短脉冲强激光驱动磁重联过程的靶后电势分布特征

超短超强激光因其极端的物理参数范围以及可用于研究相对论等离子体等特征,成为当前激光驱动磁重联物理的研究热点.通常采用两路激光与平面靶相互作用实现激光驱动磁重联,然而在实验诊断中,由于激光等离子体自身的复杂性导致很难辨别磁重联的物理特征.本文对两路短脉冲激光驱动平面靶磁重联进行了数值模拟,重点分析了靶后电势分布特征和磁重联之间的关系.模拟结果显示,靶后电势分布可以直接影响被加速离子在探测面上的空间分布,因此可用来直接诊断短脉冲激光驱动磁重联实验.     

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。     

激光驱动磁重联过程中的喷流演化和电子能谱测量

利用神光Ⅱ激光器和日本大阪大学Gekko激光器构建了激光驱动等离子体磁重联过程. 在垂直于磁重联平面方向发现了高速喷流, 从不同观测方向实验证实了该喷流的存在并测量了喷流的流体力学演化过程, 对其中的电子能谱进行了诊断分析.     

导向磁场对非对称多重X线重联演化过程的影响

采用二维可压缩电阻性磁流体力学模型,研究导向磁场(B_y0)对非对称多重X线磁场重联过程的影响。研究发现：对于未加B_y0时有多重X线重联发生的情形,加入B_y0之后,会加快次级磁岛的出现。当B_y0增加到一定程度后,该促进效应有所减弱。而对于未加B_y0时无多重X线重联发生的情形,加入一定大小的B_y0之后,会诱发次级磁岛的产生,即诱发了非对称多重X线重联。进一步研究发现,当B_y0增加到一定程度后,将无法诱发非对称多重X线重联。两种情形均表明,B_y0的加入可以促进或者诱发非对称多重X线重联,且存在一个阈值,当B_y0超过这个阈值之后,该促进或诱发效应开始减弱。所得结果可以用于解释一些卫星观测现象,尤其对空间物理中有关导向磁场非对称多重X线重联观测结果的理解有参考价值。     

双撕裂模非线性演化过程中有理面上的剪切流

在二维平板几何模型下,利用磁流体力学方程组数值模拟托卡马克装置中双撕裂模非线性演化过程中有理面上剪切流的时间和空间分布.结果表明,双撕裂模非线性演化的早期阶段,有理面上没有形成明显的剪切流.剪切流主要存在于快速磁重联阶段,随着磁重联的结束而逐渐消失,剪切流的强度和空间分布随磁岛的演化而改变.另外,较大的等离子体电阻加速磁重联,但是对剪切流的强度和变化趋势没有直接的影响.     

用于地球磁尾三维磁重联实验的脉冲电源

空间环境地面模拟装置是哈尔滨工业大学承建的国家重大科技基础设施项目,其包含的空间等离子体环境模拟与研究系统是用于提供磁重联过程等基本物理过程的时空演化规律研究的平台。在研究地球磁尾三维磁重联时,使用处于真空环境内的偶极磁场线圈和两个磁镜场线圈来提供研究所需的模拟背景磁场,其中偶极场线圈为一个总电感为17.4 mH、总电阻为30.25 mΩ的单个线圈,而磁镜场线圈为两个线圈镜像对称设置并串联连接,总电感30.16 mH,总电阻58.81 mΩ。为了产生实验所需背景磁场的幅值和持续时间,研制并测试了两套总能量3.36 MJ的脉冲电源,在进行地球磁尾三维磁重联实验时两套电源需要同时工作。用于驱动偶极场线圈的脉冲电源按照实验需求可以在充电压不大于20 kV的情况下,能够提供超过9 kA的峰值电流,95%峰值电流的持续时间超过了5 ms,由峰值时刻降低到10%峰值时刻的时间不超过130 ms;用于驱动磁镜场线圈的脉冲电源按照实验需求可以在充电压不大于20 kV的情况下,能够提供超过8 kA峰的值电流,95%峰值电流的持续时间超过了5 ms,由峰值时刻降低到10%峰值时刻的时间不超过130 ms。     

