Dielectric Targets Research Articles

Beam-generated plasma formation near a dielectric target irradiated by a pulsed electron beam in the forevacuum pressure range

We have investigated the formation of electron beam-generated (EBG) plasma near a dielectric (ceramic) target and an insulated metal target, and the compensation of the negative charge accumulated on the insulated metal target when the targets are irradiated by an intense pulsed electron beam in the forevacuum pressure range (4–15 Pa). It is shown that the density of the EBG plasma near the irradiated ceramic target (or the irradiated insulated metal target) is greater than the plasma density for a beam propagating freely in a vacuum chamber (or for the grounded metal target). The EBG plasma near the target is formed with a certain delay with respect to the electron beam current pulse, because of which the negative potential of the insulated target is also compensated by a delay. The delay time in the formation of the EBG plasma and in the compensation of the target negative potential decreases with increasing gas pressure. Expressions have been proposed for estimating this delay time.

Laser-induced fluorescence measurements of spatially resolved OH densities in a helium plasma jet interacting with three different targets in the open air

The presented research work is devoted to the measurements of the absolute densities of ground state OH radicals in an atmospheric pressure helium plasma jet launched in free air onto three different targets. The OH density is measured by using laser induced fluorescence (LIF). The plasma is generated inside a glass tube by a dielectric barrier discharge powered by a mono-polar pulsed high voltage. Estimation of the absolute density is based on a quantitative analysis of the spectral and temporal component of the LIF signal and does not require any assumption on the densities of colliding species. Based on this analysis, the spatially-resolved density distributions of OH radicals are determined in front of three different targets (metallic, dielectric, and ultra-pure water) placed in the open air at 2 cm from the exit of the glass tube. In order to also investigate the temporal behaviour of the OH densities, and the influence of the gas flow rate and of the amplitude of the voltage-pulse, planar LIF has been performed investigating the fluorescence signal (i) during the intervals of the discharge pulses, (ii) for various gas flow rates, and (iii) as a function of the amplitude of the voltage pulse. In the free jet configuration without a target, the OH densities were determined to be in the order of , whereas they increased up to a factor of 10 in contact with a target. The spatial distributions were also influenced by the dielectric properties of the target: in the case of the metallic target, the maximum of the OH density was found to be in the center between the exit of the glass tube and the target, whereas in the case of the dielectric target, the maximum was found to be close to the surface of the target. In the case of ultra-pure water, the densities were equally distributed between the exit of the glass tube and the target. With this work, the influence of a target on the production of OH radicals was demonstrated, which enables the optimisation of the plasma parameters for an efficient production of OH radicals in front of a target with various dielectric properties.

Neutralization of the Surface Charge of an Insulated Target under the Interaction of High-Energy Metal Ion Beams

The interaction of ion beams with dielectric materials is an urgent problem, both from the point of view of practical application in ion implantation processes and for understanding the fundamental processes of charge compensation and the effective interaction of beam ions with a target surface. This paper presents the results of studies of the processes of compensation of the surface charge of an insulated collector upon interaction with a beam of metal ions with energies up to 50–150 keV. At low pressure (about 10−6 torr), removing the collector from the region of extraction and beam formation makes it possible to reduce the floating potential to a value of 5–10% of the total accelerating voltage. This phenomenon allows for the efficient implantation of metal ions onto the surface of alumina ceramics. We have shown that the sheet resistance of dielectric targets depends on the material of the implanted metal ions and decreases with an increase in the implantation dose by 3–4 orders of magnitude compared with the initial value at the level of 1012 Ω per square.

Composite Backscatter Characteristics of Conductive/Dielectric Ships and Sea Surfaces with Breaking Waves under High Sea Conditions.

