FDTD Analysis of Reflection of Electromagnetic Wave From a Conductive Plane Covered with Inhomogeneous Time-Varying Plasma

A finite-difference time-domain (FDTD) algorithm is applied to study the electromagnetic reflection of conduction plane covered with inhomogeneous time-varying plasma, homogeneous plasma and inhomogeneous plasma. The collision frequency of plasma is a function of electron density and plasma temperature. The number density profile follows a parabolic function. A discussion on the effect of various plasma parameters on the reflection coefficient is presented. Under the one-dimensional case, transient electromagnetic propagation through various plasmas has been obtained, and the reflection coefficients of EM wave through various plasmas are calculated under different conditions. The results illustrate that a plasma cloaking system can successfully absorb the incident EM wave.     

Power Absorption of High Frequency Electromagnetic Waves in a Partially Ionized Plasma Layer in Atmosphere Conditions

We have studied the absorption, reflection, and transmission of electromagnetic waves in an unmagnetized uniform plasma layer covering a metal surface in atmosphere conditions. Instead of the absorption of the electromagnetic wave propagating only once in previous work on the plasma layer, a general formula of total power absorption by the plasma layer with an infinite time of reflections between the atmosphere-plasma interface and the metal surface has been derived for the first time. Effects of plasma parameters, especially the dependence of the fraction of positive lons, negative ions and electrons in plasmas on the power absorption processes are discussed. The results show that the existence of negative ions significantly reduces the power absorption of the electromagnetic wave. Absorptions of electromagnetic waves are calculated.     

Influence of Plasma Pressure Fluctuation on RF Wave Propagation

Pressure fluctuations in the plasma sheath from spacecraft reentry affect radiofrequency(RF) wave propagation.The influence of these fluctuations on wave propagation and wave properties is studied using methods derived by synthesizing the compressible turbulent flow theory,plasma theory,and electromagnetic wave theory.We study these influences on wave propagation at GPS and Ka frequencies during typical reentry by adopting stratified modeling.We analyzed the variations in reflection and transmission properties induced by pressure fluctuations.Our results show that,at the GPS frequency,if the waves are not totally reflected then the pressure fluctuations can remarkably affect reflection,transmission,and absorption properties.In extreme situations,the fluctuations can even cause blackout.At the Ka frequency,the influences are obvious when the waves are not totally transmitted.The influences are more pronounced at the GPS frequency than at the Ka frequency.This suggests that the latter can mitigate blackout by reducing both the reflection and the absorption of waves,as well as the influences of plasma fluctuations on wave propagation.Given that communication links with the reentry vehicles are susceptible to plasma pressure fluctuations,the influences on link budgets should be taken into consideration.     

Propagation characteristics of microwaves in dusty plasmas with multi-collisions

In this study, we consider three main collisions in dusty plasmas and investigate the effects of dust grains on the propagation of electromagnetic(EM) waves through uniform, unmagnetized and weakly ionized dusty plasma. The Drude model is improved to describe the dielectric property of dusty plasmas, which accounts for collisions including electron–molecule, electron–ion, and electron–dust particles. Based on the improved Drude model, the propagation characteristics of microwaves in dusty plasmas have been numerically calculated and studied.The results show that the propagation characteristics of microwaves through dusty plasmas are different from those through normal plasmas. The effects of dust density and size are mainly studied. Numerical results indicate that the momentum transfer between electrons and dust grains makes more energy loss. The dust density and dust size have a similar influence on EM wave propagation, resulting in less transmission and more absorption.     

Numerical and Experimental Investigation on the Attenuation of Electromagnetic Waves in Unmagnetized Plasmas Using Inductively Coupled Plasma Actuator

The attenuation of electromagnetic(EM) waves in unmagnetized plasma generated by an inductively coupled plasma(ICP) actuator has been investigated both theoretically and experimentally. A numerical study is conducted to investigate the propagation of EM waves in multilayer plasma structures which cover a square flat plate. Experimentally, an ICP actuator with dimensions of 20 cm×20 cm×4 cm is designed to produce a steady plasma slab. The attenuation of EM waves in the plasma generated by the ICP actuator is measured by a reflectivity arch test method at incident waves of 2.3 GHz and 10.1 GHz, respectively. A contrastive analysis of calculated and measured results of these incident wave frequencies is presented, which suggests that the experiment accords well with our theory. As expected, the plasma slab generated by the ICP actuator can effectively attenuate the EM waves, which may have great potential application prospects in aircraft stealth.     

