Population inversion and radiation density in a Q-switched CO2 laser

The mechanism of lasing in a steady-state CO₂ laser has been investigated in [1–3]. In this paper we present a numerical analysis of the processes which occur in a CO₂ laser when the resonator is rapidly Q-switched. It is shown that the transition of the laser into the state with a new Q has an oscillatory form.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 6, pp. 18–23, November–December, 1972.We wish to thank R. I. Soloukhin for his interest and help.     

Some laws for precipitation of aerosols on a charged collector in the reynolds number range 10–100

Experimental data are presented on the efficiency of electrostatic precipitation of aqueous aerosol particles on a strongly charged sphere in the medium Reynolds number range (Re = 10–100). The asymptotic solutions for the problem are presented, and typical errors allowable in interpreting this type of experiment are discussed.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 190–193, May–June, 1976.     

Errors in stress measurements in soils under short-term loads

The results of experimental investigations of systemic errors in stress measurements in sandy soils by strain gauges under short-term loads produced by detonation of a plane charge are presented. The effect of rigidity of sensitive elements of the gauges and the effects of stress concentrations around the gauge body on the stress field measured are analyzed. A comparison of the experimental results with the theoretical calculations of [1, 2] is offered. It is shown that the systemic errors will not exceed ± 3–7% if certain easily achieved requirements with respect to the gauges are fulfilled. The question of evaluating systemic error in stress measurement in soils under low intensity static loads has been examined in [3–6].Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 146–154, July–August, 1972.In conclusion, the authors thank N. V. Zvolinskii and A. M. Skobeev for their valuable advice and evaluation of the study, and A. I. Kotov, V. P. Sutyrin, and L. G. Romanov for participation in the conduct of the experiments and processing of the results.     

Effect of a laser beam on plexiglas

An analysis is given of the optical effects during the focusing of a laser beam inside a transparent dielectric (plexiglas). Two types of damage were established. One is connected with the appearance of microcracks and the other with large plane cracks. Transition from one type of damage to the other is observed when the focal length of the lenses (energy density in the specimen) is varied and the pulse length is changed from 10^–3 to 10^–8 sec. The effect of a laser beam on metals and ionic crystals was investigated in [1–3]. A number of effects associated with structural changes and a specific material damage were found. There is also some interest in the effects of laser light on transparent dielectrics (glasses and polymers). In the present paper we report results of an investigation of the effect of laser beams on plexiglas.The authors are grateful to G. I. Barenblatt and B. Ya. Zel'dovich for valuable advice in formulating the problem and discussions of the results and to V. V. Kireev, G. F. Kuzmin, and O. E. Marin for assistance in the experiments.     

Modeling the dynamics of refractive index variation associated with the absorption of radiation of wavelength λ=10.6 mm by water vapor

The analysis of the dynamics of the changes in the relative index, necessary for determining the best conditions for ensuring minimum energy losses when CO₂ laser radiation is transmitted through the atmosphere, is investigated.Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.3, pp. 125–134, May–June, 1992.     

Simulations of hot, dense iron plasma opacity at 89 eV and comparison with experiment

Predictions of hot, dense iron plasma opacity at 89 eV photon energy are compared with experimental determinations from the transmission of laser-heated iron to extreme ultra-violet (EUV) laser radiation. The EUV laser was pumped using six beams of an Nd-Yag laser in a refraction compensating geometry, while another beam irradiated a tamped solid iron target with an intensity of 10¹⁴ W cm⁻². The Ehybrid hydrodynamic and atomic physics code was used to predict temperatures, densities and ionisation throughout the evolving iron plasma. The iron opacities were deduced taking into account free–free, bound–free and bound–bound absorption. Bound–bound absorption was considered using atomic data generated by the Opacity Project. Reasonable overall agreement between theory and experiment was obtained for the iron layer transmission. The simulations indicated the dominance of bound–bound absorption throughout most regions of the iron plasma, but also the potential importance of photoionisation from core levels where energetically possible.     

Calculation of the characteristics of a gas-dynamic laser

The dependence of the radiated power on the characteristics of optical cavities in the case of flow systems has been investigated in a number of papers [1–3], in which it is assumed that population inversion of the laser levels is obtained until entry into the cavity. The operation of a cavity is analyzed in [1] in the geometric-optical approximation with allowance for vibrational relaxation in the gas flow. A simplified system of relaxation equations is solved under steady-state lasing conditions and an expression derived for the laser output power on the assumption of constant temperature, density, and flow speed. The vibrational relaxation processes in the cavity itself are ignored in [2, 3]. It is shown in those studies that the solution has a singularity at the cavity input within the context of the model used. In the present article the performance characteristics of a CO₂-N₂-He gas-dynamic laser with a plane cavity are calculated. A set of equations describing the processes in the cavity is analyzed and solved numerically. Population inversion of the CO₂ laser levels is created by pre-expansion of the given mixture through a flat hyperbolic nozzle. The dependence of the output power on the reflectivities of the mirrors, the cavity length, the pressure, and the composition of the active gas medium is determined.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi FiziM, No. 5, pp. 33–40, September–October, 1972.     

