Magnetic Field In Free Space Research Articles

Generalized magnetostatic target field method for shielded magnetic field coils in a separable coordinate system.

A theoretical method is described to analytically calculate a pair of surface current densities, which produce a desired static magnetic field in one region of the space and zero magnetic field in another. The analysis is based on the known relationship between a surface current density and a stream function, the equivalence of stream functions and surface magnetic dipole density, and the scalar potential representation of the associated magnetic field in free space. From these relations, we formulate the magnetostatic problem, which is often treated as a vector field problem, as a scalar field problem in which a two-dimensional scalar field (stream function) is related to a three-dimensional one (magnetic scalar potential) via the differentiation of the electrostatic Green's function 1/|r-rs|. It is shown that, in a coordinate system in which a separated form of the Green's function exists (separable coordinate system), there exists a simple relationship between a harmonic component of a stream function and a harmonic component of the magnetic scalar potential. The method is applied to calculate idealized surface current patterns for actively shielded, linear gradient field coils in the Cartesian, cylindrical, and spherical coordinates.

Tailoring Magnetic Fields in Inaccessible Regions.

Controlling magnetism, essential for a wide range of technologies, is impaired by the impossibility of generating a maximum of magnetic field in free space. Here, we propose a strategy based on negative permeability to overcome this stringent limitation. We experimentally demonstrate that an active magnetic metamaterial can emulate the field of a straight current wire at a distance. Our strategy leads to an unprecedented focusing of magnetic fields in empty space and enables the remote cancellation of magnetic sources, opening an avenue for manipulating magnetic fields in inaccessible regions.

Experimental demonstration and operating principles of a multiferroic antenna

This paper experimentally demonstrates the operating principles of a near-field multiferroic antenna. The antenna uses a piezoelectric lead-zirconate-titanate stack to apply a time varying strain to a magnetoelastic iron gallium Fe80.77Ga19.23 (FeGa) rod. The voltage induced strain controls the net magnetization of the FeGa rod to generate oscillating magnetic fields in free space surrounding the antenna. Direct experimental measurements of the mechanical force, strain, magnetic field, and magnetic flux inside the FeGa rod were collected and show a strong correlation with the dynamic magnetic field measured in free space. Additionally, an analytical dipole model is used to demonstrate that the free space signal originates from the changing magnetization in the FeGa rod, providing a clear multiferroic antenna proof-of-concept demonstration. Finally, analysis is provided to highlight the importance of device geometry and demagnetization fields when optimizing the antenna response.

A Quantum-Based Microwave Magnetic Field Sensor.

In this paper, a quantum-based method for measuring the microwave magnetic field in free space is presented by exploring atomic Rabi resonance in the clock transition of 133Cs. A compact cesium glass cell serving as the microwave magnetic field sensing head was used to measure the spatial distribution of microwave radiation from an open-ended waveguide antenna. The measured microwave magnetic field was not restricted by other microwave devices. The longitudinal distribution of the magnetic field was measured. The experimental results measured by the sensor were in agreement with the simulation. In addition, a slightly electromagnetic perturbation caused by the glass cell was investigated through simulation calculations.

Effects of an external circuit on a MHD slider bearing with couplestress fluid between conducting plates

A MHD Slider bearing lubricated with conducting couplestress fluid (CCSF) between two electrical conducting plates under the influence of magnetic field in free space is theoretically investigated. A closed form solution for the film pressure and load carrying capacity is obtained analytically in terms of inlet-outlet (IO) film height ratio of slider bearings. The results are presented graphically for different values of operating parameters. The results suggest that the bearings with couplestress fluid as lubricant provide significant load carrying capacity than Newtonian lubricant case. Further, it is observed that the influence of applied magnetic field and induced magnetic field is to increase the load carrying capacity substantially while, the load decreases with increase in IO film ratio. Besides, the conductivity increases the load carrying capacity significantly. The results are compared with the Newtonian Fluid case.

Polarized neutron imaging at the CONRAD instrument at Helmholtz Centre Berlin

Neutrons are highly sensitive to magnetic fields because of their intrinsic magnetic moment. At the same time, their zero net electrical charge allows them to penetrate thick layers of matter. In combination with standard radiographic and tomographic imaging techniques, these properties have been exploited to visualize magnetic fields in free space and within the bulk of solid, massive, opaque samples. In this paper, the basic principle of this new technique and its realization at the CONRAD instrument at the Helmholtz Centre Berlin (formerly Hahn-Meitner Institute Berlin) are explained. Two application examples will be given: the visualization of the magnetic field around a permanent magnet and the investigation of the displacement of the current density from the core of a conductor to the surface (the skin effect).

Nonlinear transformation of electromagnetic wave in time-varying plasma medium: Longitudinal propagation

The nonlinear transformation of a linearly polarized source plane electromagnetic wave, propagating along an external static magnetic field in free space for t<0, due to the sudden creation of homogeneous plasma at t=0 and sudden switching off the external magnetic field after an elapsed time interval τ, is studied. This transformation has been analyzed by using the second order perturbation theory in radio approximation. For t>τ, the stationary (rectification and space-varying) modes, the electron and electromagnetic plasma oscillations and traveling electron and electromagnetic plasma waves have been created. Among the large variety of nonlinear longitudinal electric modes the appearance of rectification mode was observed. The efficiency of excitation of the longitudinal electric rectification mode has been studied for different values of source wave angular frequency, electron cyclotron angular frequency, and duration time τ of external static magnetic field.

