ELMs and Negative Triangularity

Edge-localized modes eject heat from high-confinement plasmas; negative triangularity is chosen partly to soften or avoid them.

The edge relaxation problem

High-confinement (H-mode) plasmas build a steep pressure pedestal at their edge. When that pedestal exceeds a stability limit it relaxes in a burst called an edge-localized mode (ELM), throwing a pulse of heat and particles onto the first wall and divertor. Large ELMs impose transient loads that can damage plasma-facing surfaces, so controlling them is essential in a burning machine.

Why negative triangularity helps

Negative triangularity, Hyperion's delta -0.30, alters the edge stability so that good core confinement can be reached without the same steep, ELM-prone pedestal that positive-triangularity H-mode requires. In experiments, negative-triangularity plasmas have accessed high confinement with weak or absent ELMs. Softening these transients protects the first wall, which for the breeder is also the neutron window into the blanket.

The trade and the test

The benefit is not free: negative triangularity is harder to hold at high current and pressure and can narrow the operating window. Whether the edge behaves as favorably at Hyperion's 9.66 MA and compact geometry as it has in smaller experiments is an open confinement question. It is one of the specific reasons the shaping was chosen, and one of the specific things the first-of-a-kind machine is meant to confirm.

• ELMs eject heat from the H-mode edge onto surfaces
• Negative triangularity can give good confinement with weak ELMs
• Its behavior at Hyperion's parameters is an open test

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.