Minimum-B Anchor Coils

Non-axisymmetric anchor coils bend the field into a minimum-B shape that MHD-stabilises the connected plasma, including the symmetric central cell.

Stabilising the whole chain

A minimum-B (baseball or yin-yang) coil produces a field that increases in all directions from the centre — a magnetic well. Placed as anchors in the end regions, these coils stabilise not just their own plasma but the axisymmetric central cell connected to them, because the plasma is a single connected object and MHD stability is a global property.

The trade-off is geometric complexity and field errors. Non-axisymmetric coils create field ripple that can enhance radial transport, and they are mechanically intricate to build at high field. The design must place just enough minimum-B stabilisation to hold the plasma while keeping the transport penalty and the magnet engineering tractable.

An open balance

How to combine strong minimum-B anchors with the extreme axisymmetric plug field, without the ripple destroying confinement or the stresses exceeding limits, is unresolved at the burner's parameters. It is one strand of the broader plug-regime gate.

The historic yin-yang and baseball coils proved the principle at modest field; scaling that shaping to sit alongside a 26.49 T axisymmetric plug, without the ripple ruining radial confinement or the combined stresses exceeding limits, is unproven. The anchor problem is thus a specific, nameable piece of the general plug-regime gate rather than a separate concern.

• Baseball / yin-yang coils make a minimum-B well
• Anchors stabilise the connected central cell
• Penalty: field ripple, geometric complexity
• Balance with the plug field remains open