The self-sustaining state in which energy released by fusion reactions alone maintains the plasma temperature, requiring no external heating—the ultimate goal of fusion energy research.
Ignition is the condition in which a fusion plasma is entirely self-heating: the energy deposited by fusion reaction products (primarily alpha particles in D–T fuel) is sufficient to maintain the plasma temperature against all energy losses. In terms of the fusion energy gain factor Q, ignition corresponds to Q → ∞.[1]
In a D–T reaction, the alpha particle carries 3.5 MeV and is confined by the magnetic field, depositing its kinetic energy in the plasma through collisions. For ignition, this alpha heating rate must exceed all loss channels. This requires n · T · τE ≥ 3 × 1021 keV·s·m−3 at the optimal temperature of ~14 keV.[1]
A burning plasma is one in which alpha-particle self-heating provides a significant fraction of the total heating power but does not necessarily replace external input entirely. ITER is designed for Q = 10, where alpha heating provides approximately two-thirds of the total power. This is a necessary precursor to ignition but is not ignition itself.[4]
In December 2022, NIF achieved target energy gain greater than unity—3.15 MJ of fusion energy from 2.05 MJ of laser energy on target. This is a verified experimental result, though the overall energy balance (laser system consumed ~300 MJ) remains far from engineering breakeven.[2]
No magnetic confinement device has yet achieved ignition. Demonstrating controlled ignition remains a central goal of the international fusion programme.[4]