Structure and phase composition of a chromium-silicon system modified by high-current electron beams

The results of studies of the structure-phase state of a chromium-coated silicon substrate system’s subsurface layer treated with low-energy high-current electron beams, 50–200 μs in duration and with an energy density of 15 J/cm², are reported. The data of raster electron microscopy and X-ray structural and spectral microanalysis revealed the formation of a chromium-doped silicon layer with a thickness of 2–38 μm, chromium-enriched silicon dendrites, chromium disilicide CrSi₂, and an amorphous eutectic layer (the characteristic cross-section size of the chromium-enriched phase extrusions is ∼50 nm). The structure-phase transformations are discussed taking into account the peculiarities of the distribution of temperature, diffusion and convective mass-transfer in the modified layer.     

Three-dimensional simulation of laser–plasma-based electron acceleration

A sequential three-dimensional (3D) particle-in-cell simulation code PICPSI-3D with a user friendly graphical user interface (GUI) has been developed and used to study the interaction of plasma with ultrahigh intensity laser radiation. A case study of laser–plasma-based electron acceleration has been carried out to assess the performance of this code. Simulations have been performed for a Gaussian laser beam of peak intensity 5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 1 × 10¹⁹ cm^− 3, and for a Gaussian laser beam of peak intensity 1.5 × 10¹⁹ W/cm² propagating through an underdense plasma of uniform density 3.5 × 10¹⁹ cm^− 3. The electron energy spectrum has been evaluated at different time-steps during the propagation of the laser beam. When the plasma density is 1 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~14 MeV, with an energy spread of ±7 MeV. On the other hand, when the plasma density is 3.5 × 10¹⁹ cm^− 3, simulations show that the electron energy spectrum forms a monoenergetic peak at ~23 MeV, with an energy spread of ±7.5 MeV.     

Structural features of aluminium alloy 1441 irradiated by Ar+ ions

Using electron microscopy it was found that irradiation of clad cold-worked specimens made of commercial aluminium-lithium alloy 1441 by the Ar ⁺ ions of energy 40 keV at low doses of irradiation (10¹⁵ cm⁻², irradiation time 1 s, T < 70 °C) and ion-current density of about 100 μA/cm² results in the transformation of the cellular structure formed in the alloy under deformation. As the dose of irradiation is increased up to 10¹⁶ cm⁻², a transition from a cellular to a subgrain structure close to a polygonal one is observed. The efficiency of the process is increased with ion-current density. Furthermore, under ion irradiation at increased ion-current densities, the β′(Al ₃ Zr) and Al ₈ Fe ₂ Si particles present in the deformed alloy dissolve, and disperse particles of a new Al ₂ LiMg phase of platelet shape are formed. The changes in the dislocation structure and phase composition in alloy 1441 are observed several seconds after irradiation not only in the surface layer adjacent to the ion incorporation band but also through the thickness of the specimen tens of thousands times greater than ion projective ranges. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 73–81, February, 2007.     

Structural phase changes in a titanium-silicon system modified by high-current electron beams and compression plasma flows

Structural phase changes in a titanium-silicon system treated by low-energy high-current electron beams (HCEBs) and compression plasma flows (CPFs) with the duration 100 μs and the energy density 12–15 J/cm² are studied. Scanning electron microscopy, X-ray diffraction and electron microprobe analysis are used in this work. The formation of a titanium-doped silicon layer 10–25 μm thick, titanium silicides (TiSi₂ under HCEBs and Ti₅Si₃ under CPF treatment), silicon dendrites, and needle-like eutectics (typical size of precipitates is about 50 nm) is revealed. It is shown via the results of numerical simulation that the thickness of the metal-doped layer is mainly controlled by the power density value and the surface nonuniformity of the heat flow over the target surface. The thermodynamic regularities of phase formation are discussed, taking into account heat transfer between the silicide nuclei and solid silicon.     

