The Beta-Limit Benchmark

The breeder's normalized pressure is checked against the Troyon beta limit, the empirical ceiling on how much plasma pressure a given field and current can hold.

How much pressure the field can hold

Fusion power scales steeply with plasma pressure, so a design is tempted toward high beta — the ratio of plasma pressure to magnetic pressure. The empirical ceiling is the Troyon limit, expressed through normalized beta, beta_N. Exceeding it invites large-scale instability. The breeder (Hyperion) design point is checked against this limit as part of the 81-analysis record.

# Troyon normalized-beta limit
# beta[%] <= beta_N * Ip[MA] / (a[m] * B[T])
# beta_N ~ 2.8-4 (conventional); STs access higher values.
# Design inputs: Ip = 9.66 MA, B_axis = 8 T, low aspect ratio.
# The benchmark reports the design beta_N and its margin to the limit.

Spherical tokamaks change the number

Low-aspect-ratio machines are known to reach higher beta_N than conventional tokamaks, so the breeder benchmark uses the ST-appropriate range rather than the textbook conventional value. The reported quantity is the design's beta_N together with the margin between it and the applicable limit.

What a margin means

A comfortable margin means the design is not relying on operating at the ragged edge of a stability boundary; a thin margin is flagged as a risk to retire at FOAK. The record states the margin rather than asserting the design is 'stable', because stability is a boundary the machine must be shown to respect, not a property claimed by fiat.

This page describes a design-and-simulation study, not a built machine. Construction of the breeder (Hyperion) begins Q2 2027; first-of-a-kind first tritium is targeted near 2030. No hardware net-gain claim is made before FOAK.