Nuclear fusion powers every star in the universe, including our Sun, and humans have harnessed it in weapons, experimental energy reactors, and several less obvious applications. While large-scale fusion power plants remain under development, fusion already plays active roles in defense, scientific research, medical isotope production, and space mission planning.
Stars: The Original Fusion Engines
The most widespread use of nuclear fusion is entirely natural. Every main-sequence star generates energy by fusing hydrogen into helium through a process called the proton-proton chain. Four hydrogen nuclei combine to form one helium nucleus, and 0.7 percent of the original mass converts into energy. That small fraction, multiplied across trillions of reactions per second, is what makes the Sun shine. The core temperature required to sustain this process is about 15 million °C, with immense gravitational pressure keeping the fuel compressed.
Heavier stars fuse progressively heavier elements, producing carbon, oxygen, silicon, and eventually iron. When massive stars explode as supernovae, they scatter these elements into space, seeding the raw materials for planets and, eventually, life. Nearly every element in your body heavier than hydrogen was forged by fusion inside a star.
Thermonuclear Weapons
The first human-engineered use of fusion was the hydrogen bomb, tested in the early 1950s. A thermonuclear weapon is a two-stage device. The first stage is a conventional fission bomb (splitting atoms of uranium or plutonium), which generates intense X-ray radiation. That radiation compresses and heats a secondary stage filled with fusion fuel, typically isotopes of hydrogen. The compressed fuel reaches temperatures high enough for hydrogen nuclei to fuse, releasing far more energy than fission alone. The largest thermonuclear weapons have yields measured in megatons, thousands of times more powerful than the bombs dropped on Hiroshima and Nagasaki.
Experimental Energy Reactors
The promise of fusion energy is enormous: virtually limitless fuel, no carbon emissions, and far less long-lived radioactive waste than fission. Two main approaches are being pursued to make it work.
Magnetic Confinement (Tokamaks)
The leading approach uses powerful magnets to confine a superheated plasma of hydrogen isotopes (deuterium and tritium) inside a doughnut-shaped chamber called a tokamak. ITER, the international fusion project under construction in southern France, is designed to heat plasma to 150 million °C, ten times hotter than the Sun’s core. The goal is to inject 50 megawatts of heating power and get 500 megawatts of fusion power out, a tenfold energy return. ITER is an experimental reactor, not a power plant, but it is intended to prove that sustained, net-positive fusion energy is physically achievable at scale.
A smaller, faster-track project called SPARC, developed by MIT and Commonwealth Fusion Systems, uses high-temperature superconducting magnets to achieve similar plasma conditions in a more compact device. Its developers have projected that ARC, a follow-on power plant design, could produce electricity continuously by the early 2030s.
Inertial Confinement (Lasers)
The other major approach fires extremely powerful lasers at a tiny capsule of fusion fuel, compressing it so rapidly that the fuel ignites. The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in California has achieved ignition multiple times. In December 2022, NIF produced 3.15 megajoules of fusion energy from 2.05 megajoules of laser energy delivered to the target, crossing the ignition threshold for the first time. By April 2025, a single shot yielded 8.6 megajoules from 2.08 megajoules of laser energy, a target gain of 4.13. These results demonstrate that fusion ignition is repeatable and improving, though the total electrical energy needed to run the lasers is still far greater than the fusion energy produced.
Fuel: Where Fusion Gets Its Hydrogen
Most fusion reactor designs burn deuterium and tritium. Deuterium is abundant in seawater (roughly 1 in every 6,500 hydrogen atoms is deuterium), so supply is essentially unlimited. Tritium is far rarer. It is radioactive with a half-life of about 12 years, so it doesn’t accumulate naturally in useful quantities.
The solution is to breed tritium inside the reactor itself. When fusion reactions produce high-energy neutrons, those neutrons can be absorbed by lithium in a surrounding “breeding blanket,” generating fresh tritium that gets recycled back into the plasma as fuel. ITER plans to test this process using experimental blanket modules inside its vacuum vessel. If tritium breeding works reliably, a fusion power plant would need only lithium and water to run, both of which are plentiful.
Medical Isotope Production
Fusion reactors produce large numbers of high-energy neutrons, and researchers are exploring how to use those neutrons to manufacture medical isotopes. A feasibility study modeled on the China Fusion Engineering Test Reactor found that fusion-produced neutrons could generate isotopes used in diagnostic imaging and cancer therapy. Among the most significant is molybdenum-99, the parent isotope of technetium-99m, which is used in tens of millions of medical imaging scans worldwide each year. Other candidates include copper-64 for PET imaging and immunotherapy, strontium-89 and phosphorus-32 for treating bone cancer pain, and copper-67 for targeted cancer therapy.
Fusion reactors are particularly well suited for producing therapeutic isotopes that require fast neutrons, because the neutrons from a deuterium-tritium reaction carry much higher energies than those from conventional fission reactors. This could diversify the supply chain for critical medical isotopes, which currently depends on a small number of aging fission reactors.
Compact Neutron Sources
You don’t need a full-scale reactor to get useful fusion. Small tabletop devices called fusors use electric fields to accelerate and collide deuterium ions, producing a modest but steady stream of neutrons. These devices don’t generate net energy, but that’s not their purpose. University labs and engineering departments use fusors as neutron generators for materials testing, specifically to study how metals and alloys weaken after prolonged neutron exposure (a problem called embrittlement). Fusors also enable neutron activation analysis, a technique that identifies the elemental composition of a sample by bombarding it with neutrons and measuring the resulting radiation signatures.
Space Propulsion
Chemical rockets are powerful but inefficient for deep-space travel. Fusion engines could change that. The Direct Fusion Drive concept, developed from Princeton’s field-reversed configuration reactor, would use a compact fusion reaction to heat and expel propellant at far higher velocities than any chemical engine. Spacecraft studies have designed missions using this technology for destinations ranging from Mars to Pluto and even the Alpha Centauri star system. A fusion-powered spacecraft could cut travel time to Mars significantly compared to conventional propulsion, while also generating onboard electricity for instruments and life support. None of these engines have flown yet, but the physics is being actively tested on the ground.