等离子体光栅靶的表面粗糙度对高次谐波产生的影响

极紫外光和软X射线由于其波长和脉冲持续时间极短,可用于超快物理过程和物质微观结构的探测.最近几年,研究人员发现激光和等离子体相互作用可以产生持续时间极短(阿秒)且相干性较好的高次谐波辐射,其波长可接近甚至达到水窗波段.然而,实验研究指出,理论上应出现的一些谐波在实验中并没有出现.本文针对超短超强激光与非理想条件下的等离子体光栅靶相互作用产生高次谐波的物理过程进行了理论分析和粒子模拟.研究结果表明,等离子体光栅的周期性结构对于高次谐波的频谱和辐射角分布存在显著调制效果.光栅靶表面粗糙度直接影响光栅的光学调制效果,改变高次谐波的频谱分布和辐射角分布.理想光栅条件下,满足光栅匹配条件的特定阶数谐波明显获得增强,且辐射张角集中在平行靶面的方向.靶表面粗糙度的出现,导致光栅匹配条件失效,高次谐波能量向各阶分散且辐射张角逐渐偏离靶表面方向.研究结果较好地解释了实验中观测到的谐波频谱分布,为进一步的研究提供了一定参考.     

Tunable THz Sources and Their Applications

In this paper, the technology aspects of the THz laser and spectrometric devices, developed over the last decade are described and their applications in various fields of science and technology are explained. The THz sources of importance are the optically pumped lasers, solid state devices and the backward wave oscillator (BWO) based systems. The BWO system offers the electronic tuning and multiplexing facility like Fourier transform technology. The time resolved and frequency domain aspects with relative merits are discussed. Non-linear optical methods of production of monochromatic radiation using second harmonic generation (SHG) are also considered. The important areas of space applications are described in detail. Present status and future directions are indicated.     

Harmonic balance/Galerkin method for non-smooth dynamic systems

Models of non-linear systems frequently introduce forces with bounded continuity resulting in non-smooth (even discontinuous) flow. Examples include systems with clearances, backlash, friction, and impulses. Asymptotic methods require smooth (differentiable) flow and are therefore ill-suited for analyzing non-smooth systems. In these cases, the traditional harmonic balance method may be used to obtain approximate periodic solutions, but the method suffers from extremely slow convergence in general. Generalizations of the traditional harmonic balance method are introduced in this paper that result in superior convergence rates and superior modes of convergence. These improvements derive from the introduction of one or more expansion functions that possesses the same degree of continuity as the exact solution. In particular, forming an infinite series of such functions results in an expansion in the same function space of the exact solution. This expansion converges pointwise to the exact solution and to all derivatives thereof. These improvements are illustrated by example upon re-evaluating a classical single degree-of-freedom model for friction-induced vibration.     

Recent developments in radiative detection of nuclear magnetic resonance

A review is given of various methods for the detection of nuclear magnetic resonance using nuclear radiations. After a discussion of the physical basis of the different methods, emphasizing their common features, some remarks are made on the development of the methods and their applications up to the present. Finally, a summary of the most recent new aspects and future perspectives of each method is given.This article is based on a lecture originally given at the XVII Winter School on Physics (Hyperfine Interactions) in Bielsko-Biala, Poland, 1979, and is a revised and updated version of the lecture printed in the Proceedings of that school.     

Continuum three-body states using the hyperspherical adiabatic basis set

In this paper we investigate the feasibility of employing the hyperspherical adiabatic (HA) basis set to describe continuum three-nucleon states. In particular, the HA expansion is compared with the simpler expansion on hyperspherical harmonics (HH). A practical numerical application is presented using the MT-III potential.     

Power Series Expansion of Propagator for Path Integral and Its Applications

In this paper we obtain a propagator of path integral for a harmonic oscillator and a driven harmonic oscillator by using the power series expansion. It is shown that our result for the harmonic oscillator is more exact than the previous one obtained with other approximation methods. By using the same method, we obtain a propagator of path integral for the driven harmonic oscillator, which does not have any exact expansion. The more exact propagators may improve the path integral results for these systems.     

Approaching the strongly anharmonic limit with ab initio calculations of materials’ vibrational properties – a colloquium*

Despite ab initio computational techniques have opened new possibilities to interpret experimental results and predict the properties of new materials, their applications are limited by the adopted approximative schemes. Consequently, the first-principles calculation of many physical properties and phenomena is hindered and ab initio methods need to be further developed to overcome such limits. For example, the standard harmonic approximation used to assess the vibrational properties of materials often completely breaks down, so that the vibrational properties need to be calculated including strong anharmonic effects. The harmonic approximation has also intrinsic failures as it cannot estimate the lattice thermal conductivity of materials nor the temperature dependence of the phonon frequencies, crucial to account for temperature driven second-order phase transitions. Several methods developed in the last years to account for anharmonicity in the non-perturbative regime and overcome such difficulties are briefly reviewed in this colloquium paper. In particular, the stochastic self-consistent harmonic approximation, a variational method that allows calculating vibrational properties in strongly anharmonic systems, is described in further detail. Applications of the latter method to superconducting palladium, platinum, and sulfur hydrides are discussed, where anharmonicity has a huge impact on their vibrational and superconducting properties.     

