How Fusion Energy Works

In a research facility in southern France, 35 nations are collaborating on the most ambitious energy project in human history. The ITER tokamak aims to recreate the process that powers stars, here on Earth. If successful, it could provide humanity with an almost limitless source of clean energy. The challenge is that the sun achieves fusion at 15 million degrees Celsius in its core, and replicating those conditions on our planet has taken scientists decades of painstaking research.

The short answer

Fusion energy works by forcing two light atomic nuclei, typically forms of hydrogen, to collide and merge at extremely high temperatures and pressures. When they fuse, the resulting nucleus is slightly lighter than the sum of the original particles, and that lost mass converts directly into enormous amounts of energy according to Einstein’s famous equation E=mc2. Unlike fission, which splits heavy atoms, fusion joins light ones together, and it produces no long-lived radioactive waste.

The full picture

How stars work as fusion reactors

Stars are natural fusion reactors. In the core of our sun, immense gravitational pressure forces hydrogen nuclei to overcome their natural repulsion and merge into helium. This process, called the proton-proton chain, releases energy that travels outward as light and heat. A single kilogram of hydrogen fused into helium releases about 275 trillion joules of energy, equivalent to burning 8,000 tonnes of oil.

The key to making fusion work on Earth is achieving what scientists call “ignition”: the point where the fusion reactions generate enough energy to sustain themselves. This requires extremely high temperatures (over 100 million degrees Celsius), sufficient plasma density, and long enough confinement time for the nuclei to collide and fuse.

The challenge: containing plasma

At fusion temperatures, matter exists not as a gas but as plasma: a fourth state where electrons are stripped from atoms, leaving a soup of positively charged ions and free electrons. This plasma is hotter than the core of the sun, and no material on Earth can contain it directly.

Instead, scientists use magnetic fields to confine the plasma in a donut shape called a torus. Since charged particles spiral around magnetic field lines, powerful magnets can trap the plasma in a contained space without it touching the walls of the reactor. The challenge is that plasma is notoriously unstable, prone to turbulence and sudden energy losses that have confounded physicists for decades.

Magnetic confinement vs inertial confinement

There are two main approaches to achieving fusion on Earth. Magnetic confinement uses powerful magnetic fields to contain plasma in devices called tokamaks or stellarators. The magnetic approach has received the majority of research funding because it can theoretically sustain continuous fusion reactions.

Inertial confinement takes a different approach: firing multiple powerful lasers at a tiny fuel pellet from all directions simultaneously. The lasers compress the pellet to extreme densities in a fraction of a second, creating the conditions for fusion before the pellet flies apart. The National Ignition Facility in California achieved a major milestone in 2022 by producing more energy from fusion than the laser energy input, a breakthrough called net energy gain.

Tokamaks and the progress

The tokamak, invented by Soviet physicists Andrei Sakharov and Igor Tamm in 1951, remains the leading concept for fusion power plants. In a tokamak, magnetic field coils confine plasma particles while a central solenoid induces an electric current that heats the plasma. The first tokamak began operating in Russia in 1958.

The Joint European Torus (JET) in England achieved a major milestone in December 2021, producing 59 megajoules of fusion energy over 5 seconds, more than doubling the previous record set in 1997 Department of Energy. ITER, currently under construction in France, aims to produce 500 megawatts of fusion power and demonstrate that a burning plasma is achievable at scale. A prototype commercial fusion reactor called DEMO is expected by 2040, with widespread electricity generation anticipated in the second half of the century.

Fusion vs fission

Nuclear fission, used in today’s power plants, splits heavy uranium atoms to release energy. Fusion, by contrast, joins light hydrogen atoms. The differences are profound. Fusion produces no carbon dioxide emissions, and its fuel sources (hydrogen and lithium) are abundant worldwide. Most importantly, fusion is inherently safe: unlike fission, it cannot sustain a chain reaction. If the plasma cools or loses containment, the reaction stops instantly. According to the International Atomic Energy Agency, fusion reactors are considered inherently safe because they require continuous external heating and magnetic confinement to maintain the reaction International Atomic Energy Agency.

Fission produces radioactive waste that remains dangerous for thousands of years. Fusion produces helium, an inert gas, and the small amounts of tritium fuel are used in a closed circuit within the reactor. According to the International Atomic Energy Agency, fusion reactors are considered inherently safe because they require continuous external heating and magnetic confinement to maintain the reaction.

Why it matters

The global energy demand is projected to nearly double by 2050 as developing nations industrialize and populations grow. Fusion energy could provide a virtually limitless source of baseload power without the carbon emissions driving climate change. Unlike solar and wind, fusion produces consistent power regardless of weather or time of day, making it an ideal complement to renewable energy sources.

The economic stakes are enormous. A single fusion power plant could generate electricity for millions of homes without producing greenhouse gases or long-lived radioactive waste. Countries and private companies that crack fusion first could dominate the global energy market for decades. Over $6 billion in private capital has flowed into fusion startups since 2022, reflecting confidence that commercial fusion is now a matter of “when” not “if”.

Beyond electricity, fusion could transform space exploration. Nuclear thermal rockets using fusion principles could cut travel times to Mars in half, and fusion-powered spacecraft could reach distant destinations that chemical rockets cannot.

Common misconceptions

“Fusion is just around the corner, like it has been for 50 years”

While fusion has been “30 years away” for decades, the progress is real and accelerating. The past five years have seen multiple historic breakthroughs: net energy gain in inertial confinement, record fusion power in magnetic confinement, and unprecedented private investment. ITER is no longer a speculative project but an under-construction facility with concrete milestones. The phrase “always 30 years away” ignores the genuine scientific and engineering advances that have brought fusion closer to reality.

“Fusion requires temperatures hotter than the sun”

This is technically true but misleading. While fusion reactions require extreme temperatures, the “temperature” of plasma is not the same as the heat we experience. Plasma temperature measures particle velocity, not thermal energy in the everyday sense. A plasma at 100 million degrees Celsius contains far less thermal energy than boiling water, which is why containing it with magnetic fields is possible. The real challenge is not just heat but maintaining the right combination of temperature, density, and confinement time.

Key terms

Plasma: The fourth state of matter, where electrons are stripped from atoms, creating a soup of ions and free electrons that can undergo fusion.

Tokamak: A donut-shaped device that uses magnetic fields to confine plasma for fusion reactions, the leading approach for magnetic confinement fusion.

Net energy gain: The point where a fusion reaction produces more energy than the energy input required to maintain it, a critical milestone achieved in 2022.

Ignition: When fusion reactions become self-sustaining, generating enough energy to maintain the high temperatures needed for continued fusion without external heating.

Tritium: One of three hydrogen isotopes used as fusion fuel, radioactive but short-lived, produced within fusion reactors in a closed cycle.