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)     

Properties and structural state of the surface layer in a zirconium alloy modified by a pulsed electron beam and saturated by hydrogen

A pulsed action of an electron beam on a Zr-1% Nb zirconium alloy is studied. Alloy samples are irradiated by three 50-μs pulses at an energy density of 15–25 J/cm², a power of (3–6) × 10⁴ W/cm², a current density of 10–50 A/cm², and an electron energy of 18 keV. This method of processing is found to modify the surface layer of the alloy without changing the structure-phase state of its volume. This surface modification increases the hydrogen saturation resistance of the alloy.     

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.     

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)     

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)     

Doppler spectroscopy of a beam in the channel of a H− ion source

It is shown that high-accuracy contact-free measurements of the divergence and emittance of an accelerated H⁻ ion beam at the exit from the source can in principle be performed by passive Doppler spectroscopy of a beam of excited hydrogen atoms produced by neutralization of the ions with excitation on the residual gas in the source channel. The intensity of the H_α-line radiation detected by the Doppler system is calculated, taking into account the principal processes leading to the excitation and deexcitation of the 3s, 3p, and 3d levels of the hydrogen atoms in the beam, for residual gas densities of the order of 10⁻⁴–10⁻⁵ Torr in the source channel. The computed H_α-line intensity was confirmed experimentally, making it possible to perform photoelectronic detection of the spectral contour of the line in the current mode rather than the photon-counting mode. Zh. Tekh. Fiz. 68, 15–18 (June 1998)     

Focusing a compensated proton beam with a high compression coefficient

An analysis is made of the possibility of focusing 10–30 keV charge-compensated proton beams with a current of 1 kA and an area compression coefficient >1.5×10³. To obtain a high degree of compression a combination of ballistic focusing and magnetic compression is used. The parameters of the focusing channel are determined and an estimate is made of the possible effect of the compensating electron background on the effectiveness of focusing of the proton beam. Zh. Tekh. Fiz. 67, 73–80 (May 1997)     

Calculation of the electrostatic tracking force in the transport of relativistic electron beams in Ohmic plasma channels of low conductivity

The interaction force between a paraxial relativistic electron beam and a preformed Ohmic plasma channel of low conductivity is calculated in the electrostatic limit. The dependence of this force on the channel conductivity and the distance from the beam front is found for concrete parameters of the relativistic electron beam and various values of the beam current rise rate. Zh. Tekh. Fiz. 67, 78–80 (December 1997)     

Mechanism for the influence of the geometric parameters on the electrical characteristics of Penning ion sources

A steady-state Penning ion source is studied experimentally. Depending on the geometric parameter l _a (the anode length to diameter ratio) and pressure, maxima are observed in the discharge current and in the ion beam current extracted from an aperture at the center of the cathode. It is shown that at pressures of the order of 10⁻⁴ Torr, two maxima appear in these currents: one, for short discharge gaps with l _a =1–1.5, corresponds to a diverging ion beam, and the other, for longer anodes with l _a =4–5, to a collimated ion beam. At pressures of the order of 10⁻⁵ Torr, only one maximum appears in these currents, for short anodes l _a =2–3 with a diverging ion beam. A physical explanation is proposed for these findings. Zh. Tekh. Fiz. 68, 29–32 (September 1998)     

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)     

Theoretical investigation of the bunching of an electron beam in relativistic power amplifiers

The modern approach to designing an injector for the driver of a two-beam accelerator is based on the use of a bunched electron beam. The results of simulation and comparison of the processes leading to bunching of a relativistic electron beam in a free-electron laser and in a traveling-wave tube at low electron beam energies are discussed. The simulation and existing experimental results for bunching of an electron beam in a free-electron laser are compared. Zh. Tekh. Fiz. 69, 98–102 (February 1999)     

Temporal evolution of a long-pulse high-density electron beam

The temporal evolution of a long-pulse high-density electron beam in a high-power traveling-wave tube with a periodic magnetic focusing system is investigated. The relation between the variation of the characteristics of the electron beam and ionization of the residual gas and particles desorbed from the electrode surfaces is shown. A strong influence of the formation of the collector plasma and the flux of ions from its surface on the beam dynamics is revealed. The effect of microwave fields in the transport channel on the characteristics of the beam during its evolution is studied. Zh. Tekh. Fiz. 67, 54–58 (December 1997)     

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.     

