Competition between Langmuir and upper-hybrid turbulence in a high-frequency-pumped ionosphere

We show how the secondary escaping radiation, also known as stimulated electromagnetic emission (SEE), from the ionosphere irradiated by a high-intensity radio beam, can be used to study both reflection altitude ponderomotive parametric instabilities and upper-hybrid altitude thermal parametric instabilities. This has allowed us to observe the transfer of energy from smaller to higher sideband frequency offsets and to identify a new transient SEE feature.     

Shear instabilities in granular mixtures

Dynamical instabilities in fluid mechanics are responsible for a variety of important common phenomena, such as waves on the sea surface or Taylor vortices in Couette flow. In granular media dynamical instabilities have just begun to be discovered. Here we show by means of molecular dynamics simulation the existence of a new dynamical instability of a granular mixture under oscillating horizontal shear, which leads to the formation of a striped pattern where the components are segregated. We investigate the properties of such a Kelvin-Helmholtz-like instability and show how it is connected to pattern formation in granular flow and segregation.     

Parametric excitation of magnetic electron drift vortex modes by Langmuir waves

A new multi-dimensional parametric interaction coupling Langmuir waves with magnetic electron drift vortex modes is presented. A general dispersion relation for the parametric instabilities is derived, and the growth rates of the decay and modulational instabilities are obtained. The present instabilities can be the source of large-scale magnetic fields near the critical surface of a laser-produced plasma.     

Friction of wrinkles

Sufficiently thin elastic sheets wrinkle when they are in contact with a small adhesive counterbody. Despite significant progress on the dynamics of wrinkle formation and morphology, little is known about how wrinkles impede the relative sliding motion of the counterbody. Using molecular dynamics we demonstrate that instabilities are likely to occur during sliding when the wrinkle pattern has asymmetries not present in the counterbody. The instabilities then cause Coulomb's friction law. The behavior can be rationalized in terms of simple models for multistable elastic manifolds.     

Limit cycles of the ballast resistor caused by intrinsic instabilities

Spatio-temporal patterns of the ballast resistor are investigated. It is well known that in a voltage-controlled ballast resistor an electrothermal instability leads to stable stationary states consisting of hot and cold domains. Such states may become oscillatory unstable, giving rise to the bifurcation of limit cycles. These limit cycles are not caused by the external circuit but by a recently proposed novel intrinsic mechanism. There are two types of oscillatory instabilities: bulk instabilities and boundary-induced instabilities. The bulk instabilities are caused by resistivities which are not monotonically increasing functions of the temperature. The boundary-induced instabilities occur in small systems with Neumann boundary conditions. To find the bulk instability, experiments with materials showing a metal-semiconductor transition or high-temperature superconductors are suggested. To understand these new phenomena, the equation of motion is reduced to ordinary differential equations where the instabilities can be discussed analytically.     

General theory of microscopic dynamical response in surface probe microscopy: from imaging to dissipation

We present a general theory of atomistic dynamical response in surface probe microscopy when two solid surfaces move with respect to each other in close proximity, when atomic instabilities are likely to occur. These instabilities result in a bistable potential energy surface, leading to temperature dependent atomic scale topography and damping (dissipation) images. The theory is illustrated on noncontact atomic force microscopy and enables us to calculate, on the same footing, both the frequency shift and the excitation signal amplitude for tip oscillations. We show, using atomistic simulations, how dissipation occurs through reversible jumps of a surface atom between the minima when a tip is close to the surface, resulting in dissipated energies of 1.6 eV. We also demonstrate that atomic instabilities lead to jumps in the frequency shift that are smoothed out with increasing temperature.     

Stabilization of some dissipative instabilities by means of h.f. electromagnetic field

The stabilizing effect of an h.f. electromagnetic field is studied in the case of dissipative instabilities. It is shown theoretically that a decrease of the growth rate of several instabilities takes place. An explanation of the new stabilizing mechanism is presented.     

蒸发两层流系统的对流不稳定性分析

关于蒸发液层的Rayleigh-Marangoni-Benard不稳定性的研究中,早期文献中普遍采用的是单层流模型。近年来,一些学者采用两层流模型对蒸发稳定性进行了理论分析,有的文献中没有考虑蒸发率与饱和蒸汽压的耦合关系,所以得到的结果不能完全反应蒸发对系统稳定性的影响。本文建立了一种新的两层流模型,考虑了界面变形对系统稳定性的影响。采用线性稳定性方法对带有蒸发界面的两层流的Rayleigh-Marangoni-Benard对流不稳定性进行了分析,得到了临界 Marangoni数与波数的关系,重点讨论了蒸发系数以及重力对汽液两层流系统的不稳定性的影响。     

Numerical studies of QGP instabilities and implications

Because of the flat initial shape of the QGP in a heavy-ion collision, the momentum distribution becomes anisotropic after a short time. This leads to plasma instabilities, which may help explain how the plasma isotropizes. We explain the physics of instabilities and give the latest results of numerical simulations into their evolution. Non-Abelian interactions cut off the size to which the soft unstable fields grow, and energy in the soft fields subsequently cascades towards more ultraviolet scales. We present first results for the power spectrum of this cascade.     

