pellet injection is fueling the 📝plasma core by firing frozen fuel pellets into it at high speed.
The pellets are millimeter-scale ice — 📝deuterium, or deuterium and 📝tritium together — frozen at cryogenic temperature and accelerated to kilometer-per-second speeds by gas guns or centrifuges. Speed is the point: a neutral gas ionizes and stops at the plasma edge, but a solid pellet survives long enough to penetrate toward the core before it ablates, depositing fuel where the 📝fusion actually happens. Injecting from the machine's inboard side deepens the delivery further, because the ablated material drifts toward lower field — inward, in that geometry.
Its near-neighbor is gas puffing, the simpler edge-fueling method, and the division of labor is depth: gas feeds the boundary, pellets feed the core. Pellets have also become control instruments in their own right. Small pellets fired rhythmically pace edge localized modes, triggering many small eruptions instead of fewer damaging ones, and massive pellets of frozen impurities are the basis of 📝disruption mitigation systems that quench a failing discharge before it can damage the machine. 📝ITER's fuel cycle is designed around pellet injection as the primary means of getting tritium deep into the burning core.