Dynamics and instability of electron phase-space tubes

Two-dimensional simulations of beam-driven turbulence in the auroral ionosphere have shown the formation and instability of phase-space tubes. These tubes are a generalization of electron phase-space holes in a one-dimensional plasma. In a strongly magnetized plasma, such tubes vibrate at frequencies below the bounce frequency of the trapping potential. A theory for these vibrations yields quantitative agreement with kinetic simulations. Furthermore, the theory predicts that the vibrations can become unstable when resonantly coupled to electrostatic whistlers-also in agreement with simulations.     

A new multi-path scheme for adaptive computation-aware motion estimation

Ability to work in a computation-limited and computation-variant environment described as “computational awareness” is a desirable feature of a real-time motion estimation system. In this paper, a new multi-path computation-aware algorithm is proposed. One-pass scheme, presented in the prior work by Chen et al., considers only one candidate point which is the median of neighbouring motion vectors (MVs). However, when computational resources are abundant, it is possible to investigate more search paths around points from the prediction set and get a significant improvement both in terms of quality and utilization of available search points (SPs). There are also other enhancements introduced regarding the way of selection of the starting SP and the search strategy, and allocation of resources, which leads to higher PSNR and a better utilization of computational resources.     

Optical phase-space reconstruction of mirror motion at the attometer level

We describe an experiment in which the quadratures of the position of an harmonically-bound mirror are observed at the attometer level. We have studied the Brownian motion of the mirror, both in the free regime and in the cold-damped regime when an external viscous force is applied by radiation pressure. We have also studied the thermal-noise squeezing when the external force is parametrically modulated. We have observed both the % theoretical limit of squeezing at low gain and the parametric oscillation of the mirror for a large gain. Received 9 July 2002 Published online 29 October 2002 RID="a" ID="a"e-mail: heidmann@spectro.jussieu.fr RID="b" ID="b"Unité mixte de recherche du Centre National de la Recherche Scientifique, de l'école Normale Supérieure et de l'Université Pierre et Marie Curie. Website: www.spectro.jussieu.fr/Mesure     

Numerical studies on the phase-space evolution of relative motion of two heavy ions

The two-dimensional Fokker-Planck equation describing the phase-space evolution of the reduced density matrixf(r, v; t) with respect to radial distancer and radial velocityv is numerically solved for various cases of inelastic heavy-ion collisions. A comparison of the exact solution with the global moment approach (GMA) used so far by several authors clearly shows that the GMA is not applicable for deeply inelastic collisions. A local moment approach (LMA) is proposed as an approximate solution to the Fokker-Planck equation.     

An action principle for the motion of particles

An action principle is set up for the motion of charged matter in the presence of the Einstein gravitational field. It can be applied to the motion of an extended particle and avoids the difficulties of a point particle arising from the singularities in the field.     

Averaging scheme for atomic resolution off-axis electron holograms

All micrographs are limited by shot-noise, which is intrinsic to the detection process of electrons. For beam insensitive specimen this limitation can in principle easily be circumvented by prolonged exposure times. However, in the high-resolution regime several instrumental instabilities limit the applicable exposure time. Particularly in the case of off-axis holography the holograms are highly sensitive to the position and voltage of the electron-optical biprism. We present a novel reconstruction algorithm to average series of off-axis holograms while compensating for specimen drift, biprism drift, drift of biprism voltage, and drift of defocus, which all might cause problematic changes from exposure to exposure. We show an application of the algorithm utilizing also the possibilities of double biprism holography, which results in a high quality exit-wave reconstruction with 75 pm resolution at a very high signal-to-noise ratio.     

Alternative equations of motion for the radiating electron

We have previously written down a simple second-order equation for the radiating electron and pointed out that its solutions are well behaved. A key feature of this equation is the presence of a term involving the time derivative of the external fieldf(t). Here, we show that a completely equivalent, but less elegant, equation may be written down which contains no derivatives off(t) but, instead, an infinite number of derivatives of the coordinate. However, it has the merit of displaying explicitly how our result differs from that of Abraham-Lorentz.I have benefited enormously from the stimulating and friendly atmosphere of the Max-Planck-Institute for Quantum Optics, which I experienced during many summer visits. I would like to thank H. Walther for his gracious hospitality on these occasions and also take great pleasure in wishing him a very Happy 60th Birthday     

Amplitude phase-space model for quantum mechanics

We show that there is a close relationship between quantum mechanics and ordinary probability theory. The main difference is that in quantum mechanics the probability is computed in terms of an amplitude function, while in probability theory a probability distribution is used. Applying this idea, we then construct an amplitude model for quantum mechanics on phase space. In this model, states are represented by amplitude functions and observables are represented by functions on phase space. If we now postulate a conjugation condition, the model provides the same predictions as conventional quantum mechanics. In particular, we obtain the usual quantum marginal probabilities, conditional probabilities and expectations. The commutation relations and uncertainty principle also follow. Moreover Schrödinger's equation is shown to be an averaged version of Hamilton's equation in classical mechanics.     

