Fusion gain

The fusion energy gain factor, usually expressed with the symbol Q, is the ratio of fusion power produced in a nuclear fusion reactor to the power required to maintain the plasma in steady state. The condition of Q = 1, when the power being released by the fusion reactions is equal to the required heating power, is referred to as scientific breakeven, one of four types breakeven measurements for fusion plasmas.

The power given off by the fusion reactions may be captured within the fuel, leading to self-heating. Most fusion reactions release at least some of their energy in a form that cannot be captured within the plasma, so a system at Q = 1 will cool without external heating. With typical fuels, self-heating does not match the external sources until about Q = 5.

As Q increases past this point, the increasing amount of self-heating eventually removes the need for external heating. At this point the reaction becomes self-sustaining, a condition known as ignition. Ignition corresponds to infinite Q, and is generally regarded as highly desirable for a practical reactor design.

As of 2017^[update], the record for scientific breakeven is held by the JET reactor in the UK, at Q = (16 MW)/(24 MW) ≈ 0.67, first attained in 1997. ITER was originally designed to reach ignition, but is currently designed to reach Q = 10, producing 500 MW of fusion power from 50 MW of injected thermal power.

Q is simply the comparison of the power being released by the fusion reactions in a reactor, P_fus, to the constant heating power being supplied, P_heat. However, there are several definitions of breakeven that consider additional power losses.

In a successful fusion reactor design, the fusion reactions generate an amount of power designated P_fus. Some amount of this energy, P_loss, is lost through a variety of mechanisms, mostly convection of the fuel to the walls of the reactor chamber and various forms of radiation that cannot be captured to generate power. In order to keep the reaction going, the system has to provide heating to make up for these losses, where P_loss = P_heat to maintain thermal equilibrium.

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