电磁波在磁化等离子体表面的功率反射系数计算研究

采用等效输入阻抗方法计算金属平板前非均匀磁化等离子体层对垂直入射电磁波的功率反射系数，结果表明，电子数密度大小、等离子体层厚度、入射波频率和外磁场是功率反射系数的主要影响因素.电子数密度取值必须合适，外加磁场才能明显降低等离子体对入射电磁波的功率反射系数.入射电磁波频率增加时，必须加大外磁场强度，才能明显降低功率反射系数. 关键词： 等离子体中电磁波的传输 等离子体特性 电磁波     

等离子体的SO-FDTD算法和对电磁波反射系数的计算分析

将一类色散介质的介电常数写成有理分式函数形式，进而导出FDTD中电位移矢量D和电场强度E之间的关系，形成SO-FDTD方法. 应用该方法计算了冷等离子体平板对电磁波的反射系数，通过与解析结果的比较，验证了该算法的高效性和高精度，同时，应用SO-FDTD方法，计算了等离子体层对垂直入射电磁波的反射系数，结果表明：等离子体厚度、电子密度、电子密度的分布形式和入射波频率是影响反射系数的重要因素. 关键词： 等离子体 电磁波 FDTD方法     

进气道内衬筒形等离子体隐身性能三维模拟

针对飞机进气道等离子体隐身问题,建立了三维筒形进气道模型,采用有限元求解波动方程,计算了腔体内壁覆盖均匀等离子体时的雷达散射截面。研究表明:腔体内覆盖等离子体时可以有效吸收入射电磁波能量;吸收随电磁波频率增加而减弱,但由于腔体结构作用,会存在几个吸收峰;吸收随电子数密度增加而增强,但电子数密度过高时,吸收效果会变差;最佳碰撞频率虽然与电磁波频率和电子数密度有关,但其值可约为9GHz;吸收随等离子体厚度增加而变大,但在较大电子数密度时,由于电磁波在等离子体与空气交界面处反射导致厚度增加,从而使得吸收变小;选取合适的入射角度和等离子体数密度可以在1~3GHz频段实现明显的隐身效果。     

电磁波与非磁化等离子体的相互作用

研究了频率为0．5-10HGz的电磁波在非磁化等离子体中的传播。在厚度为10cm的、密度n=10^10或10^11cm^-3的均匀和密度分布n=n0exp[2(z/d-1)]非均匀的等离子体中，计算了等离子体中的电子与中性气体的碰撞频率为0．1-10GHz间的电磁波的反射功率，吸收功率 透射功率的变化。结果表明当等离子体密度比较大（n=10^11cm^-3)、电子与中性气体的碰撞频率比较大时，无论是均匀还是非均匀等离子体对电磁波能量的吸收都比较大，最大可达90％。     

等离子体鞘层附近尘埃颗粒特性的数值模拟

为了研究尘埃等离子体中尘埃颗粒以及鞘层中粒子密度分布等特性,对尘埃颗粒存在条件下等离子体鞘层结构的采取数值模拟.采用稳态无碰撞的尘埃等离子体鞘层模型,对玻姆判据、尘埃颗粒的荷电性质、平板鞘层区域的电势分布及鞘层内粒子分布特性进行了系统的数值模拟研究.计算结果显示,鞘层边缘尘埃颗粒数密度的增加、尘埃温度的升高,将引起孤立尘埃颗粒对电子吸附能力的减弱,集体效应也受到一定程度的影响;二者同时对离子玻姆速度以及鞘层厚度的增加都有着极大的促进作用.鞘层电势在靠近下极区处降落迅速,主要聚集在接近阴极极板的鞘层区域,各种微粒数密度的空间分布满足准中性条件.     

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.     

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。     

磁化等离子体对平面电磁波吸收特性的数值研究与全波仿真

研究了平面电磁波在磁化、稳定、二维、非均匀等离子体中的传播特性;采用等效输入阻抗方法计算非均匀磁化等离子体层对不同模式入射电磁波的功率吸收情况。结果表明,电子数密度、碰撞频率和外磁场大小是等离子体对电磁波功率吸收的主要影响因素。采用等效介电常数的方法模拟等离子体特性,代入有限元软件进行平面电磁波入射等离子体仿真,得到了非寻常波与右旋极化波的吸收特性。根据数值计算和全波仿真结果可知,当等离子体密度为1017 m-3、碰撞频率2.5 GHz、外加磁场的磁感应强度为0.15 T时,磁化等离子体对电磁波有强烈的吸收特性。     

非均匀等离子体对平面电磁波的衰减

利用荧光灯排列形成非均匀等离子体层(面积约60cm×52cm,消耗功率约400W),研究了其对1～8GHzE波(电矢量方向平行于灯轴方向的入射波)的反射和透射的影响。结果表明,该等离子体对1～4GHz的E波具有强吸收和弱反射的特性,单程衰减最高可达8dB。利用2维分段线性电流密度递归卷积时域有限差分计算式,模拟了E波传播及其在非均匀等离子体内推进的瞬态过程,计算了等离子体对电磁波的反射和透射衰减,并与实验结果拟合,得到等离子体电子数密度峰值约9.72×1016m-3,电子与中性粒子碰撞频率约4GHz。     

