Why a mirror, not a tokamak?

Because Kronos runs two different jobs on two different fuels — and each fuel wants a different machine. The rule is simple: fuel follows purpose, not platform.

Breeder

Spherical tokamak · D–T · gain at modest current

Burner

Tandem mirror · D–³He · direct conversion

Governing rule

Fuel follows purpose, not platform

Why not one machine

Closed vs open geometry serve opposite goals

The HYPERION breeder only needs gain: burn D–T, which lights at a modest ion temperature, and capture the neutrons to breed tritium. A closed torus is the proven way to reach gain at a small current, so the breeder is a spherical tokamak.

The burner has the opposite job. Its fuel is D–³He, whose energy comes out mostly as charged particles rather than neutrons — but which only becomes reactive near 90 keV, far hotter than D–T. To profit from charged products you want to collect them directly as electricity, and to do that you want field lines that are open at the ends. A tokamak's field lines close on themselves; a mirror's run straight out onto collector plates. So the burner is a tandem mirror.

Put the two reactivity curves side by side and the split is obvious: D–T peaks near 70 keV and is usable at 15 keV; D–³He needs ~90–250 keV and pays back in charged power. One fuel wants a compact high-gain torus; the other wants a long, hot, open channel with direct conversion. Forcing both onto one platform would compromise both.

This is why Kronos does not describe itself by a single reactor type. It builds the machine each stage needs: a tokamak to make the fuel, a mirror to burn it.