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Plasma characteristics and mode transition of atmospheric pressure gas–liquid discharge oxygen plasma
Abstract
In this paper, a capacitor assisted AC high-voltage was employed to generate a gas–liquid discharge in pure oxygen at atmospheric pressure. The discharge images, waveforms of voltage and discharge current, and optical emission spectra of plasma were diagnosed for the purpose of investigating the discharge modes. The gas temperature (Tg), excitation temperature of hydrogen (Texc), and electron density (ne) were calculated by the spectra of OH (A²Σ–X²Π), the intensity ratio of Hα and Hβ, and the Stark broadening of Hβ, respectively. The effects of applied voltage and capacitance value on the mode transition of discharge were also discussed. It is found that due to the presence of capacitor, not only is the unlimited growth of discharge current restrained, but the transition of discharge mode is also controllable. There are three discharge modes of gas–liquid discharge oxygen plasma (GLDOP), and with the increase of applied voltage or capacitance value, discharge modes are transited from the streamer mode, to the glow-like mode, and to the abnormal glow/arc mode. With the mode transition, the Tg and Texc of GLDOP increase and the ne decreases. In contrast, the change of Tg and ne is negligible when GLDOP maintains one kind of discharge mode.
... Hoffer et al used high-speed shadowgraphy for the study of the disturbances at the plasma-liquid interface in the pin to liquid pulsed discharge in air [121]. Depending on the discharge conditions, they recorded the [123]. https://doi.org/10.1063/5.0008941. ...
... https://doi.org/10.1063/5.0008941. Reprinted from [123], with the permission of AIP Publishing. ...
... On the other hand, when grounding electrode is cowered with the dielectric-indirect grounding, the discharge is more stable, consumes less energy and operates in streamer mode. The mode transitions are also recorded in the work of Yuan et al [123]. In this study of pin-to-liquid AC discharge in oxygen at atmospheric pressure, three discharge modes were found which depended on the applied voltage: streamer mode, glowlike mode and abnormal glow/arc mode, see figure 12. ...
The study of plasma–liquid interactions has evolved as a new interdisciplinary research field driven by the development of plasma applications for water purification, biomedicine and agriculture. Electrical discharges in contact with liquids are a rich source of reactive species in gas and in liquid phase which can be used to break polluting compounds in water or to induce healing processes in medical applications. An understanding of the fundamental processes in plasma, and of the interaction of plasma with liquid, enables the optimization of plasma chemistry in large-scale plasma devices with liquid electrodes. This article reviews recent progress and insight in the research of low-temperature plasmas in contact with liquids at atmospheric pressure. The work mainly focuses on the physical processes and phenomena in these plasmas with an attempt to provide a review of the latest and the most important research outcomes in the literature. The article provides an overview of the breakdown mechanisms in discharges in contact with liquid, emphasizing the recently studied specifities of plasma jets impinging on the liquid surface, and discharge generation with a high overvoltage. It also covers innovative approaches in the generation of plasma in contact with liquids. Novel phenomena detected by the imaging techniques and measurement of discharge parameters in the reviewed discharges are also presented. The results, the techniques that are applied, and those that may be applied in further studies, are listed and discussed. A brief overview of the applications focuses on the original approaches and new application fields. Future challenges and gaps in knowledge regarding further advancement in applications are summarized.
... By connecting a capacitor in series to the high-voltage end of the power supply, charge accumulates due to the external electric field and is promptly discharged when the gap breaks down. The capacitor is then charged by primary discharge, creating a reverse electric field that weakens the original field and partially suppresses an excessive discharge current [30]. The primary discharge, caused by ionization and heating effects, facilitates the Figure 5a indicates that the discharges occur in both the positive half-cycle (0 to 60 µs) and the negative half-cycle (60 to 120 µs). ...
... By connecting a capacitor in series to the high-voltage end of the power supply, charge accumulates due to the external electric field and is promptly discharged when the gap breaks down. The capacitor is then charged by primary discharge, creating a reverse electric field that weakens the original field and partially suppresses an excessive discharge current [30]. The primary discharge, caused by ionization and heating effects, facilitates the breakdown of existing discharge channels and the generation of secondary discharges [31]. ...
Atmospheric pressure gas–liquid discharge plasma has garnered considerable attention for its efficacy in wastewater contaminant removal. This study utilized atmospheric oxygen gas–liquid discharge plasma for the treatment of ammonia nitrogen wastewater. The effect of applied voltage on the treatment of ammonia nitrogen wastewater by gas–liquid discharge plasma was discussed, and the potential reaction mechanism was elucidated. As the applied voltage increased from 9 kV to 17 kV, the ammonia nitrogen removal efficiency rose from 49.45% to 99.04%, with an N2 selectivity of 87.72%. The mechanism of ammonia nitrogen degradation by gas–liquid discharge plasma under different applied voltages was deduced through electrical characteristic analysis, emission spectrum diagnosis, and further measurement of the concentration of active species in the gas–liquid two-phase system. The degradation of ammonia nitrogen by gas–liquid discharge plasma primarily relies on the generation of active species in the liquid phase after plasma–gas interactions, rather than direct plasma effects. Increasing the applied voltage leads to changes in discharge morphology, higher energy input, elevated electron excitation temperatures, enhanced collisions, a decrease in plasma electron density, and an increase in rotational temperatures. The change in the plasma state enhances the gas–liquid transfer process and increases the concentration of H2O2, O3, and, ⋅OH in the liquid phase. Ultimately, the efficient removal of ammonia nitrogen from wastewater is achieved.
