The SPARC Toroidal Field Model Coil Program is the 2024 IEEE Transactions on Applied Superconductivity paper in which 📝Zachary Hartwig and 97 co-authors set down the full technical record of the three-year effort that produced a 20-📝tesla fusion magnet.
The numbers are measurements, which in this corpus is worth saying plainly. Between 2018 and 2021 the 📝MIT Plasma Science and Fusion Center and 📝Commonwealth Fusion Systems jointly designed, built and tested the 📝Toroidal Field Model Coil (TFMC), a representative-scale coil roughly three meters across. It reached 20.1 T peak field on conductor at 40.5 kiloamps of terminal current, with 815 kilonewtons per meter of Lorentz loading on the 📝REBCO stacks and close to a gigapascal of mechanical stress carried by the structural case.
Three subsidiary results carry as much weight as the headline field. Fifteen internal demountable pancake-to-pancake joints ran at 0.5 to 2.0 nanoohms at 20 kelvin in fields up to 12 T, which is the practical proof that a magnet can be taken apart. The REBCO current lead and feeder system was qualified to 50 kA, and the cryocooler-based system delivered 600 watts of cooling at 20 K. And the team deliberately drove the coil into an open-circuit 📝quench at 31.5 kA to test whether a fusion-scale 📝no-insulation coil can protect itself passively — the experiment nobody performs by accident.
This is the paper we point to when someone asks what we have actually built rather than modeled. Eighteen coils of this lineage are going into 📝SPARC.