利用离子声波朗道阻尼测量氧化物阴极放电中的离子温度

作为等离子体重要参数之一, 特别是在低温等离子体中离子温度的测量一直较为困难. 在磁化线性等离子体装置氧化物阴极脉冲放电条件下, 利用栅网激发离子声波, 通过测量波幅在朗道阻尼作用下随空间的演化, 利用阻尼长度是离子温度和电子温度的函数, 计算得到离子温度为0.3 eV. 测量值与国外类似装置利用光谱诊断所得结果基本相同.     

Relativistic degenerate effects of electrons and positrons on modulational instability of quantum ion acoustic waves in dense plasmas with two polarity ions

The nonlinear propagation of quantum ion acoustic wave(QIAW) is investigated in a four-component plasma composed of warm classical positive ions and negative ions,as well as inertialess relativistically degenerate electrons and positrons.A nonlinear Schrodinger equation is derived by using the reductive perturbation method,which governs the dynamics of QIAW packets.The modulation instability analysis of QIAWs is considered based on the typical parameters of the white dwarf.The results exhibit that both in the weakly relativistic limit and in the ultrarelativistic limit,the modulational instability regions are sensitively dependent on the ratios of temperature and number density of negative ions to those of positive ions respectively,and on the relativistically degenerate effect as well.     

掺杂和结构对光纤中去极化型声波导布里渊散射频移与散射效率的影响

利用全矢量有限元法研究了单模光纤及光子晶体光纤中掺杂材料及浓度、光纤结构对去极化型声波导布里渊散射频移和散射效率的影响.结果表明增大包层掺氟浓度或纤芯掺锗浓度均会减小光纤布里渊频移,也会增大单模光纤散射效率.光子晶体光纤中整体掺氟浓度不变时,空气孔层数的增加会减小布里渊频移;而纤芯掺锗浓度不变时,随空气孔层数的增加TR2,3模式的布里渊频移增大,而TR2,7模式的布里渊频移减小.     

声单向操控研究进展

电子二极管的发明标志着现代电子学的诞生, 在整个人类社会中引起了科技的深刻变革. 声波是一种具有非常悠久的研究历史的经典波, 却始终被认为仅具有对称的传播形式. 若能制造出可像电子二极管控制电流般实现声波单向导通的声学器件, 显然将对整个声学研究领域产生重大影响, 具有重要的科学意义及应用价值. 第一个基于非线性媒质与声子晶体的声二极管利用非线性突破声学互易原理的局限, 首次实现了将声能流限制在单一方向上的声整流效应. 针对非线性系统转换效率低下的固有缺陷, 在线性体系内围绕声单向传播这个重要科学问题开展了一系列理论和实验研究, 设计与制备了多种具有特殊结构和性能的线性声学单向结构, 在器件的效率、带宽及尺寸方面产生了突破. 在声二极管研究的基础上, 第一个可以像电子三极管操控电流般对声流进行操控与放大的声三极管理论模型也被提出. 本文介绍了声单向传播这一新兴且富有蓬勃生机的研究领域中的主要进展.     

超声复合电弧声调控特性研究

超声复合电弧作为一种新的焊接热源, 在电弧焊接过程中可利用超声实现对熔融金属的深度处理, 但是超声与电弧等离子体间相互作用机理还不清晰, 这成为阻碍该技术工程应用的关键问题. 本文通过实验与相应理论针对外加超声场对焊接电弧调控特性进行了研究. 为说明电弧特性, 针对试验中高速摄像采集的电弧图片进行了处理. 对比未加超声情况, 超声复合电弧受内外声场共同作用等离子体拘束程度明显提高, 电弧亮度增强, 弧柱高温区范围扩展至阳极, 中间粒子出现团聚并以一定频率上下抖动. 通过改变超声激励电流大小和声发射端高度, 电弧结构产生显著变化, 在谐振点附近, 电弧挺直度最强, 脉动频率最大. 试验结果显示通过外加超声可以达到对焊接电弧热等离子体调控的目的. 最后结合波动方程和二维声边界元模型, 分析了电弧内部声传播过程以及声场结构对等离子体粒子的作用规律, 这为进一步理解超声对电弧的调控机理打下良好基础.     

