The paper is devoted to the experimental study of the characteristics of continuous optical discharge (COD) sustained by high power continuous wave laser radiation at a wavelength λ = 1.08 μm in high pressure argon. New data on COD threshold laser power dependence on argon pressure in the range 20-50 bar are obtained. COD threshold laser power is shown to be in good agreement with the data obtained by other authors and theoretical evaluations provided the contribution of plasma energy loss due thermal radiation is taken into account properly. A study of the convective plume oscillations around COD in argon has been carried out. It was found that in a pressure range 25-35 bar the growth of the laser radiation power leads to a decrease in convection oscillation frequency from 33 to 29 Hz, while the radius of the convective plume grows accordingly. The oscillation frequency ν and characteristic radius of the convective plume r0 were found to obey the similarity relation previously established in experiments with COD in xenon.

Quiet optical discharge in high-pressure xenon is visually stable pre-breakdown stage of optical discharge supported by pulse-periodic short-wavelength infrared laser radiation with intensity of the order of 10(9) W/cm(2). Increasing the laser intensity leads to the transition of the quiet discharge into a pulse-periodic optical breakdown. In this study, quasi-stationary directional flows generated by both the quiet discharge and the optical breakdown are observed. The flows and their initial stages under limited number of discharge pulses are visualized by shadow imaging method using a point-like laser-plasma radiation source. It is shown that the gas streams outflow points in a quiet discharge correspond to the positions of the laser radiation intensity local maxima in the focal beam waist with astigmatism complicated by defocusing effect of thermal lens arising in the discharge zone. The pulse-periodic optical breakdown emits a turbulent gas flow toward the laser beam. The beam refraction on density gradients in the turbulent flow causes the breakdown instability from pulse to pulse. It was found that time-average absorption of laser radiation in a quiet discharge is about 3% (also in a pulse), while that in optical breakdown is 15% and higher (from 70% to 100% in a pulse). Laser beam intensity for quiet discharge at pulse repetition rate nu(p )= 20 kHz ranges from 1.3 x 10(9) to 1.5 x 10(9 )W/cm(2). At intensities above 4 x 109 W/cm(2) and repetition rates nu(p) > 50-55 kHz, laser breakdowns are observed in each laser pulse with the focal ratio f/d <= 7. At f/d = 10.6 and nu(p) > 28 kHz, the quiet discharge does not transit to laser breakdown due to defocusing by the thermal lens induced.

This publication presents an experimentally obtained video file for visualizing changes in the brightness of a radiation source based on a combination of continuous and repetitively pulsed optical discharges on a microsecond scale. The brightness was recorded with an electron-optical camera in the visible region of the spectrum in a nine-frame scan mode. A detailed description of the conditions and results of the experiments performed, the characteristics of the radiation source, as well as a discussion of the results obtained are published in [1, 2].

The problem of flight safety of high-speed aircrafts is primarily related to the reliability of engines, which, in turn, depends on the correct understanding of the processes of ignition and combustion of fuel in the combustion chambers of engines during their design. In this work, a new method for registration the ignition of shock heated gaseous fuels using a thermoelectric detector is proposed. The detector well measures the ignition delay time of fuels in the microsecond range, which is characteristic of detonation processes in the combustion chambers of promising aircraft engines operating on detonation combustion. The efficiency of the detector is demonstrated by the ignition of a propane-air mixture behind a reflected shock wave as an example. During the experiments, the thermoelectric detector showed such properties as the ability to register high heat flux values, low inertia, high signal-to-noise ratio, and high temporal resolution. The data obtained are compared with the measurement data of the ignition delay time by optical and piezoelectric methods obtained in this work, as well as in the studies of other authors.

