双频激光辐照的等离子体中的稳态电场结构和密度分布

本文导出了法向入射时,双频激光辐照的等离子体中的稳态电场强度和局域标度长度的解析表达式。结果表明,双频辐照的有质动力可以导致临界面附近出现密度凹坑和临界密度处的局域标度长度在某个强度范围内的反常增加和起伏。 关键词：     

A theory of a receiving antenna in a moving isotropic plasma

We analyze the response of a dipole antenna to the noise-like and/or regular (quasimonochromatic) plasma oscillations and waves. The antenna is immersed in an isotropic plasma moving with velocity greater than the electron thermal velocity. In the case of a noise field, we calculate the squared spectral power density of the noise voltage at the input of a receiving antenna for frequencies close to the electron plasma frequency. It is shown that the main contribution to the noise is made by the radiation due to the excitation of waves at anomalous Doppler frequencies. In the case of an incident monochromatic wave, the mean square voltage at the antenna input is calculated as a function of the wave frequency and angle of arrival. It is shown that the effective antenna length can differ strongly from the geometrical length of the dipole. This fact results from the dispersion of longitudinal waves ensuring that many plane waves (a continuum, in the limiting case) contribute to the re-radiated field for a given direction of propagation of the radiation energy.     

激光等离子体受激散射的线性理论研究

利用包含两种阻尼(Landau阻尼和碰撞阻尼)成分的受激拉曼散射(SRS)、受激布里渊散射线性分析程序LIP,在给定的等离子体状态下分析了阻尼成分、激光强度以及等离子体组分对点火尺度等离子体中受激散射不稳定性发展的影响.详细评估了碰撞阻尼的效应,发现碰撞阻尼在接近1/4临界密度处能有效抑制受激散射的增长,从而造成SRS光谱上的"缝"现象.同时,电子温度的升高会在特定的密度区域促进SRS的发展.研究结果可为点火实验的设计提供参考.     

基于一种新型超腔的高亮度激光同步辐射分析

 提出了一种由一对抛物面构成的超腔的技术方案,计算了超腔汇聚点处的总光子密度。利用康普顿散射理论对基于抛物面超腔的激光同步辐射及其光子产额进行了讨论和计算。结果表明:当超腔镜面的反射率等于99.99%时,在超腔碰撞点处的总光子数密度大约是初始激光束在碰撞点处光子数密度的5 000倍,对应康普顿垂直散射的光子产额大约是电子束与初始激光束在碰撞点处发生康普顿垂直散射时的5 000倍。     

Electromagnetic-wave reflectivity of the surface of a cubic-ferrite plate

The electromagnetic-wave reflectivity of the surface of an insulating cubic-ferromagnet (ferrite) plate is calculated analytically and numerically with allowance for spin-wave damping. The frequency and magnetic-field dependences of the reflectivity are found at the orientational phase transition point and in the vicinity of this point for various values of the plate thickness. It is shown that the reflectivity exhibits anomalous behavior when the dynamic magnetic permeability becomes equal to the dielectric constant, as well as when dimensional resonances occur for electromagnetic or acoustic waves or ferromagnetic resonance takes place. At frequencies below the magnetoelastic gap in the quasi-spin-wave spectrum, the reflectivity may have anomalously low (down to zero) or anomalously high (up to unity) values. Such frequencies can lie in the microwave region.     

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)     

双频激光在不均匀等离子体中的耦合

本文研究了具有不同频率的两束激光在不均匀等离子体拍频共振点的耦合系数,计算了该系数对于等离子体拍频波加热的影响以及它对于双频光束频率比和等离子体定标长度的依赖关系。 关键词：     

On the nonlinear theory of current instability of short-wave drift oscillations

The paper deals with the studies of current instability in the inhomogeneous plasma resulting in excitation of short-wave drift oscillations with a frequency near the low-hybrid resonance.It is shown that the saturation of such an instability is associated with the spectral pumping of oscillations into the short-wave region that occurs due to the modulation instability; and maximum amplitudes of the electrical fields of oscillations are determined. The effective frequency of electron collisions due to current instability is calculated. It is indicated that the diffusion model of the Parker-Sweet magnetic field reconnection modified taking into account the anomalous resistance mechanism studied here leads to the estimate of the magnetopause width being in satisfactory agreement with the experiment.     

