Low Aspect Ratio A = 2.5

Aspect ratio A is the major radius divided by minor radius; Hyperion's A 2.5 is what makes it a spherical rather than conventional tokamak.

What A measures

Aspect ratio A is the ratio of the major radius (from the machine axis to the plasma center) to the minor radius (the plasma's own radius). Conventional tokamaks sit around A 3 to 4; spherical tokamaks push below A 2. Hyperion's A 2.5 with R0 1.2 m places it firmly in spherical-tokamak territory while keeping a workable central column.

Lowering A changes the field geometry the plasma sees. Field lines spend more of their length on the low-field outboard side and wrap tightly on the inboard side, which improves stability against several pressure-driven instabilities and raises the beta the plasma can hold.

The design trade

A cannot be pushed arbitrarily low. The smaller the central hole, the less space for the toroidal-field conductor and inboard shielding, and the harder the center post's job. A 2.5 is a compromise: low enough to capture most of the spherical-tokamak stability and beta benefit, high enough to leave a buildable, if highly stressed, center column. Pushed lower, the machine would gain a little more beta headroom but lose the space needed to route conductor and coolant through the center; pushed higher, it would forfeit the very stability margin that lets it stay compact.

• A = R0 / a; Hyperion A 2.5, R0 1.2 m
• Below conventional tokamaks (A ~ 3-4)
• Trade: stability and beta gains vs a tighter, harder center post

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. No hardware net-gain is claimed before FOAK.