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。     

磁流体中Helmholtz和Kelvin力的界定

磁流体磁彻体力的两种简化形式Helmholtz力和Kelvin力具有一定的适用范围.在推导磁流体中的磁彻体力表达式基础上,分析Helmholtz力和Kelvin力在磁流体中的起源,得出两种形式的成立条件.计算结果表明:当磁流体磁导率与外磁场强度无关时,磁流体磁彻体力可由Helmholtz力表示;当磁流体中磁性颗粒的平均磁矩与磁流体比体积无关时,Kelvin力为磁彻体力的简化形式;在磁流体磁化系数与其密度成正比情况下,Helmholtz力可转换为Kelvin力. 关键词： 磁流体 磁彻体力 Helmholtz力 Kelvin力     

水中金属丝电爆炸拉氏磁流体动力学模拟方法

基于拉格朗日描述,建立了水中金属丝电爆炸的单温磁流体动力学模型,并给出一种高阶混合有限元离散求解方法。拉氏可压缩流体方程组中,速度定义在H1连续有限元空间,内能定义在L2间断有限元空间实现物质界面的精确捕捉,存在激波的区域引入张量人工粘性抑制数值振荡。磁扩散方程仅考虑周向磁通量密度,简化为标量方程,使用H1连续有限元方法离散求解。焦耳热和洛伦兹力作为源项引入实现磁流体方程的耦合。数值算例表明:磁扩散求解器能够求解存在不同电导率的多介质磁扩散问题;拉氏流体求解器能够精确追踪物质界面,具有较好的激波分辨能力;耦合RLC电路的磁流体求解器能够复现水中金属丝电爆炸加热相变、冲击波的产生与传播、放电模式转变等物理过程。     

非均匀磁场中磁流体热磁对流的实验研究

建立了测量非均匀磁场条件下圆柱腔体内磁流体热磁对流特性的实验系统,实验结果显示,磁流体热磁对流特性受磁场强度、温差以及磁场梯度方向与温度梯度方向之间关系的控制,当磁场梯度方向与温度梯度方向一致时,外加磁场强化了磁流体的热磁对流过程,且随着磁场强度和温差的增大,热磁对流强度加强。当磁场梯度方向、温度梯度方向以及重力方向三者相同时,磁流体的热磁对流强度最剧烈。     

磁流体中微粒运动情况的初步分析

针对磁流体显示器件的结构特征,设计了对称双光束激光散斑剪切实验,着重研究电磁场驱动条件下磁流体中磁微粒的运动状况.解决了激光散斑剪切干涉实验中磁流体液面影响观察磁微粒运动的问题,并分析了磁流体液面在电磁场驱动下的变化规律和影响范围.在此实验基础上,观察到磁流体的聚集、堆起和碰撞现象,在电磁场驱动下,磁微粒表现出周期性运...     

温度敏感型磁流体热磁对流特性研究

建立了描述外磁场作用下温度敏感型磁流体热磁对流特性的数学模型,数值模拟了回路中热磁对流的流动与传热特性.搭建了磁流体热磁对流回路装置,采用粒子示踪测速技术(PIV)测量了磁流体的流速,用热电偶测量了磁流体的温度分布.通过实验值和数值模拟结果,分析了不同磁场大小、不同热负荷,以及不同冷却温度下回路的运行特性.     

自由表面磁流体射流不稳定性分析（Ⅱ）

在“自由表面磁流体射流不稳定性分析(Ⅰ)”一文中，分析了圆形截面自由表面磁流体射流在一个均匀或非均匀磁场中的不稳定性行为。本文以类似的方法分析了圆形截面射流(下文“射流”亦特指“圆形截面射流”)在两个相互垂直且非均匀的横向磁场中的磁流体不稳定性行为；这是磁聚变中液态金属偏滤器和限制器所处的情况，即，环向和极向磁场同时作用于自由表面射流。     

纳秒激光烧蚀固体靶产生的等离子体在外加横向磁场中膨胀时的温度和密度参数演化

利用等离子体光学波段自发光成像、光学光谱和光学探针干涉等诊断手段, 观察了纳秒脉冲激光烧蚀固体靶产生的等离子体在外加横向磁场中的膨胀过程. 根据实验参数特征建立了简化的磁流体物理模型, 结合自发光强度的时间演化, 理论计算了等离子体温度和密度参数的时间演化, 理论计算结果与实验测量结果基本符合, 证实了碰撞磁扩散过程在等离子体演化中发挥了关键作用.     

磁场对激光驱动Rayleigh-Taylor不稳定性影响的数值研究

Rayleigh-Taylor不稳定性(RTI)作为流体和等离子体中基础的物理现象,在天体物理、空间物理以及工程领域扮演着重要角色.尤其在惯性约束核聚变(ICF)研究中, RTI等宏观流体不稳定性是不可回避的物理问题.本文利用开源的辐射磁流体模拟程序FLASH对激光驱动调制靶产生的RTI进行了二维的数值模拟,系统地考察和比较了RTI在无磁场、Biermann自生磁场、不同外加磁场情况下的演化.模拟结果表明, Biermann自生磁场和平行流向的外加磁场在RTI演化过程中基本不会改变RTI的界面动力学,而垂直流向的外加磁场对RTI以及RTI尖钉尾部的Kelvin-Helmholtz涡旋有致稳作用,其中磁压力起主导作用.研究结果为后续开展和ICF相关的靶物理研究提供借鉴,也有助于加深对流体混合过程的理解.     

