Ambipolar Potential

The plug plasma builds an electrostatic potential peak that reflects central-cell ions along the axis — the mechanism that makes a tandem mirror confine.

In any mirror plasma, electrons escape faster than ions because they are lighter, leaving the plasma slightly positive. That charge imbalance builds an ambipolar potential. In a tandem mirror the plugs are engineered to make this potential peak sharply at each end, high enough to reflect central-cell ions electrostatically as they approach the loss cone.

The plug potential must exceed the central-cell ion energy — near 90 keV — for the barrier to hold the fuel ions. Building and sustaining a potential that tall requires a dense, hot plug plasma confined by the 26.49 T field. This is the physical reason the plug field is so extreme: the potential it must support is what confines the whole machine.

The potential profile

Along the axis the electrostatic potential rises from the central cell to a peak in each plug and falls again into the expander. Ions born in the central cell see a hill at each end and are reflected; only ions that scatter into the loss cone and climb the hill escape. Electrons see the mirror image of this profile. The shape of the potential is set by the plug plasma density and temperature.

Why it is fragile

• The potential must stay above central-cell ion energy at all times
• It depends on maintaining the dense plug plasma
• The plug plasma sits in an un-post-dictable regime (166–830x)
• Loss of plug potential collapses central-cell confinement

The ambipolar barrier is elegant and well grounded in mirror theory, but sustaining it at burner parameters is exactly what the plug-regime gate says is unproven. All figures are design-and-simulation values.