Circuit-field coupled finite element analysis method for an electromagnetic acoustic transducer under pulsed voltage excitation

This paper presents an analytical method for electromagnetic acoustic transducers (EMATs) under voltage excitation and considers the non-uniform distribution of the biased magnetic field. A complete model of EMATs including the non-uniform biased magnetic field, a pulsed eddy current field and the acoustic field is built up. The pulsed voltage excitation is transformed to the frequency domain by fast Fourier transformation (FFT). In terms of the time harmonic field equations of the EMAT system, the impedances of the coils under different frequencies are calculated according to the circuit-field coupling method and Poynting’s theorem. Then the currents under different frequencies are calculated according to Ohm’s law and the pulsed current excitation is obtained by inverse fast Fourier transformation (IFFT). Lastly, the sequentially coupled finite element method (FEM) is used to calculate the Lorentz force in the EMATs under the current excitation. An actual EMAT with a two-layer two-bundle printed circuit board (PCB) coil, a rectangular permanent magnet and an aluminium specimen is analysed. The coil impedances and the pulsed current are calculated and compared with the experimental results. Their agreement verified the validity of the proposed method. Further- more, the influences of lift-off distances and the non-uniform static magnetic field on the Lorentz force under pulsed voltage excitation are studied.     

Avoiding eddy-current problems in ultra-low-field MRI with self-shielded polarizing coils

In ultra-low-field magnetic resonance imaging (ULF MRI), superconductive sensors are used to detect MRI signals typically in fields on the order of 10-100 μT. Despite the highly sensitive detectors, it is necessary to prepolarize the sample in a stronger magnetic field on the order of 10-100 mT, which has to be switched off rapidly in a few milliseconds before signal acquisition. In addition, external magnetic interference is commonly reduced by situating the ULF-MRI system inside a magnetically shielded room (MSR). With typical dipolar polarizing coil designs, the stray field induces strong eddy currents in the conductive layers of the MSR. These eddy currents cause significant secondary magnetic fields that may distort the spin dynamics of the sample, exceed the dynamic range of the sensors, and prevent simultaneous magnetoencephalography and MRI acquisitions. In this paper, we describe a method to design self-shielded polarizing coils for ULF MRI. The experimental results show that with a simple self-shielded polarizing coil, the magnetic fields caused by the eddy currents are largely reduced. With the presented shielding technique, ULF-MRI devices can utilize stronger and spatially broader polarizing fields than achievable with unshielded polarizing coils.     

Analytical solutions of transient pulsed eddy current problem due to elliptical electromagnetic concentrative coils

The electromagnetic concentrative coils are indispensable in the functional magnetic stimulation and have potential applications in nondestructive testing.In this paper,we propose a figure-8-shaped coil being composed of two arbitrary oblique elliptical coils,which can change the electromagnetic concentrative region and the magnitude of eddy current density by changing the elliptical shape and/or spread angle between two elliptical coils.Pulsed current is usually the excitation source in the functional magnetic stimulation,so in this paper we derive the analytical solutions of transient pulsed eddy current field in the time domain due to the elliptical concentrative coil placed in an arbitrary position over a half-infinite plane conductor by making use of the scale-transformation,the Laplace transform and the Fourier transform are used in our derivation.Calculation results of field distributions produced by the figure-8-shaped elliptical coil show some behaviours as follows:1) the eddy currents are focused on the conductor under the geometric symmetric centre of figure-8-shaped coil;2) the greater the scale factor of ellipse is,the higher the eddy current density is and the wider the concentrative area of eddy current along y axis is;3) the maximum magnitude of eddy current density increases with the increase of spread angle.When spread angle is 180,there are two additional reverse concentrative areas on both sides of x axis.     

