The spatio-temporal dynamics of plasma in a symmetrical dual-frequency 81 MHz/1.76 MHz capacitive discharge under the influence of a low-frequency 1.76 MHz field has been studied. Using phase-resolved optical emission spectroscopy, the dynamics of argon and xenon emission intensity in the plasma was obtained. Measurements of the electron energy probability function (EEPF) at the center of the discharge were performed using a Langmuir probe, and electron density measurements were made using a hairpin probe. The main result is the dynamics of the intensity ratios of selected argon and xenon lines under different conditions: at pressures of 40, 200, and 400 mTorr, low-frequency voltage amplitudes of 100, 200, and 400 V, and input power at 81 MHz of 3 and 15 W. The dynamics of high-energy electrons was investigated based on a two-temperature approximation of the electron energy probability function.

The objective of this paper is to conduct a comprehensive investigation of the thermal and gas dynamic flow fields generated during the interaction of pulsed volume discharge plasma with high-speed channel flow. A comparative visualization was carried out using high-speed infrared thermography and shadowgraphy techniques. We examined phenomena related to both plasma and gas dynamic interactions within a special test section of the gas dynamic channel. The spatial-temporal characteristics of the thermal fields associated with these plasma and gas dynamic interactions were analyzed, together with infrared radiation intensity diagrams. The dynamics of discontinuities and inhomogeneities resulting from the interaction of shock waves with the pulsed volume discharge plasma-referred to as discontinuity breakdown-were also investigated. We compared two physical mechanisms of energy conversion into infrared radiation recorded by the thermal imager in the range of 1.5-5.1 mu m. These mechanisms include low-temperature plasma emission from a sub-microsecond localized volume discharge and the sub-millisecond radiation from the inner surfaces of glass walls heated due to thermal conductivity at the interface with the gas flow boundary layer.

We study the spatial structure of nonstationary inhomogeneous supersonic airflows as shock wave diffraction on an obstacle occurs in a shock tube of a rectangular cross section. The Mach numbers of shock waves were 2.7–4.4 at initial air pressures of 10–30 Torr. The supersonic flow in the discharge chamber was visualized by high-speed shadowgraphy and by the registration of radiation of combined volume discharge by photo camera and by ICCD camera. In experiments, a combined volume discharge with a current duration of ~ 500 ns was initiated 40–150 μs after the initial shock wave have passed an obstacle. It has been established that the radiation of the volume phase of discharge lasts 400–700 ns, and the displacement of the flow during this time does not exceed 0.6 mm. A correlation is established between the spatial distribution of discharge radiation and the low-density local areas determined as a result of two-dimensional Navier-Stokes based numerical simulation of the flow. As visualized by the glow of the discharge, the shape of the shock wave front is in good agreement with the results of shadowgraphy at different stages of diffraction and with the numerical simulation results.

The aim of the work is an experimental and numerical investigation of the interaction between the pulse volume discharge with a high-speed flow in the rectangular profiled channel (obstacle on the bottom wall). The special type of combined discharge—pulse volume discharge with preionization by an ultraviolet radiation from plasma sheets—is used. The flow around the obstacle influences the pulse discharge plasma distribution. The short-pulse initiation of a high power discharge leads to the effects observable in the time range up to millisecond. Ultrafast local heating of the medium with the formation of blast (shock) waves is carried out during the creation of a high nonequilibrium sub microsecond pulsed plasma. The duration of the shock-wave effect of the pulsed discharge is from 20 to 120 μs in supersonic and transonic flow. The spatially inhomogeneous distribution of energy input in a supersonic flow associates with the density lowest areas, which occur in a gas flow regime in a channel with an obstacle on the bottom. Discharge localization regions are sources of more intense wall surface local heating observed in the infrared range. A numerical calculation is carried out in order to match the calculated and experimental gas dynamical configurations.

The results of studies of the effect of volume and surface pulse discharges on high-speed gas flow in a rectangular shock-tube channel with a change in the profile (obstacle on the lower wall) are given. A single nanosecond surface discharge or a discharge with preionization induced by the plasma electrodes (combined discharge) was initiated in flow downstream of the shock wave with the Mach numbers Ms = 3.2–3.4. The obstacle determines the distribution of the parameters of flow past the obstacle and the pulse discharge plasma redistribution. The density fields of gas dynamic flow under the experimental conditions are obtained and compared with the discharge plasma distribution. It is shown that the shock-wave effect of the discharge on flow behind the obstacle continued from 25 to 70 µs.

