Helium Atmospheric Pressure Plasma Jet Research Articles

A new optimization strategy, the influence of hollow electrode chamfers on the development of helium atmospheric pressure plasma jets

This work suggests applying chamfering treatment to the plasma generator of the empty electrode structure. Enhancing the electrodes’ physical structure can significantly improve plasma characteristics without requiring intricate control systems. Experiments have shown that changes in the electrode’s shape can lead to changes in the formation of the atmospheric pressure plasma jet. Specifically, our observations indicate that an increase in the chamfer radius leads to an increase in the ignition voltage and a greater density of reactive species inside the jet. We developed a multi-channel equivalent circuit model to describe the discharge process of a plasma jet. Then, using the mixed layer theory, we investigated the effect of the chamfer radius on the plasma jet. Our findings suggest that chamfering increases the effective discharge area, resulting in more discharge channels in the model. This leads to a higher density of reactive species. Additionally, chamfering improves the mixing of helium and air, increasing the concentration of N2 and O2. This consumes some of the avalanche electrons and raises the ignition voltages, ultimately enhancing the chemical reactivity of the plasma jet. This work provides new ideas for the optimization strategy of atmospheric pressure plasma radiation devices.

Enhancement of OH spectra in atmospheric pressure plasma jet by femtosecond laser

A helium (He) atmospheric pressure plasma jet (APPJ) system with a single electrode was configured. A pulsed light of femtosecond (fs) laser was irradiated at the guided streamer of He APPJ through an objective lens to generate the laser induced plasma (LIP) inside the He APPJ. The optical emission spectra of LIP were measured as the light energy of the fs laser increased. The spectra of hydroxyl molecules and atomic oxygen were enhanced when the fs laser energy exceeded 114 μJ. The plasma parameters of LIP inside the APPJ were determined using He collisional-radiative model. Electron temperature and density increased to ∼7.2 eV and 1.7×1014 cm−3, respectively. The dominant processes underlying the enhancement were discussed in the interaction of fs laser and He APPJ.

Numerical simulation of dynamics behavior of pulsed-DC helium plasma jet confined by parallel magnetic field at atmospheric pressure

A two-dimensional axisymmetric fluid model is used to simulate the dynamics behavior of an atmospheric-pressure helium plasma jet in the presence of a parallel magnetic field. The plasma jet is generated in a coaxial dielectric barrier discharge (DBD) driven by pulsed direct-current voltage. Comparative analysis of the plasma jet with and without the parallel magnetic field indicates that a slightly thinner plasma sheath inside the tube is present with the parallel magnetic field as a result of the decreased accumulated electrons on the inner surface of dielectric tube. After the streamer propagates outside the tube, a little more concentrated electron distribution in the annular wall is observed by applying the magnetic field because of the reduced electron diffusion in the radial direction and the confinement effect of the magnetic field on the electrons in the avalanche heads. The tiny reduction in the length of plasma jet is attributed to the E × B drift of charged particles. These results demonstrate that the parallel magnetic field has no apparent effect on the propagation of the plasma jet, and it contributes little to the performance improvement of the coaxial DBD, which agrees well with the previous experimental observations. This little impact of the parallel magnetic field on the coaxial DBD plasma jet may result from negligible contribution of the memory effect to the sole discharge pulse as well as the weak confinement effect of the applied magnetic field on the surface electrons that moves along the magnetic field lines under electrostatic repulsion. Published by the American Physical Society 2024

Interaction of atmospheric pressure helium plasma jet with non-planar substrates: path selectivity of surface ionization wave

