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Nuclear fusion would give us cheap and abundant energy. Energy is a $8.5 trillion energy industry. It also doesn't emit greenhouse gases (once built), and its residue has a half-life of a few hundred years (much less than nuclear fission). (source) It's also safer as it seems the fusion reactor would be incapable of generating the dangerous runaway chain reactions that lead to a meltdown (source).
On the other hand, "fusion reactors have other serious problems that also afflict today's fission reactors, including neutron radiation damage and radioactive waste, potential tritium release, the burden on coolant resources, outsize operating costs, and increased risks of nuclear weapons proliferation" (source). Also see: Pure fusion weapon — Wikipedia.
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 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. Ignition corresponds to infinite Q, and is generally regarded as highly desirable for practical reactor designs.
(source: Fusion energy gain factor)
When will a fusion reactor reach ignition?
This question will resolve on the date when a nuclear fusion reactor has sustained a reaction which produces more energy and heat than the external energy delivered to the system; or in other words, when a fusion reaction has become self-sustaining. This reaction may last for any length of time and release any non-zero amount of energy. Statements by the laboratory achieving this feat will be used for resolution, and such statements should be corroborated by credible scientific media reports.
[May 21, 2022, casens-- The resolution criteria have been edited. See the previous criteria in the fine print]
If a relevant Wikipedia page states that a fusion reactor has reached ignition, and continues to state that for at least 10 months in a 12 months period, this question gets resolved with the date mentioned on Wikipedia. If Wikipedia isn't active anymore, a paper replicating the fusion reactor ignition will confirm the resolution as the date at which the first paper reported ignition.