Magnetic Expander & Flux Expansion

Flux expansion trades the plasma's gyration energy for directed streaming and spreads the flow, setting the beam the converters receive.

The magnetic funnel in reverse

A magnetic expander is a diverging field that acts like a funnel run backward: plasma entering the narrow, strong-field throat spreads out as it follows the widening field lines into weak field. Because the magnetic moment of each ion is conserved as the field changes slowly, the perpendicular energy tied to gyration decreases as the field drops, and that energy reappears as parallel, directed motion.

What the ratio buys you

The expansion ratio — the field at the throat divided by the field at the collector plane — sets how much the flow spreads and how directional it becomes. A large ratio spreads the power over a big collector area, lowering the heat flux and the space-charge density that would otherwise limit the converters, and it improves the parallel-to-perpendicular energy ratio the converters prefer.

Design tensions

• A larger expansion ratio eases collector loads but makes the converter structure physically larger.
• The field must fall smoothly — abrupt changes break magnetic-moment conservation and scatter ions.
• Stray fields at the collector plane can deflect low-energy ions and must be controlled.

Tie to the burner magnets

The expander field is graded down from the 17 T throat provided by the burner's superconducting magnets. Because the same magnet system sets both the confining throat and the expanding funnel, the expansion ratio is chosen alongside the confinement design, not independently — another instance of the burner's tightly coupled magnetics and conversion.