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Halbach Array

Yesterday I learned about something really cool called a Halbach array. It's a special arrangement of magnets that has a magnetic field on one side, but not the other.

The array and its effect were discovered in 1973 by John C. Mallinson, but it was named after physicist Klaus Halbach, who invented the array 7 years later - not knowing someone else had already gone to the trouble.

Halbach array, showing approximate field lines.
The way the Halbach array works is similar to a bunch of lined up horseshoe magnets. There are North and South poles, but they are all on the same side of the row. Horseshoe magnets are clumsy, so in the Halbach array, regular bar magnets are used instead. The magnets are oriented so that their magnetic fields match those of the horseshoe magnets they're replacing; that way, the effect is nearly the same.

A perfect Halbach array is only magnetic on one side, greatly reducing stray magnetic fields that can interfere with other equipment; and since the magnetic field is all on the same side, it is twice as strong as a regular magnet's field. Because of those properties, Halbach arrays are useful in a variety of applications, including:
  • Refrigerator magnets
  • Inductrack Maglev train system
  • Particle accelerators
  • Free electron lasers
Then there's the Halbach cylinder, where the magnetic field is in a bore down the center, and the Halbach sphere, where the magnetic field is in an empty spot inside the sphere. They aren't as simple as the array, so I won't explain them in this post.
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