一种“类耗散系统”中的“类Ⅴ型阵发”

一类不连续不可逆保面积映象可以展示类似耗散的行为,因此可称其为“类耗散系统”.在一种类耗散系统中观察到了椭圆周期轨道及其周围的椭圆岛与映象不连续边界碰撞而消失的现象.周期轨道消失后,经过一系列过渡椭圆周期轨道之后,系统的行为由一个混沌类吸引子主导.在混沌类吸引子刚刚出现时,混沌时间序列呈现层流相与湍流相的无规交替.这一切都与不连续耗散系统中发生的Ⅴ型阵发的相应性质十分相似,因此可称为“类Ⅴ型阵发”.然而,当混沌类吸引子刚刚出现时,仅可以找到最后一个过渡椭圆岛的“遗迹”,并不存在它的“鬼魂”,因此类Ⅴ型阵发不遵从Ⅴ型阵发的特征标度规律.反之,混沌类吸引子的鬼魂却存在于最后一个过渡椭圆周期轨道的类瞬态过程中,因此在类Ⅴ型阵发导致混沌运动的临界点之前,由此“类瞬态混沌奇异集”中逃逸的规律就成为标志这一种临界现象的标度律.这与Ⅴ型阵发又根本不同. 关键词： 类耗散性 类混沌吸引子 类Ⅴ型阵发     

这是否只是一种"美丽的传说"——对伽利略是否在比萨斜塔公开做过落体实验的探讨

一次偶然的机会笔者发现现行高中历史教材（人教版必修3）,在第四单元“近代以来世界的科学发展历程”的第11课“物理学的重大进展”导言部分写道：“17世纪初的一天,在意大利比萨斜塔上进行了一场著名的实验．物理学家伽利略从塔顶抛出了质量不一的两个球,结果这两个球同时落地．这一实验证明了物体的下落速度与物体的质量无关,     

Calculation of a "narrowed" autocorrelation function

A method of calculating an autocorrelation function with extremely narrow peaks is described. This is done by including terms in the autocorrelation expression corresponding to delays at 2 tau, 3 tau, etc., in addition to the usual term with delay tau. Implications in the frequency domain are explored. Graphs of this autocorrelation function for a number of violin sound samples, including a two-octave scale, vibrato, and glissando, are presented. Graphs of the autocorrelation function for some synthetic sound samples are also included.     

One-dimensional "turbulence" in a discrete lattice

We study a one-dimensional discrete analog of the von Karman flow, widely investigated in turbulence. A lattice of anharmonic oscillators is excited by both ends in order to create a large scale structure in a highly nonlinear medium, in the presence of a dissipative term proportional to the second order finite difference of the velocities, similar to the viscous term in a fluid. In a first part, the energy density is investigated in real and Fourier space in order to characterize the behavior of the system on a local scale. At low amplitude of excitation the large scale structure persists in the system but all modes are however excited and exchange energy, leading to a power law spectrum for the energy density, which is remarkably stable against changes in the model parameters, amplitude of excitation, or damping. In the spirit of shell models, this regime can be described in terms of interacting scales. At higher amplitude of excitation, the large scale structure is destroyed and the dynamics of the system can be viewed as resulting from the creation, interaction, and decay of localized excitations, the discrete breathers, the one-dimensional equivalents of vortices in a fluid. The spectrum of the energy density is well described by the spectrum of the breathers, and shows an exponential decay with the wave vector. Due to this exponential behavior, the spectrum is dominated by the most intense breathers. In this regime, the probability distribution of the increments of velocity between neighboring points is remarkably similar to the experimental results of turbulence and can be described by distributions deduced from nonextensive thermodynamics as in fluids. In a second part the power dissipated in the whole lattice is studied to characterize the global behavior of the system. Its probability distribution function shows non-Gaussian fluctuations similar to the one exhibited recently in a large class of "inertial systems," i.e., systems that cannot be divided into mesoscopic regions which are independent. The properties of the nonlinear excitations of the lattice provide a partial understanding of this behavior.     

Imaging with a metamaterial ＂Im-Perfect＂ lens

There is widespread and strong interest in trying to fabricate a metamaterial in which both the permittivity and permeability are equal to - 1 in order to achieve sub-wavelength imaging. Several metamaterial constructs have been proposed with varying degrees of suceess because of inherent losses, limited bandwidth and scattering from the abstracted circuit elements constituting the artificial material itself. A further limitation is the need to capture evanescent components from the object to be imaged that requires the lens to be located near the object. We have studied the underlying models and constraints that influence the design of a negative index lens and present this analysis as well as reviewing the opportunities. There are inevitable and well-known trade-offs between lens thickness, wavelength, dispersion and absorption. However, these can be eharaeterized both numerically and experimentally, suggesting that a computational imaging approach to the recovery of sub-wavelength features might be effective. Depending on the specific details of the metamaterial employed for imaging, one can consider the data acquired to represent a set of coded apertures.     

High-resolution NMR "chromatography" using a liquids spectrometer

NMR spectroscopy is an excellent tool for the structural analysis of pure compounds. However, for mixtures it performs poorly because of overlapping signals. Diffusion can be used to separate compounds of widely differing molecular weight but the amount of separation is usually insufficient.Addition of a solid medium, analogous to the stationary phase in chromatography, can preferentially slow the diffusion of some components of a mixture permitting separation in the diffusion dimension. However, this would usually require a solid-state NMR spectrometer otherwise the signals would be too broad.Susceptibility matching the solvent to the solid medium yields a spectrum with narrow signals allowing the measurement of a DOSY spectrum with enhanced separation in the diffusion dimension.     

"Snowflake" H mode in a tokamak plasma

An edge-localized mode (ELM) H-mode regime, supported by electron cyclotron heating, has been successfully established in a "snowflake" (second-order null) divertor configuration for the first time in the TCV tokamak. This regime exhibits 2 to 3 times lower ELM frequency and 20%-30% increased normalized ELM energy (ΔWELM/Wp) compared to an identically shaped, conventional single-null diverted H mode. Enhanced stability of mid- to high-toroidal-mode-number ideal modes is consistent with the different snowflake ELM phenomenology. The capability of the snowflake to redistribute the edge power on the additional strike points has been confirmed experimentally.