Making Superconducting Magnets
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What are these two types of superconducting magnets used for?
The main advantage to CICC magnets: They can be turned off and on relatively easily. They can go from no magnetic field to, say, 15 tesla in a matter of minutes. This is called "ramping up" or "ramping down" a magnet. When magnetic fields increase or decrease, they actually create electrical currents in the conductors around them; with that current comes heat. Excess heat can endanger a magnet, but CICC magnets can handle it because the superconducting wires are bathed in liquid helium. Wire-wound magnets, however, can't tolerate rapid changes in field and the consequent heat because the coolant is further from the superconducting wires. In fact, when wire-wound magnets are first turned on, engineers ramp them up very gradually, over several hours, days or sometimes even weeks; anything faster would create heat that the system is not designed to tolerate.
In other words, CICC magnets allow scientists to put their experiments in a changing magnetic field, to see how that affects their sample. And because they're more heat tolerant, CICC magnets can be combined with resistive magnets to build hybrid magnets, together creating a magnetic field that neither the superconducting magnet nor the resistive magnet alone could achieve.
Scientist Victor Schepkin monitors an experiment in the 900 MHz NMR superconducting magnet.
Wire-wound magnets, however, have their own advantage over CICC magnets: They create a field that is far more uniform. That's because they are not powered from an outside source. After they are first ramped up, they are unplugged from their power supply. The superconducting current continues to run on its own: Our 900 MHz has been conducting electricity continuously, uplugged, since 2004. That smooth current makes a smooth, homogenous magnetic field that travels a precisely controlled path.
CICC magnets, on the other hand, are always plugged in; they use only a little electricity, but enough to cause irregularities in the magnetic field caused by the directions of the current paths. This results in variations in magnetic field, both over time and across the space inside the magnet where the experimental samples are placed. In CICC magnets, this "field homogeneity" is measured in parts per thousand. In wire wound magnets, it is so high it is measured in parts per million, and in some cases billion.
A uniform field is unimportant for many experiments, but for scientists doing nuclear magnetic resonance (NMR), it is critical; Even the tiniest fluctuation in magnetic field creates "noise" in their experiments, which means their data will be less clear.
"They're looking at complex molecular structures, doing very precise measurements," explains Iain Dixon, MS&T research associate and project leader for the 900 MHz magnet. "For that, very high-quality instrumentation, high-quality probes and high-quality magnetic fields are needed. If you have variation in the magnetic fields, then the scientists are unsure of what they're getting; it's got to be more or less exact."
These powerful magnets are a tough act to follow. But MagLab scientists and engineers are working to do just that. New high-temperature superconductors with a higher upper critical field are on the horizon. These new materials, some of which are being studied here at the lab, may soon allow engineers to create even higher field superconducting magnets.
"You could call it a revolution in superconducting magnets," says MagLab Director Greg Boebinger. "These new materials may free superconducting magnets from the 'niobium jailhouse.'"
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Thanks to the Magnet Academy's scientific advisers on this article, Iain Dixon, Lee Marks and Tom Painter of the lab's Magnet Science & Technology program.