电子脉冲在飞秒电子衍射系统中的传输特性

优化设计了一套超快电子衍射系统.用轨迹追踪法讨论了光电子在阴极上有150 fs的时间弥散量，及其静态、动态特性(电子束斑、时间弥散、方位角、倾角)在超快电子衍射系统中的传输情况.用Monte Carlo方法对光电子的初始状态进行抽样、用有限差分法计算二维、三维空间电场分布，用有限元法计算磁场分布.     

量子等离子体中波的色散关系以及朗道阻尼

利用动理学理论研究量子等离子体中波的色散关系和电子朗道阻尼.从电子的量子流体动力学方程和动理学描述下的光子运动方程出发,研究量子效应对光子朗道阻尼的修正.研究发现量子效应只对纵波模式,即电子等离子体波的色散关系有修正,对横向电磁波的色散关系没有影响.量子效应减小了朗道阻尼,起着朗道增长的作用. 关键词： 量子等离子体 朗道阻尼 电子等离子体波 色散关系     

声子色散对量子点中弱耦合磁极化子声子平均数的影响

利用线性组合算符和幺正变换相结合的方法,研究了声子色散对抛物量子点中弱耦合磁极化子电子周围光学声子平均数的影响。计及纵光学（ LO）声子色散,在抛物近似下导出了基态能量与量子点有效受限长度、声子色散系数、回旋共振频率以及电子－声子耦合常数之间的关系,电子周围光学声子平均数与声子色散系数以及电子－声子耦合常数的关系。数值计算结果表明在弱耦合情况下抛物量子点中磁极化子的基态能量随声子色散系数的增大而减小;电子周围光学声子平均数随声子色散系数增大而增大,随电子－声子耦合常数的增大而增大。     

声子色散对量子点中弱耦合磁极化子声子平均数的影响

利用线性组合算符和幺正变换相结合的方法,研究了声子色散对抛物量子点中弱耦合磁极化子电子周围光学声子平均数的影响．计及纵光学(LO)声子色散,在抛物近似下导出了基态能量与量子点有效受限长度、声子色散系数、回旋共振频率以及电子-声子耦合常数之间的关系,电子周围光学声子平均数与声子色散系数以及电子-声子耦合常数的关系．数值计算结果表明在弱耦合情况下抛物量子点中磁极化子的基态能量随声子色散系数的增大而减小;电子周围光学声子平均数随声子色散系数增大而增大,随电子-声子耦合常数的增大而增大．     

Electromagnetic renormalization of the plasmon spectrum in a laterally screened two-dimensional electron system

It is demonstrated theoretically that the effects of lateral electromagnetic screening of plasmons in a two-dimensional electron system by side metallic electrodes modify the plasmon spectrum. The effects of lateral screening become more pronounced at larger values of the plasmon wave vector. This leads to the deviation of the dispersion of laterally screened plasmons from the square-root dispersion law characteristic of unscreened plasmons. The results allow interpreting the recent experimental data of other authors on the dispersion of the plasma oscillations in a laterally screened two-dimensional electron system.     

Ordinary-wave Propagation in Mirror-Confined Plasmas

The non-relativistic dispersion relation for ordinary-waves propagating in a hot plasma confined by magnetic mirrors is derived under small gyroradius approximation. The hot electron velocity distribution is modelled by a bi-Maxwellian with a cutoff at the loss-cone boundary. The dispersion relation is analyzed from propagation point of view. It is seen that the presence of loss cones in the electron velocity distribution reduces the high temperature effects in the ordinary-wave propagation characteristics.     

Purely quadratic dispersion of surface plasmon in Ag/Ni(111): the influence of electron confinement

Surface plasmon dispersion in nanoscale thin Ag films deposited onto the Ni(111) surface was investigated by angle‐resolved electron energy loss spectroscopy. We found that the dispersion curve contains only the quadratic term. The vanishing of the linear term was ascribed to the presence in the film of Ag 5sp‐related quantum well states. Screening effects enhanced by electron confinement in Ag quantum well states push the position of the centroid of the induced charge of the surface plasmon less inside the interface compared to other Ag systems, rendering null the linear coefficient of the dispersion curve. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Plasma bands in graphite

The π + σ plasmons in a strongly oriented pyrolitic graphite were studied experimentally using the characteristic electron energy loss technique with an angular resolution in the reflection mode. It was established that the bulk intralayer π + σ plasma bands have two branches with a positive and a negative dispersion, respectively. Each dispersion branch of the π + σ plasmons is a doublet. The interlayer plasmons are characterized by a positive dispersion coefficient only. The results are interpreted qualitatively based on the special features of the electronic band spectrum of graphite. The doublet arises because both dipole and multipole interband transitions similar in intensity are excited in graphite exposed to electron bombardment.     

