Superconducting magnet
461227
225189263
2008-07-12T10:43:23Z
TomyDuby
2575906
/* Construction */ Added link to Type II superconductors and fixed a number of typos with units
{{Unreferenced|date=April 2008}}
'''Superconducting magnets''' are [[electromagnet]]s that are built using [[superconductivity|superconducting]] coils.
==Construction==
[[Image:20mag.gif|thumb|Schematic of a 20 tesla superconducting magnet]]
===Composition===
====Coil windings====
The coil windings of a superconducting [[magnet]] are made of wires or tapes of [[Type II]] [[superconductivity|superconductor]]s (e.g.[[niobium-titanium]]).
====Coils====
The coil itself may be made of tiny [[Electrical filament|filament]]s (about 20 [[micrometre|micrometers]] thick) of a type II [[superconductivity|superconductor]] in a [[copper]] matrix. The copper is needed for adding mechanical stability, and thermal stability in case the temperature rises above [[superconductivity|''T''<sub>c</sub> or the current rises above ''I''<sub>c</sub>]] at which point superconductivity is lost. These [[Electrical filament|filament]]s need to be this small because in this type of superconductor the current only flows skin-deep. The coil must be carefully designed to withstand (or counteract) [[magnetic pressure]] and [[Lorentz force]]s that could otherwise cause wire fracture or crushing of insulation between adjacent turns.
====Cooling====
Liquid [[helium]] is used as a [[Heat pump|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 [[superconductivity|critical temperature]] of 10 [[kelvin]]s and remains in this state until about 15 [[Tesla (unit)|teslas]]. More expensive [[electromagnet|magnets]] can be made of [[niobium-tin]] (Nb<sub>3</sub>Sn). These have a [[superconductivity|''T''<sub>c</sub>]] of 18 K. When operating at 4.2 K they are able to withstand a much higher [[field strength|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 Nb<sub>3</sub>Sn for the high field sections and Nb<sub>3</sub>Ti 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 Nb<sub>3</sub>Sn 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.
In 2007 [[YBCO]] was used to demonstrate a world record of 26.8 [[Tesla (unit)|Tesla]] for a superconductive magnet<ref>http://www.superpower-inc.com/index.php?p=26</ref>.
==Use==
Superconducting magnets have a number of advantages over [[Electrical resistance|resistive]] electromagnets. The field is generally more stable, resulting in less noisy measurements. They can be smaller, allowing more freedom in the configuration of the rest of the device (such as a [[cryostat]]), and consume much less power - in fact, power consumption is negligible in the steady field state. Higher fields, however can be achieved with cooled resistive and [[hybrid magnets]], as the superconducting coils will enter the normal (non-superconducting) state (see quench, below) at high fields.
===Magnet quench===
A quench occurs when part of the superconducting coil enters the normal state. This can be because the field inside the magnet is too great, the rate of change of field is too great (causing [[eddy current | eddy currents]] and resultant [[Joule heating|heating]] in the copper support matrix), or a combination of the two. More rarely a defect in the magnet can cause a quench. When this happens, that particular spot is subject to rapid joule heating, which raises the temperature of the surrounding regions. This pushes these into the normal state as well, which leads to more heating. The entire magnet rapidly (in less than a second) becomes normal. This is accompanied by a loud bang and rapid boil-off of the cryogenic fluid. Permanent damage to the magnet is rare, but components can be damaged by localised heating or large mechanical forces.
==References==
{{Reflist}}
==See also==
[[Fault current limiter]]
<!-- Unsourced image removed: [[image:12T-2.jpg|thumb|An 11.5 T magnet with electronics used at [[NIST]] for [[neutron scattering]].]] -->
[[Category:Types of magnets]]
[[Category:Superconductivity]]
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