Making Resistive Magnets

Forces to Be Reckoned With

Heat isn't the only devil in the details of resistive magnet design.

End-turn (top) and mid-turn discs.

Inside any magnet coil, there are really two levels of magnetic fields. There's the larger field of the magnet generated by the multiple megawatts of power surging through it, and there are the much smaller fields generated by each of the countless electrons in motion. When these smaller fields interact with the larger field, a force (called the Lorentz force) is exerted, which pushes the plates outward. This isn't a problem in the middle of the coil (more on that in a minute), but it is at the top and bottom.

Because of this, holes in the top and bottom plates must be a bit wider so that they don't plug up the flow of cooling water when they shift out of alignment with the middle plates.

The middle plates have a secret weapon to counter the Lorentz force: magnetic clamping. This happens when wires (or in this case, coils) conduct current in parallel. (Our interactive tutorial on parallel wires illustrates this phenomenon). The direction of magnetic fields circling around these wires or coils is such that they attract each other. So discs in the middle of the coil attract each other like opposing magnets. This keeps the discs clamped together, even as the Lorentz force tries to force them outward, as illustrated below. Because magnetic clamping isn't strong at the ends of the coil, the Lorentz force can only be dealt with by making those holes wider.

MagLab engineers were the first to figure out how best to design around the interacting forces of magnetic clamping and the Lorentz force, giving them a 10 percent increase in efficiency. This advance occurred in 2000 while engineers were trying to set a new record of 45 tesla with our hybrid magnet (part superconducting magnet, part resistive magnet). The innovation of wider holes at the ends of the coils helped them achieve this goal.

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