HL-1M等离子体中ECW传播和吸收的数值分析

计算了频率在电子回旋频率附近O波和X波在HL－1M等离子体中的传播轨迹和吸收系数。研究了等离子体温度、密度和共振层位置等参数及波的入射对波迹和功率沉积分布的影响。结果表明,O波和X波能有效地被等离子体吸收,其吸收具有定域性,波的单次吸收系数较高,功率的沉积强烈地依赖等离子体参数。     

圆柱形等离子体对微波散射的数值模拟与实验研究

模拟和实验研究了非均匀圆柱形等离子体及阵列对微波的散射作用。利用有限时域差分(FDTD)方法仿真得到了等离子体柱的密度、碰撞频率对微波传播系数的影响,并利用低气压放电产生的等离子体柱对微波的吸收和散射作用进行了验证。结果表明:电子密度中心高、周围低的非均匀等离子体柱可将微波散射至两个侧向;等离子体频率越大,散射的微波功率越强;增加碰撞频率使等离子体柱的微波散射功率减小、吸收增大。等离子体必须具有合适的密度,才能对微波反射产生较大影响。     

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.     

Frequency shift and pulse compression of an ultrashort pulse in a large-amplitude plasma wave

By calculating the momenta of a coupled set of nonlinear equations of laserpulse-plasma wave interaction in the weak relativistic approximation,the conditions for fre-quency up-shift have been found.That the energy change of the pulse due to frequency shiftis compensated by the change of plasma wave energy results in photon number conservation.Some factors that affect the frequency up-shift are studied.It is also found that the laser pulsecan be compressed when it is located near the valley of plasma density perturbation and ifsome threshold value of the plasma wave field is exceeded.     

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.     

Study of the terahertz wave propagate in high temperature plasma band gap

In this paper, the phenomenon of band gap transmission in high temperature plasma is studied. Cold plasma has high pass property, so the low-frequency electromagnetic wave cannot penetrate plasma and propagate in it. Simulation results show that in high temperature plasma, a transmission peak will be produced in the low frequency band where the band gap is supposed to occur due to the influence of the external magnetic field and the electron temperature. Through the study of the frequency and the amplitude of the transmission peak, it is found that the frequency of the peak is related to the collision frequency and the magnetic field, and the amplitude of the peak is related to the electron temperature and the thickness of the plasma slab. The calculation formula of peak frequency is obtained by fitting the obtained data. The fitting formula is validated by the analytic solution, and the results show that the two methods are in good agreement.     

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.     

Resonant interaction of an electromagnetic wave with an inhomogeneous plasma slab

Resonant interaction at oblique incidence of an electromagnetic wave on an inhomogeneous plasma slab is studied. The time evolution of this interaction is solved numerically from two-fluid equations, adiabatic equation for electron pressure and from Maxwell equations. It is shown that the electromagnetic energy of an incident wave is transformed both into the heat energy and into the energy of plasma oscillations in the direction of density gradient. The distribution of the transformed energy between the heat energy and the energy of plasma oscillations is strongly dependent on the plasma temperature. The ratio of heat energy to the energy of plasma oscillations is growing with growing temperature. The plasma oscillations are generated by magnetic induction of the penetrating wave. In a cold plasma they are generated especially in the overdense region and their frequency is equal to local plasma frequency. The electric field in the direction of plasma gradient has a form of a wave packet whose envelope reaches a maximum at resonance. The characteristic wavelength in the wave packet decreases and the amplitude of the packet increases with the time.     

Attenuation characteristics of obliquely incident electromagnetic wave in weakly ionized dusty plasma based on modified Bhatnagar–Gross–Krook collision model

The attenuation characteristics of obliquely incident electromagnetic(EM) wave in L-Ka frequency band in weakly ionized dusty plasma are analyzed based on the modified Bhatnagar-Gross-Krook(BGK) collision model.According to the kinetic equation and the charging theory,the total complex dielectric constant of the weakly ionized dusty plasma is derived by considering that the minimum velocity of the electron accessible to the dust particle surface is non-zero and the second potential part of the collision cross-section contributes to the charging.The attenuation characteristics within the modified model are compared with those within the traditional model.The influence of the dusty plasma parameters and the incident angle of EM waves on the attenuation in weakly ionized dusty plasma is further analyzed.Finally,the influence of different reentry heights on the attenuation characteristics of the obliquely incident EM wave is discussed.The results show that the effect of the minimum electron velocity and the second term of the collision cross-section on the attenuation characteristics of EM waves cannot be ignored.When the dust density and dust radius are changed,the trends of the attenuation of obliquely incident EM waves are consistent,but the influence of dust density is weaker than that of dust radius due to the constraint of orbit-limited motion(OLM) theory.The plasma thickness,electron density,and incident angle are proportional to the attenuation amplitude of EM waves.The effect of different reentry heights on the attenuation obliquely incident EM waves is related to the electron density and plasma thickness.