Detection procedures of ultrasound contrast agents

In the early days, it was believed that ultrasound contrast agents (UCA) could be sufficiently detected and imaged with the conventional imaging methods nowadays referred to as fundamental imaging. Newer imaging techniques proved to be more sensitive and are based on specific properties of the UCA. In general, these new characteristics involve non-linear and transient characteristics of contrast agents that appear at the high end of the diagnostic acoustic intensity. Imaging modalities used today for UCA are, besides fundamental imaging, second harmonic imaging, power Doppler, harmonic power Doppler, pulse inversion and pulse inversion Doppler, multi-pulse imaging and subharmonic imaging. Although the results of conventional second harmonic imaging are still not optimal for perfusion imaging applications, in combination with Doppler techniques (colour Doppler, power Doppler) it is one of the most sensitive techniques currently available in terms of agent-to-tissue ratio. Further improvements in current and future detection methods demand a complete understanding of the ultrasound-UCA interaction.     

Summation of power series in particle physics

The large-order behaviour of power series used in quantum theory (perturbation series and the operator-product expansion) is discussed and relevant summation methods are reviewed. It is emphasised that, in most physically interesting situations, the mere knowledge of the expansion coefficients is not sufficient for a unique determination of the function expanded, and the necessity of some additional, extra-perturbative, input is pointed out. Several possible nonperturbative inputs are suggested. Applications to various problems of quantum chromodynamics are considered.This lecture was presented on the special Memorial Day dedicated to Professor Ryszard Ra̧czka at this Workshop. The last section is devoted to my personal recollections of this remarkable personality.     

The structure of liquid methanol revisited: a neutron diffraction experiment at −80 °C and +25 °C

Pulsed neutron diffraction with isotope substitution on the hydroxyl hydrogens (H) is used to study the structure of pure liquid methanol at ?80 °C and +25 °C. Although this liquid has been studied with neutrons several times in the past this is the first time that the composite partial structure factors, XX, XH and HH, are derived from the diffraction data. Here X represents a weighted sum of correlations from carbon (C), oxygen (O), and methyl hydrogen (M) atoms on the methanol molecule. The data are used in an empirical potential structure refinement (EPSR) computer simulation of the liquid at both temperatures. Model distributions of molecules consistent with these data are used to estimate the individual site—site radial distribution functions, the coefficients of the spherical harmonic expansion of the orientational pair correlation function, and the length of possible chains of methanol molecules formed in the liquid. Although the results are qualitatively similar to those of earlier computer simulation studies of this liquid, they do differ in detail from previous work. In particular, although most molecules in the liquid are found as part of chains up to 10 molecules long, the average chain length is only about 2.7 molecules at both temperatures, there are about 1.3 hydrogen bonds per molecule on average, and the chains are highly nonlinear on average.     

Wave collapse in plasmas and fluids

This lecture is a review of recent results (obtained mainly by the author and his co-authors) in the wave collapse theory with applications to plasma physics, fluid dynamics and nonlinear optics as well. The main attention in the review is paid to the qualitative reasons of the wave collapse and to the exact methods based on the integral estimations. Both approaches are applied to both the nonlinear Schrodinger equation and the two-dimensional generalized Benjamin-Ono equation which describes self-focusing of low-frequency oscillations in the boundary layer. (c) 1996 American Institute of Physics.     

幂级数展开法计算周期阻尼振子路径积分短时传播子

文章用Schwinger方法与Makri等人发展的幂级数展开法研究了周期阻尼简谐振子的传播子.研究表明,这两种方法所获得的短时传播子非常接近,但是后者比前者要简单得多,这给研究复杂系统的传播子提供了一个简单可靠的方法.     

Solution to the equations of the moment expansions

We develop a formula for matching a Taylor series about the origin and an asymptotic exponential expansion for large values of the coordinate. We test it on the expansion of the generating functions for the moments and connected moments of the Hamiltonian operator. In the former case the formula produces the energies and overlaps for the Rayleigh-Ritz method in the Krylov space. We choose the harmonic oscillator and a strongly anharmonic oscillator as illustrative examples for numerical test. Our results reveal some features of the connected-moments expansion that were overlooked in earlier studies and applications of the approach.     

The Gamow shell model with realistic interactions: a theoretical framework for ab initio nuclear structure at drip-lines

Ab initio approaches are among the most advanced models to solve the nuclear many-body problem. In particular, the no-core–shell model and many-body perturbation theory have been recently extended to the Gamow shell model framework, where the harmonic oscillator basis is replaced by a basis bearing bound, resonance and scattering states, i.e. the Berggren basis. As continuum coupling is included at basis level and as configuration mixing takes care of inter-nucleon correlations, halo and resonance nuclei can be properly described with the Gamow shell model. The development of the no-core Gamow shell model and the introduction of the $\hat{\bar{Q}}$-box method in the Gamow shell model, as well as their first ab initio applications, will be reviewed in this paper. Peculiarities compared to models using harmonic oscillator bases will be shortly described. The current power and limitations of ab initio Gamow shell model will also be discussed, as well as its potential for future applications.     