In the article, an automatic system for radiation power control of lasers excited by а nonself-sustained glow discharge is studied using industrial lasers of the Lantan series as an example. They are designed for cutting, welding and surface modification of various materials as part of laser machines. The power of laser radiation is one of the most important parameters of a laser that determines its technological capabilities. The radiation power is controlled by changing the ionization pulses frequency of high voltage pulses with duration of 100 ns, given with a frequency of 1-5 kHz. The step response of the laser is experimentally obtained. Laser radiation is fed to a thermoelectric mirror-detector with thermo-EMF anisotropy, which measures its power. After preliminary amplification, the differential signal from the mirror-detector is recorded by a digital oscilloscope. A delay in the change in the laser radiation power relative to the control signal was established. The delay is 1487 ms that is explained by the fact that several ionization pulses are required for the initial exciting of the gas volume before the start of radiation generation. The initial section of the step response and damped oscillations are explained by the presence of a protective choke in the main discharge source connection circuit. The choke slows down the rise in current during the short circuit of discharge, allowing circuit breakers to turn off the power supply. To simulate the transient process, the step response of the oscillating circuit is used. The original signal was filtered to remove noise that does not al[1]low determining the parameters of the step response. To determine the spectrum of the step response, fast Fourier transform is carried out, frequencies introducing noise were cut out, and the inverse fast Fourier transform is performed. According to the step response obtained after filtering, the parameters of the modeling step response are determined. Based on the parameters of the step response, the laser transfer function is calculated. It makes possible to proceed to the calculation of the optimal radiation power controller, which ensures the best quality of the transient process.

The effects of subsonic plasma flows of carbon dioxide and pure nitrogen combined with additional laser irradiation on silicon carbide material were investigated. Silicon carbide samples were exposed to temperatures from 1480 to 1620°C (in the carbon dioxide plasma flow) and from 1600 to 1960°C (in the pure nitrogen plasma flow) at a constant pressure in the test chamber of 1 × 104 Pa and gas mass flow rate of 2.4 g/s. Significant differences in the heat transfer and behavior of the samples were observed depending on the plasma composition. The microstructure and surface profiles of the samples before and after different exposures were compared. Surface peculiarities were detected in the areas where the maximum laser irradiation intensity was applied.

The heat transfer to a cylindrical water-cooled copper model was experimentally investigated in an induction VGU-4 high-frequency (HF) plasmatron of the Institute for Problems in Mechanics of the Russian Academy of Sciences. The model, 30 mm in diameter, equipped with a calorimetric transducer with a heat-adsorbing graphite surface, 13.8 mm in diameter, was exposed to the surface heating in the combined regime by nitrogen plasma and laser radiation and in the cases of the heating with only laser radiation or a nitrogen plasma jet. The experiments in the HF-plasmatron jets were performed at the pressure in the setup low-pressure chamber p = 1 × 10 4 Pa, nitrogen mass flow rate G = 2.4 g/s, and the plasmatron HF-generator anode power N a.p. = 22 kW. It is established that in the chosen experimental regimes the dissociated-nitrogen jet and the high-frequency induction discharge do not produce a considerable effect on the laser beam passing through them. The values of the heat flux density are obtained as functions of the laser radiation power delivered. The subsonic nitrogen plasma flow in the quartz discharge channel and in the low-pressure chamber of the VGU-4 setup is numerically modeled under the experimental conditions basing on the solution of the complete Navier–Stokes equations using the Patankar–Spalding method.

The aim of the work was to determine the effective ultraviolet (UV) doses required for the disinfection of surfaces contaminated with the SARS-CoV-2 coronavirus using a low-pressure mercury lamp. Materials and methods. To carry out prompt disinfection of surfaces, a specially designed source of UV radiation with a power of 7.5 W at a wavelength of 254 nm in the form of a portable flashlight was employed, which has a high efficiency of UV radiation output and the possibility of long-term autonomous operation from a compact battery. In the studies, a suspension culture of the SARS-CoV-2 coronavirus with biological activity of 5.3∙106 PFU/ml was used. The objects of testing were plastic Petri dishes (disposable) and office paper (grade C, density 80 g/m2 ). Results and discussion. Doses of UV radiation that provide disinfection of surfaces contaminated with the COVID-19 pathogen with an efficiency of 99.0 % (paper) to 99.95 % (plastic) have been determined. The results obtained make it possible to recommend a portable UV irradiator for use in the practice of preventive measures to combat the spread of the disease caused by the SARS-CoV-2 coronavirus.