When a radar detects marine targets, the radar echo is influenced by the shape, size and dielectric properties of the targets, as well as the sea surface under different sea conditions and the coupling scattering between them. This paper presents a composite backscattering model of the sea surface and conductive and dielectric ships under different sea conditions. The ship scattering is calculated using the equivalent edge electromagnetic current (EEC) theory. The scattering of the sea surface with wedge-like breaking waves is calculated using the capillary wave phase perturbation method combined with the multi-path scattering method. The coupling scattering between ship and sea surface is obtained using the modified four-path model. The results reveal that the backscattering RCS of the dielectric target is significantly reduced compared with the conducting target. Furthermore, the composite backscattering of the sea surface and ship increases significantly in both HH and VV polarizations when considering the effect of breaking waves under high sea conditions at low grazing angles in the upwind direction, especially for HH polarization. This research offers valuable insights into optimizing radar detection of marine targets in varying sea conditions.

Electron-beam synthesis of ceramic- and boron-based coatings

We report the results of our experimental investigations on the synthesis of coatings based on various ceramics and boron. The coatings were synthesized by electron-beam evaporation of solid dielectric targets of Al2O3, YSZ, Al2O3Cr, and B. The deposition rates for coatings based on these materials were measured. The deposition rate of defect-free coatings was 1200–1600 nm min−1. The obtained coatings, up to 5 μm thick, were studied in terms of their morphology, structure, and tribological properties. The coatings are uniform in thickness, composition and free of impurities. The coatings, with the exception of the zirconium oxide film, have a predominantly amorphous structure. Comparisons of their mechanical and tribological characteristics are presented. The hardness of the coatings formed from YSZ was 0.4 GPa, from aluminum oxide ceramics 4–6 GPa, and from boron 11 GPa. Boron-based coatings were found to be the most wear-resistant (with wear coefficient about 8.9·10−6 mm3 N−1·m−1).

Deposition of layered iron-nickel-alumina ceramic magneto-dielectric thin films using a forevacuum electron source

We present the results of our investigation of layered micrometer-thin magneto-dielectric Fe–Ni–Al2O3 coatings deposited by successive electron-beam evaporation of metal (Fe, Ni) and dielectric (alumina ceramic) targets in an inert gas atmosphere (helium) at forevacuum pressure (3.5 Pa). The low-lying alumina ceramic layer provides the dielectric properties (real parts of the permittivity of 4.34 ± 1.8 and conductivity of 0.143 ± 0.001 S/m), while the magnetic properties are provided by the nickel and iron layers. Studies of the magnetic properties using a ferromagnetic resonance technique show that the effective saturation magnetization of the coatings is 5.5 ± 1.0 kG, which is close to the known value for a fully-metallic nickel layer (6.05 kG).

Near-Field IPO for Analysis of EM Scattering from Multiple Hybrid Dielectric and Conductor Target and High Resolution Range Profiles

Aiming at improving the accuracy and efficiency of scattering information from multiple targets in near-field regions, this paper proposes a near-field iterative physical optics (IPO) method based on a modified near-field Green’s function for the composite electromagnetic scattering analysis of multiple hybrid dielectric and conductor targets. According to the electric field and magnetic field integral equation, the electric and magnetic current were updated utilizing the Jacobi iteration method. Then, by introducing an expansion center lying in the neighborhood of the source point, Green’s function was modified for near-field scattering between multiple hybrid dielectric and conductor targets. To accelerate the implementation of the procedure, the multilevel fast multipole method, the fast far-field approximation, and parallel multicore programming were introduced. Numerical results indicate that there is good agreement between the results calculated by the near-field IPO method and MLFMM solver in commercial software FEKO while significantly reducing the computational burden. To fully exploit the scattering information, the high resolution range profiles (HRRP) of different targets under different conditions were analyzed, which can be further applied for automatic target detection and recognition.