Kinetic theory of fluctuations near a convective instability

The kinetic theory of density and velocity fluctuations below and above the first convective instability in a Benard cell is presented. Results are given in a form valid for all fluid densities. In particular, the singular behavior near the instability point is computed and a detailed comparison with that near a critical point is made. Mode coupling effects, involving the viscous and the heat mode, that determine the singular behavior of the thermal conductivity near the critical point, are also responsible for the singular behavior of the pair correlation function and in particular the density-density correlation function near the instability point. The anomalous density fluctuations could be measured directly by microwave scattering experiments, while the velocity fluctuation could be measured by laser Doppler velocimetry.     

亚密度冷等离子体的低功率激光吸收特性分析

当电子密度远低于入射小功率激光所对应的临界密度时,冷等离子体对激光的吸收特性与高密度热等离子体将有很大的差别。在亚密度冷等离子体中,电子与中性粒子间的碰撞将占主导地位,对应的频率远大于电子-离子碰撞频率,这使得碰撞吸收本质发生了变化。亚密度等离子体的电子密度和碰撞频率均较小,它在单位长度传输路径内对常用的工作在可见光、红外波段内的小功率激光的碰撞吸收可以忽略不计。但是对于非正常吸收机制的影响尚需深入研究。     

Interpretational problems of interferograms of asymmetric laser produced plasmas

Computer simulations have been made of interferograms produced in typical short pulse laser produced plasma experiments. It is shown that for certain plasma profiles and values of scale length the reconstructed profile is an inaccurate representation of the original profile. It is also shown that the interferometer focussing accuracy affects the maximum measurable electron density and the implied plasma scale length.     

Optical conductivity of a hydrogen plasma: weak localization effects

To what extent can collective electron-ion interaction effects influence the electric conductivity of a Coulomb system? This paper is aimed to answer this question for the case of the solid density hydrogen plasma. The thermodynamic Green's function approach is combined with the generalized hydrodynamic theory to develop a self-consistent approximation for the dynamic (frequency dependent) electron-ion collision frequency. On this basis the electric conductivity, skin depth and reflectivity of a dense hydrogen plasma are evaluated at frequencies below the plasma frequency. The results indicate that the percolation metal-insulator transition becomes possible at low plasma densities.     

Coherent nonlinear electromagnetic response in twisted bilayer and few-layer graphene

The phenomenon of Rabi oscillations far from resonance is described in bilayer and few-layer graphene. These oscillations in the population and polarization at the Dirac point in n-layer graphene are seen in the nth harmonic term in the external driving frequency. The underlying reason behind these oscillations is attributable to the pseudospin degree of freedom possessed by all these systems. Conventional Rabi oscillations, which occur only near resonance, are seen in multiple harmonics in multilayer graphene. However, the experimentally measurable current density exhibits anomalous behaviour only in the first harmonic in all the graphene systems. A fully numerical solution of the optical Bloch equations is in complete agreement with the analytical results, thereby justifying the approximation schemes used in the latter. The same phenomena are also described in twisted bilayer graphene with and without an electric potential difference between the layers. It is found that the anomalous Rabi frequency is strongly dependent on twist angle for weak applied fields – a feature absent in single-layer graphene, whereas the conventional Rabi frequency is relatively independent of the twist angle.     

Noise and output power in the CW 4416 Å HeCd laser

It is shown that the anomalous output power-discharge current dependence observed on a 50 Hz a.c. excited 4416 Å HeCd laser is due to the appearance of plasma oscillations, which cause an increase of He metastable density.     

