Making Resistive Magnets
Table of Contents
Stacking Up
With all the plates finished and in tip-top shape, it's time to assemble them into a tower.
On its journey from the top of the magnet coil to the bottom, an electrical current wants to find the shortest, quickest route. That would not make for a very powerful magnet, however, so the coil stacking process is designed to make sure the current takes the longest possible path from point A to point B without melting the magnet. The idea is to have the current travel all around one loop, then move to the next loop, go all around it, then move to the next one, etc., etc., until the current has spiraled all the way up or down the coil. No shortcuts allowed.
To prevent the current from avoiding the long series of rotaries that make up a magnet coil, pieces of insulator are inserted at strategic spots between the Bitter plates. Like traffic cones lining a parade route, they make sure the electricity takes the long and winding path the magnet designers intended by blocking all possible shortcuts.
The insulators are interwoven with the Bitter plates in groupings called magnet turns. Generally from four to 16 plates, along with one or more insulators, make up a turn, depending on the turn's location within the coil and the magnet's design. Each turn is one loop in the current's path through the magnet. In the innermost of the three coils that make up our world record 35 T resistive magnet, for example, there are about a thousand Bitter plates. These plates are grouped into a total of 93 turns of four to nine discs each. So the current enters the coil and makes a total of 93 magnetic field-producing loop-de-loops before exiting.
At the MagLab, most coils are stacked in one of two ways: helix stacking or flat stacking. In helix stacking, a group of discs is woven with one disc-shaped insulator into a turn, as illustrated in this brief helix-stacking slideshow.
In flat stacking, short sections of insulator are stacked in a staggered pattern between Bitter discs in a way that forces the current along a short section of each disc. For example, a turn of six discs begins with a plate, on top of which is stacked a small insulator covering the disc between 12 o'clock to 2 o'clock. Another disc is stacked on this, followed by another insulator positioned between 2 o'clock to 4 o'clock. The third disc comes next, then the next insulator, placed in the 4 to 6 o'clock position … and so on until the last insulator (10 to 12 o'clock) is in place. This short flat-stacking slideshow illustrates how this is done.
All turns within a magnet coil are not created equal. Remember how the discs at the end of the coils have wider holes? There's another way these end-coil turns differ from mid-coil turns: they contain more disks. Turns with more disks have lower current density. It's important for the mid-coil turns to have the highest possible current density as this will yield the highest possible field for the experiments to be run at the center of the magnet.
But at the ends of the coil, the field can afford to be a little weaker, meaning less current density and more plates and a bit of savings on operating costs.
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