用磁探针反演磁岛二维结构的方法及其在HL-2A装置上的应用

介绍了HL-2A装置中利用磁探针数据反演磁岛极向二维结构的新方法,以及在反演基础上建立的撕裂模动态分析方法。在实验中通过磁探针测量确定作为扰动磁场来源的扰动电流。将扰动磁通与由EFIT重建的平衡磁通叠加反演出磁岛的结构,并给出磁岛的宽度。然后,按时间顺序建构二维结构图并依次记录,之后依次展现图像就可以对磁岛进行动态分析。应用此方法进行撕裂模分析,得出了磁岛旋转与电子抗磁漂移方向相同,验证了磁岛宽度与扰动场的关系及ECRH对磁岛的抑制作用。这显示了磁探针反演磁岛结构方法的直观性,对观察并控制MHD不稳定性非常有利。     

消磁场对纳米铁磁线磁畴壁动力学行为的影响

为了实现基于磁畴壁运动的自旋电子学装置, 掌握磁畴壁动力学行为是重要争论之一.研究了在外磁场驱动下L-型纳米铁磁线磁畴壁的动力学行为. 通过微磁学模拟,在各种外磁场的驱动下考察了纳米铁磁线磁畴壁的动力学特性; 在较强外磁场的驱动下, 在不同厚度纳米线上考察了纳米线表面消磁场对磁畴壁动力学行为的影响. 为了进一步证实消磁场对磁畴壁动力学的影响, 在垂直于纳米线表面的外磁场辅助下分析了磁畴壁的动力学行为变化. 结果表明, 随着纳米线厚度和外驱动磁场强度的增加, 增强了纳米线表面的消磁场的形成, 使得磁畴壁内部自旋结构发生周期性变化, 导致磁畴壁在纳米线上传播时出现Walker崩溃现象. 在垂直于纳米线表面的外磁场辅助下, 发现辅助磁场可以调节消磁场的强度和方向. 这意味着利用辅助磁场可以有效地控制纳米铁磁线磁畴壁的动力学行为.     

磁记忆检测的力磁耦合型磁偶极子理论及解析解

磁偶极子理论在缺陷漏磁场解释中被成功广泛使用.由于磁荷密度等参数不易定量,磁偶极子理论在应用中常常进行归一化处理,被认为不适用于对应力相关的磁记忆信号做量化分析.本文通过建立力磁耦合型磁偶极子理论模型,以适用于分析磁记忆检测中应力对磁信号的影响.基于铁磁学理论确定应力和磁场联合作用下的等效场强度,基于弱磁化状态的一阶近似,获得了各向同性铁磁材料微弱环境磁场下的应力磁化解析解.结合磁信号二维问题中矩形和V形磁荷分布假定,建立了光滑与破坏试件表面磁信号、矩形和V形表面缺陷所诱导磁信号的力磁耦合型磁偶极子理论分析模型,并获得其解析解.基于力磁耦合型磁偶极子理论的解析解,对拉伸实验中试件破坏前后的信号差异、矩形和V形表面缺陷诱导磁信号,以及磁信号的影响因素和规律等进行了详细分析.理论研究表明,基于本文理论模型的解析解可实现对磁记忆检测中的一些基本实验现象和规律的解释.     

