Burner Plug Control in a Tandem Mirror
A tandem mirror confines plasma between high-field end plugs; controlling the plug potentials is the heart of running the burner.
How a tandem mirror confines
A tandem mirror is a linear device with a long central cell and strong magnetic mirrors at each end. Additional end cells raise the electrostatic potential at the ends, forming a barrier that keeps ions in the central cell. Confinement depends on maintaining those end-plug conditions, which is a control problem distinct from a tokamak's.
The control variables
- End-plug magnetic field, up to 26.49 T at the plug and 17 T at the throat.
- Plug electrostatic potential, set by heating and fueling in the end cells.
- Central-cell density and temperature.
- Balance between the two ends to keep the trap symmetric.
Why it is demanding
The plug potential must be held against losses, and the two ends must stay balanced. Small imbalances degrade confinement, so the controller continuously adjusts end-cell heating and fueling based on an estimate of the plug state, using reduced-order models fast enough for real time.
No tokamak disruption
A tandem mirror does not carry a large plasma current, so it does not disrupt the way a tokamak does. The failure modes are loss of plug confinement rather than a current collapse, which changes what the control system watches for and how it protects the machine.
D-3He and low neutron fraction
The burner runs on D-3He with a neutron fraction of only 5.44 percent, so its control also manages the direct energy conversion that captures charged-particle energy, a feature a D-T tokamak lacks.
Shared discipline
Despite the different physics, plug control uses the same sense-estimate-act loop and the same twin-based training as the breeder.