Hydrogen permeability of protective coating formed by electron treatment of zirconium alloys

The results of studying the hydrogen permeability and physicomechanical properties of zirconium alloy irradiated with a pulse electron beam are presented. It is established that, with an increase in the beam energy, a surface layer of this material is hardened to depths of about 2.5 μμm. The depth distribution of Young’s modulus values of zirconium alloy confirms the obtained results. The study of the hydrogen permeability of this material has shown that, after hydrogen charging of samples, the hydrogen concentration in a sample treated at an energy density of 18 J/cm² is 2.5–3 times lower than that of the initial material charged with hydrogen.     

Erosion of the GaAs target under irradiation by a high-power pulsed ion beam

The results of examination of the GaAs-target erosion under irradiation by a high-power pulsed ion beam are reported. In the experiments, use was made of a high-power pulsed ion source with the following parameters: ion energy — 250 keV, target current density — 350 A/cm², pulse duration — 80 ns, target energy density — up to 7 J/cm². The target erosion coefficient and its dependence on the number of successive pulses are measured. It is found that the surface roughness parameter is increased with the number of successive beam pulses. A regular structure of surface relief is observed to form in the case where the number of pulses > 20–40. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 66–70, January, 2007.     

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)     

Interaction between oppositely directed compression plasma flows

The dynamics of the interaction of two oppositely directed plasma flows generated by miniature gas-discharge magnetoplasma compressors is studied. The maximum plasma electron temperature and density in the region where the plasmas interact are found to be 4.5 eV and 1.4⋅10¹⁷ cm^–3. Exposure of samples to this kind of plasma near the interaction region leads to an energy flux at the surface of 2–8 J/cm², which is sufficient for modification of the surface properties of various materials.     

Nanostructuring of surfaces of metalloceramic and ceramic materials by electron-beams

An electron-emitting source generating a low-energy beam measuring 1–3 cm in diameter, with current up to 300 A, pulse duration within 50–200 μs, and pulse repetition frequency up to 10 Hz is investigated in a gas-filled diode with a mesh plasma cathode at the accelerating voltage up to 25 kV. The beam is transported in a longitudinal pulsed magnetic field to a distance of up to 30 cm towards the region of its interaction with a solid. For the current densities up to 100 A/cm², it provides the power density as high as 10–100 J/cm² sufficient to melt surfaces of metals, alloys, and composite (metalloceramic) materials within one or a few pulses. This makes this beam useful for modification of material surfaces and articles made thereof. Using the methods of optical, scanning and diffraction electron microscopy, by building micro-and nanohardness profiles, and via identification of the treated surface roughness, the phase composition and the substructure state of the materials subjected to pulsed low-energy e-beam of sub-millimeter durations are investigated. Formation of submicro-and nanocrystalline multi-phase structure is observed, which ensures a multiple increase in physico-mechanical and tribological characteristics of the treated material. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 60–70, May, 2008.     

Pulsed electron-beam melting of a copper — steel 316 system: Evolution of the chemical composition,microstructure, and properties

The surface topography, chemical composition, microstructure, nanohardness, and tribological characteristics of a Cu (film, 512 nm)-stainless steel 316 (substrate) system subjected to pulsed melting by a low-energy (20–30 keV), high-current electron beam (2–3 μs, 2–10 J/cm²) were investigated. The film was deposited by sputtering a Cu target in the plasma of a microwave discharge in argon. To prevent local exfoliation of the film due to cratering, the substrate was multiply pre-irradiated with 8–10 J/cm². On single irradiation, the bulk of the film survived, and a diffusion layer containing the film and substrate components was formed at the interface. The thickness of this layer was 120–170 nm irrespective of the energy density. The diffusion layer consisted of subgrains of γ-Fe solid solution and nanosized particles of copper. In the surface layer of thickness 0.5–1 μm, which included the copper film quenched from melt and the diffusion layer, the nanohardness and the wear resistance nonmonotonicly varied with energy density, reaching, respectively, a maximum and a minimum in the range 4.3–6.3 J/cm². As the number of pulsed melting cycles was increased to five in the same energy density range, there occurred mixing of the film-substrate system and a surface layer of thickness ∼2 μm was formed which contained ∼20 at. % copper. Displacement of the excess copper during crystallization resulted in the formation of two-phase nanocrystal interlayers separating the γ-phase grains. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 6–13, December, 2005.     