Interaction of atmospheric pressure helium plasma jet with tilted dielectric target: split and deviation of ionization waves

Atmospheric pressure plasma jets (APPJs) are commonly used plasma sources in many biomedical applications. Typically, the APPJs are not always perpendicular to the target to be treated. In this paper, the effects of tilted angle (), relative permittivity () of the target, the target potential, the distance () and the discharge parameters on the interaction of a helium APPJ with the target are investigated. Spatio-temporal resolved images show that the ionization wave (IW) split into a surface ionization wave (SIW) and a continued propagate IW when it approaches the target. The split leads to a position deviation of the IW interaction with the target. The measurements show that the split and deviation of IWs are strongly dependent on and . For 9, the split can be only observed with 60°. The deviation gradually decreases as increases from 15° to 90° with 9. With the increase of from 9 to 56, the split and deviation are more pronounced with = 30°. The deviation gradually increases as applied voltage and values increase from 5.5 to 10.0 kV and 19–24 mm, respectively. However, the deviation is independent of the gas flow rate of 400–2000 standard cubic centimeters per minute. The split of IW depends on the combined effect of enhanced electric field due to target polarization and mismatch between the propagation direction of IW and the direction of enhanced electric field caused by variation. An increase in results in a stronger dielectric polarization effect. Changes in cause a misalignment between the propagation direction of IW and the direction of enhanced electric field. Additionally, decreasing shortens the perpendicular distance () between IW and the target surface, further enhancing the electric field in the perpendicular direction. After the split, there is an interaction between the SIW and the IW. The propagation of the SIW is restricted to the local electric field of IW.

Transient Laser-Induced Breakdown of Dielectrics in Ultrarelativistic Laser-Solid Interactions

For high-intensity laser-solid interactions, the absolute density and surface density gradients of the target at the arrival of the ultrarelativistic laser peak are critical parameters. Accurate modeling of the leading edge-driven target preexpansion is desired to strengthen the predictive power of associated computer simulations. The transition from an initial solid state to a plasma state, i.e., the breakdown of the solid, defines the starting point of the subsequent target preexpansion. In this work, we report on the time-resolved observation of transient laser-induced breakdown (LIB) during the leading edge of high-intensity petawatt-class laser pulses with peak intensities of up to 5.7×1021W/cm2 in interaction with dielectric cryogenic hydrogen jet targets. LIB occurs much earlier than what is typically expected following the concept of barrier suppression ionization. The observation is explained by comparing a characterization study of target-specific LIB thresholds with laser contrast measurements. The results demonstrate the relevance of the laser pulse duration dependence of LIB for high-intensity laser-solid interactions. We provide an effective approach to determine the onset of LIB and thereby the starting point of target preexpansion in other laser-target systems.Received 28 July 2022Revised 2 December 2022Accepted 16 December 2022DOI:https://doi.org/10.1103/PhysRevApplied.19.014070Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHigh intensity laser-plasma interactionsLight-matter interactionPlasma production & heating by laser beams, laser-foil, laser-clusterPhysical SystemsDielectricsRelativistic plasmasTechniquesFemtosecond laser irradiationLaser ablationOptical plasma measurementsPlasma PhysicsCondensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

The effect of Penning ionization reactions on the evolution of He with O2 admixtures plasma jets

In this study, the effect of the O2 and N2 Penning ionization reactions on the evolution a capillary helium atmospheric pressure plasma jet (APPJ) with and without the presence of oxygen admixtures is investigated numerically using a two dimensional axi-symmetric model. In order to better understand potential applications of plasma surface interaction, the He and He + O2 (1000 ppm) APPJ is launched into a dielectric target. Moreover, the numerical model is compared with experimental measurements showing that the model captures the right physics and can elucidate the discharge dynamics. The comparison is realized for axial and radial measurements in order to have a complete picture of the plasma jet evolution. In this paper, the velocity and the shape of the plasma guided streamer were chosen as indicators of plasma jet evolution. In particular, the investigation is accomplished experimentally by tracking the spatio-temporal resolved emission of 706.5 nm (3s3S → 2p3P) line and numerically by calculating the reaction rate for the same transition. The O2 and N2 Penning reactions are then independently considered in the numerical model in order to isolate their contribution to the plasma evolution. It is shown that for the pure helium plasma, the N2 Penning reactions are more important to the plasma evolution. However, when oxygen admixtures are added to the plasma gas, the O2 Penning reactions become more important than the N2 ones.

