How Does Nuclear Fusion Work?

Nuclear fusion is the process that powers the Sun and every star. It works by forcing light atomic nuclei to merge into a heavier one, releasing a tremendous amount of energy in the process. It is the opposite of fission, which splits heavy nuclei apart.

The core idea: mass becomes energy

When two light nuclei — such as the hydrogen isotopes deuterium and tritium — fuse, the resulting nucleus weighs slightly less than the two originals. That missing mass is converted into energy, following Einstein’s E = mc². Because c² is a huge number, even a tiny mass loss releases enormous energy.

The barrier: nuclei repel

Nuclei are positively charged, so they push each other apart. To fuse, they must collide fast enough to overcome that repulsion — which means extreme temperature and pressure:

• In the Sun, immense gravity provides the pressure, and the core sits at ~15 million °C.
• On Earth, we can’t match the Sun’s pressure, so we compensate with even higher temperatures — over 100 million °C — turning the fuel into a plasma.

How we try to do it on Earth

Two main approaches:

1. Magnetic confinement — hold the hot plasma in magnetic fields inside a tokamak or stellarator.
2. Inertial confinement — blast a tiny fuel pellet with powerful lasers to compress and heat it in an instant. In 2022, the National Ignition Facility used this method to achieve fusion ignition — getting more energy out of the reaction than the lasers delivered to the fuel.

Why we want it

Fusion fuel comes from water and lithium, produces no carbon emissions and no long-lived radioactive waste, and cannot melt down. The challenge is sustaining the reaction long enough, and cheaply enough, to make electricity. It is one of the great engineering quests of the century.

Related: what is a tokamak?

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