Control of a Tandem-Mirror Burner
How control differs for an open-ended mirror machine, where confinement rests on end-plug fields and potentials rather than a closed current.
A different confinement geometry
A tandem mirror confines plasma in a straight central cell capped by strong magnetic mirrors at each end, with end plugs that raise the potential to electrostatically confine ions. There is no large toroidal plasma current and no closed flux surface, so the control problems are different from a tokamak's: no vertical instability of an elongated current, no q profile, but new problems of plug fields and potentials.
What must be controlled
- End-plug magnetic field strength, which sets the mirror ratio and confinement
- The confining electrostatic potential in the plugs
- Central-cell density and pressure
- MHD stability of the open geometry, which lacks the tokamak's average good curvature
- Direct energy conversion of escaping charged particles at the ends
Stability of the open field
Open mirror geometries are prone to interchange and other MHD instabilities because the field can curve unfavorably. Stability is engineered through field shaping (minimum-B or tandem configurations) and can be aided by controlling the plasma pressure and the potentials. Maintaining MHD stability is a central design and control concern distinct from anything in a tokamak.
Potential and density control
In a tandem mirror the confining potential depends on the plug density and temperature, so controlling confinement means controlling the plug plasma - through heating and fueling of the plugs - as much as the central cell. This coupling between plug conditions and central-cell confinement is the machine's characteristic control loop.
Design context
The Kronos burner is a D-3He tandem-mirror generator with a 26.49 T plug field, a 17 T throat field, and direct energy conversion, in the Aegis (fixed defense) and MetroVolt (data-center) housings. These are simulated design parameters; the machine is not built, and the neutron fraction of about 5.44 percent reflects the design point, not measured hardware.