Glossary / Nuclear fusion
Q value
Fusion energy released divided by the energy put in from outside to heat the plasma. Q = 1 is breakeven; Q becoming infinite corresponds to ignition.
At Q = 1 input and output balance (scientific breakeven). ITER aims for Q = 10. The magnetic-confinement record is JET's in 1997, at Q = 0.62[1] — a figure often quoted as 0.67.
The catch is that the number depends heavily on where the input is measured. NIF's 2022 gain of 1.5 is relative to the laser energy reaching the target, not the electricity the laser facility drew, which was more than a hundred times larger.
A power plant needs an engineering Q — counting all the electricity the facility uses — well above 1. When a headline announces breakeven, it is worth asking which Q is meant[2].
Related terms
- IgnitionThe state in which a plasma keeps burning on the heat of its own fusion products — the helium nuclei — without any outside heating.
- Lawson criterionThe condition that a plasma's density, confinement time and temperature must meet for a fusion reactor to produce net energy, derived by John Lawson in Britain in 1955.
- D-T fusionThe fusion of deuterium (D) and tritium (T) into a helium-4 nucleus and a neutron. Almost every fusion reactor now under development is built around this reaction.