载流长直密绕螺线管磁场的全空间解

很多《普通物理学》教材讲到稳恒磁场时，都以长直密绕螺线管为例，为求解其磁场分布，一般都假设导线很细，将长直密绕螺线管简化为一串共轴圆电流，得到管内磁场是一均匀磁场，管外磁场为零的结论．但这样的螺线管模型不可能使电流从管的一端流到另一端，与实际情况相差较远．本文在不做这种简化假设的条件下，求得长直密绕螺线管磁场在全空间的精确解．ｌ长直密绕螺线管的磁场 设长直密绕螺线管的半径为Ｒ，单位长度上的匝数为ｎ，电流环绕轴线流动并与轴线方向成一角度ａ，如图１所示．将管上任一段的管壁展开，如图２（为清楚起见，图…     

也谈长直螺线管磁场的全空间解

本文证明了长直螺线管上电流密度沿螺线管圆周切线方向分量和电流密度沿螺线管轴线方向分量与θ无关。并给出了长直密绕通电螺线管磁场的全空间解。     

非密绕螺线管轴线上的磁场

利用旋转直角坐标系,根据Biot-Savart定律给出非密绕螺线管轴线上的磁场分布,最后由第二类变型Bessel函数的积分得到非密绕螺线管轴线上的磁场分布精确解。     

镜像对称的载流导线磁场方向的分析

电磁场的对称性分析在大学物理教学中有着极其重要的地位,但是现有的大学物理教材很少给出如何利用稳恒磁场的对称性来分析磁感强度的方向.文章针对此问题应用毕奥-萨伐尔定律并结合矢量的分解更加便捷地分析了一对镜像对称的载流导线在中间面上任意点处的磁场方向:其方向必定垂直于该面.应用这一结论可以很容易判断出像密绕螺线管、直螺线管和圆柱形导线等这一类电流分布具有镜像对称的载流导线在其中间面上的磁感强度的方向.     

直接计算载流密绕直螺线管任一点的磁场

本刊在84年11期上刊登了“论证安培环路定律的两点补充”一文.该支引用了参考文献[1]中的结果,即载流密绕宜螺线管内的磁感应强度为　0nI,管外为零,从而得出包围国电流时安培环路定律　B·dl=u0∑I亦正确.但[1]用毕奥-萨伐尔定律只计算直螺线管轴线上的磁场,管内任一点和管外的磁场则是在忽略轴向电流的近似条件下由磁感应线推理求得的.显然,利用这个结果去论证安培环路定律是欠严格的.本文介绍一个直接计算载流密绕直螺线管任一点的磁场的方法. 设螺线管的半径为a,其轴线沿z轴,单位长度的匝数为u,每匝的电流为I,源点(a、θ’、z’)处的电…     

疏松载流螺线管轴线上磁场的分布研究

疏松载流螺线管轴线上磁场实验的设计可以测量螺线管轴线不同位置上的轴向磁场和径向磁场,实验结果与数值计算结果吻合。另外,从该实验测量结果发现疏松型螺线管轴线上的轴向磁场与螺距呈近似线性关系。这些工作不仅可作为大学物理实验课的选做内容,还可以为设计基于测量磁场的位移和应力传感器提供依据。     

无限长密绕载流螺线管中磁场分布教学的不同设计

无限长密绕载流螺线管是磁场教学中的一个重点和难点. 通过几种不同方法的教学设计, 对无限长密 绕载流螺线管磁场分布进行具体分析, 达到启迪学生思维, 领悟物理思维方式精髓的目的     

对厚壁螺线管电流分布的讨论

厚壁螺线管壁内电流分布通常所用的两个简化假设，即电流沿圆周方向和电流密度均匀，被证明是不符合电磁学的普遍原理的。同时也证明了只要电流不存在径向分量，就能够自然地推论出壁内的电流分布，从而也可望由此算得的磁场能更接近于实际。     

一种磁流变液制动演示装置的磁路设计

设计了一种磁流变液制动演示装置,为了使磁流变液在匀强磁场下工作,采用多层密绕螺线管的方式产生磁场,并对螺线管所用铜线的规格和缠绕的匝数进行选择和计算,这种方式使得在通入相同的电流时能产生更大的磁场。最后,应用MATLAB软件对励磁线圈所产生的磁场的分布与大小进行仿真,结合磁场的实际测量数据对比分析,证明外加磁场符合演示实验要求,能够达到理想的演示目的。     

