The Density-Limit Benchmark

The breeder's operating density is compared to the Greenwald limit, an empirical density ceiling above which tokamaks tend to disrupt.

An empirical density ceiling

Tokamaks show an empirical maximum line-averaged density, above which the plasma tends to disrupt. The standard reference is the Greenwald density limit, which rises with plasma current and falls with plasma size. Fusion power benefits from high density, so a design must sit at a defensible fraction of this limit, not beyond it.

# Greenwald density limit
# n_GW[1e20 m^-3] = Ip[MA] / (pi * a[m]^2)
# Operating at a fraction f_GW = n / n_GW, typically <= ~1.
# Design input: Ip = 9.66 MA. The benchmark reports f_GW for the
# design density and flags any point approaching unity.

The current advantage

Because the Greenwald limit rises with plasma current, the breeder's 9.66 MA supports a higher absolute density before the limit bites. The benchmark reports the Greenwald fraction f_GW at the design point; a value comfortably below one indicates headroom, while a value near one is recorded as an operating risk.

Coupled, not isolated

Density, confinement, and beta are not independent knobs. The record evaluates them together so that a design point does not satisfy each limit in isolation while violating a joint operating boundary. Cross-checking the coupled constraints is part of the sensitivity analysis.

The Greenwald limit is empirical, not a hard law of nature, and some machines exceed it under particular conditions. The breeder record does not lean on such exceptions: it targets a defensible fraction below one and treats any operating point that would rely on beating the empirical limit as a risk to retire at FOAK rather than a capability to assume in design.

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.