Editing Electromagnetic shielding (section)

== Magnetic shielding ==
Equipment sometimes requires isolation from external magnetic fields.<ref>{{cite journal | last1 = Hobson | first1 = P. J. |display-authors=et al | year = 2022 | title =  Bespoke magnetic field design for a magnetically shielded cold atom interferometer | journal = Sci. Rep. | volume = 12 | issue = 1 | pages = 10520 | doi= 10.1038/s41598-022-13979-4 | pmid = 35732872 | pmc = 9217970 | arxiv = 2110.04498 | bibcode = 2022NatSR..1210520H | s2cid = 238583775 }}</ref>  For static or slowly varying magnetic fields (below about 100&nbsp;kHz) the Faraday shielding described above is ineffective.  In these cases shields made of high [[magnetic permeability]] metal [[alloy]]s can be used, such as sheets of [[permalloy]] and [[mu-metal]]<ref>{{cite web |url=http://www.magnetic-shield.com/literature/pdf/mu-2.pdf |title=MuMETAL |id=Catalog MU-2 |publisher=Magnetic Shield Corp. |date=2012 |access-date=26 June 2016 }}{{Dead link|date=March 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref>{{Cite web|url=http://tsdr.uspto.gov/#caseNumber=73410422&caseType=SERIAL_NO&searchType=statusSearch|title=Trademark Status & Document Retrieval|website=tsdr.uspto.gov|language=en|access-date=2017-08-02}}</ref> or with nanocrystalline grain structure ferromagnetic metal coatings.<ref>{{cite web |url=http://www.nxtbook.com/nxtbooks/item/emcdirectory-design_2009/index.php#/134 |title=Interference Technology Magazine Whitepaper on Ferromagnetic Nanocrystalline Metal Magnetic Shield Coatings |url-status=dead |archive-url=https://web.archive.org/web/20100315010701/http://www.nxtbook.com/nxtbooks/item/emcdirectory-design_2009/index.php#/134 |archive-date=March 15, 2010 }}</ref>  These materials do not block the magnetic field, as with electric shielding, but rather draw the field into themselves, providing a path for the [[magnetic field lines]] around the shielded volume.  The best shape for magnetic shields is thus a closed container surrounding the shielded volume.  The effectiveness of this type of shielding depends on the material's permeability, which generally drops off at both very low magnetic field strengths and high field strengths where the material becomes [[magnetic saturation|saturated]].  Therefore, to achieve low residual fields, magnetic shields often consist of several enclosures, one inside the other, each of which successively reduces the field inside it. Entry holes within shielding surfaces may degrade their performance significantly.

