太赫兹波在飞行器等离子体鞘套中的传输特性

针对临近空间飞行器的黑障问题,根据模拟的RAM C-Ⅲ飞行器周围的流场分布结果,计算了等离子体电子密度和碰撞频率,并根据其分布建立了非均匀的等离子体模型。在此基础上,利用散射矩阵方法分析了太赫兹波在等离子体中的传输特性随着等离子体密度、等离子体厚度、等离子体碰撞频率的变化以及外加磁场对传输特性的影响。结果表明,太赫兹波的传输损耗随着等离子体电子密度和等离子体厚度的增加而增加,而碰撞频率的增加会使得透射率先减小后增加。在外加磁场的作用下,左旋太赫兹波的传输特性会得到改善;而对于右旋太赫兹波,磁场的施加会引入吸收峰,并且随着磁感应强度的增加向高频方向移动。     

太赫兹波斜入射到磁化等离子体的数值研究

针对“黑障”问题,借助于电磁波斜入射到磁化等离子体的传输模型,研究了太赫兹(THz)波在磁化、均匀等离子体中的传输特性,分析了太赫兹波在磁化等离子体中传播的反射、透射以及衰减。仿真结果表明,等离子体碰撞频率、电子密度、入射角度以及磁场强度,对衰减产生不同的影响。单纯的增加碰撞频率,衰减值呈现出先增大后减小的变化趋势;电子密度的增加,与衰减峰值的大小有关;外加磁场加入,衰减值降低。通过调节外加恒定磁场,可以有效地解决“黑障”问题。     

Impact of nonlinear absorption on propagation of microwave in a plasma filled rectangular waveguide

In collisional and ponderomotive predominant regimes, the propagation of microwave in rectangular waveguide filled with collisional plasma is investigated numerically. The dominant mode is excited through an evacuated waveguide and then enters a similar and co-axis waveguide filled with plasma. In collisional predominant regime, the amplitude of electric field is oscillated along propagation path; outset of propagation path due to the electron-ion collision, the intensity oscillations are reduced. Afterward, under competition between the collisional nonlinearity and absorption, the intensity is increased, so the electron density peak is created in middle of waveguide. In ponderomotive predominant regime, the intensity is slowly decreased due to collision, so the electron density is ramped. Control parameters, like the frequency, input power, collision frequency, and background electron density are surveyed that can be used to control propagation characteristics of microwave. This method can be used to control heating of fusion plasma and accelerate charged particle.     

Terahertz radiation generation by beating of two chirped laser pulses in a warm collisional magnetized plasma

Analytical equations of terahertz(THz) radiation generation based on beating of two laser beams in a warm collisional magnetized plasma with a ripple density profile are developed. In this regard, the effects of frequency chirp on the field amplitude of the terahertz radiation as well as the temperature and collision parameters are investigated. The ponderomotive force is generated in the frequency chirp of beams. Resonant excitation depends on tuning of the plasma beat frequency,magnetic field frequency, thermal velocity, collisional frequency, and effect of the frequency chirp with the plasma density.For optimum parameters of frequency and temperature the maximum THz amplitude is obtained.     

磁化碰撞等离子体对雷达波的共振吸收

使用平板几何对雷达工作频段的电磁波在磁化碰撞等离子体中的传播作了数值计算,计算结果表明,在均匀等离子体中,当等离子体碰撞频率f∞=0.1,0.5,1GHz及电磁波频率接近高混杂频率时,电磁波衰减和被吸收功率出现最大峰值,即出现共振吸收;当fen=1,10GHz时,电磁波衰减、被吸收功率和透射功率曲线变得很平坦;衰减和吸收功率随等离子体密度的增大而增大,在n=10^11cm^-3时,衰减可达100dB,吸收比可达80％。在非均匀等离子体中,电磁波的反射功率峰值比在均匀等离子体中大。因此,磁化均匀等郭了体更有利于等离子体隐身。     

太赫兹波在磁化等离子体中传输特性

建立了电磁波穿过磁化等离子体鞘套的一维模型,并采用数值分析方法对太赫兹（THz）电磁波在磁化等离子体鞘套中传播时的反射率、透射率和衰减值进行了计算仿真,分析了磁化条件下磁场强度、太赫兹波频率、等离子体鞘套厚度、等离子体碰撞频率和等离子体密度对太赫兹波在磁化等离子体鞘套中传播特性的影响。仿真结果表明:排除衰减波峰对应的频率范围,外加磁场有效降低了太赫兹波传输的衰减,在固定磁场强度下,衰减波峰两侧的衰减值均达到了5 dB以下,有效地解决了通信黑障问题。     

时空非均匀等离子体鞘套中太赫兹波的传播特性

高超声速飞行器再入地面的过程中,其周围等离子体的电子密度是非均匀且随时间变化的.对于不同的再入高度,飞行器周围的温度和压强也会发生改变.因此,研究电磁波在时空非均匀等离子体鞘套中的传播特性意义重大.首先建立了时变非均匀的等离子体鞘套模型,然后通过经验公式得到温度、压强与碰撞频率三者的关系.采用时域有限差分方法计算了太赫兹波段中不同电子密度弛豫时间、温度、压强时的反射系数、透射系数和吸收率.研究结果表明:在太赫兹波段中,电子密度的弛豫时间越长,温度越高,压强越大,电磁波越容易穿透等离子体;弛豫时间越短,温度越低,压强越小,等离子体对电磁波吸收率的变化越明显.这些结果为解决"黑障"问题提供了理论依据.     

