电子束与等离子体相互作用中的反常效应

本文对强流相对论性电子束与靶等离子体相互作用的反常吸收效应进行了理论研究。从符拉索夫方程出发,导出了强流相对论性电子束双流不稳定性效应解,并进行了弱碰撞修正,得到了相应的等离子体波增长率,对不同情况下出现反常效应的条件及机制进行了研究和讨论。     

磁层顶中的低混杂漂移不稳定性

本文讨论了具有简单结构磁层顶中的低混杂漂移不稳定性,假定在磁层顶中磁场方向是相互平行的,电子与离子的密度处处相等,总压力为一常数,采用1971年Alpers建立的分布函数做为零级分布,计算了下混杂漂移不稳定性的增长率和饱和时相应的反常电阻,计算表明,当磁层顶厚度接近两个质子迴旋半径时,低混杂漂移不稳定性的增长率约为0.26ω_LH(ω_LH为低混杂频率),反常电阻率为10^-5sec,随着磁层顶厚度成倍增加,反常电阻率以指数形式下降。 关键词：     

A simplified approximate analytical model for Rayleigh–Taylor instability in elastic–plastic solid and viscous fluid with thicknesses

A simplified theoretical model for the linear Rayleigh-Taylor instability of finite thickness elastic-plastic solid constantly accelerated by finite thickness viscous fluid is performed.With the irrotational assumption,it is possible to consider viscosity,surface tension,elasticity or plasticity effects simultaneously.The model considers thicknesses at rigid wall boundary conditions with the velocity potentials,and deals with solid elastic-plastic transition and fluid viscosity based on the velocity continuity and force equilibrium at contact interface.The complete analytical expressions of the amplitude motion equation,the growth rate,and the instability boundary are obtained for arbitrary Atwood number,viscosity,thicknesses of solid and fluid.The thicknesses effects of two materials on the growth rate and the instability boundary are discussed.     

Linear Growth of Rayleigh-Taylor Instability of Two Finite-Thickness Fluid Layers

The linear growth of Rayleigh-Taylor instability(RTI) of two superimposed finite-thickness fluids in a gravitational field is investigated analytically. Coupling evolution equations for perturbation on the upper, middle and lower interfaces of the two stratified fluids are derived. The growth rate of the RTI and the evolution of the amplitudes of perturbation on the three interfaces are obtained by solving the coupling equations. It is found that the finite-thickness fluids reduce the growth rate of perturbation on the middle interface. However, the finite-thickness effect plays an important role in perturbation growth even for the thin layers which will cause more severe RTI growth. Finally, the dependence of the interface position under different initial conditions are discussed in some detail.     

Discrete modulational instability and bright localized spin wave modes in easy-axis weak ferromagnetic spin chains involving the next-nearest-neighbor coupling

We report a theoretical work on the properties of modulational instability and bright type nonlinear localized modes in one-dimensional easy-axis weak ferromagnetic spin lattices involving next-nearest-neighbor couplings.With a linear stability analysis, we calculate the growth rates of the modulational instability, and plot the instability regions.When the strength of the next-nearest-neighbor coupling is large enough, two new asymmetric modulational instability regions appear near the boundary of the first Brillouin zone.Furthermore, analytical forms of the bright nonlinear localized modes are constructed by means of a quasi-discreteness approach.The influence of the next-nearest-neighbor coupling on the Brillouin zone center mode and boundary mode are discussed.In particular, we discover a reversal phenomenon of the propagation direction of the Brillouin zone boundary mode.     

R-T不稳定性发展的二维和三维对比研究

 应用摄动方法研究不同密度流体交界面上的Rayleigh-Taylor（R-T）不稳定性问题，分别研究了二维（2D）与三维（3D）R-T不稳定性的线性与非线性增长，并进行了对比。得出的结论为：（1） 三维不稳定性的线性增长比二维情况要快，且同维度上相同的初始扰动波长，其时间增长指数受另一维度上的初始波长影响。（2） R-T不稳定性发展在考虑了非线性增长的影响之后，二维与三维扰动增长的图像也不一样。     

Hydrodynamic analysis of electrostatic counter-streaming instability in a spin-polarized electron–positron–ion plasma

In this study, we present linear analysis of electrostatic counter-streaming instability in spin-polarized electron–positron–ion (e-p-i) plasma. With the aid of the separate spin evolution-quantum hydrodynamic (SSE-QHD) model, we derive the dispersion relation of counter-streaming instability. We numerically solve the dispersion and find four wave solutions: Langmuir wave, positron acoustic mode, and two electron and positron spin-dependent waves. It is noted that coupling of streaming and spin effects excites Langmuir instability and positron acoustic mode instability. However, in the absence of spin effect, only Langmuir instability will survive in e-p-i plasma. We have also discussed the effects of positron concentration, streaming speed, and spin polarization on the real frequency of waves and the growth rate. The present study may be helpful for understanding longitudinal wave propagation and instabilities in dense magnetized environments.     

