Femtosecond laser-induced heating of electron gas in aluminium

The distribution function of free electrons in metal is calculated for irradiation of aluminium with an ultrashort laser pulse of moderate intensity. We consider inverse Bremsstrahlung absorption, electron-electron and electron-phonon interaction. Our theoretical model is based on Boltzmann equations and describes each considered process by a corresponding collision integral. The results show the excitation and relaxation of the free electron gas. Energy transfer to the phonon gas is calculated. Our model predicts linear absorption for intensities up to damage threshold. The calculated absorbed energy compares very well with known absorption characteristics.     

高重复频率激光能量沉积减小超声速波阻的数值研究

采用数值模拟的方法研究了激光重复频率、点火位置及来流马赫数等参数对激光能量沉积减小超声速钝头体波阻的影响。数值模拟结果表明,由于激光能量的沉积产生的低密度区与弓形激波相互作用,在钝头体前形成了类似虚拟尖锥的回流区,使原弓形激波逐渐向阻力较小的斜激波转变。阻力随着频率的增加而减小,当频率增加到200 kHz时,阻力减小到约为原来的17%,能量效率的最大值出现在频率为50 kHz处。说明控制参数的选择对减阻性能起着关键的作用。     

高重复频率激光能量沉积减小超声速波阻的数值研究

 采用数值模拟的方法研究了激光重复频率、点火位置及来流马赫数等参数对激光能量沉积减小超声速钝头体波阻的影响。数值模拟结果表明,由于激光能量的沉积产生的低密度区与弓形激波相互作用,在钝头体前形成了类似虚拟尖锥的回流区,使原弓形激波逐渐向阻力较小的斜激波转变。阻力随着频率的增加而减小,当频率增加到200 kHz时,阻力减小到约为原来的17%,能量效率的最大值出现在频率为50 kHz处。说明控制参数的选择对减阻性能起着关键的作用。     

Plasmon effects in electron energy loss and gain spectra in aluminium

Oberservations of the low energy secondary and Auger electron spectra and the electron energy loss spectra from a clean aluminium surface have been made and the results are compared with other recent studies including that of Jenkins and Chung (1971). Low energy emissions at 5.7 eV and 10.3 eV are associated with the creation of single electron excitations in the valence band by plasmon decay. An apparent anomaly in the plasmon loss and gain peaks associated with the Auger spectrum is discussed.     

飞秒激光-等离子体相互作用中快电子能量分布

采用不同量程的电子谱仪与LiF热释光探测器相配合,测量了飞秒激光 等离子体相互作用中产生的快电子能量分布。结果显示快电子能量分布的一致性和多个重要特征与国外同类实验和计算机模拟结果相似。快电子能谱在低能处产生凹陷是由于冷电子的回流产生的;几种加速机制共同作用是能谱在100~350 keV范围内出现平台的原因;快电子的有效温度较好地满足共振吸收的温度定标律是由于反射激光加速与共振吸收机制均是通过朗道阻尼或波破对电子进行加速的。     

飞秒激光-等离子体相互作用中快电子能量分布

 采用不同量程的电子谱仪与LiF热释光探测器相配合,测量了飞秒激光 等离子体相互作用中产生的快电子能量分布。结果显示快电子能量分布的一致性和多个重要特征与国外同类实验和计算机模拟结果相似。快电子能谱在低能处产生凹陷是由于冷电子的回流产生的;几种加速机制共同作用是能谱在100~350 keV范围内出现平台的原因;快电子的有效温度较好地满足共振吸收的温度定标律是由于反射激光加速与共振吸收机制均是通过朗道阻尼或波破对电子进行加速的。     

