The fraction of injected fuel that successfully reaches the plasma core and participates in fusion reactions — a deceptively simple metric that governs tritium economy and burn performance.
Fuelling efficiency is the ratio of the fuel (deuterium and tritium in a D-T reactor) that is deposited in the plasma core to the total amount injected into the vacuum vessel. It sounds straightforward, but achieving high fuelling efficiency in a burning plasma is one of the more challenging engineering problems in fusion. The plasma edge acts as a barrier: neutral gas injected at the periphery is ionized in the scrape-off layer and swept into the divertor before it can reach the core, while pellets must penetrate deep enough to survive ablation in the hot edge plasma.[1]
Gas puffing: The simplest method — neutral gas is released through valves at the vessel wall. However, in reactor-scale devices with dense, hot edge plasmas, gas puffing fuelling efficiency is very low, typically 1–5%. Most of the injected gas is ionized in the scrape-off layer and recycled through the divertor without ever reaching the confined plasma. Gas puffing remains useful for edge density control and divertor detachment but is inadequate as the primary core fuelling method.[2]
Pellet injection: Frozen hydrogen-isotope pellets (typically 3–6 mm diameter) are launched into the plasma at speeds of 200–1,000 m/s. As a pellet traverses the hot plasma, its surface ablates and the released gas is ionized. The penetration depth depends on pellet size, velocity, and the local plasma temperature and density. High-field-side (inboard) injection exploits a favorable drift that carries the ablated fuel deeper into the core, improving efficiency to 20–50% or more.[1]
Compact toroid injection: An advanced concept in which small, self-contained magnetized plasma structures (compact toroids or spheromaks) are accelerated into the plasma core. Because the injected fuel is already ionized and magnetically confined, it can in principle penetrate deeper than a pellet and deposit fuel at precisely controlled radial locations. This method remains experimental.[3]
In a D-T reactor, tritium is bred in the lithium blanket at a rate that must slightly exceed the burn rate to maintain fuel self-sufficiency. Every tritium atom that is injected but fails to reach the core is wasted — it is pumped out, reprocessed, and reinjected, adding to the tritium inventory circulating through the fuel cycle. Low fuelling efficiency increases the required tritium throughput, enlarges the fuel processing plant, raises the in-vessel tritium inventory (with attendant safety implications), and demands a higher tritium breeding ratio from the blanket.[2]
The connection between fuelling efficiency and burn fraction is direct: if only 30% of injected fuel reaches the core, and only ~1–2% of that core fuel actually fuses before being exhausted, then the overall fuel utilization is a fraction of a percent. Maximizing fuelling efficiency is one of the few levers available to ease the stringent demands on every other element of the tritium fuel cycle.[1]
Fuelling is not merely a supply problem — it couples to plasma stability and confinement. Deep pellet fuelling can trigger edge-localized modes (ELMs) or modify the pressure profile in ways that affect MHD stability. Conversely, the fuelling profile shapes the density profile, which influences bootstrap current, fusion power density, and impurity transport. Designing a fuelling system for a reactor therefore requires simultaneous optimization of fuel penetration, plasma stability, and burn performance.[3]