Numerical coupling of Landau damping and Raman amplification

In this paper, we present a numerical model for laser-plasma interaction involving Raman instability and Landau damping. This model exhibits three main difficulties. The first one is the coupling of PDE’s posed both in Fourier space and in physical space. The second one is a three-waves resonance condition that has to be verified. The third one is the boundary conditions. We overcome these difficulties using, respectively a splitting scheme, a numerical dispersion relation and absorbing boundary conditions. We present some comparison between several phenomena that are involved and the influence of the Raman amplification and the Landau damping.     

Acoustical power amplification and damping by temperature gradients

Ceperley proposed a concept of a traveling wave heat engine ["A pistonless Stirling engine-The traveling wave heat engine," J. Acoust. Soc. Am. 66, 1508-1513 (1979).] that provided a starting point of thermoacoustics today. This paper verifies experimentally his idea through observation of amplification and strong damping of a plane acoustic traveling wave as it passes through axial temperature gradients. The acoustic power gain is shown to obey a universal curve specified by a dimensionless parameter ωτα; ω is the angular frequency and τα is the relaxation time for the gas to thermally equilibrate with channel walls. As an application of his idea, a three-stage acoustic power amplifier is developed, which attains the gain up to 10 with a moderate temperature ratio of 2.3.     

Error field amplification and rotation damping in tokamak plasmas

Toroidal rotation is normally very weakly damped in plasmas that are magnetically confined in the nominally toroidally symmetric tokamak. However, a strong damping of toroidal rotation is observed as such plasmas approach marginal stability for perturbations that produce a kinklike distortion of the plasma. It is shown that the damping of toroidal rotation by very small departures of the magnetic field from toroidal symmetry is greatly enhanced as marginal stability is approached. The response of a plasma to perturbations is studied using a set of electrical circuit elements, which provide an equation for the rotational damping that requires minimal information about the plasma.     

Interaction, damping and possible amplification of 2D plasmons in asymmetric bilayer structures

It is shown that the asymmetry of a double quantum well structure considerably affects the dispersion relations for the optical and intersubband plasmons. A specific mechanism of the plasmon damping arises due to a resonance of the plasmon energy and that of single-particle intersubband transitions. If the population of the subbands is inverted, an amplification of the plasma waves becomes possible.     

The separability of two-mode Gaussian state under amplification and symmetric damping

The performances of a two-mode Gaussian state under parametric amplification, symmetric amplitude damping and thermal noise are studied. The time-dependent complex correlation matrix of the state in evolution is given. The separability of the final two-mode Gaussian state is examined under symmetric amplification and asymmetric amplification separately.     

Vibration amplification, damping, and self-oscillations in micromechanical resonators induced by optomechanical coupling through carrier excitation

Carrier-induced dynamic backaction in micromechanical resonators is demonstrated. Thermal vibration of an n-GaAs/i-GaAs bilayer cantilever is amplified by optical band-gap excitation, and for the excitation power above a critical value, self-oscillations are induced. These phenomena are found in the [1[over ˉ]10]-oriented cantilever, whereas the damping (deamplification) is observed in the [1[over ˉ]10] orientation. This optomechanical coupling does not require any optical cavities but is instead based on the piezoelectric effect that is generated by photoinduced carriers.     

Reduction of bridge dynamic amplification through adjustment of vehicle suspension damping

This paper presents a novel approach to the reduction of short-span bridge dynamic responses to heavy vehicle crossing events. The reductions are achieved through adjustment of the vehicle suspension damping coefficient just before the crossing. Given pre-calculations of the response of a vehicle-bridge system to a set of ‘unit’ road disturbances, it is shown that a single optimum damping coefficient may be determined for a given velocity and any specified road profile. This approach can facilitate implementation since the optimum damping is selected prior to the bridge and there is no need to continuously vary the damping coefficient during the crossing. The concept is numerically validated using a bridge-vehicle interaction model with several road profiles, both measured and artificially generated. The bridge-friendly damping control strategy is shown to reduce bridge dynamics across a typical range of vehicle velocities, proving most effective for road profiles that induce large vibrations in the vehicle-bridge system.     