Laser pulse modulation instabilities in partially stripped plasma

The laser pulse modulation instabilities in partially stripped plasma were discussed based on the phase and group velocities of the laser pulse and the two processes that modulation instabilities excited. The excitation condition and growth rate of the modulation instability were obtained. It was found that the positive chirp and competition between normal and abnormal dispersions play important roles in the modulation instability. In the partially stripped plasma, the increased positive chirp enhances the modulation instability, and the dispersion competition reduces it.     

Magneto-Rayleigh-Taylor instability in compressible Z-pinch liner plasmas

In this paper, the characteristics of magneto-Rayleigh-Taylor(MRT) instability of liner plasmas in Mag LIF is theoretically investigated. A three-region slab model, based on ideal MHD equations, is used to derive the dispersion relation of MRT instability. The effect of compressibility on the development of MRT instability is specially examined. It is shown that the growth rate of MRT instability in compressible condition is generally lower than that in incompressible condition in the presence of magnetic field. In the case of zero magnetic field, the growth rate in compressible assumption is approximately the same as that in incompressible assumption. Generally, MRT instability in(x, y) plane can be remarkably mitigated due to the presence of magnetic field especially for short-wavelength perturbations. Perturbations may be nearly completely mitigated when the magnetic field is increased to over 1000 T during liner implosions. The feedthrough of MRT instability in liner outer surface on inner surface is also discussed.     

Universal Behaviour of (2+1)-Dimensional Stochastic Equations for Epitaxial Growth Processes

We investigate spatially discretized versions of a class of nonequilibrium continuum equations for epitaxial growth processes in (2+1)-dimensions using numerical integration. The epitaxial growth models include the most well-known Villain-Lai-Das Sarma (VLDS) equation and a stochastic differential equation recently proposed by Escudero (Phys. Rev. Lett. 101:196102, 2008). To suppress the instability in the VLDS equation, the nonlinear term is replaced by exponentially decreasing functions. The critical exponents in different regions are obtained. The roughness distributions at the steady states of the growth models show that the two equations are in good agreement with each other. Our results imply that the modified version of the VLDS equation with controlled instability and the equation proposed by Escudero belong to the same universality class. Anomalous scaling behaviour in these growth models are also discussed, and the nontrivial scaling properties are found very weak in (2+1)-dimensions.     

Effects of dust-charge gradient and polarization forces on the waves and Jeans instability in strongly coupled dusty plasma

The effects of dust charge gradient (DCG) force and polarization force have been investigated on the properties of dust acoustic wave (DAW) and linear Jeans instability in strongly coupled dusty plasma. In the kinetic regime, DCG and polarization forces modify the DAW mode and couple with compressional viscoelastic wave mode. The Jeans instability criterion and critical wavenumber have been modified due to DCG force, polarization force and strong coupling effects. The results have been discussed in the warm photodisassociation region and in the laboratory complex plasmas. The strong correlation effect and the charge variation parameter stabilize the growth rate of Jeans instability. But, the polarization parameter stabilize the growth rate for positively charged dust grains and destabilize for negatively charged dust grains. The implications of charge gradient and polarization parameters are discussed for lower and higher charges in the laboratory complex plasma which decreases the growth of the propagating DAW.     

Drift wave turbulence in a dense semi-classical magnetoplasma

A semi-classical nonlinear collisional drift wave model for dense magnetized plasmas is developed and solved numerically. The effects of fluid electron density fluctuations associated with quantum statistical pressure and quantum Bohm force are included, and their influences on the collisional drift wave instability and the resulting fully developed nanoscale drift wave turbulence are discussed. It is found that the quantum effects increase the growth rate of the collisional drift wave instability, and introduce a finite de Broglie length screening on the drift wave turbulent density perturbations. The relevance to nanoscale turbulence in nonuniform dense magnetoplasmas is discussed.     

Uniform plasma model of shot noise in gyroklystrons

Theoretical estimates of electron cyclotron shot noise in gyroklystrons have recently been confirmed at low currents. However at high beam current, the noise temperature is always reduced. We examine the effect of transverse collective effects on the shot noise. There are two collective effects; shielding, which reduces the noise; and instability, which increases it. It is shown that the effect of transverse shielding is negligible unless the gyrotron beam is extremely cold. Regarding instability, if the bare shot noise amplitude is denoted Ξ, then the shot noise, including the effect of instability, can be expressed as Ξ(1+A exp Γ), where Γ is the integrated growth. The effect of instability is then measured by two parameters, Γ and A. For a cold gyrotron beam, A is about 0.3, meaning about 10 dB of power growth is needed for the instability to manifest itself. Thermal effects both reduce A and Γ. For realistic gyrotron beams, about 20-25 dB of power e folds would be necessary for instability to manifest itself. To summarize, the theory developed explains the absence of instability in the measurements, but indicates that phenomena other than transverse shielding are responsible for the noise reduction     

