Optical Discharge Research Articles

Gas flows generated by pulse-periodic optical breakdown and quiet optical discharge

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 109 W/cm2. 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 νp = 20 kHz ranges from 1.3 × 109 to 1.5 × 109 W/cm2. At intensities above 4 × 109 W/cm2 and repetition rates νp > 50–55 kHz, laser breakdowns are observed in each laser pulse with the focal ratio f/d ≤ 7. At f/d = 10.6 and νp > 28 kHz, the quiet discharge does not transit to laser breakdown due to defocusing by the thermal lens induced.

Properties of a continuous optical discharge sustained by short-wave infrared laser radiation in high pressure argon

Abstract 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.

Wear Mechanisms of the Working Surface of Gears after Scuffing Tests.

Identification of changes occurring on the working surface of lubricated gears using analytical equipment, e.g., an FE-SEM scanning electron microscope with an EDS microanalyzer, a WLI interferometric microscope, or a GDEOS optical discharge spectrometer, enables the characterisation of wear mechanisms of this surface. Definition of the phenomena occurring on the surface of tribo-couples after scuffing tests enables a comparative analysis of scuffing resistance and surface properties of the micro- and nanostructure, and elemental composition of the tested gears. Recognition and analysis of the wear mechanisms occurring on the working surface of gears will reduce the risk of damage and losses resulting from the need for maintenance and repair. The study concerned the working surfaces of gears made of 17HNM and 35HGSA steels on which a W-DLC/CrN coating was deposited. Shell Omala S4 GX 320 commercial industrial oil with a synthetic PAO (polyalphaolefin) base was selected for the lubrication of the gears. Tribological tests employed an FZG gear scuffing under severe conditions test method and they were carried out on a T-12U test rig for cylindrical gear analysis.

Pulsed THz radiation under ultrafast optical discharge of vacuum photodiode

In this paper, we first present an experimental demonstration of terahertz radiation pulse generation with energy up to 5 pJ under the electron emission during ultrafast optical discharge of a vacuum photodiode. We use a femtosecond optical excitation of metallic copper photocathode for the generation of ultrashort electron bunch and up to 45 kV/cm external electric field for the photo-emitted electron acceleration. Measurements of terahertz pulses energy as a function of emitted charge density, incidence angle of optical radiation and applied electric field have been provided. Spectral and polarization characteristics of generated terahertz pulses have also been studied. The proposed semi-analytical model and simulations in COMSOL Multiphysics prove the experimental data and allow for the optimization of experimental conditions aimed at flexible control of radiation parameters.Graphical

Using a coupled optical and electrical monitoring method to follow the R-HiPIMS TiO2 deposition process drifts

In this work, coupled optical and electrical discharge measurements have been implemented to investigate the plasma state of a reactive HiPIMS TiO2 deposition process running at a fixed duty cycle of 2% and at a repetition rate of 1 kHz. Investigations focus on both the effect of the erosion target and substrate-holder temperature in an Ar/O2 gas mixture at fixed working pressure. First, as the racetrack shape evolves with the use of the target, the deposition rate is modified, in the same way as the emission intensity measured. Second, with the heater set at 400 °C in pure Ar, the coating appears thinner, and the optical emission spectroscopy measurement reveals the presence of oxygen atoms. Thus, the results from the coupled optical and electrical measurements are sensitive enough to track potential drift of the process.

Non-Symmetry in the Shock Refraction at a Closed Interface as a Recovery Mechanism

The possibility of a shock wave recovery at a discrete closed interface with a heated gas has been investigated. A two-dimensional model applied to conditions of optical discharges featuring spherical, elliptical, and drop-like configurations demonstrated that non-symmetry in the shock refraction contributes to the specific mechanism of recovery other than simply its compensation. Even though the full restoration of the hypersonic flow state does not occur in a strict sense of it, clear reverse changes toward the initial shape of the shock front eventually take place, thus creating an appearance of a full recovery seen in experiments. From analysis of different interface symmetries, the factors determining the recovery dynamics are identified. The results are directly applicable to the problem of energy deposition into a hypersonic flow; however, it can be useful anywhere else where the flow modifications following the interaction are important. The dimensionless form of the equations allows applications on any scale other than that demonstrated for the optical discharges.

