Non-Kolmogorov湍流大气中小尺度热晕效应线性理论

从小尺度热晕线性理论出发,在non-Kolmogorov谱的基础上,得到了non-Kolmogorov谱湍流下热晕相位补偿的Strehl比表达式,分析了湍流谱对高能激光的相位补偿的影响.研究结果表明湍流谱对湍流热晕效应的相位补偿有重要的影响.在相同的湍流菲涅耳数下,当谱指数越接近于3时补偿效果越差,谱指数接近于4时补偿效果越好.在相同大气相干长度条件下或在相同湍流折射率常量条件下,当谱指数接近于3时,Strehl比随热晕效应的增强而下降变快,当湍流谱指数逐渐接近于4时,Strehl比下降速度变慢.其原因是随着湍流谱指数的增大,湍流热晕相互作用引起的对数振幅起伏增长变慢.     

Selective ion heating by energetic ion beam injection into a plasma

High energy neutral beam injected into a plasma has been predicted to be susceptible to microinstabilities. If the growth time of these microinstabilities is shorter than the collisional relaxation time, then the energy transfer from the beam to the plasma may take place, in the initial phase, by collisionless wave-particle interaction. We report here an experiment where this effect has indeed been observed. Here ion Bernstein waves excited by an energetic ion beam lead to selective ion heating of the plasma in a collisionless manner.     

大气湍流对多抖动法相干合成技术中相位调制信号的影响

分析了大气湍流对采用多抖动法实现的相干合成阵列光束中相位调制信号的影响. 文章首先根据广义惠更斯-菲涅耳原理,采用折射率结构函数对大气湍流进行描述,推导了多抖动法相干合成中阵列光束通过大气湍流后相位调制信号的一般表达式. 在此基础上进行数值模拟,分析了传输距离、湍流强度、光束阵列占空比和光斑尺寸等因素对相位调制信号的影响. 研究发现随着传输距离的增大,相位调制信号强度会先增大后减小,存在一个极大值点;随着湍流强度的增强,相位调制信号强度极大值点的出现距离不断缩短;当光束阵列占空比一定时,随着光斑尺寸的增大 关键词： 大气湍流 相干合成 多抖动法 相位调制     

库仑作用对同位旋分馏过程的影响

利用同位旋相关的量子分子动力学模型研究了中能重离子碰撞中库仑作用对同位旋分馏过程的影响.研究结果表明,在所研究的能区,无论是丰(缺)中子碰撞系统或者轻(重)反应系统,库仑作用都使同位旋分馏过程减弱,而这种影响主要来自于库仑作用对质子的排斥作用,使更多的质子发射,从而降低了气相中子–质子比所导致     

Formation of Suprathermal Particle Fluxes in a Strongly Turbulent Plasma

We consider the problem on the formation of suprathermal particle fluxes by electrostatic structures in strongly turbulent cosmic plasmas. It is shown that regions with a strong plasma turbulence can be large accelerators of charged particles. We give solutions of the stationary kinetic equation in a turbulent layer for different acceleration regimes and estimate the efficiency of diffusion over the longitudinal and transverse velocities of particles with respect to the magnetic field. The transverse diffusion in velocity space is more efficient for ions and leads to strong isotropization of ion fluxes. Electrons move almost along the magnetic field. We reveal the conditions under which the regular force in a nonuniform magnetic field influences the stochastic-acceleration process. The average energy of axial motion of the particles and the particle fluxes at large distances from the injection region are estimated. Ions and electrons can be accelerated up to comparable energies. We analyze the characteristic features of the motion of the relativistic-particle beams. It is shown that strong plasma turbulence can form particle beams with specific energies. The proposed mechanism is useful for explanation of the properties of energetic particles in cosmic plasmas with magnetic-field-aligned currents, e.g., in high-latitude regions of planetary magnetospheres, force-free configurations of the solar corona, and the solar wind.     

