强激光辐照非均匀等离子体引起临界点密度变陡

本文在临界密度区域内激光场趋于零的假设下,系统地研究了由强激光辐照非均匀等离子体引起临界密度变陡的现象,给出了较为精确、完备的描述等离子体密度结构的各种参数和等离子体密度的坪台结构。     

四层不对称波导的一些近似关系式

本文在四层不对称波导的归一化电场方程和本征方程基础上,导出了归一化有效折射b表达式和限制因子Γ_x等的近似关系式,并提出了不对称近场和远场高斯分布的近似表达式。应用上述分析,研究了四层不对称波导的动态特性。文中导出了注入载流子分布,增益分布的解析表达式,可方便地计算模增益,阈值电流密度,最佳有源区厚度和最佳阈值电流密度。并进一步研究了结构参数对器件的影响。 关键词：     

Field theoretical approach to inhomogeneous ionic systems: thermodynamic consistency with the contact theorem,Gibbs adsorption and surface tension

Using a statistical field approach we investigate the structure of an electrolyte solution in contact with a neutral impenetrable wall. The Hamiltonian contains the Coulomb interaction and the ideal entropy. At the level of the quadratic approximation, the Hamiltonian yields the Debye-Hückel theory in the bulk. Analytic expressions of the charge-charge and potential-potential inhomogeneous correlation functions are obtained. Exact asymptotic results for point ion charge correlation functions are obtained and the profile for the fluctuation of the electric potential is calculated. We also consider the term beyond the quadratic expansion of the ideal entropy in the Hamiltonian. With this term a higher order coupling between charge density and number density produces a non-trivial profile for the total ion density. This density profile is consistent with the contact theorem and the related surface tension calculated from the Gibbs adsorption isotherm.     

The space-time-averaging procedure and modeling of the RF discharge

An approach to modeling RF discharges and the ensuing analysis of fast electron and ion motions for the case of electrode sheaths in the high-pressure RF discharge is discussed. Time-averaging over fast electron motions with the applied voltage frequency gives analytic expressions for the average electric field and average ionization density. The resulting relatively simple equations for the ion density profile describe drift, diffusion, ionization, and recombination processes. The simple scaling rules, the approximate expressions for the density profile in various regions, the sheath length, the ion density at the plasma-sheath boundary, and the dimensionless criteria for various discharge regimes can be deduced. For the non-self-sustained discharge, it is demonstrated that the ion drag towards the electrode and the diffusion results in significant lowering of the ion density in the sheath compared with the positive column at not too high a pressure. The analytic transition criterion from α to γ forms of the self-sustained discharge is obtained. The numerical solution of the averaged ion equations yields the results which nearly coincide with the results of full-scale modeling     

System of magnetic particles in an external magnetic field: Surface and bulk structure

A system of particles with spin interaction in an external magnetic field is studied using a general formalism applicable to the solid, liquid, or glassy state. Explicit results are given in the mean spherical, LOGA, and EXP approximations. The Laplace transform of the wall-particle correlation function near a surface is obtained. From it, analytical expressions for the orientational density profile are derived and used to calculate the magnetization and magnetic susceptibility in the bulk. Magnetostriction is also discussed.     

Density correlations and analog dynamical Casimir emission of Bogoliubov phonons in modulated atomic Bose-Einstein condensates

We present a theory of the density correlations that appear in an atomic Bose-Einstein condensate as a consequence of the emission of correlated pairs of Bogoliubov phonons by a time-dependent atom-atom scattering length. This effect can be considered as a condensed matter analog of the dynamical Casimir effect of quantum field theory. Different regimes as a function of the temporal shape of the modulation are identified and a simple physical picture of the phenomenon is discussed. Analytical expressions for the density correlation function are provided for the most significant limiting cases. This theory is able to explain some unexpected features recently observed in numerical studies of analog Hawking radiation from acoustic black holes.     