A quantitative study has been made of the flow with shock waves generated in air by a sliding surface discharge lasting less than one microsecond. The high-speed flow was visualized using the shadowgraph method, the process was recorded at a rate of 124 000 frames/s, the exposure time was 1 μs. The aim of this work is to study the dynamics of the two discontinuities: the cylindrical shock wave and the contact surface generated by the discharge. Each experiment allowed several hundred images to be taken of a short-lived gas-dynamic process lasting up to 1 ms. A YOLOv8 convolutional neural network was trained and used to determine the positions of the discontinuities. A data set of 984 markups was labeled. The model on the mAP50 metric achieved 0.887 and the mAP50-95 was 0.557. The model was used to automatically measure the vertical dimensions of the contact discontinuity. It expands at times up to 0.4 - 0.8 ms to a vertical size of 5 - 11 mm. The x-t plots and the velocities of the cylindrical shock waves were measured. It is shown that up to 1 ms after the discharge, the flow development is due to the blast wind motion behind the shock wave. It is shown that the use of computer vision can significantly speed up the analysis of high-speed flow visualizations and the extraction of quantitative information.

Optical spectroscopy and microscopy techniques are widely used for basic studies of living systems. However, their application in clinical practice has two fundamental limitations. First, the depth of probing biological tissues with light is small and varies from tenths to several of millimeters. Secondly, it is difficult to use exogenous labels, which increase the sensitivity and specificity of pathological tissue detection, in vivo measurements on patients. This raises the question of the place of biomedical photonics among other physical diagnostic methods used in clinical practice. This article presents an review of optical methods and relatively new certified commercially available medical devices that use photonics to solve intraoperative diagnostic problems, i.e., the discrimination between pathological and healthy tissue sites in vivo and ex vivo by an endogenous optical response. This work discusses a wide range of medical fields in which researchers and engineers have been able to achieve high rates of sensitivity and specificity in solving the problem of classifying such tissues. The advantages and disadvantages of optical imaging and diagnostic methods, which determine their place in clinical practice, are discussed by the example of intraoperative diagnostics.

The present work demonstrates the possibility of contactless registration of changes in the thermal radiation of the metal outer surfaces of shock tunnels during the formation and passage of high-velocity gas-dynamic flows with planar shock waves inside the channels. The representative curves of the temperature response of the channel outer wall to thermal processes inside the shock tunnels have been studied. The dependence of the parameters of these curves on the shock wave Mach number (in the range of 2.0–4.5), the position of the detection zone (10–90 calibers from the diaphragm), and the thickness of the copper walls of the channel (1.5–2.0 mm) were also studied.

This paper presents the results of studies on unsteady gas-dynamic flows up to six milliseconds in duration in the shock tube channel. The results were acquired by continuous high-speed shadowgraphy and subsequent big data processing based on machine vision (edge detection and the Hough transform) and machine learning (convolutional neural networks). The evolution of flows with discontinuities in the rectangular shock tube channel behind a shock wave at Mach numbers 2–3.5 was studied with a recording frame rate of 150 000 fps. The time dependence of the angle of oblique shock inclination was plotted, and the time for the flow to reach a subsonic regime was estimated. This paper shows the possibility of conducting research in gas dynamics based on big data analysis of digital recordings using the approach suggested.

We report on the effect of nanosecond surface sliding discharge glow redistribution near a dielectric ledge and high-speed post-discharge flow dynamics. The discharge energy localization is shown to be supplementary to plasma glow inhomogeneity along the surface. The process is studied in a discharge chamber with two $$100 \,\mathrm{mm} \times 30 \,\mathrm{mm}$$ surface sliding discharges (plasma sheets) placed on the top and bottom walls and a dielectric ledge, $$48\,\mathrm{mm} \times 6 \,\mathrm{mm} \times 2 \,\mathrm{mm}$$ in size, mounted on the bottom plasma sheet. The dynamics of the discharge-induced flow is captured using high-speed shadowgraphy during the first 40–50 $$\mu \mathrm{s}$$ after the discharge ignition. Computational fluid dynamics (CFD) simulations of the induced flow are also conducted to gain more insight into the energy release area configuration. Based on the numerical and experimental shadow images matching, the pulsed discharge energy redistribution is quantitatively analyzed.

