MagLev Trains: Superconductivity Explained

Watch science videos online and learn how superconductors, magnetic fields and magnetic levitation work, from flux-pinning to maglev trains at The Emergent Universe, an online interactive science museum about emergence.

Superconducting Levitation

Floating Train

This model train contains a piece of superconducting material. The superconductor repels the magnetic track, causing the train to float.

How does superconducting levitation work?

No Strings!

At room temperature, when the superconducting disk is in its normal non-superconducting state, the magnet's field – indicated in the video by field lines – can pass through the disk; consequently, there is no repulsion and the magnet sits on the disk. When the disk is cooled into its superconducting state, it expels the magnet's field, pushing up the magnet, and causing it to float. This is a demonstration of the Meissner-Ochsenfeld effect.

Making a (Bad) Train

Because superconductors expel magnetic fields, a track created from rows of magnets, each 3 magnets across with alternating North-South-North poles, as shown, can levitate a superconductor. But while this effect can keep the superconductor suspended, it doesn't have the forces necessary to keep the superconductor "train" on the track; instead the superconductor easily falls off. What's needed to make a good superconductor train that will stay on track? A phenomenon called flux pinning.

Trick of Attachment: Flux Pinning

When certain types of superconductors are cooled in the presence of a magnet (of appropriate strength), tube-like areas of the material remain in the non-superconducting state. These tubes continue to allow the magnetic field to pass, even as the surrounding material becomes superconducting and expels the field. Because the details of these tubes are specific to the magnetic field they were formed around, any change in the superconductor’s position relative to the field would disrupt the field’s flow through the tubes and cost energy. As a result, the superconductor remains pinned to the magnet.

Staying on Track: A Good Train

Flux-pinning keeps a good train on track. Because the magnetic field emanating from the track is the same all along the track, the flux-pinned superconductor can move along the track without disrupting the trapped field lines. Movements of the superconductor off or away from the track, however, would require disrupting these field lines, which costs energy. This cost keeps the train from leaving the track, enabling it to follow around curves.*

*Note that while commercial superconducting magnetic levitation trains do exist, they operate on different principles than the model trains shown here.

What Are Magnetic Field Lines?

Recall that all magnets, like the one shown here, have a north (N) and south (S) pole. When two magnets are brought together, their “unlike” poles (N and S) will attract each other. Conversely, their “like” poles (N and N, or S and S) will repel each other. Scientists draw “field lines” around a magnet to indicate how a compass (which is just another tiny magnet) would respond to that magnet at each point. Its north pole (blue) traces out the magnet's field lines and illustrates the magnet's force field.

How Does a Superconductor Expel a Magnetic Field?

When a superconductor is placed nest to a magnet, electric currents form near the surface of the superconductor. These currents form in just the right way to generate a magnetic field inside the superconductor that exactly cancels the field from the nearby magnet, leaving the interior of the superconductor with zero magnetic field. These currents also change the field outside of the superconductor, and the net effect is the expulsion of field lines from inside the superconductor to outside the superconductor.

Tubes and Vortices

The boundaries between the tube-like, non-superconducting areas and the surrounding superconducting region are created by tiny, whirlpool-shaped electric currents, called vortices. These whirlpool currents generate a canceling field in the surrounding region (effectively expelling the magnet’s field), but not in their central cores (which become the tubes through which the magnet’s field can pass). While in a perfectly pure material, these vortices could move around freely, the defects and impurities found in most materials trap the vortices, making it energetically costly for them to move in response to changes in the magnet’s field.

