The 26.49 T Plug Magnets
The end-plug coils reach a 26.49 T peak field — the strongest in the machine — set by the confinement the tandem-mirror scheme demands.
Why 26.49 T
The plug field is not chosen for spectacle; it falls out of the confinement requirement. The mirror ratio — the ratio of peak plug field to central-cell field — governs how many ions are reflected versus lost through the loss cone. A high peak field gives a large mirror ratio and a deep confining well. The reference design point places that peak at 26.49 T, with 17 T at the throat.
Only high-temperature superconductor can do this
A 26.49 T DC field at usable scale is beyond low-temperature superconductors like Nb3Sn, which quench above roughly 23 T. The plug coils use REBCO high-temperature superconducting tape, which carries current at high field and can operate at elevated temperature headrooms. REBCO is the enabling technology for both the burner plugs and the breeder's high-field magnets.
What high field demands of the engineer
- Lorentz forces scale with field squared — the coil must be heavily structurally reinforced
- Quench protection must dump large stored magnetic energy safely
- Field gradients drive eddy and screening currents in the tape stack
- Neutron flux is low here (5.44% fraction) but not zero — see shielding
The plug magnet is the single most stressed component in the machine, and its design is coupled to the cryogenics, the structural cage, and quench management. Because MetroVolt is a design study, these coils exist as engineering models validated against REBCO conductor data, not yet as a built plug.