The condition a fusion plasma must meet — enough density, held long enough — before a reactor can produce more energy than it consumes.
The Lawson criterion is the benchmark that tells you whether a fusion plasma can, in principle, power a reactor. First set out by the British engineer John D. Lawson in a 1955 classified memo and published in 1957,[1] it expresses a simple idea: to get net energy out, a hot fusion fuel must be both dense enough and confined long enough that the energy released by fusion reactions exceeds the energy lost from the plasma.
In its original form, the criterion is a threshold on the product of the plasma density n and the energy confinement time τE. For deuterium–tritium (D–T) fuel near the optimal temperature (~10–20 keV, i.e. ~100–200 million °C), the commonly cited threshold is:
Modern fusion research uses an extended figure of merit, the fusion triple product, which adds the plasma temperature T:
The triple product is the single number most often used to compare devices, because it captures the three levers a reactor must win at once: how much fuel, how hot, and how well held.[2] Exact threshold values depend on temperature and on whether the goal is break-even, ignition, or a full power-producing plant — distinctions this entry keeps explicit rather than blurring.
Every confinement approach — tokamaks, stellarators, inertial confinement, and the rest — is ultimately a different strategy for clearing the same bar. It reframes “is fusion working?” into a measurable question: how close is a given machine's triple product to the threshold, and is the result a claim or an independently confirmed measurement?
In December 2022, the National Ignition Facility reported the first laboratory fusion ignition, producing more energy from the fuel than the laser energy delivered to it — a milestone defined against exactly this class of criterion.[3]
Lawson derived the condition while at the UK's Atomic Energy Research Establishment. Because early fusion research was classified, his analysis circulated internally in 1955 before its 1957 publication, where it became one of the founding quantitative results of fusion energy science.[1]