A quasistatic magnetic cloak

Cloaking a three-dimensional object in free space from electromagnetic waves has recently become a theoretical possibility, although practical implementations can only be made in reduced schemes. If static fields are involved, requirements are less restrictive and some practical realizations have been possible. Here we present a third regime between the full wave and the static cases. We experimentally demonstrate that a cloak constructed under the dc conditions can keep cloaking properties for applied magnetic fields oscillating at low frequencies (up to hundreds of Hz). Because electromagnetic technology works at these frequencies, applications of our ideas to present technology are discussed

A magnetic wormhole

Wormholes are fascinating cosmological objects that can connect two distant regions of the universe. Because of their intriguing nature, constructing a wormhole in a lab seems a formidable task. A theoretical proposal by Greenleaf et al. presented a strategy to build a wormhole for electromagnetic waves. Based on metamaterials, it could allow electromagnetic wave propagation between two points in space through an invisible tunnel. However, an actual realization has not been possible until now. Here we construct and experimentally demonstrate a magnetostatic wormhole. Using magnetic metamaterials and metasurfaces, our wormhole transfers the magnetic field from one point in space to another through a path that is magnetically undetectable. We experimentally show that the magnetic field from a source at one end of the wormhole appears at the other end as an isolated magnetic monopolar field, creating the illusion of a magnetic field propagating through a tunnel outside the 3D space. Practical applications of the results can be envisaged, including medical techniques based on magnetism

Enhancing the sensitivity of magnetic sensors by 3D metamaterial shells

Magnetic sensors are key elements in our interconnected smart society. Their sensitivity becomes essential for many applications in fields such as biomedicine, computer memories, geophysics, or space exploration. Here we present a universal way of increasing the sensitivity of magnetic sensors by surrounding them with a spherical metamaterial shell with specially designed anisotropic magnetic properties. We analytically demonstrate that the magnetic field in the sensing area is enhanced by our metamaterial shell by a known factor that depends on the shell radii ratio. When the applied field is non-uniform, as for dipolar magnetic field sources, field gradient is increased as well. A proof-of-concept experimental realization confirms the theoretical predictions. The metamaterial shell is also shown to concentrate time-dependent magnetic fields upto frequencies of 100 kHz

Can Resistive-Type Fault Current Limiter Operate in Cryogen-Free Environment?

Superconducting fault current limiters are unique devices that offer fast response to fault without need of an external triggering system. Therefore, they are interesting for industrial use, although their price is high. The principle of the resistive fault current limiter is based on the steep current-voltage characteristic of superconductors. When the current rises over the critical one, voltage on the superconductor rises steeply, and this mechanism blocks the increase of the current. Simultaneous appearance of voltage and current during this limiting period of operation generates a substantial amount of heat that is dissipated in the superconducting wire causing a rapid increase of its temperature. In this contribution, we further develop the idea that, during a limiting period, there is only a small difference between cooling by liquid nitrogen and adiabatic conditions. Using this approach, one can think about a current limiter free from a liquid coolant using a cryocooler. In this contribution, we discuss main differences in cooling conditions and test the idea on a short sample of the high-temperature superconductor REBCO conductor. We compare the behavior of identical samples cooled to the same temperature by liquid nitrogen and conduction cooled. If the realization can be achieved, the huge benefit would be an FCL with a tunable triggering current via its operating temperature

Analysis of coupling losses in multifilamentary untwisted BSCCO/Ag tapes through a.c. susceptibility measurements

Losses as a function of the a.c. magnetic field amplitude (B/sub 0/) have been evaluated at 77 K in untwisted BSCCO(2223)/Ag tapes, at different frequencies, by measuring the imaginary part of a.c. magnetic susceptibility. In particular, the measurements were performed on different portions of the same tape, obtained by cutting it in pieces with different length, starting from around 120/spl ap/mm down to 10/spl ap/mm. The experimental results show that the losses depend on the sample length, but this observed behavior can not be always ascribed to the coupling mechanism among filaments. In this work we discuss the observed experimental behavior for different typologies of tapes. The data are analyzed by comparing them with the results obtained by means of analytical models allowing us to characterize the tapes with respect to the coupling mechanisms