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.     

Inversion of speckle interferometer fringes for hole-drilling residual stress determinations

Speckle interferometric fringe patterns record stress-relief displacements induced by the drilling of blind-holes into prestressed objects. The quantitative determination of residual stress state from such stress patterns is difficult because of the ambiguity in the order of the observed fringes. The plane stress magnitudes are provided directly from selected fringe positions using a stochastic, iterative least squares minimization approach. The inversion requires prior knowledge of the experimental geometry and an appropriate uniaxial stress-relief displacement basis function derived from three-dimensional finite element calculations. Superpositioning of the rotated and scaled displacement basis functions allows the stress-relief relaxation for any biaxial state of stress to be determined. In this paper, fringe patterns were forward modeled from a large ensemble of calculated biaxial stress-relief displacement fields. Inversion of these noise-free fringe patterns reproduced the biaxial stresses with negligible error. Analysis of more realistic fringe patterns that include speckle noise gave stress magnitude errors that diminished rapidly with the number of selected points to better than 3 percent for 100 points. Sensitivity of the optical method is influenced by a number of factors, but the ensemble of model fringe patterns studied indicates that the stress magnitudes (nomalized with respect to the material's Young's modulus) from 3×10^–4 to 10^–2 can accurately be determined with visible laser radiation. The method is amenable to automation and can easily be extended to study near surface gradients in the residual stresses or applied to other optical recording techniques such as moiré and phase-shifting interferometry.     

Experimental and theoretical/numerical investigations of thin films bonding strength

A laser spallation facility has been developed to measure the strength of planar interfaces between a substrate and a thin coating. This quantity is a central requirement in contemporary thin film and protective coatings technology and its successful measurement should improve the scientific/technological potential for the design of advanced composites, protective coatings of composites that operate in hostile environments, and in joining of dissimilar materials. The technique involves impinging a laser pulse of ultra short duration on the rear surface of the substrate, which is coated by a thin layer of energy absorbing metal such as Sn and Pb. The explosive evaporation of the metallic layer, confined between a fused quartz crystal and the substrate, induces a compressive shock wave, which propagates through the substrate toward the material interface. Upon reflection from the free surface of the coating, the pressure pulse is converted into a tensile wave which, under certain conditions, can lead to spallation at the interface. It is shown by mathematical simulation that atomic bond rupture is the mechanism of separation in this experiment. Since the interaction of laser energy with matter is a complicated, highly non-linear process, our investigations, at first, were based on measurement of the pressure pulse generated by the threshold flux level that leads to spallation, by using a micro-electronics device with a piezo-electric crystal, and on computation of the tensile stress experienced at the material interface, by numerical simulation of the induced stress wave propagation. Several substrate/coating (ceramic/ceramic and ceramic/metal) systems have been investigated such as, 1–15 μm SiC by CVD, 1–4 μm TiC and TiN by PVD coatings on sapphire substrates, as well as 1–2 μm Au, Sn and Ag coatings by sputtering on sapphire, fused quartz and glass substrates. For identically prepared specimens, the measured threshold energy levels are reproducible, thus leading to reproducible bond strength values, while the spall size, as expected, is dependent on the laser pulse energy level. Finally, the bond strength values obtained are in very good agreement with similar data derived by direct experimental techniques based on Laser-Doppler-Interferometry.     

Laser diagnostics of molecular states in nonequilibrium gas flows

Laser diagnostics techniques are developed, and some typical results are presented on the vibrational CO₂ molecule level population measurements under highly nonequilibrium conditions for two types of flows: (i) in a conventional gasdynamic lasers and (ii) in a flow mixing gasdynamic laser. The measuring procedure is based on recording spectral gain coefficient distributions at several resolved rotational transitions of different vibrational bands. The laser optical system with spatial selection of single lines is described. The system allows fast (about 10^–5–10^–4 s) line tuning, thus providing a variable and arbitrary choice of the selected line sequences. The optimum choice of the laser generation spectrum is discussed, and the sources of measuring uncertainties are analyzed. Typical illustrations and results are given and discussed.     

