Fueling: Pellets and Gas
Deuterium and helium-3 are fed by frozen pellets and gas puffing, matched to burn-up and end losses to hold density steady.
Feeding the burn
A burning plasma consumes fuel and loses particles out the ends, so it must be continuously refueled to hold density. The burner uses two complementary methods: gas puffing at the edge and injection of frozen fuel pellets that penetrate deeper into the plasma. Neutral beams also add some fuel as they deposit.
Two methods, different reach
- Gas puffing: simple edge fueling, good for controlling the plasma boundary
- Pellet injection: frozen D and 3He pellets deposit fuel in the interior
- Beam fueling: fast neutrals add particles where they ionize
- Feed rate is trimmed to follow load — see load following
Balancing the fuel mix
The D-to-3He ratio must be held near the design point to keep the reaction rate and the neutron fraction where they are wanted — too much deuterium raises D-D side reactions and neutrons. Helium ash from the burn must also be removed by the vacuum system so it does not dilute the fuel. Fueling, pumping, and burn form a balanced particle loop the control system manages continuously.
Helium-3 is the scarce component, so fuel efficiency matters: pellet penetration and burn-up fraction affect how much 3He is used per unit of energy. Fueling parameters are set in the design study and tie directly into the machine's fuel-supply and tritium-handling accounting.
Fuel economy is not a minor detail here: because helium-3 is scarce, the fraction of injected 3He that actually fuses is a design figure of merit, and pellet penetration, density control, and ash removal are tuned together to use the fuel as fully as possible.