Eddy current simulation in thick cylinders of finite length induced by coils of arbitrary geometry

Eddy currents are inevitably induced when time-varying magnetic field gradients interact with the metallic structures of a magnetic resonance imaging (MRI) scanner. The secondary magnetic field produced by this induced current degrades the spatial and temporal performance of the primary field generated by the gradient coils. Although this undesired effect can be minimized by using actively and/or passively shielded gradient coils and current pre-emphasis techniques, a residual eddy current still remains in the MRI scanner structure. Accurate simulation of these eddy currents is important in the successful design of gradient coils and magnet cryostat vessels. Efficient methods for simulating eddy currents are currently restricted to cylindrical-symmetry. The approach presented in this paper divides thick conducting cylinders into thin layers (thinner than the skin depth) and expresses the current density on each as a Fourier series. The coupling between each mode of the Fourier series with every other is modeled with an inductive network method. In this way, the eddy currents induced in realistic cryostat surfaces by coils of arbitrary geometry can be simulated. The new method was validated by simulating a canonical problem and comparing the results against a commercially available software package. An accurate skin depth of 2.76 mm was calculated in 6 min with the new method. The currents induced by an actively shielded x-gradient coil were simulated assuming a finite length cylindrical cryostat consisting of three different conducting materials. Details of the temporal-spatial induced current diffusion process were simulated through all cryostat layers, which could not be efficiently simulated with any other method. With this data, all quantities that depend on the current density, such as the secondary magnetic field, are simply evaluated.     

基于二阶矢量位的矩形截面回折线圈阻抗和脉冲磁场的解析建模与计算

本文基于二阶矢量位建立了回折线圈的阻抗和磁场计算的解析模型,并提出了计算磁场脉冲响应的方法.根据回折线圈用于涡流检测和电磁超声检测时的通用模型,将回折线圈的阻抗和磁场计算问题转化为多个单匝矩形线圈阻抗和磁场的叠加问题.基于二阶矢量位和时谐电磁场方程,推导了回折线圈的频域标势表达式;利用标势与矢量磁位和磁通密度间的关系,推导了计算区域的磁通密度和式样表面涡流的频域解析计算公式;通过计算线圈的感应电势和阻抗变化表达式,得到了线圈阻抗的频域解析表达式;采用FFT-IFFT方法计算了脉冲磁场的时域响应.以一双层双 关键词： 无损检测 回折线圈 二阶矢量位 解析建模     

Detecting magnetic field direction by a micro beam operating in different vibration modes

A new method to detect the magnetic field direction by using a silicon structure is presented in this paper. The structure includes a micro beam and an in-plane coil electrode. When the electrode under a magnetic field is applied with an alternating current, the micro beam is actuated under the effect of the Lorentz forces. Magnetic fields of different directions cause different vibration profiles. The direction of the magnetic field is obtained by measuring the vibration amplitudes of the micro beam, which is driven to work at first- and second-order resonant modes. A micro structure has been fabricated using the bulk micromachined silicon process. A laser Doppler vibrometer system is implemented to measure the vibration amplitudes. The experimental results show that the amplitude of the structure, which depends on the different modes, is a sine or cosine function of the angle of the magnetic field. It agrees well with the simulation result. Currently a resolution of 10° for the magnetic field direction measurement can be obtained using the detecting principle.     

基于反射与折射理论的电涡流检测探头阻抗解析模型

应用电磁波的反射与折射理论建立了多层导电结构电涡流检测探头阻抗解析模型.电涡流检测中空间电磁场可看作线圈激励场与涡流场的叠加,而涡流场是线圈激励场的反射与折射效应,因此应用线圈激励场的级数型解析解和电磁波的反射与折射理论推导了级数型探头阻抗解析解.最后,比较了推导的级数型与传统的积分型探头阻抗解析解的计算结果,并进行了实验验证.计算结果与实验结果基本吻合,表明推导的级数型探头阻抗解析解是正确的,且与积分解析解相比具有不需要确定积分上限、耗时少、计算精度容易控制等优点. 关键词： 电涡流检测 多层导电结构 反射与折射理论 探头阻抗     

准分子激光用矩形脉冲磁场线圈的研制

 叙述用于准分子激光实验的矩形磁场线圈的设计、加工工艺及实验结果。该线圈内表面尺寸为925×425×100mm³，用此线圈在激光腔内得到了磁感应强度为0.225T的磁场，在准分子激光实验中明显地改善了电子束的均匀性。     

Compensation of gradient-induced magnetic field perturbations

Pulsed magnetic field gradients are essential for MR imaging and localized spectroscopy applications. However, besides the desired linear field gradients, pulsed currents in a strong external magnetic field also generate unwanted effects like eddy currents, gradient coil vibrations and acoustic noise. While the temporal magnetic field perturbations associated with eddy currents lead to spectral line shape distortions and signal loss, the vibration-related modulations lead to anti-symmetrical sidebands of any large signal (i.e. water), thereby obliterating the signals from low-concentration metabolites. Here the measurement, characterization and compensation of vibration-related magnetic field perturbations is presented. Following a quantitative evaluation of the various temporal components of the main magnetic field, a digital B0 magnetic field waveform is generated which reduces all temporal variations of the main magnetic field to within the spectral noise level.     

