Formation of a shock wave in aerogel irradiated with a high-current pulsed electron beam

The propagation of pressure jumps excited by a high-current pulsed electron beam in SiO₂ aerogels with density ranging from 0.025 to 0.25 g/cm³ is investigated using a laser differential interferometer and optical methods. Spallation on the back side of the aerogel targets is observed and the velocity of the spallation fragments is measured. The expansion velocity of the aerogel in the direction of the electron beam is determined. The parameters of the shock adiabat are established in a wide range of aerogel porosities. The depth of the energy-deposition zone of the electron beam is determined experimentally as a function of the aerogel density in the range from 0.015 to 0.25 g/cm³. A model describing highly porous materials which reflects the fractal properties of highly porous aerogels is developed on the basis of the experimental data. Numerical calculations of the observed phenomena are performed. Zh. Tekh. Fiz. 69, 18–25 (December 1999)     

Dynamics of an electron beam from a high-current picosecond accelerator

The dynamics of a high-current (10²–10⁴ A) electron beam with energies of 10⁵–10⁶ eV and picosecond duration (10⁻¹⁰ s) at the output of the accelerator tube is investigated. The slowing of electrons by the residual positive charge on the surface of the tube is found to have a significant influence in the case of short pulse durations. The distance of the electron beam from the surface of the tube in vacuum is estimated on the basis of a one-dimensional model. It is shown that the electron radiation can travel to a distance of several centimeters from the surface at current densities below 20 A/mm², whereas at high current densities the beam is trapped near the surface. Zh. Tekh. Fiz. 69, 111–115 (May 1999)     

Evolution of vacancy defects in the surface layers of a metal irradiated with a pulsed electron beam

The distribution of vacancy defects in the surface layers of α-Fe after irradiation with a high-current pulsed electron beam is studied experimentally by unique nuclear-physical methods — low-energy positron annihilation, Rutherford backscattering (RBS), and proton-induced x-ray emission (PIXE). Regions with low local density, which are sources of crater formation on the surface of the irradiated sample, are observed by scanning a proton microbeam. Positron lifetime measurements reveal that as the electron beam power increases, nonequilibrium vacancies tend to be captured by carbon impurity atoms. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 618–622 (25 April 1997)     

Determination of the dynamic characteristics of aerogels in the energy-release zone of a high-power electron beam

The dynamics of the interaction of a high-current electron beam with SiO₂ aerogels of different density and porosity are investigated by optical methods. A model for describing highly porous materials is developed on the basis of the information obtained regarding the unloading of aerogels in the energy-release zone of an electron beam, as well as the measured energy-release profiles. A corresponding nonlinear self-consistent equation of state is obtained which reflects the fractal properties of aerogels and permits determination of the thermodynamic characteristics of aerogels as the porosity varies tens of times. The influence of electric space charge on the energy-absorption profile of a high-power electron beam in aerogels of different density is discussed. Zh. Tekh. Fiz. 68, 112–120 (October 1998)     

Measurements of the transverse electron velocities in high-current microsecond relativistic electron beams in a strong magnetic field

An analyzer is created for time-resolved measurements of the electron pitch-angles in high-current microsecond relativistic electron beams in a strong magnetic field. The electron pitch-angles in a 500-keV relativistic electron beam with a current density of ∼1 kA/cm² and a 1-μs flat-top current profile are measured. The diode proposed previously by the authors allows one to produce a high-current electron beam in which pitchangles vary only slightly with time and over the beam cross section.     

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)     

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)     

Asymptotic form of the radial profile of a relativistic electron beam propagating in a gas-plasma medium in the presence of an external magnetic field and a neutralizing ion background

Boltzmann’s H theorem and variational methods are used to find the temporally asymptotic form of the radial current density profile of a paraxial relativistic electron beam propagating in a scattering gas-plasma medium along a static external magnetic field and a neutralizing ion background. It is shown that in this case the radial profile is Gaussian. Zh. Tekh. Fiz. 67, 62–65 (November 1997)     

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.     

Interaction of a modulated electron beam with a magnetoactive plasma

Experimental results concerning the interaction of a modulated electron beam with a magnetoactive plasma in the whistler frequency range are reported. It was shown experimentally that when a beam is injected into the plasma, waves can be generated by two possible mechanisms: Cherenkov emission of whistlers by the modulated beam, and transition radiation from the beam injection point. In the case of weak beam currents (N _b/N ₀)≪⁻⁴) the Cherenkov resonance radiation is more than an order of magnitude stronger than the transition radiation; the Cherenkov emission efficiency decreases at high beam currents. The transformation of the distribution function of the beam is investigated for the case of weak beam currents. It is shown that in the case of the Cherenkov interaction with whistlers the beam is retarded and the beam distribution function becomes wider and acquires a plateau region. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 378–382 (25 March 1998)     

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)     

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)     

Nonuniform distribution of absorbed energy in high-resistance materials excited by an electron beam

