Lawson criterion is the threshold condition on density, temperature, and confinement time that a 📝plasma must exceed for 📝fusion to produce net energy.
The criterion comes from a power balance worked out by the British engineer John D. Lawson in 1955 and published in 1957: for a fusion plasma to come out ahead, the fusion power generated must exceed the power the plasma loses through radiation and heat leakage. From that balance Lawson derived a minimum value for the product of plasma density and confinement time at a given temperature — too thin a plasma fuses too rarely, and one that cools too fast never pays back the energy spent heating it. The modern field usually quotes the condition in its three-factor form, the triple product of density, temperature, and confinement time.
The criterion is a threshold with tiers rather than a single line. A lower value of the product yields 📝Q (fusion energy gain factor) of 1 — breakeven — while a value several times higher reaches 📝ignition, where the plasma heats itself entirely. Every fusion concept, magnetic or inertial, is ultimately a strategy for satisfying Lawson: tokamaks hold moderate densities for seconds, while laser fusion compresses fuel to enormous density for a fraction of a nanosecond.