Formation,Dynamics, and Characterization of Nanostructures by Ion Beam Irradiation

Ion beam irradiation is a potential tool for phase formation and material modification as a non-equilibrium technique. Localized rise in temperature and ultra fast (～10^?12 s) dissipations of impinging energy make it an attractive tool for metastable phase formation. As a matter of fact, a major component of materials science is dominated by ion beam methods, either for synthesis of materials or for its characterization. The synthesis of nanostructures, and their modification by ion beam technique will be discussed in this review article. Formation of nanostructures using ion beam technique will be discussed first. Depending on species (e.g., mass and charge state) and energy range, there are various modes for an energetic ion to dissipate its energy. The role of the electron will also be covered in this article as a basic principle of its interaction with matter, which is same as for an ion. By using a simple reactive ion beam or electron induced deposition, a secondary phase can be nucleated by ion beam mixing techniques, either by using inert gas irradiation or reactive gas implantation on any desired substrate. Nucleation of secondary phase can also be executed by electron irradiation and direct implantation of either negative or positive ions. Post implantation annealing processes are required for the complete growth of clusters formed in most of these ion irradiation techniques. Implantation processes being inherently a non-equilibrium technique, defects always have a role to play in phase formation, amorphization, and beyond (blister formation). When implanted with large energy, even electrons, one of the lightest charged particles, also manifest these properties. Electronic and nuclear energy losses of the impinging charged particle play a crucial role in material modification. Doping a nanocluster, however, is still a controversial topic. Some light will be shed on this topic with a discussion of focused ion beam.     

Correlations of conductance peaks and transmission phases in deformed quantum dots

We investigate the Coulomb blockade resonances and the phase of the transmission amplitude of a deformed ballistic quantum dot weakly coupled to leads. We show that preferred single-particle levels exist which stay close to the Fermi energy for a wide range of values of the gate voltage. These states give rise to sequences of Coulomb blockade resonances with correlated peak heights and transmission phases. The correlation of the peak heights becomes stronger with increasing temperature. The phase of the transmission amplitude shows lapses by between the resonances. Implications for recent experiments on ballistic quantum dots are discussed. Received 17 July 1998     

Renormalized phase dynamics in oscillatory media

Based on the complex Ginzburg-Landau equation (CGLE), a new mapping model of oscillatory media is proposed. The present dynamics is fully determined by an effective phase field renormalized by amplitude. The model exhibits phase turbulence, amplitude turbulence, and a frozen state reported in the CGLE. In addition, we find a state in which the phase and amplitude have spiral structures with opposite rotational directions. This state is found to be observed also in the CGLE. Thus, one concludes that the behaviors observed in the CGLE can be described by only the phase dynamics appropriately constructed.     

Beam-Plasma Heating Model

A one-dimensional ionization and heating model is applied to results of several electron-beam-plasma interaction experiments. Beam energy is deposited resistively in the plasma at a rate ?j2, where j is the return current density and ? the plasma resistivity both classical and anomalous due to ion acoustic or e-e-mode turbulence. Principal energy losses include ionization, line radiation, inelastic electron impact excitation, bremsstrahlung and radiative recombination. The level of ionization and plasma heating are computed as a function of neutral gas pressure, beam rise time, pulsewidth and current density, and resistivity model. Plasma dynamics and kinetic effects such as expansion and end loss are not explicitly included in the model.     

电子-离子碰撞对超热电子影响的PIC模拟计算

超热电子-离子的产生和输运在传统的ICF方案和“快点火”方案中都是很重要的问题.讨论了电子-离子碰撞对参量不稳定性产生超热电子的过程和超热电子输运过程的影响.指出电子-离子弱碰撞项的加入增强了碰撞吸收,提高了热电子温度,降低了静电波破裂时的场能.这些改变了超热电子的总能量和分布,使之更集中于静电波的相速;电子-离子碰撞的存在还增强了自洽电场,阻碍了超热电子的输运,同样也是超热电子能量下降的原因之一.同时,为确保计算结果的可靠,讨论了初条件对PIC模拟计算的影响,指出空间位置随机热启动容易引入非物理因素,对 关键词：     

Coupling Instability of a Warm Relativistic Electron Beam with Ion-Channel Guiding