Формирование плазмы в атмосфере азота импульсным электронным пучком вблизи диэлектрической мишени при форвакуумных давлениях

Features of the processes of beam plasma formation near a dielectric (aluminum oxide ceramic) target being irradiated by an intense pulsed electron beam in the forevacuum pressure range (4–15 Pa) have been investigated. It has been established that the density of the beam plasma near the irradiated dielectric target is higher than in the case of "free" propagation of the electron beam. The observed increase in plasma density depends on the emission current (electron beam current), gas pressure, and accelerating voltage. The influence of the dielectric target on the beam plasma density is due to emission of electrons from the target surface and the uncompensated negative potential on the target surface, which determines the energy of emitted electrons. An increase in gas pressure leads to a smaller increase in the beam plasma density due to a decrease in the absolute value of the negative potential. Varying the electron beam current and accelerating voltage provide to control the beam plasma density.

МАГНИТНЫЕ, ПРОВОДЯЩИЕ И МАГНИТОПРОВОДЯЩИЕ СВОЙСТВА КОМПОЗИТНЫХ ПЛЁНОК (CoFeB+SiO2+N2) В ИНТЕРВАЛЕ ТЕМПЕРАТУР 2–400 К И МАГНИТНЫХ ПОЛЕЙ 0, 1 И 5 Тл

Composite films with compositions [(CoFeB)x+(SiO2)(1–x)+N2), x = 0,33–0,52] were obtained by ion-beam sputtering using metal alloy and dielectric targets in a nitrogen atmosphere. The thickness and content of atomic elements, and the concentration of metal alloy x composite films were found using a TESCAN MIRA3 electron microscope. Images of the magnetic phase contrast of the surface of composite films were obtained using magnetic force microscopy (Integra Prima atomic force microscope NT-MDT, Russia) with an MFM10 cantilever. Magnetic phase contrast images of x = 0,48–0,52 films were studied. All the composite films have granular and percolation structures, the volume ratio of these structures varies depending on x. The percolation structure of films with high concentrations x > 0,48 manifested in the presence of extended isolated areas (more than 1 μm) of the accumulation of metal granules. The temperature dependences of magnetic susceptibility, specific conductivity, and specific magnetic conductivity of the composite films with metal alloy concentrations x = 0,33–0,52 were studied in a temperature range of 2–400 K and in magnetic fields of 0 T, 1 T, and 5 T. All measurements of magnetic susceptibility and electrical resistance of the composite films in this temperature and magnetic field range were carried out at the center for diagnostics of functional materials for medicine, pharmacology, and nanoelectronics at St. Petersburg State University. In composite film No.1(x = 0,46) in the temperature range 100–300 K, the highest positive magnetic conductivity had a value of 1,2 at 200 K. The highest positive magnetic conductivity (about 1,2) was observed in composite granular films with small granule sizes. The results of temperature studies of the magnetic conductivity of the composites show that the ratio of positive and negative magnetic conductivity depends on the temperature of the film and on the concentration of the metal alloy x, which sets the ratio of the granular and percolation structures. Large negative magnetic conductivity (about –0,2) occurs in composite films No.3 and 5, the structure of which, along with the granular structure, includes extended (more than several micrometers) metallic areas of metal granules.

Research on SAR Imaging Simulation Based on Time-Domain Shooting and Bouncing Ray Algorithm

The signal source of synthetic aperture radar (SAR) usually adopts the linear frequency modulation (LFM) signal, which exhibits the characteristics of a wide pulse. Hence, in the case of usage of the LFM signal by time-domain shooting and bouncing ray (TDSBR) to simulate the SAR echo signal, numerous time sampling points are generated, resulting in huge computational efforts; thereby, it is hard to exploit the TDSBR algorithm for simulating the SAR echo. In order to unlock this dilemma, the hybrid approach, the transfer function in conjunction with the range frequency-domain pulse coherence, is developed, in which the transfer function is stated by the radar cross-section. The proposed methodology is capable of enhancing the computational efficiency through avoiding the massive time sampling, so that the suggested TDSBR approach could be more conveniently applied to the SAR echo simulations. Furthermore, because of the efficiency advantage of the TDSBR in the calculation of wideband scattering field, the proposed methodology exhibits higher computational efficiency than the frequency-domain shooting and bouncing ray in the SAR image simulation. Finally, the key equation of the TDSBR for the transient scattering field of dielectric targets based on a closed-form integration formula is analytically derived, which is the basis of SAR imaging simulation for the composite scene of ships above the sea surface.