By now technologies, employing optical discharges find ever expanding applications in metering and diagnostic equipment in science, engineering and medicine. Based on the original experimental results authors look into some manifestations of spatial and temporal instabilities of the continuous and periodic pulse optical discharges (COD, POD). Set of the phenomena considered makes a great impact on a performance of laser produced plasmas essential for many applications, such as high brightness broadband light sources, for instance. Performance instability of continuous optical discharges followed by laser beam refraction on the refraction index gradients exhibit themselves first in spatial inhomogeneity of plasma thermal radiation luminosity and the other parameters related. Spatial inhomogeneity is accompanied by temporal instability of the plasma. The paper reports criteria for the appearance of instabilities related to the refraction followed by the limitations on supporting of elongated plasma. One of the main reasons of temporal instabilities of COD is thermal gravity convection. Instability of a heated gas zone surrounding optical discharge is followed by regular self-sustained oscillations leading in turn to pulsing of brightness and position of radiated plasma. Simple physical model proposed gives estimations correspondent to the observed pulse frequency dependence on gas pressure. In the case of periodic pulse optical discharges forced convection may be put forward as one of the main discharge instability reasons. Pulse optical discharge induces convective flows due to asymmetrical gas expanding following gasdynamic effect of the energy release zone shape determined by laser beam focusing system configuration.

The reasons for the observed propagation velocities of stationary laser-supported combustion (LSC) waves in laser plasmatron scheme in argon and air to exceed the calculated ones in assumption of heat-conductive propagation mechanism are considered. Earlier obtained analytical solution of the hydrodynamic problem of flowing around model low density heated gas volume with step-like spherical boundary is used for interpretation experimental results. It is shown that when laser power is 2–3 times above LSC threshold power heat-conductive mechanism with correction factor predicted by the model gives satisfying description of the LSC wave velocities observed. At higher laser power radiative heat transfer factor should be taken into account. It is shown that flowing around spherical hot gas boundary model can also be applied to describe gas flow in thermal gravitational convection around continuous optical discharge (COD). An estimate is given for the pulsation frequency of the convective plume from COD, leading to the similarity relation common for optical discharges and flickering flames.

The possibility of using a thermoelectric detector (TD) to measure the ignition delay time of a combustible mixture behind the front of a reflected shock wave is demonstrated. This device has already been used to record heat flows in shock tubes and is used for the first time to record the process of ignition of combustible mixtures. In the course of experiments with propane-air mixtures, it is found that the TD is capable of recording high heat flows, low inertia, high signal-to-noise ratios, and high time resolution. It is also shown that, due to the faster response time, the use of the sensor makes it possible to refine the ignition delay time of the mixture, and the sensor itself can serve as a useful device for studying the ignition processes of combustible mixtures in shock tubes in more detail.

The study is devoted to assessing the applicability of the manufactured thermoelectric sensor to measure pulsed heat fluxes in shock-wave processes. It is shown that the created thermoelectric sensor has fast response time and sufficient level of electric signal and can be successfully used in short duration high speed gas dynamic experiments.

Quasi-stationary flows under the effect of focused periodic pulse femtosecond laser radiation were generated and observed in liquid solvents: water, heavy water, alcohols, ketones, chloromethanes. The mechanism inducing directional flows appears to be directional collapse of the gas bubbles produced by multiphoton dissociation in a focused laser beam. Laser pulses of 450 fs length, up to 220 μJ pulse energy at repetition rates up to 10 kHz have induced stationary flows of liquid originated from the laser beam waist directed along or transversely to the beam axis. The streams along the beam axis were observed under low pulse power (10-20 μJ), provided precise lens adjustment. Lens displacement transversely to the beam axis led to splitting beam waist in two astigmatic foci. Both foci generate the streams along the beam axis. Counter directed streams have collided in the gap between foci, forming the flow spreading transversely to the laser beam. The increase of the pulse energy was followed by formation of the filament of self-focusing. Repeating cycles of focusing and defocusing along the filament produced several beam energy dissipation zones, each one generating separate streams along the beam axis. Colliding of the counter directed streams gave rise to complex flow pattern transversely and upward with respect to the beam axis.

