General intense electron beams by means of a contracted arc discharge

Two types of contracted arc discharge are investigated with a view to generating intense electron beams over a wide pressure range (1–10^–3 Pa). For an arc discharge with a hollow cathode and anode, an electron beam corresponding to a current of up to 300 A and a pulse length of 25 µsec is obtained at a pressure of 1–10^–1 Pa in the accelerating gap with an accelerating voltage of up to 15 kV. At pressures of 10^–2–10^–3 Pa, emitting plasma is created by a low-pressure arc discharge on the basis of a Penning cell. Three discharge systems operating in parallel are used to increase the working life of the cathode and improve the current density distribution of the beam. An electron beam of diameter 200 mm with a current of up to 125 A and a pulse length of 50 µsec is obtained.Institute of High-Power Electronics, Siberian Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshkikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 76–82, March, 1994.     

Dissociative electron attachment to CF2Cl2

Using a recently constructed high resolution crossed electron/molecular beam apparatus consisting of a hemispherical electron monochromator and a quadrupole mass spectrometer we have measured the relative production cross sections for CI^– and F^– via electron attachment to CF₂Cl₂. The relative Cl^– cross section is placed on an absolute scale by reference to an absolute rate coefficient using a calibration method involving integration of the measured anion signal. The most efficient Cl^– production process is at about zero energy and its magnitude is resolution limited. The present high resolution value of 6 × 10^–16 cm² compares well with an earlier value reported by Chen and Chantry. A second peak is detected at around 0.8 eV in accordance with some of the earlier beam and swarm measurements. The observed production of F^– has an appearance energy of 1.9 eV and the energy of maximum cross section is 3.36 eV, the latter value comparing well with several previous studies.     

Anomalous plasma phenomena of hollow cathode arc discharge

Magnetically confined argon plasma produced by hollow cathode arc discharge has been studied in different experimental conditions, with discharge current from 10–50 A, vessel argon pressure between 10^–3 and 10^–4 torr (1 torr=133·32 Pa) and axial magnetic field up to 0·12 T. The plasma density measured by a cylindrical Langmuir probe is found to be 10¹⁹ to 4 × 10¹⁹ m^–3 and the electron temperatureT _e varies between 2·5 and 4·8 eV. When an external axial magnetic field is applied the plasma temperature decreases with the increase in the magnetic field intensity until it reaches a minimum value at 0·075T and then increases with the same rate. This has been interpreted as high frequency waves excitation due to electron beam-plasma interaction, which explains the electron density jumps with the magnetic field intensity. Enhanced plasma transport across the magnetic field is studied and classified as anomalous diffusion.     

Measurement of laser light thomson-scattered from a cooling electron beam

First results are presented from an experiment scattering laser light from a relativistic electron beam. The 5 cm diameter continuous electron beam of 28 keV kinetic energy and 2.6 A current presents an electron gas of a density of 8×10⁷ cm^–3, from which 20 ns pulses of laser light (490 nm) were scattered at a repetition rate of 15 Hz and an average power of 20 mJ per pulse. The Doppler-shifted wavelength of photons backscattered under 180° was analysed with a Fabry-Perot interferometer. This technique provides, for the first time, a non-destructive measurement of the velocity distribution in an electron beam radially resolved in space. The results presented here comprise the direct measurement of the absolute electron energy and the degree of space-charge compensation in the electron beam. The determination of an upper bound of 10^–2 for the ratio of longitudinal to transverse electron temperature implies the first direct measurement of a flattened velocity distribution.     

Surface strain-hardening of iron alloys with an intense pulsed electron beam

Results are presented from study of surface strain-hardening and measurements of the structure of carbon (St. 45, U7A, 40Kh) and alloy (R6M5, Kh6VF) steels subjected to surface fusion by pulsed electron beams with the following parameters: electron energy 20–250 keV, pulse duration 5·10^–8–3·10^–4 sec, power density 10⁵–10⁹ W/cm³. It is shown that the microhardness of the surface of most alloys increases by a factor of 1.2–1.7 on quenched specimens and by a factor of 2.5–3.5 on unquenched specimens, depending on the regime. Microhardness increases in the surface layer due to quenching from the liquid state. An increase in electron energy from 40 to 250 keV with a pulse duration of 6·-10^–8 sec leads to a severalfold increase in the thickness of the strengthened layers and to a shift of the microhardness peak from the surface to a depth of 70 m. Here, microhardness reaches 2000 kgf/mm². This is due to an increase in the mean free path of the electrons in the metal and displacement of the energy-release maximum of the bundle deeper into the specimen.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, No. 6, pp. 38–43, June, 1985.     

