Laser heating of its erosional low-temperature plasma in the process of gasdynamic motion

A study is made of the gasdynamical and optical properties of erosional laser plasma jets in the presence and absence of laser radiation. It is shown that in processes of plasma formation during the action of laser radiation of moderate intensity (q 10⁷ W/cm²) on absorbing materials, the heating of the disintegration products by the attacking laser radiation plays an important role. The temperature distribution is obtained along the plasma jet which forms during the laser attack counter to its propagation in a quartz tube confining the dispersion. The temperature maximum is found at the exit from the tube, is caused by the heating of the erosional laser plasma by the incident laser radiation in the process of its one-dimensional gasdynamical motion, and indicates the screening of the surface from the laser radiation. It is established that the screening is affected by the gasdynamical structure of the plasma jet and by the spacing of the plasma clusters corresponding to the regular pulses of laser radiation.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 13–18, January–February, 1974.The authors are grateful to M. A. El'yashevich for his interest in the work.     

Calculation of the kinetics of propagation of a powerful light beam in a transparent dielectric with impurities

A large number of studies have been dedicated to the interaction of powerful optical radiation with transparent dielectrics in the prebreakdown or breakdown regimes. However, the mechanism by which the material is destroyed has not been determined decisively, as indicated by the constant flow of new publications on this theme. Attempts to obtain destruction as the result of electron avalanche [1] give threshold power values orders of magnitude greater than experiment [2]. In connection with this, in recent years the accent has been to deal with the concept of microimpurities of foreign particles or inhomogeneities within the medium having dimensions so small that their presence and concentration is difficult to monitor. As it absorbs optical radiation, the microimpurity (inhomogeneity) is heated and warms the areas of the medium adjacent to itself, which areas then commence to absorb light with significantly more intensity than they did in the initial state [3]. As a result, increase in absorption within the medium commences, terminating in breakdown or destruction of the material around the inhomogeneity. In [4, 5] it was noted that an important role may be played in such a destruction mechanism by thermoelastic stresses in the medium, which factor was not considered in [3]. In [4, 5] it was proposed that the basic effect of thermoelastic stresses reduces to development of microfissures in the medium. However, thermoelastic stresses can lead to yet another effect — narrowing of the forbidden zone of the medium and increase (together with the analogous action of temperature growth) in the coefficient of absorption of the medium. In the present study, the kinetics of interaction of optical radiation with a dielectric medium containing spherical metal particles as an impurity will be calculated, and it will be shown that thermoelastic stresses produce a significant contribution to the increase in light absorption by the medium around a particle.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 60–66, September–October, 1978.     

Destruction of coarse and fine water drops by monopulses of a ruby laser

In addition to the known method of evaporating water drops in the intensive radiation field of a CO₂ laser, papers have recently appeared wherein the destruction of water drops without the conversion of the light energy they absorbed into heat is investigated (surveys [1, 2], for example). Papers devoted to nonthermal methods of destroying a water aerosol, although still few in number, indicate the proposal of three methods of destroying the drops: optical breakdown in water, excitation of mechanical vibrations of the drops, and photochemical destruction of the water molecules [1, 2]. The optical breakdown phenomenon, when intense destructive shocks occur in a water drop subjected to a laser monopulse, has been investigated more fully than the other methods but also clearly insufficiently. Experiments on destroying millimeter- and micron-sized drops by ruby laser monopulses are described in this paper, values of the parameters characterizing this process are determined, and an approximate estimate of the energy and power of the laser pulses required to destroy a water aerosol in a track of definite length is also given.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 26–35, May–June, 1979.     

Transfer of energy from an evaporating drop in a vapor medium

The article considers the temperature distribution around an evaporating drop in a vapor medium. The transfer of energy is effected by molecular thermal conductivity, convection, and radiation. The mean length of the free flight path of the radiation considerably exceeds the characteristic distance at which the temperature changes. The times required for relaxation of the temperature to a steady-state value are determined, as well as the characteristic distances at which the temperature distribution changes.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 74–78, January–February, 1972.The authors thank V. G. Levich for his evaluation of the results obtained.     