Wave motion and dispersion phenomena: veering, locking and strong coupling effects

The dispersion curves describe wave propagation in a structure, each branch representing a wave mode. As frequency varies the wavenumbers change and a number of dispersion phenomena may occur. This paper characterizes, analyzes, and quantifies these phenomena in general terms and illustrates them with examples. Two classes of phenomena occur. Weak coupling phenomena-veering and locking-arise when branches of the dispersion curves interact. These occur in the vicinity of the frequency at which, for undamped waveguides, the dispersion curves in the uncoupled waveguides would cross: if two dispersion curves (representing either propagating or evanescent waves) come close together as frequency increases then the curves either veer apart or lock together, forming a pair of attenuating oscillatory waves, which may later unlock into a pair of either propagating or evanescent waves. Which phenomenon occurs depends on the product of the gradients of the dispersion curves. The wave mode shapes which describe the deformation of the structure under the passage of a wave change rapidly around this critical frequency. These phenomena also occur in damped systems unless the levels of damping of the uncoupled waveguides are sufficiently different. Other phenomena can be attributed to strong coupling effects, where arbitrarily light stiffness or gyroscopic coupling changes the qualitative nature of the dispersion curves.     

The role of linear damping for explosive instabilities

It is shown that conservation laws of energy and momentum contain often sufficient information on various weakly nonlinear interactions. The changes in the temporal development of an explosive instability or its stabilization due to the linear damping of the interacting waves are discussed in more detail.Nademlýnská 600, Praha 9, Czechoslovakia.     

Relaxation,amplification, and the KMS conditions

We formulate the dynamics of an oscillator, $\text{gS}$, harmonically coupled to an infinite quasifree bose system Σ, the initial states of Σ and Σ being mutually uncorrelated and the initial state of Σ being quasifree, primary, and stationary. It is shown that, subject to certain regularity conditions, the oscillator $\text{gS}$ will either relax towards a terminal state, whose value is independent of the initial state of the oscillator, or else will become amplified, according to whether the energy quantum for $\text{gS}$ is positive or negative. In the former case, the weak-coupling limit for the terminal state of $\text{gS}$ is a Gibbs state, corresponding to a temperature which takes the same value for all $\text{Σ-}\text{gS}$ couplings of the prescribed class, if and only if the initial state of Σ satisfies the KMS conditions for that temperature. We argue that this result provides an operational justification for identifying equilibrium conditions with KMS conditions, at least for the class of stationary states of Σ that we consider.     

激光驱动的冲击波自生磁场以及外加磁场的冲击波放大研究

冲击波是天体物理观测中常见的现象, 其对粒子的加速被认为是高能宇宙射线的来源. 宇宙中冲击波周围往往存在很强的磁场, 但人们对于此类强磁场的产生放大过程的理解并不充分. 本文利用二维粒子模拟程序研究了激光与磁化或者非磁化等离子体相互作用产生的冲击波现象, 给出了冲击波波前处磁场的产生放大特性. 研究发现, 作用过程中的自生磁场可以储存能量, 从而进一步加速电子; 当存在外加磁场时, 由冲击波加速的电子和离子的能量都比同条件下非磁化等离子体的能量高; 而且外加磁场藉由冲击波放大倍数则与其值有极大关系. 与天文观测中推断的磁场与背景磁场相比放大千倍这一研究结果的比较可以看出, 天体冲击波周围磁场放大主要是由局域内生磁场导致的.     

