Evolution of the elastoplastic bending of whisker crystals of NaCl due to nanosecond exposure to a high-density electron beam

High-speed photography is used to study the evolution of the plastic bending of whisker crystals (WCs) of NaCL caused by nanosecond exposure to a pulsed high-density electron beam. The total time of irreversible bending of a WC, reckoned from the radiation pulse to attainment of the quasisteady state, is equal to 1.5–2.5 msec and nearly coincides with the characteristic relaxation time of the quasistatic thermoelastic mechanical stresses that develop due to the nonuniform distribution of the dose and the corresponding temperature through the thickness of the specimen. These stresses relax when the temperature distribution is equalized by heat conduction. There is an induction period of 0.3–0.6 msec between the radiation pulse and the beginning of intensive bending, this period being much longer than the period of the thermoelastic bending vibrations. One interesting result of the study is that the bending is nonmonotonic. Tomsk Polytechnic Institute, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 41, No. 12, pp. 25–30, December, 1998.     

Numerical analysis of the spatial behaviour of nanosecond optical parametric oscillators of beta-barium borate

We report on a numerical analysis of the temporal and spatial beam properties of nanosecond optical parametric oscillators (OPOs). The analysis is performed for a 355-nm-pumped critically phase-matched OPO of beta-barium borate. The calculations provide detailed information on the dependence of the OPO beam quality (measured by the quality factor M ²) on pump energy. An important result is the strong increase of the M ² value for pump energies exceeding 1–2 times the energy at threshold. Furthermore, a temporal analysis of single OPO pulses indicates that the M ² value strongly increases during the first few nanoseconds of the OPO oscillation. This increase is understood by considering the temporal dynamics of the spatial profiles of the OPO signal beam and the depleted pump radiation. Received: 1 April 1999 / Revised version: 26 July 1999 / Published online: 20 October 1999     

Critical (burst) electron emission from dielectrics, induced by injection of a dense electron beam

A dense pulsed electron beam and nanosecond pulse length has been used to inject negative electric charge into various dielectric materials (single crystals, glasses, composites, plastics) for initiation of electron field emission from the dielectric into a vacuum. It has been shown that upon reaching a critical electric field in the bulk and at the dielectric surface there is intense critical electron emission. The local current density from the emission centers reaches a record value (for dielectrics) of the order of 10⁶ A/cm². The emission occurs in the form of a single gigantic pulse. The measured amplitude of the emission current averaged over the emitting surface is the same order of magnitude as the injected electron current: 10–1000 A. the emission current pulse lages behind the current pulse of the primary electron beam injected into the sample. The delay time is in the range 1–20 nsec and decreases with increasing current density of the injected beam. Direct experimental evidence is found for intense generation of carriers (band or quasifree electrons) in the near-surface layer of the dielectric in a strong electric field due to the Frenkel-Poole effect and collisional ionization of traps, usually various donor levels. This process greatly strengthens the field emission from the dielectric. It has been shown experimentally that the emission is nonuniform and is accompanied by “point bursts” at the surface of the dielectric and ionized plasma spikes in the vacuum interval. These spikes are the main reason that the transition of the field emission into “bursts” is critical, similar to the current which has been previously observed in metals and semiconductors. However there are a number of substantial differences. For example the critical field emission current density needed for the transition into “bursts” is three orders of magnitude less than for metals. If we provide sufficient electron current at the surface or from the bulk of the dielectric to the emission centers, then the critical emission is always accompanied by a vacuum discharge between the surface of the dielectric and a metallic collector. A detailed computer model of the processes in the dielectric during injection of a high-density electron beam has been developed which allows one to understand the complex physical pattern of the phenomenon. Tomsk Polytechnic University. Institute of High-Current Electronics, Siberian Section, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 45–67, November, 1997.     

