True energy-minimal and finite-size biplanar gradient coil designfor MRI

Finite-sized high-performance planar magnetic field gradient coils in today's open configuration magnetic resonance imaging (MRI) systems have always been desirable for ever demanding imaging applications. The authors present a Lagrange multiplier technique for designing a minimum-energy gradient coil under a finite-size planar geometry constraint in addition to a set of magnetic field constraints. In this new design methodology, the surface current density on a finite size plane is represented by a two-dimensional (2-D) Fourier series expansion. Following the standard approach, the authors construct a functional F in terms of the stored magnetic energy and a set of field constraint points which are chosen over the desired imaging volume. Minimizing F, the authors obtain the continuous current density distribution for the finite-size planar gradient coil. Applying the stream function technique to the resulting continuous current distribution, the discrete current pattern can be generated. Employing the Biot-Savart law to the discrete current loops, the gradient magnetic field has been re-evaluated in order to validate the theory. Using this approach, the authors have been able to design a finite-size biplanar z-gradient coil which is capable of generating a gradient field of 40 mT /m @ 266 A. The excellent agreement between the analytical and numerical results has been achieved     

平面波在任意磁化方向旋磁媒质中传播时的极化特性

在铁氧体基片平面电路和发射天线中必须研究平面波在任意磁化方向旋磁媒质中的传播。这时平面波的特征波为ＴＥ波,其磁通密度、磁场、电通密度、电场的极化是不相同的,磁场的极化方向为左、右椭圆极化。     

Scattering of TM excitation by coupled and partially buried cylinders at the interface between two media

An analysis of scattering from coupled conducting cylinders near the planar interface between two semi-infinite, homogeneous halfspaces of different electromagnetic properties and from partially buried conducting cylinders is presented. The perfectly conducting cylinders of general cross sections are of infinite extent and the excitation is transverse magnetic to the cylinder axes. Coupled integral equations for the unknown current induced on the cylinders are derived and a numerical method for solving them is described. In addition, a simple technique is employed to determine the far-zone scattered field from knowledge of the cylinder current. Data displaying the distribution of the induced current and the scattered field patterns for cylinders of interest are presented and discussed.     

Magnetic flux density produced by finite-length twisted-wire pairs

Twisting a wire pair is often used to reduce the wire's low frequency magnetic flux density impinging upon spatially adjacent circuitry. Typically, predictions of the magnetic flux density due to the twisted-wire pair are based upon mathematical formulations for infinite-length wires. In the present work, expressions for predicting the quasistatic magnetic flux density near a finite-length, current-carrying twisted-wire pair are derived. These equations are numerically integrated, and the results compared with the magnetic flux density from the corresponding straight-wire pair, thereby yielding results useful for electromagnetic compatibility analyses. Additionally, approximate far-zone magnetic flux density equations are developed, and their range of validity established. These equations are useful for magnetic flux density estimations at distances further removed from the twisted-wire pair than the usual ones encountered in a typical enclosure. Also, they show that the large near-zone reduction of the magnetic flux density magnitude obtained by twisting the wire pair, relative to the straight-wire pair, is not available in the far-zone     

Magneto-optical visualization of magnetic processes in superconducting strip with the application of magnetic fields and currents

The magneto-optical imaging (MOI) technique is widely employed to characterize the magnetic properties of superconductors and other magnetic materials because it can easily show the variation of flux density over a plane with relatively high resolution. In this paper, flux profiles visualized by the MOI technique will be presented for different experimental conditions. The sample investigated is a high-temperature superconducting single strip. Three cases are investigated: 1) application of external currents; 2) application of external magnetic fields; and 3) simultaneous application of external magnetic field and current. The behavior of complicated vortex penetration into the samples for different conditions will be discussed in detail. The magnetic flux profiles are calculated taking into account the sample thickness, as well as the interval between the sample and the MOI indicator film. The calculated and the experimental results are compared and discussed.     

Ground level current density and ionised flux lines of DC wire-to-plane corona

The nonlinear differential equations of unipolar DC corona in air are solved iteratively using a finite-element based algorithm. The distortion of the flux lines pattern by the corona-generated space charge is accurately demonstrated for the first time. Numerical results for both the current density and electric field on the ground plane show good agreement with experimental data.     

Magnetization Loss in a Striated YBCO Coated Conductor Considering the Intrinsic Critical Current Distribution

In this work we present numerical analysis results of the magnetization loss in a perpendicular AC magnetic field. The numerical analysis is based on the relation of the linkage magnetic flux to current distribution, and takes into account the$J_ c-B$characteristic, and$n$value-$B$characteristic. We discuss the relation between the magnetization losses with striated superconducting tape and the semicylindrical intrinsic critical current density distribution across the width of the superconducting tape. The result shows that the magnetization loss of monocore superconducting tape in a low magnetic field is affected by the intrinsic semicylindrical critical current density distribution. With a striated structure, the influence of the intrinsic critical current density distribution is reduced.     

