Radiation propagating transverse to the external magnetic field from an electromagnetic source in an unbounded plasma

The radiation characteristics of a line source of magnetic current embedded in an unbounded plasma are investigated for the case in which a uniform magnetic field is impressed externally throughout the medium in the direction of the source. The plasma is assumed to be a homogeneous and macroscopically neutral mixture of compressible gas of electrons and ions. A two-fluid continuum theory of plasma dynamics is employed. It is shown that it is possible to define three suitable wave functions which satisfy separately simple wave equations whose solutions are written down by inspection. These wave functions specify the three possible modes which are identified, respectively, to be the modified forms of the electromagnetic, the electron plasma and the ion plasma modes. The limiting behavior of these modes are discussed for the following two cases: 1) infinite source frequency and 2) vanishing external magnetic field. The dispersion relations for the three modes are examined in detail for the general case using a perturbation procedure. It is shown that the modified ion plasma (MIP) mode propagates for all frequencies whereas both the modified electron plasma (MEP) mode and the modified electromagnetic (MEM) mode have a low-frequency cutoff. Explicit expressions for the cutoff frequencies are obtained. The power radiated in each of the three modes is also evaluated. It is found that the power radiated in the MEM mode is always lower than that due to the line source in free space, whereas the power radiated in the two plasma modes is higher than that value for certain ranges of frequency.     

Radiation from a uniformly moving distribution of electric charge in an anisotropic, compressible plasma

The radiation characteristics of a linear distribution of electric charge moving with a uniform velocity in a homogeneous electron plasma of infinite extent are investigated for the case in which a uniform static magnetic field is impressed externally throughout the medium. The linear distribution of charge and its direction of motion are assumed to be parallel and perpendicular, respectively, to the direction of the external magnetic field. Of the two possible modes of waves of small amplitude, namely, the modified electromagnetic mode and the modified electron plasma mode, the uniformly moving charge distribution excites the modified electron plasma mode. The emitted radiation has no frequencies less than the plasma frequency. For a particular value of the ratio of the gyrotropic to the plasma frequency of the electrons, the frequency and the angular spectrum of the emitted radiation are determined for two values of the velocity of the charge.     

The effects of electron collisions on electric dipole radiation through a conical plasma sheath

The radiated power of an electric dipole antenna in a thin, dissipative, infinite conical plasma sheath is analyzed. The electromagnetic fields are represented by contour integrals, and the radiated power is calculated according to the method of Baños et al. The sheath is characterized by a single parameter, which is proportional to the integrated electron density and depends inversely on the electron collision frequency. The radiation pattern is distorted from the form of the free-space dipole pattern because of the interaction between the primary dipole fields and the fields induced on the surface of the plasma sheath. In most cases it is found that for small electron densities, electron collisions attenuate the induced surface fields so that the patterns approach their free-space form. For larger electron densities, the pattern has a characteristic heart shape for zero collision frequency. The pattern increases in magnitude, while retaining the same form, as the collision frequency increases from zero. As the collision frequency is further increased, the pattern approaches the free-space pattern in magnitude and form.     

Radiation from electromagnetic sources in a plasma

The radiation from electromagnetic sources in an unbounded, isotropic plasma is treated. Using a two-fluid magnetohydrodynamic approach, the problem is formulated in terms of three orthogonal modes, namely, i) the electromagnetic mode, ii) the electron plasma mode and iii) the ion plasma mode. When formulated in this manner, the radiation from simple electromagnetic sources are obtained easily. Three specific sources are treated, namely, i) a point source of electric current, ii) a uniformly moving charge and iii) a short filament with prescribed current distribution. For the case of the electric dipole and the current filament, the total power radiated in each of the three modes are obtained. A point charge moving uniformly with the velocity of sound, for example, is found to excite only the ion plasma mode. The frequency and the angular spectrum of the emitted radiation are obtained. It is found that at frequencies less than a critical frequency which is approximately equal tosqrt{2}times the ion plasma frequency, the radiation has the character of a simple sound wave.     

