Superconducting Coils

The electromagnets that confine plasma at hundreds of millions of degrees — without melting themselves.

Superconducting coils are electromagnets wound from materials that carry electric current with zero resistance when cooled below a critical temperature. In magnetic-confinement fusion devices — tokamaks, stellarators, and mirror machines — these coils generate the intense magnetic fields (typically 5–13 T on axis) needed to confine plasma hot enough for deuterium-tritium or advanced-fuel reactions.

Why Superconductors Matter for Fusion

Resistive copper magnets can produce comparable field strengths, but they dissipate enormous ohmic heating losses — hundreds of megawatts in a reactor-scale device. A superconducting coil, once energized, circulates current indefinitely with negligible power input. This single fact transforms fusion from a net-energy impossibility into a plausible power source. The recirculating-power fraction of a fusion plant drops from >50% with copper coils to <10% with superconductors.¹

Low-Temperature vs. High-Temperature Superconductors

Historically, fusion magnets used low-temperature superconductors (LTS) such as NbTi and Nb₃Sn, which require liquid-helium cooling to 4.2 K. ITER's toroidal-field coils, the largest superconducting magnets ever built, use Nb₃Sn cable-in-conduit conductors operating at roughly 4.5 K and producing 11.8 T.²

A newer generation of devices — notably MIT/CFS's SPARC and Tokamak Energy's ST-HTS — employ high-temperature superconductors (HTS), primarily REBCO tape, which can operate at 20 K and sustain fields above 20 T. Higher field strength shrinks the required plasma volume roughly as B⁻⁴, dramatically reducing machine size and cost.³

Coil Configurations

A tokamak typically employs three coil systems: toroidal-field (TF) coils that wrap around the torus, a central solenoid (CS) that drives plasma current inductively, and poloidal-field (PF) coils that shape and position the plasma. Stellarators replace the plasma-current drive with complex, non-planar coils whose precise 3-D geometry produces the confining field entirely externally.⁴

Engineering challenges include quench protection, joints between conductor lengths, nuclear heating from 14.1 MeV neutrons in D-T devices, and the mechanical loads imposed by Lorentz forces on current-carrying conductors in high fields.

Sources

1. Bottura, L. et al., 'Superconducting Magnets for Particle Accelerators and Fusion Devices,' CERN Yellow Reports, 2020.
2. Mitchell, N. et al., 'The ITER Magnets: Design and Construction Status,' IEEE Transactions on Applied Superconductivity, vol. 22, no. 3, 2012.
3. Whyte, D. et al., 'Smaller and Sooner: How High-Field Superconducting Magnets Can Accelerate Fusion Energy,' Journal of Plasma Physics, vol. 82, 2016.
4. Bromberg, L. et al., 'Structural Design of Superconducting Coils for Fusion Applications,' Fusion Engineering and Design, vol. 87, 2012.

Related

REBCO Tape Nb₃Sn Superconductor NbTi Superconductor Magnet Quench Toroidal Field