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.     

T.W.T.-type radiation from a uniformly moving charge in an isotropic plasma environment

The characteristics of the travelling-wave-tube-type radiation produced by a point charge moving with uniform velocity along the axis of an infinite cylindrical cavity of free space in an otherwise unbounded, isotropic, cold and homogeneous plasma are investigated. For sufficiently small radius of the free-space column, the frequency of the emitted radiation is close to the plasma frequency, and the radiated energy is of significant strength and progresses in a direction opposite to that of the motion of the charge.     

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.     

Excitation of Plasma Waves in an Unbounded Homogeneous Plasma by a Line Source

The radiation characteristics of a line source of magnetic current embedded in a homogeneous electron plasma of infinite extent are investigated for the case in which a uniform magnetic field is impressed externally throughout the medium in the direction of the source. The single-fluid theory of magnetohydrodynamics is employed. A very simple model is assumed for the plasma. Under this assumption, it is found that there are two modes of propagation of waves of small amplitude. By examining the behavior of these modes in the limiting cases of vanishing external magnetic field or infinite source frequency, they are identifiable as the modified forms of the usual plasma and optical modes which exist in an isotropic electron plasma. The dispersion relations for these two modes are discussed. The power radiated in each of the two modes is also evaluated. It is found that the power radiated in the optical mode is always lower than that due to the line source in free space, whereas the power radiated in the plasma mode is higher than that value for certain ranges of the source frequency.     

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.     

等离子体圆柱波导慢电磁波的激励

本文在考虑等离子体厚度效应的情况上,详尽地推导了充填圆环状等离子体的圓柱波导中TM模慢电磁波的色散方程;利用相对论空间电荷波理论,导出了相对论电子注在轴向磁场引导下沿等离子体内表面传输时的空间电荷波方程;求得了电子注和慢空间电荷波相互作用的线性增益和频率漂移;讨论了等离子体厚度、密度对色散特性和互作用增益的影响。     

相对论电子注与等离子体柱互作用产生微波辐射的研究

本文利用线性理论，对相对论电子注与等离子体柱互作用产生微波辐射进行了研究，且考虑了纵向均匀磁场的作用。     

Radiation from an aperture in a conducting cylinder coated with a moving plasma sheath

The electromagnetic radiation from an aperture on a conducting cylinder coated with a moving isotropic plasma sheath is considered. Numerical results are presented to illustrate the radiation patterns as functions of sheath velocity and plasma frequency for the circumferential slot and axial slot apertures. It is found for the circumferential slot aperture that the radiation is enhanced in the direction of sheath motion when the plasma is overdense and that relatively little change occurs when the sheath is underdense. For the axial slot, it is found that an electromagnetic field is radiated whose polarization is normal to that of the field radiated under stationary conditions, in addition to a field of the usual polarization. Significant alterations of radiation patterns from their form when the sheath is stationary can occur at relatively small velocities if the wave frequency is near the plasma frequency.     

Linear theory of an electron cyclotron maser operating at the fundamental

The linear theory of an electron cyclotron maser (ECM) operating at the fundamental is developed. A set of analytic expressions, valid for all TE cavity modes, is derived for the starting current and frequency detuning using the Vlasov-Maxwell equations in the weakly relativistic limit. These results are applicable for an arbitrary electron velocity distribution as well as any longitudinal distribution of the RF field. It is shown that the starting current can be expressed in a simple form which contains the Fourier trans-form of the longitudinal field distribution. Analytic results are presented for specific longitudinal field variations, including uniform, sinusoidal, and Gaussian. It is found that the starting characteristics of an ECM are strongly influenced by the axial dependence of the RF field, but weakly affected by the velocity spread of the electron beam. The problem of multimode oscillation is treated in the linear theory by using a Slater expansion of the cavity field. The complete formulation for mode competition based on this expansion is presented and preliminary results are derived. This comprehensive analysis of ECM linear theory should be useful as a diagnostic of ECM performance and should facilitate comparison between theory and experiment.     

磁化环形等离子体填充波导中注-波互作用的研究

该文利用线性理论,对相对论环形电子注与环形等离子体互作用产生微波辐射进行了研究。在考虑有限磁场的基础上,利用匹配场法求得色散方程。重点分析了等离子体密度、厚度,外加磁场对等离子体切伦可夫脉塞注-波互作用的影响,并计算了各参量变化时的波增长率以及色散关系。     

相对论电子在周期磁场中的辐射频谱

分析了相对论电子在空间周期磁场中的运动轨迹.在一定条件下,电子在垂直原始入射方向上的运动是简谐的,利用谐振子模型求出了它的辐射频谱和频宽;最后讨论了运动的非谐性.得到辐射频谱分布将是与电子的初速度、磁场强度有关.     

A General Reciprocity Theorem

A general reciprocity theorem based on the Onsager relations is developed which applies to all casual and linear media, including those whose ac susceptibilities depend on an applied dc magnetic field and on the dc drift velocity of charge carriers. Applications are made to the scattering matrix for microwave junctions and to the mode orthogonality relations for uniform and periodic waveguides.     

