Q_plasma vs Q_engineering names the distinction between plasma-level energy gain and whole-plant gain — the same letter Q drawn at two different boundaries, and the source of most public confusion about 📝fusion milestones.
Q_plasma is the conventional 📝Q (fusion energy gain factor): fusion power produced divided by the heating power absorbed by the 📝plasma, with Q_plasma of 1 marking 📝scientific breakeven. Q_engineering — also written Q_total or wall-plug Q — divides the electricity a facility delivers by the electricity the entire facility draws, and reaching 1 there is 📝engineering breakeven.
The gap between the two is large by construction. Heating systems convert wall-plug electricity into absorbed plasma heating at fractional efficiency; magnets, cryogenics, pumps, and controls draw power continuously; and turning fusion heat back into electricity loses roughly two-thirds of it. The National Ignition Facility's December 2022 shot makes the arithmetic vivid: a Q_plasma equivalent of about 1.5, but a Q_engineering near 0.01 once the roughly 300 megajoules feeding the lasers is counted. A claim about "net energy" is meaningless until the speaker says which Q it refers to — which is why careful writing in the field always names the boundary.