 A "Q" of at least one is needed for the reactor to generate net energy, but for practical reasons, it is desirable for it to be much higher.Once breakeven is reached, further improvements in confinement generally lead to a rapidly increasing "Q".That is because some of the energy being given off by the fusion reactions of the most common fusion fuel, a 50-50 mix of deuterium and tritium, is in the form of alpha particles.