Optical Partial Discharge Detection and Diagnosis Method Based on PHOG Features

Optical Partial Discharge Detection and Diagnosis Method Based on PHOG Features

Efficient laser methods for producing spherical powders

A new process of laser gas atomization has been developed to obtain a spherical metal powder with a particle size of 30-100 μm using laser beams of conical geometry. A process for obtaining a spherical powder in a wide size range of 50 nm - 100 μm, in which a continuous near-surface optical discharge with a temperature of 20 kK is formed using conical laser beams in an inert gas flow, into which the material is introduced in the form of a wire or a powder flow has been designed. Particle condensation is strongly and rapidly quenched by the inert gas flow, resulting in high supersaturation. The efficiency of the proposed laser methods for producing powders is up to 0.5 kg/kWh at an electric power of 16 kW, while the existing, most efficient methods of plasma and gas spraying provide an efficiency of no better than 0.1-0.25 kg/h kW. The method can be used to obtain powders from various materials - metals, ceramics, plastics, suspensions. The new process of laser centrifugal atomization has been elaborated. Melting or evaporation of the end of a rotating hollow cylinder by a laser system ensures a highly efficient process for obtaining spherical powder in the nano and micro size range

Stabilization of the Hydrogen–Air Flame in a High-Velocity Flow by an Optical Discharge

Stabilization of the Hydrogen–Air Flame in a High-Velocity Flow by an Optical Discharge

Optical Discharge Regimes Sustained by a Continuously Pumped, Q-Switched Pulsed CO2 Laser

Optical Discharge Regimes Sustained by a Continuously Pumped, Q-Switched Pulsed CO2 Laser

Visualization of an Optical Discharge Behavior in the Combined Mode of Sustaining

Visualization of an Optical Discharge Behavior in the Combined Mode of Sustaining

Laser Cutting of Aluminum Alloys Using Pulsed Radiation from a CO2 Laser under Conditions of an Optical Discharge in an Argon Jet

Laser Cutting of Aluminum Alloys Using Pulsed Radiation from a CO2 Laser under Conditions of an Optical Discharge in an Argon Jet

GIS partial discharge data enhancement method based on self attention mechanism VAE-GAN

The reliability of geographic information system (GIS) partial discharge fault diagnosis is crucial for the safe and stable operation of power grids. This study proposed a data enhancement method based on a self-attention mechanism to optimize the VAE-GAN method and solve the problem of the lack of partial discharge samples and the unbalanced distribution between different defects. First, the non-subsampled contourlet transform (NSCT) algorithm was used to fuse the UHF and optical partial discharge signals to obtain a photoelectric fusion phase resolved partial discharge (PRPD) spectrum with richer information. Subsequently, the VAE structure was introduced into the traditional GAN, and the excellent hidden layer feature extraction ability of the VAE was used to guide the generation of the GAN. Then, the self-attention mechanism was integrated into the VAE-GAN, and the Wasserstein distance and gradient penalty mechanisms were used to optimize the network loss function and expand the sample sets to an equilibrium state. Finally, the KAZE and polar coordinate distribution entropy methods were used to extract the expanded samples. The eigenvectors of the sets were substituted into the long short-term memory (LSTM) network for partial discharge fault diagnosis. The experimental results show that the sample generation quality and fault diagnosis results of this method were significantly better than the traditional data enhancement method. The structure similarity index measure (SSIM) index is increased by 4.5% and 21.7%, respectively, and the average accuracy of fault diagnosis is increased by 22.9%, 9%, 5.7%, and 6.5%, respectively. The data enhancement method proposed in this study can provide a reference for GIS partial discharge fault diagnosis.

Computational and Experimental Modeling in Magnetoplasma Aerodynamics and High-Speed Gas and Plasma Flows (A Review)

This paper provides an overview of modern research on magnetoplasma methods of influencing gas-dynamic and plasma flows. The main physical mechanisms that control the interaction of plasma discharges with gaseous moving media are indicated. The ways of organizing pulsed energy input, characteristic of plasma aerodynamics, are briefly described: linearly stabilized discharge, magnetoplasma compressor, capillary discharge, laser-microwave action, electron beam action, nanosecond surface barrier discharges, pulsed spark discharges, and nanosecond optical discharges. A description of the physical mechanism of heating the gas-plasma flow at high values of electric fields, which are realized in high-current and nanosecond (ultrafast heating) electric discharges, is performed. Methods for magnetoplasma control of the configuration and gas-dynamic characteristics of shock waves arising in front of promising and advanced aircraft (AA) are described. Approaches to the control of quasi-stationary separated flows, laminar–turbulent transitions, and static and dynamic separation of the boundary layer (for large PA angles of attack) are presented.

