Helium-3 Purity Requirements
Fusion-grade helium-3 must be separated from ordinary helium-4 and other gases; even small contamination changes plasma behavior and fuel accounting.
Why purity matters
The burner plasma is tuned to a specific fuel mix. Contaminants dilute the reacting species, radiate energy away, and shift the plasma's effective charge. Helium-4 is the chief concern: it is chemically identical to helium-3 and cannot be removed by chemistry, only by isotope separation. Other light gases — residual hydrogen, deuterium, nitrogen — must also be held to low levels. Purity is specified because impurities cost confinement and confuse fuel accounting.
Where impurity comes from
Helium-3 from tritium decay is intrinsically clean at birth, but it accumulates alongside the parent tritium and any helium-4 already present, and it can pick up gases during storage and transfer. Recovered helium-3 from a burner's exhaust is mixed with the helium-4 ash the reaction produces. So purity is maintained by separation both upstream (before delivery) and downstream (in exhaust processing).
- Helium-4 dilutes fuel and cannot be removed chemically
- Impurities radiate energy and raise effective charge
- Decay-born helium-3 is clean but co-located with tritium and 4He
- Purity is enforced by isotope separation, upstream and downstream
Specifying and verifying purity is therefore tied directly to the isotope-separation and fuel-accounting systems described elsewhere in this section.