Caldeira-Leggett model, Landau damping, and the Vlasov-Poisson system

The Caldeira-Leggett Hamiltonian describes the interaction of a discrete harmonic oscillator with a continuous bath of harmonic oscillators. This system is a standard model of dissipation in macroscopic low temperature physics, and has applications to superconductors, quantum computing, and macroscopic quantum tunneling. The similarities between the Caldeira-Leggett model and the linearized Vlasov-Poisson equation are analyzed, and it is shown that the damping in the Caldeira-Leggett model is analogous to that of Landau damping in plasmas (Landau, 1946 [1]). An invertible linear transformation (Morrison and Pfirsch, 1992 [18]; Morrison, 2000 [19]) is presented that converts solutions of the Caldeira-Leggett model into solutions of the linearized Vlasov-Poisson system.     

Gravitational radiation,radiation damping,and the fast-motion approximation

The sign of the gravitational energy radiated by a material system, as calculated by one or another appropriate stress-energy pseudotensor, is shown always to be positive. A fastmotion approximation that finds this quantity to be negative is shown to violate one of the conditions that ensure its positivity. As a consequence the calculation based on this approximation is shown to neglect terms of the same order (G ²) as it retains.     

The role of magnetic damping in the r-mode evolution of accreting neutron stars

The magnetic damping rate was introduced in the evolution equations of r-modes,which shows that r-modes can generate strong toroidal magnetic fields in the core of accreting millisecond pulsars inducing by differential rotation.With consideration of the coupling evolution of r-modes,spin and thermal evolution,we investigated the influence of the magnetic damping on the differential rotation of nonlinear r-modes of accreting neutron stars.We derived the coupling evolution equations of the star involving the magnetic damping rate in the framework of second-order r-mode theory.The numerical results show that the magnetic damping suppressed the nonlinear evolution of r-modes since the saturation amplitude is reduced to a great extent.In particular,because of the presence of the generated toroidal magnetic field,the spin-down of the stars is terminated and the viscous heating effects are also weakened.Moreover,we could obtain a stronger generated toroidal magnetic field in the second-order r-mode theory.The gravitational radiation may be detected by the advanced laser interferometer detector LIGO if the amount of differential rotation is small when the r-mode instability becomes active and the accretion rate is not very high.     

Collective modes,damping, and the scattering function in classical liquids

An exact representation for the density-density response function is presented. This representation is a generalization of the result obtained in the mean field approximation and amounts to replacing the static, effective potential by one which is both wavenumber- and frequency-dependent. This potential possesses both a real and an imaginary part; the latter describes the collisional damping of collective modes. Analyticity and sum rule arguments are used to describe the basic properties of this complex potential. The formalism allows us to write an exact formula for the scattering functionS(k, ) in which the basic unknown is the collisional damping function. Using a small portion of the recent experimental data on coherent neutron scattering in liquid argon, we are able to calculateS(k, ) and other quantities of interest and to make comparisons with the rest of the data.     

Radiation-spin interaction, Gilbert damping, and spin torque

Magnetization relaxation processes, which are represented by the Gilbert damping term and the spin torque term in the Landau-Lifshitz-Gilbert (LLG) equation, are described by the radiation-spin interaction (RSI), where the radiation field is produced by magnetization precessional motion itself. It is shown that the LLG equation including the Gilbert damping term and the spin torque term is derived from the spin Hamiltonian containing the RSI. The derivation of the LLG equation is given in a self-consistent method. It is also shown that, according to RSI, the magnitude of the magnetization vector deviates from the magnetization saturation with the order of O(alpha(2)), where alpha is the Gilbert damping parameter.     

Generation,stabilization, and amplification of subpicosecond pulses

A sync-pumped cw dye laser system has been used to produce subpicosecond pulses. Pulses as short as 0.7 ps, assuming a single-sided exponential pulse shape, were observed. A set of experiments was performed to investigate the origin and effects of noise in the sync-pumped system. A digital and an analog feedback loop have been designed to optimize the pulse width. The noise has been lowered by 10 dB for frequencies up to 10 kHz; long-term drift is also controlled by this method. A four-stage dye laser amplifier, pumped by a Nd:YAG laser which operates at a 10-Hz repetition rate, is synchronized electronically to the dye-laser picosecond pulses. A gain of 3×10⁶ has been achieved.This work was supported by the Joint Services Electronics Program