What is fusion ignition?
Fusion ignition is the point at which a fusion reaction heats its own fuel faster than the fuel loses heat, so the burn sustains itself. In laser experiments it is commonly reported as getting more fusion energy out than the lasers delivered.
Also known as: ignition, scientific breakeven
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How it works
Fusion forces light nuclei together to form helium and release energy. These experiments use the hydrogen isotopes deuterium and tritium. Some of that energy is carried by the helium nuclei, called alpha particles. They can deposit heat back into the fuel.
Lawrence Livermore National Laboratory (LLNL) defines ignition as the point at which this alpha heating in the hot center of the fuel outpaces cooling. It says cooling comes through x-ray emission, heat conduction and expansion. A wave of fusion burn then spreads into the surrounding fuel, LLNL says. The U.S. Department of Energy (DOE) describes the Sun as a plasma that has reached ignition, meaning fusion energy alone keeps it hot.
LLNL's National Ignition Facility (NIF) fires as many as 192 laser beams into a hohlraum, a hollow cylinder about a centimeter across. The resulting x-rays strip the outer layer from a peppercorn-sized fuel capsule. The implosion that follows compresses and heats the hydrogen fuel inside.
One word, two yardsticks
LLNL's FAQ describes ignition at NIF more simply: the fuel releases at least as much fusion energy as the lasers delivered to the target. LLNL calls the ratio of the two target gain. DOE also calls the milestone scientific energy breakeven.
Neither yardstick means net electricity. Osaka University physicist Shinsuke Fujioka wrote a February 2024 commentary in Physics Magazine, published by the American Physical Society. It says running NIF's lasers takes 300 to 500 megajoules (MJ) of electricity. That is far more than the fusion energy reported from the 2022 experiment, Fujioka wrote.
Where things stand in 2026
NIF first reached ignition on December 5, 2022, DOE announced that month. It said 2.05 MJ of laser energy delivered to the target produced 3.15 MJ of fusion energy.
LLNL's list of milestones reports that the result has been repeated. The highest yield it lists is 8.6 MJ from 2.08 MJ of laser energy, a target gain of 4.13, on April 7, 2025. The latest entry as of October 2026 is an 11th ignition shot, on June 20, 2026. LLNL says it yielded 7.9 MJ at a gain of about 3.8.
The international ITER project has a different target. DOE says the facility aims for a burning plasma in which self-heating power is twice the external heating power. Under DOE's definitions that falls short of ignition, which means fusion energy alone keeps the plasma hot.
Sources
- Achieving Fusion Ignition, Lawrence Livermore National Laboratory
- FAQs, Lawrence Livermore National Laboratory
- DOE National Laboratory Makes History by Achieving Fusion Ignition, U.S. Department of Energy
- DOE Explains...Burning Plasma, U.S. Department of Energy, Office of Science
- Nuclear-Fusion Reaction Beats Breakeven, Physics Magazine, American Physical Society
