Plasma losses of electron energy and secondary electrons of discrete energies in aluminum

Discrete energy' loss is examined for electrons of energy 30–116 V. The losses due to excitation of three types of plasma wave are found; these plasmons have energies of 15.5, 9.5, and 7 eV. The energy distribution curves for the secondary electrons contain groups with discrete energies, which in aluminum coincide with the plasma energies, and so arise by decomposition of plasmons.     

Aluminum Hugoniot from model calculations including semicore electrons based on density functional theory

We present a method to obtain Hugoniot from model calculations based on density functional theory, and apply the method to aluminum Hugoniot. Technological advances have extended the experimental research of high energy density physics, and call for quantitative theoretical analysis. However, direct computation of Hugoniot from density functional theory is very difficult. We propose two step calculations of Hugoniot from density functional theory. The first step is molecular dynamics simulations with an ambient temperature for electrons. The second step is total energy calculations of a crystal with desired high temperatures for electrons and with the ambient temperature for electrons. We treated the semicore 2s and 2p electrons of aluminum as valence electrons only for the total energy calculations of the aluminum crystal. The aluminum Hugoniot from our model calculations is in excellent agreement with available experimental data and the previous density functional theory calculations in the literature.     

Ionization Potential Depression in Hot Dense Plasmas Through a Pure Classical Model

The ionization potential of an ion embedded in a plasma, lowered due to the whole of the charged particles (ions and electrons) interacting with this ion, is the so‐called plasma effect. A numerical plasma model based on classical molecular dynamics has been developed recently. It is capable to describe a neutral plasma at equilibrium involving ions of various charge states of the same atom together with electrons. This code is used here to investigate the ionization potential depression (IPD). The study of the IPD is illustrated and discussed for aluminum plasmas at mid and solid density and electron temperatures varying from 50eV to 190eV. The method relies on a sampling of the total potential energy of the electron located at an ion being ionized. The potential energy of such electron results from all of the interacting charged particles interacting with it. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

铝中气泡在电子束辐照下的异常放热现象

在室温下,利用离子加速器对纯铝透射样品分别注入He⁺,Ne⁺和Ar⁺三种惰性气体离子,通过透射电子显微镜原位观察分析了纯铝中三种气体气泡在电子束辐照下形貌及电子衍射花样的变化.实验表明,在200 keV电子束辐照下,三种惰性气体气泡均会合并长大,亮度逐渐增强,最终破裂,气泡内部产生许多约几个纳米的黑色斑点衬度像,选区电子衍射花样由单晶斑点衍射花样变为多晶衍射环.这一现象的原因可能是气泡在电子束辐照过程中发生了放热反应,使气泡附近铝熔化后再结晶产生多晶,从而在电子衍射花样中观察到了多晶衍射环.然而,氦气泡在80 keV电子束辐照下氦气泡形貌和选区电子衍射花样保持不变,辐照后衍射花样中无多晶衍射环产生;氦氩混合气体气泡在200 keV电子束辐照下气泡形貌和选区电子衍射花样同样保持不变;这可能与电子束能量和气泡内气体压力有关.     

X-ray bremsstrahlung and fast-ion measurements from picosecond laser-produced plasmas

Fast-ion and suprathermal electron energies have been measured from plasmas generated by 25 ps duration neodymium laser pulses focussed onto solid targets. The relationship between the fast-ion energy and the energy of the suprathermal electrons in the plasma has been demonstrated and these energies have been found to increase in proportion to I^0.35±0.05 for targets of aluminum and copper, with I the laser intensity.     

Energy spectra of electrons in plasma accelerators

The acceleration of electrons in the fast (relativistic) plasma wave, generated, e.g., by intense laser pulse in an underdense plasma, is studied theoretically and numerically. The analytical method, developed to describe the energy spectrum of electrons accelerated in one-dimensional (1-D) plasma wave of an arbitrary form, predicts the “bunching” of electrons in the energy space for linear (harmonic) plasma wave in contrast to the nonlinear one. The results of one- and two-dimensional (2-D) numerical simulations of the resonant and nonresonant electron bunch acceleration are presented and discussed     

A novel “sandwich” method for observation of the keyhole in deep penetration laser welding

A sandwich method was used to observe the keyhole in deep penetration laser welding, which provided an effective way to analyze both the Fresnel and inverse Bremsstrahlung absorption. In the transparent metal-analog system, different densities of metal vapor, ionized atoms, and free electrons in the keyhole can be simulated by changing the thickness of aluminum films. The research results show that inverse Bremsstrahlung absorption exerts a tremendous influence on the energy absorption of the laser beam for CO₂ laser welding. Low density of keyhole plasma benefits the incident laser energy coupling to the materials. However, excess density of keyhole plasma baffles the transmission of the incident laser beam to the interior material. By comparing inflow energy and outflow energy, there exits an energy balance on the keyhole wall by balancing the absorbed laser intensity and heat flux on the wall.     