Magnetic In-circuit Testing of Multiple Power and Ground Pins for Open Faults

The viability of magnetically detecting open faults among multiple power and ground pins of an integrated-circuit package is investigated. The experimental technique is based on injecting current to the leadframe through the chip, and individually sensing the lateral magnetic field of each VDD or GND lead on top of the package. Field distribution in sensor plane is analyzed in order to determine the interference conditions under which correct classification of pin continuity is possible. It is shown that field-based classification benefits from a mildly subtractive interference. Also proposed is a model-based algorithm by which lead currents are calculated from field readings. Classification on the basis of current is shown to be robust against interference. Predictions are verified with experimentation conducted on a PQFP package with a moving Hall sensor.     

Filter functions for computing multipole moments from the magnetic field normal to a plane

A technique for calculating the magnetostatic multipole moments (and therefore complete biomagnetic information) from measurements of the normal component of the magnetic field on a plane is described. The technique involves multiplying the measured values by a set of unique filtering functions and integrating. A new formulation and method for calculating the filtering functions is introduced. The method uses expansions in basis sets and matrix inversion. Proofs are given of the nonsingularity of the matrices. Computer simulations of the application of the technique to a circular current loop and a current dipole in a spherical conductor are described.     

Self Field Effect Compensation in an HTS Tube

It is well known that the critical current density ${J}_{c}$ of a superconducting material depends on the magnetic flux density $B$ . There exists an electric method to measure the ${J}_{c}({B})$ deduced from the $U(I)$ measurements. The problem with this method is the self field effect because the magnetic flux density is always the sum of the applied magnetic flux density and the self magnetic flux density. This paper presents a special experimental arrangement, compensating fully or partially the self magnetic flux density in an HTS tube. It allows characterizing the true zero magnetic flux density behaviour of the superconducting material. The experimental results of the compensation are discussed. A theoretical analysis based on Bean's model is presented and gives results close to the experimental ones. The proposed compensation is not perfect but the experiments and the theoretical analysis allow validation of the compensation principle.      

螺线管结构参数对脉冲磁场的影响

研究了线圈间距、匝数、个数以及不锈钢套筒对脉冲磁体产生磁场的影响规律。在储能电容和电压不变的前提下,研究结果表明：增加线圈间距会导致磁感应强度降低,磁力线包络增大,但总电流达到峰值时刻减小;增加线圈匝数,峰值电流明显减小,会降低磁感应强度,但有利于抑制磁力线包络;增加并联线圈个数,有利于产生较长的均匀区,但是在供能一定的条件下,磁场强度有所降低,同时总电流达到峰值时刻减小。总体来看,在一定均匀区长度的设计要求下,减少单个线圈匝数,增加并联线圈个数,能够得到磁感应强度更大、均匀性更好的磁场,但要考虑线圈承载电流的能力。另外,不对称的阴阳极金属结构会导致磁场不对称分布,且磁感应强度达到峰值时刻要晚于总电流达到峰值的时刻。     

Measurement of nonuniform current density by magnetic resonance

A noninvasive tissue current measurement technique and its use in measuring a nonuniform current density are described. This current density image is created by measuring the magnetic field arising from these currents and taking its curl. These magnetic fields are proportional to the phase component of a complex magnetic resonance image. Measurements of all three components of a quasistatic nonuniform current density in a phantom are described. Expected current density calculations from a numerical solution for the magnetic field which was created by the phantom are presented for comparison. The results of a numerical simulation of the experiment, which used this field solution and which included the effects of slice selection and sampling, are also presented. The experimental and simulated results are quantitatively compared. It is concluded that the principle source of systematic error was the finite slice thickness, which causes blurring of boundaries.     

A simple method of calculating the minority-carrier current in heavily doped silicon

It is shown that the calculation of the one-dimensional minority-carrier current density in heavily doped silicon can be described by two coupled differential equations of the first order. These equations are derived with a minimum of assumptions and approximations and without the explicit use of an electric field. The relevant input parameters to these equations are the product of the equilibrium hole density with the diffusion coefficient and the product of the equilibrium hole density with the reciprocal value of the lifetime. These equations can very easily be solved numerically and the solution gives the minority-carrier density and the current density as a function of space coordinate. It is shown that values of the band gap narrowing cannot be derived from current measurements alone.     

A computational methodology and results for quasistaticmultilayered magnetic shielding

A methodology for accurate calculations of shielding factors for quasistatic multilayered magnetic shields is described. “Transfer relations” for individual layers with specified magnetic permeabilities and electrical conductivities are spliced together. Specific transfer relations for four layer geometries (planar, cylindrical with transverse fields, cylindrical with axial fields, and spherical) and constraints at source and shielded surfaces for six source-shield configurations involving both externally applied fields and enclosed sources are developed. Limiting cases are extracted for magnetically permeable, nonconducting layers, and for thin, magnetically nonpermeable, conducting layers. Reciprocity conditions are identified for interchanged source and shielded regions in planar, transverse field cylindrical, and spherical geometries. Variations of magnetic field and flux density with position are shown for a specific planar example involving alternating layers of aluminum and steel, with the same total shield thickness occupied by either one or five layer pairs. Simulations with alternating layers of aluminum and steel for the four layer geometries are used to study the effects of material composition, number of layer pairs, and air gaps. An optimal number of layer pairs for a given total shield thickness is identified. Results from simulations where induced currents in the steel layers are neglected are compared with those for simulations with a realistic conductivity value for steel to assess the relative effects of flux shunting and induced current shielding     

