critical magnetic field is the magnetic field strength above which a superconductor stops superconducting and reverts to ordinary resistive conduction.
The limit takes two forms. Type I superconductors — most pure superconducting elements — expel magnetic fields entirely and lose superconductivity at a single, low critical field. Type II superconductors, which include every magnet-grade material, have two thresholds: above a lower critical field the magnetic field begins threading through the material in quantized filaments, and superconductivity survives in this mixed state up to a much higher upper critical field, written B_c2. It is the upper critical field that matters for engineering, because it sets the strongest magnet a material can ever be wound into. Like the 📝critical current, it rises as the conductor is cooled further below its 📝critical temperature — the three limits together fence in the superconducting state.
The spread between materials is what reshaped 📝fusion. Niobium-titanium runs out near 10 tesla and niobium-tin in the low twenties, which is why magnets built from them plateau around 12 to 13 tesla in practice. REBCO's upper critical field at low temperature is measured in the hundreds of tesla, so the practical ceiling of a 📝superconductor magnet becomes mechanical stress rather than the conductor itself — the opening exploited by 📝High-Temperature Superconducting (HTS) Magnets at roughly 20 tesla.