ignition is the condition in which a 📝fusion plasma's own self-heating sustains the reaction without external heating power, achieved in a laboratory for the first time at the National Ignition Facility in December 2022.
Ignition is stronger than its near-neighbor, breakeven. 📝scientific breakeven means fusion power out equals heating power in — 📝Q (fusion energy gain factor) of 1. Ignition means the alpha particles born in the reaction deposit enough energy in the 📝plasma to keep it hot entirely on their own; external heating can be switched off, and Q formally rises toward infinity. Between the two lies the burning plasma, heated mostly but not wholly by its own reactions.
Usage differs by confinement approach, and the difference is worth attributing. In inertial confinement, the community adopted a 1997 National Academy of Sciences definition — fusion yield exceeding the laser energy delivered to the target — and by that standard the National Ignition Facility ignited on December 5, 2022, producing 3.15 megajoules from 2.05. In magnetic confinement, ignition retains the stricter self-sustaining sense, and no magnetic device has approached it. Power-plant designs do not actually require ignition: a controlled burn with modest external heating is easier to regulate and entirely sufficient for net power.