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What does plasma ignition mean in a fusion reactor?

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3D illustration of the interior reaction chamber of a tokamak fusion reactor, showing a large, cylindrical, metallic structure with curved walls and bright light panels. — fusion reactor A tokamak is a device used in nuclear-fusion research for magnetic confinement of plasma.

In a fusion reactor, plasma ignition occurs when the energy released by nuclear fusion reactions is sufficient to maintain the plasma's temperature against all energy losses, such as convection, radiation, and conduction.1

When this state is achieved, the system no longer requires external energy input to sustain the fusion process. In magnetic confinement approaches, this allows for a steady-state operation, provided that fuel is replenished and fusion "ash"—specifically helium—is removed to prevent the plasma from quenching. In inertial confinement fusion (ICF), ignition results in a rapid temperature excursion where the energy yield significantly exceeds the energy required to initiate the process.1

Achieving ignition requires meeting specific conditions regarding plasma temperature and confinement quality, often measured by the product of ion density and energy confinement time ( $n τ$ ). An ignited plasma is characterized by an energy gain ( $Q$ ) of infinity, meaning the fusion reactions themselves provide all the necessary heating to keep the plasma burning.12