Ambipolar Potential Confinement

Hot plug plasma builds an electrostatic potential that traps central-cell ions in a well — the idea that makes a tandem mirror viable.

Plugging the ends with voltage

In any plasma, electrons are far more mobile than ions and try to escape first. Charge separation immediately builds an electric field that holds electrons back and, at the ends of a tandem mirror, that field can be shaped into a potential barrier for ions. By making the plug plasma hot and dense, the design raises the potential at each end above the central cell.

The confining well

Central-cell ions then live at the bottom of an electrostatic well with a peak at each end. An ion must climb that potential to leave. Confinement time is set by how deep the well is relative to the ion temperature — a Boltzmann-like factor. This is fundamentally different from magnetic confinement: the barrier is a voltage, not a field-line topology.

Thermal barriers

To raise the ion-confining potential without also over-heating the passing electrons, tandem mirrors use a thermal barrier — a local dip in potential that thermally isolates plug electrons from central-cell electrons, letting the plug electrons run hotter and the potential peak higher. Electron-cyclotron heating and sloshing ions are the tools that build and maintain this structure.

• Potential well depth scales confinement time exponentially
• Plug density and electron temperature set the well depth
• Thermal barrier decouples plug and cell electron populations
• This is the physics that distinguishes a tandem mirror from a plain mirror

The ambipolar scheme is elegant but demanding: it requires steady power into the plugs to hold the potential. That power, and the physics of holding the barrier in steady state, is one of the open questions the burner program treats honestly rather than assuming solved.