An equation describing what conditions the plasma must achieve for thermonuclear fusion to generate enough usable energy. It was first formulated in 1955 by engineer John D. Lawson. It is a relationship of three quantities: temperature (T), density (n) and confinement time (τ), which is the time the fusion device can keep particles of the desired density and temperature in the reaction space. The product of density and confinement time is a function of temperature. For example, for the fusion of deuterium and tritium we obtain the relation:

n* τ > 1,5 * 10²⁰ s*m⁻³

From the results of this Lawson criterion, there are two ways how to reach economically viable fusion. The first is to keep plasma with relatively low density (n ≈ 10²⁰ m⁻³) but with a long confinement time (τ ≈ few seconds). This way leads to magnetic confinement. The second approach will need a shorter confinement time (τ ≈ 10⁻¹⁰s), but will require a higher density (n ≈ 10³⁰ m⁻³). This is exploited by inertial confinement.