A Helicon Plasma Source

The parameters of a helicon plasma source was studied for an rf power to 500 W and a magnetic field ranging from 0 to 200 G under an argon pressure of 3 mtorr. The electron density in the plasma was found to reach 10¹² cm^–3. Axial and radial electron distributions were studied. Helicon waves were excited by antennas of three types, a loop antenna being the most efficient. It is shown that the electron density in the plasma can significantly be increased if the system is under a nonuniform magnetic field such that its lower-value (relative to the rest of the system) part is applied to the antenna.     

Radiation From a Uniformly Moving Charge in an Anisotropic Plasma

The radiation from a point charge moving uniformly in a plasma is investigated when the charge is moving in the direction of an external magnetic field. In general there are two modes, for each of which all the components of the electric and magnetic field are present. The two parameters of interest in this problem are the ratio u/c/sub 0/ of the velocity of the charges to the free-space velocity of electromagnetic waves and the ratio R of the gyromagnetic frequency to the plasma frequency of the electrons. For two sets of values of these parameters the frequency and the angular spectrum of the emitted radiation are obtained. In certain cases, as many as three Cerenkov rays are found to propagate in the same direction; these multiple rays, however, correspond to different frequency components and to different modes of propagation. The motivation for this investigation is indicated briefly.     

Study of Disk-Loaded Waveguide Filled with Plasma

Recently disk-loaded waveguide has been widely used in high-power traveling wave tubes (HPTWT). It has been shown in many experiments that the interaction efficiency and output power can be increased greatly by injecting plasma into the vacuum microwave device. Using field analysis, the dispersion characteristics and the interaction impedance of the disk-loaded waveguide filled with plasma are analyzed in a strong longitudinal magnetic field. In conclusion, the frequency of the TM₀₁ mode increases as the density of the plasma increases. It also can be found when the plasma density increases to a large scale, the TM₀₁ mode of the disk-loaded waveguide overlaps the TG mode, these two modes couple each other and form the new hybrid modes.     

Helicon Plasma Source

A spiral-antenna helicon plasma source is characterized experimentally at applied magnetic flux densities below 60 G and applied RF powers below 800 W. It is shown that, for a given RF power, a magnetostatic field applied to an inductively coupled plasma increases the electron concentration fourfold.     

Some electromagnetic aspects of coreless PCB transformers

In this paper, some EMI aspects of using coreless PCB transformers are addressed. Based on the antennas theory, the radiated power of a coreless PCB transformer is estimated and found to be negligible. The electromagnetic field plot of a power electronic circuit using a gate drive circuit isolated by a coreless PCB transformer has been recorded. The major radiated EMI source in the frequency range of 30 MHz to 300 MHz is found to be the copper tracks of the power circuit, where switching transients occur, rather than the coreless PCB transformer. Coreless PCB transformers essentially operate at relatively low frequency (8 MHz in this case) by near-field magnetic coupling. Experimental results have confirmed that the application of coreless PCB transformer in gate drive circuit will not impose any serious EMI problem on the power electronic circuit     

The effects of a moving anisotropic grounded layer on radiation from a line source

The radiation from a magnetic or an electric current line source embedded in an uniaxially anisotropic grounded dielectric or plasma layer moving perpendicular to the line source is described. The radiation pattern is obtained, and the effects of the motion of the layer on the radiated power distribution are examined with several numerical examples.     

High-gain plasma Cerenkov maser

The linearized fluid and Maxwell's equations are used to calculate the dispersion relation for the transverse magnetic modes of a partially filled, plasma-lined, cylindrical waveguide interacting with a relativistic electron beam. Both the plasma liner and the electron beam are assumed to be cold, and the system is immersed in an infinite axial magnetic guide field. The dispersion relation is then used to calculate the growth rate for the instability between the slow-wave modes (Trivelpiece-Gould modes) of the plasma guide and the slow-space charge mode of the electron beam. The calculation is done in the high-gain, strong coupling limit     

