Review of collective beam instabilities in electron-positron storage rings

The review of studies of collective beam instabilities in electron-positron storage rings is presented. The processes of excitation and suppression of the longitudinal microwave instability, transverse mode coupling instability, longitudinal and transverse multi-bunch instabilities, and instabilities induced by an interaction of a beam with ions or electron clouds are discussed. Important equations for estimation of the threshold beam currents and the rise time of instabilities, as well as the references to the major original works are given. The methods for diagnostics and suppression of instabilities are considered using specific examples.     

Observation of transverse instabilities in optically induced lattices

We study experimentally the Bloch-wave instabilities in optically induced photonic lattices. We reveal two different instability scenarios associated with either the transverse modulational instability of a single Bloch wave or the nonlinear interband coupling between different Bloch waves. We show that the transverse instability is greatly enhanced in the induced lattice in comparison with homogeneous media.     

等离子体交换不稳定性的模耦合理论

本文研究部分电离等离子体中交换不稳定性的爆炸型模耦合。从磁流体动力学方程出发,导出交换模相干三波相互作用的模耦合方程,分析产生爆炸不稳定的可能的耦合类型,具体讨论产生爆炸不稳定的条件。结果表明,非线性效应能使交换模达到饱和状态表现为小幅度的正弦型扰动,也能通过模耦合而导致大幅度扰动的爆发。本文的分析,适用于磁镜位形的等离子体和电离层等离子体。 关键词：     

Nonlinear Theory of the Instability of a Modulated Electron Beam of Low Density in a Plasma. V. Excitation of Waves in Strong Dissipative Plasmas by a Monoenergetic Beam

A system of nonlinear equations derived in a previous paper which describes the evolution of the beam-plasma instability in strong dissipative plasmas is solved numerically. It is shown that there are three characteristic solutions of the system of equations: the resonant dissipative instability, the nonresonant instability with strong dissipation and the nonresonant dissipative instability. A physical interpretation of essential features of these instabilities is given. The interaction of resonant and nonresonant waves in the system electron beam-strong dissipative plasma is examined. Some conclusions for the transport problem of electron beams in strong dissipative plasmas are obtained in this paper.     

Theoretical and simulation research of hydrodynamic instabilities in inertial-confinement fusion implosions

Inertial fusion energy(IFE)has been considered a promising,nearly inexhaustible source of sustainable carbon-free power for the world's energy future.It has long been recognized that the control of hydrodynamic instabilities is of critical importance for ignition and high-gain in the inertial-confinement fusion(ICF)hot-spot ignition scheme.In this mini-review,we summarize the progress of theoretical and simulation research of hydrodynamic instabilities in the ICF central hot-spot implosion in our group over the past decade.In order to obtain sufficient understanding of the growth of hydrodynamic instabilities in ICF,we first decompose the problem into different stages according to the implosion physics processes.The decomposed essential physics processes that are associated with ICF implosions,such as Rayleigh-Taylor instability(RTI),Richtmyer-Meshkov instability(RMI),Kelvin-Helmholtz instability(KHI),convergent geometry effects,as well as perturbation feed-through are reviewed.Analytical models in planar,cylindrical,and spherical geometries have been established to study different physical aspects,including density-gradient,interface-coupling,geometry,and convergent effects.The influence of ablation in the presence of preheating on the RTI has been extensively studied by numerical simulations.The KHI considering the ablation effect has been discussed in detail for the first time.A series of single-mode ablative RTI experiments has been performed on the Shenguang-Ⅱlaser facility.The theoretical and simulation research provides us the physical insights of linear and weakly nonlinear growths,and nonlinear evolutions of the hydrodynamic instabilities in ICF implosions,which has directly supported the research of ICF ignition target design.The ICF hot-spot ignition implosion design that uses several controlling features,based on our current understanding of hydrodynamic instabilities,to address shell implosion stability,has been briefly described,several of which are novel.     

Secondary transverse instabilities in optical parametric oscillators

We investigate the effect of coupling between diffraction and walk-off on secondary instabilities in nondegenerate optical parametric oscillators. We show that traveling waves that propagate in the walk-off direction, which are generated at the onset of absolute instability, experience Eckhaus and zigzag phase instabilities. Each of these secondary instabilities splits into absolute and convective instabilities that modify the Eckhaus and zigzag instability boundaries. As a consequence, the stability domain of modulated traveling waves is enlarged and may coexist with uniform steady states. The predictions are consistent with the numerical solutions of the optical parametric oscillator model.     

Measurement of the current sheet during magnetic reconnection in a toroidal plasma

The current and magnetic-field fluctuations associated with magnetic-field-line reconnection have been measured in the reversed field pinch plasma configuration. The current sheet resulting from this reconnection has been measured. The current layer is radially broad, comparable to a magnetic-island width, as may be expected from current transport along magnetic-field lines. It is much larger than that predicted by resistive MHD for linear tearing modes and larger than prediction from two-fluid linear theory.     