具有条纹磁畴结构的NiFe薄膜的制备与磁各向异性研究

具有条纹磁畴结构的磁性薄膜表现出面内转动磁各向异性,对于解决高频电子器件的方向性问题起着至关重要的作用.本文采用射频磁控溅射的方法,研究了NiFe薄膜的厚度、溅射功率密度、溅射气压等制备工艺参数对条纹磁畴结构、面内静态磁各向异性、面内转动磁各向异性、垂直磁各向异性的影响规律.研究发现,在功率密度15.6 W/cm~2与溅射气压2 mTorr(1 Torr=1.33322×102Pa)下生长的NiFe薄膜,表现出条纹磁畴的临界厚度在250 nm到300 nm之间.厚度为300 nm的薄膜比250 nm薄膜的垂直磁各向异性场增大近一倍,从而磁矩偏离膜面形成条纹磁畴结构,并表现出面内转动磁各向异性.高溅射功率密度可以降低薄膜出现条纹磁畴的临界厚度.在相同功率密度15.6 W/cm~2下生长300 nm的NiFe薄膜,随着溅射气压由2 mTorr增大到9 mTorr,NiFe薄膜的垂直磁各向异性场逐渐由1247.8 Oe(1 Oe=79.5775 A/m)增大到3248.0 Oe,面内转动磁各向异性场由72.5 Oe增大到141.9 Oe,条纹磁畴周期从0.53μm单调减小到0.24μm.NiFe薄膜的断面结构表明柱状晶的形成是表现出条纹磁畴结构的本质原因,高功率密度下低溅射气压有利于柱状晶结构的形成,表现出规整的条纹磁畴结构,高溅射气压会导致柱状晶纤细化,面内转动磁各向异性与面外垂直磁各向异性增强,条纹磁畴结构变得混乱.     

Magnetic reconnection in the two-dimensional Kelvin-Helmholtz instability

Magnetic reconnection in the two-dimensional Kelvin-Helmholtz instability is studied. The flow is modeled by the reduced MHD equations with constant resistivity and viscosity. For super-Alfvénic flow, localized transient reconnection is observed on the Kelvin-Helmholtz time scale (this is not new). We study this transient reconnection and consider the peak reconnection rate which occurs with the initial vortex formation. Over the range of resistivities considered, it is shown that this peak reconnection rate is not a function of resistivity, and is a function of the initial flow shear. Additionally, it is demonstrated that there is a fundamental difference between the evolution of a problem at S = 200 and S = 10,000.     

Two-scale structure of the electron dissipation region during collisionless magnetic reconnection

Particle-in-cell simulations of collisionless magnetic reconnection are presented that demonstrate that reconnection remains fast in very large systems. The electron dissipation region develops a distinct two-scale structure along the outflow direction. Consistent with fast reconnection, the length of the electron current layer stabilizes and decreases with decreasing electron mass, approaching the ion inertial length for a proton-electron plasma. Surprisingly, the electrons form a super-Alfvénic outflow jet that remains decoupled from the magnetic field and extends large distances downstream from the x line.     

Super-Alfvénic propagation of substorm reconnection signatures and Poynting flux

The propagation of reconnection signatures and their associated energy are examined using kinetic particle-in-cell simulations and Cluster satellite observations. It is found that the quadrupolar out-of-plane magnetic field near the separatrices is associated with a kinetic Alfvén wave. For magnetotail parameters, the parallel propagation of this wave is super-Alfvénic (V(∥) ～ 1500-5500 km/s) and generates substantial Poynting flux (S ～ 10(-5)-10(-4) W/m(2)) consistent with Cluster observations of magnetic reconnection. This Poynting flux substantially exceeds that due to frozen-in ion bulk outflows and is sufficient to generate white light aurora in Earth's ionosphere.     

Evidence for an elongated (>60 ion skin depths) electron diffusion region during fast magnetic reconnection

Observations of an extremely elongated electron diffusion region occurring during fast reconnection are presented. Cluster spacecraft in situ observations of an expanding reconnection exhaust reveal a broad current layer ( approximately 10 ion skin depths thick) supporting the reversal of the reconnecting magnetic field together with an intense current embedded at the center that is due to a super-Alfvénic electron outflow jet with transverse scale of approximately 9 electron skin depths. The electron jet extends at least 60 ion skin depths downstream from the X-line.     

