Superconducting magnet

A magnet whose coils carry current with no electrical resistance once cooled far below room temperature, so it can hold a strong field for long periods without the heat and power bill of copper coils. Most large fusion magnets use low-temperature superconductors such as niobium-tin, cooled with liquid helium to about 4 kelvin, as ITER does. Newer designs such as Commonwealth Fusion Systems' SPARC use high-temperature superconducting (HTS) tape, which carries more current in stronger fields and allows a more compact machine. Field strength matters because, at the same beta, a tokamak's fusion power per cubic metre rises with the fourth power of the field.

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