Plasma circular cylindrical waveguide in lossy material

The surface modes in the plasma circular cylindrical waveguide in a lossy material are analyzed, particularly for the variations of their propagation properties with plasma parameters and surrounding material. The characteristic equations of surface modes are derived, and their relevant approximate solutions are given. The limit results of this paper are consistent with that given in literature’s. The analysis show that it can be used to improve the properties of mode suppressors and fabricat cut-off attenuators.     

Ray chaos and ray clustering in an ocean waveguide

We consider ray propagation in a waveguide with a designed sound-speed profile perturbed by a range-dependent perturbation caused by internal waves in deep ocean environments. The Hamiltonian formalism in terms of the action and angle variables is applied to study nonlinear ray dynamics with two sound-channel models and three perturbation models: a single-mode perturbation, a randomlike sound-speed fluctuations, and a mixed perturbation. In the integrable limit without any perturbation, we derive analytical expressions for ray arrival times and timefronts at a given range, the main measurable characteristics in field experiments in the ocean. In the presence of a single-mode perturbation, ray chaos is shown to arise as a result of overlapping nonlinear ray-medium resonances. Poincare maps, plots of variations of the action per ray cycle length, and plots with rays escaping the channel reveal inhomogeneous structure of the underlying phase space with remarkable zones of stability where stable coherent ray clusters may be formed. We demonstrate the possibility of determining the wavelength of the perturbation mode from the arrival time distribution under conditions of ray chaos. It is surprising that coherent ray clusters, consisting of fans of rays which propagate over long ranges with close dynamical characteristics, can survive under a randomlike multiplicative perturbation modelling sound-speed fluctuations caused by a wide spectrum of internal waves.     

Resonant positronium formation in atomic gases

A method is presented for calculation of quasistationary excited states of positronium ions. These states are treated as bound positron states in the quadrupole field of an excited valence electron, which are capable of decaying both with positron emission into the continuous spectrum, and with positronium emission. The existence of such states should manifest itself in resonances in the positronium formation section upon motion of positrons with energy in the eV range. The resonant section parameters are calculated for a number of atoms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 6–12, March, 1984.     

颗粒气体团簇行为实验研究

颗粒体系由于非弹性碰撞和摩擦等内秉的能量耗散特性,由宏观粒子形成的颗粒气体体系经常会有局部凝聚现象,这是颗粒气体体系与分子气体体系的最大区别之一.理解和预测这一现象的发生将有助于人们对远离平衡态体系的复杂现象,如有序结构、斑图和团簇形成的认知.这种局部凝聚现象可以类比于分子气体中亚稳分解形成的液滴,将气液相分离用于解释和寻求局部凝聚现象的此模型得到了分子动力学模拟的校验.但是实验的校验却由于宏观粒子运动受重力作用的影响难以在实验室中实现.作为实践十号卫星的前期实验,本文利用国家微重力实验室落塔装置,以水平激振装有不同尺寸和数目的颗粒样品,在短时微重力条件下,成功观察到颗粒气体团簇的形成;并将实验结果与颗粒气体类范德瓦耳斯气体分子相分离模型对比,由形成团簇样品的颗粒数密度条件,来实验确定了所选颗粒的恢复系数,得到直径为0.5 mm的钛珠颗粒的恢复系数在0.6—0.8之间,直径为1 mm的钛珠颗粒的恢复系数约为0.8,直径为2.5 mm的钛珠颗粒的恢复系数应大于0.8.     

Discrete X-wave formation in nonlinear waveguide arrays

We present experimental evidence for the spontaneous formation of discrete X waves in AlGaAs waveguide arrays. This new family of optical waves has been excited, for the first time, by using the interplay between discrete diffraction and normal temporal dispersion, in the presence of Kerr nonlinearity. Our experimental results are in good agreement with theoretical predictions.     

Wave-field formation in a hollow x-ray waveguide

Diffraction and refraction phenomena at the entrance of a hollow x-ray waveguide with weakly absorbing dielectric cladding layers are investigated using two independent approaches: (a) analytical and (b) numerical solutions of the wave equation in the paraxial (parabolic) approximation. It is shown that the wave penetrating through the cladding material substantially modifies the wave field near the waveguide entrance. It results in a significant increase of the total energy flux inside the guiding layer and in additional spatial modulation of the electromagnetic field.     

Excited state positronium formation in low density gases

Excited state Ps atoms formed in low density Ne, Ar and H₂ gases have been observed for the first time. The maximum yield was estimated to be ≈ 5.7 × 10^?2 excited Ps atoms per stopped positron of energy ≈ 16 eV in H₂. This is about 14 times greater than previous maximum yields.     