Mechanism of structural changes of Si(111) surfaces subjected to low-energy ion pulses during molecular-beam epitaxy

Reflected high-energy electron diffraction (RHEED) and detection of the intensity oscillations of the specular reflection have been used to investigate morphological changes in Si(111) associated with the two-dimensional layer-by-layer mechanism of silicon growth from a molecular beam under conditions of pulsed (0.25–1 s) bombardment with low-energy (80–150 eV) Kr ions in the interval of small total radiative fluxes (10¹¹–10¹² cm²²), for which the density of radiation-generated defects is small in comparison with the surface density of the atoms. After pulsed ion bombardment an increase in the intensity of the specular reflection is observed if the degree of filling of the monolayer satisfies 0.5<θ<1. No increase in the intensity occurs during the initial stages of filling of the monolayer. The maximum amplitude increment of the oscillations is reached at θ≈0.8. The magnitude of the amplitude increment of the RHEED oscillations increases with temperature up to 400°C and then falls. At temperatures above 500°C amplification of the reflection intensity essentially vanishes. Experiments on multiple ion bombardment of each growing layer showed that the magnitude of the amplitude increment of the oscillations decreased as a function of the number of deposited layers (the order of the RHEED oscillation). A mechanism for the observed phenomena is proposed, based on the concept of surface reconstruction by pulsed ion bombardment accompanied by formation of a (7×7) superstructure, which corresponds to a decrease of the activation energy of surface diffusion of the adatoms. Within the framework of the proposed mechanism the results of Monte Carlo modeling agree with the main experimental data. Zh. éksp. Teor. Fiz. 114, 2055–2064 (December 1998)     

Effect of the electron beam and ion flux parameters on the rate of plasma nitriding of an austenitic stainless steel

The method of nitriding of metals in an electron beam plasma is used to change the current density and energy of nitrogen ions by varying the electron beam parameters (5–20 A, 60–500 eV). An electron beam is generated by an electron source based on a self-heated hollow cathode discharge. Stainless steel 12Kh18N10T is saturated by nitrogen at 500°C for 1 h. The microhardness is measured on transverse polished sections to obtain the dependences of the nitrided layer thickness on the ion current density (1.6–6.2 mA/cm²), the ion energy (100–300 eV), and the nitrogen-argon mixture pressure (1–10 Pa). The layer thickness decreases by 4–5 μm when the ion energy increases by 100 V and increases from 19 to 33 μm when the ion current density increases. The pressure dependence of the layer thickness has a maximum. These results are in conflict with the conclusions of the theory of the limitation of the layer thickness by ion sputtering, and the effective diffusion coefficient significantly exceeds the well-known reported data.     

The effect of temperature on the pulsed conductivity of a KCl crystal excited by picosecond electron pulses

This paper discusses the temperature dependence of the pulsed conductivity of a KCl crystal in the interval 12–300K when it is excited by an electron beam (0.2 MeV, 50 ps, 300A/cm²) with a time resolution of 150 ps. It is shown that the electron lifetime is τ<100 ps in the entire interval under consideration, while the conductivity increases with temperature. The experimental results make it possible to obtain the temperature dependences of the effective electron-hole recombination cross section S∼T ^3.5 and the separation probability of genetic pairs. Fiz. Tverd. Tela (St. Petersburg) 41, 429–430 (March 1999)     

Thermal and optical properties of electron beam irradiated cellulose triacetate

Samples from Cellulose triacetate (CTA) sheets were irradiated with electron beam in the dose range 10–200 kGy. Non-isothermal studies were carried out using thermogravimetric analysis (TGA) to obtain the activation energy of thermal decomposition for CTA polymer. The CTA samples decompose in one main break down stage. The results indicate that the irradiation by electron beam in the dose range 80–200 kGy increases the thermal stability of the polymer samples. Also, the variation of melting temperatures with the electron dose has been determined using differential thermal analysis (DTA). The CTA polymer is characterized by the appearance of one endothermic peak due to melting. It is found that the irradiation in the dose range 10–80 kGy causes defects generation that splits the crystals depressing the melting temperature, while at higher doses (80–200 kGy), the thickness of crystalline structure (lamellae) is increased, thus the melting temperature increases. In addition, the transmission of these samples in the wavelength range 200–2500 nm, as well as any color changes, were studied. The color intensity ΔE* was greatly increased on increasing the electron beam dose, and accompanied by a significant increase in the blue color component.      

