外加磁场微波等离子体喷流对平面电磁波衰减的实验研究

采用空间透射波测量方法,实验研究透波密闭石英玻璃容器内等离子体喷流对垂直和水平极化电磁波的衰减,在有和无外加磁场条件下分析实验参数对等离子吸波效应的影响,分析等离子体的吸波机理.实验结果表明在非磁和本实验条件下,平面电磁波在等离子体中的衰减机理为碰撞吸收;在有磁和本实验条件下,平面电磁波在磁等离子体中的衰减机理同样为碰撞吸收,但是外加磁场的存在有限地改善了等离子体的吸波效应.为了使磁等离子体最有效地吸收电磁波,应该提高磁场感应强度,把高频混杂频率提高到测试微波频率范围内,或降低微波测试频率至本实验中的高混 关键词： 等离子体相互作用 电磁波 电磁波在等离子体中的传输     

Numerical analysis of the reflection and absorption of electromagnetic wave in nonuniform magnetized plasma slab

In this paper, a model for calculating the reflection and absorption powers of electromagnetic wave (EM wave) in nonuniform magnetized plasma slab is given out based on layer propagation theory. The effects of various plasma parameters and different values of magnetic field intensity on the reflected and absorbed powers are discussed. The results illustrate that the thickness of plasma seldom affects the reflection of radar wave, but it can broaden or reduce the absorption width. Meanwhile, the background magnetic field intensity has an influence upon the results, and it could change the resonance spectrum of magnetized plasma. We also find out that, with appropriate plasma density, collision frequency and magnetic field intensity, more than 90% of radar wave power can be absorbed and the resonant absorption band is about 2 GHz.     

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。     

Eigen electromagnetic waves of a coaxial waveguiding structure filled by a non‐uniform dissipative plasma with azimuthal magnetic field

This paper studies the propagation and spatial attenuation of high‐frequency eigen‐symmetric and dipolar electromagnetic waves along a coaxial plasma–metal waveguiding structure that contains a slightly axial and strong radial non‐uniform cylindrical plasma slab in an external azimuthal non‐uniform magnetic field. The influence of such parameters as the effective electron collision frequency, the direct current value producing the external azimuthal magnetic field, parameters that characterize plasma density radial profile, and waveguide geometric parameters on the dispersion, spatial attenuation, and radial field structure of the waves is considered. The regions of waveguiding structure parameters where the electromagnetic wave properties can be effectively controlled are studied and analyzed.     

飞秒光丝阵列对10 GHz电磁波的吸收特性

为了研究飞秒光丝阵列对10 GHz电磁波的吸收特性,建立了飞秒光丝阵列吸收电磁波的有限元模型,研究了光丝内电子温度、电子数密度、光丝直径和电磁波的极化等参数对吸收系数的影响。研究结果表明:当电磁波偏振方向与光丝轴向垂直时,阵列对电磁波是透明的;增加光丝内电子数密度或提高电子温度,吸收系数先增大后减小;当光丝直径与电磁波趋肤深度相等时,吸收系数达到最大值。对于S极化电磁波,当光丝直径为50 μm时,吸收系数随入射角的增大而变大;当光丝直径为100~200 μm时,在入射角较小时,吸收系数随入射角的增大而变大;在入射角较大时会出现吸收峰值,最高可达0.45,且光丝直径越大,吸收峰值对应的入射角就越小;对于P极化电磁波,吸收系数随入射角增大而降低。     

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.     

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。     

Propagation of terahertz waves in nonuniform plasma slab under "electromagnetic window"

The application of magnetic fields, electric fields, and the increase of the electromagnetic wave frequency are up-and-coming solutions for the blackout problem. Therefore, this study considers the influence of the external magnetic field on the electron flow and the effect of the external electric field on the electron density distribution, and uses the scattering matrix method (SMM) to perform theoretical calculations and analyze the transmission behavior of terahertz waves under different electron densities, magnetic field distributions, and collision frequencies. The results show that the external magnetic field can improve the transmission of terahertz waves at the low-frequency end. Magnetizing the plasma from the direction perpendicular to the incident path can optimize the right-hand polarized wave transmission. The external electric field can increase the transmittance to some extent, and the increase of the collision frequency can suppress the right-hand polarized wave cyclotron resonance caused by the external magnetic field. By adjusting these parameters, it is expected to alleviate the blackout phenomenon to a certain extent.     

