Superconducting magnet
From Free net encyclopedia
Superconducting magnets are electromagnets that are partially built from superconducting materials and therefore reach much higher magnetic field intensity.
Contents |
Construction
Composition
Coil windings
The coil windings of a superconducting magnet are made of wires of a type 2 superconductor (e.g.niobium-titanium).
Coil
The coil itself is made of tiny filaments (about 20 micrometers thick) of a type II superconductor, coated with copper. The copper coating is needed for adding mechanical strength to the fiber, and in case the temperature rises above Tc or Ic at which point superconductivity is lost. These filaments need to be this small because in this type of superconductor, the current only flows skindeep.
Cooling
Liquid helium is used as a coolant for superconducting materials with critical temperatures around 4.2 K. Liquid nitrogen is used for higher critical temperatures, or (being significantly cheaper) to cool a jacket around the helium.
Materials
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 kelvins. When operating at 10 kelvins they are able to withstand a much higher magnetic field intensity. 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 energy parts and Nb3Ti for the lower energy parts is used.
Use
Superconducting magnets can reach a much higher magnetic field intensity than normal magnets and they can hold this field for a long time. This is particularly needed in particle accelerators and nuclear magnetic resonance. For economic reasons these magnets are also used when a field of more than 1 tesla must be maintained for a long time.
