C60团簇与等离子体相互作用的分子动力学模拟

用分子动力学模拟方法研究了C60团簇与稠密等离子体的相互作用。在线性化的伏拉索夫-泊松理论框架下,借助于经典的等离子体介电函数,推导了团簇的阻止本领的一般表达式。用数值方法求解了团中离子的运动方程,研究了团簇的库仑爆炸过程,讨论了入射速度、等离子体密度和电子温度对阻止本领和库仑爆炸的影响。结果发现尾流效应降低了C60团簇离子的库仑爆炸速度,甚至压缩了团簇的结构,在高速、低密、高温的情况下库仑爆炸进行得更快。     

Electrodynamics of effective charge

A portion of the charge induced in an isotropic plasma by an injected charged particle is shown to combine with the particle to form a charged quasi-particle, the effective charge, that is different from the conventional “dressed” charge. The remainder of the induced charge is found in a wake behind the effective charge. Properties of effective charge are described, and the application of this concept to coherent ion acceleration and stopping power is discussed.     

Dependence of FLR Effects on RF Power During ICRF Heating in Tokamak Plasmas

The characteristics of FLR effects and their dependence on RF power are studied during ICRF minority heating in tokamak plasmas. With the ion energy losses being taken into consideration, a critical heating power above which FLR effects may play an important role is derived. The quasilinear Fokker-Planck equation is calculated n umericallp It is shown that the analytically derived critical power is in good agreement with the numerical calculation.     

Interactions of a Charged Particle with Parallel Two-Dimensional Quantum Electron Gases

By using the linearized quantum hydrodynamic (QHD) theory, electronic excitations induced by a charged particle moving between or over two parallel two-dimensional quantum electron gases (2DQEG) are investigated. The calculation shows that the influence of the quantum effects on the interaction process should be taken into account. Including the quantum statistical and quantum diffraction effects, the general expressions of the induced potential and the stopping power are obtained. Our simulation results indicate that a V-shaped oscillatory wake potential exists in the electron gases during the test charge intrusion. Meanwhile, double peaks will occur in the stopping power when the distance of two surfaces is smaller and the test charge gets closer to any one of the two sheets.     

Effects of Hall Currents and Gyroviscosity on the Instability of a Self-Gravitating Stratified Plasma

The instability in a horizontal layer of a partially ionized self gravitating plasma has been studied to include simultaneously the effects of Hall currents, viscosity and finiteness of Larmor radius (FLR). The ambient magnetic field is assumed to be uniform and vertical. Proper solutions have been obtained through the variational methods for a semi-infinite plasma in which the density has an exponential gradient along the vertical. The dispersion relation obtained has been solved numerically and it is found that the growth rate of unstable perturbations decreases with the effects of viscosity, neutral gas friction and FLR. The influence of effects of viscosity, neutral gas friction and FLR are consequently stabilizing. It is found that the Hall currents have a destabilizing influence as the growth rate is found to increase with this effect.     

Absence of complete finite-Larmor-radius stabilization in extended MHD

The dominant finite-Larmour-radius (FLR) stabilization effects on interchange instability can be retained by taking into account the ion gyroviscosity or the generalized Ohm's law in an extended MHD model. However, recent simulations and theoretical calculations indicate that complete FLR stabilization of the interchange mode may not be attainable by ion gyroviscosity or the two-fluid effect alone in the framework of extended MHD. For a class of plasma equilibria in certain finite-beta or nonisentropic regimes, the critical wave number for complete FLR stabilization tends toward infinity.     

Gravitational Instability of a Rotating Viscous Finitely Conducting Plasma

The gravitational instability of infinite homogeneous plasma has been studied to include simultaneously the effects of rotations, Hall currents, viscosity, finite electrical conductivity and the finite Larmor radius (FLR). Both the longitudinal, and transverse modes of wave propagation have been studied. It is found that the gravitational instability is determined by Jeans' criterion even in the presence of effects of rotation, Hall currents, FLR, viscosity and finite conductivity whether included separately or jointly.     

Finite Larmor radius and Hall effects on thermal instability of a compressible plasma

The effects of compressibility, finite Larmor radius (FLR) and Hall currents are considered on the thermal instability of a plasma in the presence of a uniform horizontal magnetic field. For stationary convection, the compressibility has a stabilizing effect whereas FLR and Hall currents have stabilizing as well as destabilizing effects. For (C _pβ/g)<1, the system is stable. The magnetic field, FLR and Hall currents introduce oscillatory modes in the system for (C _pβ/g)>1.     