Axial structure of plasma column sustained by non‐symmetric electromagnetic surface wave

This article presents the results of theoretical study of the axial structure of gas discharge that is sustained by a non‐symmetric electromagnetic surface wave (SW) with azimuth wavenumber m = –2. The phase and attenuation characteristics of the wave, and the spatial wave field structure were studied as well. The wave considered propagates along the magnetized slightly nonuniform in axial direction plasma column and sustains it. The influence of external magnetic field value and finite plasma column radius on the discharge stability, on the plasma density axial profile and on the SW properties has been studied in the case of the diffusion regime of gas discharge sustaining.     

磁场对螺旋波等离子体波和能量吸收影响的数值研究

本文考察在径向电子数密度呈抛物形分布的情况下,外加稳恒磁场,射频通过螺旋波天线在等离子体中激发电磁波的传播性质.采用线性扰动波假设,数值求解Maxwell方程组,得到80—800 G(1 G=10-4T)磁场条件下等离子体中径向电、磁场强度及能量沉积密度的分布情形.计算结果表明,磁场增大(80→800G)时,螺旋波受到的阻尼较小,可深入等离子体传播;Trivelpiece-Gould(TG)波受到的阻尼增大,在等离子体-真空边界处衰减增强;整体的能量吸收向边界集中.磁感应强度小于100 G时,TG波可深入主等离子体区传播,等离子体径向能量吸收相对均匀.     

Electromagnetic Reflection from Conductive Plate Coated with Nonuniform Plasma

Based on the theories of electromagnetic (EM) field and generalized reflection coefficient, a simple method is put forward to deal with the EM reflection coefficient of conductive plate coated with nonuniform plasma. The plasma slab is modeled by a series of subslabs, in which the electron-number density is assumed to be constant. The overall number density profile across the whole slab follows any practical distribution function. The reflection coefficient is then deduced and its functional dependence on the number density, incident wave frequency, collision frequency, and background magnetic field is discussed.     

Low-threshold parametric decay instabilities in the experiments on the electron cyclotron resonance heating in tokamaks and stellarators

The experimental conditions have been analyzed for a significant reduction of the threshold of the reflective parametric decay instabilities under electron cyclotron resonance heating (ECRH) of a plasma in magnetic devices in the absence of the upper hybrid resonance for the pump wave. The role of the nonmonotonic profile of the plasma density near the O point of the magnetic island, which allows for the localization of ion Bernstein waves in the direction of the density gradient and the suppression of convective losses from the decay region has been discussed. It has been shown that the threshold of the instability of the induced backscattering near the local maximum of the density profile is decreased by four orders of magnitude and is easily exceeded in present-day ECRH experiments at a power of several hundred kilowatts.     

Polarization Equation for a High-Frequency EM Wave Propagating through a Cold Plasma

An equation is derived for the polarization of a high-frequency EM wave propagating through a cold plasma. The polarization is determined in a coordinate system fixed to the wave-normal. It is assumed that the plasma density varies slowly in only one direction and that the wave-normal and the external magnetic field B? make arbitrary angles with that direction.     

Interaction of electromagnetic waves with a magnetized nonuniform plasma slab

The absorption, reflection, and transmission of electromagnetic waves by a nonuniform plasma slab immersed in an ambient uniform magnetic field of various strengths are studied in this paper. The effects of the plasma parameters and magnetic field strength on the absorbed, reflected, and transmitted power are discussed. The magnetized nonuniform plasma slab is modeled by a series of magnetized uniform plasma subslabs. The calculation results show that the effects of the magnetic field strength and density gradient on the absorbed power, as well as the frequency band of resonant absorption, are significant. A complete analysis utilizing the scattering matrix method is also used to compare the above calculation results which neglect multiple reflections between subslab interfaces. Broadband absorption of electromagnetic waves can be achieved by changing the magnetic field strength and plasma density. More than 90% of the electromagnetic wave power can be absorbed in a magnetized nonuniform plasma slab with width of 12 cm and the absorption bandwidth can range from 1 to 20 GHz with different plasma parameters and external magnetic field strengths.     

