Skip to content
ContentLora

    Tip: press / anywhere to search.

    Explainer

    How fusion works

    Fusion releases energy when light nuclei such as deuterium and tritium merge into helium and a neutron, but only when the fuel is a plasma at around 150 million degrees Celsius.[1][2] Progress is measured by the Lawson triple product and by gain figures (target gain, plasma Q), which are easy to confuse with net electricity.[3][4][5]

    Editor reviewedUpdated Fusion energyPhysicsEnergy and climate

    The reaction

    Fusion happens when two light atomic nuclei merge into a heavier one.[6] A small amount of mass disappears in the process and comes out as energy, the same process that powers the Sun.[7] The easiest reaction to drive on Earth joins deuterium and tritium, two heavier forms of hydrogen. It produces helium and one fast neutron.[1] Kilogram for kilogram, this releases nearly four million times more energy than burning coal, oil or gas.[8]

    The deuterium-tritium (D-T) reaction is the most efficient one available in the laboratory.[1] Its neutron carries an energy of 14 MeV, which creates a materials problem for any plant built around it.[9] Deuterium can be distilled from any water.[10] Tritium has to be bred from lithium by neutrons, because the world’s civilian stock is only around 20 kilograms.[11][12] Alternative fuels such as deuterium-helium-3 are being pursued by some companies, including helion-energy.[13]

    Why it needs a plasma

    Nuclei are positively charged, so they push each other away. To make them collide hard enough to fuse, the fuel must be heated until electrons are stripped off the atoms, producing a plasma.[14] For D-T fuel in the laboratory, that means about 150 million degrees Celsius.[2] Keeping it that hot and dense for long enough is the central problem, solved either with magnetic fields or by rapid compression.[15][16]

    At high enough temperature, ions overcome their mutual electrostatic repulsion and fuse.[14] The practical temperature for D-T fusion is about 150 million degrees Celsius.[2] Confinement then splits the field into two families: magnetic fields that hold the plasma long enough for reactions to occur, as in a tokamak, and rapid heating and compression of a fuel capsule.[15][16]

    The Lawson criterion and the triple product

    In 1955 John D. Lawson estimated what it would take for fusion to give out more energy than is put in. He set a minimum for the product of plasma density, confinement time and temperature.[17] Researchers today track this “triple product” as their main performance metric.[3] Records are now set both for peak values and for how long they can be held. In 2025 the stellarator wendelstein-7-x held a record triple product for long discharges for 43 seconds.[18]

    Gain: target gain, Q and electricity

    “Gain” means energy out divided by energy in, but which energy counts makes a big difference. In December 2022 the national-ignition-facility got 3.15 megajoules out of a capsule hit by 2.05 megajoules of laser light. That was ignition.[19] The lasers, however, needed about 300 megajoules of electricity to fire.[5]

    Three gain figures are commonly quoted. Target gain is fusion yield divided by laser energy on target; NIF’s best is 4.13.[20] Plasma Q is fusion power divided by external heating power; iter aims for Q of at least 10.[4] Engineering or plant gain counts all the electricity a facility consumes. On that measure NIF falls far short, because its flash-lamp lasers have a wall-plug efficiency of only about 0.5 percent by one estimate.[5] Diode-pumped lasers could reach about 20 percent.[21]

    Why it is still hard

    Reaching fusion conditions is no longer the only obstacle. The US Department of Energy’s 2025 roadmap lists the remaining gaps: structural materials, plasma-facing components, confinement, the fuel cycle, blankets and plant integration.[22] The fuel cycle and materials explainer covers these in detail.

    Questions readers ask

    Why does fusion need such high temperatures?

    Nuclei are positively charged and repel each other. Only at very high temperatures, around 150 million degrees Celsius for deuterium-tritium fuel in the laboratory, do they move fast enough to overcome that repulsion and fuse.[14][2]

    What is the triple product?

    It is plasma density multiplied by temperature and energy confinement time. John Lawson showed in 1955 that it must exceed a minimum value before fusion gives out more energy than it takes in.[3][17]

    What does Q mean?

    Q is the ratio of fusion power produced to heating power put into the plasma. ITER aims for Q of at least 10, which means 500 MW of fusion power from 50 MW of heating.[4]

    Does ignition mean fusion now produces net electricity?

    No. At NIF, ignition means the fuel capsule released more energy than it absorbed. The lasers themselves draw about 300 to 400 megajoules of electricity per shot.[23][5]

    Sources

    Each numbered claim is a statement we checked against the sources listed with it. Status shows how well established it is.

