The Thermal Barrier

A local potential dip isolates plug electrons from central-cell electrons, letting the plug run hotter and the confining potential peak higher.

Why a barrier is needed

The ambipolar potential that confines central-cell ions is tied to the plug electron temperature — hotter plug electrons make a higher potential peak. But if plug and central-cell electrons are thermally connected, heating the plug electrons drags the whole electron population up and the advantage is lost. The thermal barrier breaks that connection.

How it works

Between the central cell and the plug the design engineers a dip in the electrostatic potential. This dip reflects lower-energy electrons, thermally insulating the plug electrons from the cooler central-cell electrons. The plug electrons can then be heated hard by electron-cyclotron resonance heating and reach a high temperature, driving a tall ion-confining peak just outboard of the barrier.

Maintaining the dip

The barrier dip is sustained by pumping ions out of the barrier region — otherwise ions fill the dip and wash it out. Charge-exchange pumping and carefully aimed neutral beams that create a sloshing-ion population keep the barrier region depleted of trapped ions. The interplay of ECRH, beam geometry, and pumping is the core of tandem-mirror potential control.

• Barrier isolates plug electrons so ECRH is effective
• Requires active ion pumping to stay open
• Sloshing ions and beam aiming shape the barrier
• Directly sets achievable central-cell confinement

The thermal barrier was the central advance that turned early tandem mirrors from marginal to promising. It is also one of the burner's most physics-intensive subsystems, and its steady-state control is treated as an open engineering question in the design study.