Floating double probe in non‐Maxwellian plasmas: Determination of the electron density and mean electron energy

A theoretical study of the floating double probe based on the Druyvesteyn theory is developed in the case of non‐Maxwellian electron energy distribution functions (EEDFs). It is used to calculate the EEDF in the electron energy range larger than –e(V_f ? V_p) from the I–V double probe characteristics. V_f and V_p are the floating and plasma potential, respectively. The analytical distribution function corresponding to the best fit of EEDF in the energy range larger than e(V_f ? V_p) allows the determination of the total electron density (n_e) and the mean electron energy (<?_e>). The method is detailed and tested in the case of a theoretical Maxwell–Boltzmann distribution function. It is applied for experiments that are performed in expanding microwave plasmas sustained in argon. Analytical EEDFs determined by this method are compared with those measured by means of single probes under the same experimental conditions. A good agreement is observed between single and double probe measurements. Results obtained under different experimental conditions are used to define the best conditions to obtain reliable results by means of the double probe technique.     

Electron distribution function and recombination coefficient in ultracold plasma in a magnetic field

The electron distribution function and diffusion coefficient in energy space have been calculated for the first time for a weakly coupled ultracold plasma in a magnetic field in the range of magnetic fields B = 100?50000 G for various temperatures. The dependence of these characteristics on the magnetic field is analyzed and the distribution function is shown to depend on the electron energy shift in a magnetic field. The position of the “bottleneck” of the distribution function has been found to be shifted toward negative energies with increasing magnetic field. The electron velocity autocorrelators as a function of the magnetic field have been calculated; their behavior suggests that the frequency of collisions between charged particles decreases significantly with increasing magnetic field. The collisional recombination coefficient α _B has been calculated in the diffusion approximation for a weakly coupled ultracold plasma in a magnetic field. An increase in magnetic field is shown to lead to a decrease in α _B and this decrease can be several orders of magnitude.     

Non-equilibrium distribution function in polar direct gap semiconductors

Electron distribution function (EDF) for non-equilibrium configurations in polar direct gap semiconductors is obtained in the temperature range k_BT? ? ω_Lo. The sample is assumed to be free of impurities and crystal defects. Electron-phonon coupling is the main relaxation mechanism. Master equation is solved approximately and analitycal expressions for EDF are obtained. Average electron energy as a function of temperature is discussed.     

Energieverlustmessungen mit 60 keV Elektronen an amorphem und polykristallinem Selen und Tellur und Bestimmung optischer Konstanten

The energy loss functions Im—(1/?) of amorphous and polycrystalline Selenium and Tellurium are determined from energy loss spectra of 60 keV electrons in the energy range up to 30 eV. The optical constants ?₁ and ?₂ are calculated from the energy loss function by Kramers Kronig analysis. The energy difference in the position of the volume plasma loss of amorphous and polycrystalline foils is compared with the calculated change in free electron plasma energy resulting from density change. Characteristic structure dependence of the optical constants are found to be similar for Selenium and Tellurium. They are discussed in terms of the results of band structure calculations.     

Electroexcitation of 208Pb,distribution of electric dipole and quadrupole strength and fragmentation of the isoscalar quadrupole giant resonance

Low momentum transfer, high-resolution inelastic electron scattering on ²⁰⁸Pb has been used to study the distribution of E1 and E2 strength in the region of excitation energy E_x = 8?12MeV. The E1 and E2 strength is very fragmented and the EWSR strength amounts to (10⁺⁷_?6)% and (29⁺¹¹_?8)% in the investigated energy region, respectively. The E2 strength found is less than most current theoretical predictions but agrees qualitatively with a 1p - 1h +2p - 2h model calculation presented in this paper. The E2 strength is also smaller than what is known from hadron scattering and the shape of the strength distribution is also markedly different in electron and hadron scattering.     