Mechanisms for second harmonic generation in the interaction of a superintense ultrashort laser pulse with cluster plasma

The effects of the interaction of an intense femtosecond laser pulse with large atomic clusters are considered. The pulse intensity is of the order of 10¹⁸ W cm^?2. New effects appear when the magnetic component of the Lorentz force is taken into account. The second harmonic of laser radiation is generated. The second harmonic generation (SHG) efficiency is proportional to the square of the number of atoms in a cluster and the square of the laser radiation intensity. The resonance increase in the SHG efficiency at the Mie frequencies (both at the second harmonic frequency and fundamental frequency) proved to be insignificant because of the fast passage through the resonance during cluster expansion. The mechanisms of the expansion and accumulation of energy by electrons and ions in the cluster are discussed in detail. The energy accumulation by electrons mainly occurs due to stimulated inverse bremsstrahlung upon elastic reflection of the electrons from the cluster surface. The equations describing the cluster expansion take into account both the hydrodynamic pressure of heated electrons and the Coulomb explosion of the ionized cluster caused by outer shell ionization. It is assumed that both inner shell and outer shell ionization is described by the over barrier mechanism. It is shown that atomic clusters are more attractive for the generation of even harmonics than compared to solid and gas targets.     

Derivation of collective potential and mass from the generator coordinate method

The Hill-Wheeler equation of the generator coordinate method is approximated by a local collective Schrödinger equation. General expressions for the potential and the mass parameter are obtained by a symmetrized moment expansion. The validity of the approximation is tested for several examples where the exact solution is known. These include the Gaussian overlap with harmonic and anharmonic interaction, the Lipkin model, and monopole resonances of spherical light nuclei. In all cases, surprisingly close agreement with the exact solution is found. Other possible applications of the formalism are indicated.     

SFC新谐波电源的研制

简要描述了扇聚焦回旋加速器 (SFC)中使用谐波场的意义和对谐波电源的要求 .根据长期运行的经验 ,对原有谐波电源的缺点和不方便处进行了分析 .在此基础上 ,提出了使用稳流电源和控制系统相结合的方法提供所要求的谐波电流 .描述了新电源的优点 ,在运行过程中也充分显示了它的优越性. The significance of using a harmonic magnetic field in SFC (Sector Focusing Cyclotron) and the requirements for the harmonic power supplies are described roughly. According to the experience of the operation in the long period, the shortages and some un conveniences for the original harmonic power supply were analyzed. In the case, a new kind of harmonic power supply was fabricated successfully in which the DC power supplies and the control technology were. ...     

新型负热膨胀氧化物材料的研究

综述了这年来国际上兴起的氧化物ZrW2O8,Y2W3O12等新型负热膨胀材料的研究及其进展。对研究此类负热膨胀氧化物的重要性,其优异的负热膨胀性能以及它的结构特点、机制及可能的应用作了论述。建议我国加大此项课题的研究力度。     

Transformation Coefficients of Hyperspherical Harmonic Functions of an -Body System

Two algorithms are presented for calculating the transformation coefficients between hyperspherical harmonic functions constructed with different sets of Jacobi vectors. They have been tested in the case , where the transformation coefficients of states with grand angular quantum number up to have been studied. The applicability of the two algorithms to larger systems is discussed. The numbers of independent hyperspherical-spin-isospin states with given values, entering the expansion of the alpha-particle ground-state wave function, are also evaluated. The use of complete non-redundant bases is important for future accurate applications of the hyperspherical harmonic technique. Received December 23, 1997; revised May 25, 1998; accepted for publication May 30, 1998     

Efficient semi-analytical methods for the vibration response of and acoustic radiation from a periodical orthogonally rib-stiffened plate

In order to theoretically predict and analyze the vibration response and acoustic radiation characteristics of a periodical orthogonally rib-stiffened plate,its vibro-acoustic equations of an underwater infinite model are established.The rib-stiffened plate is stimulated by a harmonic plane pressure.By using the Fourier transforms,Poisson's summation formula and space harmonic method,the structural vibration response and acoustic radiation pressure are expressed as functions of displacement harmonic components.Efficient semi-analytical methods are proposed in this work,and then approximate solutions for finite terms of the harmonic components are obtained by employing the truncation technique.Effects of the vibration response,rib spacing and torsional moment of the ribs on the radiation pressure are examined,and the validity of the present methods is also verified.Theoretical results show that the torsional moment of the ribs affects the modal frequencies of the stiffened plate,which should not be neglected in engineering applications with high precision requirement.With attachment of the ribs to the thin plate,its far field radiation pressure can be reduced in the low frequency range by adjusting rib spacing and cross sectional size of the ribs.