The Fusion Record — Fusion Energy News ← Home · Knowledge base
Glossary

Energy Gain (Q)

The ratio of fusion power output to external heating power input — the single most important performance metric in fusion, with Q = 1 marking scientific breakeven and Q ≥ 10 the threshold for a practical power plant.

Reviewed Last reviewed: 9 Aug 2026 · Category: Glossary

Definition

Fusion energy gain Q is defined as the ratio of fusion power produced (Pfus) to the external heating power injected into the plasma (Pheat): Q = Pfus/Pheat. It is the standard metric for comparing fusion experiments and reactor designs.[1]

Key milestones: Q = 1 is “scientific breakeven” — the plasma produces as much fusion energy as is put in to heat it. ITER targets Q = 10 (500 MW fusion from 50 MW heating). Q = ∞ is “ignition” — the plasma sustains itself with no external heating.

Records

JET achieved Q ≈ 0.67 in its 1997 D–T campaign (16.1 MW peak fusion from ~24 MW heating). NIF achieved target gain Q ≈ 1.5 in December 2022 (3.15 MJ from 2.05 MJ laser on target), but the overall system gain (including laser efficiency) was about Q ≈ 0.01. No magnetic confinement device has yet achieved Q ≥ 1.[2]

Q vs. Engineering Q

The physics Q counts only heating power. Engineering Q (QE) is the ratio of gross electrical output to total electrical input (including magnets, pumps, cryogenics). A commercial power plant requires QE > 1, which typically demands physics Q ≥ 10–25 depending on recirculating power fraction and thermal efficiency.[3]

Sources

  1. Wesson, J. Tokamaks. 4th ed., Oxford University Press, 2011.
  2. Keilhacker, M. et al. "High fusion performance from deuterium-tritium plasmas in JET." Nuclear Fusion, 39, 209, 1999.
  3. Freidberg, J.P. Plasma Physics and Fusion Energy. Cambridge University Press, 2007.

Related

Fusion Power Gain Triple Product ITER Ignition Lawson Criterion