Linear theory of beam–wave interaction in double-slot coupled cavity travelling wave tube

A three-dimensional model of the double-slot coupled cavity slow-wave structure(CCSWS) with a solid round electron beam for the beam–wave interaction is presented. Based on the "cold" dispersion, the "hot" dispersion equation is derived with the Maxwell equations by using the variable separation method and the field-matching method. Through numerical calculations, the effects of the electron beam parameters and the staggered angle between adjacent walls on the linear gain are analyzed.     

Electron exchange-correlation effects on dispersion properties of electrostatic waves in degenerate quantum plasmas

Based on the quantum Magnetohydrodynamic (QMHD) model, the obliquely propagation of electrostatic waves in degenerate magnetized quantum plasmas with electron exchange-correlation effects are theoretically investigated. The modified linear dispersion relations of electrostatic waves are obtained and discussed in some specific cases. The analytical results clearly show that the dispersion properties of the high frequency electron waves (including the Langmuir wave and upper-hybrid wave) and the low frequency ion acoustic wave are modified by the quantum effects together with the electron exchange-correlation effects. The numerical results depict that the Langmuir wave and upper-hybrid wave can be unstable in the presence of the electron exchange-correlation effects, and it is also evidently indicated that the electron exchange-correlation effects can reduce the phase velocity of the waves, especially in the high wave number region. The corresponding results should be of relevance for identifying electrostatic fluctuations which transport in an inhomogeneous and magnetized quantum plasmas.     

相对论返波管注-波作用不稳定区的研究

 以环形电子注驱动的、正弦型周期慢波结构的相对论返波管（RBWO）为模型,理论推导出RBWO慢波结构的色散方程,画出了不包括和包括电子注的色散曲线,然后采用在复平面上绘制等高线图进行逼近的方法对注-波作用不稳定区工作频率的实部和虚部进行了求解,绘出了完整的包括电子注的慢波结构色散关系曲线,并估算了微波的时间增长率和能量转换效率。研究得出：当a（电子注电流参数）较大时,电磁波与慢电荷波的相互作用是不稳定的;而在a较小时,注-波相互作用具有明显的三波作用的特点;微波增长是通过注-波不稳定作用引起的;随着a的增大,微波时间增长率先增大后减小,在a为0.02时有最大值1.37,此时的能量转换效率为19%;增大电子注半径或慢波结构的波纹深度都可以增强注-波不稳定作用。     

Electric field driven plasmon dispersion in AlGaN/GaN high electron mobility transistors

We present a theoretical study on the electric field driven plasmon dispersion of the two-dimensional electron gas(2DEG)in AlGaN/GaN high electron mobility transistors(HEMTs).By introducing a drifted Fermi–Dirac distribution,we calculate the transport properties of the 2DEG in the AlGaN/GaN interface by employing the balance-equation approach based on the Boltzmann equation.Then,the nonequilibrium Fermi–Dirac function is obtained by applying the calculated electron drift velocity and electron temperature.Under random phase approximation(RPA),the electric field driven plasmon dispersion is investigated.The calculated results indicate that the plasmon frequency is dominated by both the electric field and the angle between wavevector and electric field.Importantly,the plasmon frequency could be tuned by the applied source–drain bias voltage besides the gate voltage(change of the electron density).     

电子束电流对介质切伦科夫脉塞色散关系的影响

采用等离子体流体理论,从线性扰动方程出发,研究了电子束电流大小对介质切伦科夫脉塞中束波相互作用色散关系的影响,得到了当等离子体返回电流能有效中和与不能有效中和电子束电流两种情形下色散关系和波的增长率,并讨论了电子束电流对束波相互作用色散关系和波增长率的影响。研究表明,等离子体返回电流能有效中和与不能有效中和电子束电流两种情况下有着不同的色散关系和波的增长率。在波导参数和背景等离子体参数固定的情况下,电子束电流并不是越大越好,电子束电流过大不利于提高器件效率,它们之间存在最佳匹配关系。     

Anisotropy of plasmon dispersion in Al: An electron correlation effect

Electron energy-loss spectroscopy of epitaxial Al films shows a small, but well-established anisotropy of the plasmon dispersion. This cannot be explained within the random-phase approximation (RPA). Anisotropic electron exchange correlations, caused by the lattice pseudopotential, account qualitatively for the observed differences of dispersion.     

