Burn Control
Regulating fusion power in a burning plasma, where self-heating couples temperature and reaction rate in a potentially unstable loop.
The self-heating loop
In a burning plasma, the charged fusion products heat the plasma themselves. This self-heating couples fusion power to temperature: more power raises temperature, which can raise the reaction rate, which raises power. Whether this loop is stable or runs away depends on how the fusion rate and the losses each vary with temperature - the essence of the burn-control problem.
Stability of the operating point
A burning-plasma operating point is stable if a small temperature rise increases losses faster than it increases self-heating, so the plasma settles back. Some operating points are naturally stable; others are thermally unstable and need active control to hold. Burn control keeps the plasma at a chosen fusion-power level by adjusting the levers that shift this balance.
The control levers
- Fueling rate: sets the fuel density available to react
- Auxiliary heating: trims the power balance and can stabilize the point
- Fuel mix: adjusting the ratio of reacting species changes the rate
- Impurity or density control: modulates losses
Why it is subtle
The reaction rate depends steeply and nonlinearly on temperature, the response of transport losses to temperature is uncertain, and the actuators act with delay through slow transport. Burn control is therefore a hard regulation problem on an uncertain, possibly unstable plant, and it becomes fully testable only in a plasma actually producing significant fusion self-heating.
Design context
Burn control matters for the D-T breeder Hyperion, whose reference design targets a fusion power of 88.7 MW at a fusion gain Q of 3.424 in simulation. These are computed design figures; the machine is not built, and no hardware net-gain claim is made before first-of-a-kind first tritium. Burn control here is a design and simulation study.