Most surfaces treated by atmospheric pressure plasma jets (APPJs) in practical applications are notably three-dimensional. However, non-planar surfaces exhibit a diverse array of geometries, such as variations in curvature, roughness, and texture, complicating the prediction of surface ionization waves (SIWs) propagation behavior across varied surface shapes, in the absence of sufficient experimental data. In this study, we made measurements of APPJ interactions with the non-planar substrates using the spatio-temporal resolved image method. Non-planar substrates encompassed wavy surfaces, arrayed hemispheres, and randomly textured raised surfaces. We tracked the morphology and velocity of SIW propagation over these surfaces. The results indicate that the SIW propagation on non-planar surfaces is significantly influenced by surface geometry and displays path selectivity, i.e. the SIW tends to propagate along valleys. The average propagation velocity of the SIW increases with the increasing radius of the wavy surface, as well as with the increased height of the arrayed hemispheres. This is attributable to the surface geometry constraining the dispersion of the SIW, causing it to concentrate and propagate in a singular direction. Moreover, the surface geometry markedly affects the distribution of the plasma treatment area, with the plasma inclined to enter valleys (where the light emission is significantly stronger than that of peaks) and to closely adhere to hemispherical surfaces. These patterns suggest a potential positive impact on treating skin surfaces such as pores, reducing bacteria in wrinkles, and addressing pimples.

Numerical investigation on the discharge formation in micrometer pores in structured catalyst irradiated by a helium atmospheric pressure plasma jet

Abstract Non-thermal plasma catalysis is a promising way to achieve high efficiency in applications such as energy conversion and chemical engineering. Although synergistic effects between plasmas and catalysts have been preliminarily considered as an underlying mechanism of this type of catalysis, the formation of discharges in small-size catalyst pores, which is possibly a crucial factor in plasma-activated catalysis, is still not well understood. In this paper, investigations on the interactions between a helium atmospheric pressure plasma jet (APPJ) and catalysts with micrometer-sized pores of different shapes and sizes are conducted with a 2D fluid model. Simulation results show that the existence of pores makes a subtle difference to the APPJ by changing the equivalent capacitance, indicating the potential to achieve moderate and stable APPJ-catalyst interactions. Traces of air impurities in helium can promote discharges in catalyst pores, and thus allow discharges to form in smaller pores. In the case when the catalyst channel is too small for direct APPJ penetration, we propose a method by producing a prior discharge in a relatively large cavity to supply seed electrons to ignite discharges inside the channel. The effects of channel and cavity sizes are discussed from the perspectives of discharge behavior and plasma-surface interactions. This work will contribute to the preparation of structured catalysts to potentially achieve higher efficient plasma catalysis, and better understanding of the physical processes in plasma-surface interactions inside micrometer pores.

High time resolution diagnosis of electron density in helium plasma jets with impurity gas

Atmospheric pressure helium plasma jets are widely used in biomedical applications. Researchers normally introduce small amounts of nitrogen and oxygen (0.2–1.0%) into helium to enhance the electron density and electron energy, thus increasing the concentration of active species in plasma. To further explore why the combination of impurity gases N2/O2 leads to an increase in the electron density from the discharge mechanism, we used a microwave Rayleigh scattering method with excellent time-varying characteristics to monitor the temporal electron density changes when different concentrations of N2/O2 were mixed. The research revealed that even trace amounts of N2/O2 (0.2%) can increase the peak electron density, with this effect being more pronounced when N2 is added, increasing from 3.3 × 1019 to 4.6 × 1019 m−3 in pure helium. As the concentration increases, the introduction of O2 leads to a rapid decrease in the electron density. When 1.0% oxygen is mixed, the electron density decreases from 3.3 × 1019 to 2.4 × 1019 m−3. However, the situation is different when N2 is added, at 0.5% proportion of nitrogen, the electron density increases to its maximum at 6.5 × 1019 m−3. These effects are due to the electronegativity of the oxygen-containing particles or the Penning ionization related to excited nitrogen species.

Effects of operational parameters on plasma characteristics and liquid treatment of a DBD-based unipolar microsecond-pulsed helium atmospheric pressure plasma jet

A dielectric-barrier-discharge-based square unipolar microsecond-pulsed helium atmospheric pressure plasma jet (APPJ) was characterized by combining a simplified equivalent circuit model with a transferred charge (Q) measured by introducing an additional capacitance in series with the reactor. From Q-V plots, Cd and Ccell for DBD reactors under pulsed excitation were determined. Q-V plots were drawn for varying operational parameters, and the dissipated energy per cycle was evaluated. Operational parameters, such as the gas flow rate and pulse frequency, were varied, and the resulting changes in the plasma plume length, gas temperature, excitation temperature, discharge current, dissipated power, and optical emission spectra were examined. As an example of the application of the plasma jet, liquid media, including de-ionized water, were exposed to the APPJ, and their properties (pH and electrical conductivity) and concentrations of reactive species generated in the media were measured as functions of the operational parameters. Furthermore, changes in the plasma-activated media after storage for different durations and under different conditions were examined. The correlation between plasma characteristics and properties of plasma-treated liquid is discussed.

