Making Resistive Magnets

The Hole Story

There is quite a bit of science and engineering that goes into determining the precise size, shape, number and pattern of holes on any particular plate, which vary from magnet to magnet.

Old-style Bitter plate.

The larger, round holes accommodate rods that are used to keep the plates in place as they are stacked one on top of the other. There is no wiggle room: Wiggling could translate into blocked holes, and blockages, as we will see, could trigger a meltdown.

The narrower, elongated holes prevent the plates from melting. They do this by funneling vast amounts of cold water right through the magnet, made quite hot by tremendous amount of current (we're talking megawatts). Were it not for the cold water rushing through at the rate of up to 15,000 liters (4,000 gallons) a minute, our magnets would quickly melt into copper puddles. (Deionized water is used to cool the magnets; unlike tap water, it contains no salt or other impurities, and therefore won't conduct electricity).

Florida Bitter plate.

An MIT physicist named Francis Bitter came up with the idea for these holes back in the 1930s, an innovation that made more powerful magnets possible. That explains why the plates are commonly called Bitter plates. Back in Bitter's day, these water holes were round. But in the mid-1990s, MagLab engineers figured out that using elongated rather than round holes, and staggering rather than aligning the rows of holes, would greatly increase the coil's ability to withstand stress, meaning even more current could be pumped through the magnet resulting in a higher magnetic field – an incredible 40 percent increase in efficiency. Prior to this innovation, 20 megawatts of power could generate a field of 28 tesla (tesla is a measure of magnetic field strength – a fridge magnet, by comparison, is a mere .03 tesla). But since the invention of the Florida Bitter plate, that same 20 MW could yield 35 tesla.

The MagLab's Florida Bitter plate was quickly adopted by magnet makers worldwide; the design paved the way in 1995 for the lab's world-record 30 tesla resistive magnet. This was surpassed in 2005 by our 35 tesla resistive magnet, which remains the most powerful magnet of its kind on earth. The secret in designing these holes is to find the right balance between the amount of copper used (maximizing the current) and the amount of copper sacrificed to the cooling holes (preventing a melt-down).

Every plate within a coil is identical, and they are stacked in such a way as to create within the coil dozens of very narrow tubes through which cold water can be flushed.

The above statement is 90 percent true. Here we must make a small detour to account for that other 10 percent and explain the Lorentz Force.

Next Page Forces to Be Reckoned With

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