横向三角脉冲磁场下悬臂导电薄板的动力稳定性分析

对于在仅有外加横向三角脉冲磁场下的悬臂导电薄板,给出了受到脉冲磁场影响时其动力稳定性的定量分析方法。揭示了在三角脉冲磁场下由于受到涡电流与总磁场相互作用所产生的面内磁体压力的作用,悬臂导电薄板会发生失稳。得到了在不同稳态磁场作用下受到三角脉冲磁场作用时导电薄板的磁弹性动力响应的数值模拟结果。在此基础上,给出了关于稳态磁场值、横向脉冲磁场峰值和脉冲持续时间等参数范围的磁弹性动力稳定性区域。     

导电流体中气泡径向振动行为的磁场控制

液态金属中气泡行为是磁流体力学的重要方面。为对磁场条件下导电流体中气泡动力学行为作全面理解,基于磁流体动力学方法建立了磁场条件下导电流体中气泡径向振动的无量纲化动力学方程,数值研究了磁场对导电流体中气泡径向非线性振动稳定性、泡内温度、泡内气压及液体空化阈值的影响。结果显示:磁场增强了气泡非线性振动的稳定性,随着磁场增强且当作用在泡上的电磁力与惯性力数量级可比时,气泡运动为稳定的周期性振动;同时,磁场引起泡内温度、泡内压力及液体空化阈值变化。研究表明,可用磁场调节和控制液态金属中气泡的运动使其满足工程应用需求。      

运动感应涡流磁场检测中的激励磁场优化设计方法

在生产或使用过程中,薄金属板中会产生微缺陷(微颗粒或气孔),这些微缺陷的数目及尺寸决定了薄金属板的品质,影响其使用寿命和安全系数.因此,对微缺陷进行定量和准确的表征是保证薄金属板产品品质与使用安全的必要条件.电磁检测中的运动感应涡流磁场检测可应用于检测导电非导磁材料中的缺陷.本文通过模拟和实验结果显示,当提离距离、永磁体表面剩磁确定时,永磁体尺寸与缺陷检测信号幅值正相关.另外,提出一种运动感应涡流磁场检测中激励磁场的优化方法,由该方法产生的两种永磁体阵列相比于同体积、同表面剩磁的简单永磁体,产生的缺陷检测信号幅值更高,因而可以提高检测的灵敏度.     

一种外磁作用下球壳系统空间磁场的计算方法

为了快速分析球形铁磁壳体间的相互磁化影响,提出一种能够计算外磁作用下球壳系统空间磁场的镜像解算法。从外磁作用下的单个球壳与单个球体产生的磁场规律出发,通过引入磁场比例系数把球壳间的相互磁化等效为球体间的相互磁化,并根据镜像法推导出球壳间的相互磁化规律,然后计算球壳间的相互磁化产生的附加空间磁场,从而得到球壳系统空间磁场的镜像解。通过分析衰减程度随阶数变化,讨论了镜像解的计算阶数如何选择。最后仿真实验结果表明,本文镜像解的计算误差较小,且适用于不同厚度的球壳系统空间磁场的计算。     

磁场微重力效应的研究

将导电的液体置于磁场中,使导电液体中浮力驱动的自然对流减弱甚至消失,在导电液体中制造出了二级微重力效应,将这种情况称为磁场微重力效应.通过研究导电液体自然对流驱动力的无量纲Grashof数的变化,发现微重力效应的水平可以用公式g_m＝（β₀/β_m）（ν₀ /ν_m）²g₀ 计算,如果略去一次项,则可用g_m＝（ν₀ /ν_m）²g₀来估算.测量研究了不同磁场条件下硅熔体的磁黏度,估算不同磁场强度对应的磁场微重力水平后,发现估算结果与实验结果基本符合. 关键词： 磁场 二级微重力效应 Grashof数 微重力水平     

能量输运对柱状等离子体开关磁场穿透过程的影响

 对于高密度、导通时间为μs级的柱状等离子体开关，利用磁流体动力学理论(MHD)，对其导通阶段的磁场穿透过程进行了模拟，得到了磁场分布随时间的变化；研究了开关导通过程中能量输运导致的温度不均匀分布对磁场穿透过程的影响。模拟结果表明：对于高密度等离子体开关，磁场以远大于磁扩散速率的速度穿透到等离子体中；在磁压对等离子体产生的压缩效应和欧姆加热效应共同作用下，激波区域的等离子体温度显著升高，这进一步加速了磁场穿透；当考虑能量输运方程时，开关导通时间为0.87 μs，比等温模型的结果0.92 μs短，与实验结果0.87 μs相一致。     

斜向静磁场中光导波模式转换与衍射理论

应用经典场论和耦合模理论计算了斜向静磁场作用下，磁性薄膜波导中静磁波与导波光的相互作用.计算结果表明，与垂直静磁场情形相比，斜向静磁场时的相匹配条件有所变化，由于磁性薄膜波导中法拉第磁光效应增强等因素，导波光的Bragg衍射效率得以显著增加.这有利于应用YIG等低损耗磁性薄膜做成高衍射效率的磁光波导器件，亦有利于降低Bi∶YIG等高比法拉第旋转的磁光薄膜波导器件的体积和损耗.此外，理论指出的在斜向静磁场条件下所具有的一些磁光特性与实验结果亦符合得很好. 关键词：     

