An Efficacious Target-Field Approach to Design Shim Coils for Halbach Magnet of Mobile NMR Sensors

The main magnetic fields of mobile nuclear magnetic resonance (NMR) magnets differ from those of conventional NMR and magnetic resonance imaging (MRI) magnets. In the Halbach magnet, the main field B ₀ is perpendicular to the longitudinal axis, the symmetry of the current distribution with respect to the symmetry of the magnetic field differs from that in conventional target-field applications, and the current distribution on the coil surface cannot be expressed in terms of periodic basis functions. To obtain the winding pattern of the coil, an efficacious target-field approach. The surface of a coil is divided into small discrete elements, where each element is represented by a magnetic dipole. From the stream function of the elements, the resultant magnetic field is calculated. The optimization strategy follows an objective function defined by the power dissipation or efficiency of the coil. This leads to the optimum stream function on the coil surface, whose contour lines define the winding patterns of the coil. This paper shows winding patterns designed of shim coils for Halbach magnet and illustrates the craft of a shim coil using flexible printed circuit board. The performance of the coils is verified by simulating the fields they produce over the sensitive volume.     

矩阵梯度线圈研究现状与发展趋势

梯度和匀场线圈性能的好坏直接影响磁共振成像质量．常规线圈在成像过程中存在一些固有的不足,如产生的磁场形态单一、不灵活,需要的线圈种类较多,结构较复杂等．而新型矩阵梯度线圈可以较好地弥补这些缺点．本文首先介绍了矩阵梯度线圈的概念及其特性,然后根据结构和功能对其研究现状进行分类汇总,在此基础上对矩阵线圈未来的发展趋势进行分析．此外,本文还对矩阵梯度线圈的前期研究基础进行了介绍．     

球形线圈磁场均匀性的研究

研究非密绕球形线圈内部的磁场均匀性质.从圆形电流线圈轴线上的磁场出发,利用叠加原理和Mathematica软件的计算和符号推导功能,得到一个关于线圈内部磁场均匀性与其匝数的定量关系式,在此基础上通过实例说明设计球形线圈磁场均匀区的方法.     

霍尔推力器磁场位形及其优化的数值研究

基于麦克斯韦方程,在轴对称假设下建立了霍尔推力器磁场的数学模型.用有限差分方法对模型进行了离散.给出了数值求解模型的迭代法.通过对模型的数值求解,得到了相应的数值结果.通过对所得数值结果的分析,研究了磁场线圈电流变化对霍尔推力器磁场位形的影响.通过调整磁场线圈电流的大小找到了理想磁场位形.研究表明,对于理想磁场位形,内通道的磁镜比在3—3.5之间,外通道的磁镜比在0.4—0.9之间;增加磁场线圈的电流,出口的磁场强度随着增加,但不能增加磁镜比.通道内部的磁场强度几乎不随着磁场线圈电流的变化而变化. 关键词： 霍尔推力器 磁场位形 磁场线圈电流 磁镜比     

Design of SC solenoid with high homogeneity

A novel kind of SC (superconducting) solenoid coil is designed to satisfy the homogeneity requirement of the magnetic field. In this paper, we first calculate the current density distribution of the solenoid coil section through the linear programming method. Then a traditional solenoid and a nonrectangular section solenoid are designed to produce a central field up to 7 T with a homogeneity to the greatest extent. After comparision of the two solenoid coils designed in magnet field quality, fabrication cost and other aspects, the new design of the nonrectangular section of a solenoid coil can be realized through improving the techniques of framework fabrication and winding. Finally, the outlook and error analysis of this kind of SC magnet coil are also discussed briefly.     

Shim coil design for Halbach magnet by equivalent magnetic dipole method

Low-field nuclear magnetic resonance magnet(2 MHz) is required for rock core analysis. However, due to its low field strength, it is hard to achieve a high uniform B_0 field only by using the passive shimming. Therefore, active shimming is necessarily used to further improve uniformity for Halbach magnet. In this work, an equivalent magnetic dipole method is presented for designing shim coils. The minimization of the coil power dissipation is considered as an optimal object to minimize coil heating effect, and the deviation from the target field is selected as a penalty function term. The lsqnonlin optimization toolbox of MATLAB is used to solve the optimization problem. Eight shim coils are obtained in accordance with the contour of the stream function. We simulate each shim coil by ANSYS Maxwell software to verify the validity of the designed coils. Measurement results of the field distribution of these coils are consistent with those of the target fields.The uniformity of the B_0 field is improved from 114.2 ppm to 26.9 ppm after using these shim coils.     