Optical phase-space distributions for low-coherence light

Using a novel heterodyne technique, we measure optical phase-space distributions in momentum and position for low-coherence light. Quantitative information is obtained simultaneously about the longitudinal and the transverse coherence properties as well as the wave-front curvature of the light field. This method can be used to monitor these optical parameters directly for signal fields scattered from samples of interest, for tomographic imaging.     

Phase motion of particles upon formation of subpicosecond electron bunches in a traveling wave under heavy beam loading

The possibility of efficient bunch compression due to the phase motion of particles in the field of a traveling wave is demonstrated. The calculated bunch length is 120 μm, which makes it possible to expect spontaneous coherent radiation from the undulator at a wavelength of 240 μm.     

Pion entropy and phase-space densities in A+A collisions

We propose a method to estimate the entropy of thermal pions in A+A collisions irrespective of the unknown form of freeze-out (isothermal) hypersurface and transverse flows developed. We analyse the average phase-space densities and entropies of the thermal pions vs, their multiplicities and collision energies. The behaviour of these values apparently indicates the deconflnement and chiral phase transitions in relativistic nucleus-nucleus collisions.     

一种绳约束两质点的运动研究

研究一种绳约束下两质点运动的动力学模型和运动规律,分别采用泰勒级数展开的方法和四阶龙格-库塔数值方法,得到了质点运动微分方程的近似解和数值解.通过对系统的动力学方程计算,给出了位置矢量随时间的演化曲线、运动相图和运动轨迹.研究发现系统的运动与初始位置和速度有关,在初速度较大和较小时,系统作非线性周期性运动.存在临界初始速度,使得系统处于平衡态,在微小扰动下系统在初始位置附近作近似简谐振动.以上研究结果对微小振动力学问题的教学和科研具有一定的参考价值.     

Thermostatistics of overdamped motion of interacting particles

We show through a nonlinear Fokker-Planck formalism, and confirm by molecular dynamics simulations, that the overdamped motion of interacting particles at T=0, where T is the temperature of a thermal bath connected to the system, can be directly associated with Tsallis thermostatistics. For sufficiently high values of T, the distribution of particles becomes Gaussian, so that the classical Boltzmann-Gibbs behavior is recovered. For intermediate temperatures of the thermal bath, the system displays a mixed behavior that follows a novel type of thermostatistics, where the entropy is given by a linear combination of Tsallis and Boltzmann-Gibbs entropies.     

Mapping attosecond electron wave packet motion

Attosecond pulses are produced when an intense infrared laser pulse induces a dipole interaction between a sublaser cycle recollision electron wave packet and the remaining coherently related bound-state population. By solving the time-dependent Schr?dinger equation we show that, if the recollision electron is extracted from one or more electronic states that contribute to the bound-state wave packet, then the spectrum of the attosecond pulse is modulated depending on the relative motion of the continuum and bound wave packets. When the internal electron and recollision electron wave packet counterpropagate, the radiation intensity is lower. We show that we can fully characterize the attosecond bound-state wave packet dynamics. We demonstrate that electron motion from a two-level molecule with an energy difference of 14 eV, corresponding to a period of 290 asec, can be resolved.     

General equations of motion for test particles in space-time with torsion

The momentum and spin equations of motion for test particles possessing different spins in space-time with torsion are derived from the most general functional form of _M . The same kinds of equations in general relativity and in Kibble's gauge theory of gravitation are special cases of our equations.     

Equations of motion from field equations and a gauge-invariant variational principle for the motion of charged particles

New, gauge-independent, second-order Lagrangian for the motion of classical, charged test particles is proposed. It differs from the standard, gauge-dependent, first-order Lagrangian by boundary terms only. A new method of deriving equations of motion from field equations is developed. When applied to classical electrodynamics, this method enables us to obtain unambiguously the above, second-order Lagrangian from the general energy-momentum conservation principle.