Z箍缩等离子体MHD不稳定性的简化分析

在Haines的理论模型基础上,将金属丝阵负载Z箍缩内爆等离子体MRT不稳定性发展的全过程划分为四个部分加以描述,并利用ZP-0D程序定性分析了丝阵参数和驱动器条件对不稳定性的影响。研究结果表明,增加丝阵的丝根数,能够明显改善Z箍缩的稳定状态,有利于提高箍缩停滞时等离子体的温度、密度及X射线产额。     

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针对实验室等离子体的辐射物理条件和等离子体箍缩三温的数值模型,提出一种定量计算Z箍缩等离子体辐射能谱的近似方法。在给出计算能谱结构的理论方法之后,再将这种近似计算方法编入到Kr喷气Z箍缩等离子体X射线辐射的二维三温辐射磁流体力学模型中,得出典型Kr喷气Z箍缩X射线辐射的能谱分布。对照"强光一号"加速器Kr喷气箍缩实验测量得到的辐射能谱分布,分析了理论与实测结果之间存在的偏差。     

轴向流对Z箍缩等离子体稳定性的影响

利用理想磁流体力学(MHD)模型对有轴向流参与的Z箍缩等离子体不稳定性进行了分析.对可压缩平板等离子体模型的色散关系进行了推导，讨论了三种不同等离子体状态下的不稳定性增长率.结果显示，等离子体的可压缩性对磁瑞利-泰勒/开尔文-亥姆霍兹(MRT/KH)杂化不稳定性有抑制作用，改善了轴向剪切流对长波长扰动的影响.分析了不同轴向流速度分布对系统稳定性的影响.结果表明，对于峰值相同的不同轴向流，其对不稳定性的抑制效果只依赖于扰动集中区域内速度剪切的大小，与其他位置的速度剪切无关. 关键词： Z箍缩 磁瑞利-泰勒不稳定性 轴向剪切流 MHD方程     

Nonlinear development of capillary waves in a viscous liquid jet

The one-dimensional approximate equations describing the dynamics of a Newtonian viscous fluid are used to analyze the nonlinear development of capillary waves in a jet. It is shown that the size of satellite droplets resulting from a nonuniform jet breakup decreases with the Reynolds number at a constant wavenumber. The satellite-droplet formation ceases at a certain value of the Reynolds number, which depends on the wavenumber and initial perturbation amplitude.     

Solitons in random media with long-range correlation

Abstract

A statistical approach of the propagation of solitons in media with a spatially random potential is developed. Applying the inverse scattering transform several regimes are demonstrated which are determined by the mass and the velocity of the incoming soliton as well as by the correlation length of the random potential. Namely, the mass of the soliton is conserved if its initial amplitude is large enough. If the initial mass is small, then the mass decays with the length of the system. The decay rate is exponential in the case of a white noise perturbation, but it obeys a power law if the carrier wavenumber of the soliton lies in the tail of the spectrum of the potential. Furthermore, the scattered radiation propagates in a backward direction in the case of a white noise perturbation, while it propagates in a forward direction (with the same carrier wavenumber as the soliton) in the case of a coloured noise with long-range correlation.     

大气边界层模式中随机参数的反演与不确定性分析

在大气边界层气象中湍流黏性系数是一个很重要的参数,通过直接观测往往无法得到其准确值,仅能通过间接观测获得大致范围.本文选用随机广义Ekman动力近似模式中的湍流黏性系数进行反演研究与不确定性分析.首先利用风速观测数据,并采用基于混沌多项式的集合Kalman滤波方法对系数进行反演,降低其不确定性,缩小可能取值的范围,该方法的核心思想是将集合Kalman滤波方法中求解模式不确定性传播的方法由蒙特卡罗法改为混沌多项式展开,从而避免大规模采样带来的计算资源耗费.然后进行数值实验,结果表明该方法能够有效且快速地求解出湍流黏性的后验概率分布,从而达到降低系数不确定性的目的.根据系数的先验分布计算出风速的先验分布,从而找到风速不确定性大的区域,且揭示了在不确定性大的区域内的观测数据进行系数反演可得到十分明显的效果,这对于观测点位置的选择提供了重要的指导.     

