Analysis of plasma sheath propagation attenuation based on ray tracing

Based on the complex Snell's law of lossy media, a ray tracing method is proposed to study the propagation attenuation characteristics of electromagnetic (EM) waves in plasma sheaths. The plasma sheath is modelled as layered media. This method considers the complex ray characteristics of inhomogeneous plane EM waves, tracks the propagation rays of EM waves in each layer of media, and calculates the propagation attenuation of EM waves in each layer of media according to the propagation direction of the complex rays. The attenuation during numerical cumulative propagation is the total attenuation of EM waves through the plasma sheath. By comparing the results with that obtained from the WKB method, the accuracy of the ray tracing algorithm is proved. The results of the propagation attenuation of a blunt cone model are calculated by the proposed method, and the effects of different parameters on the EM wave propagation attenuation in the plasma sheath are analysed at different heights, velocities, incident angles, and incident positions. Studying the propagation characteristics of EM waves in the plasma sheath is of importance in application for radar target tracking, blackout communication, and other issues.     

用有限元法解龟磁波穿透非均匀有耗等离子体鞘套时的损耗

本文用有限无法求解了电磁波穿透具有任意密度剖面的非均匀等离子体鞘套时的反射和透射问題。鞘套的剖面参数选自典型再入飞行器的粘性激波层平衡流场数据^[1]。利用虚功原理和线性插值函数从Helmholtz方程导出有限元方程。鞘套按对数剖分为31个单元。然后,分别对频率f=400MHZ,4000MHZ和10GHZ时,计算了鞘套内的场值及反射,透射和吸收系数。并估算了信号的总功耗。误差分析表明,计算结果是令人满意的。与其它方法相比较,本文所给出的有限无法是简捷而精确的,所以该方程适于计算波穿透具有任意的而且变化剧烈的剖面的等离子体鞘套时的损耗     

Reflection–refraction of attenuated waves at the interface between a thermo-poroelastic medium and a thermoelastic medium

Phenomenon of reflection and refraction is considered at the plane interface between a thermoelastic medium and thermo-poroelastic medium. Both the media are isotropic and behave dissipative to wave propagation. Incident wave in thermo-poroelastic medium is considered inhomogeneous with deviation allowed between the directions of propagation and maximum attenuation. For this incidence, four attenuated waves reflect back in thermo-poroelastic medium and three waves refract to the continuing thermoelastic medium. Each of these reflected/refracted waves is inhomogeneous and propagates with a phase shift. The propagation characteristics (velocity, attenuation, inhomogeneity, phase shift, amplitude, energy) of reflected and refracted waves are calculated as functions of propagation direction and inhomogeneity of the incident wave. Variations in these propagation characteristics with the incident direction are illustrated through a numerical example.     

A note on Snell laws for electromagnetic plane waves in lossy media

Based on Maxwell's equations and Ohm's law, we rederived the Snell laws for reflection and transmission of harmonic inhomogeneous plane electromagnetic (EM) waves propagating through planar lossy interfaces. The present results are new, simple and exact and they recover the ordinary Snell laws in the case of lossless media. Besides, these results show that the wave propagation direction strongly depends on the polarization state of the EM wave and the lossy media can behave as a polarizing device. Moreover, we verify that in low frequency regime these traveling waves do not exhibit total internal reflection at interfaces between two adjacent lossy media.     

Propagation characteristics of oblique incidence terahertz wave through non-uniform plasma

The propagation characteristics of oblique incidence terahertz(THz) waves through non-uniform plasma are investigated by the shift-operator finite-difference time-domain(SO-FDTD) method combined with the phase matching condition.The electron density distribution of the non-uniform plasma is assumed to be in a Gaussian profile. Validation of the present method is performed by comparing the results with those obtained by an analytical method for a homogeneous plasma slab.Then the effects of parameters of THz wave and plasma layer on the propagation properties are analyzed. It is found that the transmission coefficients greatly depend on the incident angle as well as on the thickness of the plasma, while the polarization of the incident wave has little influence on the propagation process in the range of frequency considered in this paper. The results confirm that the THz wave can pass through the plasma sheath effectively under certain conditions,which makes it a potential candidate to overcome the ionization blackout problem.     

激光激发水中产生超声波的实验研究

采用压电陶瓷水听器研究了Nd：YAG脉冲激光激发水中产生瞬态超声波的特性,给出了声压信号随距离的变化关系。研究结果表明：用激光产生水下声波是完全可行的。当观测点与光击穿区的距离r远大于柱体长度时,垂直于光传播方向的激光瞬态超声波幅值与r成反比;当观测点与光击穿区的距离r很小时,垂直于光传播方向的声压幅值与产成反比。此外,当激光入射角度发生变化,超声脉冲的幅值也随之发生变化,其幅值在激光束垂直入射的时候最大。     

Partially coherent optical waves in random gradient fibres

The excitation characteristics and spatial coherence of partially coherent optical waves in gradient fibres are demonstrated for incident light waves radiated from semiconductor lasers and light emitting diodes. Lower modes are efficiently excited in the case of coherent laser beams, while incident waves of low coherence such as lightwaves of LEDs excite higher modes. Pulse propagation of partially coherent optical waves in dispersive gradient fibres is also discussed for random index fluctuations. Mode coupled equations for temporal correlation functions of the electric field that are generalizations of coupled power equations are found. Mode filtering and pulse improvement with a lossy inhomogeneous cladding are described.     