电子能量对收集极中电子束能量沉积的影响

理论分析了收集极中运动电子的失能机制和电子能量对电子束能量沉积的影响, 用蒙特卡罗方法计算了不同能量下入射电子的能量沉积分布, 分析了电子能量对电子束在收集极中能量沉积的影响, 并据此提出了提高收集极耐电子束轰击能力的两种途径。结果表明：激发和电离是收集极中入射电子的主要失能机制;电子的能量越高, 在材料中的穿透能力越强, 收集极中被收集电子束的最大能量沉积密度越低。综合考虑束流密度分布对能量沉积的影响, 可通过两种途径来提高收集极耐电子束轰击的能力：一是通过结构设计增大电子束的收集面积, 减小收集极上被收集电子束的束流密度;二是设计高阻抗器件, 增大被收集电子束的电子能量, 减小收集极上被收集电子束的束流密度。     

电子能量对收集极中电子束能量沉积的影响北大核心CSCD

理论分析了收集极中运动电子的失能机制和电子能量对电子束能量沉积的影响,用蒙特卡罗方法计算了不同能量下入射电子的能量沉积分布,分析了电子能量对电子束在收集极中能量沉积的影响,并据此提出了提高收集极耐电子束轰击能力的两种途径。结果表明:激发和电离是收集极中入射电子的主要失能机制;电子的能量越高,在材料中的穿透能力越强,收集极中被收集电子束的最大能量沉积密度越低。综合考虑束流密度分布对能量沉积的影响,可通过两种途径来提高收集极耐电子束轰击的能力:一是通过结构设计增大电子束的收集面积,减小收集极上被收集电子束的束流密度;二是设计高阻抗器件,增大被收集电子束的电子能量,减小收集极上被收集电子束的束流密度。     

A possible measure of the energy deposition profile in additional plasma heating experiments

We suggest measuring the energy deposition profile in additionally heated plasmas by modulating, at an appropriate frequency, a fraction of the supplementary power. If the frequency is high enough the temperature oscillations occur, with appreciable amplitude, only where the energy is directly deposited. Operating at lower frequency, plasma transport properties can be studied, through the measure of the phase delay of the temperature oscillations.     

Elemental electron energy loss mapping of a precipitate in a multi-component aluminium alloy

The elemental distribution of a precipitate cross section, situated in a lean Al-Mg-Si-Cu-Ag-Ge alloy, has been investigated in detail by electron energy loss spectroscopy (EELS) and aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). A correlative analysis of the EELS data is connected to the results and discussed in detail. The energy loss maps for all relevant elements were recorded simultaneously. The good spatial resolution allows elemental distribution to be evaluated, such as by correlation functions, in addition to being compared with the HAADF image.The fcc-Al lattice and the hexagonal Si-network within the precipitates were resolved by EELS. The combination of EELS and HAADF-STEM demonstrated that some atomic columns consist of mixed elements, a result that would be very uncertain based on one of the techniques alone. EELS elemental mapping combined with a correlative analysis have great potential for identification and quantification of small amounts of elements at the atomic scale.     

Resistive collimation of electron beams in laser-produced plasmas

Intense relativistic electron beams, produced by high-intensity short-pulse laser irradiation of a solid target, have many potential applications including fusion by fast ignition. Using a unique Fokker-Planck code, supported by analytic calculations, we show that fast electrons can be collimated into a beam even when the fast electron source is not strongly anisotropic, and we derive a condition for collimation to occur.     

Silicon surface passivation by aluminium oxide studied with electron energy loss spectroscopy

The origin behind crystalline silicon surface passivation by Al₂O₃ films is studied in detail by means of spatially‐resolved electron energy loss spectroscopy. The bonding configurations of Al and O are studied in as‐deposited and annealed Al₂O₃ films grown on c‐Si substrates by plasma‐assisted and thermal atomic layer deposition. The results confirm the presence of an interfacial SiO₂‐like film and demonstrate changes in the ratio between tetrahedrally and octahedrally coordinated Al in the films after annealing. These observations reveal the underlying origin of c‐Si surface passivation by Al₂O₃. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fast electron energy loss spectrum of an amorphous tungsten trioxide film

The forward scattering energy loss spectrum of 300 KeV electrons transmitted through thin films of WO₃ has been measured in the energy range of 0.8–225 eV with an energy loss resolution of 0.1 eV. Energy losses due to excitation of color centers, inter band transitions and core elestrons in WO₃ have been identified. Measurement of the elastic electron difraction pattern revealed the amorphous nature of the film with structural differences depending upon the preparative methods.     

