Axial Confinement and the Loss Cone

In an open machine the hard problem is holding plasma along the axis; the burner solves it with a mirror force plus an electrostatic well.

The open-ends problem

A toroidal device closes its field lines so plasma has nowhere to leak axially. A linear machine cannot do that — its field lines run out the ends. Axial confinement must therefore be built, and it is the defining challenge of every mirror design. The burner attacks it two ways at once.

Mechanism one: the magnetic mirror

Rising field toward each throat reflects ions by conserving their magnetic moment. Ions whose velocity vector lies outside the loss cone turn back; those inside it escape. The mirror ratio sets the loss-cone width. Alone this leaks too fast for a burning plasma.

Mechanism two: the ambipolar potential

The plugs raise the electrostatic potential at each end of the central cell. Central-cell ions then sit in a potential well and are reflected electrostatically, not just magnetically. This ambipolar plugging is what makes the tandem mirror confine well enough to burn.

The residual leak is deliberate and useful. Plasma that does escape flows out into the expander and is captured by the direct converter, so the loss channel becomes a power-extraction channel. The design tunes the balance so confinement is adequate for the burn while a controlled outflow feeds conversion.

• Magnetic mirror: fast, geometric, but leaky alone
• Ambipolar potential: closes the confinement gap electrostatically
• Residual axial loss is recovered as electricity in the converter

This dual scheme is why the burner has no divertor in the tokamak sense — see the divertor-free exhaust. The ends are the exhaust, and the exhaust is the generator.