利用“几何画板”探索螺线管的磁场分布

应用"几何画板"模拟了螺线管内轴线上磁场的分布,给出了螺线管磁场的数据计算表,绘制了螺线管磁场的分布图。     

轴线起爆式螺线管型爆磁压缩发生器理论模型

 对轴线起爆式螺线管型爆磁压缩发生器进行了理论模型研究，建立了爆炸管的一维爆轰驱动模型、螺线管内空间磁场强度分布模型、爆炸管外表面磁压力模型和发生器系统的等效电路模型等，对此类发生器的物理过程进行系统描述。在此基础上，编制了相应的零维数值模拟程序CEMG 1.0，利用该程序分别对四种不同模型参数的发生器进行了理论计算和参数优化，并对其中一模型发生器爆炸管外表面的磁压力及其引起的剩余电感进行了计算，给出了剩余电感与初始输入条件及负载电感的关系，从而得到该模型的输出性能极限。对理论模型的正确性进行了实例验算证明。     

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。     

Ultra-broadband NMR probe: numerical and experimental study of transmission line NMR probe

We have reinvestigated a transmission line NMR probe first published by Lowe and co-workers in 1970s [Rev. Sci. Instrum. 45 (1974) 631; 48 (1977) 268] numerically and experimentally. The probe is expected to be ultra-broadband, thus might enable new types of solid-state NMR experiments. The NMR probe consists of a coil and capacitors which are connected to the coil at regular intervals. The circuit is the same as a cascaded LC low-pass filter, except there are nonzero mutual inductances between different coil sections. We evaluated the mutual inductances by Neumann's formula and calculated the electrical characteristics of the probe as a function of a carrier frequency. We found that they were almost the same as those of a cascaded LC low-pass filter, when the inductance L of a section was estimated from the inductance of the whole coil divided by the number of the sections, and if C was set to the capacitance in a section. For example, the characteristic impedance of a transmission line coil is given by Z=(L/C)(1/2). We also calculated the magnitude and the distribution of RF magnetic field inside the probe. The magnitude of RF field decreases when the carrier frequency is increased because the phase delay between neighboring sections is proportional to the carrier frequency. For cylindrical coils, the RF field is proportional to (pinu/2nu(d))(1/2)exp(-nu/nu(d)), where the decay frequency nu(d) is determined by the dimensions of the coil. The observed carrier frequency thus must be much smaller than the decay frequency. This condition restricts the size of transmission line coils. We made a cylindrical coil for a 1H NMR probe operating below 400 MHz. It had a diameter 2.3mm and a pitch 1.2mm. Five capacitors of 6pF were connected at every three turns. The RF field strength was 40 and 60 kHz at the input RF power 100 W by a calculation and by experiments, respectively. The calculations showed that the RF field inhomogeneity along the coil axis was caused by a standing wave of current, which arose from the reflections at the coil ends. The calculation showed that the homogeneity could be improved by decreasing the pitch near the both ends and making their impedance close to that at the center.     

Excitation of a cylindrical cavity by a helical current and an axial electron beam current

The explicit expressions (in the Vainshtein and Markov forms) are derived for the excitation of a cylindrical cavity with perfectly conducting walls and with impedance end faces. Excitation of a cylindrical cavity and a cylindrical waveguide with a preset nonuniform axial electron-beam current and a helical current with a variable pitch, which is excited by a concentrated voltage source and is loaded by a preset pointlike matched load, is considered. For the helical current, the integro-differential equation is formulated. The traveling-wave tube (TWT) is simulated in the preset beam current approximation taking into account the nonuniform winding of the spiral coil, nonuniform electron beam, and losses.     