Vibrational relaxation of diatomic molecules in a non-Boltzmann thermostat

The action of resonance IR laser radiation on a molecular gas leads, at high-power absorption intensity, to a breakdown in the equilibrium (Boltzmann) energy distribution in the internal degrees of freedom [1]. Under realistic conditions, molecular gases usually are (due to small amounts of impurities or isotopic components) multicomponent systems. In this case resonance IR laser radiation (or other methods of selective action), disturbing the distribution function of the primary gas, does not interact directly with impurities. The problem thus arises of determining the distribution function of the impurity gas interacting with the nonequilibrium (non-Boltzmann) thermostat. The present paper, devoted to the solution of this problem, treats the distribution function of harmonic oscillators A, consisting of a small amount of impurities in a system of harmonic oscillators B with given nonequilibrium distribution functions of vibrational energy. The behavior of a system in a nonequilibrium thermostat was first considered in [2, 3] where, as well as in [4, 5], it was shown that in a non-Maxwellian thermostat with a small amount of harmonic oscillator impurities, a Boltzmann distribution in harmonic oscillator vibrational energies is established under stationary conditions, with a temperature differing from the gas-kinetic temperature of the thermostat, defined in terms of the mean-square velocity. The behavior of a small amount of impurities (heavy monoatomic particles and harmonic oscillators) in a non-Maxwellian thermostat of a light gas was further investigated in [6–8]. Unlike the papers mentioned, the present one considers the behavior of a small amount of harmonic oscillator impurities in a thermostat with a Maxwellian velocity distribution and with a nonequilibrium (non-Boltzmann) distribution in vibrational energies.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 3–10, September–October, 1978.     

Sound generation by a laser beam in a liquid with absorbing particles

Sound generation by a laser beam within a liquid, related to liquid boiling at absorption centers, has been studied relatively little. Acoustic effects upon liquid boiling in a laser beam were first reported in [1]. A change in the index of refraction of a liquid has been observed under the action of acoustical radiation of microbubbles formed on absorbing particles in the zone irradiated by a laser beam [2–4]. However direct measurements of acoustical radiation from a set of microbubbles uniformly distributed over the volume of laser beam-liquid interaction has yet to be performed, to the authors' knowledge. The present study is a preliminary report of results of an experimental study of this class of opticoacoustical phenomena.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 9–12, January–February, 1984.In conclusion, the authors consider it their pleasant duty to thank G. A. Askar'yan for evaluating the study and S. V. Luk'yanov for assistance in measuring the particle size distribution.     

Noncontacting strain measurements during tensile tests

The application of an innovative noncontracting Doppler laser extensometer is presented. True axial strain has been measured during tensile tests conducted on stainless-steel metal sheets over a range of strain rates (from 10^–4 to 10² 1/s) and temperatures (from –40°C to 400°C). The laser radiation scattered at the surface of the specimen is recorded during the duration of the experiment. The signals are then used to determine the evolution of the axial strain, which is subsequently combined with the load signal to construct the stress-strain curve for the material. Excellent agreement has been obtained between the total elongation predicted by the laser measurements and the actual values measured from the specimens. This technique offers several advantages over traditional strain-measuring technologies.     

Reduction of gas breakdown voltage with pulsed ionizing radiation

An algorithm is described for computer calculation of the dynamic breakdown voltage of a gas gap affected by a spatially uniform pulse of ionizing radiation. The algorithm is based on numerical integration of a system of nonlinear equations with integral boundary conditions. The program is used to calculate the breakdown voltage of an air gap affected by a bell-shaped ionizing pulse. It is shown that the relative reduction in breakdown voltage can amount to tens of percent for a radiation exposure dose rate P₀ 10⁸ R/sec.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 6, pp. 52–60, November–December, 1973.     

Water vapor cooling when radiation of wavelength λ=2.8 μm is absorbed

Investigations of the processes arising under the influence of electromagnetic radiation on resonantly absorbing gaseous media have now been widely developed. Particular interest is shown in the penetration of a pulse of laser radiation through the atmosphere. The main component absorbing the radiation of both CO₂ and HF lasers (wavelengths, respectively, 10.6 and 2.8 m) in the earth's atmosphere is water vapor [1]. Numerous experimental investigations show that the integrated coefficient of laser radiation absorption by water vapor is fairly large [1–3], while at the same time the energy absorption leads to the heating of the medium in a channel around the beam and, as a consequence, to its defocusing. However, all these investigations were carried out with continuous sources of laser radiation or with pulses of fairly great duration. It will be shown below that gas cooling in the channel around the beam is possible when a pulse of radiation with wavelength 2.8 m whose duration is less than the vibrational-translational (V-T) relaxation time of the energy absorbed by the H₂O molecules passes through a stationary medium containing water vapor.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 141–151, May–June, 1986.     