Ion acoustic solitons in magnetized collisional non-thermal dusty plasmas

The oblique propagation of ion-acoustic solitary waves (IASWs) is considered, in a magnetized non-thermal collisional dusty plasma, composed of non-Maxwelian κ-distributed electrons, inertial ions, and stationary dust. The reductive perturbation approach is adopted to derive the damped Korteweg de-Vries (dKdV) equation, and the dissipative oblique ion-acoustic wave properties are investigated in terms of different key plasma parameters via the numerical solution of the dKdV equation. The collisional effect, describing the ion-neutral collision in the plasma, is taken into account, and seen to influence the dynamics of IASWs significantly. The basic features of IASWs are observed to modify, and the polarity of the wave is seen to change due to the variation of dust to that of ion number density and also due to the variation of the supethermality index κ in the considered plasma system.     

Plasma characteristics and broadband electromagnetic wave absorption in argon and helium capacitively coupled plasma

A one-dimensional self-consistent calculation model of capacitively coupled plasma(CCP) discharge and electromagnetic wave propagation is developed to solve the plasma characteristics and electromagnetic wave transmission attenuation.Numerical simulation results show that the peak electron number density of argon is about 12 times higher than that of helium, and that the electron number density increases with the augment of pressure, radio frequency(RF) power, and RF frequency. However, the electron number density first increases and then decreases as the discharge gap increases. The transmission attenuation of electromagnetic wave in argon discharge plasma is 8.5-d B higher than that of helium. At the same time, the transmission attenuation increases with the augment of the RF power and RF frequency, but it does not increase or decrease monotonically with the increase of gas pressure and discharge gap. The electromagnetic wave absorption frequency band of the argon discharge plasma under the optimal parameters in this paper can reach the Ku band. It is concluded that the argon CCP discharge under the optimal discharge parameters has great potential applications in plasma stealth.     

毫米波与太赫兹波在等离子体中传输特性

针对黑障区高速飞行器测控通信研究需求,在激波管设备上开展了Ka波段毫米波与太赫兹波在等离子体中传输特性的实验研究,获得了不同电子密度和碰撞频率的等离子体中传输衰减特性实验数据。采用辅助差分方程的时域有限差分（ADE-FDTD）方法对实验进行了数值模拟,数值模拟结果和实验结果有很好的一致性。实验结果和数值模拟结果均表明:太赫兹波信号在相同的实验等离子体中传输衰减比毫米波信号小得多,具有更强的穿透等离子体能力;毫米波与太赫兹波信号在等离子体中传输衰减量随着等离子体电子密度的增加而增加,二者的差值也增加;太赫兹波能够作为解决高密度等离子体中电磁波传输的有效技术手段。     

高功率微波输出窗内侧击穿动力学的PIC/MCC模拟研究

高功率微波在受控热核聚变加热、微波高梯度加速器、高功率雷达、定向能武器、超级干扰机及冲击雷达等方面有着重要的应用.本文针对高功率微波输出窗内侧氩气放电击穿过程,建立了二次电子倍增和气体电离的一维空间分布、三维速度分布（1D3V）模型,并开发了相应的PIC/MC程序代码.研究了气压、微波频率、微波振幅对放电击穿的影响.结果表明：在真空情况下,介质窗放电击穿只存在二次电子倍增过程;在低气压和稍高气压时,二次电子倍增和气体电离共存;在极高气压时,气体电离占主导.给出了不同气压下电子、离子的密度和静电场的空间分布.此外还观察到,在500 mTorr时,随着微波振幅或微波频率的变化,气体电离出现的时刻和电离产生的等离子体峰值位置有较大差异,尤其是当微波频率（GHz）在数值上是微波振幅（MV/m）的2倍时,气体电离出现的较早.     

THz电磁波在时变非磁化等离子体中的传播特性研究

本文建立了时变非磁化等离子体平板的一维模型,并采用时域有限差分(FDTD)方法对太赫兹(THz)电磁波在时变等离子体中传播时的反射、透射系数及吸收率进行了计算.然后根据计算结果分析了时变等离子体的上升时间、电子密度、温度以及等离子体平板厚度等参数对不同频段THz波在等离子体中传播特性的影响.分析结果表明:THz波在时变等离子体中传播时,其反射系数受等离子体电子密度和上升时间的影响较大;而吸收率则随着上升时间的减小、电子密度及平板厚度的增加而增大;此外,THz电磁波能够穿透量级为1020m-3的高密度等离子体层,可以作为再入段飞行器通信以及高密度等离子体诊断的理想工具.     