Efficiency of an electron-beam-pumped chemical laser with an SF6-H2 working mixture

Results are presented from a study of HF lasers pumped by non-chain chemical reactions initiated by a radially convergent and by a planar electron beam. The main channels of formation of vibrationally excited HF molecules are analyzed. The distribution of the energy density of the radiation in the output beam of a wide-aperture laser is measured. In 30 liters of a mixture of SF₆:H₂=8:1 at a pressure of 1.1 atm an output energy of ∼200 J is obtained at an ∼11% efficiency with respect to the energy deposition. It is shown that the admixture of a buffer gas of neon or argon improves the uniformity of the radiation energy distribution in the output beam of an HF laser pumped by a non-chain chemical reaction and initiated by an electron beam, and it also increases the output energy. Zh. Tekh. Fiz. 69, 76–81 (January 1999)     

Classical trajectory calculations for state-resolved Penning ionisation reactions of polycyclic aromatic hydrocarbon C10H8 in collision with He*(23S)

ABSTRACT

The reaction dynamics of Penning ionisation of a polycyclic aromatic hydrocarbon (PAH), naphthalene C₁₀H₈, in collision with the metastable He*(2³S) atom is studied by classical trajectory calculations using an approximate interaction potential energy surface between He* and the molecule, which is constructed based on ab initio calculations for the isovalent Li?+?C₁₀H₈ system. The ionisation width (rate) around the molecular surface are obtained from overlap integrals of the He 1s orbital and the molecular orbital. The calculated collision energy dependences of partial Penning ionisation cross sections (CEDPICS) in the range 50–500?meV at 300?K have reproduced the experimental results semi-quantitatively. The opacity functions, which represent the reaction probability with respect to the impact parameter b, are discussed in connection with collision energy, interaction with He* and the exterior electron density of molecular orbitals. They indicate that the collisional ionisations of C₁₀H₈ can be classified into three types: π electron ionisations with negative collision energy dependences which are predominantly determined by attractive interaction with He*; σ orbitals ionisations of the hardcore type; σ orbital ionisations which reflect interaction potentials around CH bonds. The critical impact parameters b_c become larger with increasing collision energy due to the centrifugal barrier.     

Ionization Dynamics in a Pulse Disturbed Magnetically Confined Helium Plasma

The equilibrium of a magnetized Helium plasma is disturbed by a pulsed Trivelpiece-Gouldwave. The electrons obtain the energy by linear collisionless wave absorption. The relaxation phenomena of density and energy are explained in terms of two relaxation times τ_E, τ₁ and a quantity giving the additional ionization. These quantities are derived from a small signal fluid model based upon energy and particle balance equations. In the experiment they are taken from the transient curves of Langmuir-probe current, optical line radiation and the noise power at the electron cyclotron frequency. The experimental conditions are: Helium-gas, p = 1 …? 5 Pa, T_e = 4 eV, n = 1 …? 5 · 10¹⁰ cm^?3, B = 6,5 · 10^?2 T, 27 MHz rf plasma source, low frequency fluctuation level < 1%, classical losses. The energy relaxation time …?_E = 10 …? 15 μs is given by inelastic collision losses. The ionization time constant τ₁ is related to the instantaneous ionization frequency during the transient state. It shows a high value at the very beginning of the pulse which must be explained by a tail formation in the distribution function and enhanced radial losses becoming Bohm-like in the transition phase.     

Kinetic electron reflection coefficient in a low-voltage Knudsen arc

The influence of the escape of fast plasma electrons on the electron distribution function (EDF) in a low-voltage cesium Knudsen arc is discussed. It is shown that even with a large Knudsen parameter l _e /h∼5–10 (where h is the gap and l _e is the mean free path of electrons with energy of the order of the anode barrier) the electron flux from the plasma to the anode is virtually identical to that calculated with a Maxwellian EDF. Zh. Tekh. Fiz. 68, 61–64 (May 1998)     

Non-Maxwellian Electron Velocity Distribution Resulting from Electron-SF6 Attachment Collisions

A model reaction system is considered consisting of a heat bath of argon atoms (c) and small concentrations of SF₆-molecules (m) and electrons (e) (number densities: n_e ? n_m ? n_c). Within the model the time behaviour of the electron velocity distribution function is studied under the influence of elastic electron-argon collisions and electron SF₆ attachment collisions. On the basis of the distribution function time-dependent macroscopic quantities (kinetic electron temperature. nonequilibrium rate constant of the attachment reaction) are calculated.     