Leaky waves supported by uniaxial plasma layers

Each discrete complex wave supported by a uniaxial layer corresponds to an isolated singularity in an appropriate Green's function. The location of a singularity depends on the geometry, plasma density, and the direction of the magnetic field, but not on the source. We have derived the discrete solutions for an infinite magnetic field that is parallel or perpendicular to the slab interfaces. For a parallel field, analytic expressions are obtained that yield the exact locations of the complex pole Ioci. Although similar expressions cannot be obtained if the field is perpendicular, the resonance relation which locates the poles can be solved by methods developed for an isotropic plasma. We find that improper modes and surface waves can be supported in the case of a parallel field, while a perpendicular field permits spectral complex modes as well. These results are applied to a Kirchhoff-Huygens integration of the leaky-wave distribution excited by a magnetic line source or an annular slot. A close correlation is shown between the radiation pattern derived by this procedure and the exact pattern calculated by the method of steepest descent. In particular, it is shown that the appearance of a peak in the pattern can be analytically correlated with the contributing pole, provided it corresponds to a dominant leaky wave.     

Effect of substrate on the efficiency of an arbitrarily shaped microstrip patch antenna

A general approach is presented for the determination of power radiated via the space wave and surface wave from the aperture of an arbitrarily shaped microstrip antenna. The magnetic current model is used for this, and the analysis is carried out in the Fourier domain to determine the effect of the substrate. It has been shown that, in the Fourier domain, the longitudinal components of electric and magnetic displacement vectors follow the transmission line equation and can be solved by inspection. An expression for total radiated power has been derived. The singularities of the integral for power radiation indicate the presence of surface wave modes, and the associated power has been obtained using a singularity extraction technique. The effect of the substrate on space wave power has also been determined. This theory has been applied to a rectangular patch antenna. The results are in conformity with those reported in the literature. It has been found that for the frequency range (h/lambda, < 0.02), the effect of dielectric substrate can be neglected.     

均匀非磁化等离子体与雷达波的相互作用

采用平板几何金属衬底模型对雷达波在均匀等离子体中传播所发生的吸收、反射和衰减进行了数值分析研究,结果表明:等离子体对电磁波的吸收衰减取决于等离子体密度和碰撞频率的共同作用;通过适当选择等离子体密度和等离子体碰撞频率,可以使均匀等离子体对某一雷达波段的吸收达到90%以上,隐身效果显著。     

Effects of leaky-wave propagation in metamaterial grounded slabs excited by a dipole source

In this paper, dispersive propagation and radiation properties of leaky waves on metamaterial grounded slabs are investigated. The proper or improper nature of leaky modes supported by such structures is shown to be related to the metamaterial being /spl epsi/-negative, /spl mu/-negative, or double-negative, and to field polarization, giving rise to backward or forward radiation depending on the frequency range of operation. These spectral features and the associated frequency scan of the radiated beam are illustrated by considering the field excited by a dipole source in the presence of an infinite metamaterial grounded slab. The possibility to achieve nearly equal values for the phase constants of a TE and a TM leaky mode on a large frequency range is shown; this allows us to obtain a conical radiation pattern and, also, for suitable values of the attenuation constants, the radiation of a pencil beam at broadside. Conditions for achieving maximum power density at broadside are derived, when one constitutive parameter is much smaller than the other. In order to illustrate these novel features, numerical results based on experimentally tested dispersion models for permittivity and permeability of the metamaterial media are provided, concerning leaky-wave modal properties and near and far fields excited by a dipole source.     