Elucidating alkane adsorption in sodium-exchanged zeolites from molecular simulations to empirical equations

Configurational-bias Monte Carlo (CBMC) simulations provide adsorption isotherms, Henry coefficients and heats of adsorption of linear alkanes in sodium-exchanged MFI- and FAU-type zeolites. These simulations were carried out using our newly developed force field that reproduces experimental sodium positions in the dehydrated zeolites, and successfully predicts alkane adsorption properties over a wide range of sodium cation densities, temperatures, and pressures. We derived empirical expressions from the simulation data to describe the adsorption of linear alkanes in MFI- and FAU-type zeolites. These expressions afford a suitable substitute for complex CBMC simulations. In the low coverage regime we provide simple expressions that adequately describe the Henry coefficient and adsorption enthalpy of n-alkanes as a function of sodium density and temperature. The predicted Henry coefficients and heats of adsorption compare extremely well to available experimental data. In the high coverage regime we provide an expression for saturation capacities of linear alkanes in the zeolite. This expression, combined with the expression for the Henry coefficients, provides of the complete adsorption isotherms of pure adsorbents and mixtures, in good agreement with the adsorption isotherms obtained from CBMC.     

Using the Steepened Plasma Profile and Wave Breaking Threshold in Laser‐Plasma Interaction

In this work we evaluate the interaction of high intense laser beam with a steepened density profile. During laser interaction with underdense plasma by freely expanding plasma regime, modification of density profile is possible. In this paper we have investigated the ultra short laser pulse interaction with nonisothermal and collisionless plasma. We consider self–focusing as an effective nonlinear phenomenon that tends to increase when the laser power is more than critical rate. By leading the expanded plasma to a preferred location near to critical density, laser reflection is obtained, so the density profile will be locally steepened. The electromagnetic fields are evaluated in this new profile. We show the amplitude and period of electrical field oscillation are increased by reducing the steepened scale length. Also our numerical results identify that by reducing the steepened scale length, the electrical field is increased to wave breaking threshold limit. This high gradient electrical field causes the effective beam loading during the wave breaking phenomenon. The wave breaking can be the initial point for other acceleration regime as cavity or channel guiding regime. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Profile of photorefractive one-dimensional bright spatial solitons

Analytic solutions for the profile of one-dimensional bright photorefractive steady-state spatial solitons generated as a result of screening of drift or photogalvanic currents are found by means of a power series development. The solutions are valid over all ranges of intensities. We also give simple analytic expressions that describe the soliton full width at half-maximum as a function of all important experimental parameters.     

Density profile of a harmonically trapped ideal fermi gas in arbitrary dimension

Closed-form analytic expressions are derived for the density profile of a harmonically trapped noninteracting Fermi gas in d dimensions. Shell structure effects are included to leading order in 1/N, where N is the number of particles. These corrections to the local density approximation scale as deltan/n approximately N-alpha, where alpha=(1+1/d)/2.     

Exact kinetic transport equation solutions in the particle propagation theory in the scattering medium

Charged particle kinetics in an inhomogeneous medium (stochastic magnetic field) is investigated. Exact analytic expressions for Green function of kinetic equation in relaxation-time approximation are derived in one and three dimensions with arbitrary particle absorption. We separately consider the case of isotropic particle injection as well as the case of unidirectional instantaneous particle injection. The new way of solution makes it possible to get off any Cauchy-Principal Value integrals in some solutions which arise in the inverse Fourier-Laplace transform. Weak scattering regime and diffusion approximation is considered, and particle density is derived in three dimensions and arbitrary particle source.     

Effects of inhomogeneous broadening on two-frequency longitudinal wiggler brightness

The two-frequency longitudinal wiggler brightness is further extended to include the effects of betatron oscillations. The analytic expressions and the results with relevant physical consequences are presented for a comparison with the linear two-frequency undulator field, a popular device proposed earlier for side band suppression and quasi-continuous behaviour.     

Diffusion-controlled processes among partially absorbing stationary sinks

A general linear response theory is presented to calculate the zero-wavevector and zero-frequency reaction rate coefficient for particles diffusing into absorbing spheres. Allowance is made for possible incomplete particle absorption. A Faxén-like theorem for chemical reactions is derived. The problem is solved completely for a simple regular array of sinks. Exact analytic expressions for the rate coefficient as a function of sink volume fraction are obtained for the sc and fcc lattices. The case of a disordered array of sinks is also considered and the leading order nonanalytic density dependence of the rate coefficient is calculated. In both cases an increase in the rate coefficient with sink density in a local region of the system is found. The general formalism is extended to examine the modification to the particle diffusion coefficient due to the presence of the spheres. For regular arrays of spheres, the mean field result is reproduced.Research supported in part by a grant from the National Research Council of Canada.     

