Optical Pulsating Discharge Research Articles

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.

The influence of the thermal wake due to pulsating optical discharge on the aerodynamic-drag force

The influence of a thermal wake due to gas injection and due to a pulsating optical discharge (POD) on the aero-dynamic-drag force of a body in a supersonic air flow with Mach number M = 1.45 are experimentally examined. With the help of a single-component aerodynamic balance, the influence of the injected subsonic jet and the thermal wake produced by POD on the aerodynamic drag of a hemisphere-on-cylinder model was studied. It is shown that the observed aerodynamic-force reduction phenomenon can be made more pronounced by increasing the laser power and pulse repetition frequency, or by decreasing the distance between the model and the pulsating optical discharge. The maximum aerodynamic-force reduction (up to 15%) due to the thermal-wake action was observed at a maximum laser-radiation power of W = 2.3 kW and at a pulse rate of f = 90 kHz. The joint effect due to the argon jet and due to the POD caused an aerodynamic-drag force reduction reaching 30%.

New laser plasma process to obtain solid coatings and their structural characteristics

Carbon nitride coatings are synthesized in the new laser plasma of powerful optical pulsating discharge using acetonitrile as a precursor. A high-pressure and -temperature C3N4 cubic phase with a spinel structure type is obtained. The microhardness of coatings of this structure type is ~45 gPa. The chemical composition and structure of the coatings are characterized by a complex of spectroscopic (IR, Raman, EDS) techniques as well as microscopic (AFM and XRD) techniques.

Localisation of intense sound produced by an optical pulsating discharge in the air

It is shown that the localisation radius and the spectrum of intense sound produced by an optical pulsating discharge in the air depend on the power and repetition rate of the repetitively pulsed laser radiation, which is associated with a manifestation of the wave merging mechanism and sound absorption in the air. The experiment makes use of a CO2 laser with a power of ∼1.5 kW and a repetition rate of microsecond pulses of ∼50 kHz.

Study of spectral characteristics of radiation from a thermal wake of a pulsating optical discharge in a supersonic air flow

The spectral characteristics of the thermal wake of a pulsating optical discharge (POD) in a supersonic air flow are studied. The POD is stimulated by radiation of a mechanically -switched, repetitively pulsed laser with a pulse repetition rate of and a power up to . The flow is produced by means of the supersonic aerodynamic MAU-M setup having a conic nozzle with a critical cross-section size of , the Mach number being 1.3 – 1.6. We describe in detail the system of optical diagnostics that allows the detection of the spectrum of the weak thermal wake glow against the background of high-power POD radiation. The glow of the thermal wake is due to the emission of light by atoms and ions of nitrogen and oxygen, carried by the flow in the form of hot low-density gas clouds (caverns). The wavelengths of the thermal wake emission and the data on the transitions, corresponding to the spectral lines are presented.

Laser plasmochemical synthesis of hard protective SiCN films

A new method of laser plasma deposition had been applied to synthesise the silicon carbonitride (SiCN) coatings on stainless steel substrates and cutting devices using hexamethyldisilazane Si2NH(CH3)6 as a precursor. The precursor vapours were activated by powerful optical pulsating discharge in a high velocity gas stream (Ar). Characterisation of the films has been carried out by complex methods. The deposited coatings are amorphous, uniform in thickness and decorate surfaces of different engineering materials quite well. The nanohardness of SiCN coatings was estimated as 15–24 GPa.

Influence of an optical pulsed discharge on the structure of a supersonic air flow

We present the results of investigation of the parameters of an optical pulsed discharge (OPD) and their relation with gas-dynamic parameters of a supersonic flow and with characteristics of laser radiation. For the first time the discrete objects are detected in the OPD by an optical method, namely, low-density caverns moving along with the flow. The propagation velocity of the thermal track arising in a supersonic flow under the action of the OPD is measured. It is found that at a pulse repetition rate of the caverns unite into a single plasma jet.

Formation of an optical pulsed discharge in a supersonic air flow by radiation of a repetitively pulsed CO2 laser

Results of optimisation of repetitively pulsed CO2-laser generation are presented for finding physical conditions of forming stable burning of an optical pulsed discharge (OPD) in a supersonic air flow and for studying the influence of pulse parameters on the energy absorption efficiency of laser radiation in plasma. The optical discharge in a supersonic air flow was formed by radiation of a repetitively pulsed CO2 laser with mechanical Q-switching excited by a discharge with a convective cooling of the working gas. For the first time the influence of radiation pulse parameters on the ignition conditions and stable burning of the OPD in a supersonic air flow was investigated and the efficiency of laser radiation absorption in plasma was studied. The influence of the air flow velocity on stability of plasma production was investigated. It was shown that stable burning of the OPD in a supersonic flow is realised at a high pulse repetition rate where the interval between radiation pulses is shorter than the time of plasma blowing-off. Study of the instantaneous value of the absorption coefficient shows that after a breakdown in a time lapse of 100 — 150 ns, a quasi-stationary 'absorption phase' is formed with the duration of ∼1.5 ms, which exists independently of air flow and radiation pulse repetition rate. This phase of strong absorption is, seemingly, related to evolution of the ionisation wave.

