The 17 T Throat Coils
At each mirror throat the field is 17 T — the point of maximum field the escaping plasma must pass, and where the mirror force is strongest.
The narrow point
The throat is the constriction between the central cell and the plug where the magnetic field lines are packed most tightly. At the burner design point the throat field is 17 T. This is where a mirror does its mirroring: as an ion moves into rising field, its perpendicular energy grows at the expense of parallel energy, and if the field is high enough the ion's parallel motion reverses — it is reflected.
Mirror force and the loss cone
Whether an ion reflects depends on its pitch angle. Ions with too much parallel velocity fall into the loss cone and escape; the rest are turned back. The throat-to-cell field ratio sets the width of that loss cone. The 17 T throat combined with the low central-cell field yields a large mirror ratio, narrowing the loss cone so magnetic confinement carries much of the load and the electrostatic potential handles the rest.
Engineering at the throat
- High field and steep gradient — large local forces on the REBCO throat coil
- Peak plasma-facing heat flux from the streaming loss population
- Sets the geometry the expander must match to spread the exhaust
The throat and the 26.49 T plug are designed as a pair: the plug provides the confining well, the throat sets the mirror ratio, and together they define how much plasma leaks axially into the direct-conversion end. That controlled leak is not a loss to be eliminated — it is the fuel for direct energy conversion.