Influence of operating conditions on electron density in atmospheric pressure helium plasma jets

In recent years, atmospheric-pressure plasma jets have emerged as valuable tools in many application areas, including material modification, environmental remediation and biomedicine. Understanding the discharge characteristics of these plasma jets under various operating conditions is crucial for optimizing process outcomes. This paper presents a two-dimensional fluid model for numerical simulation to study the variation in electron density within an atmospheric-pressure helium plasma jet under different operating conditions. The investigated parameters include helium gas flow rate, voltage amplitude, needle-to-ring discharge gap, and relative permittivity of the dielectric tube. The results reveal that the peak electric field and electron density initially occur at the wall of the dielectric tube and subsequently shift towards the head of the propagating jet. Gas flow rate has minimal impact on the electron density throughout the plasma jet, whereas increasing the needle-to-ring discharge gap significantly decreases the average electron density within the jet. In addition, an increase in the voltage amplitude and the relative permittivity of the dielectric tube enhances the electric field within the discharge space, thereby increasing the electron density in the plasma jet. These findings underscore the importance of understanding the correlation between electron density and operating conditions to precisely control plasma jets and enhance material treatment effectiveness for specific applications.

Investigation of electric field distribution on dielectric exposed to DC-pulsed He plasma jet with shielding gas

Atmospheric pressure plasma jets (APPJs) produce reactive species and electric fields for biomedical applications. Gas shields control plasma plume-surrounding gas interactions, regulating reactive species generation and electric field strength. However, the surface electric field distribution is still unclear and needs urgent attention. Here, the electric field distribution on the surface exposed to a helium APPJ with shielding gas is investigated using the Pockels technique. This study considers the influence of the type of shielding gas (ambient air, dry air, nitrogen, oxygen, nitrogen-oxygen mixture) and the flow rate (2000–6000 sccm). The results show that the surface electric field develops in three phases: establishment, maintenance, and dissipation. Both flow rate and oxygen content of the shielding gas significantly influence surface discharge behavior and the maximum electric field value. The analysis suggests that the establishment phase of the electric field results from charge transfer by ionization waves to the dielectric, while the maintenance of the electric field depends on pulse duration. During the dissipation phase, the positive surface charge attracts negatively charged species to the surface (electrons and negative ions), which causes charge neutralization at the surface. The oxygen content in the shielding gas impacts the electric field establishment phase, with a low oxygen content leading to lower photo-ionization rates and, consequently, surface discharges with branching. Shielding gas flow rates affect the amount of shielding gas mixed into the helium channel. Mixing less oxygen into the APPJ increases the electric field strength, as the ionization potential is lower than nitrogen. Excessive oxygen mixing traps more free electrons due to electronegativity, causing fewer ionized collisions and more negative ions in APPJ, ultimately lowering the electric field strength. This study shows that shielding gas type and flow rates can adjust surface charging, aiding in optimizing biomedical APPJ.

Effect of CF4 additive on dynamic surface charge deposited on dielectric by helium atmospheric pressure plasma jet