磁流体结构的Lattice-Boltzmann方法模拟

本文运用Lattice-Boltzmann(LB)方法建立了宏观静止磁流体模型,提出磁性聚集体概念,充分考虑了磁性颗粒受到的各种内力与外力包括重力、布朗力、vanderWaals相互作用及静磁相互作用,对无外加磁场及外加竖直均匀磁场时磁流体的结构进行了模拟,并分析了磁能与热能之比对磁流体系统中粒子分布形态的影响。模拟结果表明:无外加磁场时磁流体结构易失去稳定性,外加均匀磁场时磁流体粒子沿外磁场方向排列,随着磁热能比例的增大,外磁场对粒子分布的影响逐渐明显。     

Secondary instabilities and vortex formation in collisionless-fluid magnetic reconnection

It is shown that the pattern of current layers formed within a magnetic island in the nonlinear phase of magnetic field line reconnection in a collisionless two-dimensional fluid plasma is subject to the onset of a secondary instability, the effect of which increases with decreasing electron temperature. In the cold electron limit the saturation of the island growth is accompanied by a turbulent redistribution of the current layers and by the development of long lived fluid vortices while, in the opposite limit, the current layer structure remains regular.     

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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.     

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。     

Electron acceleration during magnetic islands coalescence and division process in a guide field reconnection

The magnetic merging process related to pairwise magnetic islands coalescence is investigated by two-dimensional particle-in-cell simulations with a guide field.Owing to the force of attraction between parallel currents within the initial magnetic islands,the magnetic islands begin to approach each other and merge into one big island.We find that this newly formed island is unstable and can be divided into two small magnetic islands spontaneously.Lastly,these two small islands merge again.We follow the time evolution of this process,in which the contributions of three mechanisms of electron acceleration at different stages,including the Fermi,parallel electric field,and betatron mechanisms,are studied with the guide center theory.     

Simplified method for describing plasma temperature evolution in a tokamak with a growing magnetic island

A new simplified method is proposed for solving the heat conduction equation in a tokamak plasma with an unsteady magnetic island with allowance for heat transfer along the magnetic field. The method, whose efficiency is illustrated by a simple example, allows one to reduce the three-dimensional problem of plasma temperature evolution in the presence of several magnetic islands to a set of one-dimensional equations.     

Structure and magnetic properties of epitaxial cobalt islands

Growth, structural and magnetic properties of cobalt island structures grown by molecular beam epitaxy on 4 nm thick island shaped Ru buffers are studied. The islands are characterized by a narrow distribution of sizes, and are separated on average by spacing of around one hundred nanometers. Growth of the islands into continuous films is found to proceed in three well defined stages, and corresponding magnetic behaviors are identified. The particles formed in the first stage of growth appear to be similar in shape and size, and their magnetic behavior suggest the presence of superparamagnetism. Particles in the second stage of growth are ferromagnetic and display a reorientation transition of their magnetization out of the film plane, consistent with that seen in patterned magnetic dot arrays. Particles coalesce in the last stage of growth and magnetic properties of continuous films are observed. Received 24 August 2001     

The dynamic behavior of magnetic fluid adsorbed to small permanent magnet in alternating magnetic field

The dynamic behavior of a magnetic fluid adsorbed to a small NdFeB permanent magnet subjected to an alternating magnetic field was studied with a high speed video camera system. The directions of alternating magnetic field are parallel and opposite to that of the permanent magnet. It was found that the surface of magnetic fluid responds to the external alternating magnetic field in elongation and contraction with a lot of spikes. Generation of a capillary magnetic fluid jet was observed in the neighbourhood of a specific frequency of alternating field. The effect of gravitational force on surface phenomena of magnetic fluid adsorbed to the permanent magnet was revealed.     

Role of kinetic effects on the polarization current around a magnetic island

The polarization current due to a magnetic island rotating in a tokamak plasma is believed to play a central role in the initial evolution of the neoclassical tearing mode. Monte Carlo delta f simulations are performed which also cover a parameter range that is not amenable to analytic treatment but is relevant to experiments. It is shown that the polarization current can change sign when the island rotation frequency is of the order of the ion parallel streaming around the island. Moreover, the current is reduced when the island width is of the order of the ion banana width. Finally, the transition to the enhanced high-collisionality regime is shown to occur for collision frequencies higher than those typical of today's experiments.     

Rigidly connected magnetic lines: twisting and winding of magnetic lines

The dynamical process of magnetic flux variation in a fluid’s stream tube is described by constructing \(1+1+ (2)\) decomposition of the gradient of fluid’s 4-velocity. The necessary and sufficient conditions are obtained for a spacelike congruence to be a congruence of rigidly connected spacelike curves. The evolution of magnetic flux in a magnetic tube is explored under the assumptions that magnetic lines are rigidly connected and the chemical potential of the fluid is constant along a magnetic tube. The interplay between magnetic and stream tubes is demonstrated. It is shown that the growth of magnetic energy in a magnetic tube cannot exceed to that of a stream tube. It is found that the proper time variation of twist of magnetic lines is caused by gravitation inside a neutron star if magnetic lines are rigidly connected and charge neutrality condition holds. Helmholtz-like magnetic vorticity flux conservation in a magnetic tube constituted by rigidly connected geodetic magnetic lines is derived under the assumption that the charge neutrality condition holds. It is shown that the winding of frozen-in poloidal magnetic field due to differential rotation requires meridional circulation in an axisymmetric stationary hydromagnetic configuration.     

Phase mixing and island saturation in Hamiltonian reconnection

The nonlinear evolution of a Hamiltonian magnetic field line reconnection in a two-dimensional fluid plasma leads to a macroscopic equilibrium with a finite-size island and fine-scale spatial structures. The latter arise from the phase mixing of the Lagrangian invariant fields. This equilibrium is the analog of the Bernstein-Greene-Kruskal equilibrium solution for electrostatic Langmuir waves.