二维声光互作用理论及实验分析

本文在理论上推导了二维声光互作用的耦合波方程的普适形式,并借以对正常声光互作用下的喇曼-奈斯(Raman-Nath)型衍射进行了求解,所得结果与两个一维喇曼-奈斯型声光调制器组成的两维器件的实验结果进行比较,两者相符.     

Magnetic breakdown at high fields: semiclassical and quantum treatments

The effects of finite temperature and noninteracting spins on magnetic breakdown (MB) in a quasi-two-dimensional organic conductor have been determined by computing the field-dependent free energy using a realistic crystal structure with no adjustable parameters. The de Haas-van Alphen oscillation spectra, including the MB phenomena, are thereby obtained microscopically. Within the range of computed magnetic field, from 170 to 1400 T, we find remarkable agreement between the predictions of the semiclassical and quantum treatment. We also find that the Zeeman effect leads to splitting of a frequency corresponding to the fundamental orbit.     

电磁场统一性质的三维描述

论述了电场与磁场本质上的统一性质,构造了三维空间中电磁场的统一场量,给出它的方程、场量的模方与能量和动量的关系,发现三维空间中的统一场量自身的标积竟然是相对论变换下的不变量．     

On the breakdown of axisymmetric smooth solutions for the 3-D Euler equations

We refine the Beale-Kato-Majda criterion for the breakdown of smooth solutions of the 3-D incompressible Euler equations in the case of axisymmetry. In this case the angular component of vorticity in the cylindrical coordinates alone controls blow-up of the higher Sobolev norms of the velocity.     

Remarks on the breakdown of smooth solutions for the 3-D Euler equations

The authors prove that the maximum norm of the vorticity controls the breakdown of smooth solutions of the 3-D Euler equations. In other words, if a solution of the Euler equations is initially smooth and loses its regularity at some later time, then the maximum vorticity necessarily grows without bound as the critical time approaches; equivalently, if the vorticity remains bounded, a smooth solution persists.Partially supported by O.N.R. Contract No. N00014-76-C-0316 and N.S.F. Grant No. MCS-81-01639Partially supported by N.S.F. Grant No. MCS-82-00171Partially supported by N.S.F. Grant No. MCS-81-02360     

Ambipolar drift and breakdown of oscillistor effect in strong magnetic fields

The drift mechanism of helical instability suppression (oscillistor) in electron-hole plasma Ge was, for the first time, investigated. It was found that in strong magnetic fields ($\text{γ}{}_{\mathrm{i=e,h}}\text{= μ}{}_{\mathrm{i}}\text{H/c}\text{a}\text{?}\text{1}$) due to anisotropy of mobility (μ_i ≠ μ_i⊥) the ambipolar drift of helical disturbances of density along the electric field occurs even in quasineutral plasma (n = p), provided μ_e ≠ μ_h. To corroborate the drift mechanism of oscillistor effect breakdown in strong fields the experiments were carried out under the conditions of uniaxial crystal deformation oriented towards 〈111〉 (H, E  〈111〉) resulting in the intervalley redistribution of electrons and additional anisotropy of mobility. Variation in the ambipolar drift velocity connected with the deformation influences essentially the conditions of the helical instability breakdown in strong fields.     

Bipolar model for electrical breakdown in polyethylene materials under dc high electrical fields

In this paper, we have applied our bipolar transport model for studying the charge dynamics in the insulating polyethylene materials, from nano to micro -scales and under dc high applied field. We have also applied the model for studying the electrical breakdown phenomenon for these mentioned scales. The principal results are dedicated to the evolution of the external current in low density polyethylene samples. Under notable dc high applied field, the electrical pre-breakdown phenomenon is indicated by an abrupt increase that occurs during the steady state of the external current.     

