How fuel is delivered to the core of a fusion plasma — from simple gas puffing at the edge to supersonic pellet injection and neutral beam fueling, each with different penetration depth and efficiency.
A fusion plasma continuously burns its fuel and must be continuously refueled. The challenge is getting fuel into the hot, dense core: the plasma edge is much cooler and denser than the core, and simply adding gas at the edge may not penetrate deeply enough for efficient fueling.[1]
Deuterium or tritium gas is released through valves at the plasma edge. Gas molecules are ionised in the scrape-off layer and enter the confined plasma through transport. Gas puffing is simple and reliable but only fuels the plasma edge, relying on inward particle transport to refuel the core. It is the primary fueling method for most current tokamaks.[2]
Frozen hydrogen-isotope pellets (1–5 mm diameter) are launched into the plasma at 100–1000 m/s. They ablate as they penetrate, depositing fuel deeper in the plasma. High-field-side injection (launching from the inboard side) achieves better penetration due to favourable magnetic curvature. ITER will use pellet injection as its primary fueling method.[3]
Neutral beam injectors simultaneously heat and fuel the plasma, as the injected neutral atoms are ionised and trapped. However, the fueling rate from NBI is typically insufficient alone, and it is primarily valued for heating and current drive.