Negative Triangularity: What Is and Is Not Settled

The breeder adopts negative triangularity (delta -0.30) for edge and exhaust reasons; the record separates the well-motivated choice from the parts that FOAK must still confirm.

A deliberate, unusual shape

Most tokamaks run positive triangularity, a D-shaped cross section with the corners pointing toward the center column. The breeder (Hyperion) runs negative triangularity, delta -0.30 — the D bows outward. The motivation is edge and exhaust behavior: negative triangularity is associated with a benign edge that avoids certain large edge instabilities.

What is well-motivated

The choice draws on experiments at conventional aspect ratio that observed favorable edge behavior with negative triangularity. The breeder record cites that basis and includes the shape in its equilibrium and stability analyses among the 81.

What is not yet settled

• Negative triangularity at low aspect ratio (A 2.5) is far less explored than at conventional aspect ratio.
• The interaction of the outward-bowed edge with the breeder's exhaust and divertor loading.
• Confinement at negative triangularity in this regime, where the ST database is thin.
• Whether the equilibrium is as controllable in hardware as in simulation.

The record's position is that the shape is a reasoned design choice with a real experimental basis, and that its behavior in this specific low-aspect-ratio, D-T breeding context is a simulation result to be confirmed, not an established fact.

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.