Self-modulation of acoustic waves in resonant bars

Observations of the induced transparency in the oscillations of a glass bar containing an “artificial crack” in the form of a saw-cut with a tightly inserted small metal plate are reported. In such a configuration, the increase of the resonator quality factor with increasing wave amplitude (denoting a decrease of dissipation which will be referred to as self-induced transparency) has been observed indicating an important role of the amplitude-dependent losses introduced by the inter-surface contacts. The self-induced transparency manifests itself also by the discontinuities (jumps) in the acoustic wave amplitude measured as a function of sweeping excitation frequency around the sample eigenfrequencies and by a self-modulation instability of the primary acoustic wave. This instability leads to the generation of side-lobes in the wave spectrum near the fundamental excitation frequency. The developed theoretical model confirms that all these observations can be self-consistently attributed to nonlinearity of the sound dissipation process. Possible physical mechanisms of the nonlinear dissipation are discussed. Although self-modulation has already been observed in nonlinear acoustical systems, to the knowledge of the authors, the reported data constitute the first observation of the instabilities due to essentially dissipative system behaviour that requires neither nonlinear elasticity nor multimode interaction.     

3维Mortar谱元法模拟Kelvin-Helmhtz不稳定性混合层

采用Mortar谱元法和多处理器并行计算技术模拟了Kelvin-Helmhtz界面不稳定性湍流的混合发展过程,通过对混合层动量厚度、能谱和总动能的计算,评估了Kelvin-Helmhtz混合层的演化机理。计算结果表明:3维Mortar谱元法具有高计算精度和光滑区域的指数收敛特性,可以有效模拟混合层流动的湍流混合和演化,能够捕捉到涡的合并现象和大涡到小涡的级联过程;初期的混合层层流运动发展成具有连续谱结构的湍流运动过程,实现了Kelvin-Helmhtz界面不稳定性混合层流动从2维发展到3维的转捩特征,总湍流统计动能的变化反映了粘性耗散过程的作用。通过对Kelvin-Helmhtz 3维界面不稳定性混合层流动和3维层流向湍流转捩过程的数值模拟,程序的有效性得到了验算,表明谱元法应用于湍流混合模拟是可行的。     

耦合失稳模式与控制策略综合验证技术

为实现宽速域大空域飞行,面对称布局逐渐成为新型航天飞行器的典型特征,随之而来的还有横航向耦合问题,以及由此导致的失稳现象等.文章阐述了新型航天飞行器横航向耦合问题的成因和研究重要性,给出了针对耦合失稳模式和新型控制策略的综合验证方案,通过CFD-RBD仿真方法验证了荷兰滚失稳模式和副翼操纵耦合失稳模式的正确性,通过风洞虚拟飞行试验技术验证了新型控制策略的有效性,它可降低舵偏需求45%以上.      

Three-dimensional evolution of a relativistic current sheet: triggering of magnetic reconnection by the guide field

The linear and nonlinear evolution of a relativistic current sheet of pair (e(+/-)) plasmas is investigated by three-dimensional particle-in-cell simulations. In a Harris configuration, it is obtained that the magnetic energy is fast dissipated by the relativistic drift kink instability (RDKI). However, when a current-aligned magnetic field (the so-called "guide field") is introduced, the RDKI is stabilized by the magnetic tension force and it separates into two obliquely propagating modes, which we call the relativistic drift-kink-tearing instability. These two waves deform the current sheet so that they trigger relativistic magnetic reconnection at a crossover thinning point. Since relativistic reconnection produces a lot of nonthermal particles, the guide field is of critical importance to study the energetics of a relativistic current sheet.     

Instability Entanglement in the Electron Cyclotron Maser

Investigation of absolute instability (AI) and convective instability (CI) of the electron cyclotron maser has independently led to the birth of gyrotron oscillators and amplifiers. Here, it is demonstrated that these instabilities can form a cooperative relationship owing to the nonlinear behavior of the stimulated electron beam. The CI can be induced in a zero-drive system with the assistance of AI, and the ohmic losses of all the excited waves inside the system are greatly reduced, which is called “instability entanglement” here. According to the theoretical and experimental study of a 167/330 GHz gyrotron, when instability entanglement occurs, the ohmic dissipation decreases to one-ninth of the AI-only condition, and the output power is enhanced by 20%. This discovery is promising for surpassing the ceiling of output power and frequency of gyrodevices placed by ohmic losses.     