Distribution function and electron state density in nonequilibrium plasma created by dye laser

Ultracold nonequilibrium plasma created by a dye laser has been studied by the molecular dynamics method. Electrons and protons in this model of nonequilibrium plasma interacted according to the Coulomb law. In the case of electron-proton interaction and a distance between particles r < a ₀ (Bohr radius), the interaction energy is constant, e ²/a ₀ (e is the charge of electron). An initial proton kinetic energy is set randomly so that the average kinetic energy is 0.01–1 K. Initial full electron energy is also set randomly, but at the same time it is positive; i.e., all the electrons according to our task are located in the continuous spectrum. Average kinetic electron energy per one particle varies from 1 to 50 K. The motion equations in periodical boundary condition for this system have been solved by molecular dynamics method. We have calculated the distribution function in the region near the ionization threshold. The distribution function is being described using electron state density in the nearest neighbor approximation with activity correction.     

Radiation Efficiency and Kinetic Calculations for the Positive Column in Hg/Ar Mixtures. I. Thg Variation

Properties of the positive column of low pressure Hg/Ar discharge have been investigated from the point of view of its application in fluorescent lamps. The results obtained are based on experimental methods to determine both UV resonance radiation fluxes of the 6³ P₁ and 6¹P₁ Hg levels and to measure the electron distribution function by probes; in addition a kinetic model is applied for the theoretical interpretation of the experimental results and to give a deeper insight into the particle and energy budget. In the present first part of the paper the effects of recently determined τ_eff-values on radiation, net collision rates, power transfer and the electron energy distribution for extended parameter values are investigated especially in dependence on mercury partial pressure.     

Significance of Hall measurements in Ga1−xAlxAs alloys at 300 K

The Hall mobility, electron concentration and resistivity have been measured as a function of alloy composition for Ga_1?x Al _x As alloys at 300 K. The data have been explained on the multiconduction band structure of the alloys. The alloy composition for the direct-indirect conduction band minima cross-over, the electron mobility in theX minima and the activation energy of the deep level below these minima have been determined.     

In situ TEM observations of dislocation/anti-phase boundary interactions

L2₁-ordered Fe₅₉Mn₁₇Al₂₄ (in at%) single crystals were in situ strained at either 300?K or 700?K in a transmission electron microscope. At 300?K, the strain was accommodated by the glide of four-fold dissociated super-dislocations, whereas, at 700?K, the strain was accommodated by the glide of a/2?111? partials. Dislocation pile-ups occurred at a/4?111? thermal anti-phase boundaries (APBs). Screw super-dislocations frequently cross-slipped when they encountered the thermal APBs, while mixed dislocations tended to be pinned at them. This impediment is attributed to the creation of new APB segments when dislocations pass through the curvy thermal APBs.     

Energy absorption buildup factors of some potential bioactive compounds in the energy region 0.015–15 MeV

Kinetic energy released per unit mass relative to air and energy absorption buildup factors has been calculated for some potential bioactive compounds in the energy region of 0.015–15?MeV. The bioactive compounds of 1-aryl-3-dibenzylamino-propane-1-on hydrochloride type Mannich bases were used in this work. Aryl part was changed as C₆H₅ (1), 4-CH₃C₆H₄ (2), C₄H₃S-2-yl (3), 4-FC₆H₄ (4), 4-BrC₆H₄ (5), 4-ClC₆H₄ (6), and 4-NO₂C₆H₄ (7). The energy absorption buildup factors have been calculated for penetration depth of 40 mean free paths. It is observed that kinetic energy released per unit mass relative to air depends on the photon energy and chemical content of compounds. The compounds with least mean atomic number possess the maximum value of energy absorption buildup factors. Also, the energy absorption buildup factors are found the highest in intermediate energy, whereas the lowest in low as well as high energies.     