High-energy passive-ionization electrons and holes in dielectrics with pulsed irradiation by high-density electron beams

This article is a survey of works by the author and colleagues on the investigation of charging and discharging dynamics in solid dielectrics exposed to dense electron beams with subnanosecond resolution. Small high-current electron accelerators of theDzhin type, which were developed and fabricated at the Nonlinear Physics Laboratory, were used as the source of the primary electron beam. The primary electron beam parameters were: 0.25–0.45 MeV, 1–30 nsec, 0.1–10,000 A/cm³. The dielectric is investigated experimentally with its surface covered by a metallic electrode and with critical electron emission into the vacuum eliminated. In this case, the total current in the dielectric consists of three components: the primary beam current, the displacement current, and the conduction current. The first and last are responsible for charging and discharging of the dielectric volume. It is shown that the bulk nonequilibrium radiation conduction mechanism depends greatly on the dose intensity. For small dose intensities, the principal current carriers are the low-energy electrons of the conduction band and holes of the valence band, which are in quasi-equilibrium with the lattice phonon field before capture by defects or merging into excitons preceding recombination. This type of conduction has been well studied in the physics of semiconductors and dielectrics. However, over a broad interval of intermediate and high dose intensities, another type of nonequilibrium conduction dominates — the high-energy, which was discovered and studied by the author and colleagues. The principal carriers are then passive-ionization electrons and holes with energies of 0.1–10 eV in the process of phonon relaxation in which phonon emission dominates absorption. The high-energy conduction differs considerably from the low-energy in many properties, which determines the unusual dynamics of dielectric charging and discharging with irradiation by the dense electron beam of a high-current accelerator. Institute of High-Current Electronics, Siberian Section, Russian Academy of Sciences. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 109–119, November, 1996.     

Experimental and computational study of the passage of an electron beam through the curvilinear element of the Drakon stellarator system in a tenuous plasma

Results are presented from the first stage of studies on the passage of an electron beam with energy 100–500 eV in a magnetic field of 300–700 Oe through the curvilinear solenoid of the KRéL unit, the latter being a prototype of the closing segment of the Drakon stellarator system, in the plasma-beam discharge regime. The ion density at the end of the curvilinear part of the chamber, n _i ≈8×10⁸–10¹⁰ cm⁻³, the electron temperature T _e ≈4–15 eV, and the positions at which the beam hits the target for different distances from it to the electron source are determined experimentally. The motion of the electron beam is computationally modeled with allowance for the space charge created by the beam and the secondary plasma. From a comparison of the experimentally measured trajectories and trajectories calculated for different values of the space charge, we have obtained an estimate for the unneutralized ion density of the order of 5×10⁷ cm⁻³. Zh. Tekh. Fiz. 69, 22–26 (February 1999)     

Generation of currents in gases by fast, highly charged ions

It is shown that an exit asymmetry of the electrons and recoil ions formed during ionization of atoms in elementary collision events with fast, highly charged ions can give rise to macroscopic electron and recoil ion currents during the bombardment of a gaseous target by a beam of fast, highly charged ions. Zh. Tekh. Fiz. 68, 20–23 (September 1998)     