Local surface segregations of implanted aluminum in an iron crystal with a low density of defects

The surface distribution of elements is studied by scanning a 3-MeV proton beam along the surface of a bcc-Fe sample implanted with aluminum ions in the dose interval (1–50) · 10¹⁶ cm⁻². Ring-shaped regions, up to 30 μm in diameter, with a high density of aluminum, which appear at implantation doses (5–20) · 10¹⁶ cm⁻², are observed. These regions appear as a result of radiation-stimulated segregation processes. A mechanism based on the existence of a low density of dislocations in the initial crystal is proposed to explain the implanted impurity segregation processes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 86–89 (10 January 1997)     

Relaxation of the conductivity of CsI-Tl after excitation by subnanosecond electron pulses

The pulsed conductivity is investigated for a CsI-Tl crystal having a Tl⁺ concentration N=8×10¹⁷cm⁻³ and excited by an electron beam (0.2 MeV, 50 ps, 10²–10⁴ A/cm ²). It is shown that the amplitude of the conduction current pulse is almost an order of magnitude lower than for “pure” CsI crystals irradiated under like conditions. The conduction current relaxation time is preserved up to τ=100 ps in this case. Under the experimental conditions, therefore, the lifetime of electrons in the conduction band is controlled by trapping at Tl⁺ centers. The electron capture cross section at a Tl⁺ center is determined: σ=7×10⁻¹⁶ cm², which agrees in order of magnitude with estimates of the capture cross section for a neutral trapping center. Fiz. Tverd. Tela (St. Petersburg) 40, 66–67 (January 1998)     

Wet bone ablation with mechanically Q-switched high-repetition-rate CO2 laser

2 laser using a miniature water spray is demonstrated. An ablation threshold of 1.4 J/cm², an optimal energy density of 9–10 J/cm², and a corresponding specific ablation energy of 25–30 J/mm³ are found for pig thighbone compacta at λ=9.57 μm and a beam waist diameter of 0.5 mm. The water spray alleviates tissue carbonization even at high laser pulse repetition rates and increases ablation efficiency. Received: 9 March 1998/Revised version: 6 July 1998     

Effects of irradiation parameters on metal surface erosion by pulsed ion beams

Potential application of high-power ion beams of submicrosecond and microsecond durations with the initial particle energy from 50 to 1000 keV and power density 10⁷–10⁹ W/cm² to ensure metal surface erosion are investigated. Evaporation is treated as a major erosion mechanism and the erosion coefficient is taken as an efficiency indicator. Dependences of the erosion coefficients of several metals on beam parameters obtained via calculations using a technique based on the solution of thermal conductivity equation with phase transitions are presented. The ion species, their initial energy, current pulse duration and power density are used as the beam parameters controlling the result of irradiation. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 49–54, August, 2007.     

Sheath and electron density dynamics in the normal and self-pulsing regime of a micro hollow cathode discharge in argon gas