Abstract The addition of active trace components to non-thermal plasmas effectively enhances their chemical activity and has attracted considerable academic attention in the plasma community. It is essential to quantitatively estimate the effect of active addition on the properties of the plasma–surface interaction (PSI). In this study, we focus on the fundamental properties of dynamic surface charge deposited by a He-atmospheric pressure plasma jet (APPJ) with 0%–2% CF4 additive. The charge distribution is measured by a reflective optical measurement platform based on linear electro-optic effect, and is calculated with surface charge density inversion algorithm. Results show that a higher CF4 concentration can shrink the region of surface charge accumulation. Polarity effect is observed in aspects of maximum charge density when the CF4 concentration gradually rises. The negative charge deposition is suppressed by the addition of CF4, nearly disappearing at 2% concentration. While the maximum density of positive charge rises to ∼25 nC cm−2 at 0.5% CF4 before it decreases to ∼10 nC cm−2. The dispersions of total surface charge at 0.5% and 1.5% additives indicate the unstable periodic bombardment of ionization waves in He/CF4 APPJ. The double effect of CF4 additive on the surface charges by He-APPJ is also discussed. These results provide fresh and deep insights into the interaction between dielectric surface modification and ionization waves that occurs in other plasmas processing fed with composite gases.

Development of an experimental system for cell viability assays of yeasts using gas-temperature controllable plasma jets

The characteristics of a gas-temperature-controllable atmospheric-pressure helium plasma jet and the development of an experimental system for cell viability assays of yeasts (fission yeast Schizosaccharomyces pombe and budding yeast Saccharomyces cerevisiae) are reported. The physicochemical properties of the plasma plume, which can maintain the temperature of the irradiated object at a temperature suitable for yeast, were not significantly different from those of a typical helium plasma jet. Furthermore, good reproducibility of cell viability was observed when gas temperature, gas flow rate, applied high voltage, and irradiation distance remained fixed, and only irradiation time was used as a parameter. This experimental system allows us to carry out various experiments, such as the search for plasma-resistant mutants that will contribute to the identification of genes involved in resistance to direct plasma irradiation.

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.

On the gas heating effect of helium atmospheric pressure plasma jet

Plasma medicine is an emerging research field which has been driven by the development of plasma sources suitable to generate low temperature plasmas. In many cases, such devices can operate without a gas flow, producing a plasma discharge from the ambient air. However, the most common case is the use of a working gas at a given flow rate to produce a plasma jet. Helium (He) is one of the gases commonly used as the carrier gas to generate cold atmospheric pressure plasma jets (CAPPJs) due mainly to the easiness to ignite a gas discharge with it. However, in this work it was observed that most of the heating of a He CAPPJ can come just from the expansion of the He gas. This was found through measurements of gas temperature (T gas ), using fiber optic temperature (FOT) sensors, and thermal output, using both FOT and infrared imaging with the He flow impinging on a copper (Cu) plate. Such findings were achieved through comparisons of T gas and the temperature on the Cu (T Cu ) plate in the conditions with and without discharge ignition, as well as comparing T gas in the free gas/jet mode with and without discharge ignition. It was verified that the T gas values increased as the distance from the gas outlet was enlarged, especially at low He flow rates, even without discharge ignition. Despite the temperature increase with distance, it is possible to produce plasma jets with temperatures lower than 40 °C at low He flow rates.

Effects of duty ratio on liquid- and polymer-surface treatment by a unipolar microsecond-pulsed helium atmospheric-pressure plasma jet

Effects of duty ratio of a unipolar microsecond-pulsed helium atmospheric-pressure plasma jet (APPJ) on liquid- and polymer-surface treatments were investigated. In addition, changes in the plasma plume length, gas temperature, excitation temperature, discharge current, absorption power, and optical emission spectra were examined by varying the other operating parameters, such as applied voltage and additive flow of oxygen or water vapor. As an example of liquid sample, de-ionized water (DW) was exposed to an APPJ, and the concentrations of the reactive species generated in the DW were measured as functions of the operating parameters. Polycarbonate, polypropylene, and polymethylmethacrylate were employed as exemplary substrate materials to investigate the effect of plasma treatment on polymeric surfaces. The APPJ treatment increased the surface energy and changed the wetting characteristics of the surface from hydrophobic to hydrophilic. X-ray photoelectron spectroscopy results showed that a short-time plasma treatment with He and/or He/O2, He/H2O affects the surface wettability owing to the introduction of polar groups.

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.

