Ignition
The state in which a plasma keeps burning on the heat of its own fusion products — the helium nuclei — without any outside heating.
The neutrons from D-T fusion carry no charge and escape straight through the magnetic field. The helium nuclei (alpha particles) stay in the plasma and heat the surrounding fuel. Once that self-heating outweighs the heat being lost, the reaction sustains itself like a spreading fire. That is ignition; in magnetic confinement it corresponds to Q becoming infinite.
The word is used a little differently across approaches. NIF, an inertial confinement facility, reported exceeding the Lawson criterion for ignition in an August 2021 shot[1], and in December 2022 released more fusion energy than the laser energy delivered to the target[2]. NIF calls the latter ignition.
In either sense, ignition is not the same as generating electricity. Ignition is a physics hurdle; beyond it lies the engineering hurdle of the whole plant's energy balance.
Related terms
- Lawson criterionThe condition that a plasma's density, confinement time and temperature must meet for a fusion reactor to produce net energy, derived by John Lawson in Britain in 1955.
- Q valueFusion energy released divided by the energy put in from outside to heat the plasma. Q = 1 is breakeven; Q becoming infinite corresponds to ignition.
- Inertial confinementCrushing a small pellet of fuel with powerful lasers or similar drivers, so that fusion happens in the tiny interval before the fuel flies apart.
On the timeline
Sources
- H. Abu-Shawareb, et al. (Indirect Drive ICF Collaboration), “Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment,” Phys. Rev. Lett. 129, 075001 (2022).
- H. Abu-Shawareb, et al. (Indirect Drive ICF Collaboration), “Achievement of Target Gain Larger than Unity in an Inertial Fusion Experiment,” Phys. Rev. Lett. 132, 065102 (2024).