Optical Discharge Plasma Research Articles

Effect of an Electric Field on an Optical Breakdown in the Air Stream

Experimental data on the effect of an electric field on the plasma of an optical discharge in an air flow have been obtained. Two configurations of an external electric field under the action of an optical discharge on the plasma are considered. To create an electric field, flat (the field along the beam and across the flow) and ring electrodes (the field across the laser beam and along the flow) were used. It was found that there are two modes of combined discharge (optical and electrical). When the field was created symmetrically with respect to the flow axis, an electrical breakdown was observed from the nozzle exit (positively charged electrode) to the focusing point of the laser beam, while no streamers were observed in the optical discharge wake. In another case, an electric discharge is realized between flat electrodes simultaneously with optical breakdown. In a field of constant strength above 3 kV/cm, the presence of an optical discharge plasma promoted electrical breakdown of the medium. In this case, the parameters of the electrical breakdown depended on the shape of the electrodes, the polarity of the applied voltage, and the air flow rate.

Влияние внешнего электрического поля на оптический разряд в скоростном потоке

The influence of an electric field on the plasma of an optical discharge in subsonic and supersonic air flows has been studied experimentally. The presence of a weak electric field practically does not affect the size of the plasma formation, but, regardless of the configuration of the field lines and the polarity of the applied voltage, it leads to a decrease in the probability of optical breakdown. The experiment has shown that the plasma created by focused laser radiation is very sensitive to the presence of an electric field. When a voltage exceeding 22 kV was applied to the ring electrodes, powerful quasi-stationary streamers were formed in the flow. The presence of an optical discharge plasma made it possible to create an electric discharge in fields with an intensity below the breakdown threshold of the medium. The effect of quenching and the processes of development of an optical discharge were studied depending on the speed and characteristics of the electric field. Quenching of the optical discharge was observed when a voltage of 22 kV and higher was applied. Despite the preservation of the geometric dimensions of the optical discharge, the high-temperature region in the flow can be increased by using electric streamers. This leads to an increase in the energy supplied to the flow, and thus allows combustion to be initiated and flame stabilized at higher flow rates. Key words: experimental modeling, laser radiation, optical breakdown, electric field, electric discharge, sub- and supersonic air flow.

Influence of external electric field on optical breakdown in high-speed flow

The influence of an electric field on the plasma of an optical discharge in subsonic and supersonic air flows has been studied experimentally. The presence of a weak electric field practically does not affect the size of the plasma formation, but, regardless of the configuration of the field lines and the polarity of the applied voltage, it leads to a decrease in the probability of optical breakdown. The experiment has shown that the plasma created by focused laser radiation is very sensitive to the presence of an electric field. When a voltage exceeding 22 kV was applied to the ring electrodes, powerful quasi-stationary streamers were formed in the flow. The presence of an optical discharge plasma made it possible to create an electric discharge in fields with an intensity below the breakdown threshold of the medium. The effect of quenching and the processes of development of an optical discharge were studied depending on the speed and characteristics of the electric field. Quenching of the optical discharge was observed when a voltage of 22 kV and higher was applied. Despite the preservation of the geometric dimensions of the optical discharge, the high-temperature region in the flow can be increased by using electric streamers. This leads to an increase in the energy supplied to the flow, and thus allows combustion to be initiated and flame stabilized at higher flow rates. Keywords: experimental modeling, laser radiation, optical breakdown, electric field, electric discharge, sub- and supersonic air flow.

Spectral diagnostics of an optical discharge propagating along a hollow-core optical fibre

The results of spectral diagnostics of an optical discharge (OD) plasma propagating through a hollow-core revolver fibre under the action of repetitively pulsed laser radiation are presented. The revolver fibre is made of silica glass, with a diameter of the hollow core being 20 μm. The peak values of the laser radiation intensity reach 1012 W cm−2at an average Nd : YAG laser power of about 2 W. The time-averaged spectrum of the OD plasma in the visible range has the form of a continuum and is close to the blackbody spectrum with a temperature of 13.3 – 17 kK. The form of the spectrum does not depend on the gas filling of the fibre core (air or argon). In the UV part of the spectrum, lines of neutral silicon are observed, which are indicative of the partial evaporation of the reflecting cladding of the fibre.