Frequency-modulated ratchet with autoresonance

Motion of an ensemble of non-interacting classical particles in a space-periodic potential subjected to a weak external wave-like perturbation is considered. With large values of the wavenumber, the perturbation causes a resonance-induced chaotic layer in a certain area of phase space. Different scenarios of the emergence of chaos are considered. Adiabatic space or time modulation of the phase of the perturbation changes the location of the chaotic layer. A significant number of particles trapped within the chaotic layer is retained inside in course of the adiabatic modulation, revealing autoresonant behavior. We show that this phenomenon can be used to generate a directed ballistic current using a weak perturbation, even if particle’s energies are initially close to the minimal value.     

Z箍缩等离子体不稳定性的数值研究

利用二维辐射磁流体程序模拟了钨丝阵Z箍缩等离子体腊肠不稳定性的演化：分析了存在不稳定性与没有不稳定性条件下等离子体内爆的差异；研究了不同初始密度扰动对x射线输出 功率和能量的影响.数值结果表明，在一定初始密度扰动范围内，这种不稳定性对x射线输 出的总能量影响不显著，但对x射线功率随时间变化的曲线有明显的影响.此外，还探讨了Z 箍缩等离子体数值模拟中低密度区的数值处理对数值结果的影响. 关键词： Z箍缩 腊肠不稳定性 数值模拟     

Z箍缩动态黑腔形成过程和关键影响因素数值模拟研究

在Z箍缩动态黑腔研究中, 认识黑腔形成物理过程及主要特征, 明确优化黑腔辐射的关键参数, 是实验物理设计的重要基础. 本文针对W丝阵填充CH泡沫转换体的负载构型, 利用一维辐射磁流体程序, 在8 MA驱动电流条件下开展了动态黑腔形成过程和关键影响因素的数值模拟研究. 结果表明, 丝阵等离子体与泡沫转换体相互作用产生局部高压力区是驱动冲击波传播和形成动态黑腔的关键物理过程. 由于辐射超声速传播及其与冲击波波阵面的空间分离, 产生了辐射温度较高而物质未受明显压缩的动态黑腔区域. 丝阵等离子体碰撞泡沫转换体前的状态分布决定了动态黑腔辐射场的主要特征, 可以通过改变负载参数调整并优化黑腔辐射波形. 综合考虑黑腔峰值辐射温度和有效维持时间两个参数, 选择匹配质量的丝阵和泡沫, 使丝阵质量略小于泡沫, 可以获得相对优化的动态黑腔辐射波形. 同时, 合适的丝阵/泡沫初始半径比也是优化动态黑腔辐射的重要影响因素.     

MHD Stokes flow in a corrugated curved channel

Magnetohydrodynamic (MHD) flow through a corrugated curved channel is modelled. The flow is perpendicular to the corrugations and applied magnetic field. A boundary perturbation analysis for small corrugation amplitude is used to find the expressions for the stream function and the flow rate. It is found that the flow is inevitably decreased by the corrugations. For a given Hartmann number, the flow reduction varies with the channel radius of curvature. The effect of the phase difference between the corrugated walls is distinct, with minimum and maximum effects when the corrugated curved walls are in-phase and out-of-phase, respectively, for small corrugation wavenumber. However, when the corrugation wavenumber is large enough, the flow is independent of the phase difference. Generally, the study shows that the Hartmann number decreases the effect of the corrugations on the flow rate.     

