HTS magnet

An electromagnet wound from high-temperature superconducting tape, producing very strong fields with no electrical resistance.

• HTS tape, usually a rare-earth copper oxide, runs at around 20 K (−253 °C) in fusion magnets, versus about 4 K for older superconductors.
• Fusion power density rises roughly with the fourth power of field strength, so a stronger field lets a tokamak shrink dramatically.
• In 2021 MIT and Commonwealth Fusion Systems ran a large HTS coil at 20 tesla, the enabling step for their compact SPARC tokamak.
• Tape is costly, and a 'quench', where a spot loses superconductivity, can release stored energy fast enough to damage the coil.