Chiral magnetic effect in SU(2) lattice gluodynamics at zero temperature

The chiral magnetic effect is the appearance of a quark electric current along a magnetic-field direction in topologically nontrivial gauge fields. There is evidence that this effect is observed in collisions between heavy ions at the RHIC collider. The features of the chiral magnetic effect in SU(2) lattice gluodynamics at zero temperature have been investigated. It has been found that the electric current increases in the magnetic-field direction owing to quantum fluctuations of gluon fields. Fluctuations of the local charge density and chirality also increase with the magnetic field strength, which is a signature of the chiral magnetic effect.     

Low-threshold absolute parametric decay instabilities in experiments on electron cyclotron resonance heating in tokamaks

We have analyzed experimental conditions for the excitation of absolute parametric decay instabilities accompanying the electron cyclotron resonance heating (ECRH) of plasma at the second harmonic of resonance in tokamaks. It has been shown that, in the case of a nonmonotonic radial profile of the plasma density, when a heating beam passes near the equatorial plane of a tokamak, the parametric excitation of resonances of ion Bernstein waves accompanied by the generation of the backscattered microwave radiation can occur. The threshold of absolute instability thus developed is determined by the dissipation of an ion Bernstein wave and can be exceeded in current experiments on the ECRH of a plasma in tokamaks.     

Universal properties of the nonadiabatic acceleration of ions in current sheets

The electric-field-induced acceleration of ions in current sheets in a collisionless plasma is investigated. The analysis of nonadiabatic ion dynamics provides a universal property of the ion acceleration mechanism, which is independent of the magnetic-field model and the initial particle distribution function. The width of the resonance region is estimated. The theoretical results are compared with the experimental and numerical simulation data.     

Splitting of thin current sheets in the Earth’s magnetosphere

An analytic model of a one-dimensional self-consistent anisotropic thin current sheet is proposed. This model describes the sheet with a split (or bifurcated) structure, where the current density is minimal at the center and maximal at the edges. The model is specified by the set of Vlasov-Maxwell equations that reduces to the Grad-Shafranov equation. Under the assumption that particles move quasi-adiabatically, i.e., that the approximate integral of motion I_z is conserved, the slow evolution of the system in the course of diffusion of the distribution function in I_z is analyzed. Scattering processes can give rise to the partial capture of flying ions near the current sheet. Since the current of such quasi-trapped particles is directed oppositely to the current of flying particles, the local current at the center of the sheet is fully or partially compensated. As a result, the ordinary single-peak shape of the current density profile changes to the bifurcated shape. Such a structure is characteristic of the thin current sheet before the total destruction, when the tension of the magnetic field is unbalanced. Numerical calculations are corroborated by the observations of split current sheets in the magnetotail by the Cluster and Geotail satellites. The obtained results indicate that a possible mechanism of the destruction of the thin current sheet is not necessarily associated with the development of plasma instabilities but can be evolutionary.     

Current sheets in magnetic configurations with singular X-lines

The possibility of the formation of current sheets in 3D magnetic configurations with singular X-lines was studied experimentally. It is shown that a sheet can be formed in the presence of the longitudinal magnetic-field component directed along the X-line, in which case the longitudinal component can exceed the transverse component everywhere inside the plasma. Characteristic of the CS formation in 3D magnetic configurations with X-lines are an increase in the longitudinal magnetic-field component inside the sheet and a decrease in the plasma compression ratio as compared to 2D configurations with null-lines. If the longitudinal component exceeds a certain critical value, a sheet cannot be formed: instead of a sheet, there appear two sheaths separated by a cavity with a local minimum in the electron density.     

Nonlinear Evolution of Lower-Hybrid Drift Instability in Harris Current Sheet

We perform 2.5-dimensional particle-in-cell simulations to investigate the nonlinear evolution of the lower hybrid drift instability (LHDI) in Harris current sheet. Due to the drift motion of electrons in the electric field of the excited low hybrid drift (LHD) waves, the electrons accumulate at the outer layer, and therefore there is net positive charge at the inner edge of the current sheet. This redistribution of charge can create an electrostatic field along the z direction, which then modifies the motions of the electrons along the y direction by E×B drift. This effect strongly changes the structure of the current sheet.     

Instabilities in Yukawa Liquids

A strongly coupled Yukawa liquid is a system of charged particles which interact via a screened Coulomb interaction and in which the electrostatic energy between neighboring particles is larger than their thermal energy but not large enough for crystallization. Various plasma systems including ultracold neutral plasmas and complex (dusty) plasmas can exist in this strongly coupled liquid phase.Here we investigate instabilities driven by the relative streaming of plasma components in three‐dimensional Yukawa liquids with a focus on complex plasmas. This includes a dust acoustic instability driven by weakly coupled ions streaming through the dust liquid, and a dust‐dust instability driven by the counter‐streaming of strongly coupled dust grains. Compared to the Vlasov behavior we find there can be a substantial modification of the unstable wavenumber spectrum due to strong coupling effects (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)