Energy characteristics of a carbon monoxide gasdynamic laser

The characteristics of gasdynamic lasers based on mixtures of carbon monoxide with nitrogen and inert gases were investigated and the populations of vibrational levels of CO molecules, the gain of the mixture, and the generation power were determined in [1–8]. But the parameters of a gasdynamic laser (GDL) in the optimum emission mode have not been determined up to now. The difficulties in calculating the optimum energy characteristics are connected with the complexity of the calculating model and the large number of parameters of the system. The energy characteristics of a CO gasdynamic laser are calculated and optimized in the present report on the basis of a simple model.Translated from Zhurnal Prikladnoi Mekhaniki Tekhnicheskoi Fiziki, No. 5, pp. 16–23, September–October, 1978.     

Closed-form solution of the direct kinematics of the 6–3 type Stewart Platform using one extra sensor

A procedure for the determination of the actual configuration of the 6–3 type Stewart Platform is presented. The procedure makes use of one extra sensor in addition to the six that provide the mechanism leg lengths. The use of one extra sensor suitably located on the mechanism allows the direct position analysis to be solved in closed form and makes it possible to obtain a one-to-one correspondence between the sensor measurements and the configuration of the mechanism in almost the entire workspace. Particular configurations where the correspondence is not one-to-one are outlined and criteria to identify them given. Sensitivity to sensor measurement errors is investigated.Paper presented at the 12th Italian Congress on Theoretical and Applied Mechanics AIMETA 1995, Napoli, 3–6 October 1995     

Measurement of the velocity profile in liquid streams of large volumes using a laser Doppler velocimeter

The use of the laser Doppler velocimeter (LDVM) in fluid mechanics already has a solid past history and work on its development is proceeding rather intensively [1, 2], This method has been used successfully to measure various parameters of flows in channels or chutes of small size [3–6]. Fundamental difficulties due to possible local variations in the index of refraction along the path of the laser beam [7], in addition to technical difficulties, have not been eliminated for its use in other cases of practical importance, particularly in basins or reservoirs of large size. The aim of the present paper is the investigation of the velocity structure of streams in large volumes using an LDVM on the example of a turbulent submerged water jet. In order to estimate the effect of the thickness of the water layer on the applicability of the method we tested three schemes: based on direct [8] and back scattering [9] and a scheme with reflection of forward-scattered light from a mirror [10]. The results of the investigation of a submerged turbulent jet using an LDVM are presented.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 170–173, September–October, 1977.     

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.     

Role of the Knudsen layer in the problem of droplet evaporation

A study is made of the part played by the Knudsen layer in the problem of weak unsteady evaporation of a spherical droplet in its own vapor. It is shown that use of the classical Hertz—Knudsen formula may lead to appreciable errors, in particular, in the determination of the time required by the droplet temperature to relax to the state corresponding to steady evaporation.Translated from Izvestiya Akademii Nauk SSSR; Mekhanika Zhidkosti i Gaza, No. 1, pp. 127–131, January–February, 1984.     

Shift of chemical equilibrium in a binary reacting mixture of molecular gases under the action of resonance laser radiation

At the present time, great attention is being paid to the problem of the final state of molecular gas systems originating from a steady external effect. This is associated, first of all, with intensive investigations of the properties of molecular gas lasers and also laser initiation of chemical reactions and the isotope separation. The theoretical feasibility is shown in [1, 2] for effectively shifting the chemical equilibrium in dissociation—three-particle recombination reactions, initiated in a single-component gas by resonance laser radiation. The purpose of this paper is to analyze the shift of chemical equilibrium in a binary mixture of molecular gases as a function of the intensity and nature of the laser pumping. It is well known [3] that the equilibrium constant of a chemical reaction can be expressed in terms of the dissociation equilibrium constant of the molecules participating in the reaction This factor permits the problem to be reduced to calculation of the dissociation equilibrium constants.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 3–9, January–February, 1978.The authors express thanks to B. F. Gordiets for numerous discussions of the results obtained and also to R. V. Khokhlov and P. K. Khabibullaev for support and interest in the work.     

Optimization of the parameters of a CO2 gasdynamic laser

In order to maximize the gain coefficient of a CO₂-N₂-He gasdynamic laser, a total optimization of the stagnation parameters, the concentration coefficient, and the parameters which determine the shape of the nozzle (assuming a quasi-one-dimensional flow model) was carried out. The dependence of the optimized parameters on the limitations imposed by the stagnation temperature of the flow and the critical cross section have been determined.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 3–8, September–October, 1974.     

Three-dimensional percolation from a bounded stratum to a single borehole

Three-dimensional percolation of a liquid from a nonuniform bounded stratum to a single borehole is considered. It is shown that the method leads to insignificant errors, even in those cases when the percolation zone differs substantially from the circular and the percolation differs from radial flow.Translated from Ivestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 122–127, November–December, 1971.     

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