高磁雷诺数下剪切流对双撕裂模中二级磁岛的影响

在高磁雷诺数下,当双撕裂模发展进入快速磁场重联阶段时,会发生二级磁岛不稳定性,加剧磁场能量的释放。本文基于扰动形式的守恒磁流体方程组发展高精度的数值模拟程序,在平板位形下研究反对称位形剪切流对双撕裂模中二级磁岛的影响。结果表明：随着剪切流强度和剪切梯度的增加,二级磁岛的数目以及电流片横纵比变小。此外,较强的极向剪切流能够抑制二级磁岛不稳定性的发生。     

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通过求解二维热输运方程,数值分析了横越单磁岛的热输运现象。仅有背景加热时,当磁岛宽度小于临界磁岛宽度时,磁岛对能量约束影响不明显;当磁岛宽度大于临界磁岛宽度时,磁岛降低的能量损失随着磁岛宽度呈线性增大,有效径向热导系数在磁岛区域呈高斯分布且最大值在有理面处。当等离子体背景加热与ECRH共同加热时,ECRH对磁岛区域的温度分布及能量约束均有别于背景热源,这为进一步研究ECRH抑制撕裂模的问题提供了基础。     

电流驱动对磁岛的稳定化

大破裂限定了托卡马克的运行。实验指出，撕裂模或者说磁岛的发展是这种大破裂的主要原因。考虑到无感电流驱动效应，我们重新推导了电阻撕裂模的三维非线性方程组?数值计算表明，跟随磁岛旋转的ＲＦ波电流驱动有效地抑制磁岛的发展。     

托卡马克中共振螺旋场对撕裂模的影响

本文在共振螺旋场(RHF)条件下,对撕裂模的时间演化过程进行了数值计算。在等离子体内部磁扰动与外加RHF的相位差不同时,分析了撕裂模的不同发展过程和结果。认为RHF可以最终改变托卡马克中的饱和磁岛宽度。     

小磁岛转动引起的沿磁力线电流对撕裂模的影响

利用离子香蕉轨道中心坐标和香蕉轨道平均算符,讨论了磁岛转动引起的沿磁力线电流分布及其对新经典撕裂模演进的影响。结果显示,考虑此电流的影响,当磁岛旋转频率ω=ω_*i 时,被离子香蕉轨道宽度效应削弱的自举电流驱动项基本恢复;当ω=ω_*e时,离子香蕉轨道宽度效应对自举电流驱动项的削弱则加剧。这意味着磁岛转动方向不同时,此沿力线电流能显著增大或减弱离子香蕉轨道宽度效应对自举电流驱动项的削弱作用。     

通过软X射线信号研究芯部等离子体的结构

介绍了如何从软X射线原始信号上分析等离子体芯部磁场结构的方法. 在HT-7托卡马克上,通过分析一炮典型的放电数据,直接从软X射线原始信号上分析了磁岛的位置和旋转方向. 通过软X射线图像反演的结果和Mirnov信号上观察到的m=2模的走向验证了磁岛旋转的方向和位置. 关键词： 软X射线 MHD不稳定性 磁岛     

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利用离子香蕉轨道中心坐标和香蕉轨道平均算符,讨论了磁岛转动引起的沿磁力线电流分布及其对新经典撕裂模演进的影响.结果显示,考虑此电流的影响,当磁岛旋转频率ω=ω*i时,被离子香蕉轨道宽度效应削弱的自举电流驱动项基本恢复;当ω=ω*e时,离子香蕉轨道宽度效应对自举电流驱动项的削弱则加剧.这意味着磁岛转动方向不同时,此沿力线电流能显著增大或减弱离子香蕉轨道宽度效应对自举电流驱动项的削弱作用.     

