Fusion Q&A
What is Q in fusion?
Short answer
Q (the fusion energy gain factor) is the ratio of fusion power produced to external heating power supplied. Q = 1 is scientific breakeven; Q > 5 is needed for a commercial plant; ITER targets Q = 10.
The Q ladder
Q_plasma = fusion power released ÷ external heating power into the plasma. NIF reached Q_plasma = 1.5 in 2022, then 1.9 in 2023 — the first Q > 1 in history.
Q_engineering = electrical power out ÷ electrical power in (counts laser/magnet inefficiencies). For a power plant, Q_engineering ~5 is the rule-of-thumb minimum.
ITER targets Q_plasma = 10. Commonwealth Fusion's SPARC targets Q > 2 sustained.
Frequently asked
- Has any fusion device achieved Q > 1?
- Yes — NIF, in December 2022 and again in July 2023.
- What Q does a commercial fusion plant need?
- Q_plasma > 10 is typical for a commercial reactor target.
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