Superconducting magnet

A superconducting magnet is an electromagnet that is built using superconducting coils. They must be cooled to cryogenic temperatures during operation. Their advantages are that they can produce stronger fields than ordinary iron-core electromagnets, and can be cheaper to operate, since no power is lost to ohmic resistance in the windings.

Liquid helium is used as a coolant for superconducting windings with critical temperatures around its boiling point of 4.2 K. The magnet and coolant are contained in a thermally insulated container (dewar) called a cryostat. To keep the helium from boiling away, the cryostat is usually constructed with an outer jacket containing (significantly cheaper) liquid nitrogen at 77 K.

The superconducting portions of most such magnets are composed of niobium-titanium. This material has critical temperature of 10 Kelvins and remains in this state until about 15 Teslas. More expensive magnets can be made of niobium-tin (Nb3Sn). These have a Tc of 18 K. When operating at 4.2 K they are able to withstand a much higher magnetic field intensity, up to 25 to 30 Teslas. Unfortunately, it is far more difficult to make the required filaments from this material. This is why sometimes a combination of Nb3Sn for the high field sections and Nb3Ti for the lower field sections is used. High temperature superconductors (BSCCO or YBCO) may be used for high-field inserts when magnetic fields are required which are higher than Nb3Sn can manage. BSCCO, YBCO or magnesium diboride may also be used for current leads, conducting high currents from room temperature into the cold magnet without an accompanying large heat leak.