Controlled processes at laser coating deposition

A description of the method is given of coating deposition under irradiation with a horizontal laser beam of a gas-dust medium which is an ensemble of micro-particles moving in the gas in the form of a free vertical jet. A number of coatings are obtained on various substrates and the results of measurements of their characteristics are presented. A setup based on a cw CO2-laser for the synthesis of diamond coatings in optical discharge plasma in the laser plasmatron mode has been created. The effect of spherical lens aberrations on the discharge maintenance thresholds is established. A diamond film was obtained on the surface of a tungsten substrate when a plasma jet was expelled into atmospheric air. An automatic system for measuring surface temperature in real time has been developed and created. The system was tested for laser cladding of metal powder in a pilot production.

Laser breakdown model in the absorption mode behind a light supported detonation wave

The light supported detonation wave (LSDW) propagation with the laser radiation absorption in a narrow layer behind wave’s front is considered in the paper alternatively to the volumetric energy source model usually used for the energy deposition simulation [1-7]. The next laser plasma flow features were revealed in [8] by methods of numerical simulation:1) laser plasma forms high-speed jet flow behind the LSDW front along the light beam propagation direction,2) jet parameters are close to an isentropic flow mode at significant distances from the LSDW front, therefor can be determined with a good approximation using the unsteady pressure solution for the point explosion model with a kinematic x-t transformation. These features allow one to determine the plasma momentum value (in the direction of a laser beam) of the optical breakdown as an additional factor of effect on gas flow, first indicated in [9]. For an argon flow, we used a comparative analysis of the results of numerical simulations obtained both accounting the absorbed energy only and the breakdown plasma momentum additionally acquired at the absorption of radiation behind the LSDW front.The experimental results of an optical discharge plasma in a subsonic argon flow are also presented. Pulse-periodic radiation with a frequency of 40 kHz and average power of 1.6 kW (peak power more than 30 kW) was generated by CO2-laser created in ITAM SB RAS. In order to obtain the LSDW mode by increasing a length of the breakdown plasma a focusing system f / d ≈ 9 was applied. A high-speed video camera with the exposure time of 1.0 μs, and the shooting speed of 200,000 frames / sec was used to record process. It has been established that the plasma dynamics has two successive stages: from the initial high-speed (of the order of 1 μs) propagation of the optical discharge to the subsequent lower-velocity gas-dynamic stage.

Synthesis of silicon carbonitride films by activation gas flow in laser power optical pulsating discharge plasma

A new method has been developed for the plasmochemical deposition of hard protective silicon carbonitride coatings from a hexamethyldisilazane (HMDS) Si2NH(CH3)6 vapor and HMDS+ benzene activated by a powerful optic pulse discharge (POPD) in a high-velocity gas flow of argon. The method allows depositing silicon carbonitride coatings with a rate of 0.5−1.2 μm·min−1 that is (1−5)·102 times higher than in conventional CVD processes. It has been found that coating deposition rate and structure of coatings depend on the process parameters: flow rates of HMDS and HMDS+benzene and plasma generating gas (argon). The method allows depositing SiCN coatings containing Si–C, Si–N and Csp3–N bonds with high velocity and microhardness 24 GPa.

Laser-Plasma Treatment of Structural Steel

To conduct high production hardening modification of iron-carbon and titanium alloy surface layers a laser-plasma method (LPM) is developed. The method is based on the use of optical pulse discharge plasma. A discharge is ignited with laser pulses repeated with a high frequency by a CO2-laser oscillator - amplifier system. A laser pulse is focused on the treated surface. To form plasma in alignment with the beam in the laser head, a high velocity gas flow (air, nitrogen, argon, and carbon dioxide) is created. The pressure of the plasma-forming gas can reach 0.5 MPa, and the output speed of the laser head can be 300 m/s.The results of the experiment on the impact of laser-plasma action on the structure and microhardness of the structural steel surface are presented. Laser-plasma treatment leads to the formation of a layer with the martensitic structure on the surface of structural low-alloyed steel 40Kh. This layer is formed due to quenching in a liquid state (QLS) and quenching in a solid state (QSS). The microhardness of the martensitic layer is 11-13 GPa, the hardened zone depth reaches 0.3 mm. It is proposed to use laser-plasma treatment of structural steel as a method for the local surface hardening of machine parts and tools.

