Why a spherical tokamak for the breeder?

A spherical tokamak's low aspect ratio (A = 2.5) lets it hold high plasma pressure (β_N = 1.532) for a given field, making a compact, economical machine. Since the breeder's job is to be a dense neutron and tritium source, that compactness is exactly what it needs.

The trade is a tight central column, which Kronos treats as a named design focus.