Vacuum energy: Quantum hydrodynamics vs. quantum gravity

We compare quantum hydrodynamics and quantum gravity. They share many common features. In particular, both have quadratic divergences, and both lead to the problem of the vacuum energy, which, in quantum gravity, transforms to the cosmological constant problem. We show that, in quantum liquids, the vacuum energy density is not determined by the quantum zero-point energy of the phonon modes. The energy density of the vacuum is much smaller and is determined by the classical macroscopic parameters of the liquid, including the radius of the liquid droplet. In the same manner, the cosmological constant is not determined by the zero-point energy of quantum fields. It is much smaller and is determined by the classical macroscopic parameters of the Universe dynamics: the Hubble radius, the Newton constant, and the energy density of matter. The same may hold for the Higgs mass problem: the quadratically divergent quantum correction to the Higgs potential mass term is also cancelled by the microscopic (trans-Planckian) degrees of freedom due to the thermodynamic stability of the whole quantum vacuum.     

Nonohmic Coulomb drag in the ballistic electron transport regime

We developed the theory of Coulomb drag current induced in a one-dimensional nanowire by the ballistic nonohmic current in a nearby parallel nanowire under the ballistic transport regime. As in the ohmic case, we predict sharp oscillations of the drag current as a function of gate voltage or chemical potential of electrons. We also study the dependence of drag current on the voltage V across the driving wire. For relatively large values of V, the drag current is proportional to V ².     

Low-temperature spin coulomb drag in a two-dimensional electron gas

The phenomenon of low-temperature spin Coulomb drag in a two-dimensional electron gas is investigated. The spin transresistivity coefficient is essentially enhanced in the diffusive regime, as compared to conventional predictions. The origin of this enhancement is the quantum coherence of spin-up and spin-down electrons propagating in the same random impurity potential and coupled via the Coulomb interaction. A comprehensive analysis of spin and interlayer Coulomb drag effects is presented.     

Phonon drag effect in three- and two-dimensional electron systems

The nonequilibrium phonon flow drags the electrons, and depending upon experimental conditions manifests itself in the acoustoelectric current, acoustomagnetic field or acoustoelectric field. The results of these phenomena in Sn, Al, Ga, Ag measured with SQUID technique are discussed. In the two-dimensional (2D) case the phonon drag is studied on the interface of bicrystals and on the cleavage (111) surface of Ge and on the inversion layer on (111) (100) planes of Si. In all these cases the phonon drag is about two orders of magnitude larger than in metals with the same charge density. This is due to the drag of surface electrons by nonequilibrium phonon of the whole specimen. The Kohn resonance of phonons with Fermi surface and topological transitions on Fermi surface of 2D electrons produced sharp singularities of phonon drag effect in 2D cases.     

Semiclassical theory of electron drag in strong magnetic fields

We present a semiclassical theory for electron drag between two parallel two-dimensional electron systems in a strong magnetic field, which provides a transparent picture of the most salient qualitative features of anomalous drag phenomena observed in recent experiments, especially the striking sign reversal of drag at mismatched densities. The sign of the drag is determined by the curvature of the effective dispersion relation obeyed by the drift motion of the electrons in a smooth disorder potential. Localization plays a role in explaining the activated low temperature behavior but is not crucial for anomalous drag per se.     

Defect production rates in electron irradiated aluminium

Abstract

The cross section for atomic displacement has been determined at 7.5°K by means of electrical resistivity measurements in electron irradiated aluminium up to transferred energies of 1100 eV. These data and those from the literature have been evaluated with respect to the displacement function (DF). Below 200 eV the DF could be derived from the data with sufficient accuracy by solving the integral equation which describes the displacement cross section. Above 200 eV a family of non-linear and linear DF's has been found which fits the data within the experimental error. From the linear DF's one obtains an average threshold energy of (62±6) eV according to Kinchin and Pease.