Response of a Lorentzian gas to an a. c. pulse

The response of a “Lorentz gas” to a pulsed a.c. electric field and its relaxation after the cessation of this field has been studied by solving the Boltzmann's transfer equation. Explicit expressions for the electron distribution function and the current density are obtained under the assumption that the collision frequency is independent of the electron velocity.     

电磁脉冲作用下PIN二极管的响应

采用自主开发的二维半导体器件效应模拟软件,对电磁脉冲作用下PIN二极管的响应进行了数值模拟研究,分析了PIN管在脉冲电压上升沿时间内出现的电流过冲现象。结果表明:过冲电流与高频下PIN二极管的电容性有关,过冲电流的峰值与上升沿时间有关,上升沿时间越短,峰值越大;PIN管的掺杂也会对过冲电流产生影响,P层、N层的掺杂浓度越高,过冲电流的峰值越大,过冲电流的波形下降越快;I层掺杂浓度对过冲电流也有一定影响,但并不显著。     

Response of an ideal gas in an external potential

We construct the exact solution for the time evolution of the local mass density in an ideal gas in which an external short-ranged separable potential is switched on at time t = 0. The redistribution of matter in strong potentials causing bound states or resonances is studied in great numerical detail.     

Triggering and guiding of an upward positive leader from a ground rod with an ultrashort laser pulse. II. Modeling

We have used the Bondio-Gallimberti model of positive leader propagation to simulate laboratory experiments of laser triggering and guiding of upward leaders initiated from a ground rod. The model proves to be capable of reproducing all the important features of laser-guided leader propagation that have been observed experimentally. The leader guiding effect of the laser-created plasma channel is taken into account in the model by adjusting the value of the charge per unit length of the leader, which has been measured in the laboratory to be lower for a laser-guided leader than for an ordinary one. The charge per unit length of the leader is related in the model to the critical temperature at which the air in the transition region at the leader tip must be heated to be conductive enough to become a new leader portion. For an ordinary leader, this critical temperature is 1500 K, at which the electrons all detach from the negative ions in the leader corona, increasing the air conductivity. We give the interpretation that in the case of the laser-guided leaders, because of the relatively high density of negative ions per unit length in the laser-ionized channel, the right conditions of conductivity can be met in the transition region without the electrons being all detached from the ions, allowing a reduction of the critical temperature and of the charge per unit length.     

The vibration of a box-type structure II. Response to a travelling pressure wave

A finite element method is formulated for determining the transient response of a box-type structure to a travelling, arbitrarily shaped pressure wave. The method is illustrated by considering an example of practical concern-the sonic boom. The acceleration-time histories of a closed box are compared with those obtained experimentally from a simulated boom. Satisfactory agreement is obtained with only four rectangular elements per individual face and a simplified loading of the box.     

Local transient phenomena induced in an inert gas plasma by a short pulse

A microsecond periodic pulse was applied on thin probes in a positive column of a neon or helium glow discharge. The local transient phenomena provoked by this pulse were detected by a photomultiplier and presented in a time-distance display. They are qualitatively described and explained. The results are in good agreement with previous theories.     

Impurities in an imperfect bose gas II. Quasi-particle scattering

The problem of quasi-particle scattering by a hard sphere in an imperfect Bose gas is examined in order to gain insight into the behavior of ions in He II. Wave functions and phase shifts are calculated exactly using a simple approximation for the condensate wave function. For wavelengths characteristic of temperatures down to about 0.1°K the consequent cross sections are much like those of classical sound waves. (The depression of the condensate near the sphere plays a crucial role in this result.) At longer wavelengths the total cross section approaches a constant. Such an effect in He II would lead, for temperatures less than about 0.1°K, to ion mobilities substantially smaller than one would predict using acoustic cross sections. The more complicated problem of the scattering of density fluctuations (phonons) is considered briefly, and, with the help of further approximations, it is verified that the phonon scattering exhibits the acoustic characteristics calculated for the single-particle scattering.     

Magnetization of optically polarized gas atoms by an optical pulse train

The properties of the density matrix and the multipole moments arising in oriented and aligned atoms with zero nuclear spin through the interaction with strong resonant ultrashort pulses with wave vector k ₀ and circular or linear polarization have been found. Calculations have been made for the time-dependent light-induced magnetization μ(t′) of a gas of pre-oriented and prealigned atoms following the passage of a weak resonant elliptically polarized pulse with frequency ω and wave vector k collinear with k ₀. It is shown that for oriented atoms, μ(t′) is an even function of the detuning from resonance, ω-ω _ba, and can be split into two terms whose directions are a consequence of symmetry and are determined by the vectors k ₀ and k as well as by the direction of rotation of the electric fields corresponding to the pulses. For aligned atoms the vector μ(t′) is collinear with k, and the first term is an even function of ω-ω _ba. However, the second term is an odd function of ω-ω _ba and reverses direction when the sign of ω-ω _ba changes, as well as when the orientation of the axes of the polarization ellipse is changed. It is shown that if a series of weak linearly polarized pulses pass through the gas, the light-induced magnetization of the oriented and aligned gas atoms can be decomposed into three factors: the first determines the direction and is a consequence of the symmetry; the second (with the dimensions of magnetic moment) depends on the characteristics of the resonant transitions; and the third is a universal function of t′ and ω-ω _ba that does not depend on the underlying characteristics of the resonant transition. These vector factors and the universal functions are in principle different for oriented and aligned atoms. Zh. éksp. Teor. Fiz. 111, 63–92 (January 1997)     