Globally optimal superconducting magnets part II: symmetric MSE coil arrangement

A globally optimal superconducting magnet coil design procedure based on the Minimum Stored Energy (MSE) current density map is outlined. The method has the ability to arrange coils in a manner that generates a strong and homogeneous axial magnetic field over a predefined region, and ensures the stray field external to the assembly and peak magnetic field at the wires are in acceptable ranges. The outlined strategy of allocating coils within a given domain suggests that coils should be placed around the perimeter of the domain with adjacent coils possessing alternating winding directions for optimum performance. The underlying current density maps from which the coils themselves are derived are unique, and optimized to possess minimal stored energy. Therefore, the method produces magnet designs with the lowest possible overall stored energy. Optimal coil layouts are provided for unshielded and shielded short bore symmetric superconducting magnets.     

基于永磁恒定磁场激励的起始磁化曲线测量

现有起始磁化曲线测量系统需绕制励磁线圈和感应线圈,在线应用受限.为此,本文提出了一种基于永磁恒定磁场激励的起始磁化曲线测量原理并搭建了相应测量系统.该系统采用永磁磁化器作为激励磁源,以对称磁化方法在圆柱棒状构件上激励出随轴向位置变化的恒定磁场作为激励磁场;采用阵列霍尔探头测量构件表面不同提离下的轴向和法向磁感应强度;并基于多项式外推法和磁场高斯定理外推法,推算构件与空气分界面上的轴向和法向磁感应强度;进一步地,根据分界面上的磁感应强度获取构件的起始磁化曲线.系统测量结果表明,在永磁恒定磁场激励下,无须励磁线圈和感应线圈即可方便地获取棒状构件的起始磁化曲线,测量误差小于10%,测量误差标准差小于0.01,重复性较好.该系统可为便捷地在线测量棒状构件起始磁化曲线提供新途径.     

3个共轴圆线圈形成的匀强磁场

分析了3个共轴圆线圈形成磁场的均匀性，得到了3个共轴圆线圈形成匀强磁场的条件，并且通过数值计算，全面展示3个共轴圆线圈磁场的均匀性．只要合理配置3个共轴圆线圈的大小、线圈中的电流强度和线圈之间的距离，3个共轴圆线圈的磁场在强度和均匀性方面都优于亥姆霍兹线圈的磁场。     

Multi-objective optimization of gradient coil for benchtop magnetic resonance imaging system with high-resolution

Significant high magnetic gradient field strength is essential to obtaining high-resolution images in a benchtop mag- netic resonance imaging （BT-MRI） system with permanent magnet. Extending minimum wire spacing and maximum wire width of gradient coils is one of the key solutions to minimize the maximum current density so as to reduce the local heating and generate higher magnetic field gradient strength. However, maximum current density is hard to optimize together with field linearity, stored magnetic energy, and power dissipation by the traditional target field method. In this paper, a new multi-objective method is proposed to optimize the maximum current density, field linearity, stored magnetic energy, and power dissipation in MRI gradient coils. The simulation and experimental results show that the minimum wire spacings are improved by 159% and 62% for the transverse and longitudinal gradient coil respectively. The maximum wire width increases from 0.5 mm to 1.5 mm. Maximum gradient field strengths of 157 mT/m and 405 mT/m for transverse and lon- gitudinal coil are achieved, respectively. The experimental results in BT-MRI instrument demonstrate that the MRI images with in-plane resolution of 50 ~tm can be obtained by using the designed coils.     

HL-2M 装置极向场线圈的结构设计与制造

介绍了 HL-2M 装置极向场(PF)线圈的结构设计和制造。16 饼 PF 线圈布置在环向场(TF)线圈空腔之 内、真空室外,沿中平面对称分布。PF 线圈采用中空矩形铜导体绕制,其中 PF1~PF4 线圈为双层螺旋绕制结构, 最大运行电流 14.5kA;PF5~PF8 线圈为多层盘式绕制结构,最大运行电流 38~42kA。      

制作简易直流电动机

为了演示“磁场对通电导线的作用力”实验,设计了简易直流电动机.该简易直流电动机是将绕成不同形状的导线放置在磁场中,通过线圈的旋转,即可演示磁场对通电导线的作用.     

