Q (fusion gain, use with caution)

Q

The letter commonly used in coverage and by companies for fusion gain, energy out divided by energy in. It is ambiguous on its own, because a fusion experiment spends energy at several different stages and each stage gives a different answer: the same NIF shot in December 2022 was a target gain (Q_target) of 1.5 and a wall-plug gain below 0.01. By convention Q usually means plasma gain in magnetic confinement and target gain in laser fusion, but companies also use it for other ratios, and 'Q > 1' is often stated without saying which. This site never publishes a bare Q. Every gain figure is filed as one of six named kinds, each with its numerator and denominator stated: target gain, capsule gain, plasma gain, equivalent gain, engineering gain or wall-plug gain. The tokamak safety factor, lowercase q, is a different quantity.

Vocabulary id

concept:q-factor-fusion

Scheme

Concepts

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