About two dozen private firms globally are striving to harness a groundbreaking energy technology that could save the planet from climate disaster. One operates in a repurposed factory that once housed a defunct U.S. Department of Energy-funded research project in Cambridge, Massachusetts. Another is located in an industrial area near a Costco in Vancouver, and a third is found close to a self-storage facility in Orange County, California.

These companies aim to commercialize fusion energy, which holds immense promise. Fusion represents the most energy-rich power source: a single liter of fusion fuel can equate to 55,000 barrels of oil. The primary fuel source is virtually limitless—derived from water. In fact, just 2 cubic kilometers of seawater could theoretically yield energy comparable to all the oil reserves on Earth. "It’s a ubiquitous, inherently safe, zero-carbon energy source capable of powering the globe," states Matt Miller, president of Stellar Energy Foundation Inc., a nonprofit advocating for fusion energy development. "This is definitely worth pursuing."

It was approximately a century ago that scientists first recognized fusion as the process fueling the sun. Shortly after, efforts began to replicate it on Earth. From small-scale experiments, fusion quickly transitioned into large-scale scientific endeavors. Since 1953, the U.S. government has invested over $30 billion in fusion research, encompassing both fundamental science and military applications, according to Fusion Power Associates, another nonprofit. European nations, Russia, China, and Japan have also heavily invested in the quest for this ultimate energy solution.

Despite decades of research, past expectations for imminent breakthroughs have consistently fallen flat. Today, however, technological advancements are making fusion more attainable than ever.

Transforming theoretical concepts into practical technologies is becoming possible due to improvements in supercomputing and advanced modeling, explains Steven Cowley, director of the Princeton Plasma Physics Laboratory and former head of the U.K. Atomic Energy Authority. Historically, fusion was described as "the ideal method for energy generation, except for one issue: we didn’t know how to achieve it." Now, he asserts, "we do."

So, what exactly is fusion? The concept is straightforward: collide two atoms so they merge into a single, heavier element, releasing energy in the process. This is the antithesis of fission, the method employed in current nuclear reactors and the atomic bombs dropped on Hiroshima and Nagasaki.

In fission, a large, unstable atomic nucleus is split into smaller components, releasing energy. Conversely, fusion begins with light atoms. For instance, two hydrogen nuclei typically repel one another due to their positive charges. However, by applying sufficient heat and pressure, they can be brought close enough for the strong nuclear force to engage, merging them into a single helium nucleus. The resulting mass of this new nucleus is slightly less than the combined mass of the original hydrogen nuclei, with the mass difference converted into energy, as explained by Einstein’s famous equation E=mc². This process is fundamental to the universe and occurs in stars, including our sun.

Early attempts to harness fusion often led to disappointment and skepticism. Following World War II, an Austrian scientist who had worked in Germany convinced Argentine dictator Juan Perón to fund his fusion research. On a remote island in an Andean lake, Ronald Richter established a complex facility. In February 1951, he claimed to have detected heat from a thermonuclear reaction. The following month, Perón announced that Argentina had achieved unlimited energy through atomic harnessing. An investigation later revealed that Richter’s instruments had malfunctioned, leading to his discredit.

Many physicists doubted the initial claims, but the news sparked research initiatives in the U.S., U.K., and Soviet Union. At Princeton, a covert U.S. project focused on the H-bomb began exploring fusion technology, resulting in the development of a device called a stellarator, which used magnetic fields to confine superheated plasma. This effort eventually became the Princeton Plasma Physics Laboratory after declassification.

In the U.K., research on a machine named Zeta, which "pinched" fusion fuel using a substantial current, led to another premature announcement about entering the fusion age in 1958. Ultimately, researchers realized that instabilities in the fuel were responsible for their incorrect conclusions.

The Argentine announcement also accelerated efforts on a concept proposed by Soviet physicist Andrei Sakharov, who suggested confining fusion fuel in a toroidal configuration using a tokamak.

Since the 1960s, when government labs and universities began constructing tokamaks, over 200 operational machines have been built. A significant indicator of progress in fusion is the "triple product," which measures reactor performance based on temperature, density, and confinement time. When plotted over time, this improvement resembles Moore’s Law, suggesting that fusion technology is advancing even more rapidly. "Tokamaks have surpassed Moore's law," claims Bob Mumgaard, CEO of Commonwealth Fusion Systems, a spin-off from MIT.

Why is the temperature of a fusion system so crucial? Consider the sun: the immense gravitational force within it creates a pressure equivalent to that of approximately 333,000 Earths and a temperature around 15 million degrees Celsius (27 million degrees Fahrenheit)—conditions under which fusion naturally occurs.