Thick-grating non-divergent focussing device: 2-D analysis

The two-dimensional analysis of a recently reported thick grating Bragg diffraction focussing device is carried out using Solymar's coupled-wave approach. It is shown that the device is capable of operating as a non-divergent lens with beam contraction ratios of better than 100:1, and conversion efficiencies of nearly 100%. It is also shown that it can operate as a novel kind of Fourier spectrum analyser, the focussed diffracted intensity being proportional (as a function of Bragg condition violation) to the modulus squared of the Fourier transform of the incident finite beam. The focussing properties are studied as functions of incident beam profile, width and position, grating strength and Bragg condition violation. Poynting vector optics is used successfully to predict (in conjunction with the dispersion surfaces of x-ray dynamical theory) the off-Bragg behaviour in the focal plane. It is likely that the device could be used as avariable ratio beam contractor in integrated optics, where the grating strength could be controlled (interdigital electrode system) electrooptically.     

利用激光诱导击穿光谱分析土壤成份

搭建了一套激光诱导击穿光谱实验装置,并通过配置特定样品,开展了一系列激光诱导击穿光谱探测实验。根据含有不同质量分数的同种元素样品的激光诱导击穿光谱实验结果,获得元素质量分数与谱线强度的关系曲线(定标曲线)。对中南民族大学附近的土壤进行激光诱导击穿光谱实验,发现土壤中含有Mg,Ca,Na等18种元素,对河南云台山茱萸峰岩石的激光诱导击穿光谱实验结果仅获得Fe,Mg,Ca 3种金属元素。比较这2种实际样品的激光诱导击穿光谱结果表明,检测样品的物理结构影响激光诱导光谱的实验结果。     

利用激光诱导击穿光谱分析土壤成份

 搭建了一套激光诱导击穿光谱实验装置,并通过配置特定样品,开展了一系列激光诱导击穿光谱探测实验。根据含有不同质量分数的同种元素样品的激光诱导击穿光谱实验结果,获得元素质量分数与谱线强度的关系曲线(定标曲线)。对中南民族大学附近的土壤进行激光诱导击穿光谱实验,发现土壤中含有Mg,Ca,Na等18种元素,对河南云台山茱萸峰岩石的激光诱导击穿光谱实验结果仅获得Fe,Mg,Ca 3种金属元素。比较这2种实际样品的激光诱导击穿光谱结果表明,检测样品的物理结构影响激光诱导光谱的实验结果。     

Fermi liquid breakdown and evidence for superconductivity in YFe2Ge2

In the d‐electron system YFe₂Ge₂, an unusually high and temperature dependent Sommerfeld ratio of the specific heat capacity C /T ～ 100 mJ/(mol K²) and an anomalous power law temperature dependence of the electrical resistivity signal Fermi liquid breakdown, probably connected to a close‐by quantum critical point. Full resistive transitions and DC diamagnetic screening fractions of up to 80% suggest that pure samples of YFe₂Ge₂ superconduct below 1.8 K. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Electron neutral collision frequency from breakdown measurements in crossed electric and magnetic fields

A simplified method of determining the electron-neutral collision frequency at unit pressurev ₀ is presented in a number of gases (hydrogen, argon and air) in crossed electric and magnetic fields utilising the sparking voltage data and equivalent pressure concept. The values ofv ₀ for lowH/p are found to agree with those reported by other authors. But at highH/p, the estimated values ofv ₀ are appreciably higher for all the three gases. The increase in the values ofv ₀ possibly occurs due to the change of the electron energy distribution at higherH/p.     

Overlayer-substrate spacing for argon and krypton on graphite determined by leed intensity analysis

Low-energy electron diffraction has been used to measure the spacing between monolayers of Ar or Kr and the outermost basal plane of a graphite substrate. The intensity of the (00) beam is recorded as a function of energy for graphite with and without the overlayer present. Using the analysis method of Cohen, Unguris and Webb, we find that the ratio of the two intensity curves shows an oscillation whose period is inversely proportional to the desired spacing. For Ar, we find a value of 3.2 ± 0.1 Å at 42 K. The results for Kr are somewhat ambiguous but suggest a value of 3.3 ± 0.1 A at 52 K.     

