Center Post vs Breeding Trade

The spherical tokamak's inboard center post takes space that cannot breed and must be shielded; this geometry directly limits inboard TBR.

The cost of a compact machine

Hyperion is a spherical tokamak with aspect ratio A 2.5 and major radius R0 1.2 m, which makes it compact and efficient but places a slim center post on the inboard side carrying the toroidal-field conductors up to a peak field of 16.84 T. That center post is central to why the machine works — and a real constraint on breeding.

Inboard space occupied by the center post and its shielding is space that cannot hold a full breeding blanket. Because inboard coverage contributes significantly to TBR, this geometry caps how much can be bred there. The trade is fundamental to the spherical-tokamak choice, not a flaw to be engineered away.

Two coupled limits

• Breeding: reduced inboard blanket lowers achievable net TBR
• Shielding: the center post must be protected from neutron damage
• Lifetime: even shielded, the center post has a limited life near 0.01 fpy
• Both compete for the same scarce inboard volume

How the design responds

The response is to maximize outboard and top/bottom breeding, use neutron multiplication to recover budget, and accept scheduled center-post replacement. Whether the resulting net TBR meets the 1.8 target is precisely the open reconciliation. The trade is a design and validation subject, and the center-post lifetime is stated candidly as a constraint on availability and on what the machine can make over time.

Construction begins Q2 2027; first-of-a-kind (FOAK) first tritium is targeted near 2030. No net-gain claim is made before FOAK.