A 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 a 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 breakeven, or in some sources, scientific breakeven. The energy 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 in fusion reactors is not expected to match the external sources until at least Q ≈ 5. If Q increases past this point, increasing self-heating eventually removes the need for external heating. At this point the reaction becomes self-sustaining, a condition called ignition, and is generally regarded as highly desirable for practical reactor designs. Ignition corresponds to infinite Q. Over time, several related terms have entered the fusion lexicon. Energy that is not captured within the fuel can be captured externally to produce electricity. That electricity can be used to heat the plasma to operational temperatures. A system that is self-powered in this way is referred to as running at engineering breakeven. Operating above engineering breakeven, a machine would produce more electricity than it uses and could sell that excess. One that sells enough electricity to cover its operating costs is sometimes known as economic breakeven. Additionally, fusion fuels, especially tritium, are very expensive, so many experiments run on various test gasses like hydrogen or deuterium. A reactor running on these fuels that reaches the conditions for breakeven if tritium was introduced is said to be at extrapolated breakeven. The current record for highest Q in a tokamak (as recorded during actual D-T fusion) was set by JET at Q = 0.67 in 1997. The record for Qext (the theoretical Q value of D-T fusion as extrapolated from D-D results) in a tokamak is held by JT-60, with Qext = 1.25, slightly besting JET's earlier Qext = 1.14. In December 2022, the National Ignition Facility, or NIF, an inertial confinement facility, reached Q = 1.54 with a 3.15 MJ output from a 2.05 MJ laser heating. NIF achieved ignition seven times. The highest gain as of 2025 of Q = 4.13 yielded 8.6 MJ from 2.08 MJ of laser energy.
Concept Q is the comparison of the power being released by the fusion reactions in a reactor, Pfus, to the constant heating power being supplied, Pheat, in normal operating conditions. For those designs that do not run in the steady state, but are instead pulsed, the same calculation can be made by summing all of the fusion energy produced in Pfus and all of the energy expended producing the pulse in Pheat. However, there are several definitions of breakeven that consider additional power losses.
Breakeven In 1955, John Lawson was the first to explore the energy balance mechanisms in detail, initially in classified works but published openly in a now-famous 1957 paper. In this paper he considered and refined work by earlier researchers, notably Hans Thirring, Peter Thonemann, and a review article by Richard Post. Expanding on all of these, Lawson's paper made detailed predictions for the amount of power that would be lost through various mechanisms, and compared that to the energy needed to sustain the reaction. This balance is today known as the Lawson criterion. In a successful fusion reactor design, the fusion reactions generate an amount of power designated Pfus. Some amount of this energy, Ploss, 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 Ploss = Pheat to maintain thermal equilibrium. The most basic definition of breakeven is when Q = 1, that is, Pfus = Pheat.
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