Performance Analysis of 2-D GRAPPA and Partial Fourier GRAPPA for 3-D MRI

The acquisition time of three-dimensional magnetic resonance imaging (3-D MRI) is too long to tolerate in many clinical applications. At present, parallel MRI (pMRI) and partial Fourier (PF) with homodyne detection, including 2-D pMRI (two-dimensional pMRI) and PF_pMRI (the combination of PF and pMRI), are often used to accelerate data sampling in 3-D MRI. However, the performances of 2-D pMRI and PF_pMRI have been seldom discussed. In this paper, we choose GRAPPA (generalized auto-calibrating partially parallel acquisition) as a representative pMRI to analyze and compare the performances of 2-D GRAPPA and PF_GRAPPA, including the noise standard deviation (SD), root mean-square error (RMSE) and g factor, through a series of in vitro experiments. A series of phantom experiments show that the SD, RMSE and g-factor values of PF_GRAPPA are lower than those of 2-D GRAPPA under the same acceleration factor. It demonstrates that the performance of PF_GRAPPA is better than that of 2-D GRAPPA. PF_GRAPPA can be used in any thickness of imaging slab, while 2-D GRAPPA can only be used in thick slab due to the difficulties in determination of the fitting coefficients which result from imperfect RF pulse. In vivo brain experiment results also show that the performance of PF_GRAPPA is better than that of 2-D GRAPPA.     

OCDMA系统中2-D码和3-D码

光码分多址(OCDMA)技术主要应用在接入网中。设计大容量的地址码是该技术实用化的前提。本文在构造2 D素数码的基础上提出了一种用于OCDMA系统的空域/频域/时域3 D地址码。这种码基于素数序列运算。理论分析和数学计算结果表明，3 D码比2 D码的容量更大，系统带宽效率更高，误码率(BER)更低，表现出的性能更好。     

Rapid increase in the delay time for the breakdown of gas-filled gaps in high electric fields

An experimental study is reported of the delay-time statistics for electrical breakdown in air in intense electric fields E = 100–200 kV/cm. The dependence of the statistical delay time t_st for the breakdown on the gap width is obtained for various fields E. With E = 100 kV/cm and with an air pressure of p = 1 atm, the delay time is 20 nsec and depends weakly on in the range 5 = 3–10 mm. As the gap width is reduced from 3 to 2 mm, t_st increases sharply, reaching 3000 nsec. The gap width at which the sharp increase in t_st sets in decreases with increasing E. The gap-width dependence of t_st has the same nature at different pressures. The minimum delay time has been measured to be 8 nsec.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 12, No. 2, pp. 36–39, February, 1969.     

The electrical breakdown of gases in the presence of crossed electric and magnetic fields

The breakdown characteristics of a gas in the presence of crossed electric and magnetic fields are discussed in terms of the Townsend ionization coefficients. The “equivalent pressure” concept is used to assess the effect of a transverse magnetic field on the first Townsend coefficient and the objections which have been raised to the application of this approach to breakdown potentials are shown to be removed by a consideration of the dependence of the second Townsend coefficient upon electric and magnetic field strengths.     

Method for analysis of focused electromagnetic fields

Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 33, No. 10, pp. 1144–1149, October, 1990.     

Calibration-free laser-induced breakdown spectroscopy for quantitative elemental analysis of materials

The application of calibration-free laser-induced breakdown spectroscopy (CF-LIBS) for quantitative analysis of materials, illustrated by CF-LIBS applied to a brass sample of known composition, is presented in this paper. The LIBS plasma is produced by a 355?nm pulsed Nd:YAG laser with a pulse duration of 6?ns focussed onto a brass sample in air at atmospheric pressure. The time-resolved atomic and ionic emission lines of Cu and Zn from the LIBS spectra recorded by an Echelle spectrograph coupled with a gated intensified charge coupled detector are used for the plasma characterization and the quantitative analysis of the sample. The time delay where the plasma is optically thin and is also in local thermodynamic equilibrium (LTE), necessary for the elemental analysis of samples from the LIBS spectra, is deduced. An algorithm relating the experimentally measured spectral intensity values with the basic physics of the plasma is developed. Using the algorithm, the Zn and Cu concentrations in the brass sample are determined. The analytical results obtained from the CF-LIBS technique agree well with the certified values of the elements in the sample, with an accuracy error <1%.