斜向静磁场中的薄膜磁光效应和静磁波传播特性

理论计算表明，与垂直静磁场情形相比，斜向静磁场可以明显增强磁性薄膜波导中的法拉第效应，同时会引起静磁波共振频率和带宽等许多静磁波特性的变化.合理利用这些变化特性，可望大幅度提高导波光的衍射效率，改善磁光波导器件的特性. 关键词：     

等效磁荷方法及其在三维磁场计算中的应用

本从普通物理教学角度讨论了等效磁荷概念；以及等效磁荷方法在包含有非线性高磁导介质的三维磁场计算中的应用，给出了用此方法计算电磁除铁器磁场的实例，最后讨论此方法的优缺点。     

电、磁场作用于扁球边界面上的力矩

本文研究电、磁场作用于扁球导体、磁导体上的力矩。根据基本解答和把体积力公式运用于扁球体面上后(包括平面上有半扁球形突面的边界问题〕,得出静电问题的较为普遍的解式。又由于静电、静磁张力公式的简单对应关系存在,故很容易把所得到的公式直接推广到对应的静磁问题上去,最后根据所得的解式,我们得到了若干有益的结论。     

Radial magnetic field in magnetic confinement device

The intrinsic radial magnetic field(B r) in a tokamak is explored by the solution of the Grad–Shafranov equation in axisymmetric configurations through an expansion of the four terms of the magnetic surfaces. It can be inferred from the simulation results that at the core of the device, the tokamak should possess a three-dimensional magnetic field configuration, which could be reduced to a two-dimensional one when the radial position is greater than 0.6a. The radial magnetic field and the amzimuthal magnetic field have the same order of magnitude at the core of the device. These results can offer a reference for the analysis of the plasma instability, the property of the core plasma, and the magnetic field measurement.     

Variable magnetic field and temperature magnetic force microscopy

Magnetic force microscopy (MFM) studies of epitaxial MnAs films on GaAs(001) have been performed as a function of the applied magnetic field and the sample temperature. For this purpose, we combined a stable variable-temperature sample stage with a compact magnet assembly to fit a commercial magnetic force microscope. In order to keep the thermal drift that affects MFM measurements low, we employed a permanent magnet that can be rotated in a yoke assembly guiding the magnetic flux to the sample. PACS 68.37.Rt; 68.35.Rh; 75.70.-i; 75.70.Kw     

论静电场和静磁场的相对性

电场和磁场形式不同但本质相同,在不同的惯性系下,二者可以相互转变,转变的规律遵循最基本的力学原理.结合狭义相对论和基本的力学定律,可以更加直观地表达电场与磁场的相互转变过程和转变形式,从而更深刻地理解电场与磁场同宗同源的本质.     

Critical magnetic field ratio of anisotropic magnetic superconductors

The upper critical field, the lower critical field and the critical magnetic field ratio of anisotropic magnetic superconductors are calculated by Ginzburg–Landau theory analytically. The effect of the Ginzburg–Landau parameter (κ₀), magnetic susceptibility (χ) and magnetic-to-anisotropic parameter ratio (θ) on the critical field ratio are considered. We find that the value of critical field ratio increases with increasing κ₀ and θ, and decreases with increasing χ. The highest and the lowest value of critical field ratio is found in the diamagnetic superconductors and the ferromagnetic superconductors, respectively.     

Concomitant magnetic field gradients and their effects on imaging at low magnetic field strengths

Low-field NMR imaging systems which use large amplitude field gradient pulses (e.g., in flow velocity encoding) may be subject to the undesirable effects of concomitant gradients. We demonstrate the effects of these extra gradients, which arise from Maxwell's equations, and show that the resultant image phase shifts and amplitude changes are consistent with theory.     

Effects of static and radiofrequency magnetic field inhomogeneity in ultra-high field magnetic resonance imaging

To characterize the severe static (B(0)) and radiofrequency (B(1)) magnetic field inhomogeneity in ultra-high field (> or =7 T) magnetic resonance imaging, gradient echo (GE) and spin echo (SE) images of in vivo and postmortem human brains were acquired. The B(0) and B(1) inhomogeneity were experimentally mapped and/or numerically simulated, and correlated with the image artifacts. Whereas B(0) inhomogeneity affects predominantly GE images near air/tissue interfaces, B(1) inhomogeneity affects SE images more severely and shows non-intuitive patterns. Mapping of the B(0) and B(1) inhomogeneity is important in characterizing image artifacts. This will help develop better B(0) and B(1) inhomogeneity correction methods.     

Hysteresis in rotation magnetic field

The different properties of the vector Jiles–Atherton hysteresis operator is proved under forced H- and B-field supply. Feeding the magnetic material with alternating and circular polarised rotational excitation, the different properties of the model under the input field intensity and the flux density are investigated and the results are proved in figures.     

NMR in pulsed magnetic field

Nuclear magnetic resonance (NMR) experiments in pulsed magnetic fields up to 30.4 T focused on ¹H and ⁹³Nb nuclei are reported. Here we discuss the advantage and limitation of pulsed field NMR and why this technique is able to become a promising research tool.     

Vacuum polarization in magnetic field

The dispersive properties of free space in the presence of a magnetic field are calculated in the lowest order in the fine-structure constant, but exactly in the field. An integral representation of the corresponding index of refraction is written, exhibiting the dispersive and birefringent properties of the vacuum, with absorption coming in for transverse photon momenta above the pair production threshold. The weak-field limit and the dominant behavior in strong fields are explicitly calculated.