Collision of two short laser pulses in plasma and the generation of short-lived Bragg mirrors

We consider the nonlinear excitation of small-scale electron-density perturbations when two identical short laser pulses propagating toward each other collide in plasma. Pulses with duration τ of the order of the plasma-oscillation period ω_pτ ≤ 1, ω_p is the plasma frequency) are shown to excite long-lived localized plasma oscillations in the collision region. The energy conservation laws for the nonlinear mixing of short laser pulses in plasma are analyzed. We investigate the scattering of a sounding wave by the electron-density perturbations produced in the pulse collision region (short-lived Bragg mirror).     

等离子体密度标长对高次谐波转换效率的影响

通过等离子体粒子模拟研究了在强激光与等离子体相互作用产生高次谐波的过程中,等离子体密度标长对转换效率的影响,计算了在不同密度标长下p偏振非相对论强度激光与高密度等离子体相互作用产生高次谐波的转换效率,发现等离子体密度标长对转换效率有重要的影响,这种影响与谐波级次,等离子体密度,激光脉冲宽度有关。     

激光脉冲诱导的等离子体密度调制及其产生的相位反射

理论研究和数值模拟发现入射光和反射光在低密度等离子体中形成的干涉场可以产生深度的等离子体密度调制. 对于中等强度的入射光，譬如10¹⁵W／cm² ，产生密度调制的时间尺度在几十个光周期的范围. 这样的等离子体密度调制可以起类似布拉格反射镜的 作用，使得后面的入射光在临界面以下的区域产生相位反射. 因为密度调制的周期是光在等 离子体中波长的一半，其产生的反射率可以接近100％. 相位反射也可以在不均匀的低密度 等离子体中产生，它可以极大地减少等离子体对光的吸收，因此在惯性约束核聚变中需要考 虑到它的影响. 关键词： 相位反射 密度调制 激光等离子体 粒子模拟     

Interaction of microwaves with atmospheric pressure plasmas

In this paper, the interaction of microwaves with a plasma, generated at atmospheric pressure, is studied. The refractive index, attenuation index, skin depth, attenuation coefficient, phase coefficient, and reflectivity are investigated as functions of the plasma number density, the wave frequency and type of polarization, and the grazing angle. It is found that two frequency regimes characterize these type of plasmas. The first is a range where the phase velocity and attenuation of the wave both increase with frequency. The second is a frequency range in which the phase velocity and attenuation of the wave remain constant. It is also found that to have a shallow skin depth, the plasma number density has to be in the 10¹³ cm^–3 range. The reflectivity is found to be an increasing function of the number density. In horizontal polarization, the reflectivity is a decreasing function of the grazing angle. But in vertical polarization and for grazing angle less than 20°, the reflectivity has a maximum at a frequency , where f_pe is the electron plasma frequency and v is the collision frequency.     

Quasi-two-dimensional metallic hydrogen in diphosphide at a high pressure

The average optical reflectivity of bismuth as a function of time t after irradiation by a short laser pulse has been calculated. The amplitude A of photoinduced oscillations in the average optical reflectivity is shown to have extrema under certain conditions. The time τ_j (j is a natural number) at which the amplitude A reaches the jth extremum has been calculated. The calculated dependences of the times τ₁ and τ₂ at which, respectively, the first and second extrema (the first minimum and the first maximum) of the amplitude A are reached on the maximum laser pulse energy density Q are consistent with the experimental data from [8].     

Inverse Bremsstrahlung absorption coefficient in inhomogeneous plasma with nonextensive electron velocity distribution and Tsallis exponential electron density profile

Inverse bremsstrahlung (collisional) absorption of the laser beam is studied in plasma with a generalized (q-nonextensive) electron velocity distribution and some kind of generalized electron density profile. It is shown that for some values of parameters designating the q-nonextensive electron velocity distribution function and its generalized density profile, the calculated absorption coefficient reduces to the already known cases with Maxwellian velocity distribution with linear and exponential density profiles.