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Fusion energy gain factor, Q

The ratio of the energy produced by thermonuclear fusion to the energy required for heating the plasma to fusion temperature. If Q is less than 1, the fusion releases less energy than is required to ignite it. At Q = 1, scientific breakeven occurs. This is an important milestone in fusion research, but for an electricity producing fusion power plant, Q needs to be much higher. At Q > 5, the condition is referred to as ignition. Fusion generates as much energy as is needed to sustain it, and thus burns itself out like a fire in a fireplace. All the excess energy it produces can be used, for example, to generate electricity. Scientific breakeven was surpassed in December 2022, when the laser-driven inertial confinement facility NIF reached Q = 1.5.