Beta and the Troyon Limit

Beta is the ratio of plasma pressure to magnetic pressure; the Troyon scaling caps it, and spherical tokamaks push that cap higher.

Pressure per unit field

Beta is the ratio of plasma pressure to the magnetic pressure holding it. It is the efficiency with which a magnetic field is used: high beta means a lot of fusion-producing pressure for a given, expensive magnetic field. Fusion power scales strongly with pressure, so beta is one of the most important figures of merit for a compact machine.

Beta cannot rise without limit. The Troyon scaling sets a normalized beta ceiling above which pressure-driven MHD instabilities grow and terminate the discharge. The practical operating point sits at a fraction of that limit for margin. What makes spherical tokamaks attractive is that their geometry raises the achievable beta substantially compared with conventional tokamaks at the same field.

Where Hyperion sits

Hyperion's high on-axis field (8 T) combined with the ST's elevated beta limit is what allows 85.0 MW from R0 1.2 m. The design must hold beta high enough for the target fusion power yet below the stability ceiling with margin. That balance — high beta without crossing the Troyon-type limit — is a design-and-simulation constraint the operating point is built around.

• Beta = plasma pressure / magnetic pressure
• Troyon scaling caps normalized beta before MHD onset
• STs reach higher beta limits — the basis of the compact design

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.