超导线圈绕制过程的力学行为研究

为了增加超导线圈中导线的占空比, 提高超导磁体正常运行时的机械稳定性, 通常在超导线圈绕制过程中施加一定的绕制张紧力. 绕制张紧力的大小会对超导磁体的失超特性和退化性能产生重要的影响, 因此有必要对绕制过程中的机械应力进行详细的分析. 本文仔细地分析了绕制过程中导线的受力情况, 进行了一些合理的假设和近似, 提出了研究超导线圈绕制应力的理论模型, 并根据轴对称结构的弹性力学方程式推导了计算超导线圈应力应变分布的理论公式. 基于该模型分别研究了单一绕组的超导线圈和双绕组的超导线圈的绕制应力, 分析了绕制张紧力和绕组的各向异性特性对径向应力和环向应力的影响. 在该理论模型分析结果的基础上可以进一步分析多物理场作用下的超导磁体的应力应变行为, 为高性能超导线圈的设计和建造提供理论指导. 关键词： 超导线圈 机械稳定性 绕制张力 应力     

HL-1装置的磁场测量

本文叙述了HL-1装置环向场线圈、内外垂直场线圈,欧姆变压器初级绕组和偏磁绕组的磁场测量方法和结果,初步讨论了带有切口的铜壳对磁场的影响。     

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.     

Calculation of magnetic field in the region of the superconducting winding of window frame dipole for particle accelerators

The necessity of simulations in design of superconducting dipole magnets is due to the following circumstances. First, the critical current as a function of the magnetic field I _c(B) for the multicore superconducting cable which drops strongly requires the knowledge of the value of maximum magnetic field “felt” by its coils for estimation of the working current of the magnet. Second, for choosing the optimal number of coils of the winding (1 or 2) and the working current of the magnet, the ratio of B _max for the inner and outer layers of the dipole magnet winding should be known. Since usually the length of the dipole magnet exceeds many times its transverse size, in this work all calculations of B(x, y) are performed in the transverse plane crossing the center of the magnet. The field at the central point is chosen to be B(0, 0) = 2 T (this is the characteristic working value close to the maximum value in the dipole magnet of this type). In this work, the results of calculation of B(x, y) for single-and double-layered windings with 8 and 16 coils from circular hollow cable are presented.     

Effect of large-heat-capacity substances doping of superconducting coils on their dynamic stability

The dependence of the quench currents on the ramp rate was studied for four small NbTi coils. Two pairs of superconducting coils were tested. In one pair the SC 0.85-mm-dia wire with 2970 filaments was used, in the other two coils the SC wire contained 8910 filaments of smaller size. Two coils (with different number of filaments) contained 4.9 vol % of Large-Heat-Capacity Substance (LHCS) in the form of tiny powder evenly distributed over the winding volume, therefore their heat capacity at 4.2 K was an order of magnitude larger than that for coils without LHCS. The LHCS was introduced into the winding in a mixture with epoxy resin (“wet” winding). When the self-magnetic field varied with a rate of ≥5 T/s, premature quenches were observed in the central turns of the undoped coil made of a wire with 2970 filaments. These transitions are likely to be caused by magnetic flux jumps. In the LHCS-doped coil made of the same wire, the quenches took place at currents two to three times higher, since the sample was heated up to a critical temperature because of electrical losses (as confirmed by calculations). Thus, the improved stability of the LHCS-containing coils not only against long-term (0.1–1.0 s) disturbances but also against much shorter (10–100 μs) jumps of the magnetic flux is demonstrated.     

An efficient calibration method for SQUID measurement system using three orthogonal Helmholtz coils

For a practical superconducting quantum interference device(SQUID) based measurement system,the Tesla/volt coefficient must be accurately calibrated.In this paper,we propose a highly efficient method of calibrating a SQUID magnetometer system using three orthogonal Helmholtz coils.The Tesla/volt coefficient is regarded as the magnitude of a vector pointing to the normal direction of the pickup coil.By applying magnetic fields through a three-dimensional Helmholtz coil,the Tesla/volt coefficient can be directly calculated from magnetometer responses to the three orthogonally applied magnetic fields.Calibration with alternating current(AC) field is normally used for better signal-to-noise ratio in noisy urban environments and the results are compared with the direct current(DC) calibration to avoid possible effects due to eddy current.In our experiment,a calibration relative error of about 6.89 × 10-4is obtained,and the error is mainly caused by the non-orthogonality of three axes of the Helmholtz coils.The method does not need precise alignment of the magnetometer inside the Helmholtz coil.It can be used for the multichannel magnetometer system calibration effectively and accurately.     

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.     

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