Modification of the model of charging dielectrics under electron beam irradiation

In this paper, a number of drawbacks and contradictions of the existing models of charging dielectric targets under irradiation with defocused electron beams of moderate energies (0.2–20 keV) are briefly noted. In order to streamline and coordinate all experimental results and theoretical information concerning the phenomenon of charging under electron irradiation, it was necessary to reconsider some postulates of the standard model based on a simple dependence of the electron emission coefficient σ on irradiating electron energy E0. It is shown that the decisive role in establishing the equilibrium state of charging dielectrics is played by not only establishing the equilibrium value σ = 1, but also by reaching a certain critical value of the field Fin in the near-surface area of the target. This field is due to the generation of the two-layer bipolar distribution of charges. It enhances the electron emission owing to increasing the depth at which secondary electrons emerge and also owing to the previously neglected contribution of primary thermalized electrons. As a result, there occurs a fundamental transformation of the dependence of the total electron emission coefficient σc for a charged dielectric on the energy of primary electrons as compared to the case of an uncharged dielectric. Consequently, the value of the equilibrium energy E2C, at which σ = 1, is changed. It is established that times of reaching equilibrium states for electron emission δ(t) and for surface potentials Vs(t) may differ by orders of magnitude. Also, the charging kinetics is significantly affected by the process of radiation-induced formation of defects in the irradiated dielectric, which results in two charging time constants—fast and long-term ones.

Complex Inhomogeneous Dielectric Target Modeling and Scattering Estimation using a Self-Designed Software

This paper presents a method to finely model the arbitrarily irregular-shaped and inhomogeneous dielectric target. The target is first geometrically divided into a set of homogeneous and isotropic tetrahedral regions. Each region is precisely matched with a set of electromagnetic parameters. As a result, this can accurately model the target which has an extremely complex dielectric constant distribution and an irregular shape. Regarding the electromagnetic scattering evaluation of the established model, the method of moments (MoM) is adopted in consideration of the coupling between these tetrahedral regions, and the total scattering is obtained by solving the matrix equation. The above two computational sections are integrated into a self-designed software. One can just input the spatial distribution of the dielectric constant and then the designed software automatically processes the target’s geometric information and meshes the target. Finally, the scattered electric field and radar cross section (RCS) of the target are output from the software. The designed software provides an effective and accurate way to study the electromagnetic scattering characteristics of the complex inhomogeneous objects.

Decoupled Potential Integral Equation for Electromagnetic Scattering From Arbitrarily Shaped Dielectric Objects

Recently, integral equation formulations that use potentials as opposed to fields as unknown quantities have been developed for scattering from dielectric objects. It has been shown that these formulations can be construed so that they are well-conditioned across a broad frequency spectrum, a result that has been theoretically proven for spherical systems. Unfortunately, to date, this formulation has not been implemented on practical discretizations of objects. This is the goal of this article. Specifically, we present a well-conditioned and well-tested decoupled potential integral equation (DPIE) formulation and all the necessary implementation details for electromagnetic scattering from homogeneous, dielectric, arbitrarily shaped objects. The resulting decoupled systems do not suffer from low-frequency breakdown. Results that demonstrate these properties are presented for a number of different dielectric targets. Furthermore, in order to fully validate each of the two integral equations for the potentials, we develop analytical solutions for spherical systems.

