Monday, April 18, 2016

“Giant Leaps” Mean Changes to Superconductor Theory and New Magnetic Opportunities

Experimentally verifying a prediction can be ground-breaking and extremely important, like the recent detection of gravitational waves. But finding something that disagrees with a generally accepted prediction? That can be the science of game-changing discoveries…assuming, of course, that there isn’t a problem with the experiment. As University of Houston physicist Roy Weinstein puts it, “When something new is found, the first suspect is always ourselves.”

Recently, Weinstein and his colleagues exposed significant breakdowns in a 54-year-old model describing how superconductors can trap and retain magnetic fields. Their experimental results show that exciting advances in cost-effective, very strong magnets could be within reach.

Electricity can flow around and around a superconducting loop for a long time—perhaps 100,000 years or more—without losing energy. Superconductors are defined by this property; they are materials with zero resistance to electricity when cooled below a certain temperature. The specific temperature depends on the material, but it is way below room temperature for all known superconductors. Despite the need for serious cooling systems, superconductors are used in a variety of high-tech devices.

Under certain conditions, you can make a superconductor that acts like a permanent magnet. This is called a "trapped field" magnet, or TFM. Most superconducting magnets in use today are not TFMs—rather, they are electromagnets made from coils of superconducting wire kept at a very low temperature. Superconducting electromagnets can produce extremely high, variable magnetic fields, but at an extremely high cost; superconducting wire is expensive. TFMs can be a more economic alternative for devices that require a very high magnetic field (up to about 17 Tesla—more than ten times stronger than a rare-earth magnet) and are well suited to the fixed magnetic field of a permanent magnet. In light of recent discoveries, they are likely to be even more cost-effective than previously thought.

A silver ferromagnet levitating over TFMs (black) above
a container of liquid nitrogen.
Image Credit: Weinstein/University of Houston
A few years ago, Weinstein and his team saw a surprising result during an experimental test of the critical state model, a widely accepted theory published in 1962 that describes how superconductors trap and hold magnetic fields. They used a pulsed field magnet, a brief but very strong magnetic field, to create a TFM. Throughout the process they measured the strength of the field and other variables. According to the critical state model, the magnetic field inside of a superconductor should grow slowly and steadily, and its magnetic field strength should be limited to one-half of the strength of the pulsed field—usually less.

The team saw these predictions hold true under many conditions, but in certain cases they measured “giant flux leaps” in the magnetic field inside of the superconductor. In other words, the strength of the resulting field jumped much faster and higher than the critical state model allows. They found that the TFM field can actually grow as high as the field of the pulse field magnet—the predicted one-half limit is not real.

This is significant not just because it goes against the leading theory, but because it opens doors to new applications for TFMs. The research demonstrates that TFMs can be created easier and more economically than previously thought. They require a significantly smaller magnetic field, less energy, and less sophisticated hardware to produce, factors likely to appeal to a variety of markets. However, there is still an overhead cost to cooling the TFMs that other permanent magnets don’t require, so TFMs are likely to find their niche in large devices that require very high magnetic fields.

“When we first observed the giant flux leaps that mark the newly discovered behavior, I had all work in our labs stopped,” said Weinstein. “We then had non-stop group meetings in which we reviewed and re-reviewed every aspect of the measurements.” After this careful review, the team published their results in IEEE Transactions on Applied Superconductivity in late 2014, and related work in Applied Physics Letters in 2015. Last week, they published a paper in the Journal of Applied Physics describing a number of follow-up experiments that explored the conditions for giant flux leaps, as well as some thoughts about applications and the underlying physics, which is still unclear.

Many research groups have studied the critical state model over the last 50 years, but Weinstein’s is the first to observe experimental differences on this scale. The team is now considering how all of the past experiments—theirs and others—fit together to tell a more complete story of how magnetic fields are trapped and held in superconductors. By exploring the physics behind the giant flux leaps they aim to build a model that more closely matches reality.

The collaborators include Weinstein, Drew Parks, Ravi-Persad Sawh, Keith Carpenter, and Kent Davey. All are at the University of Houston and its Texas Center for Superconductivity.

—Kendra Redmond

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