, SCIENTISTS AT THE NATIONAL IGNITION FACILITY at Lawrence Livermore National Laboratory in California fired 192 lasers at a small pellet and essentially squeezed the deuterium-tritium fuel inside. Like many times before, the inertial confinement experiment generated only a few nanoseconds of fusion, but this time, something was different. This time, the experiment got more energy out of the reaction than it had put in.
Humanity had finally achieved ignition.
“Some of that fusion energy stops in the plasma and then it makes it even hotter. … Now, it’s even more likely for there to be more fusion,” Tang says. “That’s when you have ignition, when the fusion reaction is starting to bootstrap itself enough that it’s producing more energy than the plasma uses.”
Unlike magnetic confinement reactors like tokamaks and stellarators, inertial confinement ditches magnets and instead relies on the implosion of the fuel pellet itself to sustain the reaction. After NIFs 192 infrared lasers traveled through a complicated series of laser banks and power amplifiers, increasing the lasers’ combined energy to a petawatt (one quadrillion watts), the beams were converted into ultraviolet rays that converged on a small capsule known as a hohlraum (German for “hollow area”).
This superheated capsule created an x-ray bath that finally squeezed the spherical deuterium-tritium pellet inside. Collapsing at roughly 250 miles per second, the fusion reaction took place before the fuel could disassemble, so in a sense, the plasma was effectively contained by its own inertia. The length of this “containment” lasted only 100 trillionths of a second, but it was long enough to produce a significant amount of energy.
During NIF’s now-famous ignition shot, the facility delivered 2.05 megajoules (MJ) of energy to the target and produced 3.15 MJ, which means the fusion reaction was, for a short time, fueling itself. Although these numbers seem small, Tang argues that the result is even more impressive than it seems at first glance.
Because energy is lost when transferring ultraviolet lasers into x-rays inside the hohlraum, the actual amount of energy used to start the fusion reaction was actually around 250 to 300 kilojoules, roughly twelve times less than the energy gained from the reaction. While the NIF experiment uses an indirect drive process, other inertial confinement reactors are experimenting with direct drives, which deliver the laser’s energy directly to the deuterium-tritium capsule (though these concepts also suffer other kinds of energy loss).
This entire process lasts for much less than a blink of an eye, so any future commercial reactor based on this technology would use this same process but deliver about 10 of these mini-explosions a second—essentially creating microscopic sun for just a few nanoseconds at time.
“The NIF technology is from the 80s and 90s … and it wasn’t required to run 10 times a second,” Tang says, emphasizing that one of the laser’s main missions was for actually testing nuclear weapons—not creating the next-generation of clean energy. “NIF is not efficient … if you want to go 10 times a second, you’d use laser diodes.”
Luckily, there’s already a laser that can deliver those 10 beams per second, and it’s called the High-Repetition-Rate Advanced Petawatt Laser System (HALPS). Because NIF is designed using flash lamp technology, the system requires a significant cooldown period between uses. HALPS, on the other hand, uses advanced laser diodes to deliver the same amount of energy but with hardly any cooling at all.
Denver-based Xcimer Energy is fully committed to the inertial fusion approach, focusing on the power of the lasers themselves. The team hopes to create the world’s largest laser, based on technology originally designed in the “Star Wars” Strategic Defense Initiative of the 1980s. This machine will be a krypton fluoride laser that’s a combination of “gas” optics and eximcer laser amplifiers, which are already used in semiconductor lithography and other industrial applications. The results will (hopefully) pump out high beam energies that produce “10 times higher laser energy at 10 times higher efficiency and over 30 times lower cost per joule than the National Ignition Facility (NIF) laser system,” according to a June 2024 press release.
But there’s a long road of advancements, efficiency improvements, and groundbreaking material science that needs to happen before NIF’s ignition breakthrough transforms into an inertial confinement power plant. But after finally achieving ignition, the story of inertial fusion is heading into a new era.
“This is just the end of the beginning,” Tang says. “There’s still so much to do.”
AS WITH MANY ENGINEERING CHALLENGES, things aren’t as black and white as “team magnet” and “team inertia.” Some proposals, like the magneto-inertial fusion borrows a little bit of both while some inertial fusion concepts, like magnetized liner inertial fusion, throw in a teeny bit of magnetism to sustain inertial fusion reactions for longer.
With ignition finally achieved, physics shows that bottling the stars on our planets is possible, but can we develop the exotic materials and incredible technology to make it possible?
“Really sustaining the fusion core, getting the performance as high as possible so you can make the device as efficient, compact, and cost effective as possible is really important,” Solomon says. “Now’s the time to really step up the effort on the technology side of things.”
For one, engineers are still trying to build a working blanket—the technology that essentially turns all this fusion goodness into electricity we can use. Secondly, scientists are developing materials that can withstand the intense temperatures found within these machines, whether tokamaks, stellarators, or inertial confinement reactors. “We are still lacking a breakthrough in materials,” Ferguson says.
Apart from neutron-capturing technologies and high-tech materials, fusion scientists and engineers still need to figure out how to close the deuterium-tritium fuel cycle. While deuterium is abundant on Earth, tritium is much more rare. Making more tritium could make reactors both economically viable and more efficient—two things any fusion reactor needs if it hopes to escape the lab.
Meanwhile, interest in the potential of fusion energy is heating up. Investment in fusion technology skyrocketed in 2022, and private companies are beginning to investigate ways to bring this technology to market.
General Atomics also announced plans in October 2022 to build a fusion pilot plant. ITER, the largest magnetic confinement fusion reactor, is scheduled to achieve “first plasma” by 2025. The successor of ITER, called the Demonstration power plant (DEMO), is already under construction, and is designed to bridge the gap between lab-based experiments and commercialized energy. However, this project isn’t scheduled to go online until the 2050s.
Meanwhile reactors around the world, like the Joint European Torus (JET) in the U.K., the JT-60SA tokamak in Japan, and the D-III D in the U.S. continue investigating the unknown mysteries of fusion.
But that future is coming.
“This is the energy of the universe,” Tang says. “If we figure out how to harness this effectively and efficiently, this is it. This is the end. This is the solution. Everything we understand about how the universe works, we’ve now figured out how to leverage its fundamental source of energy.
“We’ve got to keep pushing forward.”
Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.
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