An adaptive approach for the detection of contrast targets for the through-wall imaging

AbstractThrough-wall imaging is capable of detecting various living and non-living things behind the wall. The characteristics of the wall under the investigation, amount of clutter and noise govern the quality and reliability of the image as well as the detection ability of the targets using through the wall imaging system. The characteristics of the wall are not known prior, in the literature only the intensity profile is investigated for the unknown wall characteristics using a single dielectric target and the effect of the wall characteristics on the contrast imaging and impact on time or frequency domain features are not investigated. The target with less dielectric is having less reflectivity; hence its detection in the presence of a high reflective target and a noisy environment becomes difficult. In this paper, to enhance the detection ability of the imaging system attenuation constant (α) of the wall is estimated with the proposed wall parameter estimation methods and used as a normalizing factor. To achieve effective beamforming different weighting strategies are developed and the obtained images are compared with the traditional beamforming. Furthermore, a novel approach to finding the effective rank in the low-rank estimation using a statistical model and multi-objective genetic algorithm is proposed for de-noising.

Generation of beam plasma near a dielectric target irradiated by a pulsed large-radius electron beam in the forevacuum pressure range

We have investigated generation of beam plasma near a dielectric (ceramic) target irradiated by a pulsed large-radius electron beam in the forevacuum pressure range 3–15 Pa. The electron beam was accelerated with voltage up to 8 kV. Under a certain threshold accelerating voltage increase in the plasma density was observed near the dielectric target as compared with beam plasma density without the target. A decrease in gas pressure leads to smaller value of this threshold voltage. The observed dependencies are apparently caused by two factors: secondary electrons being emitted from the ceramic target and charging of surface of the ceramic target by the e-beam. The secondary electrons, emitted from the ceramic target, provide “additional” ionization of the gas. The charge on the dielectric target surface creates a field that accelerates the slow secondary electrons, which provides an increase in plasma density near the ceramic target. The negative charge on the target slows down the beam electrons, and thus increases total ionization cross section of gas by electron impact. The value of negative charge on the dielectric surface increases with decreasing gas pressure.

A grounded electrode beneath dielectric targets, including cancer cells, enhances the impact of cold atmospheric plasma jet

The effect of the presence or absence of a grounded substrate beneath dielectric targets, including cancer cells, during exposure to the cold atmospheric plasma jet is studied in the experiments and in fluid model simulations for the discharge parameters typical for the medical applications. It is shown that the dynamics of streamers generated in each positive cycle of ac voltage depends on the grounded substrate position. The streamers approach the target more often if the grounded substrate is beneath the target, that provides more intensive plasma-target interaction. In this case, the measured spectrum of plasma jet emission near the target demonstrates much higher intensity compared to an electrically isolated target case. The calculated and measured discharge currents with time demonstrate a mismatch of frequencies of the ac voltage and current over the target. The viability of A431 human skin carcinoma and MX7 mouse rhabdomyosarcoma cells treated by cold atmospheric plasma jet with/without the grounded substrate is measured with MTT assay 24 h after. The results show an enhanced suppression of the cell viability when using the grounded substrate for both cell lines. Achieving effective death of tumor cells with a shorter irradiation time can be considered an advantage of using a grounded electrode beneath the bio target.

Quantification of surface charging memory effect in ionization wave dynamics

The dynamics of ionization waves (IWs) in atmospheric pressure discharges is fundamentally determined by the electric polarity (positive or negative) at which they are generated and by the presence of memory effects, i.e. leftover charges and reactive species that influence subsequent IWs. This work examines and compares positive and negative IWs in pulsed plasma jets (1 upmu s on-time), showing the difference in their nature and the different resulting interaction with a dielectric BSO target. For the first time, it is shown that a surface charging memory effect is produced, i.e. that a significant amount of surface charges and electric field remain in the target in between discharge pulses (200 upmu s off-time). This memory effect directly impacts IW dynamics and is especially important when using negative electric polarity. The results suggest that the remainder of surface charges is due to the lack of charged particles in the plasma near the target, which avoids a full neutralization of the target. This demonstration and the quantification of the memory effect are possible for the first time by using an unique approach, assessing the electric field inside a dielectric material through the combination of an advanced experimental technique called Mueller polarimetry and state-of-the-art numerical simulations.