Electromagnetic wave propagation in spherically stratified isotropic media

The general electromagnetic field can be resolved into two components represented by electric and magnetic Hertz vectors, both vectors being parallel to the radial direction. Very few exact solutions for the radial variation of fields of the electric type have been found hitherto. Generalised refractive-index profiles permitting exact transcendental solutions are presented.     

Electromagnetic wave propagation in cylindrically stratified isotropic media

The general electromagnetic field, when independent of axial distance, may be expressed as the sum of E parallel and H parallel partial fields. Exact solutions of the wave equation for the radial variation of the H parallel field are not numerous. Generalised refractive-index profiles are presented which permit transcendental solutions to be made.     

Prolate spheroidal antennas in isotropic plasma media

The problems of a gap-excited finite-sized prolate spheroidal antenna, with and without a confocal prolate spheroidal vacuum sheath, operated in a uniform cold and in a uniform warm lossy plasma medium, have been considered by applying boundary value problem techniques. The dependence of the input admittance, the current distribution, and the radiation pattern, upon the collision frequency, the electron temperature, the length of the antenna, the length-to-radius ratio for the antenna, the ratio of the plasma frequency to the operating frequency, and the thickness of a vacuum sheath have been investigated. Admittance curves are given from which the input admittance of a finite linear antenna can be estimated for a wide range of operating conditions and length-to-radius ratios. Conductance and susceptance maxima, dependent upon the vacuum sheath thickness, have been observed when using the cold plasma model foromega < omega_{p}. When using the warm plasma model in conjunction with the boundary condition that the normal component of the fluid velocity is zero at the antenna surface and at the vacuum sheath surface, the radiation resistance of the antenna is increased significantly for the parameters considered.     

Reflection of pulses at oblique incidence from stratified dispersive media

The reflection coefficient is calculated at normal and oblique incidence for a sine-squared pulse with the middle layer having a frequency dependent complex permittivity. The two important results of the theoretical investigation are the importance of the frequency dependence of the complex dielectric constant in pulse propagation studies as well as the unique effect that occurs at some angles of incidence at parallel polarization in which the power from the second interface exceeds the primary reflection from the first interface.     

Propagation of a radio pulse in isotropic plasma

The problem of propagation of a radio pulse in isotropic collisional cold plasma is solved. The solution analytically describes the spatiotemporal evolution of the characteristics of a radio pulse with an envelope of a rather general form. Explicit expressions are obtained for the velocity of radio-pulse propagation in isotropic plasma.     

The distortion of electromagnetic pulses undergoing total internal reflection in a stratified troposphere

The treatment of total internal reflection of electromagnetic pulses at the sharp boundary of two homogeneous half-spaces is extended to cover the case of total internal reflection in a stratified nonionized half-space. It is indicated that any inhomogeneity of the reflecting medium perturbs each harmonic spectral component of the reflected pulse in phase and amplitude as both the real and the imaginary parts of the reflection coefficient actually become functions of the frequency.     

Subnanosecond laser pulses for plasma production

A laser source suitable for plasma production is described. The technique involves the use of uncoated flats that act as mode selectors when inserted inside the laser cavity. Pulse widths of 15 to 30 and 60 to 70 ps were obtained with signal-to-noise ratios greater than 20 dB for 70 to 80 percent of the laser firings.     

Some methods of solving for the coefficients of dyadic Green's functions in isotropic stratified media

Dyadic Green's functions in multi-layered isotropic media are analysed in this paper. Three different kinds of method for obtaining the coefficients of dyadic Green's functions in multi-layered media are given, these are (a) the boundary condition method, which is well-known; (b) the recurrence matrix equation method; and (c) the ray trail method. Using these methods, several examples are considered. Some results are the same as those obtain previously. Some are obtained for the first time.     

Propagation of a chirp radio pulse in isotropic plasma

The problem of space-time deformation of a chirp radio pulse propagating in cold isotropic collisional plasma is solved analytically. Pulses with rectangular and biexponential initial envelopes are considered as examples to show that, as the covered distance grows, a chirp pulse is at first compressed and then broadened and, simultaneously, the linear increase of the instantaneous frequency observed within the pulse duration changes into an approximately linear decrease. The distance at which a chirp pulse is compressed to the minimum possible duration is estimated, and the effect of absorption on the current shape of the pulse envelope is analyzed.     

Angular feature of conical emission in an isotropic amorphous medium pumped by femtosecond pulses

Broadband parametric upconverted conical emission (CE) has been investigated in an isotropic amorphous medium pumped by femtosecond laser pulses,which covers a broad range from 470 to 630 nm.Three theo...     

Admittance of a waveguide radiating into stratified plasma

A slot covered by a stratified plasma is assumed to radiate into a wide waveguide instead of free space. The slot admittance approximates the free space admittance of the slot for waveguide diameters exceeding 6 to10lambda. For thick plasma layers the computed slot admittance checks with earlier admittance calculations for a laterally unbounded plasma. When approximating a plasma profile of a typical hypersonic re-entry, a multilayer plasma model in a wide waveguide appears to provide a more accurate slot admittance than a single-layer approximation in a laterally unbounded geometry.     

