Lawson criterion
The 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.
A hotter plasma fuses more readily, but it also loses heat faster. Lawson expressed the condition for fusion output to exceed those losses as a product of density and confinement time. It first appeared in a classified 1955 report and was published in 1957[1].
Later practice multiplies in the temperature to give the triple product (density × temperature × confinement time). For D-T fuel, a triple product above roughly 3×10²¹ keV·s/m³ is the usual threshold for ignition[2].
It is almost the only common yardstick for comparing machines of different designs by how close they are to practical use. Nearly seventy years on, every fusion device is still judged by where it stands against this condition.
Related terms
- IgnitionThe state in which a plasma keeps burning on the heat of its own fusion products — the helium nuclei — without any outside heating.
- 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.
- PlasmaA gas in which electrons have been stripped from their atoms, so that electrons and nuclei move around separately. Often called the fourth state of matter, after solid, liquid and gas.