Vertical asymmetric metal-semiconductor-metal photodiode based on β-Ga2O3 thin films fabricated via solution process for arc discharge detection

Arc discharge detection is a preeminent strategy for diagnosing aging of high-power electric equipment. Therefore, the timely detection of arc discharge can prevent immense disasters, such as massive fire accidents. Ultraviolet C (UVC) photodetectors (PD) are effective devices for optical arc discharge detection because of their solar blindness, which enables noise-free and highly selective arc discharge detection. In this study, we developed a solution-processed β-Ga2O3-based vertical asymmetric metal-semiconductor-metal (MSM) UVC PD as a high-performance and inexpensive UVC detector. The high bandgap of photoactive β-Ga2O3 (4.7–5.1 eV) enables selective UVC detection. The Schottky barrier heights at two different metal-semiconductor junctions (PEDOT:PSS/β-Ga2O3 and ITO/β-Ga2O3) were systematically modulated by varying the β-Ga2O3 film annealing temperature and thickness, which enabled fast photoresponse and self-powered operation, as demonstrated by various optical and electrical analyses. The optimized device exhibited a fast photoresponse (rise/fall time of 67/53 ms) and high responsivity (41 mA W−1) and detectivity (1.5×1011 Jones) in self-powered mode. The instantaneous arc discharge was successfully detected using the β-Ga2O3 based vertical MSM PD, demonstrating its feasibility for arc discharge detection. Thus, this study provides new insights into inexpensive self-powered PDs that can be implemented for the facile and prompt monitoring of aging electric equipment and prevention of potential fire accidents.

The Post-Shock Nonequilibrium Relaxation in a Hypersonic Plasma Flow Involving Reflection off a Thermal Discontinuity

The evolution in the post-shock nonequilibrium relaxation in a hypersonic plasma flow was investigated during a shock’s reflection off a thermal discontinuity. It was found that within a transitional period, the relaxation zone parameters past both the reflected and transmitted waves evolve differently compared to that in the incident wave. In a numerical example for the non-dissociating N2 gas heated to 5000 K/10,000 K across the interface and M = 3.5, the relaxation time determined for the transmitted wave is up to 50% shorter and the relaxation depth for both waves is significantly reduced, thus resulting in a weakened wave structure. The results of the extension into larger values of heating strength and the shock Mach numbers are discussed. The findings can be useful in the areas of research involving strong shocks interacting with optical discharges or other heated media on the scale where the shock structure becomes important.

Polarization sensitive terahertz all dielectric continuous trapezoid metasurface beam splitter

In order to improve the efficiency of beam splitting, we propose an all-dielectric metasurface for efficient beam splitting. When designing metasurface splitters, most of the research focuses on constructing devices using discrete element structures. However, the interaction between discrete elements leads to change on the phase distribution. Here, we propose a continuously distributed trapezoidal structure with phase gradient metasurface and use it to function as a terahertz wave splitter. The structure is much simpler than conventional metasurfaces based on arrays of discrete nanoantennas. We designed a series of beam splitters to achieve a deflection angle of 15°∼30° by adjusting the dimensions of the trapezoidal resonator. We further experimentally prepared the continuous phase gradient trapezoidal structures and measured their far-field scattering properties. Terahertz beam splitters are one typical device with wide applications ranging from terahertz communication to system integration, such as the spectroscopy, sensing, high-speed data transmission, terahertz imaging, soft ablation, optical discharge, and many others fields.

Initiation of Homogeneous Combustion in a High-Velocity Jet by a Joint Action of Optical and Electric Discharge

Initiation of Homogeneous Combustion in a High-Velocity Jet by a Joint Action of Optical and Electric Discharge

Initiation of Homogeneous Combustion in a High-Velocity Jet by the Joint Exposure of Optical and Electric Discharge

Initiation of Homogeneous Combustion in a High-Velocity Jet by the Joint Exposure of Optical and Electric Discharge

Hardening of surface of medical instrument by laser pulse optical discharge using methods of mathematical experimental planning

The purpose of this work is to study the effect of a nanosecond laser pulsed optical discharge on the properties of the surface of medical instrument steel. As a result of the work done, a scheme was developed for hardening materials and alloys, based on the local thermal effect of the shock wave of a laser pulsed discharge, which is distributed at supersonic speed in the atmosphere. The impact of a pulsed laser with a frequency of 5 to 20 Hz, with a duration of 10 ns, with a single pulse energy of 150 to 300 mJ was carried out on the surface of a metal scalpel. Using mathematical planning methods based on a full factorial experiment, it was obtained that the maximum hardness (600 HV) of the surface of a medical instrument is achieved in the mode E = 0,225 mJ and F = 5 Hz.