Collision effects on propagation characteristics of electromagnetic waves in a sub-wavelength plasma slab of partially ionized dense plasmas

Intensive collisions between electrons and neutral particles in partially ionized plasmas generated in atmospheric/sub-atmospheric pressure environments can sufficiently affect the propagation characteristics of electromagnetic waves,particularly in the sub-wavelength regime.To investigate the collisional effect in such plasmas,we introduce a simplified plasma slab model with a thickness on the order of the wavelength of the incident electromagnetic wave.The scattering matrix method (SMM) is applied to solve the wave equation in the plasma slab with significant nonuniformity.Results show that the collisions between the electrons and the neutral particles,as well as the incident angle and the plasma thickness,can disturb the transmission and reduce reflection significantly.     

Investigation into wideband electromagnetic stealth device based on plasma array and radar-absorbing materials

Manipulation of electromagnetic waves is essential to various microwave applications, and absorbing devices composed of low-pressure gas discharge tubes and radar-absorbing materials (RAM) can bring new solutions to broadband electromagnetic stealth. The microwave transmission method is used to measure the physical parameters of the plasma unit. The designed structure exhibits superior absorption performance and radar cross-section (RCS) reduction capability in the 2–18 GHz band, with unique absorption advantage in the S and C frequency bands. It is found that the combination of the plasma and the RAM can significantly broaden the absorption frequency band and improve the absorption performance with excellent synergistic stealth capability. Experimental and simulation results present that broadband, wide-angle, tunable electromagnetic wave absorption and RCS reduction can be achieved by adjusting the spatial layout of the combined plasma layer and the type of RAMs, which creates opportunities for microwave transmission and selective stealth of equipment. Therefore, the wave manipulation by combined plasma array and RAM provides a valuable reference for developing numerous applications, including radar antenna stealth, spatial filter, and high power microwave shielding.     

Experimental and Simulational Studies on the Theoretical Model of the Plasma Absorption Probe

Plasma absorption probe (PAP) was developed for measuring the electron density in plasmas processing based on the surface-wave characteristics. In order to diagnose the plasma with lower density and higher pressure, a sensitive PAP was also developed. Both types of PAP were analyzed theoretically under the quasi-static approximation, which is highly problematic when a conductor exists in the resonance region of the probe. For this reason, a theoretical model for the PAP is presented in this paper. The model is derived from the electromagnetic wave equation. Its principle is then verified via experiments and numerical simulations. Both experimental and numerical results show that the electromagnetic theoretical model is valid as compared with the quasi-static model. Consequently, a new type of PAP, named as the electromagnetic PAP, is thus proposed for the measurement of electron density.     

Propagation properties of broadband terahertz pulses through a bounded magnetized thermal plasma

An analysis of THz waves propagation in dense, collisional, thermal, magnetized and bounded plasma is presented. By introducing the dielectric constant of a warm magnetoplasma and using the method of impedance transformation with multiple dielectrics, the coefficients of power reflection (R) and absorption (A) are derived for a bounded plasma model by a lossless plate and a conductor plate. The effects of electron temperature, collision frequency, external magnetic field, electron density and thickness of the plasma slab on the absorption coefficient are analyzed numerically. It is found that these plasma parameters can cause significant change in the value of A. Some phenomena, for example negative power absorption, upper-hybrid resonance absorption and geometric resonances absorption, are observed and the behavior of the THz wave propagation inside the plasma model is explained numerically and physically.     

Research on phase shift characteristics of electromagnetic wave in plasma

The phase shift characteristics reflect the state change of electromagnetic wave in plasma sheath and can be used to reveal deeply the action mechanism between electromagnetic wave and plasma sheath. In this paper, the phase shift characteristics of electromagnetic wave propagation in plasma were investigated. Firstly, the impact factors of phase shift including electron density,collision frequency and incident frequency were discussed. Then, the plasma with different electron density distribution profiles were employed to investigate the influence on the phase shift characteristics. In a real case, the plasma sheath around the hypersonic vehicle will affect and even break down the communication. Based on the hypersonic vehicle model, we studied the electromagnetic wave phase shift under different flight altitude, speed, and attack angle. The results indicate that the phase shift is inversely proportional to the flight altitude and positively proportional to the flight speed and attack angle. Our work provides a theoretical guidance for the further research of phase shift characteristics and parameters inversion in plasma.     

