The ratio of fusion power produced to heating power input — the single most important performance metric for a fusion device, with Q = 1 being breakeven and Q > 5 needed for a power plant.
Fusion gain Q is defined as the ratio of total fusion power (Pfus) to externally supplied heating power (Pheat): Q = Pfus/Pheat. A Q of 1 means breakeven — as much power out as in. ITER targets Q = 10 (500 MW fusion from 50 MW heating). A commercial reactor needs Q > 20–50 to be economically viable after accounting for all plant systems.[1]
Physics Q (QDT): Ratio of D-T fusion power to injected heating power — the standard metric. Engineering Q (Qeng): Ratio of net electrical output to total electrical input — must exceed 1 for a power plant to be useful. Qeng accounts for heating efficiency, magnet power, cooling, tritium processing, and all other plant loads. Target gain: In ICF, ratio of fusion energy to laser energy on target (ignoring laser wall-plug efficiency).[2]
At Q = ∞, the plasma is self-sustaining — alpha-particle heating alone maintains fusion temperature with no external heating. This is ignition. ITER’s baseline scenario (Q = 10) is not ignition but burning plasma, where alpha heating dominates but external heating is still required for control.[3]