Spherical Tokamak Geometry

A spherical tokamak squeezes the torus toward a cored apple shape, trading a slender central column for high plasma pressure at modest field.

The shape of the device

A conventional tokamak is a fat doughnut: the hole in the middle is large relative to the plasma. A spherical tokamak (ST) shrinks that hole until the plasma wraps closely around a slender central column. The result looks less like a doughnut and more like a cored apple. Hyperion is an ST with aspect ratio A of 2.5, meaning the major radius R0 (1.2 m) is only 2.5 times the plasma minor radius a (about 0.48 m).

Why the shape matters

Low aspect ratio raises the achievable normalized pressure (beta) for a given magnetic field. Field lines on the outboard side are longer and the average magnetic curvature is favorable over more of the plasma, which improves stability limits. In practice this lets an ST reach useful fusion conditions in a much smaller machine than a high-aspect-ratio design at the same on-axis field.

The trade-off of the shape

The compactness is paid for at the center. The central column, the center stack, must carry the return legs of the toroidal field conductor and, in many STs, the ohmic solenoid, all inside a very small radius while withstanding intense neutron and heat loading. There is little room for shielding. This is the defining engineering tension of the ST line and it drives the center-post lifetime limit discussed elsewhere in this section.

• A 2.5 (R0 1.2 m, a ~0.48 m) — a compact machine
• High beta at modest field is the payoff
• The slender center stack is the principal engineering constraint

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.