Minimum-B Stability

Mirror plasmas are held against interchange instability by shaping the field so its strength increases outward in every direction — a magnetic well.

The interchange problem

A simple mirror with straight field lines is unstable: the plasma sits where the field is weak, and it is favorable for plasma and field to interchange, letting the plasma bulge outward. This flute or interchange instability destroys confinement. Every practical mirror must be shaped to defeat it, and the classic solution is a minimum-B configuration — a magnetic well.

A well in field strength

In a minimum-B field, the magnitude of B increases in every direction away from the plasma center. The plasma then sits at the bottom of a well in field strength. Interchanging plasma outward would move it into stronger field, which is energetically unfavorable, so the interchange mode is stabilized. Historically this was achieved with baseball or yin-yang shaped coils that curve the field into a well.

In a tandem mirror

• Stabilizing shaping is concentrated in the end regions
• The central cell can stay simple, stabilized by the shaped ends
• Sets a stability-limited beta the design stays under
• Couples to how the plug and throat coils are shaped

Minimum-B stability is well established from decades of mirror experiments — it is one of the more settled parts of mirror physics. In the burner design it constrains the coil geometry and the achievable beta, and it is why the plug and end-cell magnets are shaped rather than simple solenoids. Achieving good MHD stability at the burner's operating point is part of what the test unit validates.