Synergistic effect of multi-parameters on NO density of a helium atmospheric pressure plasma jet in contact with skin

Nitric oxide (NO) is a crucial molecule for living organisms, which acts as a messenger in most mammalian organs. Fortunately, plasma jet can generate an abundant amount of NO. In this paper, the NO generated by a helium plasma jet when it is interacting with pig skin is diagnosed using laser-induced fluorescence (LIF) method. The multi-parameters’ effect of the distance between the nozzle and the skin surface (distance d), the gas parameters (gas flow rate, O2 and H2O percentage) and the applied voltage parameters (the amplitude and frequency of the voltage) on the NO density is explored. The results show that the NO density increases and then decreases as the gas flow rate increases. The gas flow rate corresponding to the maximum NO density is positively correlated with the distance d. In the case of high gas flow rate or small distance d, premixing a small amount of O2 or H2O content lead to higher NO density. Higher voltage amplitude and frequency can increase NO density, but high voltage amplitude at high flow rate and small distance d lead to a decrease in NO density. The generation pathway of NO is mainly affected by N2(A), N, O2, O, OH, H and NO2, and the consumption pathway is closely related to the particles N, O, OH, O3 and HO2. Among them, N and O atoms participate in both generation and consumption. Therefore, the synergistic effect of multi-parameters, the emergence of different effects is closely related to the synergistic effect of these two particles.

Experimental Investigation on Atmospheric Pressure Plasma Jet under Locally Divergent Magnet Field

Regulating the parameters of the atmospheric pressure plasma jet (APPJ) is meaningful for industrial applications. Since plasma is a typical functional fluid in the magnetic field, it is possible to control the discharge characteristics via the Lorentz force. In this study, the effects of a locally divergent magnetic field on the generation and propagation of APPJ were examined experimentally. The experiments used a coplanar dielectric barrier discharge (CDBD) device driven by a 30 kHz AC high-voltage source to generate a helium APPJ. A locally divergent magnetic field of 250 mT (maximum) was applied coupled with the electric field, and noticeable enhancement was observed. The results showed that the magnetic field changed the motion state of electrons and promoted collision ionization, leading to a 40% improvement in the APPJ length (0.6 cm) and a 23% increase in the intensity of line O (777.2 nm). In addition, the spatiotemporal evolution and flow field of APPJ were diagnosed by ICCD and schlieren technique. The combination of electric and magnetic fields may effectively optimize the APPJ in practical applications.

Numerical study of helium atmospheric pressure plasma jet interacting with a wavy substrate surface with different dielectric constants and curvature radius

In this paper, we present a two-dimensional numerical study on a helium atmospheric pressure plasma jet interacting with a wavy substrate surface, focusing on the effects of the substrate relative dielectric constant and substrate morphologies on the plasma jet behavior near the wavy surface. The results show that when the dielectric constant is small, the jet can form separate discharge channels near the wavy substrate surface and can penetrate the cavity of the wavy substrate surface. With increasing dielectric constant, the penetration distance of the discharge channels decreases. When the substrate dielectric constant exceeds a certain value, the plasma jet only propagates above the wavy substrate surface and there are no prominent separated channels near the surface. Meanwhile, the radial propagation distance along the substrate surface decreases. For a certain dielectric constant, the penetration depth of the separated channel depends on the curvature radius of the wavy substrate surface and there exists a minimum curvature radius that allows the separated channel to enter the cavity. This minimum curvature radius varies with the substrate dielectric constant. If the dielectric constant becomes larger, the minimum curvature radius increases.

Study of increment of emission intensity in a cold atmospheric pressure helium plasma jet using the ADAS

In this paper, the Atomic Data and Analysis Structure (ADAS) has been used to study the cold atmospheric pressure plasma jet. Variation of emission intensity pattern from glass nozzle to the plasma jet tip of a helium plasma jet in ambient air has been observed from the intensified charge-coupled device (ICCD) camera image. The increase of intensity pattern towards the plasma jet tip is an important and fundamental feature of the plasma jet in ambient air. The correlation of this observed pattern with electron impact processes has been studied by computing photon emissivity coefficients (PEC) using the ADAS. The role of various atomic processes involved in this study has also been discussed.

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.