    1. [1]

      The most efficient fusion reaction in the laboratory is between the hydrogen isotopes deuterium and tritium, which produces helium and one neutron. confirmedas of 2026-10-10

    2. [2]

      Deuterium-tritium fusion in the laboratory requires temperatures of about 150 million degrees Celsius. confirmedas of 2026-10-10

    3. [3]

      The triple product is the product of plasma density, energy confinement time and temperature, and is the standard figure of merit for fusion performance. confirmedas of 2026-10-10

    4. [4]

      ITER's main performance goal, Q of at least 10, means 500 megawatts of thermal fusion power from 50 megawatts of heating power put into the plasma, in 400-second pulses. confirmedas of 2026-10-10

    5. [5]

      NIF's lasers draw roughly 300 to 400 megajoules of electricity to deliver a 2-megajoule shot, so its ignition shots are far from overall energy break-even. confirmedas of 2026-10-10

    6. [6]

      Fusion, the process that powers the Sun and the stars, occurs when two light atomic nuclei combine to form a new, heavier nucleus, releasing energy. confirmedas of 2026-10-10

    7. [7]

      In fusion a tiny amount of mass is lost and converted into a large amount of energy, following E = mc². confirmedas of 2026-10-10

    8. [8]

      Controlled fusion releases nearly four million times more energy than a chemical reaction such as burning coal, oil or gas. confirmedas of 2026-10-10

    9. [9]

      A key materials challenge is building components that can withstand the 14 MeV neutrons from deuterium-tritium fusion, energies that fission-reactor experiments cannot easily mimic. confirmedas of 2026-10-10

    10. [10]

      Deuterium can be distilled from all forms of water. confirmedas of 2026-10-10

    11. [11]

      Tritium for future plants is to be bred when neutrons escaping the plasma react with lithium in a blanket lining the reactor wall. confirmedas of 2026-10-10

      • Fuelling · ITER Organization (retrieved 2026-10-10)
    12. [12]

      ITER puts the global tritium inventory at around 20 kilograms, which it will draw on during operation. confirmedas of 2026-10-10

      • Fuelling · ITER Organization (retrieved 2026-10-10)
    13. [13]

      Helion's long-term commercial fuel is deuterium and helium-3, with helium-3 to be produced in its own machines through a closed-loop fuel cycle based on deuterium-deuterium reactions and tritium decay. confirmedas of 2026-10-10

    14. [14]

      Fusion fuel is heated into a plasma, a gas so hot that electrons are freed from atomic nuclei; at high enough temperatures ions overcome their electrostatic repulsion and fuse. confirmedas of 2026-10-10

    15. [15]

      Magnetic confinement fusion holds the plasma with carefully designed magnetic fields long enough for fusion reactions to occur. confirmedas of 2026-10-10

    16. [16]

      Inertial confinement fusion heats and compresses a fuel capsule rapidly enough to reach the temperatures and densities needed for fusion. confirmedas of 2026-10-10

    17. [17]

      In 1955 the British physicist John D. Lawson estimated the parameters needed for fusion to give more energy out than is put in, setting a minimum for the product of plasma density, confinement time and temperature. confirmedas of 2026-10-10

    18. [18]

      On 22 May 2025 Wendelstein 7-X sustained a record triple product for long plasma discharges for 43 seconds, surpassing tokamak results for comparable durations. confirmedas of 2026-10-10

    19. [19]

      On 5 December 2022 the National Ignition Facility produced 3.15 megajoules of fusion energy from 2.05 megajoules of laser energy, the first laboratory fusion ignition. confirmedas of 2026-10-10

    20. [20]

      On 7 April 2025 NIF set records for yield and target gain, producing 8.6 megajoules (plus or minus 0.45) from 2.08 megajoules of laser energy, a target gain of 4.13. confirmedas of 2026-10-10

    21. [21]

      Diode-pumped lasers could reach efficiencies as high as 20 percent, and one laser-fusion company (Longview Fusion) aimed for 18 percent laser efficiency at 10 to 20 shots per second. confirmedas of 2023-01-20

    22. [22]

      The DOE roadmap identifies research, materials and technology gaps to close for a fusion pilot plant across structural materials, plasma-facing components, confinement systems, fuel cycle, blankets, and plant engineering and integration. confirmedas of 2026-10-10

    23. [23]

      LLNL defines ignition as a self-sustaining fusion reaction that produces more energy than the energy absorbed by the target capsule. confirmedas of 2026-10-10

    Revision history (2)
    1. Page created.
    2. Refresh: verbatim re-quoting of the ITER fuelling, mass-energy and tritium-supply excerpts behind this page's claims.

    Created Oct 10, 2026. Last reviewed by an editor on Oct 10, 2026. Next scheduled review: Jan 10, 2027.

    Cite this page

    "How fusion works." ContentLora, updated Oct 10, 2026. https://contentlora.com/explain/how-fusion-works

    Spotted an error? Suggest a correction or emailcorrections@contentlora.com.

    Keep exploring

    ☰ Contents