一种永磁磁共振中磁体涡流场的测量方法

磁共振成像（Magntic Resonance Imaging,MRI）技术是一种先进的医疗影像技术.在MRI系统中,通过梯度线圈电流快速切换方向,对待测区域施加梯度磁场,产生的梯度磁场会在其周围的金属体内激发出变化的涡旋电场,进而导致金属体内闭合的回路中产生对原来的梯度电流起抑制作用的感生电流,也就是我们所说的涡流.本文介绍了一种测量磁体涡流场的方法,结合电磁感应定律,设计了一种磁体涡流场测量装置,通过硬件采集以及软件处理的方法,将理想梯度场与实际磁场进行相减并将波形实时呈现,实验结果表明该方法可实现对磁体涡流场的测量.     

电子束二极管脉冲磁场的涡流分析及抑制

针对强流脉冲电子束二极管金属表面涡流引起的器件内部磁场分布不均、电子束辐照材料表面处理效果不佳等现象,提出了多板钢联接式内壁与玻璃外壁相结合的双层结构设计。基于有限元仿真软件对电子束系统的磁场与涡流进行了数值计算,分析了涡流的影响因素和涡流对于电子束源磁场的影响。通过数值计算表明,设计的新型介质壁结构能有效降低涡流对于电子束的影响。该分析为消除感应涡流对真空二极管内部磁场的影响提供了理论依据,具有良好的参考价值。     

A distributed equivalent magnetic current based FDTD method for the calculation of E-fields induced by gradient coils

This paper evaluates a new, low-frequency finite-difference time-domain method applied to the problem of induced E-fields/eddy currents in the human body resulting from the pulsed magnetic field gradients in MRI. In this algorithm, a distributed equivalent magnetic current is proposed as the electromagnetic source and is obtained by quasistatic calculation of the empty coil's vector potential or measurements therein. This technique circumvents the discretization of complicated gradient coil geometries into a mesh of Yee cells, and thereby enables any type of gradient coil modelling or other complex low frequency sources. The proposed method has been verified against an example with an analytical solution. Results are presented showing the spatial distribution of gradient-induced electric fields in a multi-layered spherical phantom model and a complete body model.     

涡流对电真空器件内磁分布的影响及抑制方法

短脉冲线圈电流励磁是高频电真空器件中实现超强磁场的重要技术途径之一,此时器件内将不可避免地产生涡流并进一步对内部磁场分布构成影响。针对使用短脉冲磁场时涡流对电真空器件内磁分布的影响进行了研究,分析了线圈电流脉冲宽度、金属电导率和金属厚度等对涡流的影响,结果表明:随着线圈电流脉冲宽度的减小、金属电导率和金属厚度的增加,涡流对内部磁场的影响也随之增加,导致管内空间无法有效励磁。提出了两种抑制涡流影响的措施,包括采用高电阻率导体进行薄层电镀和对管壁金属纵向切槽开缝。计算结果表明,这两种方法能够有效抑制涡流对器件内部磁场分布的影响,具有良好的实用性。     

电导率各向异性金属薄板感应式磁声图像的仿真

为了给金属薄板感应式磁声(MAT-MI)成像算法的研究提供数据源,提出一种电导率各向异性金属薄板表面MAT-MI图像的数值仿真方法。建立含缺陷的电导率各向异性金属薄板仿真模型,并将其置于静磁场中。将通入交变电流的折线线圈置于金属薄板上方,对金属薄板在静磁场和交变磁场共同作用下产生的感应涡流以及声源(即洛伦兹力)进行数值仿真,得到金属薄板表面波位移分布的灰度图像。仿真实验结果表明,根据表面波位移在缺陷处迅速衰减的特性,可从图像中准确地识别并定位金属薄板表面的缺陷。忽略金属材料的电导率各向异性会降低成像质量,进而导致对缺陷的误判。通过提高表面波位移信号的信噪比可改善成像质量。减小提高距离或增大激励电流频率,可提高系统对微小缺陷和不规则缺陷检测的分辨力。      

Local eddy current measurements in pulsed fields

This work presents new eddy current measurements in pulsed fields. A commercial point pick-up coil is used to detect the induction signal along the radius of Cu and Al samples with cylindrical shape and diameters between 5 and 35 mm. Local eddy current measurements were performed on the surface of conducting materials due to the small dimensions of the coil. A simple electrical circuit, used as a model, is proposed to describe the local eddy current effect in pulsed fields. The proposed model allows to calculate the phase shift angle between the signal proportional to eddy currents and the applied external field in a pulsed field magnetometer.     

