superconductor is a material that, when cooled below its critical temperature, carries electric current with zero electrical resistance.
The phenomenon was discovered in 1911 by Heike Kamerlingh Onnes, who watched the resistance of mercury vanish abruptly at 4.2 kelvin. A superconductor is not merely a very good conductor: even copper dissipates power as heat, while a current started in a superconducting loop persists undiminished for years. The superconducting state survives only inside a boundary set by three limits — the critical temperature, the critical current, and the critical magnetic field — and exceeding any one of them returns the material to ordinary resistive conduction.
Superconductors fall into two broad families. The low-temperature superconductors, metallic alloys such as niobium-titanium and niobium-tin, must be chilled to within a few degrees of absolute zero. The high-temperature superconductors, copper-oxide ceramics discovered from 1986 onward, superconduct at temperatures reachable with far less refrigeration and keep working in much stronger magnetic fields. For 📝fusion energy the property is decisive: confining a 100-million-degree 📝plasma takes electromagnets carrying tens of thousands of amperes continuously, which only resistance-free conductors can do without ruinous power loss. Winding those coils from high-temperature superconductor is what allows 📝High-Temperature Superconducting (HTS) Magnets to reach roughly 20 tesla in a machine as compact as 📝SPARC.