Parametric Mechanism of Anomalous Microwave Power Absorption in Experiments on Electron Cyclotron Heating in Toroidal Magnetic Traps

The nonlinear stage of the parametric decay instability of an extraordinary wave is analyzed in the presence of a nonmonotonic density profile. The decay excites an electron Bernstein wave, which is localized in the vicinity of a local density maximum, and an ion Bernstein wave, which leaves a nonlinear interaction region and is absorbed by ions in the vicinity of the harmonics of the ion cyclotron frequency. The main mechanism of instability saturation is considered to be a cascade of decays of a primary daughter electron Bernstein wave, which leads to the excitation of localized secondary electron Bernstein waves and ion cyclotron (Bernstein) waves. The localization of electron Bernstein waves causes a significant decrease in the secondary- decay excitation threshold, which is thought to provide saturation of the primary instability at the lowest level. The saturation of the primary parametric decay instability of a pump wave and the anomalous absorption of the pump power are analytically estimated. A numerical simulation is performed using the parameters that are typical of the experiments on the electron cyclotron resonance heating of plasma at the second resonance harmonic in TCV tokamak.     

磁化非均匀等离子体中模转换机制对电磁波吸收的数值研究

用数值方法研究磁化非均匀等离子体(磁场与密度梯度垂直)中模转换机制对p极化电磁波的吸收效应,重点考虑磁场对吸收率的影响,结果表明:与非磁化等离子体相比,磁场的加入使得吸收率曲线相对入射角的分布发生小的偏移,吸收率为零的位置不是发生在零度入射(即垂直入射)时.而是在一个负的入射角位置,垂直入射时有不为零的吸收率,且正角度范围与负角度范围的峰值吸收率不相等,磁场提高了正入射角范围内的峰值吸收率而降低了负角度范围内的峰值吸收率(或反过来,这取决于磁场方向).     

Enhancement of electron density in the interaction of high-power electromagnetic waves with inhomogeneous magnetized plasma

Propagation characteristics of a high-power electromagnetic wave through an inhomogeneous magnetized plasma is investigated. Considering the momentum transfer equations for electrons and ions and taking into account the ponderomotive force, the distribution of electron density and dielectric permittivity are obtained. Using non-linear dielectric permittivity and Maxwell's equations in the absence of external current and charge densities, non-linear wave equations are achieved. The results indicate that the external static magnetic field can modify the profiles of both the electric and magnetic fields. It is also shown that the external static magnetic field enhances the amplitude of the electron density and the non-linear dielectric permittivity.     

垂直入射电磁波在大气压非平衡等离子体层中的传播特性

用数值模拟的方法对大气压非平衡等离子体薄层中，不同的电子密度分布对微波反射、吸收和透射的影响进行了研究。所采用的理论分析方法是分层模型和镶嵌不变原理。计算中考虑了微波在子层间的多次反射和吸收。数值结果表明，对于电磁波的吸收来说，等离子体中具有二次分布的电子密度，其效果要高于线性分布10%左右；而对于反射来说，线性分布效率更高。功率反射系数随波长的增大而增大，功率吸收系数A也不是单调的，当电子密度不变时，A存在一个峰值，随着电磁波波长的增加而增加，达到最大值后，缓慢降低。     

On a new mechanism of excitation of the absolute parametric decay instability of an electromagnetic wave in experiments on electron cyclotron resonance heating in toroidal devices

Experimental conditions under which the low-threshold absolute parametric decay instability of an electromagnetic wave with extraordinary polarization at the electron cyclotron resonance heating of a plasma at the second harmonic resonance in toroidal devices are analyzed. A new mechanism is proposed for the localization of a daughter electrostatic wave in the toroidal direction in the region of a high-power pump beam. This mechanism, along with the two-dimensional localization of the daughter wave because of a nonmonotonic radial profile of the plasma density and the poloidal inhomogeneity of the magnetic field, can be responsible for the parametric excitation of a three-dimensional cavity for this wave and, as a result, low-threshold absolute decay instability of the pump wave.