Rotating relativistic electron beam-plasma interaction and formation of a field-reversed configuration

Experimental results on interaction of a rotating relativistic electron beam with plasma and neutral gas are presented. The rotating relativistic electron beam has been propagated up to a distance of 150 cm in a plasma. The response of the plasma to the rotating electron beam is found to be of magnetic diffusion type over a plasma density range 10¹¹–10¹³ cm⁻³. Excitation of the axial and azimuthal return currents by the rotating beam and subsequent trapping of the azimuthal return current layer by the magnetic mirror field are observed. A field-reversed configuration has been formed by the rotating relativistic electron beam when injected into neutral hydrogen gas. We have observed field reversal up to three times the initial field in an axial length of 100 cm.     

Effects of pulsed irradiation by low-energy ions during homoepitaxy of silicon from a molecular beam

The change in the morphology of a Si(111) surface under pulsed irradiation by 145-eV Kr⁺ ions with a pulse duration of 0.5 s during epitaxy of silicon from a molecular beam is studied experimentally by RHEED. It is found that pulsed irradiation by low-energy ions intensifies the specular reflection. This corresponds to a decrease in the roughness of the growing surface. It is shown that the observed effect depends strongly on the degree of filling of the surface atomic layer, the substrate temperature, and the ion current density. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 689–694 (25 November 1996)     

Novel compact 60-kV Mott detector for spin-polarization electron spectroscopy

A compact 60-kV Mott polarimeter designed specially for the local analysis of surface and two-dimensional magnetism by spin-resolved electron spectroscopy is developed and tested. The use of a design which combines a spherical accelerating field and the absence of a retarding potential after scattering of the electron beam ensures high stability of the measured polarization even when the potential and diameter of the beam being investigated vary. As a result of optimization of the scattering angle (118°) and the use of surface-barrier detectors with a large collection angle (∼48°), the efficiency or figure of merit of the polarimeter, which determines the signal-to-noise ratio ɛ=(I/I ₀)·(S _eff)², equals 2.5×10⁻⁴. Specially developed electronic circuits and optimum positioning of the detectors provide a maximum electron counting rate as high as 5×10⁶ counts/s. Consequently, it is possible to calibrate the polarimeter (to find the effective Sherman function S _eff) by extrapolating the measured asymmetry to a high discrimination level. This instrument can also be used in other areas of solid-state physics, atomic physics, and high-energy physics. Zh. Tekh. Fiz. 68, 125–130 (August 1998)     

Field-emission characteristics of layered systems in high electric fields

A study has been made of the field-emission characteristics of cerium and BaO films on W within a broad range of field-emission currents and fields. An anomalous broadening of the spectra and a deviation of Fowler-Nordheim characteristics from linearity have been revealed for high (above 10⁹ A/m²) field-emission current densities. The dependence of this anomalous behavior of field-emission characteristics on the work function and topography of the surface has been investigated. Possible reasons for the observed phenomena are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 732–735 (April 1999)     

Effect of interparticle interactions on radiative lifetime of photoexcited electron-hole system in GaAs quantum wells

The paper reports on an investigation of changes in the photoluminescence linewidth and lifetime of excitons and electron-hole plasma over a wide range of densities between 3×10⁷ and 3×10¹² cm⁻² at a temperature of 77 K in GaAs/AlGaAs quantum wells. The roles played by thermal ionization of excitons at low densities of nonequilibrium carriers, exciton-exciton and exciton-electron collisions, and ionization of excitons at high pumping power densities have been studied. Zh. éksp. Teor. Fiz. 112, 353–361 (July 1997)     

Mechanism of field emission from carbon materials

Field emission in diamond and graphite-like polycrystalline films is investigated experimentally. It is shown that the emission efficiency increases as the nondiamond carbon phase increases; for graphite-like films the threshold electric field is less than 1.5 V/μm, and at 4 V/μm the emission current reaches 1 mA/cm², while the density of emission centers exceeds 10⁶ cm⁻². A general mechanism explaining the phenomenon of electron field emission from materials containing graphite-like carbon is proposed. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 56–60 (10 July 1998)     

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)