Fusion ignition is the point at which the energy released by fusion reactions heats the fuel mass more rapidly than it cools, thereby becoming self sustaining and removing/reducing the need for external heating.
Fusion ignition occurs when the heating power from alpha particles produced by fusion reactions in the hot spot at the center of the target capsule overcomes the cooling effects of x-ray losses, electron conduction, and implosion expansion. When enough alpha particles are "stopped," or absorbed, in the high-density fuel layer, a process known as alpha heating, a burn wave of fusion reactions propagates into the cold fuel surrounding the hot spot. When the energy deposition by the alpha particles contributes more than 50 percent of the heating of the fuel, a self-sustaining feedback loop known as a Burning Plasma results in an explosive amplification of energy output.
Lawrence Livermore National Lab (LLNL) achieved ignition for the first time ever on December 5th, 2022 at the National Ignition Facility.