Beam-plasma coupling effects on the stopping power of dense plasmas

The stopping power for ion beams in dense plasmas is investigated on the basis of quantum kinetic equations. Strong correlations between the beam ions and the plasma particles which occur for high ion charge numbers and strongly coupled plasmas are treated on the level of the statically screened T-matrix (binary collision) approximation. Dynamic screening effects are included using a combined scheme which considers both close collisions and collective effects. Applying this approach, the ion charge number dependence of the stopping power is determined. The result is a modification of the Z(2)(b) scaling law. In particular, the stopping power is reduced for strong beam-plasma coupling. Good agreement is found between T-matrix results and simulation data (particle-in-cell and molecular dynamics) for low beam velocities.     

优化脉冲间距的多脉冲尾流加速PIC模拟

多脉冲激光尾波场加速电子方法中限制尾波场振幅的主要机理是“相位失谐”,起源于非线 性效应导致尾波波长随振幅的增长而变大,从而后续脉冲逐渐偏离加速相位. 借助2D3V PIC 模拟方法优化各脉冲之间的间距,使之等于前面脉冲激发的尾波波长,模拟结果表明激发了 更大振幅的尾波场,同时激发了更强的“前向Raman散射”,它在限制尾波场进一步增长的 过程发挥了重要作用. 关键词： 多脉冲激光尾波加速 有质动力 相位失谐 前向Raman散射     

On the possibility of wake-field effects in the conductivity of doped semiconductors

The formation of a wake field around a swift ion passing through an electron gas and the resulting contribution to the stopping power acting on the ion is an intensively studied phenomenon in metals and semiconductors. The present investigation serves to clarify whether an analogous effect, namely the formation of wake fields and a corresponding contribution to the resistivity, might occur in the Galilei-transformed case of electronic transport in doped semiconductors where the gas of drifting charge carriers passes through an array of fixed impurity ions. By use of an appropriate dynamical screening theory we show that indeed a local plateau in the current density versus field characteristic has to be expected whenever the mean drift energy per carrier exceeds the sum of the mean thermal carrier energy and the zero-point energy of the longitudinal plasma mode of the carrier gas. However, our survey of the published literature suggests that this condition might be too stringent, at least for bulk materials and standard experimental situations, where the strong carrier heating in the high-field regime of relevance in combination with other drift-limiting mechanisms or interband electron-hole avalanching would always precede and prevent the formation of the wake.Dedicated to Prof. H.-J. Queisser on the occasion of his 60th birthday     

Polarization wake of penetrating ions: oscillator model

The wake potential induced by a swift nonrelativistic ion has been studied theoretically for a random stopping medium consisting of quantal-harmonic-oscillator atoms. The primary purpose has been to study the influence of atomic binding on the frequently-studied wake potential in a Fermi gas. Quantitative comparisons at constant plasma frequency and increasing oscillator frequency show a gradual decrease in wavelength and a slight decrease in amplitude of the oscillatory part of the wake potential, as well as a systematic decrease in screening of the near-field next to the projectile. These findings can be expected on the basis of the Drude-Lorentz formula for the effective resonance frequency. We find a distinct dependence of the induced potential on the ion charge as long as the plasma frequency exceeds the oscillator frequency. In the opposite case of a dominating oscillator frequency we find little difference between the field induced by a point charge and that by a neutral atom. As an application area we briefly discuss the proximity effect in the energy loss of molecular ions. We find that the polarization wake modifies the proximity effect, in contrast to the frequently-expressed view that it causes the proximity effect.     

Stopping of Heavy Ions in Magnetized Strongly Coupled Plasmas: High‐Velocity Limit

The energy loss of a heavy ion moving in a magnetized strongly coupled electron plasma is considered within the linear response treatment and in high‐velocity regime. The analytical expressions for the stopping power have been found for the arbitrary ion incidence angle. The obtained general expression for the stopping power is analyzed for the ion which moves parallel or perpendicular to the magnetic field. It is found that in general the magnetic field and the Coulomb coupling reduce the stopping power as well as the dynamical screening length at high velocities. The influence of the magnetic field and the Coulomb coupling on the high‐velocity stopping power is discussed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

铪离子等离子体源离子注入铜基体的数值模拟

通过SRIM软件对铪离子等离子体源离子注入铜进行了模拟。模拟了铪离子注入铜的核阻止本领、电子阻止本领、入射深度随能量的变化,以及在不同注入条件下铪离子的摩尔浓度分布,并对模拟结果进行了分析。结果显示:能量低于6 MeV时核阻止本领占主导地位,高于6 MeV时电子阻止本领成为主要的能量损失,并且离子注入过程中会出现能量沉积的Bragg峰和质量沉积区域较集中的现象,入射深度随能量的增加而增加。     