To develop low-cost, high-performance light rare earth element (LRE)-substituted Nd–Fe–B permanent magnets, we systematically studied the effect of substituting Nd with 20% light rare earth elements (LRE = Ce, Y, and La) on the hard magnetic properties of (Nd0.8LRE0.2)–Fe–B hot-deformed magnets. The (Nd0.8Ce0.2)2Fe14B magnet showed a coercivity of μ0Hc = 1.41 T, which is comparable to that of an LRE-free magnet. Y substitution was found to decrease μ0Hc and μ0Mr to 1.22 and 1.32 T, respectively, as was expected from the intrinsic properties of (Nd0.8Y0.2)2Fe14B. Unlike our expectation, the thermal stability of coercivity is not degraded by the Y substitution, demonstrating its potential for elevated temperature applications. With La substitution, coercivity is substantially degraded to 0.67 T because of the low intrinsic properties and the lack of the intergranular phase. The Y and Ce co-substituted (Nd0.8Ce0.1Y0.1)2Fe14B hot-deformed magnets showed better thermal stability of coercivity compared to the (Nd0.8Ce0.2)2Fe14B sample.

Digital imaging became one of the main tools for studying unsteady flows. Modern high-speed cameras support video recording at high frame rates which makes it possible to study extended high-speed processes. We demonstrate here different animations: water temperature field evolution with a frame rate of 115 Hz; high-speed shadowgraph visualisation of different flows - water jet formation process (100 000 frames / s), shadowgraph animations of the shock waves created by the pulsed discharges (124 000 frames / s). Also, as an example of plasma flow visualization technique, we offer 9 sequential images of the shock wave - pulse gas discharge visualization obtained by the high-speed CCD camera with the 100 ns delay between frames. We developed in-house software based on the machine vision and learning techniques for automatic flow animations processing. The examples of the automatic oblique shock detection using Canny edge detection and Hough transform and thermal plume detection based on the pre-trained convolutional neural network are provided and discussed.

An experimental study was made of the dynamics of shock waves developing from a localized channel of a nanosecond surface sliding discharge in supersonic airflows past a wedge in the shock tube with Mach numbers 1.16–1.47 at a density of 0.02 kg/m3–0.20 kg/m3. It is shown that a semi-cylindrical explosive-type shock wave is formed from the discharge channel in the low-density vortex zone behind the wedge. Numerical simulations of the resulting gas-dynamic flow are performed. The shadowgraphy images of shock waves after the discharge were compared with simulations of the flow field after an energy input near the wall in a supersonic flow. The energy input region in the simulations was set in accordance with the experimental results on the geometry of the discharge channel. Based on a comparison of experimental results and simulations, it is shown that shock waves are formed when a heat energy of 0.07 ± 0.04 J is released in the channel of localized discharge in supersonic airflows.

The characteristics of a distributed sliding surface discharge with a duration of ∼300 ns (plasma sheet) in a non-uniform supersonic airflow with a vortex zone behind a thin wedge have been studied in a shock tube channel. The spatial distribution of the discharge radiation, the spectra of the discharge radiation, and the discharge current are analyzed in the flows behind plane shock waves with Mach numbers 2.4–3.5 (the Mach numbers of flows are 1.16–1.47 and the density is 0.02–0.20 kg/m3). It is shown that in an airflow with a vortex zone, the surface discharge develops as a 1–3-mm-wide channel located in the region of low density behind the wedge. The calculated electron concentration in the discharge channel is an order of magnitude higher than the electron concentration when a discharge is initiated in a homogeneous medium.

The development of a pulsed volume discharge with pre-ionization in a non-uniform supersonic air flow near the wedge in the shock tube channel and the gas-dynamic flow that occur after the discharge has been experimentally studied. Plasma configurations that occur near the bottom of the wedge were investigated for the discharge initiated at various stages of an unsteady flow. The self-localization conditions of a volume discharge in the vortex zone behind the wedge are analyzed at the incident shock wave Mach numbers of 2.6–3.5 (at the flow Mach numbers up to 1.50 and a density up to0.15 kg/m 3 ).

An analysis of plasma dynamic processes and shock waves interactions in supersonic jet at plasma spherical formation initiation was made using the high-speed digital image recording. The dynamics of the spherical plasmoid which creates discontinuities, affecting the bow shock wave in front of the model was investigated with a high temporal resolution. It was shown that during the time of plasmoid electric current (about 100-130 microseconds) the structure of supersonic flow around the model changes: the shock layer is transformed, bow shock wave detached distance on the symmetry axis is significantly increased due to a change of oncoming flow parameters and structure.

Plasmadynamic processes and rapid shock-wave processes accompanying the generation of plasma formation in a supersonic jet flow are analyzed on the basis of high-speed digital recording of images. We analyze (with a high time resolution) the dynamics of spherical plasmoid formation, discontinuities generated by it, and their action on the bow shock wave in front of the model. It is shown that during the plasmoid lifetime (about 100–120 μs), the regime of the supersonic flow past the model is modified: the shock layer is rearranged, and the departure of the bow wave from the symmetry axis substantially increases due to a change in the structure and parameters of the incoming flow.