The acceleration mechanism of shock wave induced by millisecond-nanosecond combined-pulse laser on silicon

The velocity variation law of shock wave induced by millisecond-nanosecond combined-pulse laser has been investigated experimentally. The pulse delay and laser energy are important experimental variables. The method of laser shadowgraphy is used in the experiment.Experimental results show that when the pulse delay is 2.4 ms, the ms and ns laser energy density is 301 J cm~(-2) and 12 J cm~(-2), respectively, the velocity of shock wave is 1.09 times faster than that induced by single ns pulse laser. It is inferred that the shock wave propagates in the plasma is faster than that in air. When the ms and ns laser energy density is 414.58 and 24 J cm~(-2), the velocity of shock wave shows rising trend with pulse delay in a range of 1.4 msΔt 0.8 ms. It is indicated that with the increase of ns laser energy, the laser energy absorbed by laser-supported absorption wave increases. The mechanism of inverse bremsstrahlung absorption acts with target surface absorption simultaneously during the ns laser irradiation. Thus, the phenomenon of the double shock wave is induced. The numerical results of the phenomenon were accordance with experiment. The results of this research can provide a reference for the field of laser propulsion.     

The electromagnetic instability in electron flow with ion background

Electromagnetic (EM) behavior and instability resulting from the interaction between EM wave and plasma wave are analyzed based on linear perturbation theory. It is shown that the instability is caused by the the coupling between high frequency electromagnetic field and electron transverse oscillation derived from the deflection of electron longitudinal oscillation due to self-produced magnetic field. The influences of the self-produced magnetic field and plasma density on the instability are studied. In addition characteristics of EM wave propagation at different angles are investigated. The present results are of significance to new type plasma radiation source, ion accelerator and plasma diagnostic techniques.     

Numerical Research of Mode Conversion in an Inhomogeneous Plasma

The linear mode conversion of electromagnetic waves in the hot, unmagnetized inhomogeneous plasma is studied numerically for different density profiles, and the dependence of the absorption coefficient on the incident angles and the wave frequencies are obtained for different electrons＇ temperature. The results show that the shapes of the density profiles and the electron＇s temperature create a certain effect on the coefficients of absorption, which reaches its peak value （about 50%） for appropriate parameters. Effective absorption occurs in a limited range of parameter q.     

Numerical and Experimental Investigation on Electromagnetic Attenuation by Semi-Ellipsoidal Shaped Plasma

Some reports presented that the radar cross section(RCS) from the radar antenna of military airplanes can be reduced by using a low-temperature plasma screen. This paper gives a numerical and experimental analysis of this RCS-reduction method. The shape of the plasma screen was designed as a semi-ellipsoid in order to make full use of the space in the radar dome.In simulations, we discussed the scattering of the electromagnetic(EM) wave by a perfect electric conductor(PEC) covered with this plasma screen using the finite-difference-time-domain(FDTD)method. The variations of their return loss as a function of wave frequency, plasma density profile, and collision frequency were presented. In the experiments, a semi-ellipsoidal shaped plasma screen was produced. Electromagnetic attenuation of 1.5 GHz EM wave was measured for a radio frequency(RF) power of 5 k W at an argon pressure of 200-1150 Pa. A good agreement is found between simulated and experimental results. It can be confirmed that the plasma screen is useful in applications for stealth of radar antenna.     

An electromagnetic wave attenuation superposition structure for thin-layer plasma

This work proposes a new plasma super-phase gradient metasurfaces (PS-PGMs) structure, owing to the limitations of the thin-layer plasma for electromagnetic wave attenuation. Based on the cross-shaped surface unit configuration, we have designed the X-band absorbing structure through the dispersion control method. By setting up the Drude dispersion model in the computer simulation technology, the designed phase gradient metasurfaces structure is superposed over the plasma, and the PS-PGMs structure is constructed. The electromagnetic scattering characteristics of the new structure have been simulated, and the reflectance measurement has been carried out to verify the absorbing effect. The results demonstrate that the attenuation effect of the new structure is superior to that of the pure plasma structure, which invokes an improved attenuation effect from the thin layer plasma, thus enhancing the feasibility of applying the plasma stealth technology to the local stealth of the strong scattering part of a combat aircraft.     