表面波放电狭缝天线辐射电磁场的三维数值分析

针对平板型表面波放电等离子体源,建立了表面波放电狭缝天线辐射电磁波模型,对狭缝天线辐射电磁场分布进行了三维数值计算,并与表面波电磁场进行对比分析,讨论了平板型表面波放电机理。结果表明:整个狭缝天线阵激发的电磁场是每个狭缝天线激发电磁场的线性叠加;狭缝天线阵直接激发的电磁场强度在临近波导壁面处很大,并且随着空间距离的增大迅速衰减;狭缝天线阵直接激发和表面波的电场均远大于各自的磁场,分析电、磁场对带电粒子的力作用时可以忽略磁场力的作用;表面波电磁场远大于狭缝天线阵直接辐射的电磁场,强电磁场范围也远大于狭缝天线阵直接激发的强电磁场范围,等离子体有增强电磁场强度、扩大强电磁场范围的作用。     

Excitation of surface waves on a unidirectionally conducting screen by a phased line source

The excitation of surface waves on a unidirectionally conducting screen produced by a phased line source located above the screen and perpendicular to the wire elements is considered. The screen consists of an infinite number of straight, perfectly conducting, parallel wires and conducts only in the direction of the wire elements. The phased line source consists of a periodic line current with an electric charge distributed along its length. The complete electromagnetic field is determined exactly and simple expressions are given for the scattered far field. It is shown that surface waves exist and simple expressions for the amplitudes are given. Another principal result is the determination of the magnitude of the complex Poynting vector for the radiated power. It is found that the pattern function lies on a cone independent of the presence of the screen and that the cone angle depends only on the phasing of the source. The pattern function at points below the screen is independent of the location of the source above the screen. Furthermore, the pattern function vanishes in the direction of the screen and this seems concomittant to the existence of surface waves. Two pattern functions are drawn for typical cases of interest. The power propagated by the surface waves is also determined. The method employed to solve the problem is based on the deduction that the scattered magnetic field component in the direction of the wire elements is zero. A consequence of this deduction is that the electromagnetic field can be derived from a single scalar wave function that satisfies a partial differential equation in the plane of the screen and a jump condition across the screen. This method is quite general and can be applied to a large class of interesting propagation problems arising from different types of excitation. The scattered far field is obtained using another method that is algebraic in character and does not require a complete solution of the problem.     

Radiation from an electromagnetic source in a half-space of compressible plasma-surface waves

The radiation characteristics of a line source of magnetic current are studied for the case in which the source is situated in a half-space of isotropic, compressible plasma which is bounded on one side by a perfectly conducting, rigid plane screen. In addition to the electromagnetic and plasma space waves, the line source excites a boundary wave. This boundary wave is a coupled wave. It has associated with it both a magnetic field component and the pressure term. This is in contrast to the space waves which can be decomposed into an electromagnetic (EM) mode with no pressure term and a plasma (P) mode with no magnetic field associated with it. The characteristics of this boundary wave are evaluated. The boundary wave propagates for all frequencies and the power carried by the boundary wave becomes smaller as the frequency is increased.     

Field analysis of rectangular waveguide open junction

The problem of a rectangular waveguide open junction is investigated using field theory and the relevant model of two normally intersected, infinite parallel-planes waveguides. Evanescent waveguide modes generated by waveguide wall edges and/or the discontinuity in dielectric are taken into account; an infinite set of equations is derived, where the mode coupling is given by the dielectric slab modes. Proper pole handling is discussed, and a solution for the system is given. Expressions are derived for the reflected, transmitted, and radiated power, which are shown to be sufficiently reliable in the domain of practical interest, regarding the width and the dielectric loading of the gap. The analysis shows that a substantial fraction of the microwave power leaks from the dielectric gap, confirming the absolute necessity of using a choke-flange at the waveguide junction     

Radiation from a uniaxially anisotropic plasma half space

A line source of magnetic currents is located in a half space with uniaxially anisotropic dielectric constant, with the optic axis inclined at an arbitrary angle to the interface. This two-dimensional, scalar diffraction problem is solved by Fourier integral techniques, and the solution is examined in the asymptotic range of large distance from the source. The radiation field comprises incident, reflected and refracted ray contributions whose properties are determined from a study of the refractive index curves for the medium. In addition, there may exist a lateral wave which is excited by a critically refracted incident ray, and which sheds energy back into the anisotropic medium by refraction. If the medium is a plasma with an externally impressed infinite dc magnetic field, and if the source oscillates below the plasma frequency, there exist shadow regions from which the direct and reflected rays are excluded but which can be penetrated by the lateral waves.