MHD Stability Overview

Magnetohydrodynamic instabilities set hard limits on pressure and current; staying inside them defines the safe operating space.

The limits that bound operation

Magnetohydrodynamics (MHD) treats the plasma as a conducting fluid and predicts the large-scale instabilities that can grow and terminate a discharge. The major families are kink modes driven by current, ballooning and pressure-driven modes driven by beta, tearing modes that reconnect field lines at rational surfaces, and vertical instability from elongation. Each imposes a limit on some combination of pressure, current, and shape.

These limits are not soft. Cross a beta limit or destabilize a large tearing mode and the plasma can disrupt, ending the discharge abruptly. The operating point is chosen to sit inside all the relevant MHD boundaries with margin, which is why the design point specifies pressure, current, and shape together rather than pushing any one to its individual maximum.

The ST and negative-triangularity angle

Spherical tokamaks raise the beta limit, buying room for higher pressure, and negative triangularity alters the edge stability picture. Both change where the MHD boundaries lie for Hyperion compared with a conventional tokamak. Mapping the stable operating space at the machine's parameters, and confirming the design point sits safely within it, is a central design-and-simulation task.

• Kink, ballooning, tearing, and vertical modes each set a limit
• Crossing a limit can cause a disruption
• STs and negative triangularity shift where the limits fall

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.