On the Kinetics of the Active Zone of the Stationary SF6 Beam Discharge Plasma

The paper deals with the impact of intensive electron attachment on the kinetics of the electrons in the active zone of the stationary band-like beam discharge plasma in SF₆ which is an alternative useful plasma medium for “dry etching”. The energy distribution of the electrons in this plasma was obtained by numerically solving the Boltzmann equation which includes apart from elastic collisions, different exciting collision processes, attachment in electron collisions, direct ionization, the ambipolar loss of electrons, Coulomb interaction between electrons and of electrons with ions and the power input to the electrons by the turbulent electric field. In particular, due to the needed fulfilment of the consistent electron particle balance, for an extended region of the turbulence energy density in this plasma a large impact on the electron kinetics of the intensive electron attachment, which is the prevailing electron loss process, was found enforcing independent of the turbulence energy density always a large power input to the electrons, smooth and only slowly decreasing energy distributions even in the energy region of direct ionization.     

Optical properties of Ni<Subscript>3</Subscript>Al<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys with various degrees of localization of magnetic moments

The optical properties of Ni₃Al_{1 ? x} Mn _x (x = 0–1) ferromagnetic alloys have been studied in the spectral range 0.22–15 μm by the ellipsometry method. The substitution of manganese for aluminum is shown to substantially change the optical conductivity spectra. The concentration dependences of the plasma and relaxation frequencies of electrons have been found. The behavior of the optical characteristics of the alloys under study with varying x has been analyzed with allowance for the structure of the energy spectrum of electrons and the influence of a transition from the band magnetism to the spin-localized magnetism.     

Auger spectroscopy of the electron-stimulated absorption of oxygen on surfaces of polycrystalline aluminum

Electron spectroscopic analysis of oxygen absorption on aluminum is performed at a partial oxygen pressure of ～10^?6 mm Hg, with and without external impact (electron bombardment). It is shown that under these conditions, an aluminum surface oxidizes in the time required for the content of an oxide film affected by Al radiation with electrons having an energy of 800 eV to stabilize.     

温稠密铝等离子体物态方程及其电离平衡研究

温稠密物质的物性参数在惯性约束聚变能源、Z箍缩等高能量密度物理领域的实验结果分析和物理过程数值模拟等方面有着重要的应用价值.本文应用部分电离等离子体模型,在理想自由能的基础上考虑了库仑相互作用、排斥体积作用和极化作用等非理想特性,开展了温稠密等离子体物态方程和电离平衡的研究.计算了温稠密铝等离子体的压强等物态方程数据和在密度为1.0×10^-4-3.0 g/cm^3,温度为1.0×10^4-3.0×10^4 K范围内的粒子组分.计算结果显示,铝等离子体的平均电离度在临界密度区域内随着密度的增加而突然增大.根据非理想Saha方程中有效电离能这一关键参数,分析了铝等离子体平均电离度在临界密度区域内随密度迅速增大的现象.     

Diffraction of laser-plasma-generated electron pulses

We report the observation of the Debye–Scherrer diffraction using electron pulses emitted from a fs-laser plasma. Titanium sapphire laser pulses with 1.6 mJ/45 fs at 1 kHz are focused on a moving steel tape at close to normal incidence. The laser plasma generated ejects a large number of electrons in the direction of polarization, with a continuous energy spectrum extending up to 100 keV. Selecting an energy range of these electrons and scattering them on a thin aluminium sample generates a “streaked” diffraction pattern with unique features.     

Electron distribution function relaxation in monatomic gases

The authors discuss an analytic solution of the Boltzmann equation which describes the relaxation in time of the electron distribution function for electrons in a plasma derived from the monatomic gases He, Ne, Ar, and Xe. It is assumed that there are no perturbing forces on the electrons and that at t=0 they have a Maxwellian distribution function corresponding to an average energy of 2 eV. The electrons then lose energy through elastic collisions with neutrals and eventually energy-equilibrate with the neutrals, which are assumed to be cold. The evolution of the electron distribution function in time and velocity space is calculated for each gas. This model is approximately correct for the afterglow period of an electrical discharge in a monatomic gas. It is possible to calculate a time which is a measure of the decay time of the electron energy in an afterglow plasma     

Problems of technological use of coherent Bremsstrahlung beams

To determine the possibility of technological use of coherent Bremsstrahlung beams the photonuclear transmutation doping of silicon by aluminum upon γ-quantum irradiation from the aligned single-crystal target was studied. An increase in aluminum concentration by approximately ten times was obtained upon γ-radiation of silicon from the crystalline target aligned by the crystallographic axis 〈111〉 along the direction of the electron beam with the energy of 1200 MeV. The orientation effect of an increase in the yield of γ-radiation from silicon and tungsten crystals in the range of energies of accelerated electrons of 300–1200 MeV was examined. This result is essential for the estimation of a possible minimum energy of electrons at which the use of the orientation effect is reasonable.     

