Plasma Flow in the Expander
As the field falls the exhaust converts gyration into directed motion, becoming an ordered ion beam the converter can decelerate cleanly.
From gyration to a beam
Inside the machine ions carry much of their energy as perpendicular gyration around field lines. A direct converter cannot use that — it can only decelerate ordered, forward motion. The expander performs the conversion. As field strength drops along the flow, the magnetic moment stays constant, so perpendicular energy falls and parallel energy rises. The plasma leaves as a directed stream.
Adiabatic expansion of the flux tube
The flux tube cross-section grows in inverse proportion to the field. A tube squeezed to the 17 T throat fans out enormously as the field falls toward the end wall, spreading power density and cooling the flow perpendicular temperature. This adiabatic expansion is what both protects the collector surface and orders the beam for conversion.
Keeping the flow clean
- Low perpendicular temperature at the collector improves conversion efficiency
- Large expansion ratio lowers heat flux on the end wall
- Electron and ion behavior must stay well-ordered — turbulence would blur the energy spectrum
- Neutral gas from the flow is removed by the vacuum system
The quality of the expander flow directly sets how well the staged grids can sort ions. A cold, directed beam converts efficiently; a hot, disordered one does not. So the expander is not a passive drift space — its length, expansion ratio, and field profile are engineered as part of the conversion system.