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Pulsed Magnets: Brief Shining Moments

So, 45 tesla is the greatest field strength ever achieved by a magnet?

Yes – and no. It’s the greatest continuous field strength ever achieved by a magnet. But a type of magnet called a pulsed magnet can achieve far higher strengths. But there’s a trade-off: The field lasts just a few seconds or less.

The Magnet Lab has several of these magnets, which were designed and built in Tallahassee then shipped to the lab’s Pulsed Field Facility at Los Alamos National Laboratory for operation. Designers must choose their materials carefully, taking into account the incredible stresses and heat produced by such enormous magnetic fields.

Pulsed magnets come in two types, destructive and non-destructive, the latter of which we’ll discuss first.

Non-destructive magnets can be short-pulse or long-pulse. The "short" vs. "long" is a reference to how long the magnetic field lasts. In this context, long is relative – at most 2 seconds. Short is measured in milliseconds. Long-pulse magnets are motor-generator driven, and short-pulse are fueled by capacitors. In either case, they work pretty much the same way, demonstrated by this simple pulsed magnet tutorial.

PHYSICS FACTOID: The incredible field strengths achieved by these destructive pulsed magnets are over before the operator's finger leaves the "On" button – less than 10 microseconds. A microsecond is 0.000001 second.

The magnets are impressive for the fields they generate and the amount of energy they consume doing it. The Magnet Lab’s most powerful non-destructive magnet generates a field of 88.9 tesla. When turned on, this multi-shot magnet contains the energy of more than 100 sticks of dynamite and is the most powerful magnet of its type in the world. Magnet designers are further developing that magnet, which came online in October 2006, in the hope that it will eventually reach 100 tesla. The project is funded by the U.S. Department of Energy and the National Science Foundation.

If you’re ever in the vicinity when one of these magnets is in operation, be sure to use earplugs. It emits a kind of shriek that has been compared to Godzilla on a rampage. That’s because these tremendous magnetic forces produce enormous pressure – on the order of 200,000 pounds per square inch. That’s one reason the magnet operates so briefly: Left to run any longer, it would either succumb to pressure or melt from the awesome heat generated by all the power surging in.

While scientists usually try to avoid destroying their equipment, they are sometimes willing to do just that, in exchange for a phenomenal magnetic field. In fact, in destructive magnets, scientists actually detonate explosives around the pulsed magnet at the same time they turn it on. This compresses and boosts the magnetic field to as high as 1,000 tesla – for the few microseconds before the whole shebang explodes. Now that’s what you call a temporary magnet!

You may well wonder what the point of such an exercise is: After all, you’re not just blowing up a magnet, you’re blowing up your whole experiment! Truth is, some destructive magnets (called single-turn magnets) don’t really blow up – they blow outwards – so the sample is conserved. For those magnets that do blow up, scientists have become adept at measuring and collecting data during their microseconds-long window of opportunity – so all is not lost, after all!

So there you have it: From your fridge to your hospital’s MRI clinic, from an infinitesimal electron to the MagLab’s 35-ton hybrid. Magnets and their fields are everywhere, hard at work in innumerable ways and forms. That’s our story – and we’re sticking to it!

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Thanks to the Magnet Academy's scientific adviser on this article, Dr. Eric Palm, Deputy Lab Director.

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