Numerical investigation of the growth of glowing discharges in two-dimensional geometry

It is known that a breakdown in gases can take place in two fundamental ways: by diffusion (the Townsend breakdown) or by forming a narrow current channel (the streamer breakdown). At present there are no reliable criteria for one or another of these mechanisms to occur. It is also an open question as far as the pressure region p < 10 mm Hg [I] is concerned. Even in the case of special preionization it is not always possible to avoid the streamer stage breakdown. It is obvious that the fundamental cause of a streamer breakdown is related to higher intensity of the electric field around the localized zone of higher conductivity [2]. In [3] the superiority was shown of using numerical methods in the analysis of an axisymmetric cathode directed streamer between two flat electrodes in nitrogen. In the present article the results are described of computations carried out to find out whether a mechanism is feasible for fusing the discharge at any early stage of ignition for the geometry of a flat electrode plane, which is the most favorable to an anode-oriented streamer. This effect was investigated within the framework of a nonstationary system of three equations in which the ionization processes, the recombinations in the balance of charged particles as well as the effect if space charge on the electric-field distribution have been taken into account [4], One has ignored the diffusion, which is also favorable to the streamer breakdown.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 49–54, September–October, 1978.The authors would like to express their thanks to A. A. Vedenov, A. P. Napartovich, and A. N. Starostin for their unceasing interest in this work and useful discussions.     

Different modes of heating when high-power radiation fluxes interact with a material

The gas-dynamic and thermal processes which occur when a high-power flux of laser radiation interacts with a material are investigated. Fluxes for which the sublimation energy can be neglected compared with the thermal and kinetic energy of the vapors formed are considered. The electron thermal conductivity is considered as well as the hydrodynamic dispersion. The properties of different modes of propagation of temperature waves in a moving medium are studied. The case of an infinitely large absorption coefficient is given particular attention.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 41–48, September–October, 1972.The authors thank A. A. Samarskii for useful discussions.     

Criteria for excitation of a shock wave and its intensity during an explosion in a rarefied gas

The effect of the initial pressure of the surrounding gas on the intensity of the shock wave (SW) formed during the dispersion of material vaporized by a powerful laser pulse is examined. The initial stage of expansion of the plasma generated through the focusing of powerful laser radiation on the surface of a solid material in air was studied experimentally in [1, 2]. The times of formation and the initial radii of the SW were recorded on the photo-scans of the SW front radiation presented in these reports. It is found that at an air pressure below 0.1 mm Hg the recordings of the intrinsic radiation of a flare do not differ from the corresponding recordings in a vacuum. For instance, in [2] a bright shock front was observed at a pressure of 0.18 mm Hg, while at a pressure of 0.1 mm Hg SW radiation was not detected. In [2] the hypothesis was made that at an air pressure below 0.1 mm Hg a SW is not formed and the interaction of the vaporized material with the surrounding gas has a diffusion nature. However, in [1] SW were detected by the Schlieren method at a considerably lower pressure, about 2 · 10^–2 mm Hg. It will be shown below that the sharp decrease observed in the brightness of the radiation of SW fronts generated during laser heating of a solid material in a rarefied gas is explained by the rapid decrease in the maximum SW velocity at a pressure below 0.1 mm Hg. The expansion of the vaporized material at a pressure of the surrounding gas much less than 0.1 mm Hg is also examined.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 48–51, September–October, 1973.     

Strain-induced passivity breakdown in corrosion crack initiation

Corrosion crack nucleation and growth are modelled as a moving boundary problem. The model incorporates three physical processes––dissolution, passivation and straining––into a continuum mechanical framework. The dissolution triggers surface advance; the passivation restrains the access of the environment to bare metal; the deformation causes for passivity breakdown. Plane cracks nucleating from surface pits in an elastic–plastic material body under fatigue load are considered. The problem is solved using a FEM program and a moving boundary tracking procedure. The model simulates how cracks form and grow in a single continuous course. The geometry of the developed cracks is found independent of the initial pit size. Plasticity is found to influence the curvature at the tip of the nucleated corrosion cracks. The most important evolution length parameter, the width of the corrosion crack, is found to depend on the size constraints of the tracking procedure. It is concluded that the model is deficient for determining all length scales observed in reality. Physical processes to be considered in an advanced model are proposed and discussed.     