Decay of tripositronium to an electron and two photons

The bound state of a positron and two electrons, called tripositronium, is investigated in this article. The differential probability is calculated for the decay of the ground state of tripositronium to an electron and two photons. It is established that such a decay primarily occurs with the formation of a nonrelativistic electron. It is shown that the integral decay probability is equal to 0.15 · 10⁹ sec^–1. This corresponds to a lifetime for tripositronium of 0.65 · 10^–8 sec.Translated from Izvestiya Vysshikh Uchebnykh Zavedeoii, Fizika, No. 4, pp. 69–73, April, 1978.The author thanks V. S. Vanyashin for formulating the problem and for useful discussions.     

Mechanism of self-trapping of electron excitation in alkali-halide crystals

A dynamic method is used to calculate the equilibrium positions of ions near heavy and light homologous impurities for KCl:J and KJ:Cl crystals. The most probable sites of self-traps are shown.Lattice ralaxation near an excited anion is modeled. The localization of electron excitations (EE) in alkali-halide crystals is examined from the viewpointof the formation of a covalent bond between anions as a relust of their approach toward each other. The EE localization time is determined by the lifetime of local vibrations in the crystal near the excited anions (5-10^–13–10^–11 sec).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 35–38, January, 1986.     

Determination of the electron temperature of a degenerate semiconductor in quantizing fields under heating by an electric field

Investigations of the Shubnikov-de Haas oscillation of the transverse magnetoresistance due to the magnitude of the electric field in n-type InSb with 5 · 10¹⁵, 3 · 10¹⁶ cm^–3 concentrations at a 4.2 °K temperature are carried out herein. A method is proposed to determine the electron temperature T_e; dependences of T_e on the electric and magnetic fields in the sample are obtained for a transversely oriented magnetic field. The relaxation time of the electron energy is estimated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 96–100, February, 1972.     

An “instantaneous” distribution of the ionization-passive electrons and holes under irradiation of a dielectric by intense electron or laser beams

A method is worked out for calculation of an “instantaneous” energy distribution of the ionization-passive electrons and holes resulting from the electron-electron collisions before the onset of electron-phonon relaxation under 10⁻¹⁵–10⁻¹⁴ s irradiation of a dielectric by an intense electron or laser beam. The method is based on the solution of a system of integral-differential kinetic equations of general form. The Auger and impact ionization as well as hole recoil due to the momentum conservation law are taken into account in calculations. The “instantaneous” distribution is calculated in NaCl under irradiation of the sample by a high-density electron beam. The “instantaneous” distribution of ionization-passive electrons and holes is the initial one in solutions of all kinetic equations describing further relaxation of electron excitations in irradiated materials.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 15–22, November, 2004.     

Mismatch and electron mobility in MBE GaxIn1−xas epitaxial layers on InP substrates

Composition inhomogeneity in nearly matched epilayers of the ternary semiconductor alloy Ga_xIn_1–xAs (x close to 0.47), grown by molecular beam epitaxy (MBE) on (001) InP substrates is correlated to variation of the lattice mismatch by x-ray imaging and local diffractometry. Misfit dislocations are shown to develop in areas of large misfit (above 2 × 10^–3) and are at the origin of a severe degradation of the electron mobility: an increase by a factor of about 4 in the intrinsic misfit (2.3 × 10^–3 compared to 0.6 × 10^–3) results in a 35 % reduction of the 77 K electron mobility.Part of this work has been done while this author was with Thomson-CSF Central Research Laboratory, Orsay, France     

Stretching of slow positron pulses generated with an electron linac

A facility for generating a high intensity slow positron beam using an electron linear accelerator has been constructed. A conversion efficiency of 6×10^–7 slow positrons per incident electron has been obtained for 75 MeV electrons. Storage and stretching of pulsed slow positrons have been successfully carried out with a Penning trap.     