A microplasma is generated in the microhole (400 μm diameter) of a molybdenum-alumina-molybdenum sandwich (MHCD type) at medium pressure (30–200 Torr) in pure argon. Imaging and emission spectroscopy have been used to study the sheath and electron density dynamics during the stationary normal regime and the self-pulsing regime. Firstly, the evolution of the microdischarge structure is studied by recording the emission intensity of the Ar (5p[3/2]₁–4s[3/2]₁)_{1}) line at 427.217 nm, and Ar⁺ (4p′ ²P_3/2–4s′ ²D_5/2)_{5/2}) line at 427.752 nm. The maximum of the Ar⁺ line is located in the vicinity of the sheath-plasma edge. In both regimes, the experimental observations are consistent with the position of the sheath edge calculated with an ionizing sheath model. Secondly, the electron density is recorded by monitoring the Stark broadening of the H_b_\beta-line. In the self-pulsing regime at 150 Torr, the electron density reaches its maximum value of 4 × 10¹⁵ cm^-3, a few tens of ns later than the discharge current maximum. The electron density then decays with a characteristic decay time of about 2 μs, while the discharge current vanishes twice faster. The electron density in the steady-state regime is two orders of magnitude lower, at about 6–8 × 10¹³ cm^-3.     

Conductivity of ionic crystals when they are irradiated by picosecond electron beams

This paper discusses the pulsed electron conductivity σ of KCl, KBr, and NaCl crystals when they are excited by an electron beam (0.2 MeV, 50 ps) with current densities in the interval j=(30–10⁴) A/cm². It is shown that the lifetime of the electrons in the conduction band is τ≪100 ps. To explain the experimental σ(j) dependences, a model is proposed that includes electron capture by structural defects and stable radiation defects at low excitation densities and electron capture predominantly by unstable radiation defects generated by the excitation pulse at high excitation densities. Fiz. Tverd. Tela (St. Petersburg) 41, 1200–1203 (July 1999)     

Build-up of F and M color centers in KCl single crystals under combined electron and proton irradiation

The F and M color-center build-up kinetics in KCl crystals under combined irradiation with electrons of energy 15 and 100 keV and 100-keV protons have been studied in the flux range of 10¹³–10¹⁵ cm⁻² and at a flux density of 3×10¹¹ cm⁻² s⁻¹. It is shown that consecutive irradiation with electrons and protons produces results not obtainable under electron or proton irradiation alone. Fiz. Tverd. Tela (St. Petersburg) 40, 2015–2018 (November 1998)     

Wear resistance of the ion-modified surface of zirconium-alloy tubes irradiated by He+ and Ar+ ion beams with wide energy spectra

The results of investigating the wear resistance of E110 alloy samples irradiated by a He⁺ + Ar⁺ beam with a wide energy spectrum are presented. Surface modification under irradiation by an Ar⁺ beam at doses higher than 2 × 10¹⁸ ion/cm² is shown to cause substantial enhancement of the wear resistance of samples because the structural homogeneity of near surface layers increases, the surface roughness decreases, and its microhardness increases. The application of a mechanical-geometrical wear model based on the experimental wear characteristics determined during accelerated tests indicates that the thinning of an alloy cladding can reach rates of 10⁻⁶–10⁻³ mm/s, which agree satisfactorily with data obtained in other simulation experiments. The presence of an oxide film changes a wear process characterized by an abrasive component.     

Experiments on two-step heating of a dense plasma in the GOL-3 facility

This paper presents the results of experiments on two-stage heating of a dense plasma by a relativistic electron beam in the GOL-3 facility. A dense plasma with a length of about a meter and a hydrogen density up to 10¹⁷ cm⁻³ was created in the main plasma, whose density was 10¹⁵ cm⁻³. In the process of interacting with the plasma, the electron beam (1 MeV, 40 kA, 4 μs) imparts its energy to the electrons of the main plasma through collective effects. The heated electrons, as they disperse along the magnetic field lines, in turn reach the region of dense plasma and impart their energy to it by pairwise collisions. Estimates based on experimental data are given for the parameters of the flux of hot plasma electrons, the energy released in the dense plasma, and the energy balance of the beam-plasma system. The paper discusses the dynamics of the plasma, which is inhomogeneous in density and temperature, including the appearance of pressure waves. Zh. éksp. Teor. Fiz. 113, 897–917 (March 1998)