Sputtering of solid argon by keV electrons

Thermochemical maskless etching of compound semiconductors (GaAs, InP, InSb, and GaP) has been performed by focused Ar-laser irradiation in chloride gas atmospheres. A controlled minimum linewidth of down to 0.6 m with a maximum etching rate of up to 13 m/s has been obtained. Minimum laser powers necessary for thermochemical etching in each of compound semiconductors were found to be 0.24, 0.56, and 0.06 W, corresponding to minimum local temperature rises of 190, 515, and 110°C for GaAs, InP, and InSb, respectively. Etching rates exhibited Arrhenius behavior with activation energies of 3.6–3.9 kcal/mole. Etching at excessively higher laser powers than those minimum powers was found, by microprobe photoluminescence measurements, to degrade the optical quality of the etched substrate.     

快点火中相对论电子束能量沉积的动理学研究

针对相对论快电子束在高密度压缩芯区等离子体中的能量沉积过程开展物理建模、程序研制和数值模拟研究。从等离子体粒子碰撞的基本物理出发,综合考虑了高能电子与背景等离子体之间的短程两体碰撞过程和长程集体效应,建立了相对论Fokker-Planck动理学模型,通过采用球谐展开的方法,推导得到了适于数值求解的方程形式并根据方程特点开展相应的数值算法研究及程序研制并完成了物理考核,对快点火能量沉积的典型物理算例进行了模拟研究,并针对即将在神光Ⅱ升级装置上开展的快点火物理实验进行了初步的物理分析。     

快点火中相对论电子束能量沉积的动理学研究

针对相对论快电子束在高密度压缩芯区等离子体中的能量沉积过程开展物理建模、程序研制和数值模拟研究。从等离子体粒子碰撞的基本物理出发,综合考虑了高能电子与背景等离子体之间的短程两体碰撞过程和长程集体效应,建立了相对论Fokker-Planck动理学模型,通过采用球谐展开的方法,推导得到了适于数值求解的方程形式并根据方程特点开展相应的数值算法研究及程序研制并完成了物理考核,对快点火能量沉积的典型物理算例进行了模拟研究,并针对即将在神光Ⅱ升级装置上开展的快点火物理实验进行了初步的物理分析。     

Equations for scattering of fast electrons by polyatomic molecules

The correct mathematical procedure is described for obtaining the Fourier representation of the equations for scattering of fast electrons by vibrational subsystems of polyatomic molecules with vibrational anharmonism taken into account.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 93–96, January, 1996.     

Some general problems of fast electrons transport

A generalization is given of the segments method in the form of a multistep method with generalized time for computing the transport of fast particles. The integral equation for a flow with generalized time in the phase space of variables is written under the assumption that the flow cuts the generalized time surface at right angles. The Green's function for the differential flow operator is the kernel of the integral equation. It is also shown that such an integral equation which can be obtained from a nonstationary kinetic equation provides a uniform consistent algorithm for solving either nonstationary or stationary problems. Examples of Green's functions are given for an operator of differential flow of fast electrons.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 110–114, August, 1974.The author would like to express his thanks to A. A. Vorob'ev and B. A. Kononov for their encouragement, to A. P. Yalovets for discussing the work with him, and to A. M. Kol'chuzhkin for going through the text.     

Collisions of fast electrons with positronium atoms

Summary Elastic and inelastic collisions of fast non-relativistic electrons with positronium atoms have been studies in the Born-Ochkur approximation. It has been shown that exchange plays an important role for transitions between states with the same parity as in this case the direct scattering amplitude vanishes. Numerical results for the total and ortho-para conversion cross-sections for the 1s→1s, 1s→2s and 1s→2p transitions have been presented for projectile energies varying from 0.1 to 10 keV. According to charge symmetry, presented results apply also to positron projectiles.     