Independent UHF plasma source and possibility of filling open magnetic trap

A stationary UHF plasma source, its characteristics and possibility of filling open magnetic trap with plasma injected from it have been described. Plasma is created in the source at frequency of 2400 MHz (supplied power is up to 150 W) in the electron cyclotron resonance (ECR) regime under working gas pressure 10^–5–10^–2 Torr. By changing discharge conditions one can change the injected plasma density from 10⁹ to 10¹² cm^–3, at the temperatureT _e=2–10 eV. The possibility of efficient plasma injection from the source into the open magnetic trap of various configurations is shown experimentally. Plasma characteristics in the trap are presented under various experimental conditions. It is established that plasma parameters can be easily changed in the trap.     

Decay of tripositronium with spin 3/2 and orbital angular momentum L=1 into an electron and a photon

It is shown that the probability for decay into an electron and a photon of a negative ion of positronium with the quantum numbers S=3/2, L=1, S_z=3/2, and L_z=0 is equal to 0.7·10^–7sec-^–1.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 101–105, October, 1978.The author thanks V. S. Vanyashin for suggesting this problem and for useful discussions.     

Luminescence of rare gas crystals at high excitation densities for VUV laser applications

Exciton densities of the order of 10¹⁸ cm^–3 are generated in 0.1–0.3 mm thick surface layers in an area of 10×20 mm² of optically clear rare gas crystals. The quantum efficiencies at 126 nm (Ar), 145 nm (Kr), and 172 nm (Xe) remain near 0.5 even for the highest excitation densities. The corresponding gain coefficients of 2.6 cm^–1 (Ar) to 18 cm^–1 (Xe) exceed those of high pressure gas lasers by a factor of 20. Stimulated emission is inferred by observing the line narrowing, the dependence of intensities and time courses on excitation density and amplification measurements. The net gain coefficient is reduced however to 0.5–1 cm^–1 by transient absorption of excited centers and scattering by irradiation induced defects. The results are analysed by a system of rate equations for the excitation, relaxation, quenching, and amplification processes. A peculiar temperature dependence of the quantum efficiencies and time courses is attributed to electron trapping at grain boundaries.     

Environmental influence on the IR fluorescence of Xe<Subscript>2</Subscript><Superscript>*</Superscript> molecules in electron beam excited Ar–Xe mixture at high density

We report new measurements of the near infrared (NIR) Xe₂^* excimer fluorescence in an electron–beam–excited Ar (90%)–Xe (10 %) mixture at room temperature. Previous measurements up to a density N≈2×10²⁶ m^-3 discovered a broad excimer fluorescence band at ≈7800 cm^-1, whose center is red–shifted by increasing N [A.F. Borghesani, G. Bressi, G. Carugno, E. Conti, D. Iannuzzi, J. Chem. Phys. 115, 6042 (2001)]. The shift has been explained by assuming that the energy of the optical active electron in the molecule is shifted by the density–dependent Fermi shift and by accounting for the solvation effect due to the environment. We have extended the density range up to N≈6×10²⁶ m^-3, confirming the previous measurements and extending the validity of the interpretative model. A detailed analysis of the width of the fluorescence band gives a value of 2.85 nm for the size of the investigated excimer state. Such a large value lends credibility to the validity of the proposed explanation of the experimental findings.     

Investigation of the electrical and photoelectric properties of manganese-doped gallium arsenide as a material for photoresistive detectors

An investigation was made into the electrical and photoelectric properties of gallium arsenide with an initial electron density 5×10¹⁵–4×10¹⁷ cm^–3 doped with Mn in different thermodynamic diffusion regimes characterized by diffusion temperatures of 900 and 1000°C and arsenic vapor pressure 10^–2 and 10^–3 atm. The ionization energy and defect concentration were determined by a computer analysis of the equilibrium hole density determined from the Hall effect. Centers with ionization energy 0.08–0.10 eV were found, their concentration varying from 2×10¹⁹ to 2×10²⁰ cm^–3 depending on the diffusion temperature and the doping level of the original crystals. Data obtained by investigating the stationary intrinsic photoconductivity were used to determine a hole lifetime of t_p 10^–9 sec. The photoconductivity spectra were investigated in the range 0.5–2 eV at 77°K, and defects with ionization energy E_v + 0.6 eV were found in all samples. The impurity photoconductivity at wavelength 10.6 m was investigated. It was shown that GaAsMn can be used as a material for impurity photoresistors.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 15–19, March, 1981.     

