Fusion energy gain (Q) and target gain

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There are at least three quantities that public writing calls the same thing, and conflating them is the most common error about fusion. Q_plasma is fusion power out divided by heating power delivered into the plasma — JET's 1997 deuterium-tritium peak is usually quoted around Q ≈ 0.6, with lower values under quasi-steady conditions. Target gain, the inertial-confinement analogue, is fusion yield divided by the laser energy delivered to the target: NIF reported 1.5 on 5 December 2022 and 4.13 on 7 April 2025. Q_engineering, or wall-plug gain, is net electricity out divided by all electricity in, including magnets, cryogenics, lasers, pumps and control systems — no fusion device in history has exceeded a Q_engineering of one, and none has ever produced a watt of electricity from fusion. The three numbers can differ by two orders of magnitude for the same shot. A headline reporting one and implying another is not describing a different degree of the same achievement; it is describing a different achievement entirely.

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