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A cloaking device from off the shelf superconductors and magnetic tape

Posted in Science on 25th Mar, 2012 12:24 AM by AlexMuller

The dream of turning solid objects invisible is an ancient one. In recent years a number of experiments have succeeded in "cloaking" (to borrow a term from Star Trek) specialized materials, rendering them transparent to some wavelengths of light. True invisibility, however, remains in the realm of science fiction. The challenge is to create conditions in which electromagnetic fields are the same on both sides of a solid object, meaning that the presence of that object is masked.


A new experiment reported in Science involves far simpler conditions and materials than any previous attempts. Researchers Fedor Gömöry et al. constructed a cylinder of nested magnetic and high-temperature superconducting materials that precisely manipulates an external uniform magnetic field until it is the same on both sides of the object. From a magnetic point of view, the cylinder is cloaked. The technique is far from being able to mask a large object at room temperature: the cloak uses a magnetic field that doesn't vary in space or time, and the superconductor requires that the entire system be cooled to 77 degrees above absolute zero. Nevertheless, the entire setup is a significant advance and requires much simpler conditions than prior cloaking experiments.


The essence of the cloak lies in the different ways magnetic and superconducting materials respond to magnetic fields. A simple hollow ferromagnetic cylinder attracts an external magnetic field, distorting the field lines as shown in the diagram above. (Ferromagnetic is the technical term for what is colloquially known as "permanent magnetism".) A superconducting cylinder, on the other hand, expels magnetic fields, creating a space free of magnetism inside.


Gömöry et al. built a cylinder using a high-temperature superconducting tape on the inside, surrounded by an iron-nickel-chromium (FeNiCr) sheet (which is ferromagnetic). When placed in a uniform external magnetic field, the FeNiCr layer attracts the field but the superconducting inner layer repels it. The combination of the two effects produces a magnetic field that is the same on both sides of the cylinder. The cylinder is then cloaked, as well as anything inside it.


The cylinder itself is 12 millimeters long, with an inner diameter of 12.5 millimeters. It is placed between two wire coils producing a static uniform magnetic field. The entire system is then cooled to 77 Kelvins, and the magnetic field is measured at various points around the cylinder using a sensitive device known as a Hall effect probe. While the magnetic field strength isn't huge by modern laboratory standards (about 0.04 Teslas), it's strong enough that any distortions would be detectable. To check this, Gömöry et al. separately tested control cylinders made only of ferromagnetic materials and only of superconducting materials. While the controls exhibited strong distortions of the external magnetic field, the combined ferromagnet-superconductor yielded only a tiny amount of disturbance.


Both the FeNiCr alloy and the high-temperature superconducting tape are commercially available; the magnetic field strengths are easily achievable by small labs, and cooling to 77 Kelvins requires only liquid nitrogen. This is the real achievement of the experiment: producing magnetic masking with relatively inexpensive components, as opposed to previous cloaks that require exotic materials and often much colder temperatures. While static uniform magnetic fields still constitute a special set-up (as compared to real-world scenarios, where fields vary in space and time), one can imagine laboratory conditions where masking small objects from magnetic fields would be highly desirable. Romulan spacecraft may still be the stuff of science fiction, but reliable magnetic cloaks may now be within reach.

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