Resources

Overviews

Overview from the Coalition for the Commercial Application of Superconductors: http://www.ccas-web.org/superconductivity/overview/

Popular press review of top superconducting applications (as of 2011): P. M. Grant, "Fantastic five," Phys. World Apr, 23-25 (2011); http://tiny.cc/8tjai

Non-technical review of superconductor applications: J. Clarke and D. C. Larbalestier, "Wired for the Future ," Nature Physics Commentary 2(12), 794-796 (2006); http://www.nature.com/nphys/journal/v2/n12/full/nphys472.html

Review of both current applications and those nearing commercialization (as of 2005) for high Tc superconductors (a bit technical): A. P. Malozemoff, et al., "High-Temperature Cuprate Superconductors Get To Work," Phys. Today 58(4), 41-47 (2005); http://dx.doi.org/10.1063/1.1955478

MagLev

How Superconducting MagLev works: http://www.maglev2000.com/works/how-02.html

Wikipedia compares different MagLev technologies (some of which use conventional rather than superconducting magnets): http://en.wikipedia.org/wiki/Maglev_%28transport%29

Tokyo-Osaka Superconducting MagLev Line: http://search.japantimes.co.jp/cgi-bin/nb20080826a3.html

Magnets

About Magnetic Resonance Imaging (MRI): http://www.magnet.fsu.edu/education/tutorials/magnetacademy/mri/index.html

CERN LHC: http://public.web.cern.ch/public/en/Research/Accelerator-en.html

CERN superconducting magnets: http://physicsworld.com/cws/article/indepth/31745

Original Large Hadron Rap video: http://youtu.be/j50ZssEojtM

SQUIDs and Other Sensors

How SQUIDS work (some math): http://cnx.org/content/m22750/1.3/

Magnetoencephalography (MEG): http://uuhsc.utah.edu/uumsi/what-is-msi.html#puting-it-together

Geo-prospecting CSIRO magnetometer: http://www.csiro.au/science/geophysics--ci_pageNo-7.html

UXO detection: http://www.serdp.org/Program-Areas/Munitions-Response/Land/Sensors/MR-1316

Problems and solutions for UXO detection (pdf): Report of the Defense Science Task Force on Unexploded Ordnance (Nov 2003); www.cpeo.org/pubs/UXO_Final_12_8.pdf 

Kitty litter and Gamma-ray detectors: http://www.nature.com/news/2006/060316/full/news060313-13.html

Sensors at NIST: http://qdev.boulder.nist.gov/ (click on quantum sensors)

General non-technical review of superconducting sensor applications: K. D. Irwin, "Seeing with Superconductors," Sci. Amer. Nov, 86-94 (2006); http://www.scientificamerican.com/article.cfm?id=seeing-with-superconducto

Energy

The problem with the US energy grid: http://www.nytimes.com/2008/08/27/business/27grid.html

Inside Renewable Energy interviews Greg Yurek, CEO of American Superconductor, about superconductors and the energy grid, Jan. 2011 (podcast): http://www.renewableenergyworld.com/rea/podcast/play/are-superconductors-finally-coming-of-age

Superconductors and the US energy grid: http://www.renewableenergyfocus.com/view/5926/rise-of-the-superconductor-part-2/

Superconductors and wind energy: http://www.renewableenergyfocus.com/view/3224/rise-of-the-superconductor-/

Wikipedia on the Korean Jeju Smart-Grid Project: http://en.wikipedia.org/wiki/Jeju_Smart_Grid_Demonstration_Project_in_Korea#Jeju_Smart_Grid_Test-Bed

US DOE High Temperature Superconductivity page: http://www.oe.energy.gov/hts.htm

Short article discussing the need for energy storage on the grid: http://science.howstuffworks.com/environmental/green-tech/sustainable/grid-energy-storage.htm

HTS Flywheel Energy Storage (pdf): www.osti.gov/bridge/servlets/purl/918644-iL8FDI/918644.pdf

More Technical

Review of high temperature and heavy fermion superconductors at an undergraduate physics level: D. L. Cox and M. B. Maple, "Electronic Pairing in Exotic Superconductors ," Phys. Today 48, 32–40 (1995); http://dx.doi.org/10.1063/1.881443

Technical review of SQUIDs and their commercial applications; applications section is accessible with intro physics: R. Kleiner, et al., "Superconducting Quantum Interference Devices: State of the Art and Applications," Proc. IEEE 92(10), 1534-1548 (2004); http://dx.doi.org/10.1109/JPROC.2004.833655

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