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Filamentation of high-power femtosecond pulses in a gas is of great theoretical and practical interest with relation to study of large-scale spectral and temporal transformations of laser radiation in a medium and generation of extra-wide (supercontinuum) radiation, actual for the problems of nonlinear femtosecond diagnostics of the environment, transmission of optical information through an atmospheric channel, and modern optical technologies for material processing. This paper experimentally studies the influence of pressure of a gas medium (nitrogen) in an optical cell on the characteristics of femtosecond laser radiation propagating under filamentation conditions. It is shown that under conditions of high nitrogen pressure (up to 11 atm) and sharp geometric focusing of femtosecond radiation, its Kerr self-focusing is implemented, and the single filamentation made transforms into multiple post filamentation as the gas pressure increases. In this case, due to the phase self-modulation of a femtosecond pulse and plasma generation in the gas, there is a significant enrichment of the spectral composition of the radiation, and near-linear increase in the pulse spectrum width with gas pressure in the cell. It was established for the first time that the pulse spectrum is extended asymmetrically and mainly to the long-wave region with an increase in the sharpness of the initial focusing of a laser beam. In addition, the average size of intense light post-filaments formed inside a beam decreases and can be fractions of a millimeter as the gas pressure increases in the optical cell.

The paper demonstrates possibility of remote detection of surface traces of organophosphates using the double-pulse laser fragmentation/laser-induced fluorescence (LP/LIF) method. For drop-liquid traces of triethyl phosphate on a paper surface, it is shown that the process of formation of characteristic PO fragments (phosphorus oxide molecules) of organophosphates is inertial. The formation of the maximum concentration of fragments is observed approximately 2 ms after the action of a fragmenting laser pulse (266 nm). It is found that a delay between a laser pulse (247.78 nm) a fragmenting pulse of 2 ms leads to a multiple increase in the fluorescence intensity: approximately 7 times compared to the single-pulse excitation method and approximately 2.3 times compared to simultaneous double-pulse action. The results, first, demonstrate a possibility of remote detection of surface traces of organophosphates in a condensed state by the two-pulse LF/LIF method; second, they show the need to organize optimal conditions for laser exposure to increase the efficiency of the LF/LIF process.

The control of combustion by throttling jets in a two-section channel with high-velocity flow has been studied numerically. Pulses of the first throttling jet are used to produce intense transonic combustion of hydrocarbon fuel in the first section. Side fuel supply is applied after the switching-off of the first jet to maintain the combustion mode before the channel expansion. The completeness of fuel combustion in the second section is increased using a second throttling jet. The Reynolds-averaged Navier-Stokes equations closed by the eturbulence model are solved. Combustion was modeled by the overall reaction. A pulsating mode of hydrogen combustion in the second section when exposed to the cold throttling jet was established. The influence of this jet on the completeness of hydrogen combustion was studied.

We investigated the possibility of the existence of combustion waves in CH₄/air mixtures with a methane content α = 5-8 vol.% and in CH₄/air/coal dust mixtures in a closed vertical channel 6.75 m long and 0.07 m in diameter when ignited at the top. The experiments were carried out at an initial pressure of 0.1 MPa using dust with an average volume concentration of 100-530 g/m³ and a particle size 0-200 mm. It was found that combustion was not initiated in gas and gas/coal dust mixtures with α ≤5-5.5 vol.%. The results of the study indicate that the direction of gravity influences the ignition and combustion of the mixtures and that the energy contribution of coal particles is low compared to that of methane.

This paper presents new data on the detonation parameters of lean, stoichiometric, and rich methane-oxygen (air) and hydrogen-oxygen (air) mixtures inhibited by nitrogen and carbon dioxide. By varying the ratio between the initial components, carbon dioxide was found to have a stronger inhibiting effect on the parameters of combustion and detonation products.

An experimental study was performed to investigate thermal processes during gas detonation coating. Before impact on the target, particles of the sprayed material were accelerated to velocities of 300-500 m/s by the flow of detonation products of a gas mixture whose temperature reached 4200 °C. Due to intense heat transfer, the temperature of the particles increased. The highest quality coating was obtained at a heating temperature close to the melting point. Therefore, to estimate the heating temperature of the particles, we measured the heat fluxes from the detonation products of the gas mixture to their frontal and lateral surfaces using a method based on the Seebeck effect.

An X-ray radiography method for contactless diagnostics of solid rocket motors (SRM) is presented. An experimental study was performed to visualize intrachamber processes and determine the solid propellant burning rate in a model E-5-0 SRM using an X-ray system consisting of an X-ray source and a dynamic X-ray detector located at a given distance from the object of study. The possibility of contactless determination of the propellant burning rate based on an analysis of gray level changes near the burning surface was confirmed experimentally. Radiographic results are in satisfactory agreement with the results of other methods for determining the solid propellant burning rate.

Effect of carbon additives on burning rate of model composite propellants is studied. Paste-like propellant compositions are used, which are an analog of composite solid fuel with an uncured binder. The role of nanocarbon additives is played by different substances: detonation nanodiamond (DND), which is sometimes heat-treated or crushed to a size of 4 nm, multilayer carbon nanotubes, soot, activated carbon, graphite, adamantane, and graphene. Among all the allotropic forms of carbon, the maximum increment of the burning rate (23%) is ensured by an additive of 2% DND with 2% plasticizer (by weight). At the same time, the combustion product temperature decreases by ≈200 °C, thereby reducing the probability of rocket engine burnout.

The growth of particles behind the detonation wave front in condensed explosives has been investigated based on physical estimates and experimental results obtained in recent years. The focus is on the large difference in particle size due to the presence or absence of hydrogen in the explosive composition.

A computational method has been developed to model the shock-wave mechanism of detonation wave initiation during the interaction of a fast flying body (FFB) with a combustible hydrogen-oxygen mixture diluted with 50% argon under normal conditions. The FFB velocities at Mach numbers M = 3-4 are considered, which are lower than the Chapman-Jouguet detonation velocity in the mixture under study at normal pressure and temperature. It is shown that the detonation wave is initiated at a FFB velocity exceeding M = 3.9. In this case, the shock-wave mechanism of detonation initiation occurs where a detonation wave is formed at the shock wave separated from the combustion wave by an induction zone. The modeling identified a new regime of reaction gas flow around the FFB. In the range of FFB velocities M = 3.4-3.85, a quasi-stationary mode of shock-initiated combustion occurs. The flow parameters required for direct initiation of multifront detonation of the FFB are consistent with analytical estimates of the initiation energy.