The Expander Region

Past the mirror throat the field weakens and the plasma expands and cools into a beam — the region where direct conversion actually happens.

Where confinement ends and conversion begins

In a tandem mirror the plasma is held between magnetic plugs; the small fraction that leaks out the ends flows into the expander, a region where the magnetic field falls off rapidly. As the field weakens, the plasma expands to follow it, and the physics of that expansion is what turns confined plasma into a directed beam suitable for direct energy conversion.

Flux expansion and cooling

Conservation of magnetic flux means that as field lines fan out, the plasma cross-section grows and the density drops. The parallel streaming energy of the ions is largely preserved while the perpendicular (gyration) energy is converted to parallel motion — the plasma cools transversely and streams more directionally. The result is a lower-density, more beam-like flow spread over a large area, which is precisely what the electrostatic and traveling-wave collectors need.

Why the geometry matters for DEC

• Spreading the flow lowers the power density on any single collector surface, easing heat and space-charge limits.
• Making the flow directional improves the match to velocity-sorted TWDEC bunching.
• The expanding field is also where an MHD channel can sit, drawing current from the ordered flow.

The tandem-mirror synergy

The expander is not an add-on; it is intrinsic to the tandem-mirror architecture. The same open-ended geometry that some see as the mirror's weakness — end losses — is what delivers a natural, directional exhaust to the direct converters. In the burner this turns a confinement characteristic into the front door of the DEC train.