Optical discharge with absorption of repetitive laser pulses in supersonic air flow: wave structure and condition of a quasi-steady state

We report a study of the wave structure formed by an optical discharge plasma upon the absorption of repetitively pulsed laser radiation in a supersonic air flow. Experimental data are presented on the configuration of the head shock wave and the geometry and characteristic dimensions of breakdown regions behind a laser plasma pulsating in the flow at a frequency of up to . The data are compared to calculation in a point explosion model with allowance for counterpressure, which makes it possible to identify the relationship between laser radiation and supersonic flow parameters that ensures quasi-steady-state energy delivery and is necessary for extending the possibilities of controlling the structure of supersonic flows.

Optical breakdown in supersonic air jet

The characteristics of a free supersonic air jet with a diameter of 10 mm and Mach numbers within 1.7–3.7 have been studied in the presence of energy supply via pulsed optical discharge generated by a periodic-pulsed mechanically-Q-switched CO2 laser. The coefficient of laser radiation absorption by optical discharge plasma in supersonic air jet has been studied and it is established that this parameter can reach 60%. It is demonstrated for the first time that a threshold density of air corresponding to sharp growth in the absorption efficiency amounts to 1.8–2 kg/m3 and is independent of the Mach number in airflow. Distributions of the gasdynamic parameters (including dynamic pressure and temperature) in supersonic air jet with energy supply via optical discharge have been determined.

Gas laser for efficient sustaining a continuous optical discharge plasma in scientific and technological applications

A stable high-power laser is developed for the study and technical applications of a continuous optical discharge (COD). The laser based on the technology of a combined discharge in a scheme with a fast axial gas flow emits 2.2 kW at 10.6 μm per meter of the active medium in continuous and repetitively pulsed regimes with the electrooptical efficiency 20%. The sustaining of the COD plasma in argon and air is demonstrated at the atmospheric pressure. The emission properties of the COD plasma are studied and its possible applications are discussed.

Destruction of silica fiber cladding by the fuse effect.

The phenomenon of destruction of silica fiber cladding by the fiber fuse effect has been observed for the first time to the authors' knowledge. Experiments on the optical discharge propagation along a fiber were conducted with fibers of decreased cladding thickness. The destruction of fiber cladding led to expansion of the optical discharge plasma and to a decrease of its density. This resulted in the termination of optical discharge propagation. The section of a fiber with decreased cladding thickness can act as a safety device to halt damage propagation.

Optical pulsating discharge plasma dynamics in a supersonic jet: Experiment and an analytical model of the quasistationary flow development

An analytical method for determining the wave structure and quasistationary flow parameters of an optical pulsating discharge plasma expanding in a supersonic flow is developed. The method is based on a physical model of the optical gas breakdown, a theory of the point explosion, the results of numerical modeling of the breakdown plasma dynamics along a laser beam, experimental data, and numerical estimates.

