Theoretical and experimental study of electron-optical systems in low-energy wide-aperture electron accelerators

A large-area electron accelerator based on filament cathodes that operates in a continuous mode is described. The results of calculations of electron-optical systems of triode- and tetrode-type accelerators in accordance with the proposed engineering approach are considered, and experimental and calculated parameters of the accelerator are compared.     

Formation of wide-aperture electron beams in electron accelerators with explosive emission cathodes

The problems concerning the formation of electron beams of microsecond duration, electron energy 500–600 keV, and current density up to 20–30 A/cm² with a rectangular cross section of area 0.1–1 m² in high-current electron accelerators with explosive emission cathodes are considered. The designs of vacuum diodes capable of producing such beams to be used in high-power lasers and for ionization of gas in large volumes are presented. Institute of High-Current Electronics, Siberian Division of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 87–91, April, 2000.     

Voltage calibration of low-energy accelerators

The¹²C(p, γ)¹³N reaction has been used as a method to determine the absolute proton beam energy in the region ofE _p =150–350 keV with a precision of the order of 0.4 keV. The method makes use of the resulting captureγ-ray transition, whose intensity varies smoothly with beam energy and whose varying energy can be determined to high accuracy by comparison with precisely known energies ofγ-rays from radioactive sources. The energy calibration atE _p =80–150 keV has been carried out with the use of the nonresonant capture reaction D(p, γ)³He. The results have been applied to determine the absolute energies of proton-induced resonances on several light target nuclei.     

In situ electron microscopy study of the thermal motion of liquid lead nanoparticles in thin aluminum foils

The kinetics of thermal motion of liquid lead nanoparticles in thin aluminum foils has been investigated in situ by transmission electron microscopy. Dependences of the diffusion coefficient of particles on temperature and particle size have been obtained. The results of the investigations can be interpreted on the assumption that the mobility of particles is controlled by the nucleation of steps on {111} faces of their surface.     

辐射烧蚀铝箔数值模拟研究

 利用一维多群辐射输运程序RDMG数值模拟神光II条件下辐射烧蚀铝箔的实验研究,详细描述了X光在铝箔中的辐射输运过程,清楚地显示了辐射热波在铝箔中的传播,给出铝等离子体温度密度的时空分布、铝箔背面出射的X光能谱及出射X光各能区能流随时间的变化,分析出射X光的能区对测量结果的影响,对神光II条件下辐射烧蚀铝箔的厚度范围进行初步的探讨。数值结果与实验结果相吻合。     

辐射烧蚀铝箔数值模拟研究

利用一维多群辐射输运程序RDMG数值模拟神光II条件下辐射烧蚀铝箔的实验研究,详细描述了X光在铝箔中的辐射输运过程,清楚地显示了辐射热波在铝箔中的传播,给出铝等离子体温度密度的时空分布、铝箔背面出射的X光能谱及出射X光各能区能流随时间的变化,分析出射X光的能区对测量结果的影响,对神光II条件下辐射烧蚀铝箔的厚度范围进行初步的探讨。数值结果与实验结果相吻合。     

Coulomb scattering of 1 MeV electrons in aluminum foils

Electrons of a kinetic energy of 1 MeV were used to measure the angular dependence of the scattering cross section in thin aluminum foils. After correction for multiple scattering the results were compared with theoretical calculations including the radiative corrections. The measurements were found to agree with theory within ±1%.     

Diffraction effects in low-energy electron emission

The origin of diffraction peaks in the energy distribution of intensity of low-energy (< 1000 eV) electron emission from crystals is discussed from the standpoint of the dynamical theory of diffraction. The emitted electrons are considered to originate at relatively incoherent point sources in the crystal. The two-beam approximation of dynamical theory is used. The theory accounts for the chief regularities of diffraction peaks: temperature-dependence of peak intensities like that for low-energy electron diffraction (LEED) peaks, correlation of peak energies with X-ray absorption fine structure, and correlation of peak energies with the energies of normal-incidence LEED peaks in specular reflection. It is shown that the conditions for diffraction peaks coincide with the conditions for emergence of Kikuchi lines. It is predicted that for energies just above those of diffraction peaks, such emergences should be observable in the angular distribution of emission as intensity minima for emission along low-index crystal axes. Theory of Kikuchi band profiles is developed in an Appendix.     

Transients and efficient generation of electron beams in the pulsed wide-aperture glow discharge

The quasi-continuous wide-aperture glow discharge in helium at pressures from 1.2 to 6.0 Torr is studied. It is found that electron beam generation efficiency η is higher than 96% in the pressure range 1.2–3.0 Torr at voltages from 1.0 to 2.6 kV. The maximum power was ≈ 0 kW at 6 Torr and a voltage across the discharge gap of 2.6 kV. Under these conditions, the beam generation efficiency is about 80%. The pressure and voltage dependences of main parameters of the discharge are explained from the standpoint of its photoemission nature.     

The electron microscopy of thin metal foils

In this article, the nature of the defects in crystal structures and the way in which they can be studied by electron microscopy of thin metal foils are discussed in detail. Examples of phenomena observed in metals with the electron microscope are briefly presented. These include the dislocation structures introduced by work hardening, fatigue, quenching and irradiation; examples of microstructures found in alloys; and magnetic domain walls.     

