In an MRI scanner, a superconducting magnet provides the magnetic field that starts the nuclei precessing.
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Superconductivity is not only fascinating, it is also incredibly useful. Superconductors are already used in applications as diverse as seeing inside the human body and discovering the origin of mass. As important as these achievements are, their promise for future revolutionary technologies may be even greater.
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Seeing inside the human body
Heike Kamerlingh Onnes realised that one of the most important applications of superconductors would be in making powerful electromagnets. Superconducting wire can carry immense electrical currents with no heating, which allows it to generate large magnetic fields. An electromagnet with non-superconducting copper windings would melt with the same current.
Unfortunately, the superconductors available to Kamerlingh Onnes could only carry small currents producing correspondingly small magnetic fields and so he never realised this possibility in his lifetime. It took until the late 1950s and early 1960s for the right materials to be identified and the relevant technology developed.
One of the most important applications of superconducting magnets is in medicine, with the development of . MRI is the best way to see inside the body without invasive surgery.
Today MRI is used to examine the body’s soft tissues and is especially valuable for detecting tumours, examining neurological functions and revealing disorders in joints, muscles, the heart and blood vessels. It shows up the water content of tissue, which varies throughout the body and is altered by pathological processes in many diseases.
Water molecules are composed of hydrogen and oxygen atoms, and it is the hydrogen nuclei that MRI probes. Hydrogen nuclei behave like little spinning tops and placing them in a magnetic field causes the spin axis to tip over and rotate at a well-defined frequency called the precession frequency. The nuclei can interact with a second electromagnetic field whose frequency is tuned to match the precession frequency. When this happens, the nuclei absorb energy, allowing you to work out how much water is present and where.
In an MRI scanner, a superconducting magnet provides the magnetic field that starts the nuclei precessing. To produce high-resolution images typically requires a field between 1 and 3 tesla, tens of thousands of times larger than the magnetic field at the Earth’s surface. The magnet also needs to be large enough for a person to slide inside its bore.
The exquisite 3D images of the body are accomplished using a sophisticated sequence of electromagnetic pulses and a magnetic field gradient – techniques that won the British physicist Peter Mansfield and American chemist Paul Lauterbur the .
There are now several variants of MRI, including functional MRI (fMRI)that monitors processes such as blood flow in the brain in response to particular stimuli.
Superconductors have given the medical profession something even better than X-ray spectacles, and hundreds of thousands of people a year get a much better medical diagnosis because of them.
“Superconductors have given the medical profession something even better than X-ray spectacles”
Hunting the Higgs boson
Particle accelerators need magnets to manipulate beams of highly energetic particles. As the energy of these beams has increased over the decades, so has the need for ever stronger magnetic fields. Only superconducting magnets can do the job.
In fact, accelerators have been using superconducting magnets since the 1970s and the Large Hadron Collider at CERN near Geneva, Switzerland, is no exception. Particle physicists hope that the LHC will help to explain the origin of mass by searching for a particle called the Higgs boson among the debris of collisions between high-energy protons.
Beams of protons are accelerated in opposite directions around a circular tunnel 27 kilometres round under the French-Swiss border. To steer the protons requires a large magnetic field all the way around the ring. The LHC comprises 1232 superconducting magnets, each 15 metres long and weighing 35 tonnes. The magnets contain coils of superconducting cables made from niobium and titanium and cooled to a little over 1 kelvin using 100 tonnes of liquid helium.
The stored energy in the magnets at the LHC is approximately 15 gigajoules, slightly more than the kinetic energy of a fully laden Boeing 747 at cruising speed. In September 2008, only days after the LHC was switched on, an electrical fault caused the superconducting magnets to fail. Around 100 were affected and several tonnes of liquid helium leaked into the tunnel. Repairs delayed the LHC’s operation for over a year, highlighting the critical role played by superconductors.
Now the LHC is working well again. As well as being the world’s most powerful accelerator, it is also delivering a record number of collisions per second for a proton collider. Physicists hunting the Higgs are hoping that they are homing in on their prey.
New applications
Superconductors have such remarkable properties that they find many diverse applications. Magnetically levitated () trains, for example, exploit a superconductor’s ability to repel magnetic fields. At the Yamanashi test line in Japan, a superconducting coil attached to the train keeps it floating above the magnetic track thus avoiding the slowing effects of friction. The train has reached a top speed of 581 kilometres per hour – making it the fastest land-based mass transportation system in the world. However, it remains a prototype because it is so expensive to run.
Meanwhile, circuits containing a superconducting coil provide unprecedented sensitivity to tiny magnetic fields. Any magnetic field passing through the coil generates a screening current that subtly alters the current flowing in the circuit. This idea is at the heart of SQUIDs (superconducting quantum interference devices), which are used to detect the very small magnetic fields generated by currents in the heart and brain. SQUIDs have also been used in research for sensing X-rays, gamma rays and various exotic particles. They are even playing an important part in the search for dark matter in the universe.
The properties of superconductors have led to novel energy storage systems. One is based on a large flywheel driven to high speeds by electricity during the night when it is cheaper. The flywheel has frictionless superconducting bearings, so it doesn’t lose its stock of rotational kinetic energy. As a result, it can store energy until it is needed.
A second system stores current in a superconducting magnet whose energy is released when needed by connecting the magnet to a circuit. While both of these systems are attractive in principle, and several commercial systems are available, the high cost of refrigeration has not yet made this sort of technology economically viable.
The lack of electrical resistance means that superconductivity has obvious applications in power transmission. Several companies have made cables out of high-temperature superconductors that can carry currents of 2000 amps without any losses. Similar resistance-free cables are used in electrical circuits in some MRI scanners, radio telescopes and cellphone masts.
If we can do all this with today’s superconductors, we can only speculate what applications may become possible with the next generation of materials.
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