Ignition

The point where a fusion plasma's own reactions supply enough heat to keep it hot, with no more heating from outside needed. For laser-driven fusion, Lawrence Livermore National Laboratory defines it physically, as the point where heating by the fusion reactions' own alpha particles overcomes the fuel's cooling losses, and says the National Ignition Facility reached it on 5 December 2022. That shot was also the first to release more fusion energy than the laser energy delivered to the target, the simpler test the US Department of Energy used. NIF's peer-reviewed papers report that the physical threshold had already been crossed on a shot in August 2021 that released less energy than its laser delivered, so the two definitions give different dates. In magnetic confinement, ignition would mean a plasma needing no external heating at all, which no device has reached, and a power plant would not need to reach it: a high enough plasma gain is sufficient. The word is also used loosely in the press for any energy-positive result, so check which definition a claim uses.

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