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Mr. Sebastian Webel
Mr. Sebastian Webel

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Pictures of the Future
The Magazine for Research and Innovation
 

Innovations

Cool Breakthrough Opens a Path to Industrial Use of Large Magnetic Fields

Strong magnetic field: The CT-experts are measuring the magnet power of the HTS-based magnet.

Persistent-current magnets are used almost exclusively for medical imaging in MRI systems. But the technology is usually too complex for application in other fields. Despite the doubts of some experts, Siemens researchers have now succeeded in using high-temperature superconductors to advance the technology to a stage that makes it suitable for a broader spectrum of applications.

When you think of a magnetic resonance imaging machine, a doughnut-shaped unit about the size of a tractor tire comes to mind. Most such machines are used in hospitals for a wide variety of diagnostics. Generally, however, it hasn’t been possible to use MRI technology in other fields because of the size of the machines and the complexity of their cooling systems.

In order to use a magnetic field to generate high-resolution images of the inside of a human body, MRI machines generally rely on low-temperature superconductors (LTS) such as niobium-titanium compounds. These are materials that conduct electrical current with almost no resistance at a temperature of approximately minus 270 degrees Celsius. MRI machines therefore require elaborate refrigeration technology that uses cryogenic liquid helium as a coolant.

The goal was to develop a solution that would not depend on complex and expensive cooling systems.

Moreover, because helium vaporizes extremely quickly if the cooling system fails, MRI machines are currently equipped with large, very bulky, and expensive exhaust pipes that lead the intensely cold gas out of the room in the event of a cooling system breakdown. And finally, the machines are designed to operate in persistent-current mode, which means that the current used to generate the magnetic field flows through the superconductors without the need of a constant supply of current. All of this makes the technology very complex for industrial use.

In view of these limitations, researchers at Siemens Corporate Technology have taken a step toward revolutionizing MRI technology. “Our goal was to develop a solution that would not depend on complex and expensive cooling systems and therefore would not need a bulky exhaust system. Such a solution would thus allow for a more compact MRI machine that could be used in areas beyond medical imaging,” says Dr. Tabea Arndt, who is responsible for superconductor technology development at Corporate Technology (CT).

Overcoming Obstacles

Arndt’s team therefore pursued the idea of using high-temperature superconductors (HTS) instead of LTS. The team reasoned that HTS technology would make it possible to operate MRI machines at approximately minus 220 degrees Celsius, which would in turn result in significant simplification of the associated cooling technology. However, HTS technology for industrial use likewise faced obstacles, and experts around the world therefore believed it had dim prospects. For example, it did not seem possible to use ceramic HTS to make the extremely low-resistance electrical contacts needed for persistent-current mode. The lack of a superconducting switch for the initial start-up was one of several additional challenges.

But step by step, researchers at Siemens Corporate Technology overcame these challenges and developed a demonstration unit with a magnetic field strength of 1.4 tesla. They used second generation HTS, or 2G HTS — superconductors made of rare-earth elements that transport the persistent current in a thin, ceramic film at temperatures of approximately minus 220 degrees Celsius.

Whether in industrial production, in food inspections, or in pharmaceuticals industry approval processes, the simplicity of the new technology makes it suitable for completely new applications.

Such high-temperature superconductors have been used for several years now in power transmission systems and ship engines, for example. HTS are also becoming increasingly common in fault current limiters in electrical grids. So far, however, ceramic high-temperature superconductors have not been used in MRI machines. Although there is already an MRI machine without elaborate helium cryogenic technology on the market, it reaches only a modest field strength of 0.5 tesla. Furthermore, the machine does not operate in persistent-current mode. It must therefore be constantly supplied with power to maintain its magnetic field.

Simplified Cooling

The cooling system of the new HTS-based magnet is far less elaborate than its conventional counterparts. Instead of a helium bath, the new system uses a vacuum chamber with a chiller that operates like a refrigerator — i.e., a very small amount of refrigerant (such as neon or hydrogen) circulates through a closed system and is alternately compressed and then expanded.

Thanks to this approach, the researchers significantly reduced the complexity of the cooling system and thus the size of the MRI machine while also raising associated cryogenic temperatures from approximately minus 270 to minus 220 degrees Celsius.

"Warning: strong magnetic field": the HTS superconducting Magnet

No Power Supply Needed

They accomplished a great deal more as well, says project manager Dr. Marijn Peter Oomen. According to Oomen, who is an expert in superconductivity, another outstanding feature of the demonstration unit is that the current does not fade away despite the HTS. “Resistance is so low that we only need to feed current to the magnetic field once when the machine starts up; after that we can disconnect the magnet from its power supply,” he explains. The reason is that electricity simply continues to flow in HTS magnet coils, because the electrical resistance of the HTS ceramics is extremely low —something no one thought would be possible for HTS in the past. “Losses are so low that the HTS magnet is just as powerful as the established LTS persistent-current magnets,” adds Arndt. All things considered, that’s a global first.

Thanks to this breakthrough in HTS ceramics, a range of completely new applications is now at hand. For example, the new magnet technology could be used in industry to separate different raw materials from one another for processing, which was not possible with more elaborate low-temperature superconductor systems. New MRI systems could also be employed to detect the presence of foreign particles or impurities in food, or to search for internal product damage. The technology could even be used to accelerate approval processes in the pharmaceutical industry — for example by enabling rapid verification of the effects of new osteoarthritis medications through accelerated examinations of the hands of large numbers of test subjects. This could be accomplished in a small room right next to a reception area instead of requiring a specially equipped treatment area.

Whether in industrial production, in food inspections, or in pharmaceuticals industry approval processes, the simplicity of the new technology makes it suitable for completely new applications. Even though the demonstration unit is not yet a market-ready product, it already provides a firm foundation for the use of MRI technology in new applications — in short, the unit shows what’s possible.

Sebastian Webel