The ratio of fusion power produced to the external heating power injected into the plasma — the single most cited figure of merit for whether a fusion device is approaching energy relevance.
Fusion power gain, universally denoted Q, is defined as the ratio of total thermonuclear fusion power (Pfus) to the externally supplied heating power (Pheat) used to sustain the plasma:
Q = Pfus / Pheat
A device with Q < 1 consumes more heating energy than it liberates from fusion reactions. Q = 1 is termed scientific breakeven, the point at which fusion power output equals heating power input. Q > 1 indicates net energy production from the plasma itself, though it does not yet guarantee net electricity to the grid — that requires accounting for thermal conversion efficiency and recirculating power loads.
The Joint European Torus (JET) achieved Q ≈ 0.67 in 1997, the highest value recorded in a magnetic confinement device for decades.1 The National Ignition Facility (NIF) reported Q > 1 in laser-driven inertial confinement in December 2022, measuring fusion yield exceeding the laser energy delivered to the target.2 ITER is designed to demonstrate Q ≥ 10 in a burning deuterium-tritium plasma, producing 500 MW of fusion power from 50 MW of heating input.3
For a commercial fusion reactor, high Q reduces the recirculating power fraction and improves economic viability. Most power-plant studies target Q ≥ 20–30 to keep the recirculating fraction below 15–20 % and achieve competitive levelized cost of energy. The transition from Q = 10 (ITER-class) to Q ≥ 25 (commercial-class) involves reaching or approaching ignition, where alpha-particle self-heating dominates and external heating becomes a minor perturbation.4
Designers must also distinguish between steady-state Q and pulse-averaged Q. A pulsed device may reach high instantaneous Q but deliver lower time-averaged power, which affects capacity factor and economics.