Modeling of the free-surface shape in laser cutting of metals. 2. M odel of multiple reflection and absorption of radiation

A physical model of multiple reflection of focused laser radiation propagating in narrow channels and slots is proposed to be applied in laser treatment of metals. Based on the laws of geometric optics, an algorithm for calculating the shape of the surface formed by interaction of laser radiation and metal with the use of the trajectory technique is proposed. The method used allows one to describe energy transfer inward the channel in simulation of laser-treatment processes (drilling, cutting, and welding). It is shown that the efficiency of multiple reflection is most significant in cutting thick materials with a large ratio of the plate thickness to the Gaussian beam width.Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 1, pp. 16–20, January–February, 2005     

Method of effective sections to take account of selectivity of radiation and absorption in a hot gas

A method, economical in computing time, for solving radiation transfer problems by using the integrated characteristics of the absorption spectrum, the effective sections, is elucidated. The shock layer ahead of a body around which a hypersonic gas flows is analyzed in the presence of intensive mass delivery from the surface. The machine time in the computational examples is shortened 120-fold as compared with an exact computation, and the error in calculating the radiation fluxes does not exceed 15–25%.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi FizikL, No. 5, pp. 76–83, September–October, 1972.The author is grateful to G. A. Tirskii for supervising the research and to É. S. Filippov for aid in carrying out the computations.     

Effects of the interaction between radiation and a disintegrating vitreous material

This article considers the disintegration of a vitreous semitransparent material at the critical point in a stream of radiating gas. A study is made of the effect on the rate of disintegration of the indeterminacy of the effect of interaction between the radiation and the gaseous disintegration products and of the spectral distribution of the density of the radiation flux of the gas with respect to the semitransparancy of the material. An analysis of the error of the gray approximation with calculation of the disintegration of materials with a semitransparency window is also given.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 133–137, September–October, 1978.     

Numerical Modeling of Unsteady Radiative–Convective Heat Transfer on a Flat–Plate Boundary Layer in a Selectively Emitting and Scattering Medium

A conjugation problem for radiative–convective heat transfer in a turbulent flow of a high–temperature gas—particle medium around a thermally thin plate is considered. The plate experiences intense heating from an outside source that emits radiation in a restricted spectral range. Unsteady temperature fields and heat–flux distributions along the plate are calculated. The results permit prediction of the effect of the type and concentration of particles on the dynamics of the thermal state of both the medium in the boundary layer and the plate itself under conditions of its outside heating by a high–temperature source of radiation.     

Planar gas flows with weak energy supply

Perfect gas flows in an unlimited space, which occur during rectilinear motion of a system of distributed heat sources, are investigated. The next modes in order of growth of the number M are examined: the heat conductive, convective, subsonic, transonic, supersonic, hypersonic. Examples of computations are presented. Flows with distributed heat sources attract ever-increasing attention. Such flows are important, e.g., in the problem of radiation propagation [1–5], in the analysis of a gasdynamic laser resonator and the optical characteristics of a ray [6]. Changes in the density because of absorption of the ray energy, which can result in an essential redistribution of the radiation intensity, are of great interest in these problems. Theoretical investigations of a general nature with distributed heat supply [7–10] are also important for the development of further applications. Gas flows for a given distribution of relatively weak heat sources switched on at a certain time are examined in this paper.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 95–102, September–October, 1978.     

Investigation of flow past blunt bodies with allowance for radiation by the large-particle method

Much recent work has been done on developing methods of solving gas-dynamic problems in which radiation plays a part (see, for example, [1–7]). This is because the temperature in the shock layer associated with flight in the atmosphere at hypersonic velocities can reach values exceeding 10⁴ °K. In such a case, heat transfer by radiation can make an important contribution to the total heat transfer. With increasing flight velocities, the importance of radiation in heat transfer increases and then becomes predominant. In the present paper, the large-particle method as developed by Belotserkovskii and Davydov [8] is developed to calculate flows with radiation around blunt bodies, including the case when there is distributed blowing from the surface of the bodies into the shock layer, which simulates ablation of a heat-shielding covering under the influence of strong heating by radiation. The results are given of systematic calculations of flow past blunt bodies of various shapes by a stream of radiating air, and the results are compared with the data of other methods.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 106–112, July–August, 1982.     

Heat and mass transfer at the surface of glass-graphite materials in a high-temperature flow of gas

The article discusses the mutual effect of vaporization and combustion processes during the breakdown of glass-graphite materials in a hypersonic flow of gas. It demonstrates the possibility of the appearance of a nonunique dependence of the vaporization rate on the temperature of the heated surface. The effect of the composition of a material on the principal characteristics of the process of its breakdown is established.Moscow. Translated from Izvestiya Akademii Nauk SSSR. Mekhanika Zhidkosti i Gaza, No. 6, pp. 71–87, November–December, 1972.