粘滞效应对非线性电阻撕裂模的影响

本文研究粘滞效应对非线性电阻撕裂模的影响。利用准环坐标将具有标量电阻率和粘滞系数的MHD方程简化为一组无量纲的非线性方程。并对这组方程采用二步交替隐式差分格式进行数值计算。结果给出了2/1模的演变过程对无量纲粘滞系数的依赖关系和电流密度的分布。发现粘滞效应对饱和磁岛宽度的修正大约为5％(此时R=10~(-5)),同时发现由于粘滞效应的引入导致电流分布在奇异面附近的涨落明显减小。     

局域热源对磁岛热输运的影响

研究托卡马克等离子体磁岛内的热输运行为.应用局域高斯热源对磁岛加热来模拟电子回旋共振加热.对同时存在背景热源与局域高斯热源的情况,观察局域高斯热源对径向电子温度分布及热输运产生的变化,分析局域高斯热源对温度扰动及磁岛约束能量的影响.     

垂直磁重联平面的驱动流对磁岛链影响的模拟

近20年来,大量的磁岛链现象从空间、天体物理到磁约束实验室等离子体中被观察到,并且有关磁岛链现象的许多物理特性可以直接被计算机模拟结果所证实.磁岛链理论在磁重联理论中的重要进展为快速磁重联的发生机制提供了更加具有说服力的解释.本文采用二维三分量的磁流体力学模型,数值研究了不同宽度和不同强度的垂直平面驱动流对磁重联中磁岛链不稳定性的影响,并分析了导向场和垂直平面的驱动流对磁岛链的共同作用.研究结果表明:垂直平面驱动流的宽度越宽或强度越强,越容易产生磁岛链结构.电流片中的小磁岛个数及重联率随着垂直平面驱动流宽度及强度的增加而增加.另外,导向场会改变重联平面内磁岛链的对称性.相同导向场情况下,驱动流强度越大,小磁岛的增长速度越快.     

Identification of the magnetic island structure in the HL-2A tokamak

The identification of the magnetic island structure in the HL-2A tokamak is presented. First, the perturbation current as a source for the perturbation flux can be determined by using Mirnov probe measurements. By superposing the perturbation flux and the equilibrium flux reconnected by equilibrium fitting, the structure and the width of the magnetic islands can be estimated. The method has been used in the HL-2A experiments.     

Precise AC magnetic measurement under sinusoidal magnetic flux by using digital feedback of harmonic compensation

This paper presents an important improvement for precise AC magnetic measurement under sinusoidal magnetic flux by using the digital feedback of harmonic compensation. Core loss-testing can be simply carried out with the sinusoidal magnetic flux even at a magnetic induction up to 1.87 T. The principle, circuit diagram, experimental results and its analysis are reported. Also it is found that the hysteresis loss per cycle P is smaller after correcting the magnetic flux waveform than before.     

Magnetic nanostructures as amplifiers of transverse fields in magnetic resonance

We introduce the concept of amplifying the transverse magnetic fields produced and/or detected with inductive coils in magnetic resonance settings by using the reversible transverse susceptibility properties of magnetic nanostructures. First, we describe the theoretical formalism of magnetic flux amplification through the coil in the presence of a large perpendicular DC magnetic field (typical of magnetic resonance systems) achieved through the singularity in the reversible transverse susceptibility in anisotropic single domain magnetic nanoparticles. We experimentally demonstrate the concept of transverse magnetic flux amplification in an inductive coil system using oriented nanoparticles with uni-axial magnetic anisotropy. We also propose a composite ferromagnetic/anti-ferromagnetic core/shell nanostructure system with uni-directional magnetic anisotropy that, in principle, provides maximal transverse magnetic flux amplification.     

Comparison of adjustable permanent magnetic field sources

A permanent magnet assembly in which the flux density can be altered by a mechanical operation is often significantly smaller than comparable electromagnets and also requires no electrical power to operate. In this paper five permanent magnet designs in which the magnetic flux density can be altered are analyzed using numerical simulations, and compared based on the generated magnetic flux density in a sample volume and the amount of magnet material used. The designs are the concentric Halbach cylinder, the two half Halbach cylinders, the two linear Halbach arrays and the four and six rod mangle. The concentric Halbach cylinder design is found to be the best performing design, i.e. the design that provides the most magnetic flux density using the least amount of magnet material. A concentric Halbach cylinder has been constructed and the magnetic flux density, the homogeneity and the direction of the magnetic field are measured and compared with numerical simulation and a good agrement is found.     

