内壳层跃迁--实现超短波长激光的途径

内壳层跃迁机制是实现超短波长激光的一种很有潜力的方案，随着近年来超短超强激光技术和X射线激光实验方法的进展，实现内壳层跃迁机制的超短波长硬X射线激光不再是遥不可及的梦想，文章详细介绍了内壳层跃迁机制X射线激光的原理，并讨论了开展内壳层跃迁机制X射线激光实验的一些实际相关问题。     

Spatial Evolution of Nonstationary Perturbations in Hamel Flow

The propagation of small perturbations in longitudinally inhomogeneous flows is investigated. The evolution of the perturbations is studied with reference to the radial flow of a viscous incompressible fluid between plane nonparallel walls, the simplest inhomogeneous flow. Using a generalized method of variation of constants, the corresponding boundary-value problem is reduced to an infinite-dimensional evolutionary system of ordinary differential equations for the complex amplitudes of the eigensolutions of a locally homogeneous problem. Physically, the method can be interpreted as a representation of the perturbation evolution process via two concomitant processes: the independent amplification (attenuation) of normal modes of the locally homogeneous problem and the rescattering of these modes into each other on local inhomogeneities of the base flow. The calculations show that reduced versions of the method are promising for describing the linear stage of laminar-turbulent transition in a boundary layer.     

聚焦径向调制Bessel-Gaussian光束实现亚波长尺寸纵向偏振光束

亚波长尺寸纵向偏振光束在粒子加速,单个分子偶极矩测量,二次谐波成像和纵向偏振共焦显微镜等方面有非常广泛的应用.本文提出径向调制的Bessel-Gaussian光束模型,经高数值孔径显微物镜聚焦可以获得高纯度的纵向偏振光束.利用矢量衍射理论数值模拟了焦点附近光强分布、磁场分布和能流密度空间分布.结果表明光斑半高全宽度可以达到0.4λ.该方法无需环形孔径或环形相位滤波器,光能利用率高,分辨率好,达到改进各种应用的效果. 关键词： Bessel-Gaussian光束 纵向偏振光束 聚焦 亚波长     

Longitudinally detected ESR magnetometer for wide-range measurements of low fields

A new magnetometer utilizing a longitudinally detected ESR (LODESR) method was developed. The probe head of the LODESR magnetometer is equipped with a single-turn coil (8 mm in diameter) which has a very wide bandwidth because the reactance of the coil is always smaller than the resistance of the transmission line (50 ohm) at frequencies less than 700 MHz. Thus, an absolute magnetic field could be measured over a wide range (2 to 9 mT) using this magnetometer without changing the probe head.     

Propagation of Perturbations in Plane Poiseuille Flow between Walls of Nonuniform Compliance

The evolution of small perturbations in longitudinally nonuniform flows is studied with reference to the problem of the propagation of flow perturbations in a plane channel with walls of variable elasticity. Using the solution of the problem of the receptivity of the flow to local vibrations of the walls, the problem considered can be reduced to the solution of an integral equation for a single function, namely, the complex vibration amplitude of the walls. A numerical method for solving this equation on the basis of a piecewise-linear approximation of the unknown function is proposed. It is shown that the instability wave amplitude changes discontinuously at the junction of the rigid and elastic channel sections. A second method of investigating the process of propagation of perturbations in the flow considered is proposed. This method is based on laws of evolution of perturbations in nonuniform flows and an analytic solution of the problem of perturbation scattering on the junction of walls with different compliance. On the basis of this method the classical stability theory is generalized to include the case of nonuniform flows.     

Dispersion Spectrum of an Anisotropic Waveguide with Sector-Shaped Cross Section and Fixed Boundary

The dispersion equations are derived analytically for normal elastic waves in a longitudinally anisotropic cylindrical waveguide with circular cross-section and a sector-shaped channel of arbitrary angular measure. The boundary surfaces of the channel are covered with flexible, inextensible membranes. The cylindrical portion of the lateral surface is rigidly fixed. A numerical analysis of the dispersion equations reveals spectral branches of traveling normal waves with different types of symmetry of cross-sectional displacements and different circumferential wave numbers. Some features of the spectrum due to variation in the angular measure are established __________ Translated from Prikladnaya Mekhanika, Vol. 41, No. 9, pp. 46–51, September 2005.     

On the Perturbation of Hamel Flow Due to Unevenness of the Channel Walls

A method of analyzing the receptivity of longitudinally inhomogeneous flows is proposed. The process of excitation of natural oscillations is studied with reference to the simplest inhomogeneous flow: the two-dimensional flow of a viscous incompressible fluid in a channel with plane nonparallel walls. As physical factors generating perturbations, the cases of a stationary irregularity and localized vibration of the channel walls are considered. By changing the independent variables and unknown functions of the perturbed flow, the problem of the generation of stationary perturbations above an irregularity is reduced to a longitudinally homogeneous boundary-value problem which is solved using a Fourier transform in the longitudinal variable. The same problem is investigated using another method based on representing the required solution in the form of a superposition of solutions of the homogeneous problem and a forced solution calculated in the locally homogeneous approximation. As a result, the problem of calculating the longitudinal distributions of the amplitudes of the normal modes is reduced to the solution of an infinite-dimensional inhomogeneous system of ordinary differential equations. The numerical solution obtained using this method is tested by comparison with an exact calculation based on the Fourier method. Using the method proposed, the problem of flow receptivity to harmonic oscillations of parts of the channel walls is analyzed. The calculations performed show that the method is promising for investigating the receptivity of longitudinally inhomogeneous flow in a laminar boundary layer.     

Dynamic stability of axially accelerating viscoelastic plates with longitudinally varying tensions

The dynamic stability of axially accelerating plates is investigated. Longitudinally varying tensions due to the acceleration and nonhomogeneous boundary conditions are highlighted. A model of the plate combined with viscoelasticity is applied. In the viscoelastic constitutive relationship, the material derivative is used to take the place of the partial time derivative. Analytical and numerical methods are used to investigate summation and principal parametric resonances, respectively. By use of linear models for the transverse behavior in the small displacement regime, the plate is confined by a viscous damping force. The generalized Hamilton principle is used to derive the governing equations, the initial conditions, and the boundary conditions of the coupled planar vibration. The solvability conditions are established by directly using the method of multiple scales. The Routh-Hurwitz criterion is used to obtain the necessary and sufficient condition of the stability. Numerical examples are given to show the effects of related parameters on the stability boundaries. The validity of longitudinally varying tensions and nonhomogeneous boundary conditions is highlighted by comparing the results of the method of multiple scales with those of a differential quadrature scheme.     

纵向泵浦固体激光介质热透镜效应的理论研究

 在分析了纵向泵浦、周边致冷固体激光棒内加热及冷却机理的基础上,将热传导方程合理简化为一维形式,求解出以级数方式表达的棒内温度分布。并根据热弹性力学原理,获得了激光棒中热致应力场和应变场。在考虑了与热透镜效应有关的三个因素：即热色散、热致应力双折射和端面变形后,推导出了纵向泵浦固体激光棒有效热焦距的解析表达式。将所获结果运用于常用的激光晶体Nd:YAG,发现当使用10W半导体激光器泵浦时,所产生的有效热焦距在毫米量级。     

旁轴近似描述的耦合系统模式耦合效率研究

从泵浦光与腔模的空间匹配函数出发，讨论了泵浦光的发散角、焦点位置等因素对激光系统效率的影响。发现在旁轴近似下，匹配函数的最大值随泉浦光发散角的增大而增大，在考虑到像散的影响后，得到泵浦光发散角的参考值。