Effective Parameters on Energy Losses of Fast Electron in Fusion Mediums

The stopping and scattering of fast electrons in a dense plasma relevant to inertial confinement fusion (ICF) are investigated numerically with the latest improved cross section equations. Binary and collective effects are considered to determine beam transport parameters such as range, penetration depth, spreading processes as straggling and blooming versus electron energy and plasma parameters. Blooming and straggling effects, which act as consequences of scattering with statistical assumption in collisions, lead to a non-uniform, extended region of energy deposition. Finally the mean angle of deflections is calculated for different plasma energies.     

Mean excitation energies for stopping powers in various materials composed of elements hydrogen through argon

The basic model of Lindhard and Scharff, known as the local plasma model, is utilized to study the effects of the chemical and physical state of the medium on its stopping power. Unlike previous work with the local plasma model, in which individual electron shifts in the plasma frequency were estimated empirically, the Pines correction derived for a degenerate Fermi gas is shown herein to provide a reasonable estimate even on the atomic scale. Thus, the model is moved to a completely theoretical base requiring no empirical adjustments, adjustments characteristics of past applications. The principal remaining error is in the overestimation of the low-energy absorption properties characteristic of the plasma model in the region of the atomic discrete spectrum, although higher energy phenomena are accurately represented and even excitation-to-ionization ratios are given with fair accuracy. Mean excitation energies for covalently bonded gases and solids, ionic gases and crystals, and metals are calculated using first-order models of the bonded states for which reasonable agreement with the recently evaluated data of Seltzer and Berger is obtained. Hence the methods described herein allow reasonable estimates of mean excitation energy for any physical-chemical combination of material media for stopping power applications.     

Magnetogravitational Stability of Resistive Plasma through Porous Medium with Thermal Conduction and FLR Corrections

The problem of stability of self gravitating magnetized plasma in porous medium is studied incorporating electrical resistivity, thermal conduction and FLR corrections. Normal mode analysis is applied to derive the dispersion relation. Wave propagation is discussed for parallel and perpendicular directions to the magnetic field. Applying Routh Hurwitz Criterion the stability of the medium is discussed and it is found that Jeans' criterion determines the stability of the medium. Magnetic field, porosity and resistivity of the medium have no effect on Jeans' Criterion in longitudinal direction. For perpendicular direction, in case of resistive medium Jeans' expression remains unaffected by magnetic field but for perfectly conducting medium magnetic field modifies the Jeans' expression to show the stabilizing effect. Thermal conducitivity affects the sonic mode by making the process isothermal instead of adiabatic. Porosity of the medium is effective only in case of perpendicular direction to magnetic field for perfectly conducting plasma as it reduces the stabilizing effect of magnetic field. For longitudinal wave propagation, though FLR corrections have no effect on sonic mode but it changes the growth rate for Alfvén mode. For transverse wave propagation FLR corrections and porosity affect the Jeans' expression in case of nonviscous medium but viscosity of the medium removes the effect of FLR and porosity on Jeans' condition.     

再入尾迹湍流对雷达散射截面影响分析

通过探讨高超声速再入湍流尾迹等离子体场中电磁波的散射机制,推导出在工程上描述湍流亚密等离子体雷达散射的一阶畸变波Born近似模型,分析了该模型在充分发展湍流尾迹等离子体场中的适用性,完成了适用于三维尾迹等离子体场的程序设计.以已有的湍流尾迹等离子体流场数据为基础,分析了再入尾迹湍流等离子体流动对雷达散射截面的影响.选取考察的几个有代表性的因素为:湍流模型、转捩过程、湍流尺度、电子组分脉动初始条件等.由结果可以看到,湍流转捩过程和湍流尺度对雷达散射截面值影响不大;电子组份脉动强度初始值影响较明显;湍流模型在特定条件下影响亦不大.     

重离子束在热靶中的电子阻止本领与有效电荷数

采用线性介电响应理论,研究了重离子束在热靶中的有效电荷数和电子阻止本领。为了考虑入射离子的束缚电荷分布,我们将Brandt-Kitagawa的有效电荷理论推广到热靶。在低速和高速情况下,分别得到了有效电荷数和电子阻止本领的解析表示式。数值结果表明,对于低速离子,有效电荷数随电子气的温度增加而增加;在一定的温度范围内,低速离子的阻止本领与冷靶相比有明显增加。对于高速离子,我们的理论结果与实验值符合得较好。 关键词：