cryogenics is the branch of physics and engineering that deals with producing and maintaining very low temperatures — in fusion, the refrigeration that keeps superconducting magnets cold enough to superconduct.
The word joins the Greek kryos, "icy cold," to the suffix for producing — literally the making of cold. It should not be confused with the 📝cryostat, which is the vessel that holds cold things; cryogenics is the discipline and machinery that makes them cold. Its working range begins roughly where ordinary refrigeration gives out, around −150 °C, and extends down through the boiling points of nitrogen (77 kelvin) and helium (4.2 kelvin) toward absolute zero.
A fusion plant built on superconducting magnets is, among other things, a large cryogenic facility. A 📝superconductor works only below its 📝critical temperature, so the magnets sit in a cold mass that must be chilled for weeks at startup and held cold for years of operation, with cryoplants circulating helium against every watt that leaks in. The margin matters: low-temperature superconductors demand liquid-helium conditions near 4 kelvin, while the 📝REBCO tape in 📝High-Temperature Superconducting (HTS) Magnets superconducts usefully at around 20 kelvin — still profoundly cold, but far cheaper to reach and hold, with enough thermal headroom to simplify the whole cooling plant.