Because of the above limitations of passive shielding, an alternative used with static or low-frequency fields is active shielding, in which a field created by [[electromagnet]]s cancels the ambient field within a volume.<ref>{{cite web |url=http://www.acornnmr.com/appnotes/shielding.htm |title=NMR Magnet Shielding: The seat of the pants guide to understanding the problems of shielding NMR magnets |publisher=Acorn NMR |date=22 January 2003 |access-date=27 June 2016}}</ref>  [[Solenoid]]s and [[Helmholtz coil]]s are types of coils that can be used for this purpose, as well as more complex wire patterns designed using methods adapted from those used in coil design for [[magnetic resonance imaging]]. Active shields may also be designed accounting for the electromagnetic coupling with passive shields,<ref>{{Cite journal |last1=Packer |first1=M. |last2=Hobson |first2=P.J. |last3=Holmes |first3=N. |last4=Leggett |first4=J. |last5=Glover |first5=P. |last6=Brookes |first6=M.J. |last7=Bowtell |first7=R. |last8=Fromhold |first8=T.M. |date=2020-11-03 |title=Optimal Inverse Design of Magnetic Field Profiles in a Magnetically Shielded Cylinder |url=https://link.aps.org/doi/10.1103/PhysRevApplied.14.054004 |journal=Physical Review Applied |volume=14 |issue=5 |pages=054004 |doi=10.1103/PhysRevApplied.14.054004|arxiv=2006.02981 |bibcode=2020PhRvP..14e4004P |s2cid=221538013 }}</ref><ref>{{Cite journal |last1=Packer |first1=M. |last2=Hobson |first2=P.J. |last3=Holmes |first3=N. |last4=Leggett |first4=J. |last5=Glover |first5=P. |last6=Brookes |first6=M.J. |last7=Bowtell |first7=R. |last8=Fromhold |first8=T.M. |date=2021-06-02 |title=Planar Coil Optimization in a Magnetically Shielded Cylinder |url=https://link.aps.org/doi/10.1103/PhysRevApplied.15.064006 |journal=Physical Review Applied |volume=15 |issue=6 |pages=064006 |doi=10.1103/PhysRevApplied.15.064006|arxiv=2101.01275 |bibcode=2021PhRvP..15f4006P |s2cid=230524109 }}</ref><ref>{{Cite journal |last1=Liu |first1=C. -Y. |last2=Andalib |first2=T. |last3=Ostapchuk |first3=D. C. M. |last4=Bidinosti |first4=C. P. |date=2020-01-01 |title=Analytic models of magnetically enclosed spherical and solenoidal coils |url=https://www.sciencedirect.com/science/article/pii/S0168900219312719 |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |language=en |volume=949 |pages=162837 |doi=10.1016/j.nima.2019.162837 |arxiv=1907.03539 |bibcode=2020NIMPA.94962837L |s2cid=195833040 |issn=0168-9002}}</ref><ref>{{Cite journal |last1=Mäkinen |first1=Antti J. |last2=Zetter |first2=Rasmus |last3=Iivanainen |first3=Joonas |last4=Zevenhoven |first4=Koos C. J. |last5=Parkkonen |first5=Lauri |last6=Ilmoniemi |first6=Risto J. |date=2020-08-14 |title=Magnetic-field modeling with surface currents. Part I. Physical and computational principles of bfieldtools |url=http://aip.scitation.org/doi/10.1063/5.0016090 |journal=Journal of Applied Physics |language=en |volume=128 |issue=6 |pages=063906 |doi=10.1063/5.0016090 |arxiv=2005.10060 |bibcode=2020JAP...128f3906M |s2cid=218718690 |issn=0021-8979}}</ref><ref>{{Cite journal |last1=Zetter |first1=Rasmus |last2=J. Mäkinen |first2=Antti |last3=Iivanainen |first3=Joonas |last4=Zevenhoven |first4=Koos C. J. |last5=Ilmoniemi |first5=Risto J. |last6=Parkkonen |first6=Lauri |date=2020-08-14 |title=Magnetic field modeling with surface currents. Part II. Implementation and usage of bfieldtools |url=http://aip.scitation.org/doi/10.1063/5.0016087 |journal=Journal of Applied Physics |language=en |volume=128 |issue=6 |pages=063905 |doi=10.1063/5.0016087 |arxiv=2005.10056 |bibcode=2020JAP...128f3905Z |s2cid=218719330 |issn=0021-8979}}</ref> referred to as ''hybrid'' shielding,<ref>{{Cite journal |last1=Royal |first1=Kevin |last2=Crawford |first2=Christopher |last3=Mullins |first3=Andrew |last4=Porter |first4=Greg |last5=Blanton |first5=Hunter |last6=Johnstone |first6=Connor |last7=Kistler |first7=Ben |last8=Olivera |first8=Daniela |date=2017-09-01 |title=Hybrid Magnetic Shielding |journal=APS Division of Nuclear Physics Meeting Abstracts |url=https://ui.adsabs.harvard.edu/abs/2017APS..DNP.EA034R |volume=2017 |pages=EA.034|bibcode=2017APS..DNP.EA034R }}</ref> so that there is broadband shielding from the passive shield and additional cancellation of specific components using the active system.

Additionally, [[superconductivity|superconducting]] materials can expel magnetic fields via the [[Meissner effect]].