Laser equipment for piercing holes with a diameter of 0.01–0.04 mm

The application of laser radiation with ordered time and spatial structure, in particular, the transition to a pulsedperiodic laser effect, greatly widens the possibilities and increases the efficiency of laser treatment of metals. At the same time, the efficiency of pulsed laser radiation with a high power density (W > 10 W/cm) may be restricted by the formation of optical discharge plasma in air absorbing radiation. This circumstance greatly restricts the area of technological application of the most widely used Nd: YAG lasers operating in the Q-factor modulation regime. The situation greatly changes when metal and other materials are subjected to low-power laser pulses. A series of such pulses may be generated when several radiation pulses are formed during a single pumping pulse. This makes it possible to realise formation of effective erosion layers of plasma ensuring the displacement of the screening layer of air plasma from the zone of the effect, during the period separating consecutive radiation pulses. This is accompanied by increase of the depth, accuracy of dimensions and shape of the holes. The diagram of laser equipment for piercing holes with a diameter 0.01-0.4 mm is shown in Fig. 1. The master generator (MG) was represented by LTI-503 industrial laser with acousto-optical modulation of the Q-factor whose output power in the single-mode generation regime was 2 W. The repetition frequency of the pulses of the master generator was regulated in the range 3.8-50 kHz. In this case, the pulse time changed from 0.4 to 0.66 msec. Amplifier A was represented by the circuit for rotating the cross-section of the beam around optical axes ensuring the formation of beams of high optical quality. Focusing radiation on the component and examination of the enlarged image of the area of piercing the hole was provided by modernised industrial system SOK-1. The rapid selection of the area of deposition of the hole was carried out using focusing radiation of a He-Ne laser

The influence of residual stresses on cavitation resistance of metals — an analysis based on investigations of metals remelted by laser beam and optical discharge plasma

An analysis of the changes of residual stresses in metals as well as analysis of their performance under cavitation conditions contributed to enlight the problem of the influence of residual stresses on metal resistance to cavitation attack. The samples destined for testing were processed superficially with c.w. CO 2 laser beam or an optical discharge plasma; afterwards they were subjected to cavitation erosion in the rotating disk facility. Residual stresses were determined by interferometric methods.

Cavitation erosion of steels processed with laser beam and optical discharge plasma

The response of a solid state body to cavitation attack depends on impingement parameters as well as the properties of the eroded material. Changes to steel surface structural features intended to give cavitation resistance were induced by laser methods in order to test the usefulness of laser beam and optical discharge plasma surface remelting in protecting a 0·45% carbon steel and a 13% chromium steel against cavitation damage. Some light is also shed on the correlation between the performance of hardened layers during the incubation period of cavitation erosion and their physical properties.

Laser-supported detonation in a transverse magnetic field: A qualitative theory and direct conversion of laser radiation energy into electricity

This paper presents the results of a theoretical investigation of a laser-supported detonation wave (LDW) in a transverse magnetic field. A 1D problem of a steady-state wavefront structure is considered. The magnetic structure of the LDW is determined. Specific attention is focused on the stability conditions of a shock wavefront. The value of the induced (downstream) electric field strength is obtained from numerical calculations. The possibility of direct conversion of laser radiation energy into electricity in optical discharge plasma moving across an external magnetic field is considered. Such a device could be used in the energy supply systems of space vehicles.

Steel surface processing by a continuous optical discharge plasma

The influence of a continuous optical discharge (COD) plasma generated in flowing argon and argon-nitrogen mixture on alloy and carbon steel surfaces has been investigated. Changes in the structure, element composition, and microhardness of steel surface layers under the action of a COD plasma and laser beam were observed. In the presence of a COD plasma the microhardness distribution in hardened zone reaches its maximum at the metal surface and is different from that measured for samples processed by laser beam only.

Absorption of laser radiation in an optical discharge plasma in a gas mixture stream

The results of experimental investigations of the absorptivity of laser radiation energy in an optical discharge plasma are presented. The plasma was maintained in a stream of argon and nitrogen gases. The contribution of the mixture constituents to the laser beam absorption is considered. The importance of the obtained results for metal surface processing is discussed.

Influence of an optical discharge plasma on the stability of a CO2laser with an unstable resonator

An experimental investigation was made of fluctuations in the output power of a CO2 laser when laser radiation reflected from a target was returned to the unstable resonator. It was found that a small fraction of the laser radiation (less than 10−4) reflected by an optical discharge plasma, whose density was much lower than the critical density, led to strong fluctuations (of the order of 100%) in the laser emission. A theoretical model predicting the possibility of the appearance of the experimentally observed fluctuations in the laser emission was proposed.