Phenomenon Analysis and Improvement of Magnetic Shield Fringe Effect on Wireless Power Transmission of EV

An increment of magnetic field strength inevitably appears at the shield edge if a magnetic shield is made of a soft magnetic material, and that increment becomes more serious if this shield is combined with the chassis of an electrical vehicle (EV). This phenomenon is caused by the fringe effect, which limits the transfer efficiency of the coupler for the wireless power transmission (WPT) systems of EV. This phenomenon, and its relationships with some parameters, are analyzed in this paper, and these relationships are fitted to estimate the increment for different shield structures. A magnetic shield structure to reduce the increment of the magnetic field strength and improve coupler efficiency is herein proposed. The effectiveness and correctness of the fitting curves and the advantages of the proposed shield structure are demonstrated by finite element analyses results.

Potential Reduction of Peripheral Local SAR for a Birdcage Body Coil at 3 Tesla Using a Magnetic Shield

The birdcage body coil, the standard transmit coil in clinical MRI systems, is typically a shielded coil. The shield avoids interaction with other system components, but Eddy Currents induced in the shield have an opposite direction with respect to the currents in the birdcage coil. Therefore, the fields are partly counteracted by the Eddy currents, and large coil currents are required to reach the desired B1+ level in the subject. These large currents can create SAR hotspots in body regions close to the coil. Complex periodic structures known as metamaterials enable the realization of a magnetic shield with magnetic rather than electric conductivity. A magnetic shield will have Eddy currents in the same direction as the coil currents. It will allow generating the same B1+ with lower current amplitude, which is expected to reduce SAR hotspots and improve homogeneity. This work explores the feasibility of a birdcage body coil at 3 T with a magnetic shield. Initially, we investigate the feasibility by designing a scale model of a birdcage coil with an anisotropic implementation of a magnetic shield at 7 T using flattened split ring resonators. It is shown that the magnetic shield destroys the desired resonance mode because of increased coil loading. To enforce the right mode, a design is investigated where each birdcage rung is driven individually. This design is implemented in a custom built birdcage at 7 T, successfully demonstrating the feasibility of the proposed concept. Finally, we investigate the potential improvements of a 3 T birdcage body coil through simulations using an idealized magnetic shield consisting of a perfect magnetic conductor (PMC). The PMC shield is shown to eliminate the peripheral regions of high local SAR, increasing the B1+ per unit maximum local SAR by 27% in a scenario where tissue is present close to the coil. However, the magnetic shield increases the longitudinal field of view, which reduces the transmit efficiency by 25%.

Fundamental physical and resource requirements for a Martian magnetic shield

Mars lacks a substantial magnetic field; as a result, the solar wind ablates the Martian atmosphere, and cosmic rays from solar flares make the surface uninhabitable. Therefore, any terraforming attempt will require an artificial Martian magnetic shield. The fundamental challenge of building an artificial magnetosphere is to condense planetary-scale currents and magnetic fields down to the smallest mass possible. Superconducting electromagnets offer a way to do this. However, the underlying physics of superconductors and electromagnets limits this concentration. Based upon these fundamental limitations, we show that the amount of superconducting material is proportional to $B_{\rm c}^{-2}a^{-3}$ , where Bc is the critical magnetic field for the superconductor and a is the loop radius of a solenoid. Since Bc is set by fundamental physics, the only truly adjustable parameter for the design is the loop radius; a larger loop radius minimizes the amount of superconducting material required. This non-intuitive result means that the ‘intuitive’ strategy of building a compact electromagnet and placing it between Mars and the Sun at the first Lagrange point is unfeasible. Considering reasonable limits on Bc, the smallest possible loop radius is ~10 km, and the magnetic shield would have a mass of ~ 1019 g. Most high-temperature superconductors are constructed of rare elements; given solar system abundances, building a superconductor with ~ 1019 g would require mining a solar system body with several times 1025 g; this is approximately 10% of Mars. We find that the most feasible design is to encircle Mars with a superconducting wire with a loop radius of ~3400 km. The resulting wire diameter can be as small as ~5 cm. With this design, the magnetic shield would have a mass of ~ 1012 g and would require mining ~ 1018 g, or only 0.1% of Olympus Mons.

ANALIZA UNUI ECRAN MAGNETIC POZIțIONAT ÎN SPAȚIUL DINTRE ÎNFĂȘURĂRILE UNUI TRANSFORMATOR MONOFAZAT, DE MICĂ PUTERE

"This paper aims to analyze the impact of using a thin magnetic shield placed in the space between the primary and secondary winding of a simplified, low power, single-phase transformer used in energy harvesting applications that demand power transformers not only in the energy conditioning stage but also in the energy harvesting stage. By using magnetic shields, the saturation of the ferromagnetic core and, in some particular cases, the destruction of electronic devices is avoided. For this purpose two scenarios are studied: one which doesn't take into account the magnetic shield, as it considers only the air space between the primary and secondary windings, respectively, and the second case study which considers a magnetic screen placed in the centre of the air space domain. The size of the air space domain, d, is varied as the secondary winding distance itself from the primary one until it reaches the core. The number of turns in the primary and secondary winding is equal, N1 = N2 = 300 turns. By moving the secondary winding away from the primary winding, the variation of the distance d between the coils is achieved, thus keeping the same cross-section of the secondary winding. The thickness of the magnetic shield is chosen arbitrarily, as thin as possible, with a dimension of 400 µm. The idealy, 1:1, simplified, low-power, single-phase transformer powered by a harmonic voltage supply at V1 = 20 V and at a frequency, f = 50 Hz, with load resistance of Rs = 100 Ω, is analyzed in a time-dependent study and its computational domain is taken from literature [4]. Different materials can be used for realizing this magnetic shieling, even copper and aluminum, but in this paper a magnetic sheet metal material is considered because of its small, almost nonexistent electrical conductivity. Our goal is to analyze the effect of magnetic shielding on the saturation of the ferromagnetic core, and the reactance and resistance values of the primary and secondary winding, respectively, for different dimensions of the air space, d. For comparison purposes, the second model, the one in which we have the magnetic sheet metal, an analysis is performed in the permanent harmonic regime, in addition to the one performed in the dynamic one."