In this paper, the coupling instability of warm relativistic electron beam (WREB) propagating through the ion channel guiding is investigated in detail. Obtaining the equilibrium state of the system by considering the self-electric and azimuthal magnetic field, the fluid-Maxwell equations as well as linear perturbation theory are employed to derive the dispersion relation of the excited modes in the system. Numerical analysis of the obtained dispersion relation shows that the electromagnetic (EM) instability can be induced nearly the center of the beam through coupling between the fast electron plasma wave (FEPW), originated from the longitudinal oscillation of WREB, and fast forward electromagnetic wave (FFEW). In this sense, growing the perturbation amplitude occurs due to transport the kinetic energy of WREB to the EM wave at the specific frequency range, where the phase velocity of FEPW and FFEW is coincided. The results of the present investigation will greatly contribute to the understanding of the stability of the warm relativistic electron beam in laboratory experiments, such as in free electron laser experiments, where the ion-channel guiding is used to confine the electrons against the self-repulsive forces generated by the beam itself.     

On the generation and maintenance of waves and turbulence in simulations of free-surface turbulence

One of the challenges in numerical simulation of wave–turbulence interaction is the precise setup and maintenance of wave and turbulence fields. In this paper, we investigate techniques for the generation and suppression of specific surface wave modes, the generation of turbulence in an inhomogeneous physical domain with a wavy boundary-fitted grid, and the generation and maintenance of waves and turbulence during the complex wave–turbulence interaction process. We apply surface pressure to generate and suppress waves. Based on the solution of linearized Cauchy–Poisson problem, we derive three pressure expressions, which lead to a δ-function method, a time-segment method, and a gradual method. Numerical experiments show that these methods generate waves as specified and eliminate spurious waves effectively. The nonlinear wave effect is accounted for with a time-relaxation method. For turbulence generation, we extend the linear forcing method to an inhomogeneous physical domain with a curvilinear computational grid. Effects of force distribution and computational grid distortion are examined. For wave–turbulence interaction, we develop an algorithm to instantaneously identify specific progressive and standing waves. To precisely control the wave amplitude in a complex turbulent flow field, we further develop an energy controlling method. Finally, a simulation example of wave–turbulence interaction is presented. Results show that turbulence has unique features in the presence of waves. Velocity fluctuations are found to be strongly dependent on the wave phase; variations of these fluctuations are explained by the pressure–strain correlation associated with the wave-induced strain field.     

湍流等离子体鞘套中高斯光束的传播特性分析

为了研究高斯光束在湍流等离子体鞘套中的传输特性,根据广义惠更斯-菲涅耳原理,采用基于快速傅里叶变换的功率谱反演法,用多随机相位屏来模拟湍流带来的影响.根据超声速飞行器绕流等离子体流场厚度在厘米级别的特点,光束在两个相位屏之间的传输过程中采用菲涅耳衍射积分的两次快速傅里叶变换算法(double fast Fourier transform algorithm),利用多随机相位屏模拟等离子体鞘套湍流对光束传输产生的影响,解决了多随机相位屏模拟湍流研究中的超短距离传输问题.当飞行高度为45 km,飞行速度为18马赫时,通过对超声速飞行器绕流等离子体流场的统计分析,发现在此飞行条件下折射率起伏方差的强度范围10~(–11)—10~(–14).对高斯光束在湍流等离子体流场中的传输特性进行了数值仿真.结果表明:在等离子体鞘套湍流中折射率起伏强度、波长、传输距离等都是影响高斯光束质量的重要因素.折射率方差越大,传输距离越长,光斑弥散越严重,光强起伏越大,光强减弱也越明显.光束的波长越长,高斯光束抑制湍流的能力越强,光斑弥散程度越小,光强起伏也越小.     

Fusion burning waves in degenerate plasmas

The outcome of fusion burning waves in non-degenerate plasmas is limited by the strength of ion–electron Coulomb collisions and subsequent energy loss mechanisms as electron heat conduction and radiation emission. In this Letter, an analysis is presented on the degeneracy effects in the stopping power of suprathermal charged particles and in the energy transmitted from ions to electrons by Coulomb collision. Main results of this analysis is that very powerful fusion burning waves can be launched into previously compressed degenerate plasmas. This can be specially suitable for proton–boron fusion, but it also applicable to any type of fusion reaction, where ignition can be triggered by an incoming ion beam or another external source of energy deposited in a small fraction of the compressed plasma (fast ignition).     