Fresnel diffraction model for the point spread of a laser light profile microscope (LPM)

Light profile microscopy is a new method of thin film imaging in which a microscope forms an image of a laser beam intersecting a thin layer along its depth axis. Emission (luminescence or scatter) from the beam volume is transmitted through a cross sectional view surface and used by a microscope to form an image of the beam propagating through the sample. Fresnel diffraction theory is used in this work to derive a point spread model for a microscope operating in this configuration, assuming that the emission from the source beam is completely incoherent. This theory illustrates the dependence of the image resolution on the laser beam radius, which determines the effective thickness of the system object seen by the microscope. The effect of relative displacement of the source beam center from the object plane of the microscope is evaluated. The present theory shows that radial resolution may be evaluated by means of an object focus envelope and a focal energy distribution computed for the objective lens. PACS 42.25Fx; 07.60Pb     

Vibration suppression of a cantilever beam using magnetically tuned-mass-damper

Eddy currents are induced by the movement of a conductor through a stationary magnetic field or a time varying magnetic field through a stationary conductor. These currents circulate in the conductive material and are dissipated, causing a repulsive force between the magnet and the conductor. These electromagnetic forces can be used to suppress the vibrations of a flexible structure. A tuned mass damper is a device mounted in structures to reduce the amplitude of mechanical vibrations and is one of the effective vibration suppression methods. In the present study, an improved concept of this tuned mass damper for the vibration suppression of structures is introduced. This concept consists of the classical tuned mass damper and an eddy current damping. The important advantages of this magnetically tuned mass damper are that it is relatively simple to apply, it does not require any electronic devices and external power, and it is effective on the vibration suppression. The proposed concept is designed for a cantilever beam and the analytical studies on the eddy current damping and its effects on the vibration suppression. To show the effectiveness of the proposed concept and verify the eddy current damping model, experiments on a cantilever beam are performed. It is found that the proposed concept could significantly increase the damping effect of the tuned mass damper even if not adequately tuned.     

A new walk-off compensated multipass scheme for large enhancement of conversion efficiency for fourth harmonic generation

A significant enhancement is treated in the energy of a fourth harmonic generated beam (266 nm) by employing the new technique of walk-off compensated multipass configuration without any change either in the beam parameters or in the intensity of the input fundamental laser radiation. As much as 56% conversion efficiency is obtained in twin lithium tetraborate crystals, even though the effective nonlinear coefficient of the crystal is as low as 1/6th that of KDP. Apart from crystals that have large walk-off angles, low effective nonlinear coefficients, or both, the simple experimental set-up would also be quite suitable for those crystals, especially infrared crystals that have large effective nonlinear coefficients but low laser damage thresholds.     

Towards beyond 1 GHz NMR: Mechanism of the long-term drift of screening current-induced magnetic field in a Bi-2223 coil

The screening current-induced magnetic field in the (Bi,Pb)₂Sr₂Ca₂Cu₃O_x (Bi-2223) insert coil proposed for a beyond 1 GHz nuclear magnetic resonance (NMR) spectrometer may generate a long-term field drift, resulting in a loss of field-frequency lock operation and an inability to make high resolution NMR measurements. The measured screening current-induced magnetic field of a Bi-2223 double-pancake coil exhibits a hysteresis effect at 4.2 K that is reproduced by a numerical simulation based on a finite thickness rectangular superconductor bar model. The screening current-induced field at the coil center is of opposite polarity to that generated by the coil current, and thus the apparent field intensity shows a positive drift with time. On the contrary, the field at a coil end is of the same polarity as the coil field, and the apparent field intensity decreases with time. If we wait for ∼1000 h after coil excitation, the field drift rate approaches the field decay rate of the persistent current of 10⁻⁸ h⁻¹, suitable for a long-term NMR measurement in a beyond 1 GHz NMR spectrometer.