The Magnetic Well & MHD Stability

A minimum-B magnetic well makes the plasma sit in a region where field increases outward, stabilising the interchange instability.

Curvature and stability

A plasma is unstable to interchange ("flute") modes wherever the magnetic field curves toward the plasma — so-called bad curvature. Simple axisymmetric mirrors have bad curvature and are MHD-unstable. Stability requires a magnetic well: a configuration where the field strength increases in every direction away from the plasma centre, giving good curvature everywhere.

In a tandem mirror the well is created with non-axisymmetric coils — quadrupole or "minimum-B" (baseball / yin-yang) anchor coils — that bend the field into a minimum-B shape. These anchors stabilise the whole connected plasma, including the axisymmetric central cell, provided the stabilising region is strong enough.

The tension with simplicity

Non-axisymmetric coils complicate the geometry, raise the field errors that can drive transport, and add to the already severe magnet engineering. The design must reconcile MHD stability (which wants non-axisymmetric wells) with confinement and buildability (which prefer symmetry) — a balance that remains an open part of the burner's physics.

The distinction between average and local good curvature matters: a configuration can be stable on average yet locally unfavourable, so the well must be deep enough to guarantee stability everywhere the pressure is high. Verifying that at the burner's beta and field, in the presence of the non-axisymmetric anchors, is a stability calculation that experiment has not yet checked at these parameters.

• Interchange modes grow in bad curvature
• Minimum-B well = field rises outward everywhere
• Built with quadrupole/anchor coils
• Trades symmetry for MHD stability