Generation of accelerated electrons in a gas diode with hot channel

Generation of fast electrons in an inhomogeneous medium composed of a hot channel (spark channel, laser plume, etc.) surrounded by air under normal conditions has been numerically analyzed. The model used makes it possible to carry out consistent calculation of the formation of subnanosecond gas discharge and generation of accelerated electrons under these conditions. The fast-electron current is found to consist of two pulses. One of them has an amplitude of 50 A, width of 30 ps, and electron energy of more than 100 keV. These electrons are generated in the hot channel. The other pulse has an amplitude of 170 A, width of 20 ps, and electron energy in the range of 8-50 keV. These electrons are generated in cold air. Since these pulses pass successively and barely overlap, the total width of fast-electron pulse is almost 50 ps. © 2013 Pleiades Publishing, Ltd

Interaction of the radiation of the high-power ytterbium-fiber laser with inhomogeneous dielectric targets

The action of the radiation of the ytterbium-fiber laser (λ = 1.07 μm) on the Nd3+Y2O3 target with nonuniform transparency in the course of the nanopowder production is studied. It is demonstrated that the laser irradiation leads to an extremely rough surface with the stalagmite roughness due to a relatively large melting depth. The resulting powder consists of two fractions. The first fraction (99% of the total mass of the powder) consists of nanoparticles with a mean size of 29 nm (BET data). The second fraction consists of micro- and submicroparticles that represent circular drops condensed from the melt and shapeless debris of the target. The peaks on the diameter distribution of the drops at 2, 8, and 80 μm are determined by different effects. The laser heating of the inhomogeneous target with the nonlinear refractive index is numerically analyzed. It is demonstrated that the melting of the target is initiated at a mean laser power of 700 W, a power density of 5.6 × 105 W/cm2, and an irradiation time of 150 μs. © 2013 Pleiades Publishing, Ltd

Application of a hybrid model for the numerical study of the generation of runaway electrons and the formation of high-pressure gas discharge

The paper analyses the details of the application of the hybrid model for calculation of the formation of high-pressure gas discharge in conditions where the transition of electrons into runaway mode is possible. In hybrid model, PIC MC method is used only for calculation of runaway electrons, and the standard hydrodynamic approach is used for calculation of plasma electrons. Using such model can significantly reduce computing resources. The results of calculation of kinetics of electrons emitted from a micro-spike on the cathode during the formation of the cathode layer of nanosecond and sub-nanosecond high-pressure gas discharge are presented. The conditions of transition of electrons into runaway mode at this stage and their influence on the further formation of the gas discharge are analyzed. © 2018 Institute of Physics Publishing. All rights reserved.The work was supported by RFBR, Grant 16-08-00894

Production of nanopowders of oxides by means of fiber and pulse-periodical CO2 lasers

The results of investigation of YSZ, Nd:Y2O3, Al2O3 nanopowder production by laser evaporation of oxide targets in a gas current are reported in present paper. For this purpose we used the pulse-periodical CO2 laser and the continuous fiber ytterbium laser with 550 W and 600 W radiation mean power accordingly. The powders obtained by these lasers, consisted of weakly agglomerated spherical nanopartices (≥ 99 wt%), and ≤ 1 wt% of micron sized particles (drops and target fragments). Nanoparticles from various oxides produced by CO2 laser in atmospheric pressure air had close average sizes (10ч16 nm). The productivity of nanopowder synthesis by CO2 laser from YSZ 1%Nd:Y2O3, 1%Nd:Y2O3, Al2O3, and CeGdO was 23 g/hour, 29 g/hour, 24 g/hour and 80 g/hour, respectively. Unlike CO2 laser the deep melting mode is realized during evaporation of 1%Nd:Y2O3 and Al2O3 targets by fiber laser. The crater depth increases up to 300-1000 μm in this mode. As a result, the target surface became very irregular and productivity of nanopowder synthesis was less, than in the case of CO2 laser. To reduce the effect of deep melting the evaporation of a target has been investigated experimentally and theoretically. As a result of our investigations we have obtained 1%Nd:Y2O3 nanopowder with specific surface of 70 m2/g and productivity of 23 g/hour at air pressure 70 kPa. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The influence of HfO2 additives on the optical properties of Nd3+-doped Y2O3 ceramics

