V. V. Lisenkov

Evolution of subnanosecond pulsed electric breakdown of gas gaps for uniform gas preionization

Initiation and evolution of breakdown of gas gaps by surge voltage pulses with a rise time of ≤1 ns are investigated experimentally and theoretically. The propagation of ionization waves for a uniform initial electron distribution in the gap is analyzed. The results of calculation are in qualitative agreement with the experiment. It is shown that the evolution of ionization waves leads to electric field redistribution in the discharge gap, and a region of an enhanced field with the strength sufficient for initiating emission processes and generation of a short fast electron beam in the cathode region is formed at the cathode for a very short time (up to 100 ps).

Electron-optical study of the initial phase of subnanosecond pulsed electric breakdown of gas-filled gaps

The glow accompanying the breakdown of gas gaps with a strong overvoltage by voltage pulses with 1-ns and shorter fronts is studied by electron optics methods. The filling of the gap with glow was accompanied by the development of ionization wave processes originating in the bulk of the gas and controlling the first stage of the breakdown. The dynamics of evolution of ionization waves in the electrode gap was analyzed in the 1D approximation. The results of calculations are in qualitative agreement with experiment. This leads to the conclusion that breakdown can be initiated from the bulk of the gas rather than from the surface of the electrodes. At this stage, the electrodes are mainly required for producing the electric field in the gap.

Numerical investigation of the parameters of a runaway electron beam generated in a gas-filled atmospheric-pressure hot-channel diode

The generation of runaway electrons in an inhomogeneous medium under normal conditions is investigated numerically. The medium is represented by a hot channel (spark channel, laser flame, etc.) surrounded by air. A model is suggested that makes it possible to consistently calculate the initiation of a subnanosecond glow discharge and the generation of runaway electrons under such conditions. The possibility of generating 100-ps-wide runaway electron current pulses with an amplitude of several hundred amperes is shown. The influence of an air gap and an external magnetic field on the generated beam’s parameters is studied.

Laser synthesis of nanopowders with yttrium aluminum garnet stoichiometry

A nanopowder of yttria-alumina mixture with the yttrium aluminum garnet (YAG) stoichiometry has been synthesized for the first time by the laser evaporation method. A high-power CO2 laser with pulse duration above 200 μs, repetition frequency of 500 Hz, and pulse energy of about 1 J provided a high yield of powder at a rate of 24 g/h. The obtained nanopowder has been used to prepare YAG:Nd3+ ceramics with a cubic structure possessing an optical transmittance of about 77% at a wavelength of 1.06 μm. The successful synthesis of YAG nanopowder is based on the preliminary numerical simulation of the laser evaporation of a target using a three-dimensional model.

Dynamics and spectroscopy of the laser plume from solid targets

The dynamics and the spectral kinetic characteristics of the plume emerging in the vicinity of graphite targets, pressed pellets consisting of zirconium oxide powder stabilized with yttrium (YSZ) and yttrium-aluminum oxides with neodymium (YAO:Nd), and single-crystal YAG:Cr are studied. The targets are irradiated in air at room temperature using a repetitively pulsed CO2 laser with a wavelength of 10.6 μm, a peak power of up to 9 kW, a pulse energy of 1.69 J, and a pulse duration of 330 μs at a level of 0.1. The plume propagates normally to the target surface at an angle of 45° relative to the laser radiation. The spectral kinetic characteristics of the plume luminescence are discretely measured along the entire length. It is demonstrated that the plumes of all targets (except for the single-crystal YAG:Cr) represent the flows of a weakly nonequilibrium gas plasma with a temperature of 10 kK (graphite) and 3.1–4.7 kK (YSZ and YAO:Nd pressed pellets). The plume size is determined by the peak power of the laser pulse. The luminescence of the two-atom radicals (C2 in graphite; ZrO and YO in YSZ; and YO, AlO, and NdO in YAO:Nd) dominates in all of the plumes. A single radical (YO) and the spectral lines of atoms and atomic ions are observed in the YAG:Cr plume. A relatively high temperature of the graphite plume is maintained owing to the energy of the exothermic reaction involving the association of carbon atoms and the energy of the vibrationally excited molecules resulting from this reaction.

Effect of pulses from a high-power ytterbium fiber laser on a material with a nonuniform refractive index. I. Irradiation of yttrium oxide targets

The irradiation of Nd:Y2O3 targets with an absorption coefficient of 13–1.7 × 103 cm−1 using laser pulses with a duration of 0.1–3.5 ms and peak power of 200–700 W at a power density of (0.2–1.3) × 106 W/cm2 is studied. A relatively large spread of the delay times of laser plume, spike emission of the laser plume, cleavage of the front surface of the target, and greater ejection of substance from the crater in comparison with the effect of the CO2-laser radiation with almost the same power are demonstrated. A numerical model of the effect of radiation on a target with a nonuniform refractive index is proposed to interpret the destruction of dielectric material (cleavage of the front surface) and the large spread of the delay times of the plume.

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.

Electron acceleration in a gas-filled hot-channel diode under atmospheric pressure

A new method for electron acceleration in a gaseous medium is proposed and theoretically substantiated. The method is based on using a high-temperature gas domain (laser-induced jet, arc channel, etc.) as a source of runaway electrous with their subsequent acceleration in a dense low-temperature gas. It is shown feasible to obtain accelerated electron beams with currents as high as 1 kA and an average energy of approximately (2/3)eU (where U is the accelerating voltage), which is comparable to the parameters of the beams generated by accelerators on the basis of vacuum diodes.

Effect of pulses from a high-power ytterbium fiber laser on a material with a nonuniform refractive index. II. Production and parameters of Nd:Y2O3 nanopowders

The laser ablation of the Nd:Y2O3 target with substantially nonuniform refractive index leads to the formation of a needle-shaped surface with a needle height of 6–8 mm. An increase in the displacement velocity of the laser beam on the surface to 80 cm/s and an increase in the diameter of the laser spot at the central part of the beam waist to 430 μm lead to a more uniform relief of the target surface and an increase in the nanopowder yield and production rate to 22% and 23 g/h, respectively. In addition, an excess of the mole content of the low-melting Nd2O3 in the powder decreases from 174 to 11% in comparison with the target. At an air pressure in the evaporation chamber of 0.8 bar, the mean sizes of nanoparticles (13–14 nm) are virtually independent of the displacement velocity of the beam on the surface (7–81 cm/s) and the rate of air flow above the target (13–70 m/s) in spite of significantly different nanopowder production rates.

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.