The Physics of Self-Limiting Burn
The reaction rate depends so steeply on plasma conditions that any drift out of the operating window suppresses it.
Fusion power scales roughly with the square of density and steeply with temperature, but only inside a narrow band where confinement holds. That sensitivity cuts both ways: it makes fusion hard to sustain, and it makes runaway impossible. The operating point is a peak the machine must actively balance on, with declining reaction rate on every side.
The governing balance
Sustained burn requires the triple product — density, temperature, and energy confinement time — to clear a threshold (the Lawson condition). If power rises and the plasma expands, confinement time falls and pressure limits are approached; if density rises too far, radiation losses climb and the plasma cools. Each excursion pushes the triple product back down.
| Perturbation | Immediate effect | Feedback on burn |
|---|---|---|
| Power rises | Plasma heats, expands | Confinement drops → burn falls |
| Density rises | Radiation losses climb | Plasma cools → burn falls |
| Heating lost | Temperature drops | Rate collapses in seconds |
| Fueling stops | No new fuel | Burn extinguishes |
The breeder (Hyperion) uses a negative-triangularity shape (δ = −0.30) chosen for favorable stability and edge behavior; the burner (Aegis / MetroVolt) relies on magnetic-mirror confinement whose losses rise sharply if the plug field weakens. Neither has an operating branch where more power produces still more power.
In control terms the plasma is an open-loop-unstable object we must continuously stabilize — which is precisely why it cannot run away. Stop stabilizing and it quenches. See burn-condition fragility and the density limit.