Transparent yttria ceramics were fabricated by sintering a mixture of 1 at.% Nd:Y2O3 and HfO2 nanopowders produced by laser synthesis method. The best transmittance was 80.96% at the wavelength of λ=1080 nm in 6 mol.% HfO2 doped sample (1.5 mm thick). The additives of hafnium broaden both pumping and luminescence bands of Nd3+ ion in yttria ceramics. The luminescence intensity of 4F3/2→4I11/2 transition was little affected by hafnium concentration. The effective lifetime of 4F3/2 level in Nd:Y2O3 ceramics enhanced by 30% at 10 mol.% HfO2 doping concentration and the decay kinetics of laser transition attains Förster's behavior. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Features of the Electron Avalanche Formation Process in a Strongly Inhomogeneous Electric Field under High Overvoltages

The simulation of the electron avalanche formation process in subnanosecond discharges of high pressure was carried out by means of the Monte-Carlo approach. The discharge gap under consideration was of the configuration “the finger-shaped cathode – the hemispherical anode”. The presence of a conic-shaped microprotrusion on a cathode surface was assumed. Such the electrode configuration provided the strongly inhomogeneous distribution of an electric field. A gas simulated was nitrogen at a pressure of 6 atm. An average electric field strength across the discharge gap was varied from 200 kV/cm up to 400 kV/cm. Microprotrusion height was varied from 0 um up to 30 um. The critical size and formation time of an electron avalanche were determined under various conditions simulated. The threshold electric field strength for electrons to transit into the continuous accelerating regime was calculated for various heights of the microprotrusion. The applicability of the non-self-consistent Monte-Carlo technique for the investigation of the runaway electron kinetics and the correct simulation of the runaway electron beam transport across the discharge gap was shown. © 2021 Institute of Physics Publishing. All rights reserved.This work was supported in part by the Russian Foundation for Basic Research under Project 20-38-90147 and Project 20-08-00172

Numerical Investigation of a High-Pressure Gas Medium Preionization by Runaway Electrons

A comparative simulation of the generation and acceleration of runaway electrons in the discharge gap during the initiation of the discharge by nanosecond and subnanosecond pulses is carried out. We used a numerical model based on the PIC-MCC method. Calculations were carried out for N2 6 atm pressure. Numerical simulation of a formation process of the electron avalanche initiated by an electron field-emitted from the top of the cathode micro-spike was carried out taking into account the motion of each electron in the avalanche. Characteristic runaway electron trajectories, runaway electron energy gained during the motion through the discharge gap, times required for runaway electrons to reach the anode were calculated. We compared our results with calculations using well-known differential equation of electron acceleration using braking force in Bethe approximation. We solved this equation also for braking force based on real (experimental) ionization cross section. The reasons for the discrepancy in the calculation results are discussed. © 2021 Institute of Physics Publishing. All rights reserved.The work was carried out within the framework of the state tasks of IEP UB RAS and was supported by RFBR, Grant 20-08-00172

CALCULATION OF THE KINETIC COEFFICIENTS FOR DESCRIPTION OF RUNAWAY ELECTRONS BEAM DYNAMICS

A computer program calculating the kinetic coefficients by the Monte Carlo method for subsequent description of runaway electrons beam dynamics is developed. The calculated coefficients have been compared with corresponding experimental values.Работа поддержана грантом РФФИ №16-08-00894-а

EVALUATION OF INFLUENCE OF THE RUNAWAY ELECTRONS PHENOMENON ON THEIR IONIZATION ABILITY

The previously described algorithm for numerical simulation of electrons motion through a gas-filled accelerating gap has been applied to estimate the effect of the runaway electrons phenomenon on their ionization ability. It has been shown that when the runaway frequency reaches 109 s-1 the electron ionization ability and associated impact ionization coefficient start to decrease. This fact should be taken into account when gas discharge formation process is under consideration.Работа поддержана грантом РФФИ №16-08-00894-а