Like most people who have taken basic science classes or played with refrigerator magnets and paper clips, I knew that stacking up magnets can increase their magnetic pull. Three fridge magnets stuck to each other can hold a longer chain of paper clips than one. Until recently I did not know that their is actually a special way to arrange magnets to increase their lifting power much more. Invented by the late Klaus Halbach, a physicist at Lawrence Livermore National Laboratory in the 1980s (relatively recently in my world view of engineering advancements) the Halbach Array is just such an arrangement. It was invented to focus accelerator particle beams but is now finding many other applications in brushless motors, linear motors and are critical component in a new generation of maglev trains.
This is how the magnets are arranged in a Halbach array:
It does not matter which way N or S is and long as you are consistent to the diagram. It is not intuitive (to me anyway) but it turns out that this arrangement combines the magnetic flux along one side for a much greater force, while nearly canceling the pull on the other side.
“The diagram (below) shows the field from a strip of ferromagnetic material with alternating magnetization in the y direction (top left) and in the x direction (top right). Note that the field above the plane is in the same direction for both structures, but the field below the plane is in opposite directions. The effect of superimposing both of these structures is shown in the figure at the bottom:” Wikipedia
Here is another nice diagram that shows how the arrangement combines the flux of the different magnets.
The above diagram is from an excellent article entitled Build a Halbach Array which details the construction of a simple Halbach array using a wooden bock and Neodymium-Iron-Boron cube magnets. They point out how hard it is to push the magnets into position as they will always want to flip over and align N to S poles, hence the need for the wooden block and glue.
Halbach arrays can also be constructed in several cylindrical forms which turns out to be very useful for brushless AC motors, magnetic couplings and magnetic bearings.
You can learn more at Halbach Array which includes the customary Wikipedia complement of images, diagrams, formulas and links.
There is also a nice set of links at Halbach Array links
Now you know…..