Z箍缩等离子体均匀色散晶体光谱成像

为了诊断Z箍缩等离子体X射线相关信息,利用自聚焦和均匀色散原理,研制了一种新型的均匀色散弯晶谱仪。晶体分析器采用-石英（1010）,布拉格角为43.4~72.7,利用有效面积为10 mm50 mm的X射线胶片接收光谱信号,实验在中国工程物理研究院阳加速器装置上进行,摄谱元件获得了Z箍缩铝丝阵等离子体的类H及类He谱线。实验结果表明:谱线分布遵循均匀色散条件,所研制均匀色散弯晶谱仪线色散率为-116.198 mm/nm,与理论值-120 mm/nm的相对误差为3.168%,能够用于Z箍缩等离子体X射线的光谱学研究。     

Drag current in a quantum superlattice during propagation of a high-intensity bichromatic electromagnetic wave

We study the effect of electron drag in a semiconductor superlattice during intraband absorption of a biharmonic electromagnetic wave in a process accompanied by the emission (absorption) of a phonon. The problem has been solved in the second order of perturbation theory. The effective interaction Hamiltonian method makes it possible to take into account the multiphoton nature of the electromagnetic wave absorption. With increasing field, the current increases and attains a peak value, after which it decreases in an oscillatory manner due to the ionization stabilization effect.     

X箍缩三维数值模拟

利用FOI-PERFECT程序对X箍缩进行了3D数值模拟研究,给出了X箍缩的物理图像和动力学过程,探讨了Z箍缩中出现磁重联的可能性,并指出如果双丝Z箍缩中能够出现磁重联,那么X箍缩是更有利于磁重联出现的位形,并且,X箍缩中出现多重X射线暴的一个可能原因是z轴上多个位置出现磁重联。     

轴向剪切流对Z箍缩等离子体瑞利-泰勒不稳定性的抑制

利用理想磁流体力学模型对有轴向剪切流的Z箍缩等离子体不稳定性进行了分析。给出了可压缩模型的色散关系,分别对可压缩及不可压缩模型中轴向剪切流对Z箍缩等离子体瑞利-泰勒不稳定性的抑制作用进行了比较,讨论了可压缩性对含有轴向剪切流系统不稳定性的影响。结果表明,可压缩性能够减缓瑞利-泰勒P凯尔文-亥姆霍兹(RTPKH)模扰动的增长,因而使得轴向剪切流对系统不稳定性的抑制作用表现得更为突出。计算结果还说明,在RT不稳定性线性增长阶段,等离子体温度较低,使用可压缩模型能够更真实地描述系统的状态。     

Dissipative formation of hole-like excitation in ion-acoustic plasma

A viscous plasma is analyzed by reductive perturbation theory to model dissipative soliton formation. A nonlinear Schrödinger equation with complex coefficients is derived. Such an equation can be exactly solved by the technique due to Hirota. Three types of solution can be obtained under different physical conditions: solitary waves, ion-acoustic holes, and shocks. Even in the presence of a dissipative effect like viscosity, it is possible to obtain a solitary-wave-like excitation.     

Sensitivity analysis and determination of free relaxation parameters for the weakly-compressible MRT–LBM schemes

Lattice Boltzmann methods (LBMs) are very efficient for computational fluid dynamics, and for capturing the dynamics of weak acoustic fluctuations. It is known that multi-relaxation-time lattice Boltzmann method (MRT–LBM) appears as a very robust scheme with high precision. There exist several free relaxation parameters in the MRT–LBM. Although these parameters have been tuned via linear analysis, the sensitivity analysis of these parameters and other related parameters is still not sufficient for describing the behavior of the dispersion and dissipation relations of the MRT–LBM. Previous researches have shown that the bulk dissipation in the MRT–LBM induces a significant over-damping of acoustic disturbances. This indicates that the classical MRT–LBM is not best suited to recover the correct behavior of pressure fluctuations. In wave-number space, the first/second-order sensitivity analyses of matrix eigenvalues are used to address the sensitivity of the wavenumber magnitudes to the dispersion-dissipation relations. By the first-order sensitivity analysis, the numerical behaviors of the group velocity of the MRT–LBM are first obtained. Afterwards, the distribution sensitivities of the matrix eigenvalues corresponding to the linearized form of the MRT–LBM are investigated in the complex plane. Based on the sensitivity analysis and an effective algorithm of recovering linearized Navier–Stokes equations (L-NSEs) from linearized MRT–LBM (L-MRT–LBM), we propose some simplified optimization strategies to determine the free relaxation parameters of the MRT–LBM. Meanwhile, the dispersion and dissipation relations of the optimal MRT–LBM are quantitatively compared with the exact dispersion and dissipation relations. At last, some numerical validations on classical acoustic benchmark problems are shown to assess the new optimal MRT–LBM.     