Propagation in a warm collisional magnetoplasma enclosed in aconducting cylinder

A study is presented of the propagation of electron plasma waves in a warm collisional plasma filling a conducting cylinder and magnetized strongly in the axial direction, the plasma parameters being taken to be such that the electron-ion collision is the dominant damping process. The attenuation and phase constants are derived in suitably normalized form by using the electrodynamic method. The dispersion and attenuation characteristics for propagation in the lowest order mode in a hydrogen plasma are obtained for various values of the normalized plasma radius αω_pe/c, the electron temperature and electron density being held constant, and the characteristics are compared with those given by the usual quasi-static approximation, which is found to be valid only if αω_pe /c≲1, where α is the plasma radius, ω _pe is the plasma frequency, and c is the velocity of light in free space. The effect of the plasma frequency on the characteristics is investigated     

高气压空气环状感性耦合等离子体实验研究和参数诊断

等离子体的电子密度分布, 电子碰撞频率分布, 覆盖面积, 厚度是影响其覆盖目标电磁散射特征的关键属性. 对此, 本文开展了在20 cm×20 cm×7 cm石英腔内感性耦合等离子体(ICP)的放电实验, 观察了在高气压条件下, 空气ICP的环形放电形态, E-H模式跳变现象和分层结构, 测量了其电负性核心区和电正性边缘区宽度和厚度随功率、气压的变化趋势, 并通过COMSOL Multiphysics对平板线圈磁场强度分布的分析和电负性气体扩散理论给予上述现象合理的解释, 同时, 利用微波透射干涉法测量了核心区域的电子密度随功率和气压的变化曲线, 利用理论模型计算了边缘区域的电子密度分布, 最后通过辅助气体Ar发射谱线的玻尔兹曼图形法得到了核心区和边缘区的电子激发温度.     

THz波在不同角度磁化的非均匀磁化等离子体中的传输特性分析

为利用太赫兹波解决飞行器再入过程遇到的“黑障”问题,以散射矩阵方法为基础,分别以非均匀磁化等离子体的磁化方向、电子密度、外加磁场强度和碰撞频率为变量,研究了垂直入射情形下它们对太赫兹波传输行为的影响.结果表明:这些参数对太赫兹波传输性能影响明显,例如按某一方向改变磁化角度对左极化和右极化太赫兹波的传输功率有相反的影响;降低磁化强度能一定程度地避开等离子体对右极化波的吸收;而降低碰撞频率能缩小等离子体对右极化波的吸收频带.通过调整这些参数,有望在一定程度上缓解黑障现象.     

Self-referenced method for terahertz wave time-domain spectroscopy

The method allows retrieval of the absorbance of a sample without the need for a reference measurement. The method measures the dynamic variation of frequency resolution as the waveform is being acquired. In terahertz wave time-domain spectroscopy, the frequency resolution increases as the temporal window increases. Therefore, narrow absorption peaks will appear in the spectrum when the temporal window is long enough to resolve the peak. By measuring the dynamic values of each frequency component at specific points in time, a reference value and a peak value are extracted and, hence, the self-referenced is achieved. In addition, the method provides a mechanism to remove the effects of echoes, which enables arbitrary temporal window length and, thus, achieves high-resolution frequency. Examples of extraction of the water vapor lines and resonant features in gas and semiconductors are demonstrated in transmission and reflection geometries.     

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.     

高温等离子体中太赫兹波的传输特性

通过解析方法研究了高温等离子体的太赫兹波传输特性.研究发现,高温等离子体对太赫兹波高频频段透过率较高,表现为通带;对低频频段透过率较低,表现为阻带.这与冷等离子体中电磁波的传输特性是一致的.但其透射率还受到温度与磁场的影响,当改变高温等离子体的电子温度与磁场时,在阻带内会产生一尖锐的透射峰.这种现象在冷等离子体模型中从来没有出现过.本文主要对电子温度和外加磁场两个影响因素进行讨论.研究发现,禁带内出现的透射峰频率受磁场影响,而峰值幅度受温度影响.计算得到了不同外加磁场条件下产生高透过率(透射率约为1)时的电子温度.基于该结果进一步研究了透射峰出现的规律,并通过曲线拟合的方法得到了透射峰频率所遵循的计算公式.数值结果表明透射峰频率与外磁场之间为正比例函数关系,而峰值电子温度取值与外磁场的关系表现为指数规律.最后对拟合得到的方程采用时域有限差分法进行了验证,数值结果与解析解符合较好,证明了该研究的正确性.     

Non‐linear absorption of an intense laser pulse propagating in wiggler‐assisted underdense collisional plasma

In this article, the propagation of an intense laser pulse through underdense collisional plasma in the presence of planar magnetostatic wiggler is studied. It is shown that the electron density distribution, in the presence of planar wiggler with increasing of the normalized plasma length, increases initially and then reaches a peak for different values of wiggler amplitudes. In addition, it is found that the existence of wiggler field leads to an increase in the electron density distribution and subsequently enhancement of electric field. Moreover, it is observed that by increasing the wiggler field, as a result of the increase of the electron density distribution, the dielectric permittivity constant is reduced. It is seen that while wiggler magnetic field was applied appropriately, the total absorption coefficient in the underdense collisional isothermal magnetized plasma improves. In fact, increase of wiggler magnetic field causes the enhancement of the total absorption coefficient of plasma medium.     

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)