Comparison study of energy bands and Wannier-Mott excitons in mixed Zn(P As ) and Zn Cd P crystals

Excitonic absorption, reflection and photoluminescence spectra of mixed Zn(P_1-xAs_x)₂ crystals over the full range of x ( 0 ? x ? 1) and Zn_1-xCd_xP₂ crystals at 0 ? x ? 0.05 have been studied at low temperatures (1.8 K). The decrease of the energy gap in Zn(P_1-xAs_x)₂ at the increase of x occurs slightly sublinearly. The rydbergs of excitonic series in this crystals decrease as well, and the dependences Ry ( x ) for all series are strongly superlinear at small x. In Zn_1-xCd_xP₂ crystals the energy gap and rydbergs decrease at the increase of x (at 0 ? x ? 0.05) as well. The dependences of E_g and Ry on x are considerably stronger in Zn(P_1-xAs_x)₂ than in Zn_1-xCd_xP₂. At the increase of x the half-width of excitonic absorption lines increases monotonically in both type crystals that is evidence of the increasing role of fluctuations of crystal potential. Received 13 March 2002 Published online 9 July 2002     

The singlet state in the Hubbard model with U=∞

We discuss, in connection with the problem of the ground state in the Hubbard model with U=∞, the normal (nonmagnetic) N-state of a system over the entire range of electron concentrations n≤1. It is found that in a one-particle approximation, e.g., in the generalized Hartree-Fock approximation, the energy ε ₀(n) of the N-state is lower than the energy ε _FM(n) of a saturated ferromagnetic state for all values of n. Using the random phase approximation we calculate the dynamical magnetic susceptibility and show that the N-state is stable for all values of n. A formally exact representation is derived for the mass operator of the one-particle electron Green’s function, and its expression in the self-consistent Born approximation is obtained. We discuss the first Born approximation and show that when correlations are taken into account, the attenuation vanishes on the Fermi surface and the electron distribution function at T=0 acquires a Migdal discontinuity, whose magnitude depends on n. The energy of the N-state in this approximation is still lower than ε _FM(n) for n<1. We show that the spin correlation functions are isotropic, which is a characteristic feature of the singlet states of the system. We calculate the spin correlation function for the nearest neighbors in the zeroth approximation as a function of n. Finally, we conclude that the singlet state of the system in the thermodynamic limit is the ground state. Zh. éksp. Teor. Fiz. 114, 2130–2144 (December 1998)     

TiB2-reinforced B4C composites produced by reaction sintering at high-pressure and high temperature

ABSTRACT

Obtaining both high hardness and toughness is a challenge in B₄C-nano-adhesive composites. Solving the inhomogeneous distribution of nano-adhesives in B₄C and forming the chemical bonding at grain boundary is an effective method. Here, we reported that the uniform distribution of titanium diboride (TiB₂)-reinforced B₄C composites synthesized by high-pressure and high temperature (HPHT). It is found that HPHT sintering can effectively inhibit the grain growth and increase the relative density. Moreover, HPHT sintering can cross high reaction energy barrier and effectively promote the formation of chemical bonding at grain boundary between B₄C and TiB₂. The optimal hardness and toughness value reach 30.0?±?0.9?GPa and 7.87?MPa·m^1/2, respectively. The improvement of hardness and toughness in the final products are ascribed to the strengthening of nanoTiB₂ connection of B₄C boundary and intergranular fracture mechanism. This work suggests a new way to achieve the uniform distribution of nanoTiB₂ in B₄C and form the chemical bonding at grain boundary, which is of great significance to the further development of TiB₂-reinforced B₄C composites with excellent mechanical properties.     

Growth of he bubbles in al during annealing

Abstract

The temperature dependent growth of He bubbles in Al films implanted at room temperature to various He concentrations is investigated by electron energy-loss spectroscopy (EELS) and transmission electron microscopy (TEM). EELS reveals even the weakest changes in He density within the bubbles by measuring the pressure shift of the He 1¹S₀?2¹P₁ transition. This is applied to investigate the mechanisms driving the growth of bubbles in the temperature range 20°C ? T ? 500°C. For T?200°C indication is found that bubbles relax by emission of interstitial dislocation loops. At higher temperatures bubble migration and coalescence under absorption of thermal vacancies is evident. The final state is characterized by large cavities filled with He at low pressure as evidenced by the detection of the atomic He series.