The Ambipolar Potential
Electrons leak faster than ions, leaving the central cell slightly positive; the tandem mirror amplifies this potential at the ends to confine ions.
The idea that makes the tandem mirror work
In any plasma, electrons are lighter and escape a mirror faster than ions. As they leave, the plasma is left with a slight positive charge, which builds an electric potential that holds the remaining electrons back and lets ions leak faster — the ambipolar potential that equalises the two loss rates. The tandem mirror engineers this potential to confine the central-cell ions.
The end plugs are dense, hot regions where the potential peaks above the central-cell potential. A central-cell ion trying to leave must climb this confining peak φp − φc. Only ions with enough parallel energy to surmount it escape; the rest are electrostatically reflected back into the burn region.
The demand and the gate
Sustaining a high plug potential requires a dense, energetic plasma held in an extreme magnetic field — 26.49 T. The confinement quality scales with the potential the plugs can hold, and holding it drives both the coil stress gate and the plug-regime gate. The ambipolar potential is the burner's core mechanism and the seat of its hardest physics.
A useful way to see the concept: the magnetic mirror confines electrons well and ions poorly, so the plasma builds a potential that trades some electron confinement for much better ion confinement, and the tandem mirror engineers that trade deliberately at the ends. The whole machine is, in a sense, a device for building and holding a particular electrostatic potential profile along a field line.
- Electrons escape first → plasma turns positive
- Ambipolar potential equalises loss rates
- Plugs peak the potential to reflect central ions
- Confinement ∝ the potential the plugs sustain