To the space by laser light (II)

New approach to the problem of laser jet engine (LJE) creation is based on the use of shock waves (SW) resonance association mechanism, generated by the optical pulsating discharge (OPD). For the creation OPD is proposed to use powerful pulse - periodic (P-P) laser radiation with duration of pulses ~ 100 - 150 ns and high pulse repetition frequency ~ 50 - 100 kHz. OPD is formed with the help of matrix of reflectors (MR). This is allowed: several times to increase the effectiveness of laser emission use for purposes of LJE creation, to avoid strong impact loads on the apparatus, to exclude the thermal action of laser plasma on the reflector, to decrease the laser emission screening by plasma, to remove the problem of the resonance oscillations appearance into the object moved with the help of laser. In our article the estimations of possible LJE thrust levels under the conditions of the proposed mechanism of reactive motion are given. Also, the new approaches to the creation of the high power high repetition rate laser systems for the “Impulsar” program are examined.

Gas-dynamic effects in the interaction of a motionless optical pulsating discharge with gas

The effect of energy removal from the combustion zone of a motionless optical pulsating discharge in the horizontal direction along the axis of a repetitively pulsed laser beam producing the discharge is discovered. The directivity diagram of a hot gas flow is formed during the action of hundreds of pulses. The effect is observed for short pulse durations, when the discharge efficiently generates shock waves. For long pulse durations, the heated gas propagates upward, as in a thermal source.

Stationary force produced by an optical pulsating discharge in a laser engine model

An optical pulsating discharge produced by repetitively pulses laser radiation (with a pulse repetition rate of up to 100 kHz) is studied in a cylindrical tube simulating the reflector of a laser engine. The pressure of shock waves and the propulsion produced by them are measured. The discharge produced the stationary propulsion ∼1 N kW-1.

Laser engine based on the resonance merging of shock waves

A new approach to the development of a laser jet engine (LJE) is considered which is based on the resonance merging of shock waves generated by an optical pulsed discharge (OPD). The OPD can be produced by using high-power 150—250-ns, 20—200-J laser pulses with a pulse repetition rate of 50—100 kHz. The OPD is formed with the help of a reflector array. This makes it possible to increase the efficiency of utilising laser radiation by a few times, to avoid strong shock loads in the LJE, to eliminate the thermal action of a laser plasma on a reflector, and to reduce the screening of laser radiation by the plasma. The possible thrust of the LJE based on this mechanism is estimated.

Merging of shock waves produced by a moving pulsating optical discharge

Gas-dynamic perturbations produced by a pulsating discharge are studied. The mechanism of shock-wave merging and criteria of its manifestation are confirmed.

Doppler effect for an optical discharge source of shock waves

The Doppler effect for a moving pulsating optical discharge producing periodic shock waves is considered. The manifestations of the effect are limited by the wave merging mechanism. The validity conditions were found for the effect in the case of a pulsating source of shock waves.

Mechanism of shock wave merging in a laser jet engine

A new approach based on the mechanism of the merging of shock waves generated by an optical pulsating discharge (OPD) is considered for developing laser jet engines (LJE). It is proposed to use high-power repetitively pulsed laser radiation of small duration (150–250 ns), energy 20–200 J, and a pulse repetition rate of up to 100 kHz for producing an OPD. The formation of an OPD with the help of an array of reflectors allows a manifold increase in the efficiency of laser radiation used for developing an LJE and prevents severe shock loading of the engine, excludes thermal action of laser plasma on the reflector, and decreases the shielding of laser radiation by plasma. Possible values of the LJE thrust are estimated under conditions of the proposed mechanism of merging of shock waves into a quasistationary high-pressure wave.

Interaction of an optical pulsed discharge with a gas: conditions for stable generation and merging of shock waves

The conditions under which an optical pulsed discharge stably generates periodic shock waves are determined theoretically and experimentally. It is shown that the mechanism of merging shock waves into a low-frequency quasi-stationary wave is operative in various gases (and vapours) in a wide range of laser spark energies. The application of such a wave for increasing the coupling factor in a laser engine is considered.

Generation of a low-frequency wave by an optical discharge moving in a gas at a subsonic speed

A pulsating optical discharge moving in air at a subsonic speed (~250 — 300 m s-1) and creating in front of it a high-intensity low-frequency wave propagating in the same direction is considered. The range of dimensionless parameters in which this effect is manifested is determined.

Spectrum of shock waves produced by an optical discharge at a high laser-pulse repetition rate

The acceleration of a plasma jet by an optical pulsating discharge is studied. The optical discharge generated periodic shock waves or shock waves combined in periodic trains. For a pulse repetition rate f > 50 kHz, the line at frequency f dominated in the spectrum of periodic waves. The spectrum of trains following with the frequency F0 ≪ f contained an ultrasonic component and a low-frequency component. The trains produces a strong acoustic effect. The average power of the waves amounted to 160 W, and the conversion efficiency of laser radiation to shock waves was ~10% — 25%.

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.