Making Superconducting Magnets

Superconducting Magnets Rock

The trouble, though, is worth it, because superconducting magnets boast some key advantages over resistive magnets.

For starters, they cost less money to run. Even factoring in cryogens such as liquid helium and liquid nitrogen, which aren't cheap, operating a superconducting magnet costs only about 1 percent of what it costs to operate a resistive magnet.

"Once you have it built and operating, you're good to go," says Tom Painter, an engineer with the MagLab's Magnet Science & Technology department. "You can just turn it on and operate it for years."

The magnetic fields produced by these magnets are also more steady than those produced by resistive magnets. Electricity that comes through power lines can fluctuate in strength, which results in fluctuations in the magnetic field. Most experiments benefit greatly from a steady, reliable field. "It may not seem like a big thing, but for some experiments, it is a big deal," said Painter.

Steady superconducting currents can also create a more uniform magnetic field, which is also prized by scientists. Throughout the experimental space inside the magnet, there is very little variation in field.

Superconducting magnets are also more compact. You can pack in more current per square inch of space than in resistive magnets because there's no heat to worry about. And a more compact magnet is more efficient.

Finally, superconducting magnets generally last longer. Resistive magnets suffer from a lot of heat-induced wear and tear; not a problem for their cool superconducting counterparts.

Superconducting magnets don't have all the advantages, though. They are more complicated than resistive magnets (which are basically made of metal Bitter discs stacked one on top of the other), and as a result cost more money and time to develop. Also, superconducting magnets can't reach the fields of resistive magnets. The lab's most powerful superconducting magnet, the 900 MHz Nuclear Magnetic Resonance magnet, reaches 21.1 tesla (a measure of magnetic field strength). That's close to the world record for superconducting research magnets, but pales compared to the 35 tesla produced by the world's strongest resistive magnet, built and operating here at the MagLab. All superconducting materials are limited by a critical field – a magnetic field strength above which they can no longer carry current.

Still, the benefits of these instruments far outweigh the shortcomings, particularly for applications that demand very uniform fields. That's why we spend so much time and money developing different types of superconducting magnets:

• We build wire-wound superconducting magnets, such as our 900 MHz NMR magnet.
• We build superconducting magnets using cable-in-conduit conductors (CICC). One of our CICC magnets was paired with a powerful resistive magnet to make our world-record 45 tesla magnet.
• We experiment with non-conventional, high-temperature superconductors (HTS), such as YBCO (yttrium barium copper oxide) and BSCCO (bismuth strontium calcium copper oxide). Unlike the "low-temperature" superconductors currently used in superconducting magnets, HTS don't need liquid helium to operate because they work at (relatively) higher temperatures. Lab scientists and engineers have already built a prototype high-temperature superconducting magnet that holds the world record (33.8 tesla) for a magnetic field created by a superconductor.

CICC and wire-wound magnets have different advantages, shortcomings and applications. Below we take a closer look at how we make these magnets.

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