A procedure to develop realistic numerical models of cellular phones for an accurate evaluation of SAR distribution in the human head

This paper presents an optimization procedure based on the minimization of a cost function and devoted to implement realistic numerical models of cellular phones to be used inside a finite-difference time-domain code. The adopted cost function depends on geometrical and electrical parameters of the phone and quantifies the accuracy of the model by comparing the simulation results with experimental measurements of the near electric and magnetic fields in free space, and of the specific absorption rate (SAR) in a homogeneous cubic phantom. As an example of the application of the proposed optimization method, a numerical model of a commercial phone has been implemented and the power deposition in an anatomically based model of the human head has been computed for various phone positions. The obtained results show that the use of inaccurate phone models can lead to large errors on local SAR evaluation.

Evaluation of electromagnetic interference from a cellular telephone with a hearing aid

In a collaborative effort, electromagnetic interference (EMI) is evaluated from a global system for mobile communication telephone with one model of a hearing aid used in the ear canal. Since the electromagnetic fields cannot be measured in the ear canal, a reliable method of their modeling with the finite-difference time-domain method is established. Very good agreement has been achieved between the measured and computed electric and magnetic fields in free space in very close proximity to the telephone. Subsequently, electric and magnetic fields in the ear canal are computed for two models of the ear, and three positions of the telephone. The computed fields are compared with the acoustic measurements for a small number of humans subjected to the EMI test.

Generation and Amplification of Electromagnetic Waves by a Tubular Plasma Beam in Crossed Radial Electrostatic and Axial Magnetic Fields in Free Space

A possible mechanism of generation and amplification of electromagnetic waves by plasma rotating in crossed axial magnetic and radial electrostatic fields is investigated theoretically. A dispersion relation describing the wave-electron interaction has been derived. It is shown that generation and amplification of waves is possible under the Cerenkov resonance conditions.

Finite Element Analysis of Electromagnetically Driven Flow in Sub-mold Stirring of Steel Billets and Slabs.

This paper describes a new method for solving coupled three-dimensional electromagnetic and fluid flow equations in induction systems and its application to electromagnetic stirring in continuous casting of steel. This method is based on the differential-integral potential formulation of the electromagnetic field, and is capable of computing the field for any given stirrer configuration without solving the magnetic field in free space. The turbulent flow in the melt is represented by time averaged Navier–Stokes equation together with k-e turbulent model. The finite element formulation of the governing equations is also presented. Computed results were presented for the electromagnetic force distribution, velocity and turbulent parameters in rotary stirring of billets and linear stirring of slabs during continuous casting of steel. For rotary stirring, it was found that the electromagnetic forces are confined to the region surrounded by the stirrer, and the effective flow region is about twice the length of the stirrer and the flow exhibits weak axial velocity component. It was also found that the flow is turbulent with a maximum intensity of about 15% of the mean velocity, and decays rapidly above the edges of the inductor. For linear stirring of slabs, the spatial variation of the force field along and across the slab was found to generate a more complex flow pattern than predicted by two-dimensional analysis. The turbulent intensity was found to increase monotonically in the direction of the traveling force field.

Gravitational mass of magnetostatic field

Maxwell's displacement current equation is interpreted in the light of recent work to show that static magnetic field in free space should have a colocated and contemporaneous mass that is neither embodied in, nor can be anticipated from, the mass-energy relation. Thus magnetostatic field in the universe represents an ‘invisible’ mass. Some consequences are discussed.

Generation of Predominantly Electric or Magnetic Field in Free Space

The paper presents evaluation of electromagnetic field distribution and wave impedance variation of electromagnetic field between two antennas excited from coherent sources, taking space attenuation, impedance mismatch and multiple reflections between two sources into account. The variations of electric and magnetic field distribution and wave impedance as a function of position between two antennas are evaluated. The region between the two antennas, which is most suitable for EMI measurements under high electric and low magnetic fields or low electric and high magnetic fields is found. Numerical results are presented.

Efficient computation of antenna coupling and fields within the near-field region

The theory, techniques, details of the important equations, and description of two computer programs are presented for calculating efficiently the mutual coupling at a single frequency between any two antennas arbitrarily oriented and separated in free space. Both programs emphasize efficiency and generality, and require, basically, the complex electric far field of each antenna, and the Eulerian angles designating the relative orientation of each antenna. Multiple reflections between the antennas are neglected but no other restrictive assumptions are involved. If an electric field component is desired instead of coupling, the receiving antenna is replaced by a virtual antenna with uniform far field. The first computer program computes coupling (or fields) versus transverse displacement of the antennas in a plane normal to their axis of separation. An efficient fast Fourier transform (FFT) program was made possible by "collapsing" the far-field input data and showing that inmost cases the spectrum integration need cover only the solid angle mutually subtended by the smallest spheres circumscribing the antennas. Limiting the integration to this solid angle artifically band limits the coupling function, thereby allowing much larger integration increments and reducing run times and storage requirements to a feasible amount for electrically large antennas. The second program is based on a spherical wave representation of the coupling function and rapidly computes coupling (or fields) versus separation distance between the antennas. The spherical wave representation emerged naturally from an intriguing characteristic proven for the mutual coupling function; it, like each rectangular component of electric and magnetic field in free space, satisfies the homogeneous wave equation.