Triangularity and Edge Stability

Negative triangularity can suppress edge-localized modes by removing the steep edge pedestal that drives them — protecting the first wall.

The edge problem

Conventional high-confinement operation builds a steep pressure pedestal at the plasma edge. That pedestal periodically becomes unstable and ejects bursts of heat and particles — edge-localized modes (ELMs). On a large machine ELMs are a materials threat; on a compact ST with thin inboard shielding they are especially unwelcome.

Negative triangularity, delta -0.30, tends not to form the same steep pedestal. Without the pedestal there is less free energy to drive the edge instability, so the plasma can run in a quieter edge regime. For Hyperion, whose first wall and center post already face a demanding neutron and heat load, a naturally ELM-quiet edge is a design asset rather than a curiosity.

Honest framing

Negative-triangularity edge behavior has been demonstrated on other devices at smaller scale and lower field; extrapolating it to Hyperion's parameters is a simulation exercise, not a measured guarantee. The claim here is that the geometry is favorable, not that edge stability is solved. Confirming that a delta -0.30 edge stays quiet at Hyperion's power density, heat flux, and neutron environment — rather than at the milder conditions where negative triangularity has mostly been studied — is precisely the kind of question the FOAK build exists to answer in hardware.

• ELMs are pedestal-driven edge instabilities that shed heat in bursts
• delta -0.30 tends to avoid the steep pedestal that drives them
• Extrapolation to Hyperion parameters remains a simulation result

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.