A General Spectral-Domain Formulation of the Electromagnetic Scattering by a Perfectly Conducting Circular Cylinder

A full-wave method for the two-dimensional scattering problem by a perfectly conducting circular cylinder is presented, providing an exact solution for the Helmholtz equation in very general cases. The method is based on the Fourier series expression of the boundary conditions (Dirichlet and Neumann) generated by an arbitrary, finite-power, incident beam, and the analysis is performed in the complex plane of the analytic continuation of a space spectral variable. This approach allows us to define an analytic continuation for cylindrical wave expansions, working with lossy propagation media and with a full incident spectrum, including inhomogeneous waves, both in E and in H polarization. Convergence of the modal expansion is investigated, to verify that very weak hypotheses are needed, and no geometrical or paraxial approximation is required. Extact expressions for the expansion coefficients are given, in terms of complex intergrations involving the Fourier spectrum of the incident beam.     

Penetration of an ordinary wave into a weakly inhomogeneous magnetoplasma at oblique incidence

A theory is given describing the propagation of high-frequency electromagnetic waves in a plane-stratified weakly inhomogeneous plasma. The density gradient is supposed to be perpendicular to the external magnetic field and the wave vector is expected not to be generally parallel to the plane given by both the preceding vectors. The analysis points out that the ordinary wave can penetrate through the plasma resonance region if the direction of vacuum wave vector is chosen appropriately. Analytical expressions for the reflecion and transmission coefficients are obtained and their dependence on the direction cosines of the wave vector of the incident is studied. The paper further shows in outline that, after transmission through the plasma resonance, the ordinary wave is transformed into an extraordinary wave and the latter is reflected back to the region of the hybrid resonance. In this region the extraordinary wave is fully transformed into the Bernstein modes.     

Potential surface waves at the boundary of a metal with a magnetoactive plasma at finite pressure

The propagation of surface-type potential waves along the interfacial boundary of a plasma with an ideally conducting metal in an external magnetic field perpendicular to the boundary is examined. It is shown that a necessary condition for the existence of these waves in the system is a finite gas kinetic pressure. Dispersion relations for these waves and expressions for the penetration depth of the wave fields into the plasma are obtained, and they are studied numerically for various plasma parameters. The frequency region for propagation of these waves is found. It is also shown that in a nonzero external magnetic field a system of this kind has a range of frequencies in which the wave is a generalized surface wave. Zh. Tekh. Fiz. 69, 30–33 (November 1999)     

Relativistic nonlinear waves in a magnetised plasma

Relativistic propagation of a nonlinearly-coupled circularly-polarised electromagnetic wave and a Langmuir wave along the applied magnetic field is considered. Analytical solutions are given or are indicated for some special cases like purely transverse waves and purely longitudinal waves. In the presence of an applied magnetic field, the incident circularly polarised electromagnetic wave is found to propagate further into denser regions of the plasma — a result which is in accord with the so-called inverse Faraday effect. Finally, we shall consider general coupled transverse and longitudinal waves for which we give an approximate solution. We investigate whether this system of coupled waves exhibits any internal resonances and consequent energy exchange between them.     

Numerical analysis of propagation characteristics of electromagnetic wave in lossy left-handed material media

The propagation characteristics of electromagnetic wave in lossy left-handed materials (LHM) are studied using finite-difference time-domain (FDTD) method base on auxiliary differential equation (ADE) technology. The LHM medium is realized with lossy Drude models for both the negative electric permittivity and the negative magnetic permeability. The discretized ADE-FDTD equations are derived in detail. The incident wave used in the simulation is a multiple cycle m-n-m pulses source. The term of Poynting's vector E_xH_y was calculated. These numerical results demonstrate conclusively that the phase velocity direction of electromagnetic wave propagation and the direction of the Poynting vectors are anti-parallel in LHM. The amplitude of electric field is reduced with the enhancive distance of LHM slab. It is also demonstrated that the energy of electromagnetic wave in the LHM slab is obviously attenuated, and the attenuation of energy becomes stronger with the angular plasma frequency ω_p increasing. These results indicate that LHM stealth is effective in theory, and reasonable selection of the large negative index of refraction can greatly enhance its effectiveness.     

Debye analysis applied to multiple reflections and attenuation of electromagnetic plane waves in a stratified sea substratum

Investigation of the wave problem related to multiple reflections and attenuation in marine controlled source electromagnetic is important for geophysical sounding of the sea substratum. An approach towards this goal is to study the scattering of obliquely incident plane waves by a four-layered earth: sea, sea bottom sedimentary rock, hydrocarbon reservoir, and lower sedimentary rock. The approach employs Debye theory in the frequency domain and assumes that each layer is a planar lossy medium. Explicit expressions show that the secondary waves related to the sea substratum layers multireflections carry useful information about their features.     