不可压缩等离子体的2维磁场重联模型

提出了一种2维磁场重联模型。磁场重联过程中的电荷分离在等离子体中产生静电场,等离子体在电场中的漂移运动可以解释阿尔芬速度量级的出流。该磁场重联模型给出如下结论:Sweet-Parker模型描述的重联率强烈地依赖于电子质量与离子质量之比;反常电阻率正比于离子惯性长度和电流片宽度比值的平方; 相对论效应和高温等离子体中电子-正电子对的产生可以提高重联率; 电磁波的激发对于磁能的损耗是必要的。     

Fast magnetic reconnection in the plasmoid-dominated regime

A conceptual model of resistive magnetic reconnection via a stochastic plasmoid chain is proposed. The global reconnection rate is shown to be independent of the Lundquist number. The distribution of fluxes in the plasmoids is shown to be an inverse-square law. It is argued that there is a finite probability of emergence of abnormally large plasmoids, which can disrupt the chain (and may be responsible for observable large abrupt events in solar flares and sawtooth crashes). A criterion for the transition from the resistive magnetohydrodynamic to the collisionless regime is provided.     

Out-of-plane shear flow effects on fast magnetic reconnection in a two-dimensional hybrid simulation model

The effects of out-of-plane shear flows on fast magnetic reconnection are numerically investigated by a twodimensional(2D)hybrid model in an initial Harris sheet equilibrium with flows.The equilibrium and driven shear flows out of the 2D reconnection plane with symmetric and antisymmetric profiles respectively are used in the simulation.It is found that the out-of-plane flows with shears in-plane can change the quadrupolar structure of the out-of-plane magnetic field and,therefore,modify the growth rate of magnetic reconnection.Furthermore,the driven flow varying along the anti-parallel magnetic field can either enhance or reduce the reconnection rate as the direction of flow changes.Secondary islands are also generated in the process with converting the initial X-point into an O-point.     

Merging of super-Alfvénic current filaments during collisionless Weibel instability of relativistic electron beams

The theoretical framework predicting the long-term evolution, structure, and coalescence energetics of current filaments during the Weibel instability of an electron beam in a collisionless plasma is developed. We emphasize the nonlinear stage of the instability, during which the beam density of filaments increases to the background ion density, and the ambient plasma electrons are fully expelled from the filaments. Our analytic and numerical results demonstrate that the beam filaments can carry super-Alfvénic currents and develop hollow-current density profiles. This explains why the initially increasing magnetic field energy eventually decreases during the late stage of the instability.     

Solitary electromagnetic pulses detected with super-Alfvénic flows in Earth's geomagnetic tail

Solitary nonlinear (deltaB/B>1) electromagnetic pulses have been detected in Earth's geomagnetic tail accompanying plasmas flowing at super-Alfvénic speeds. The pulses in the current sheet had durations of approximately 5 s, were left-hand circularly polarized, and had phase speeds of approximately the Alfvén speed in the plasma frame. These pulses were associated with a field-aligned current J(parallel) and observed in low density (approximately 0.3 cm(-3)), high temperature (T(e) approximately T(i) approximately 3x10(7) K), and beta approximately 10 plasma that included electron and ion beams streaming along B. The wave activity was enhanced from below the ion cyclotron frequency to electron cyclotron and upper hybrid frequencies. The detailed properties suggest the pulses are nonlinearly steepened ion cyclotron or Alfvén waves.     

Identification of the electron-diffusion region during magnetic reconnection in a laboratory plasma

We report the first identification of the electron-diffusion region, where demagnetized electrons are accelerated to super-Alfvénic speed, in a reconnecting laboratory plasma. The width of the electron-diffusion region scales with the electron skin depth [ approximately (5.5-7.5)c/omega_{pe}] and the peak electron outflow velocity scales with the electron Alfvén velocity [ approximately (0.12-0.16)V_{eA}], independent of ion mass.