Novel magnetic-field sensor using carrier-domain rotation: proposed device design

A bipolar semiconductor device is described in which a mobile domain of current rotates around a circular path at a rate proportional to a magnetic field, when this is perpendicular to the planar structure, or follows the magnetic vector when this is in the same plane. Potentially high sensitivities are possible, since the response is inherently integrating; practical limitations to this are discussed.     

Gain and power parameter measurements using planar near-fieldtechniques

Equations are derived and measurement techniques described for obtaining gain, effective radiated power, and saturating flux density using planar near-field measurements. These are compared with conventional far-field techniques, and a number of parallels are evident. These give insight to the theory and help to identify the critical measurement parameters. Application of the techniques to the Intelsat VI satellite is described     

Reconstruction of conductivity and current density images using only one component of magnetic field measurements

Magnetic resonance current density imaging (MRCDI) is to provide current density images of a subject using a magnetic resonance imaging (MRI) scanner with a current injection apparatus. The injection current generates a magnetic field that we can measure from MR phase images. We obtain internal current density images from the measured magnetic flux densities via Ampere's law. However, we must rotate the subject to acquire all of the three components of the induced magnetic flux density. This subject rotation is impractical in clinical MRI scanners when the subject is a human body. In this paper, we propose a way to eliminate the requirement of subject rotation by careful mathematical analysis of the MRCDI problem. In our new MRCDI technique, we need to measure only one component of the induced magnetic flux density and reconstruct both cross-sectional conductivity and current density images without any subject rotation.     

Flux trapping in a ring-shaped YBCO bulk by pulsed fieldmagnetization

A pulsed magnetic field can be applied using small coils to generate a strong magnetic field for the magnetization of the high-Tc superconductors (HTS) to be used as quasi-permanent magnets in flywheels and motors. The dynamic electromagnetic behavior of two melt-processed ring-shaped Y-Ba-Cu-O bulks using the pulsed field magnetization (PFM) process has been experimentally investigated and analyzed. The flux trapped in the bulk by PFM process was compared to the flux trapped by field cooling process. Both cases then have been analyzed with a numerical model based on the finite-element method (FEM). The power-law model was utilized to relate the electric field to the current density inside the superconductor. The dependence of the critical current density on the magnetic field density was taken into account. Measured and calculated results are compared and discussed     

Dual-Plane Vector Control of a Five-Phase Induction Machine for an Improved Flux Pattern

A technique to improve the flux pattern within a five-phase induction machine is presented. The technique is developed through dual-plane vector control, with synchronized fluxes. By vector space decomposition, an analytical model and vector control of the machine are accomplished in two orthogonal vector planes,d₁-q₁ and d_s-q_s . The magnitude and rotating speed of the associated fluxes (fundamental and third harmonic) can be independently controlled in each vector plane. Synchronization control locks the relative position between the two fluxes. The resultant air-gap flux density is fully controlled, preventing iron saturation. This feature is especially important in reshaping the flux and back EMF waveform of the machine. A quasi-trapezoidal air-gap flux density distribution is achieved for better iron utilization and higher torque density. It is confirmed that compared with sinusoidal fluxing, the quasi-trapezoidal flux pattern will not lead to an oversized power inverter when improving machine torque density. The basic understanding and control scheme can be extended to a multiphase induction machine with a phase number greater than five.     

Electromagnetic considerations for RF current density imaging [MRI technique

Radio frequency current density imaging (RF-CDI) is a recent MRI technique that can image a Larmor frequency current density component parallel to B(0). Because the feasibility of the technique was demonstrated only for homogeneous media, the authors' goal here is to clarify the electromagnetic assumptions and field theory to allow imaging RF currents in heterogeneous media. The complete RF field and current density imaging problem is posed. General solutions are given for measuring lab frame magnetic fields from the rotating frame magnetic field measurements. For the general case of elliptically polarized fields, in which current and magnetic field components are not in phase, one can obtain a modified single rotation approximation. Sufficient information exists to image the amplitude and phase of the RF current density parallel to B(0) if the partial derivative in the B(0) direction of the RF magnetic field (amplitude and phase) parallel to B(0) is much smaller than the corresponding current density component. The heterogeneous extension was verified by imaging conduction and displacement currents in a phantom containing saline and pure water compartments. Finally, the issues required to image eddy currents are presented. Eddy currents within a sample will distort both the transmitter coil reference system, and create measurable rotating frame magnetic fields. However, a three-dimensional electro-magnetic analysis will be required to determine how the reference system distortion affects computed eddy current images.