Mirror Ratio

The mirror ratio — throat field over central-cell field — sets the trapped fraction and, with the plug potential, the confinement of the burner.

The mirror ratio R is the field at the throat divided by the field in the central cell. With the throat at 17 T and a central-cell field on the order of the 8 T on-axis equivalent, R is large. A higher mirror ratio narrows the loss cone and traps more of the plasma magnetically, at the cost of demanding a stronger throat coil.

In a simple mirror, confinement improves only slowly with R, which is why simple mirrors leak too fast for a power plant. The tandem-mirror design does not rely on magnetic ratio alone: the plug adds an electrostatic barrier on top of the magnetic one, so the effective confinement of the central cell far exceeds what R would give by itself.

What the ratio controls

• Trapped-particle fraction in the central cell
• Width of the loss cone at each end
• Magnetic-only end-loss rate
• The base the plug potential builds on

Ratio versus potential

There is a design trade between pushing the mirror ratio higher and pushing the plug potential higher. A stronger throat gives more magnetic trapping but raises coil loads; a taller plug potential gives more electrostatic trapping but demands a denser, hotter plug plasma at 26.49 T. The burner leans on the plug potential, which is why the plug regime and coil-stress gates are the machine's hardest open problems.

All figures are design-and-simulation values for an unbuilt machine.