Prolate spheroidal antennas in isotropic plasma media

The problems of a gap-excited finite-sized prolate spheroidal antenna, with and without a confocal prolate spheroidal vacuum sheath, operated in a uniform cold and in a uniform warm lossy plasma medium, have been considered by applying boundary value problem techniques. The dependence of the input admittance, the current distribution, and the radiation pattern, upon the collision frequency, the electron temperature, the length of the antenna, the length-to-radius ratio for the antenna, the ratio of the plasma frequency to the operating frequency, and the thickness of a vacuum sheath have been investigated. Admittance curves are given from which the input admittance of a finite linear antenna can be estimated for a wide range of operating conditions and length-to-radius ratios. Conductance and susceptance maxima, dependent upon the vacuum sheath thickness, have been observed when using the cold plasma model foromega < omega_{p}. When using the warm plasma model in conjunction with the boundary condition that the normal component of the fluid velocity is zero at the antenna surface and at the vacuum sheath surface, the radiation resistance of the antenna is increased significantly for the parameters considered.     

Electromagnetic Potentials and Field Expansions for Plasma Radiation in Waveguides

In order to calculate the raditaion from plasmas placed in waveguides it is necessary to know the field produced by arbitrarily moving charged particles in a waveguide. In this paper modal expansions for the vector and scalar potentials due to arbitrarily moving charged particles in a waveguide are derived and provide the extension of the Lienard-Wiechert potentials to a waveguide environment. In addition, for a plasma filled waveguide, a modal expansion is given of the electric field directly in terms of mode coupling with the charge motion. Expressions for the spectral distribution of the radiation are given, both in general and for cyclotron radiation. Some specific results for the H/sub 10/ mode excited in a rectangular guide by cyclotron motion are also presented.     

Microwave emission from n type InSb enhanced by Doppler-shifted acoustic cyclotron resonance

The microwave radiation emitted at 77 K from n InSb samples cut along the [110] direction, and subjected to a transverse magnetic field, showed maxima due to anticyclotron damping when the Doppler-shifted acoustic frequency was equal to the harmonics of the cyclotron frequency of the electrons.     

Conditions for mode change of a rigid cavity

Initial conditions for the existence of an earlier observed change in the mode of radiation of a nonuniformly moving linear cavity with fixed contents are considered. It is shown that the imaginary phase of radiation is independent of the homogeneous medium that fills an accelerated cavity with a fixed phase structure. From the law of conservation of energy in a uniform motion, it is obtained in the operator form that the radiation phase is invariant to the constant velocity of motion of a rigid cavity. The necessary and sufficient conditions for the existence of nontrivial solutions of the equation of state of radiation are discussed. The synchronism condition for the interfering branches of radiation by a nonuniformly moving source is found. It is established that, for two fields with complex phases, the interference term exceeds eight-fold the intensity of the initial beam.     

Observation of Doppler-shifted acoustic cyclotron resonance in the frequency spectrum of the microwave emission from ntype InSb

The frequency spectra between 1.8 and 3GHz of the microwave radiation emitted from ntype InSb samples orientated along the [110] direction showed maxima in a transverse magnetic field, which can be attributed to Doppler-shifted acoustic cyclotron resonance.     

自由电子激光辐射和迴旋辐射的比较研究

在自由电子激光器中一般存在两种受激辐射形式:自由电子激光辐射和迴旋辐射。本文从理论上阐明,如果电子束的初始横向速度较大,自由电子激光器的参数选用不当,则迴旋辐射将占优势,特别当电子束能量较低时,产生迴旋辐射的倾向更大。但是迴旋辐射并不具有双重多普勒频率上漂移的特性,由于受轴向磁场强度的限制,在电子束能量较高时,其辐射频率要比自由电子激光辐射频率低得多。因此在自由电子激光器的实验研究中,如何区别这两种辐射,并有效地抑制迴旋辐射,是一个十分重要的课题。     

Pulse radiation from an insulated antenna: an analog of Cherenkovradiation from a moving charged particle

Cherenkov radiation arises when a charged particle moves with a constant velocity that is greater than the speed of light in the surrounding medium. This radiation has distinctive characteristics. Near the charge, the electric field is most intense along a conical surface with apex at the charge-the Mach cone. In the far field, the radiation occurs predominantly in one direction-at the Cherenkov angle. An insulated antenna consists of a metallic cylindrical conductor covered by a concentric sheath of dielectric. In use, this antenna is embedded in a medium whose permittivity is often much greater than the permittivity of the insulation. When the antenna is excited by a pulse of voltage, a pulse of charge appears to travel along its length. The apparent velocity of this charge is close to the speed of light in the insulation, which, because of the difference in the permittivities, is greater than the speed of light in the surrounding medium. Thus, the radiation from the pulse excited, insulated antenna should be analogous to Cherenkov radiation from the moving charged particle. In this paper, the pulse-excited, traveling-wave insulated linear antenna is accurately analyzed using the finite-difference time-domain (FDTD) method. Results are obtained for the charge on the conductor, the near field, and the far field. These results show the striking similarity of the radiation from this antenna to Cherenkov radiation from the moving charge     

Streaming plasma instability of photoexcited electrons in semiconductors

It is shown that in semiconductors, in crossed steady electric and magnetic fields, at low lattice temperatures, the streaming plasma instability of photoelectrons is possible. The instability is studied in linear and nonlinear regimes. The electric field and electron mean velocity oscillations with frequency proportional to recombination rate are obtained.