Experimental results are presented on the changes in the optical characteristics of lithium fluoride induced by an electron beam with time-varying density and pulse energies close to the threshold for destruction of the material. The spatial distribution of color centers is investigated, especially near breakdown channels. Mechanisms for nonuniform accumulation of defects are discussed, along with the fundamental causes of the inhomogeneous energy distributions induced by the high-current electron beam. Concrete results of calculations of the field intensity distribution in LiF crystals during irradiation are presented, based on models of “uniform” and nonuniform charging of the sample. An abrupt increase in the electric field intensity is predicted near the breakdown channel. Zh. Tekh. Fiz. 68, 53–59 (April 1998)     

Superfluidity of 3He in aerogel at T=0 in a magnetic field

The transition of liquid ³He to the superfluid B phase in aerogel at T=0 is considered. It is shown that in a magnetic field, the quantum phase transition with respect to pressure is split in two. The amount of splitting δP is estimated. The components of the superfluid density tensor are calculated near the critical pressures. Zh. éksp. Teor. Fiz. 115, 754–762 (February 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

High-intensity and high-brightness source of moderated positrons using a brilliant γ beam

Presently, large efforts are conducted toward the development of highly brilliant γ beams via Compton back scattering of photons from a high-brilliance electron beam, either on the basis of a normal-conducting electron linac or a (super-conducting) Energy Recovery Linac (ERL). Particularly, ERLs provide an extremely brilliant electron beam, thus enabling the generation of highest-quality γ beams. A 2.5 MeV γ beam with an envisaged intensity of 10¹⁵ photons s⁻¹, as ultimately envisaged for an ERL-based γ-beam facility, narrow band width (10⁻³), and extremely low emittance (10⁻⁴ mm² mrad²) offers the possibility to produce a high-intensity bright polarized positron beam. Pair production in a face-on irradiated W converter foil (200 μm thick, 10 mm long) would lead to the emission of 2×10¹³ (fast) positrons per second, which is four orders of magnitude higher compared to strong radioactive ²²Na sources conventionally used in the laboratory. Using a stack of converter foils and subsequent positron moderation, a high-intensity low-energy beam of moderated positrons can be produced. Two different source setups are presented: a high-brightness positron beam with a diameter as low as 0.2 mm, and a high-intensity beam of 3×10¹¹ moderated positrons per second. Hence, profiting from an improved moderation efficiency, the envisaged positron intensity would exceed that of present high-intensity positron sources by a factor of 100.     

Analysis of parameters of an electron beam from a plasma-filled diode

We analyze the properties of a high-current electron beam formed in an electron source based on a plasma-filled diode and a linear pulsed transformer. The beam parameters are determined by measuring bremsstrahlung X-rays and the beam current, as well as the photographs of the diode gap in the optical range, of the anode in X-rays, and beam autographs. A beam with a current of ～100 kA and a mean electron energy exceeding 0.7 MeV for an accelerating voltage amplitude of ～1 MV is obtained. The diameter of the generated beam is ～1 cm. The electron beam from the plasma-filled diode makes it possible to attain a high anode power density (>10¹⁰ W/cm²) for exciting shock waves, for obtaining high pressures, and for generating powerful X-rays.     

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)     

Spreading of an SR beam spot (diameter 0.5 μm, 95 eV) photoelectron image on the surface of WO<Subscript>3 − <Emphasis Type="Italic">x</Emphasis></Subscript> films

The 2D-photoemission image of the beam spot was obtained for the first time for the W⁵⁺ oxidation state on the preliminary irradiated WO_{3 − x} thin film surface, created by scanning of the SR beam over the film surface. The W⁵⁺ beam profile intensity was found to spread up to a distance of 3.2 μm for an amorphous film and 5.5 μm for a polycrystalline film, it exceeds considerably the beam spot size. The image saturation dose was reached faster for a polycrystalline film. Among the possible mechanisms explaining this phenomenon, for the case of an almost unchangeable O2s state under irradiation, a choice was made in favor of a photon-generated charge diffusion due to low-energy secondary electrons from photoemission, which produce the “coloration” effect, e⁻ + W⁶⁺ (W⁵⁺) W⁵⁺ → W⁵⁺(W⁴⁺). The O512-eV Auger peak was found to degrade at the distance of 1.5–2 mm outside the beam spot under long-time electron beam irradiation, which is attributed to electron-stimulated oxygen desorption and outdiffusion.     

Single-spin asymmetries in the Bethe-Heitler process e − + p → e − + γ + p induced by loop corrections

The single-spin beam and target asymmetries in the hard electroproduction process e ⁻ + p → e ⁻ + γ + p induced by the loop radiative corrections to the vertex part of lepton interaction are considered. The physical reason for the appearance of such asymmetries is the nonzero imaginary part of the amplitude (on the level of radiative corrections) caused by diagrams with photon radiation from the outgoing electron. We calculate the single-spin beam and target asymmetries at a longitudinally polarized electron beam or at arbitrary polarizations of the target proton for the CLAS and HERMES experimental conditions. The text was submitted by the authors in English.     

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)