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Ignition

(Science: radiobiology) in fusion, as in an ordinary (chemical) fire, ignition is the point where the temperature and confinement of heat in the fuel (plasma in the case of fusion) are such that energy released from ongoing reactions is sufficient to maintain the temperature of the system, and no external heating is needed. An ignited fusion plasma produces so much energy from fusion reactions that the plasma is fully heated by fusion reaction products (alpha particles in the case of D-T fusion), and the plasma no longer needs any external source of power to maintain its temperature. (The plasma may, however, still need something to maintain its confinement, this gives us control over the fusion reaction and helps prevent fusion reactors from having meltdown problems like fission reactors.)