A novel low-E field coil to minimize heating of biological samples in solid-state multinuclear NMR experiments

A novel coil, called Z coil, is presented. Its function is to reduce the strong thermal effects produced by rf heating at high frequencies. The results obtained at 500MHz in a 50 microl sample prove that the Z coil can cope with salt concentrations that are one order of magnitude higher than in traditional solenoidal coils. The evaluation of the rf field is performed by numerical analysis based on first principles and by carrying out rf field measurements. Reduction of rf heating is probed with a DMPC/DHPC membrane prepared in buffers of increasing salt concentrations. The intricate correlation that exists between the magnetic and electric field is presented. It is demonstrated that, in a multiply tuned traditional MAS coil, the rf electric field E(1) cannot be reduced without altering the rf magnetic field. Since the detailed distribution differs when changing the coil geometry, a comparison involving the following three distinct designs is discussed: (1) a regular coil of 5.5 turns, (2) a variable pitch coil with the same number of turns, (3) the new Z coil structure. For each of these coils loaded with samples of different salt concentrations, the nutation fields obtained at a certain power level provide a basis to discuss the impact of the dielectric and conductive losses on the rf efficiency.     

带集磁器的吸引式电磁力小管件翻边方法

针对微小铝合金管件电磁翻边工艺,现有方法将驱动线圈置于管件端部外侧,利用双频电流法产生吸引式电磁力实现翻边。然而其翻边能力不强,基于此提出一种带集磁器的吸引式电磁力翻边方法。在现有方法基础上引入集磁器,利用其能够改变磁场位形的特点,优化电磁力分布并增大轴向电磁力,达到增强翻边效果的目的。为验证该方法的可行性,通过搭建管件翻边过程的电磁-结构全耦合有限元仿真模型,对比引入不同集磁器后的翻边效果,同时分析了不同工况对电磁力分布、电磁力密度以及磁场和涡流的影响。得出阶梯型集磁器效果最佳,结果表明,该方法下管件翻边角度从38°增大到90°。进一步分析表明,其磁通密度径向分量和涡流密度环向分量分别增大到164%和135%,作用在管件上的电磁力分布改变,峰值时刻轴向电磁力体密度明显加强,增大到211%。该方法进一步完善了对微小铝合金管件的电磁翻边成形,对拓展电磁成形技术在铝合金管件翻边上的应用具有一定意义。     

OHTE: A Helical Pinch with Pitch Reversal

The OHTE configuration is obtained by surrounding a reversed field pinch (RFP) with a stellarator-like helical winding whose pitch is chosen to enhance field line pitch reversal. The helical coil current needed to form a separatrix boundary is calculated analytically for a simple plasma model, which gives results in close agreement with a numerical two-dimensional (2-D) MHD equilibrium code. Basic properties of the field line transform, which is predominantly in the axial or toroidal direction, are investigated. The 2-D equilibrium code is used to investigate the effects of current profiles and high beta, and the OHTE is compared with the RFP. These calculations show that the helical winding can significantly enlarge the parameter space for interchangestable finite-? equilibria with pitch reversal while avoiding axial current reversal.     

Performance improvement of a high-temperature superconducting coil by separating and grading the coil edge

In this paper, we establish a model to analyze the transport current performance of a high-temperature superconducting (HTS) coil, considering the dependencies of critical current and n-value of an HTS tape on magnetic field and magnetic field angles. This analysis shows that relatively large electric fields appear at the coil’s edges, preventing improvement in the transport current performance of the coil. To solve this problem, in this paper, we propose a graded coil in which several coil edges of different heights are separated and graded. Analysis of its performance shows that the coil’s critical current increases, thus confirming that there exists an optimum coil cross section at which the stored energy and central magnetic field improve 2.1 times and 45%, respectively, compared with a typical rectangular coil that employs the same total length of the HTS tape. It is recommended that these results of the coil should be applied to SMES.     

SHAPING AND ADJUSTING THE FRINGING FIELD OF IRON-FREE COILS BY USE OF SEPARATED COILS

Magnetic field distribution, expressed in terms of coil parameters (geometric and current), of various kind of ironfree current wire elements is derived. A method for shaping and adjusting fringing field profile by the use of separated current wires which have different coil parameters is suggested, and the optimization of coil parameters is realized by the use of the method of least square. As an example, magnetic field profile of an iron-free magnetic β-ray spectrometer is shaped and adjusted to illustrate the use of this method.