Studies of divacancy in Si using positron lifetime measurement

The charge state dependence of positron lifetime and trapping at divacancy (V₂) in Si doped with phosphorus or boron has been studied after 15 McV electron irradiation up to a fluence of 8.0×10¹⁷ e/cm². The positron trapping cross sections for V ₂ ^2– , V ₂ ^– and V ₂ ⁰ at 300 K were about 6×10^–14, 3×10^–14 and 0.1–3×10^–14 cm², respectively. For V ₂ ⁺ , however, no positron trapping was observed. The marked difference in the cross sections comes from Coulomb interaction between the positron and the charged divacancy. The trapping rates for V ₂ ⁰ and V ₂ ^2– have been found to increase with decreasing temperature in the temperature range of 10–300 K. These results are well interpreted by a two-stage trapping model having shallow levels with energy of 9 meV (V ₂ ⁰ ) and 21 meV (V ₂ ^2– ). The appearance of a shallow level for V ₂ ⁰ can not be explained by a conventional Rydberg state model. The lifetime (290–300 ps) in V ₂ ⁰ is nearly constant in the temperature range from 10 to 300 K, while that in V ₂ ^2– increases from 260 ps at 10 K to 320 ps at 300 K. The lifetime (260 ps) in V ₂ ^2– is shorter than that in V ₂ ⁰ at low temperature, which is due to the excess electron density in V ₂ ^2– . At high temperature, however, the longer lifetime of V ₂ ^2– than that of V ₂ ⁰ is attributed to lattice relaxation around V ₂ ^2– .     

Laser gain by electron collisional pumping of Ar VII–V XII

Gain coefficients have been calculated for transitions of singlet levels ns–np of orbital n=4 and n=5 in magnesium-like ions with atomic numbers Z=18, 19, 20, 21, 22 and 23. Population inversions for 4p and 5p levels in these ions were also calculated, via electron collisional excitation, for electron temperature range of 93–231 eV and electron density range of 10¹⁶–10¹⁷ cm⁻³. Under these plasma conditions, the maximum gain that occurred for 4s–4p transition was at electron temperature of 231 eV and electron density of 4×10¹⁷ cm⁻³. Scaling of the maximum gain coefficients with atomic number Z and the plasma parameters is also presented.     

Charge Exchange of the Centers in Electron–Irradiated Zinc Selenide

The optical absorption induced by the photosensitive centers formed upon electron irradiation (E = 5 MeV, = 1.7·10¹⁸ cm^–2) of polycrystalline ZnSe has been studied. A comparison of the optical properties of the irradiated crystals with the known data for ZnS has allowed the assumption that the 496–, 563–, and 652–nm bands in ZnSe are associated with the anion vacancies being in different charge states. The ratio between the concentrations of the optical absorption centers in the crystals photoexcited at 80 K is determined by the electron traps participating in the processes of charge exchange of the vacancies.     

Effect of electron irradiation on the properties of germanium-doped gallium arsenide

The Hall effect, electrical conductivity (77–370 K), and photoluminescence spectra (77 K) are studied in single-crystals of nuclearly doped GaAs (NDG) and GaAs doped with Ge by the metallurgical method after irradiation by electrons (E= 1 MeV, D=1.1·10¹⁵–3.8·10¹⁸ cm^–2). Initial electron concentrations were n= 1.7·10¹⁷ cm^–3 and n₀=2.6·10¹⁷ cm^–3 respectively. In the GaAs doped during crystal growth by the Czochralski method the degree of compensation related to the amphoteric impurity Ge is higher (K=0.8) than in the NDG (K=0.4) for identical initial electron concentration. It was established that the rate of charge carrier removal in GaAs is lower than in NDG, while radiation defects are more thermostable in NDG. The energy spectrum of radiation defects and radiating recombination centers, and the basic steps in reestablishment of electrophysical and optical properties in GaAs and NDG are similar, i.e., they do not depend on the method of germanium doping.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 82–86, April, 1991.