Self-consistent Kinetic Description of the Low-Pressure Solenoidal Inductively Coupled Argon Discharge

Using an one-dimensional slab model, we have studied the electron energy distribution, the anomalous skin effect, and power absorption in the solenoidal-inductively-coupled argon discharge under low pressures (≤1.33 Pa). The electron energy distribution function and rf electromagnetic field in the plasma are determined self-consistently by the linearized Bolztmann equation incorporating with the Maxwell equations. The numerical results show that, at low pressures, the electron energy distribution function exhibits a non-Maxwellian distribution with a long high-energy tail. The anomalous skin effect is greatly enhanced under low pressures and the negative power absorption is also obtained.     

地面放射性异常引起电离层扰动的计算研究

地面的放射性主要来源于空间辐射、地壳放射性元素衰变和人工核活动等,地面放射性可引起地表电场的变化,地表电场变化的区域和强度达到一定的条件后可引起电离层的扰动。基于LAIC电场机制假设,本文从地面放射性引起空气电离开始推导地表大气电导率变化、地面垂直电场至电离层底部传导过程,根据临界电场理论计算地面大气垂直电场、大气附加电流密度以及电离层准静态电场的电势分布,最后通过格林函数法求解得到电离层中水平电场的分布。建立了基于地面放射性活度的地表大气电导率公式,改进了地表异常电场传播到电离层的计算过程,给出了电离层电场扰动的计算公式。利用氡和地面电场仪的实例观测数据对地面放射性异常引起的电离层扰动的计算过程进行了验证,理论计算得到的地面电场和电离层扰动的结果与实际测量结果基本一致。     

Full-wave Analyses of Scattering of Electromagnetic Wave from the Weakly Ionized Plasma in Plane Geometry

The purpose of the present work is to present a full-wave analysis of scattering from the weakly ionized plasma in the plane geometry. We have yielded an approximate solution in an analytic form to the electromagnetic wave scattering from the weakly ionized plasma. In the normal and oblique incidence, the analytic solution works well, as compared with the exact solution and the solution based on the Wenzell-Kramers-Brillouin-Jeffreys (WKBJ) approximation to the uniform density profile.     

The Absorbing Properties of Two-Dimensional Plasma Photonic Crystals

Plasma photonic crystals composed of periodic plasma and dielectric materials have attracted considerable attention because of their tunable photonic band gaps,but only their band structures or negative refractive index properties have been addressed in previous works.In this paper,through studying the transmission and reflection characteristics of two types of twodimensional plasma photonic crystals,it is found that plasma photonic crystals play an important role in absorbing waves,and they show broader band and higher amplitude absorption characteristics than bulk plasmas.Also,the absorption of plasma photonic crystals can be tuned via plasma parameters;varying the collision frequency can make the bandwidth and amplitude tunable,but cannot change the central frequency,whereas varying the plasma frequency would control both the location and the amplitude of the absorbers.These features of plasma photonic crystals have potential for terahertz tunable absorber applications.     

Microwave method based on curve fitting method for high-precision collision frequency diagnosis

In this work, the results of plasma microwave transmission diagnosis were analyzed. According to the attenuation and phase shift of the electromagnetic wave propagating in the plasma, the electron density and collision frequency of the plasma can be diagnosed. Since part of the electromagnetic wave is reflected or diffracted when propagating in the plasma, and is not absorbed by the plasma, and this part of the attenuation is still included in the measured attenuation, the attenuation is distorted. Therefore, a curve fitting method is proposed to remove the attenuation caused by the plasma reflection, thereby improving the accuracy of the diagnosis of the collision frequency. The calibration effect of this method on plasmas with different electron densities and collision frequencies is analyzed, and a diagnostic frequency band with good calibration results is given. The curve fitting method is verified by experiment and simulation. After adopting the newly proposed method, the diagnosis accuracy of collision frequency can be increased by 30%. This method can be widely used in various types of plasma diagnosis and provides a new idea for plasma diagnosis.