Dynamic quantum revivals in phase space

We explain quantum revivals and fractional revivals in phase space of the Fermi?CUlam accelerator. We derive analytic expressions of the Wigner distribution function for the driven system describing quantum interferences in position and momentum space. We assume that the fractional revival times are nonrecurrent under certain conditions and display randomness in the occurrence of the phenomenon at these times.     

The large deviation principle and some models of an interacting boson gas

This is a study of the equilibrium thermodynamics of the Huang-Yang-Luttinger model of a boson gas with a hard-sphere repulsion using large deviation methods; we contrast its properties with those of the mean field model. We prove the existence of the grand canonical pressure in the thermodynamic limit and derive two alternative expressions for the pressure as a function of the chemical potential. We prove the existence of condensate for values of the chemical potential above a critical value and verify a prediction of Thouless that there is a jump in the density of condensate at the critical value. We show also that, at fixed mean density, the density of condensate is an increasing function of the strength of the repulsive interaction. In an appendix, we give proofs of the large deviation results used in the body of the paper.     

On the theory of the adsorption of atoms onto a metal surface in the presence of an external quantizing magnetic field

The Anderson-Newns model for the adsorption of atoms onto metals is generalized to the case of the presence of an external transverse quantizing magnetic field. Analytical expressions for the density of metal states in the external magnetic field are obtained. Analytical expressions for the density of adatom states are obtained in the approximation taking only the change in the density of substrate states into account. The charge exchange between the adatom and the metal is studied. The dependences of the density of adatom and transition-charge states on the magnetic field are studied for different values of the interaction constant. An analytical expression for the density of substrate states perturbed by adsorption is obtained.     

Kinetics of ion admixture in a native gas in an external harmonic electric field

We have considered the spatially homogeneous problem of the behavior of an ion admixture in a background gas after applying a harmonic electric field with arbitrary parameters for various laws of interaction of particles. The Boltzmann equation has been solved using the modified method of moments. The ion distribution function and its first moments have been analyzed. It has been shown that the universal analytic expressions for the current density and the ion energy that we derived earlier for a small field amplitude-tofrequency ratio have a considerably wider range of applications.     

Analytical solutions of the Rayleigh flow problem for a rarefied gas of a nonhomogeneous system of charged particles

A two-stream Maxwellian distribution function with two unknown parameters corresponding to the mean velocity and the shear stress is used to obtained an approximate analytic solution of the Rayleigh flow problem for a rarefied gas of a nonhomogeneous system of charged particles. For small magnetic field, solutions are presented for the four moments equations and the Maxwell's equations. The dynamical behaviour of the electron and ion gas is examined.     

Statistical temporal behaviour of pulse wave propagation through continuous random media

Pulse waves propagating through random media suffer distortions, such as fluctuation of arrival time, temporal broadening, and alteration of skewness and kurtosis, due to both the background medium and embedded irregularities. We carry out a study on the temporal behaviour of electromagnetic pulses propagating through random media using temporal moments and an analytic solution of a two-frequency mutual coherence function recently obtained by iteration. We treat the temporal characteristics sequentially, with general expressions obtained first. Then the concise forms are given for pulse propagation in the turbulent non-dispersive atmosphere and the ionosphere, with numerical calculations for the latter. The results show that the mean arrival time is dominated by the term propagating at group velocity, and small corrections arise from higher-order dispersion of the background medium and random scattering of irregularities, but the correction from dispersion of irregularities is neglected as it is so small. As for pulse broadening in trans-ionospheric propagation, the results show that contributions are mainly from the dispersion of the background ionosphere and scattering of electron density irregularities in most cases, and the contribution of dispersion of irregularities is so small that it can be neglected. Finally, we find that the temporal skewness of a trans-ionospheric pulse is negative and its energy is shifted to the leading edge, and the contributions from scattering and dispersion of irregularities dominate over those of background, so the latter can be neglected in most cases.