环形磁芯快脉冲动态参数测量方法

 磁开关压缩脉冲过程中，磁芯磁参数需历经非饱和与饱和两个阶段，磁滞回线变化经历半个周期，通过测量这一变化过程中通过磁开关绕组的电流和磁通量变化率，可以计算出磁芯的磁滞回线，确定饱和磁通密度、剩磁等动态参数。 讨论了基于高电压放电和脉冲压缩方法测量磁芯动态参数的原理，给出了测试装置的电路原理和电路元件参数的选择方法，测量了大型磁开关磁芯快脉冲条件下的电参数，计算了相关的磁参数，给出了实验结果。     

磁压对级联爆磁压缩脉冲发生器性能的影响

 为了研究负载为mH量级的间接馈电两级级联柱-锥构型的爆磁压缩产生器的基本物理过程和能量转换机理，利用描述爆磁压缩物理过程的2维爆轰磁流体力学程序MFCG(Ⅴ)，以实验模型结构参数为基础模拟计算了一系列模型，分析了磁压对金属套筒径向膨胀速度及膨胀过程的影响。计算结果表明：套筒的径向膨胀速度取决于爆轰压与磁压的共同作用，在爆磁压缩过程的绝大部分时间里，向外膨胀的爆轰压都远大于向内压缩的磁压，因而套筒的径向膨胀速度主要是由爆轰压决定;但是在功率放大级的后半段，也就是发生器电流增长最快阶段，磁压也迅速增长，它的增长大大降低了套筒的径向膨胀速度；在功率放大级的后期，磁压已经超过爆轰压，它对系统设计的影响已经不能完全忽略。     

Field dependent shape variation of magnetic fluid droplets on magnetic dots

The morphology of magnetic fluid droplets on magnetic thin film dots is studied experimentally, including the aspect ratio and the contact angle variation of the droplets. Under a uniform external magnetic field, the droplet's aspect ratio increases with the external field and with the diameter of the magnetic dot due to the concentrated magnetic flux inside the magnetic fluid droplet. Similar to the electrical wetting phenomenon, the induced magnetic dipoles in the magnetic film and in the magnetic fluid near the solid–liquid interface change the solid–liquid interfacial tension, and in consequence reduce the apparent contact angle of the magnetic fluid droplet.     

An optimized magnet for magnetic refrigeration

A magnet designed for use in a magnetic refrigeration device is presented. The magnet is designed by applying two general schemes for improving a magnet design to a concentric Halbach cylinder magnet design and dimensioning and segmenting this design in an optimum way followed by the construction of the actual magnet. The final design generates a peak value of 1.24 T, an average flux density of 0.9 T in a volume of 2 L using only 7.3 L of magnet, and has an average low flux density of 0.08 T also in a 2 L volume. The working point of all the permanent magnet blocks in the design is very close to the maximum energy density. The final design is characterized in terms of a performance parameter, and it is shown that it is one of the best performing magnet designs published for magnetic refrigeration.     

Superconducting film flux transformer for a sensor of a weak magnetic field

The object of study is a superconducting film flux transformer in the form of a square shaped loop with the tapering operative strip used in a sensor of a weak magnetic field. The magnetosensitive film element based on the giant magnetoresistance effect is overlapped with the tapering operative strip of the flux transformer and is separated from the latter by an insulator film. It is shown that the topological nanostructuring of the operative strip of the flux transformer increases its gain factor by one or more orders of magnitude, i.e. increases its efficiency, which leads to a significant growth of important parameters of a magnetic field sensor.