Pellet Injection Fueling Control
Frozen fuel pellets fired deep into the plasma deliver particles to the core where gas puffing cannot reach, giving efficient core fueling.
Why pellets
A pellet of frozen hydrogen isotopes is accelerated to high speed and injected into the plasma, where it ablates as it flies inward, depositing fuel well past the edge. This reaches the core directly, unlike gas puffing whose fuel must diffuse inward. Deep deposition makes pellets the efficient way to build and hold core density.
Formation and firing
A pellet injector freezes fuel gas into a solid, cuts or extrudes pellets, and accelerates them with a gas gun or centrifuge. Control sets pellet size, speed, and firing rate. Faster and larger pellets penetrate deeper; the injection rate sets the average fueling. The controller schedules pellets to hold a density target while respecting the injector's mechanical limits.
Deposition physics and launch side
Where a pellet's fuel ends up depends on ablation and a drift of the ablated material. Injecting from the high-field side of a tokamak favours inward transport of the deposited fuel, improving core fueling efficiency over low-field-side launch. Control accounts for this by choosing the launch geometry and pellet parameters for the desired deposition profile.
- Frozen pellets ablate deep in the plasma
- Set size, speed, and rate to control fueling
- High-field-side launch improves core deposition
- Combined with gas puffing for full-range control
Interactions
A pellet is a sudden local perturbation: it can trigger edge instabilities or, used deliberately, pace them to keep them small. Pellet timing therefore couples to edge and divertor control. In the Hyperion breeder design study, pellet fueling sustains the D-T core density needed for the modeled 88.7 MW fusion power and Q of 3.424; these are simulation results for a machine slated to begin construction in 2027 and reach first tritium around 2030, not present hardware.
Pellets and gas together span fast edge trim to deep core supply.