Co基金属纤维不对称巨磁阻抗效应

以直径32 μm的熔体抽拉Co基非晶金属纤维为研究对象, 分析了该纤维不同激励条件下的巨磁阻抗(giant magneto impedance, GMI)效应. 实验结果表明: 这类纤维的GMI效应具有不对称性特点, 即 AGMI (asymmetric GMI)效应. 同时, 发现AGMI效应随激励条件不同而变化, 随交流频率或者激励幅值升高而逐渐增强; 当存在一定偏置电压时, AGMI效应大幅增强. 通过研究纤维的磁化过程, 分析了Co基金属纤维的AGMI效应. 由于Co基熔体抽拉纤维具有螺旋各向异性以及磁滞的存在使得GMI效应具有不对称性, 频率升高或者激励电流幅值增加有利于壳层畴环向磁化, AGMI增强. 当在纤维两端施加偏置电压时, 偏置电流诱发环向磁场增强了环向磁化, AGMI效应提高; 偏置电压较低时磁场响应灵敏度提高, 同时磁化翻转向高场移动, 阻抗线性变化对应的直流磁场区间增大. 这一方面拓宽了GMI传感器工作区间及灵敏度, 另一方面不利于获得更大的磁场响应灵敏度. 10 MHz (5 mA)激励时, 施加1 V强度的偏置电压后, 对应的磁场灵敏度从616 V/T 提高至5687 V/T; 偏置电压为2 V时, 灵敏度降低到4525 V/T. 因此, 可以通过适当提高环向磁场的方法获得大的磁场响应灵敏度及阻抗变化线性区域.     

On thermodynamic potentials in linear thermoelasticity

The four thermodynamic potentials, the internal energy u=u(ε_ij,s), the Helmholtz free energy f=f(ε_ij,T), the Gibbs energy g=g(σ_ij,T) and the enthalpy h=h(σ_ij,s) are derived, independently of each other, by using the Duhamel–Neumann extension of Hooke's law and an assumed linear dependence of the specific heat on temperature. A systematic procedure is then presented to express all thermodynamic potentials in terms of four possible pairs of independent state variables. This procedure circumvents a tedious transition from one potential to another, based on the formal change of variables, and inversions of the stress–strain and entropy–temperature relations. The general results are applied to uniaxial loading paths under isothermal, adiabatic, constant stress, and constant strain conditions. An interplay of adiabatic and isothermal elastic constants in the expressions for exchanged heat along certain thermodynamic paths is indicated.     

Exponential decay for a von Kármán equations of memory type with acoustic boundary conditions

In this paper, we consider the dynamical von Kármán equations of memory type with acoustic boundary conditions. We show an exponential decay result of solutions under weaker assumption than the ones frequently used in the literature. In particular, the kernel we are considering is not necessarily exponentially decaying to zero as was assumed before. Copyright © 2014 John Wiley & Sons, Ltd.     

Generalized impedance boundary condition at high frequency for a domain with thin layer: the circular case

Consider a conducting disk surrounded by a thin dielectric layer submitted to an electric field at the pulsation ω. The conductivity of the layer grows like ω^1?γ, γ∈[0,1], when the pulsation ω?tends to infinity. Using a pseudodifferential approach on the torus, we build an equivalent boundary condition with the help of an appropriate factorization of Helmholtz operator in the layer. This generalized impedance condition approximates the thin membrane in the high frequency limit for small thickness of the layer. L ²-error estimates are given and we illustrate our results with numerical simulations. This work extends, in the circular geometry, previous works of Lafitte and Lebeau (Lafitte O. Lebeau G. 1993, Équations de Maxwell et opérateur d’impédance sur le bord d’un obstacle convexe absorbant. Comptes Rendus de l ' Académic dis Science, Paris, Série I, Mathématiques, 316(11), 1177–1182); (Lafitte O.D., 1999, Diffraction in the high frequency regime by a thin layer of dielectric material. I. The equivalent impedance boundary condition. SIAM Journal on Applied Mathematics, 59(3), 1028–1052 (electronic)) in which γ?identically equals zero.