Orientational dependence of luminescence yield excited by fast electrons in NaCl crystals

An investigation is made of the orientational dependence of the integral luminescence yield when NaCl(T1) single crystals are irradiated by electrons of energy 0.8–2.5 MeV in the direction 100. It is shown that the luminescence yield at small excitation densities depends linearly on the specific ionizational energy losses of the bombarding particles. The ionizational energy losses are calculated for the case when electrons of energy E=1.0 MeV pass through an NaCl single crystal in the direction 100. The minimum observed in the luminescence yield is due to electron capture in canalization conditions and decrease in energy losses. The maximum in the calculated curve at small angles of disorientation is due to the focusing of fast electrons by atomic chains.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 75–79, March, 1979.It remains to thank A. M. Taratin for performing the computer calculations and S. A. Vorob'ev for supervising the work.     

Relaxation of fast electrons in laser fusion targets

Collisional relaxation of a low density beam of non-thermal electrons injected into a high density Maxwellian plasma is investigated by computational treatment of Fokker-Planck type equation, with respect to the fast electron energy deposition and geometry configuration effects.     

Magnetically guided fast electrons in cylindrically compressed matter

Fast electrons produced by a 10 ps, 160 J laser pulse through laser-compressed plastic cylinders are studied experimentally and numerically in the context of fast ignition. K(α)-emission images reveal a collimated or scattered electron beam depending on the initial density and the compression timing. A numerical transport model shows that implosion-driven electrical resistivity gradients induce strong magnetic fields able to guide the electrons. The good agreement with measured beam sizes provides the first experimental evidence for fast-electron magnetic collimation in laser-compressed matter.     

Energy losses of fast electrons in multi-layer systems

Starting from Maxwell's equations a formula for the energy loss probability of fast electrons in systems of a given number of thin layers is derived for normal incidence. Retardation effects are neglected. For three layers of arbitrary thickness the equations are fully solved. As examples, a two-metallic-layer system with energy-dependent dielectric functions is discussed. Furthermore, the tarnish of metals e.g. Al–Al₂O₃, Be–BeO, Ag–Ag₂S is considered as three-layer systems. The energy loss spectrum of the system Al₂O₃–Al–Al₂O₃ is calculated in agreement with the experiment. It turns out that the bulk longitudinal optical mode of the covering layer is excited in the infra-red. In the case of the oxidation of a dielectric film the transversal mode of the oxide is expected to prevail in the energy loss spectrum.     

Excitation of surface plasmons in metal particles by fast electrons and x rays

The cross sections for excitation of surface plasmons by fast electrons and x rays are calculated within the random phase approximation for a spherical metal particle. The results are compared with those obtained within the hydrodynamic model of a free electron gas.     

Global simulation for laser-driven MeV electrons in fast ignition

A comprehensive examination of the interaction of a picosecond-long ignition pulse on high-density (40 times critical density) pellets using a two-dimensional particle-in-cell model is described. The global geometry consists of a 50 mum diameter pellet surrounded by a corona which is isolated by a vacuum region from the boundary. For cone-attached targets, as much as 67% of the incident laser energy is absorbed with 12% sent forward as fast electrons in a 23 degrees cone. The current filaments are driven by the Weibel instability of the forward-going fast electron flux and its return current with the ions playing an important role of neutralizing the space charge. No global current filament coalescence has been observed. The electron distribution function obeys a power law, which begins at E approximately 0.2 MeV and falls off as E-(2-3).     

The probability of MgO surface excitations with fast electrons

A scanning Transmission Electron Microscope has been used to investigate the probability of exciting surface modes in MgO cubes when fast electrons pass a certain distance outside the specimen. The experimental results give reasonable agreement with the classical theory of dielectric excitation by a moving charge providing relativistic effects are included.     

Incoherent radiation of fast electrons in a crystalline undulator

Incoherent radiation of high-energy particles in a crystal is caused by the thermal scattering of atoms relative to their equilibrium lattice positions. An earlier developed procedure for incoherent radiation simulation is applied to the case of electron motion in a sinusoidally curved crystal. The simulation results agree with the data of recent experiments at the Mainz Microtron (MAMI).