Binding energy of a hydrogen impurity in an f.c.c. lattice

Hydrogen impurities in f.c.c. lattices, Pd metal for example, can occupy two interstitial sites, with tetrahedral or octahedral symmetries. Using the Husimi cactus to represent the f.c.c. lattice and two clusters to describe the two different interstices, we calculate the densities of states and the occupation numbers at the impurity site and on its nearest neighbours. The binding energies are calculated and plotted against the hopping integral between the hydrogen and its neighbours (t_a). We found that the octahedral site is the stable one for values of t_a from 0 to 1.5 eV.     

Response of a magnetic nanoparticle lattice to a magnetic field pulse near the stability boundary

The dynamic response of a system being near the stable equilibrium boundary to an external magnetic field pulse is studied for 2D lattices of magnetic nanoparticles with cubic crystallographic anisotropy. The conditions under which magnetic moment oscillations from individual dipoles propagate to the entire system are revealed. This effect results in the lattice response are significantly larger in the external pulse duration and with an amplitude rather weakly depending on initial conditions and external field parameters, the processes during which the pulse results in reorientation of only individual lattice dipoles.     

Spectral Investigation of an a. c. Plasma Display

The work presents the spectral investigations on an a.c. plasma display, in order of a better understanding of the physical phenomena taking place in such a device. The spectral characteristics of the panel filled with a Penning mixture Ne + 0.1% Ar are presented and the influence of the nitrogen addition on these characteristics was evidentiated. The presence of the trace of nitrogen in the device may be used in order to evidentiated. The presence of the trace of nitrogen in the device may be used in order to evidentiate small leaks or imperfections in pumping and outgasing processing of the display.     

感应电压叠加器模块对方波脉冲的响应

为了解感应电压叠加器(IVA)对馈入脉冲的响应特性,尤其对上升前沿、平顶的影响,从理论和实验两个方面对IVA模块进行了研究。介绍了IVA的工作原理,利用集总参数方法建立了相应的电路模型,通过拉普拉斯变换分析了感应电压叠加器对方波脉冲上升前沿和平顶的响应,并在一个特定的IVA模块上进行了实验研究。选择输出阻抗约1.2Ω、脉宽约1μs的脉冲形成网络作为馈源,在匹配负载上得到的波形与输入波形在幅值、上升前沿方面达到了很好的吻合,平顶出现略微的顶降,与理论预期相一致。     

感应电压叠加器模块对方波脉冲的响应

为了解感应电压叠加器（IVA）对馈入脉冲的响应特性,尤其对上升前沿、平顶的影响,从理论和实验两个方面对IVA模块进行了研究。介绍了IVA的工作原理,利用集总参数方法建立了相应的电路模型,通过拉普拉斯变换分析了感应电压叠加器对方波脉冲上升前沿和平顶的响应,并在一个特定的IVA模块上进行了实验研究。选择输出阻抗约1.2 Ω、脉宽约1 μs的脉冲形成网络作为馈源,在匹配负载上得到的波形与输入波形在幅值、上升前沿方面达到了很好的吻合,平顶出现略微的顶降,与理论预期相一致。     

光电倍增管对14 MeV脉冲中子的直照响应

 采用ING-103型DPF脉冲中子源产生的14.1 MeV脉冲中子对EMI两个不同型号的光电倍增管EMI-9815B和EMI-9850B进行了直照实验。针对DPF脉冲中子源产生中子脉冲的同时也会产生X射线脉冲的特点,采取了飞行时间法及吸收衰减法来消除X射线对中子脉冲的干扰。利用中子及X射线速度的差异,将光电倍增管放置在离源较远的测点位置,测得了X射线脉冲和中子脉冲时间上错开的双峰波形。通过在辐射通道内添加5 cm厚的铅吸收体有效地抑制了X射线峰,在离源较近的测量位置测到了干净的脉冲中子波形。根据实测波形,得到的光电倍增管EMI-9815B和EMI-9850B中子直照灵敏度分别为10^-13与10^-15量级,该结果与理论计算结果在量级上一致。     

Response function including collisions for an interacting fermion gas

The response function of an interacting fermion gas is considered in the entire (k-->,omega) space. Applying a generalized linear response theory, it is expressed in terms of determinants of equilibrium correlation functions, which allow for a systematic perturbative treatment. The relation to dynamical local-field factors is given. As a special case, the dielectric function is evaluated for two-component (hydrogen) plasmas at arbitrary degeneracies. Collisions are treated in Born approximation leading to a (k-->,omega)-dependent collision integral. The link to the dynamical conductivity is given in the long-wavelength limit. Sum rules are discussed.