Characterization of electron flow in negative- andpositive-polarity linear-induction accelerators

The operation of the linear-induction accelerators HELIA and Hermes III was studied in both negative and positive polarities. The experiments in positive polarity have provided a unique opportunity to explore the consequences of multiple-cathode electron emission in magnetically insulated transmission lines. It is maintained that an examination of the total energy-canonical momentum distribution of the electrons explains the features of the magnetically insulated flow exhibited by these systems. Simple analysis based on the basic concept of pressure balance, in conjunction with particle-in-cell numerical simulations, shows how the line voltage is related to the anode and cathode currents. Two flow designations are introduced that can apply to multiple-cathode, magnetically insulated transmission lines: full-gap flow (FGF) and locally emitted flow (LEF)     

Correlation of electron self-energy with geometric structure in low-energy electron diffraction

In low-energy electron diffraction (LEED) studies of surface geometries where the energy dependence of the intensities is analyzed, the in-plane lattice parameter of the surface is usually set to a value determined by x-ray diffraction for the bulk crystal. In cases where it is not known, for instance in films that are incommensurate with the substrate, it is desirable to fit the in-plane lattice parameters in the same analysis as the perpendicular interlayer spacings. We show that this is not possible in a conventional LEED I(E) analysis because the inner potential, which is typically treated as an adjustable parameter, is correlated with the geometrical structure. Therefore, without having prior knowledge of the inner potential, it is not possible to determine the complete surface structure simply from LEED I(E) spectra, and the in-plane lattice parameter must be determined independently before the I(E) analysis is performed. This can be accomplished by establishing a more precise experimental geometry. Further, it is shown that the convention of omitting the energy dependency of the real part of the inner potential means geometrical LEED results cannot be trusted beyond a precision of approximately 0.01 ?.     

Element segregation on the surfaces of pure aluminum foils

The surface segregation trend of trace elements in pure aluminum foils was investigated by density functional theory. The model of nine-layer Al(1 0 0) slab substituted partially by trace element atoms was proposed for calculating surface segregation energy. The calculating results show that (i) B, Mg, Si, Ga, Ge, Y, In, Sn, Sb, Pb and Bi exhibit negative segregation energy and possibly move to the surface, while Be, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zr exhibit positive segregation energies and migrated into the bulk; (ii) the segregation energy was found to be related with the covalent radius, the relaxed position at the surface of the substituting atom and the surface energy; (iii) the segregation behavior of trace element generates lots of defects and dislocation, which can increase the initial pitting nucleation sites in the surface of aluminum foils; (iv) the impurity atom concentration was tested with Pb-doped surfaces, the calculated negative segregation energies in all coverage increases rapidly with the Pb coverage. These conclusions are helpful for designing of the chemical composition and to advance the tunnel etching of aluminum foils.     

Attenuation and escape depths of low-energy electron emission

Electron transport and emission is simulated by two Monte Carlo (MC) programs. The first version is based on elastic Mott cross sections and inelastic loss functions with full dispersion ΔE=ℏω(q), including electron impact and subsequent cascading processes. Surface effects like surface plasmons and the quantum mechanical surface transmittivity have been taken into account too. Especially for dielectric materials like SiO₂ and applied electric fields a second MC version is developed based on the electron scattering with acoustic and optical phonons, intra- and intervalley scattering and impact valence band ionization. A comparison of both versions results in a good agreement still in the energy region of several eV, but a predominance of the phonon-based second version is found for very low electron energies, e.g., for hot and ballistic electrons in dielectric materials.     

Unoccupied electron states in low-energy electron total-current and transmission spectroscopy of molybdenum disulfide

A theoretical interpretation of the fine structure in the low-energy electron total-current spectra and low-energy electron transmission spectra measured along the normal to the (0001)MoS₂ single-crystal surface is proposed. The calculations took into account the energy dependence of band level broadening and the electronic structure of the high final unoccupied states (above the vacuum level E_vac), which become occupied by electrons entering a solid. A comparison with the available experimental and theoretical data is performed. The effects of the bulk band structure are shown to play a dominant role in the formation of the spectra (the extrema in the spectra identify the energy position of critical points, such as the band edges or the points of extremal curvature of the dispersion branches). The proposed method makes it possible to separate the bulk effects in spectra from surface effects, this approach can be used to advantage in monitoring the state of a surface in the course of its treatment.     

Formation of narrow low-energy high-intensity electron beams

The process of formation of high-density low-energy (5–10 keV) pulsed electron beams of small diameter (on the order of a few millimeters) in a gun of the “channel-spark” type is studied. It is shown that beams with a rate of rise of the current exceeding 10¹¹ A/s and an amplitude exceeding the Alfvén current by a factor of 1.5–2.0 can be obtained in experiments with intense preionisation of the transport channel combined with a pulsed supply of the accelerating voltage to the cathode. In the optimal pressure mode, the current density at a distance of 2–3 cm from the gun outlet is 40–25 kA/cm², which will ensure ablation of most solid targets.     

Measurement of electron clouds in large accelerators by microwave dispersion

Clouds of low energy electrons in the vacuum beam pipes of accelerators of positively charged particle beams present a serious limitation for operation at high currents. Furthermore, it is difficult to probe their density over substantial lengths of the beam pipe. We have developed a novel technique to directly measure the electron cloud density via the phase shift induced in a TE wave transmitted over a section of the accelerator and used it to measure the average electron cloud density over a 50 m section in the positron ring of the PEP-II collider at the Stanford Linear Accelerator Center.