Helium Ash and Particle Exhaust

Fusion produces helium-4 ash that must be exhausted; if it accumulates it dilutes the fuel and cools the plasma.

The exhaust from the burn itself

Every D-T reaction leaves behind a helium-4 nucleus — the alpha particle. After it deposits its 3.5 MeV heating the plasma, it becomes helium ash: a fully burned, non-fusing impurity. If ash is not removed it accumulates, raising the effective charge, diluting the fuel, and eventually cooling the plasma enough to reduce the fusion rate.

So particle exhaust is as necessary as heat exhaust. The same divertor and pumping systems that handle exhaust power must also remove helium ash on a timescale short compared with how fast it would otherwise build up. The relevant figure is the helium confinement time relative to the energy confinement time — ash must leave faster than it degrades the burn.

The design consequence

Adequate helium exhaust constrains the edge and divertor design and couples to the density and impurity picture. It is one more reason a compact, high-power-density machine leans hard on its divertor and pumping. Quantifying the helium removal rate and its effect on the burn at Hyperion's parameters is part of the design-and-simulation operating-point definition. The requirement is usually stated as a ratio of helium confinement time to energy confinement time that must stay below a threshold; meeting it constrains the pumping speed and the edge conditions the divertor has to maintain during steady operation.

• Helium-4 alpha becomes inert ash after heating the plasma
• Accumulated ash dilutes fuel and cools the plasma
• Ash must be pumped out faster than it degrades the burn

This page describes a design-and-simulation study, not a built machine. Construction begins Q2 2027; first-of-a-kind first tritium is targeted near 2030. No hardware net-gain is claimed before FOAK.