Collisional Effect on Weibel Instability with Semi-Relativistic Maxwellian Distribution Function

In this paper, the Coulomb collisional effect of electron-ion on the growth rate of Weibel instability is investigated based on the semi-relativistic Maxwellian distribution function in dense and unmagnetized plasma. An analytical expression was derived for the dispersion relation of Weibel instability for two limit cases [ξ = ω＇/k‖T‖ 〉〉 1 and ｜ξ｜ 〈〈 1. In limit ｜ξ｜ 〉〉 1 the dispersion relation only includes a real part and in limit ｜ξ｜ 〈〈 1 the imaginary part of the frequency of waves＇ instability plays a role in the dispersion relation. In limit ｜ξ｜ 〈〈 1, the two quantities μ and η, that are due to the relativistic and collisional effects, will appear in the growth rate of Weibel instability. The growth rate of Weible istability will be increased through decreasing the Coulomb collisional frequency and also increasing the temperature anisotropic parameter in strong relativistic limit.     

Mean effects of turbulence on elliptic instability in fluids

Elliptic instability in fluids is discussed in the context of the Lagrangian-averaged Navier-Stokes-alpha (LANS-alpha) turbulence model. This model preserves the Craik-Criminale (CC) family of solutions consisting of a columnar eddy and a Kelvin wave. The LANS-alpha model is shown to preserve elliptic instability. However, the model shifts the critical stability angle. This shift increases (decreases) the maximum growth rate for long (short) waves. It also introduces a band of stable CC solutions for short waves.     

First observation of ion acoustic waves produced by the langmuir decay instability

Thomson scattering measurements are presented which demonstrate conclusively the occurrence of the Langmuir decay instability (LDI) in a laser-produced plasma experiment. Both products of the instability, the ion acoustic wave and the electron plasma wave, were simultaneously observed and identified with their spectral characteristics. The secondary decay of the LDI-generated electron plasma wave, into another Langmuir wave and an ion acoustic wave, has been observed for the first time. The connection with growth and saturation of the stimulated Raman instability is discussed.     

On thermomagnetic instability during ionospheric modification by powerful radio waves

The possibility of excitation of thermomagnetic instability in the ionospheric F-region with an external DC electric field on exposure to waves from powerful ground-based radio transmissions is discussed. The threshold and growth rate of the instability are determined. It is shown that such instability can appear in local regions with a rather high level of plasma waves and in the high-latitude ionosphere, especially for high ionospheric-plasma drift velocities.Scientific-Research Institute of Radiophysics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 5, pp. 674–679, May, 1994.     

BEPC上离子引起的耦合束团不稳定性分析

对在北京正负电子对撞机(BEPC)上观测到的由离子引起的电子束二极耦合束团不稳定性的实验数据作了分析处理,给出了这种二极耦合束团不稳定性的边带.首先用离子俘获的线性两束流理论定量计算了离子引起的二极耦合束团不稳定性发生的阈值束流流强和不稳定性的增长时间,然后用基于束流–离子强弱作用模型的模拟程序跟踪了在实验情况下束流与离子的相互作用过程,跟踪结果成功地再现了离子引起的二极耦合束团不稳定性边带,并给出了比较合理的不稳定性增长时间.     

Theory of stimulated nonresonant emission from relativistic beams

A study is made of the radiative Pierce instability of a relativistic electron beam in a waveguide stabilized by an infinitely strong magnetic field. Analytical and computational methods are used to determine the growth rate of the instability, as well as the efficiency for conversion of the beam energy into electromagnetic field energy as a function of the beam current, how relativistic the beam is, and the geometry of the system. The physical nature of the instability is clarified and the mechanisms for its saturation are discussed. Zh. éksp. Teor. Fiz. 115, 2037–2050 (June 1999)     

On the microscopic origin of the kinetic step bunching instability on vicinal Si(0 0 1)

A scanning tunneling microscopy/atomic force microscopy study is presented of a kinetically driven growth instability, which leads to the formation of ripples during Si homoepitaxy on slightly vicinal Si(0 0 1) surfaces miscut in [1 1 0] direction. The instability is identified as step bunching, that occurs under step-flow growth conditions and vanishes both during low-temperature island growth and at high temperatures. We demonstrate, that the growth instability with the same characteristics is observed in two dimensional kinetic Monte Carlo simulation with included Si(0 0 1)-like diffusion anisotropy. The instability is mainly caused by the interplay between diffusion anisotropy and the attachment/detachment kinetics at the different step types on Si(0 0 1) surface. This new instability mechanism does not require any additional step edge barriers to diffusion of adatoms. In addition, the evolution of ripple height and periodicity was analyzed experimentally as a function of layer thickness. A lateral “ripple-zipper” mechanism is proposed for the coarsening of the ripples.