On Earth, with significantly less gravity, achieving fusion requires much higher temperatures, such as 100 million degrees Celsius. The initial step involves heating a gas until it transforms into plasma, explains Michl Binderbauer, CEO of TAE Technologies Inc. "This occurs by adding energy until the ions and electrons that constitute the atoms disassociate into a charged soup," he elaborates. "This state is the most prevalent in the universe—what we refer to as plasma."

Most visible matter in the universe exists in plasma form. "We probably inhabit one of the few locations in the universe devoid of plasma, aside from phenomena like lightning," Binderbauer points out. Additionally, in the 1950s, the challenges posed by plasma instabilities led to the establishment of plasma physics as a discipline, which has since contributed to advancements in medicine and semiconductor manufacturing.

Heating plasma to 100 million degrees may sound intimidating. Wouldn't it incinerate anything it contacts? The concise answer is no. According to Binderbauer, the plasma consists of a handful of particles in a vacuum chamber, making it millions of times less dense than air. Its state is remarkably delicate; upon contact with any material, it rapidly cools. TAE’s Norman machine achieves plasma temperatures of 35 million degrees, and if one could hypothetically insert a hand into the vacuum shell, the plasma would not cause harm. "My arm would absorb the energy," he assures. "I wouldn’t even feel warm." Unlike fission, fusion poses no meltdown risk: "We need to shield the plasma from the surrounding environment, not the other way around," he explains.

Fusion also offers an essential advantage over solar, wind, and other intermittent renewable sources, according to Christofer Mowry, CEO of General Fusion Inc., based in Burnaby, B.C.: It provides "dispatchable" power. In most anticipated fusion applications, the energy produced will heat water to operate conventional steam turbine generators. These plants could be safely located in urban areas and other regions where power is necessary, Mowry states.

However, the significant downside to fusion, evident from its 70-year history and dashed hopes for quick breakthroughs, is its extreme complexity.

In 1983, the late Lawrence Lidsky, then an associate director at MIT’s Plasma Fusion Center, published an article titled "The Trouble With Fusion." He described fusion as "an exemplary problem for both scientists and engineers. Many consider it the most challenging scientific and technical issue ever confronted, yet it is gradually yielding to our efforts." Nevertheless, he compiled a list of obstacles that he believed made fusion unlikely to become an economically viable energy source.

More than 30 years later, the challenges identified by Lidsky persist. The principal concern remains radioactivity. Although fusion fuel does not present the same risks as fission's uranium and nuclear waste, understanding the radioactivity challenges of fusion necessitates a deeper exploration of the science.

Various light elements can be fused in a fusion reaction, but the most easily combined are two isotopes of hydrogen: deuterium and tritium (D-T). Deuterium, a heavy form of hydrogen, is abundant in seawater, while tritium is a radioactive isotope with a 12-year half-life; it is both rare and expensive but can be produced in fusion reactors.

When deuterium and tritium nuclei collide, they release energy in the form of an alpha particle (a helium nucleus) and a highly energetic neutron. These neutrons, being neutral, are not confined by the magnetic fields containing the plasma. They strike the materials of the tokamak's first wall, transferring heat and displacing atoms, which leads to damage and radioactivity.

Daniel Jassby, a retired researcher from the Princeton Plasma Physics Lab, notes that the continuous bombardment of neutrons from burning D-T will generate substantial radioactive waste. Replacing weakened first-wall materials will increase costs due to both the expense of new components and the downtime when the system cannot generate power. Additionally, the size of fusion machines could result in fusion reactors producing up to ten times more waste than traditional fission reactors. While the radiation levels may be less intense than those from spent uranium fuel rods, the byproducts from fusion systems can still be hazardous for a century rather than millennia.

True operating costs for fusion reactors may not be low enough to make them competitive with existing power plants, Jassby warns. "Why would anyone want this?"

Despite this, a certain idealistic vision has always underpinned the fusion pursuit. This enthusiasm may have spurred the 1985 agreement between U.S. President Ronald Reagan and Soviet leader Mikhail Gorbachev to collaborate on a fusion energy project, now known as ITER. This massive, long-delayed initiative involving 35 nations is currently under construction in southern France, where it is approximately 60% complete.

When ITER finally achieves its first plasma, expected in 2025, it is anticipated to reach a significant fusion milestone: producing more energy than it consumes. "There’s no one knowledgeable in the field who doubts that when they activate ITER, it will yield net energy," Mowry asserts. ITER is projected to generate 500 megawatts while consuming only 50. In fusion terminology, it will achieve a Q>1, specifically, a Q=10, as it is expected to generate ten times the energy input.

Within the plasma physics community, fusion's viability is unquestioned. Now, these startups are focused on constructing functional—and profitable—fusion power plants. "Private fusion ventures won't delve into fundamental plasma physics and fusion science," Mowry explains. "They build on decades of accumulated knowledge and are entirely focused on commercialization."

Here’s a brief overview of three such companies: