The Density Limit as a Self-Limit

Push plasma density too high and radiation losses cool the plasma — a built-in ceiling that prevents excursions.

Tokamak plasmas obey an empirical density ceiling (the Greenwald limit) above which the discharge radiates strongly, cools, and terminates. Far from being only an operational nuisance, this limit is a safety feature: it means the reaction rate cannot be driven upward without bound by adding density.

The mechanism

As density rises, radiative losses from the plasma edge grow faster than the fusion power gained. Beyond the limit the edge cools, confinement degrades, and the plasma disrupts or extinguishes. The breeder (Hyperion) is designed to operate with margin below this ceiling; any excursion toward it is self-correcting because the plasma cools itself.

Together with pressure and current limits, the density limit fences the operating space on all sides. The machine lives inside a bounded window; crossing any wall of that window reduces or ends the burn rather than amplifying it. This is the quantitative face of the self-limiting principle.

Relation to the burner

The burner (Aegis / MetroVolt) has its own confinement ceilings set by mirror physics rather than the Greenwald limit, but the qualitative story is the same: there is a maximum sustainable operating point, and exceeding it degrades confinement. See burner plug safety.

• Density above the limit → radiative cooling → termination.
• The operating window is bounded on every side by physical limits.
• Excursions toward a limit are self-correcting, not amplifying.

No operator action is needed to enforce the ceiling; it is enforced by the plasma's own radiation.