Mode-coupling assisted electron accelerations by a plasma wave

The acceleration of electrons by a plasma wave in the presence of density ripple in plasma has been investigated. Plasma density ripple can excite higher harmonics of different phase velocities of the fundamental plasma wave. The combined role of the different harmonics of the plasma wave contributes significantly in electrons energy gain during acceleration by the fundamental plasma wave. Our calculation shows that the plasma electrons gain considerable energy during the acceleration by the plasma waves in the presence of a density ripple in plasma. The initial electron energy and the ripple density play an important role for the acceleration of an electron.     

Energy Spectrum and Charge Composition of Ions of Laser-Produced Gd and Al Plasmas

The energy spectrum and the distribution of the total charge over the ion energy are studied for plasma produced by partially coherent radiation of the neodymium glass laser with a power density of ~10¹³ W/cm² in the focusing spot incident on continuous gadolinium (Gd) and aluminum(Al) plates. The spectra of plasma ions for gadolinium and aluminumtargets are presented at laser radiation energies of 2, 7, 20 J and 1 J, respectively. For the gadolinium target, an increase in the peak amplitudes in ion spectra with increasing influencing radiation energy and manifestations of higher-energy ions are observed. The difference in the ion emission for gadolinium and aluminum targets is also recorded at close radiation energies of 2 and 1 J, respectively.     

The measured energy loss of high energy electrons in copper and lead

The energy distributions of electrons of about 53, 75 and 93 MeV have been measured before and after passing through copper absorber of thickness up to 5.726 g/cm² and lead absorbers of thickness up to 2.825 g/cm². Earlier data for aluminum absorbers are reviewed. The electrons were accelerated by the LINAC of the Naval Postgraduate School. The most probable energy losses agree with the theory of Blunck and Westphal for all thicknesses; the half widths agree except for large thicknesses, where they are smaller than theoretical values for lead, in agreement for copper, and larger for aluminum. Large numbers of electrons of energy less than 30 MeV are observed in the distributions of transmitted electrons, particularly for thick absorbers and higher values of atomic number. These are apparently the result of multiple processes in the absorbers.     

Thermal analysis of intense femtosecond laser ablation of aluminum

This paper numerically simulates the process of ablation of an aluminum target by an intense femtosecond laser with a fluence of 40 J/cm 2 based on the two-temperature equation,and obtains the evolution of the free electron temperature and lattice temperature over a large temporal and depth range,for the first time. By investigating the temporal evolution curves of the free electron temperature and lattice temperature at three representative depths of 0,100 nm and 500 nm,it reveals different characteristics and mechanisms of the free electron temperature evolution at different depths. The results show that,in the intense femtosecond laser ablation of aluminum,the material ablation is mainly induced by the thermal conduction of free electrons,instead of the direct absorption of the laser energy; in addition,the thermal conduction of free electrons and the coupling effect between electrons and lattice will induce the temperature of free electrons deep inside the target to experience a process from increase to decrease and finally to increase again.     

An escape of fast electrons from a plasma cluster in subcritical electric field

A behaviour of electrons with high initial thermal velocities in plasma clusters is discussed in the paper. These electrons from the tail of the Maxwell velocity distribution can be accelerated into considerable energies by means of an external electric field. Both numerical and analytical estimates of the energy of the escaping electrons and an estimate of the total energy transferred by the electrons from the cluster are treated in the paper. The calculations are performed for a situation usual in coaxial and rail accelerators of the plasma clusters.     

Measurement of high-energy electrons by means of a Cherenkov detector in ISTTOK tokamak

The paper concerns detectors of the Cherenkov radiation which can be used to measure high-energy electrons escaping from short-living plasma. Such detectors have high temporal (about 1 ns) and spatial (about 1 mm) resolution. The paper describes a Cherenkov-type detector which was designed, manufactured and installed in the ISTTOK tokamak in order to measure fast runaway electrons. The radiator of that detector was made of an aluminium nitride (AlN) tablet with a light-tight filter on its front surface. Cherenkov signals from the radiator were transmitted through an optical cable to a fast photomultiplier. It made possible to perform direct measurements of the runaway electrons of energy above 80 keV. The measured energy values and spatial characteristics of the recorded electrons appeared to be consistent with results of numerical modelling of the runaway electron generation process in the ISTTOK tokamak.