Numerical modelling of TE and TM modes in lossy nonlinear optical waveguides

The intensity-dependent characteristics of nonlinear TE and TM waves in a thin film bounded by lossy nonlinear media are analysed by the finite-element method. In this approach, the power-dependent complex propagation constants and local complex electromagnetic field distributions of lossy nonlinear TE and TM waves are obtained directly from the given lossy nonlinear waveguides, without any approximations. Numerical results for these modes in a lossy nonlinear waveguiding system with different absorption coefficients are given. It is shown that the complex dispersion relations, for both TE and TM modes, are strongly power dependent.     

Interaction of terahertz waves propagation in a homogeneous,magnetized, and collisional plasma slab

ABSTRACT

The properties of terahertz waves propagation in a homogeneous, magnetized, and collisional plasma are studied in this paper. We first theoretically calculate the reflectance and absorbance coefficients for terahertz waves passing through this region. Then, numerous simulations are conducted to investigate the influence of the plasma on the terahertz waves propagation. According to the results, the plasma density, collision frequency, and magnetic field play important roles in dielectric spectra, and then result in large impacts on the propagation with the variation of the plasma thickness, incident angle, and the gas pressure. The absorbance increases with the increase of plasma density, and the collision frequency. With respect to the collisional absorption and electron cyclotron resonance, the absorbance first increases to its maximum peak and then decreases as the wave frequency increases. When the plasma density increases, the peak value shifts to a higher frequency. Meanwhile, the plasma slab acts as the absorber or reflector with the variation of the gas pressure and the plasma thickness. These results provide supplementary information on the terahertz waves propagation in plasma and can serve as a theoretical basis for its application.     

Spatial and temporal study of shock waves generated by laser ablation for Ti target

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Reflection of an Obliquely Incident Impulsive Plane Wave from Isotropic and Uniaxial Cold Lossy Plasma Half-Spaces

An exact solution for the transient reflected wave from a lossy plasma half-space is derived for the case of an obliquely incident impulsive plane electromagnetic wave. The plasma half-space may be isotropic or uniaxially anisotropic.     

Laser ultrasonic anomalous wave propagation imaging method with adjacent wave subtraction: Algorithm

The anomalous wave propagation imaging (AWPI) method is proposed for the laser ultrasonic propagation imaging system using a Q-switched laser and a laser mirror scanner to highlight the anomalies in complex structures. The AWPI algorithm was developed based on the observation that the waves from two adjacent scanning points are very similar, and that the propagation direction of the incident wave is different from that of the anomalous wave caused by structural anomalies including damage. The structural anomaly is highlighted by suppressing the incident waves and exaggerating the anomalous wave through adjacent waves subtraction after arrival time and amplitude matching. The variable time window amplitude mapping (VTWAM) method was also developed, based on the difference in arrival time between the residual incident wave and anomalous waves. The VTWAM method enhances anomaly visualization and sizing, notably for composite damages, by mapping the amplitudes of the confining wave within the damage. Our results showed that the AWPI increased the signal-to-noise ratio of a back-surface hole damage in a steel plate by 13.76 dB, while in another inspection of a composite wing with two impact damages, the AWPI results enhanced by the VTWAM compared favorably with the results of the immersion ultrasonic C-scan. The AWPI and VTWAM adopt implicit spatial referencing wherein all necessary data can be obtained through a single-time scan, therefore circumvent the disadvantages of conventional temporal baseline referencing.     

Power-mode theorems for guided acoustic waves in piezoelectric media

Relationships between complex power flow pseudo energy, propagation constant and complex frequency are presented for acoustic waves in piezoelectric media. These relationships are essentially energy-power equations which apply to anisotropic, nonconservative, dispersive, linear systems, analogous to those obtained by Chorney and Penfield for guided electromagnetic waves. At vanishing piezoelectric coupling the powermode theorems split into a proper electromagnetic set and a proper mechanical set. By differentiating the power-mode equations with respect to the complex frequency further results are obtained linking the group velocity with power flow and energy storage. Conclusions may be drawn from these expressions regarding the signature of the dispersion (forward or backward waves). The equipartition of pseudo energy is established at cut-off, and the vanishing of the complex power flow at resonance. Examples including wave propagation in lossless and lossy media are included.     

Electromagnetic wave propagation through frequency-dispersive and lossy double-negative slab

This study presents the electromagnetic wave propagation through the frequency-dispersive and lossy double-negative slab embedded between two different semi-infinite media. The double-negative slab is realized by using two models, the Lorentz and Drude medium models. The properties and the required equations for the frequency-dispersive and lossy double-negative slab, the Lorentz medium and Drude medium are given in detail. After the construction of the problem, the reflection and transmission coefficients are derived for both TE and TM waves. Then, the reflected, transmitted and loss powers are determined using these coefficients. Finally, in the numerical results, the mentioned powers for TE and TM waves are computed and illustrated as a function of the incidence angle, the frequency and the slab thickness when the damping frequency changes.