New instabilities of free-electron laser with an axial guide magnetic field

In this paper, the unperturbed orbits, their stable conditions and the single-pass gain of a free-electron laser have been treated with a single-particle theory to show the existence of a couple of new instabilities and moreover, to discuss their effects. Numerical analyses reveal that the new instabilities will probably affect the operation of free-electron laser when the guide field increases and get into the vicinity of its stability boundary.     

Analyse quantitative par moyens optiques des instabilities d'ecoulement en regime supersonique

Previous articles described a method of tomographic visualizations of turbulent mixing zones, stationary structures and instabilities in supersonic flows. This paper shows new improvements of this method which thus becomes quantitative and able to give the nature of such instabilities (unstationary flow) and the parameters of the shock waves oscillations.     

Einstein Gravity in Almost Kähler Variables and Stability of Gravity with Nonholonomic Distributions and Nonsymmetric Metrics

We argue that the Einstein gravity theory can be reformulated in almost Kähler (nonsymmetric) variables with effective symplectic form and compatible linear connection uniquely defined by a (pseudo) Riemannian metric. A class of nonsymmetric theories of gravitation on manifolds enabled with nonholonomic distributions is considered. We prove that, for certain types of nonholonomic constraints, there are modelled effective Lagrangians which do not develop instabilities. It is also elaborated a linearization formalism for anholonomic noncommutative gravity theories models and analyzed the stability of stationary ellipsoidal solutions defining some nonholonomic and/or nonsymmetric deformations of the Schwarzschild metric. We show how to construct nonholonomic distributions which remove instabilities in nonsymmetric gravity theories. It is concluded that instabilities do not consist a general feature of theories of gravity with nonsymmetric metrics but a particular property of some models and/or unconstrained solutions.     

Universal geometric condition for the transverse instability of solitary waves

Transverse instabilities correspond to a class of perturbations traveling in a direction transverse to the direction of the basic solitary wave. Solitary waves traveling in one space direction generally come in one-parameter families. We embed them in a two-parameter family and deduce a new geometric condition for transverse instability of solitary waves. This condition is universal in the sense that it does not require explicit properties of the solitary wave-or the governing equation. In this paper the basic idea is presented and applied to the Zakharov-Kuznetsov equation for illustration. An indication of how the theory applies to a large class of equations in physics and oceanography is also discussed.     

Nonlinear simulations of combustion instabilities with a quasi-1D Navier-Stokes code

As lean premixed combustion systems are more susceptible to combustion instabilities than non-premixed systems, there is an increasing demand for improved numerical design tools that can predict the occurrence of combustion instabilities with high accuracy. The inherent nonlinearities in combustion instabilities can be of crucial importance, and we here propose an approach in which the one-dimensional (1D) Navier-Stokes and scalar transport equations are solved for geometries of variable cross-section. The focus is on attached flames, and for this purpose a new phenomenological model for the unsteady heat release from a flame front is introduced. In the attached flame method (AFM) the heat release occurs over the full length of the flame. The nonlinear code with the use of the AFM approach is validated against analytical results and against an experimental study of thermoacoustic instabilities in oxy-fuel flames by Ditaranto and Hals [Combustion and Flame 146 (2006) 493-512]. The numerical simulations are in accordance with the experimental measurements and the analytical results and both the frequencies and the amplitudes of the resonant acoustic pressure modes are reproduced with good accuracy.     

The gyrokinetic water-bag modeling in toroidal geometry

The present paper addresses the gyrokinetic water-bag model in toroidal geometry. The previous works were focused on the water-bag concept in magnetized cylindrical plasmas. Here we report on the possibility to improve the water-bag model by taking into account the curvature and gradient drifts. After a presentation of the model, a local linear analysis with some approximations is performed. Interchange and ion temperature gradient instabilities are examined with this new gyro-water-bag model in order to show its ability and its theoretical interest in describing kinetic instabilities in toroidal geometry.     

Coherent structures and entropy in constrained, modulationally unstable, nonintegrable systems

Many studies have shown that nonintegrable systems with modulational instabilities constrained by more than one conservation law exhibit universal long time behavior involving large coherent structures in a sea of small fluctuations. We show how this behavior can be explained in detail by simple thermodynamic arguments.     

Rapid large-scale magnetic-field dissipation in a collisionless current sheet via coupling between Kelvin-Helmholtz and lower-hybrid drift instabilities

Rapid large-scale magnetic-field dissipation is observed in a full kinetic simulation of cross-field current instabilities in a current sheet even when the thickness of the current sheet is at ion scale. The Kelvin-Helmholtz instability caused by the velocity shear between the current-carrying ions and the cold background ions excites the lower-hybrid drift instability at the edges of the undulated current sheet. We show that the nonlinear coupling between these two instabilities is responsible for the observed rapid dissipation. The simulation result presents a new route for magnetic-field dissipation in an ion-scale current sheet and demonstrates the general significance of nonlinear cross-scale coupling in collisionless plasmas.