波导环境多目标时延-多普勒解卷成像

有源声呐感兴趣的参量是目标距离和径向速度,它们无法直接观测得到,需要通过估计而获得。利用波导多路径环境多目标时延-多普勒模型,可以导出采样互模糊度函数均值是发射信号自模糊度函数与广义目标反射性密度函数的两维卷积,其中广义目标反射性密度函数为信道扩展函数与目标反射性密度函数的两维卷积。依据信息理论最小Csiszar鉴别准则,可导出R-L (Richardson-Lucy)迭代解卷算法,对采样互模糊度函数均值进行两维迭代解卷积,消除发射信号和信道引入的模糊,序贯地实现时延-多普勒两维像的估计,进而获得多目标的时延和多普勒参量估计。仿真结果和海上实验数据分析验证了R-L解卷算法的可行性和有效性,较之常规的匹配滤波和维纳滤波算法,R-L算法有效地提高了时延和多普勒估计的分辨力和精度。     

Delay-Doppler deconvolution image formation for multiple targets in waveguide environment

The ranges and radial velocities of targets are interesting parameters in the active sonar, which can't be observed directly but can only be estimated. Firstly, by making use of the delay-Doppler model of multi-targets in waveguide multipath environment, one finds that sample cross-ambiguity function is a two-dimensional(2D) convolution of the auto-ambiguity function of the transmitted signal with the generalized target reflectivity density, which is a 2D convolution of the spread function of channel with the reflectivity density as well. Secondly,from the perspective of information theory, an iterative deconvolution algorithm named R-L(Richardson-Lucy) is derived based on minimum Csiszar discrimination criterion. Finally, the blurs caused by both of the transmitted signal and channel are removed by 2D deconvolution of the expectation of sample cross-ambiguity function, 2D image and then parameters of time-delay and Doppler is estimated sequentially. Results of both numerical simulation and sea experimental data processing verify the feasibility and effectiveness of R-L deconvolution algorithm, which effectively improves the resolution and precision of the time delay and Doppler estimation, when compared to the classical match filtering and Wiener filtering.     

Acoustic field and the entropy mode induced by it in a waveguide filled with some non-equilibrium gases

The non-linear propagation of an acoustic beam in a rectangular waveguide is considered. The medium of sound propagation, is a gas where thermodynamically non-equilibrium processes take place: such as exothermic chemical reactions or excitation of vibrational degrees of a molecule’s freedom. The incident and reflected compounds of the acoustic field do not interact in the leading order in the case of periodic weakly nonlinear sound with zero mean value of velocity. The acoustic heating or cooling in a waveguide is discussed.     

Evidence for trimer formation during dipole clustering in Mg doped LiF

The clustering of (Mg²⁺V^?) pairs in LiF into higher order complexes is followed using dielectric loss and ITC. The results show clearly that, even at low temperatures and with high impurity content crystals, the decay follows a third-order reaction indicating that trimers are the predominant clusters formed. This is supported by the ITC results which do not give any indication of dimer formation.Difficulties may arise with the measurement of dielectric loss. An additional transient signal at ~1 kHz is observed to grow and then decay with aging time at 25°C. This signal is thought to be associated with the presence of dislocations introduced into the crystals during rapid quenching.     

Plasma channel formation in nitrogen upon electron beam injection

A theoretical study is performed of plasma channel formation and current neutralization upon injection of a high current electron beam into nitrogen under high pressure (p 70 torr). The complex ion formation and recombination mechanism is considered together with its effect on the parameters of the plasma formed and the dynamics of beam current neutralization. Analytical expressions are obtained for the densities of simple and complex ions in the limiting cases and numerical calculations are carried out by computer for a more general case. It is shown that in nitrogen at a pressure of 70 torr when complex ions are considered the plasma channel conductivity decreases by approximately 35–40%.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 59–65, December, 1990.     

Peculiarities of spontaneous grating generation in light-sensitive waveguide layers during holographic grating formation

The generation of spontaneous (noise) gratings during the simultaneous formation of a holographic grating in thin (on the order of the cutoff thickness of the TE₀ waveguide mode) AgCl-Ag films by two laser beams with λ = 532 nm and polarization vectors that make an angle of 45° with the plane of incidence have been studied. The electron microscopy images and diffraction patterns have revealed a significant difference of the spontaneous-grating structure from the structure obtained under irradiation by one laser beam. The spontaneous gratings have a significant spread in the directions of their wave vector, and the diffraction pattern (recorded using a probe beam with λ = 337 nm) has the form of a bundle of diverging arcs that intersect at one point. This difference is caused by self-diffraction from the holographic grating, which is responsible for the main diffraction peaks with odd orders, the growth of spontaneous gratings because of the interference of the beams diffracted by the holographic grating with the waveguide TE₀ modes scattered in the film, and silver transport to the interference minima. The diffraction patterns are quantitatively analyzed and the period of the new (formed under two-beam irradiation) spontaneous gratings is calculated. The calculation results are in good agreement with the experimental data.