黑腔靶中超热电子特性研究

近几年来,在“神光”装置上进行了1.053μm激光与平面靶及一系列柱形黑腔靶相互作用实验。用一台多道滤波—荧光X光能谱仪(FFS)测得各种靶发射的超热X射线谱,由谱推导超热电子温度T_h和超热电子总能量E_h当照射靶单束激光能量E_(tar)为400～670J、脉宽τ=650～1150ps时,发现黑腔内明显存在两群服从Maxwell分布高能电子(T_h=35～45keV;T_(hh)=150～350kev),而且E_(he)占E_(tar)的份额为10％～12％。实验还表明:腔内的E_(he)与非线性过程特征量(SRS)有较好的线性关系,因此推断出腔内超热电子产生的主要机制是受激Raman散射。在相同照射条件下,黑腔靶产生的超热电子比平面靶严重。     

高功率离子束辐照膜基双层靶温度场的数值研究

采用TRIM程序模拟高功率离子束与铝基钛膜双层靶的相互作用.计算了束流在靶材内的能量沉积及分布情况,并以此沉积能量为热源项,采用有限差分方法求解非线性热传导方程,得到了温度场的分布规律,分析了不同离子流密度对界面物质状态的影响.结果表明,离子束电流密度在100—200 A/cm²之间取值时,脉冲结束后界面处两种物质均达到熔融状态.     

A free-electron laser in a uniform magnetic field

The interaction of free electrons and free electromagnetic radiation, in the presence of a uniform magnetic field, can result in stimulated emission or absorption. We analyze the dynamics of single electrons by solving the classical, relativistic Lorentz force equations of motion in these combined fields. An electron may gain energy from, or lose energy to, the radiation field, depending crucially on the phase and oscillation frequency of the electron's helical motion within the superposed, circularly polarized light wave. To first order in the radiation field strength, electrons in a monoenergetic, uniformly distributed beam become spatially bunched, but there is no net energy change. To second order, however, the beam may experience a gain or loss of energy, corresponding to attenuation or amplification of radiation. We compare the bunching of this laser process to the bunching processes involved in 1) the Stanford free-electron laser and 2) the cyclotron maser, and find significant differences in each case. Our analytic results provide a clear, simple picture of the interaction process, and can be useful in exploring light amplification in astrophysical magnetic fields, the magnetosphere, or in laboratory devices. Supported in part by Army Contract No. DASG 60-77-C-0083 and NASA Grant NSG-7490.     

Nonlinear beam shaper for femtosecond laser pulses, from Gaussian to flat-top profile

We present a straightforward method to transform a spatially Gaussian femtosecond laser beam into a flat-top shaped beam. The proposed technique takes advantage of a nonlinear phase induced in positive Kerr medium followed by a simple optical system. The variation of the refractive index with the laser intensity creates a phase plate which induces changes in the beam profile after propagation; flat-top and doughnut profiles are observed. The shaping conditions are computed numerically and confirmed experimentally. The method does not introduce energy losses. The device is very simple, self-regulated, flexible and does not need a manufactured phase plate or precise alignment. This method can be useful for light-matter interaction and laser machining.     

Anisotropic ion heating and tail generation during tearing mode magnetic reconnection in a high-temperature plasma

Complementary measurements of ion energy distributions in a magnetically confined high-temperature plasma show that magnetic reconnection results in both anisotropic ion heating and the generation of suprathermal ions. The anisotropy, observed in the C(+6) impurity ions, is such that the temperature perpendicular to the magnetic field is larger than the temperature parallel to the magnetic field. The suprathermal tail appears in the majority ion distribution and is well described by a power law to energies 10 times the thermal energy. These observations may offer insight into the energization process.     

Construction and study of a magnetoactive plasma ion source

The interaction of intense electron beams with plasmas in a nonuniform magnetic field is studied for the purpose of obtaining a magnetoactive plasma ion source. The resulting experimental data are used to trace the dynamics of the changes in the basic parameters of the source, both during the stage where it interacts with the beam and in the cooling stage. It is found that the charged particles are contained for a long time and that the efficiency of energy transfer from the beam to the source is high. This source is intended to be used for shaping and accelerating multiampere ion beams. Zh. Tekh. Fiz. 69, 44–47 (April 1999)