基于等离子体粒子模拟的喷气Z箍缩过程物理研究

给出了喷气Z箍缩动力学过程在二维柱坐标系下的等离子体粒子模拟物理模型,编写了相应的程序.对低电流驱动下的稀薄喷气Z箍缩动力学过程进行了验证性的等离子体粒子模拟,得到了许多微观的Z箍缩物理信息,如负载中的电流(密度)、电磁场、粒子位置和密度的时空演化,以及总的z箍缩拖尾质量和拖尾电流等信息.发现在Z箍缩过程中,模拟得到的等离子体电流随时间的变化反映出了等离子体箍缩到心和反弹的过程特征,磁场随径向的变化与长直导线电流给出的磁场很接近;电子所受到的电场力和磁场力(洛伦兹力)是相当的,而离子所受到的力主要是电场力;电子首先在z方向加速,然后在自身运动产生电流的磁场的作用下向轴心箍缩,而离子是在电子和离子电荷分离所产生的电场力的作用下向轴心运动;在压缩到轴心附近时,电子首先因静电排斥而飞散,而离子则在惯性的作用下继续向轴心箍缩,而后滞止飞散.Z箍缩等离子体的拖尾质量在20%左右,拖尾电流最大时在7%左右.     

紧凑型可重复运行的高功率纳秒脉冲源

讨论了一种低阻抗、高储能密度、可输出中等高压的百ns脉冲形成技术,其输出波形质量较好;采用磁感应电压叠加技术将该脉冲形成装置输出的中等高压脉冲叠加到应用需求的高电压高功率脉冲。研究表明单个感应模块可在2.8 Ω的负载上获得脉冲宽度为220 ns,前沿为50 ns的中等高压脉冲。     

飞片加载产生正弦冲击波及其波阵面演化模拟

飞片加载冲击波小扰动实验方法是目前测量高温高压下物质粘性的实用方法之一针对飞片碰撞小扰动实验,采用二维Navier-Stokes方程的差分方法,研究了金属铝中复杂压力流场的演化过程,给出了这类特定冲击波流场中正弦形扰动振幅振荡衰减特性与粘性系数之间的定量关系.结果表明:数值解比以往用于分析冲击波小扰动实验的解析解结果更...     

Gravitational Instability of a Two-Component Viscous Plasma with Finite Conductivity Flowing Through a Porous Medium with Fir Corrections

The gravitational instability of a two component plasma is studied to include the simultaneous effects of collisions, gyroviscosity, finite conductivity, viscosity and porosity of the medium within the framework of two-fluid theory. From linearized equations of the system, using normal mode analysis, the dispersion relations for parallel and perpendicular directions to the magnetic field are derived and discussed. For longitudinal wave propagation it is found that the value of critical JEANS' wave number increases with increasing density and decreasing temperature of the neutral component. For transverse wave propagation the value of critical JEANS' wave number depends on gyroviscosity, ALFVÉN number, ratio of sonic speeds and densities of the two component and porosity of the medium. It is observed that the effect of magnetic field and porosity is suppressed by finite condutivity of the plasma and similarly the effect of gyroviscosity is removed by viscosity from JEANS' expression of instability. For both the directions instability is produced when the velocity perturbations are considered parallel to wave vector. The damping effect is produced due to collisional frequency, permeability of the porous medium and viscosity. The density of the neutral component and porosity of the medium tends to destabilize the system while an increased value of FLR corrections leads the system towards stabilization.