The ratio Q of fusion power produced to external heating power supplied—the single number that defines whether a fusion device crosses the threshold from energy consumer to energy producer.
ReviewedLast reviewed: 9 Aug 2026·Category: Concepts & Physics
Definition
The fusion energy gain factor, universally denoted Q, is the ratio of the total fusion power produced by a plasma to the external heating power supplied to sustain it:[1]
Definition:Q = Pfusion / Pheating
Key thresholds: Q = 1: Scientific breakeven Q ≈ 5: Approximate minimum for net electricity Q = 10: ITER design target Q → ∞: Ignition
Physical Meaning
As Q increases, a progressively larger fraction of the plasma heating comes from internal alpha particles. At Q = 5, alpha heating and external heating contribute roughly equally. At Q = 10, alpha heating provides about two-thirds. At ignition (Q → ∞), alpha heating alone sustains the burn.[1]
Scientific Q vs. Engineering Q
The standard Q considers only heating power delivered to the plasma, not total wall-plug electricity. For a fusion power plant to produce net electricity, Qeng > 1 is required, which generally demands Q well above 10.[3]
JET (EU, 1997):Q ≈ 0.67, producing 16.1 MW peak fusion power—the record for magnetic confinement D–T.
JT-60U (Japan, 1998): Equivalent QDT ≈ 1.25 extrapolated from deuterium-only plasmas (not directly verified with tritium).
NIF (USA, 2022): Target gain > 1 (3.15 MJ from 2.05 MJ laser energy on target; laser consumed ~300 MJ wall-plug).
ITER and Future Devices
ITER is designed for Q ≥ 10, producing ~500 MW from 50 MW of external heating. DEMO-class power plants generally target Q ≥ 25–50.[3]
Sources
Lawson, J.D. "Some Criteria for a Power Producing Thermonuclear Reactor." Proceedings of the Physical Society B, vol. 70, no. 1, 1957, pp. 6–10.
Keilhacker, M. et al. "High fusion performance from deuterium-tritium plasmas in JET." Nuclear Fusion, vol. 39, no. 2, 1999, pp. 209–234.
ITER Physics Expert Groups et al. "Chapter 1: Overview and summary." Nuclear Fusion, vol. 39, no. 12, 1999, pp. 2137–2174.
Wurzel, S.E. and Hsu, S.C. "Progress toward fusion energy breakeven and gain as measured against the Lawson criterion." Physics of Plasmas, vol. 29, 2022, 062103.