Antenna pattern distortion by an isotropic plasma slab

A quantitative laboratory experimental investigation has been conducted on the distortion of the main far-field radiation pattern of a horn antenna situated behind a layer of isotropic plasma. The power radiation pattern and phase for orthogonal components of the radiation transmitted through the plasma were investigated for various plasma properties. The experimental results were compared to theory based on the reciprocity theorem and good agreement was obtained. The major features are a gradual attenuation and distortion of the radiation pattern for plasma electron densities below the critical density and a rapid change in the radiation pattern with severe attenuation in the region of the critical electron density followed by "cut-off" at still larger electron densities. No appreciable polarization change of the energy radiated by the antenna is introduced by the plasma.     

有限长激光脉冲在部分离化等离子体中的传播

为了研究有限长激光脉冲在部分离化等离子体中的传播特性,采用变分法推导出有限长激光脉冲在部分离化等离子体中的参量演化方程,分析了尾波场、相对论自聚焦和部分离化非线性极化强度的影响因素;通过分析焦斑半径和脉冲宽度满足的耦合方程,讨论了横向和纵向尾波场影响下的激光脉冲传播特性。结果表明,由于焦斑半径和脉冲宽度的耦合,激光脉冲在部分离化等离子体中传播必须满足一定条件;在部分离化等离子体中,考虑激光传播时脉冲宽度的变化是有必要的;对给定强度的激光脉冲,等离子体密度不变时,随着电离程度增大,尾波场会进一步增强激光脉冲的自聚焦,其中纵向尾波场比横向尾波场对激光脉冲的自聚焦作用更明显。这一结果对有限长激光脉冲电离诱导自注入及尾波场加速电子的方案具有理论指导意义。     

Measurements of dipole antenna impedance in an isotropic laboratory plasma

The impedances of relatively long dipoles in an isotropic laboratory plasma have been measured at 9.2 GHz (Xband). A helium plasma was generated by discharging a capacitor between electrodes in a cylindrical container ten free-space wavelengths in diameter. Impedance measurements were made in the decaying afterglow plasma at electron densities both above and below the critical electron density (10^{12}cm^-3). The dipole antennas were fed from miniature solid-jacketed coaxial cables attached to a waveguide on which the voltage reflection coefficient was measured. The effect of plasma on the impedance of the dipole antenna was derived theoretically by treating the dipole as a dissipative transmission line. The theory described in a qualitative fashion the observed impedance variation with electron density for all the dipoles investigated. The measurements indicated certain antenna impedance properties, due to the plasma, which were not accounted for by the simple theory.     

Harmonic generation by an electromagnetic wave in an inhomogeneous isotropic plasma

The generation of harmonic waves by a plane electromagnetic wave normally incident on a collision-free plasma with a linear density profile is considered. The solution for the primary wave in the plasma region is first obtained, and nonlinear polarization currents for the higher harmonics are expressed in terms of the lower harmonic fields by a perturbation method. The analysis shows that the plasma excites a longitudinally oscillating second harmonic electric field which is totally confined inside the plasma and has a singularity. The third harmonic wave excited in the plasma, however, radiates back into free space, although the effect is negligibly small in a quasi-homogeneous limit. The amplitude of this radiated field oscillates as a function ofomega^{3}, and the electron density gradient has little effect on it.     

非对称激光等离子体尾场中被加速电子的模拟

为了研究在激光驱动的等离子体尾场中被加速电子的动力学,采用数值模拟方法得到了非对称脉冲驱动的尾波场中被加速的电子的运动相图、密度分布及势能。结果表明,非对称激光脉冲驱动尾场中电子得到很高的能量。在非对称激光脉冲驱动的激光尾场中,为了有效地加速电子,要选择恰当的上升激光脉冲长度和下降激光脉冲长度。     

Terminal admittance of a thin biconical antenna in an isotropic compressible plasma

An exact expression has been derived for the terminal admittance of a thin biconical antenna in an isotropic compressible plasma. It is assumed that the ion sheath is a sphere whose radius is equal to the halflength of the antenna.     

分层等离子体包覆飞行器天线性能一体化分析

提出了一种适用于分层等离子体鞘套、高超声速飞行器以及天线一体化求解的修正多层薄介质片(Multi-Layer Thin Dielectric Sheet,ML-TDS)方法.该方法只需在相关参考面上进行面剖分,避免了对覆盖于机体表面等离子体的海量体剖分,从而大幅降低了待求未知量和计算量.详细介绍了修正ML-TDS方法,通过与体面积分方程方法和面积分方程方法的对比展示了其在分析分层等离子体鞘套覆盖下天线辐射特性问题中的优势.基于高速飞行器表面等离子体鞘套和天线间的互耦模型,分别计算了飞行器上单天线和双天线在等离子体鞘套和飞行器平台影响下的辐射特性.该方法可对等离子体鞘套包覆的飞行器和天线实现一体化的建模和计算,数值结果准确,计算复杂度低,是研究分层等离子体鞘套电磁效应较为便捷的数值分析工具.