Aspect Ratio 2.5

Aspect ratio 2.5 places Hyperion firmly in the spherical-tokamak regime, buying high beta and compactness at the expense of a crowded center.

What aspect ratio means

Aspect ratio A is the ratio of the major radius R0 to the minor radius a. Hyperion uses A of 2.5 with R0 of 1.2 m, giving a minor radius near 0.48 m. Conventional tokamaks sit around A of 3 to 4; a value of 2.5 is squarely in the spherical-tokamak family, where the plasma is nearly as wide as it is tall through the machine.

The physics lever

Beta, the ratio of plasma pressure to magnetic pressure, scales favorably as aspect ratio falls. A machine at A of 2.5 can confine a higher relative pressure for the same field than one at A of 3.5. Because fusion power density grows with pressure squared, that leverage is why an ST can produce 85.0 MW from a machine only 1.2 m in major radius.

The engineering trade-off

Low A leaves almost no room on the inboard side. The center stack must fit the toroidal-field return conductor and cooling inside a radius smaller than the plasma minor radius, with minimal neutron shielding. The narrower the column, the higher the current density and the shorter the component life. Hyperion's aspect ratio is therefore a compromise: low enough to capture the beta benefit, not so low that the center post becomes unbuildable.

• A 2.5: high-beta, compact ST regime
• Fusion power density rises with plasma pressure squared
• The trade-off is a crowded, high-duty center stack

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.