A comparative study of the full dispersion relation, Compton dispersion relation, and Raman dispersion relation of a free-electron laser in helical wiggler and guiding magnetic fields

The full dispersion relation obtained for a free-electron laser in the presence of circularly polarized, periodic, static wiggler and guiding magnetic fields by the kinetic approach incorporating the particle trajectories is reduced to Compton and Raman regime approximations in the case of a tenuous electron beam. The temporal growth rate has been compared between the full dispersion relation and the Compton dispersion relation, as well as between the full dispersion relation and the Raman dispersion relation in the microwave region. The results show the maxima of growths in the full dispersion relation and the Compton dispersion relation, as well as the full dispersion relation and the Raman dispersion relation, are at the same locations, but the growth rates in both regimes are enhanced with respect to the full dispersion relation for the same plasma frequency and cavity parameters.     

Evaluation of the dipole interaction of a pair of metal particles in the region of the plasmon resonance

We consider the electric field of an induced dipole moment of a single small particle characterized by the absence of frequency dispersion of the permittivity and the field of a metal particle, which has frequency dispersion and is described in the free electron approximation taking into account the size effects of restriction of the electron free path. The influence of the induced field on the optical properties of a system of small particles is analyzed. It is shown that, for an ensemble of particles without frequency dispersion, the effective medium theory can be used up to concentrations corresponding to filling factors ? ≤ 0.52. In the case of metal particles, with frequency dispersion of dielectric functions and, especially, for the frequency range of the plasmon resonance, this theory can be used only for concentrations not exceeding the threshold ? ≈ 0.01.     

Three-dimensional electrostatic waves in a nonuniform quantum electron-positron magnetoplasma

The dispersion properties of three-dimensional electrostatic waves in a nonuniform quantum electron-positron magnetoplasma are examined. A new dispersion relation is derived using the electron and positron densities response arising from the balance between the quantum Bohm and electrostatic forces, and from the electron and positron continuity and Poisson equations. In the local approximation regime, the dispersion relation admits both oscillatory and purely growing instabilities those depend on the quantum parameters as well as the density, velocity and magnetic field inhomogeneities.     

真空二极管辐射微波的机理分析

在研究真空开关的过程中,发现真空二极管能辐射出宽带微波.这种器件只由带触发装置的阴极和平板阳极组成,不存在金属波纹慢波结构,所以真空二极管的辐射机理与等离子体填充微波器件不同,不能直接套用等离子体填充微波器件的相关理论.本文描述了真空二极管产生辐射的物理过程,建立了真空二极管辐射的数学模型,通过求解波动方程得到产生辐射的色散关系,并绘制出了色散曲线.将理论分析得到的色散曲线与已经测得的微波辐射进行比较,两者能很好地符合.理论分析和实验结果表明,电子束和磁化等离子体的相互作用是真空二极管产生微波辐射的原因.     

Spectrum of velocities and dynamics of electron beams in divergent magnetic fields

The dynamics of an electron beam in divergent magnetic fields has been studied. The changes in the longitudinal and transverse electron velocities in the beam and the effect of the magnetic field parameters on the dispersion of longitudinal electron velocities have been analyzed. A possibility of high-efficiency conversion modes in divergent magnetic fields has been established by methods of numerical simulation.     

On the instability of electrostatic waves in a nonuniform electron–positron magnetoplasma

The dispersion properties of three-dimensional electrostatic waves in a nonuniform electron–positron (EP) magnetoplasma are analyzed. A new dispersion relation is derived by use of the electron and positron density responses arising from the electron and positron continuity and Poisson equations. In the local approximation, the dispersion relation admits two wave modes with different velocities. The growth rates of various modes are illustrated both analytically and numerically. Considering the temperature gradients produces a linearly stable transverse mode. The growth rate of the slow mode instability due to the density inhomogeneity only is the highest one, though it appears at higher thermal energy. The angle of the wave propagation affects drastically on the instability features in each case. The applications of the present analysis are briefly discussed.