One of the factors inducing instability from pulse to pulse of periodic-pulse optical discharge (POD) or continuous optical discharge with deep periodic-pulse pumping modulation, or combined POD (CPOD), is the excitation of resonant acoustic oscillations in a pressurized discharge volume. In this work authors studied the violation types of the regular pulsations of CPOD thermogravitational convection plume in a closed discharge volume with xenon at a pressure of 15-45 bar, arising at certain periodic-pulse modulation repetition rates in the range 1-50 kHz. It was found that simultaneously with the appearance of failures in the behaviour of the convective plume and plasma stability, which in some cases led to the extinction of the discharge, resonant acoustic oscillations were excited in the discharge volume. Schlieren patterns of the gas around the discharge and the frequency spectra of the excited acoustic vibrations were recorded. Several types of instabilities were found that correspond to different modes of resonant vibrations. It was also found that at certain acoustic oscillation frequencies the convection process was stabilized with the suppression of regular pulsations.

The article describes radiation power control of industrial CO2 lasers of Lantan series excited by а nonself-sustained glow discharge in the automatic mode. These lasers are closed-cycle fast gas-transport lasers excited by a nonself-sustained glow discharge with ionization by periodic-pulsed capacitively coupled auxiliary discharge. In this case, ionization and conductivity are provided by periodic-pulsed capacitively coupled discharge. The energy contribution to molecular oscillations is provided by the passage of the main discharge current through the plasma with electron density given by ionization. This permits easy laser power control, provides excellent optical homogeneity and stability of an active volume together with high laser efficiency. A system of a nonself-sustained glow discharge with ionization by periodic-pulsed capacitively coupled auxiliary discharge, the stages of creation and brief characteristics of the Lantan series lasers is presented. The method of controlling the power of laser radiation by changing the frequency of the ionization pulses is determined. This control method allows operating of the laser in continuous and in pulse-periodic modes with adjustable pulse ratio and pulse duration, and also provides switching from one mode to another. In the continuous mode, the radiation power is controlled by changing the frequency of ionization pulses, which are high voltage pulses with duration of 100 ns, given with the frequency of 1-5 kHz. Pulse-periodic radiation control is performed by modulating ionization pulses that consists of pulses being delivered in batches. The frequency of the pulses in a batch determines the radiation power in a pulse. The frequency of the batches following is the frequency of the pulse mode, and the length of the batch determines the pulses duration. Based on the experimental data, the dependence of the radiation power on the ionization pulses frequency was determined. An experimental system is presented and the measuring accuracy of the laser radiation power and the frequency of ionization pulses is determined. Data acquiring and processing of experimental results were performed using the NI 6008 USB data acquisition device in the LabVIEW programs of National Instruments. To study the dependence of the laser power on Мехатроника, автоматизация, управление, Том 21, № 4, 2020 231 the frequency of the ionization pulses, a regression analysis method was applied. Studies have shown that the dependence of the laser power on the ionization pulses frequency is linear in a wide range of parameters. The equation of the direct regression is calculated. The confidence estimates of the coefficients of the direct regression and the confidence estimates of the deviation of the theoretical direct regression from the empirical one are calculated with a confidence level of 95%.

This paper presents the results of numerical simulation of convective plume pulsations from a concentrated heat source equivalent to a continuous optical discharge (COD) in high pressure xenon compared to originally obtained experimental results. Simulated dynamic distributions of temperature, density and velocity of the gas around small spheroidal heat release zone are obtained, showing the process of formation of toroidal vortices in the convection zone, leading to the appearance of pulsations. The simulation results are qualitatively and quantitatively consistent with previously obtained experimental data.

Abstract Optical discharges find now ever expanding applications in science, engineering and medicine. In present study authors discuss the phenomenon of periodic pulsing of a continuous optical discharge (COD) on the base of original experimental results. In the literature there is no general agreement regarding the origin of the considered instability. Authors have obtained the dependence of oscillation frequency on the gas pressure and proposed semi-empirical relations to estimate pulse frequency pressure dependence observed. Regular self-sustained oscillations of COD plasma were explained by the pulsations of heated gas bubble surrounding COD followed by buoyancy driven vortex formation that was detected on schlieren images of convection zone. This kind of buoyant convection instability was determined to obey the same similarity law as cycling frequency of puffing or flickering in diffusion and premixed flames. The instability discussed can affect the performance of COD in some important applications such as high brightness broadband light sources, for instance.

