Theoretical Kryptonite?

Physicists find flaws in 50-year old superconductor theory

In what could be a major development in scientific theory and related electrical fields, University of Houston physicists recently reported finding theoretical flaws in the generally accepted understanding of how a superconductor traps and holds a magnetic field. More than 50 years ago, C.P. Bean, a scientist at General Electric, developed a theoretical explanation known as the “Bean Model” or “Critical State Model.”  When scientists and professionals are willing to challenge established theories, the results can be enlightening.

Minor problems with Bean’s Critical State Model emerged shortly after it was published, according to Roy Weinstein, professor of physics emeritus at the university, who led the study. Any “chink in theoretical armor” is worthy of an exploratory experiment, he contends, and this is what motivated Weinstein and his colleagues.

They discovered that for certain constraints on a magnetic pulse, Bean’s model is often misaligned, and a significantly different spatial distribution of field occurs. “Great increases in field occur suddenly, in a single leap, whereas Bean’s model predicts a steady, slow increase,” Weinstein said.

All of this new, unexpected behavior is repeatable and controllable. “The most encouraging is that we can now produce full-strength TFMs with a pulse strength 1.0 times that of the TFM,” he added.

“By using our newly discovered methods, the maximum TFM field is now 12 tesla,” said Weinstein. “A motor, if made in a fixed size, can produce 3.2 times the torque. Alternatively, the motor can be designed to produce the same amount of torque, but have its volume reduced by more than 10 times. This reduction in materials can result in great cost savings.”

The researchers are still within the “early days” of this work and have already disproven their first thoughts concerning what is causing their results. “We’re now essentially spelunking in a dark cave without lights — it’s frustrating, but exciting,” Weinstein said.

In terms of applications for their discovery, the researchers suggest the ability to replace a $100,000 low-temperature superconducting magnet in a research X-ray machine with a $300 TFM, or possibly replace a motor with one that is a quarter of the size of an existing one. There are many other potential applications, such as an energy-efficient ore separator, noncontact magnetic gears that will not wear or require repair, a red blood separator with 50 percent improved yield, and even an automated docking system for spacecraft.

A magnet levitating over a superconductor.

A magnet levitating over a superconductor.

Weinstein and colleagues are now searching for fast, short-term support that will allow them to continue their research to explain this new phenomenon. “While we now know enough to apply our new discovery to significantly improve a large number of devices, we don’t yet fully know what’s going on in terms of the basic laws of physics,” he noted.

Superconductors are significant because they bring no resistance to electrical circuits. In a way, they are the opposite of toasters, which resist electrical currents and thereby convert energy into heat. Superconductors consume zero energy and can store it for a long period of time. Those that store magnetic energy known as “trapped field magnets”— or TFMs — can serve as magnets.

Writing in the Journal of Applied Physics from AIP Publishing, the researchers describe experiments whose results exhibited “significant deviations” from those of the Critical State Model. They revealed unexpected new behavior favorable to practical applications, including the possibility of new uses for TFMs. Much of modern technology is already based on magnets. “Without magnets, we’d lack generators [electric lights and toasters], motors [municipal water supplies, ship engines], magnetrons [microwave ovens], and much more,” said Roy Weinstein, lead author of the study, and professor of physics emeritus and research professor at the University of Houston.

“Generally, the performance of a device based on magnets improves as the strength of the magnet increases, up to the square of the increase,” the American Institute of Physics reported in a news release April 11 which details the research. “In other words, if a magnet is 25 times stronger, the device’s performance can range from 25 to 625 times better.”

TFMs have been underutilized to this point due to the challenge of getting the magnetic field into the superconductor. The main issue is cooling down the superconductor to a temperature at which it can effectively operate.

The method widely used today is to apply a magnetic field to a superconductor via a pulse field magnet after the superconductor is cooled. Bean’s model predicted, and until now experiments confirmed, that to push as much magnetic field as possible into a superconductor, the pulsed field must be at least twice as strong, and more typically over 3.2 times as strong, as the resulting field of the TFM. It is highly expensive and difficult to produce these hefty fields. This compelled researchers to look for rerouting methods. We sometimes forget that scientific theory is not set in stone, but ongoing in development.

 

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