Solid polymeric electrolyte of PVC–ENR–LiClO<Subscript>4</Subscript>

The ionic conductivity of PVC–ENR–LiClO₄ (PVC, polyvinyl chloride; ENR, epoxidized natural rubber) as a function of LiClO₄ concentration, ENR concentration, temperature, and radiation dose of electron beam cross-linking has been studied. The electrolyte samples were prepared by solution casting technique. Their ionic conductivities were measured using the impedance spectroscopy technique. It was observed that the relationship between the concentration of salt, as well as temperature, and conductivity were linear. The electrolyte conductivity increases with ENR concentration. This relationship was discussed using the number of charge carrier theory. The conductivity–temperature behaviour of the electrolyte is Arrhenian. The conductivity also varies with the radiation dose of the electron beam cross-linking. The highest room temperature conductivity of the electrolyte of 8.5 × 10⁻⁷ S/cm was obtained at 30% by weight of LiClO₄. The activation energy, E _a and pre-exponential factor, σ _o, are 1.4 × 10⁻² eV and 1.5 × 10⁻¹¹ S/cm, respectively.     

Laser interferometry of acoustic fields generated by high-current electron beams in solids

We have developed a method for detecting acoustic fields in solids irradiated with dense electron beams. The method is based on laser Michelson interferometry. The electron source is a high-current DZhIN electron accelerator. The detection system features a short baseline Michelson interferometer located inside the experimental chamber with the sample, a stabilized initial beam pathlength difference within the interferometer, high temporal resolution, an analog-to-digital converter with output to a personal computer, and a program for reducing the interferometer data. We can measure both long pulses with minimum displacements of 10⁻¹⁰ m and durations of 10⁻⁸ sec, and flexure waves with large-amplitude displacements of 10⁻⁵ m and oscillation periods of 10⁻³ sec. We present results from studies of flexure waves in thin plates and rods of copper, silicon, alkali-halide crystals, quartz glasses, and D16T aluminum alloy irradiated by nanosecond high-density electron beams. Institute of High-Current Electronics, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 82–92, November, 1997.     

Spodumene and garnet luminescence excited by subnanosecond electron beams

Pulsed cathodoluminescence of spodumene and yttrium-aluminum garnet crystals activated by Mn²⁺ and Nd³⁺ ions, respectively, is investigated. The luminescence was excited upon crystal irradiation by electron beams with current densities of 35 and 100 A/cm² and average electron energy of ∼ 50 keV for 0.1, 0.25, and 0.65 ns. It is demonstrated that the electron beam duration decreased to several tenth of a nanosecond does not lead to essential changes of the mechanisms of pulsed cathodoluminescence excitation and character of its spectrum, but in this case, the intensity of luminescence of the hole centers increases compared with the intracenter luminescence.     

Experimental set-up for irradiation of solid H2 and D2 with charged particles of keV energies

An experimental facility was built where films of solid deuterium (and hydrogen) may be made with known thickness and irradiated with pulsed beams of electrons (up to 3 keV) and light ions (up to 10 keV). Films are made on a target plate held at 2.5–3 K. Film growth rate is calibrated with a quartz crystal film thickness monitor. The target plate, which can be heated so that films are removable by evaporation, may be used both as a calorimeter and as a beam current collector. Methods for measurement of secondary electron emission coefficients were developed, and preliminary measurements were made with electrons and hydrogen ions. For electron bombardment, the secondary electron emission coefficient of solid deuterium was much smaller than one. It was shown possible to use the set-up to study beam desorption of very thin films. Furthermore the set-up could be used for measuring the energy-reflection coefficient γ (i.e. the fraction of beam energy reflected from the target) for protons impinging on a heavy target material by using the target as a calorimeter.     

Stepped form of negative corona current pulses in hydrogen

Current wave forms of initial stages of discharge formation in a short negative point-to-plane gap have been measured with a nanosecond time resolution in hydrogen at pressures (12.5–76) kPa and for various overvoltages and cathode point radii. The measurements revealed the existence of a stepped form of negative corona current pulses in hydrogen. To test existing models for negative corona pulse formation, effects of changing cathode secondary electron emission were studied using copper and brass cathodes coated by CuI and graphite. It is concluded that a negative corona pulse is associated with the ignition of a cathode-directed streamer in the vicinity of the cathode. We report what we believe are the first experimental observations of non-Trichel oscillations of negative corona current with a frequency of (1–10) MHz. This work was supported by the Scientific Grant Agency of the Ministry of Education and Academy of Sciences of Slovak Republic (Project No. 1/5190/98).     