The paper is devoted to generation and observing quasi-stationary flows under the effect of the focused pulse-periodic femtosecond laser radiation on liquid solvents: water, heavy water, alcohols, ketones, chlormetanes.The mechanism inducing directional flows appears to be directional collapse of the gas bubbles produced by multiphoton dissociation in a focused laser beam.Formation of the flows had being observed with schlieren technique and or framing small gas bubbles illuminated by visible laser beam converted to the line.Absorption coefficients of focused and unfocused femtosecond laser radiation in the fluids were also measured.In experiments with laser pulses of 450 fs at 1.023 nm wavelength with pulse energy from 10 to 220 uJ and repetition rate 1.43 ÷ 10 kHz stationary flows of fluids were originated from the laser beam waist directed along or transversely (in most cases) to the laser beam.The streams along the laser axis in both directions were observed under low pulse power (10 ÷ 20 uJ) provided precise lens adjustment without astigmatism or self-focusing.The tests show that single beam waist of 10 ÷ 20 um in diameter and 100 ÷ 200 um long generates two narrow jets along the beam axis in both directions from the waist.Lens displacement transversely to the beam leads to splitting beam waist in two astigmatic foci.Both foci generate axial flows directed toward each other that collide in the gap between foci, forming synthetic transverse flow.The increase of the pulse energy and focal point intensity were followed by nonlinear selffocusing forming filament like beam waist up to 14 mm long.Repeating cycles of selffocusing and defocusing produce several energy deposition zones along the filament beam waist, each one generating separate streams along the beam axis.Colliding of the contradirected streams gave rise to complex flow pattern along and transversely to the beam axis.Key words: laser-induced flows in

Quasi-stationary gas streams in argon (10 bar) were observed for the first time being generated by periodic-pulse optical discharge produced by laser pulses of less than 500 fs pulse length with energy up to 200 μJ/pulse and repetition rate 1.66÷10 kHz. Optical discharge was obtained in laser beam focused by off-axis (90°) parabolic mirror. In experiments the shape of the discharge zone was varied accordingly to the laser beam waist shapes varied from astigmatic to non-aberrated ones depending on the parabolic mirror tilt. Intense convective streams flowing out of the discharge volume were observed by schlieren technique. The gas streams produced could be directed normally to the laser beam axis, at some angle to the beam axis or along the beam axis toward the laser or in opposite direction. It was found that the directions of the streams produced, dynamics of their formation and their intensity were governed by the shape of the discharge zone. It was revealed that most intense and fast forming streams produced were directed normally to the laser beam axis. Two opposite streams are induced by the discharge located in astigmatic beam waist in a form of flattened “disk” ∼10 μm thick and ∼100 μm wide. The streams were directed normally to the “disk” surfaces. The energy spent on the gas flow acceleration was estimated to be up to 30% of thermal component of energy dissipated in plasma. When the focusing mirror was aligned to get no astigmatism, the gas flow generated was directed along optical axis toward the laser or backward in some cases. Refraction of the incident laser beam on the refraction index gradients of heated and excited gas injected by backward stream was followed by oscillations of the discharge zone location and generated stream direction. Discharge became stable when the gas streams were co-directional, normal or angled to the laser beam. Further studies are required to define mechanisms and possible applications of the phenomena observed.

Unusual convection flows were observed in stabilized pre-breakdown phase of the periodic-pulsed optical discharge (POD) called “quiet” POD. The discharge was a relatively weakly glowing plasma filament sustained by focused λ = 1.064 μm laser pulses with repetition rate of fr = 50÷100 kHz at the intensity several times below than that required for the optical breakdown to occur. No strong shock waves or irregular turbulence around the discharge were observed, in contrast to breakdown types of POD. Significant laser beam refraction measured in the beam cross-section behind the discharge zone was explained by the gas heating in the discharge up to 10 kK, providing high gradients of gas density and refraction index. Intense convective flow was detected on the schlieren images as thermal traces of the laser-induced gas streams flowing from the discharge zone, directed mainly normally to the optical axis. Repeated relaxation of the gas expanding after being rapidly heated by the laser pulse is proposed to explain the effect. The periodic-pulsed discharge located in the elongated beam waist generates an anisotropic heated region with gas streams and vortices, which may form the observed regular convective flow at the late stages of expanding.