Growth mechanism of carbon nanotubes produced by pyrolysis of a composite film of poly (vinyl alcohol) and fly ash

We produced carbon nanotubes (CNTs) by pyrolysis of a composite film of poly (vinyl alcohol) (PVA) with fly ash (FA) at 500°C for 10 min under nitrogen. The composite films were prepared by a suspension of PVA and FA in deionized water and cast onto glass petri dishes. The morphologies of the CNTs were observed in the images of scanning and transmission electron microscopy, showing different types of structures, e.g. whiskers, branches, ropes and graphene sheets. The widths of the CNTs measured varied in the range 18–80 nm. X-ray photoelectron spectroscopy analysis showed five types of carbon binding peaks, C–C/C–H (∼77%), C–O–H (∼9%), –C–O–C (∼5%), C=O (∼5%) and –O–C=O (∼3%). From an image of a broken CNT, a mechanism was proposed for the formation of CNTs. The CNTs grown on FA surfaces have potential for the fabrication of high-strength composite materials with polymer and metal.     

Effect of electron irradiation on InAs〈Sn〉 microcrystals grown by chemical transport method

The results of experimental studies of the effect of electron irradiation (13 MeV, 300 K) on the electrophysical properties of n-InAs whiskers are reported. The limiting electrophysical parameters and the Fermi-level pinning in the irradiated material are discussed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 8–10, March, 2006.     

Room-temperature deposition of transparent conductive Al-doped ZnO thin films using low energy ion bombardment

Al-doped zinc oxide (AZO) films are prepared on quartz substrates by dual-ion-beam sputtering deposition at room temperature (∼25°C). An assisting argon ion beam (ion energy E _i =0–300 eV) directly bombards the substrate surface to modify the properties of AZO films. The effects of assisted-ion beam energy on the characteristics of AZO films were investigated in terms of X-ray diffraction, atomic force microscopy, Raman spectra, Hall measurement and optical transmittance. With increasing assisting-ion beam bombardment, AZO films have a strong improved crystalline quality and increased radiation damage such as oxygen vacancies and zinc interstitials. The lowest resistivity of 4.9×10⁻³Ω cm and highest transmittance of above 85% in the visible region were obtained under the assisting-ion beam energy 200 eV. It was found that the bandgap of AZO films increased from 3.37 to 3.59 eV when the assisting-ion beam energy increased from 0 to 300 eV.     

Transient electromagnetic radiation induced by an electron beam at the exit of a linear accelerator

The parameters of radio radiation generated in an air medium by an electron beam from a RELUS-1 small electron accelerator (Radiation Acceleration Center, MIFI) were studied theoretically and experimentally. Theoretical models for the generation of electromagnetic fields over the frequency range 10–3000 MHz induced by an electron beam were developed and studied. Electromagnetic fields from a beam of electrons were recorded over the frequency ranges 140–160 and 2794–2804. The discrepancy between theoretical estimates and recorded values was less than 50%.     

Production of ultra-short high-power microwave pulses in Čerenkov backward-wave systems (Review)

The article presents a review of works (mainly, of experimental ones) on production of subgigawatt and gigawatt microwave pulses of extremely short duration (5–7 RF periods) using backward-wave systems fed with nanosecond and subnanosecond high-current electron beams produced by compact accelerators. Theoretical approaches to the generation process (which is essentially non-steady-state) are briefly summarized. Using the effect of spatial accumulation of energy in a short running microwave pulse allows production of pulses with peak power notably higher then the driving electron beam power. Compact microwave sources developed for operation in the Ka-band and X-band are described. Special attention is given to the issue of high pulse repetition frequency operation of the sources.