... The vibrational temperature T vib and rotational temperature T rot of plasma can be obtained according to the temperature set in the software. The rotational energy and translational energy of molecules are generally considered to be able to reach equilibrium due to the frequent collisions in atmospheric air, which means that the gas temperature of the plasma and rotational temperature of the molecules are equal [64,65]. Therefore, the gas temperature under different conditions can be obtained by OES. ...
... In addition, the line-ratio technique of OES allows the qualitative diagnosis of electron density and electron temperature [51,64,66]. According to the proposed simplified collisionradiation model [67,68], the variation trends of electron density and electron temperature can be respectively indicated by I 371.1nm /I 380.5nm and I 391.4nm /I 380.5nm . ...
The time evolution of dielectric barrier discharge driven by nanosecond pulse high-voltage power is investigated by high-speed video analysis, electrical measurements and spectral diagnostics. It is found that the discharge mode generally goes through the evolution process of filamentary discharge → diffuse discharge → filamentary discharge with the increase in discharge cycle. The time-dependent changes in the standard deviation of image gray levels indicate that the discharge uniformity first improves and then deteriorates in this evolution process. The different pre-ionization density and modulated distribution of space charges and surface charges are considered to be the main reasons for the time evolution of discharge uniformity. In addition, the experiments under different frequencies and voltages show that the transition of the discharge mode is more likely to occur at higher frequency and higher voltage. Further measurement and calculation reveal that the discharge at high frequency and high voltage has the same characteristics, that is, high pre-ionization degree, thick filament diameter and short time lag. These characteristics usually lead to higher seed electron density, larger critical avalanche size and weaker lateral inhibition effect, which make the discharge mode transition more likely to occur.
... With the production of smellable ozone, the chemical reaction is as R11-R12 [33]: ...
Melon (Cucumis melo L.) is a nutritious fruit with high economic value. As melon seeds have a relatively thick shell, they are susceptible to bacterial pathogens, seed germination is slow and seedling growth is compromised. This research employed a dielectric barrier discharge (DBD) array electrode structure plasma with nanosecond pulse stimulation to promote the germination of melon seeds at room temperature. The effects of applied voltage and discharge time on seed germination were investigated. Under the conditions of 22 kV applied voltage and 1.5 min discharge duration, the germination potential of melon seeds increased by 47%, the germination rate improved by 14%, the seed vigor index expanded by 91%, and the average root length increased by 59%. The scanning electron microscope (SEM) demonstrates that plasma treatment induces strong corrugation of the seed shell surface and greatly reduces the content of pathogenic microorganisms. The water contact angle showed that the nanosecond pulse discharge enhanced the water uptake of seedlings. The attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy results revealed that the increased hydrophilicity of the seed shell may be due to the formation of the functional group C=O carbonyl, thus accelerating seed germination at low temperatures. This study indicates that plasma-assisted breeding is a viable strategy before melon planting.
... For instance, the effects of by-products/intermediates/impurities in the NTPD or VOC destruction process can scavenge the high-energy electrons (Vitale et al. 1997) or quench the reactive species (Ochoa and Breitkopf 2020). Also, atmospheric NTPD is susceptible and non-steady in gas flow or in the presence of liquid with condition fluctuation, prompting transition of discharge mode (Korolev et al. 2014;Yuan et al. 2020), which further affects VOC degradation performance at root due to ...
Volatile organic compounds (VOCs) have posed a severe threat on both ecosystem and human health which thus have gained much attention in recent years. Nonthermal plasma (NTP) as an alternative to traditional methods has been employed to degrade VOC in the atmosphere and wastewater for its high removal efficiency (up to 100%), mild operating conditions, and environmental friendliness. This review outlined the principles of NTP production and the applications on VOC removal in different kinds of reactors, like single/double dielectric barrier discharge, surface discharge, and gliding arc discharge reactors. The combination of NTP with catalysts/oxidants was also applied for VOC degradation to further promote the energy efficiency. Further, detailed explanations were given of the effect of various important factors including input/reactor/external conditions on VOC degradation performance. The reactive species (e.g., high-energy electrons, HO·, O·, N2⁺, Ar⁺, O3, H2O2) generated in NTP discharge process have played crucial roles in decomposing VOC molecules; therefore, their variation under different parameter conditions along with the reaction mechanisms involved in these NTP technologies was emphatically explained. Finally, a conclusion of the NTP technologies was presented, and special attention was paid to future challenges for NTP technologies in VOC treatment to stimulate the advances in this topic.
Graphical abstract
... The electron density (n e ) of He G-LD is obtained by using the Stark broadening of the H β peak at 486.1 nm, using the following equation [24,25]: ...
In this paper, shielding gas (He) and shielding quartz tube (straight tube and conical tube) is added to nanosecond pulsed He gas-liquid discharge for the purposes of enhancing the plasma volume and productions of •OH and H2O2. The plasma properties, including current-voltage waveforms, the temporal-resolved discharge images, optical emission spectra, gas temperature, electron density, and the •OH and H2O2 productions are analyzed and compared among different discharges generated under the cases of no shielding, shielding He gas, shielding straight tube, and shielding conical tube. The results show that adding shielding gas and tubes in the discharge reactor can decrease the gas temperature and electron density, but enhance the plasma volume and area of plasma-liquid interface in comparison with no shielding case. In comparison, the addition of shielding gas has the most benefit for enhancing the concentrations of •OH and H2O2 produced by gas-liquid discharge. Adding a shielding conical tube slows down the decrease extent of •OH and H2O2 productions caused by increasing discharge gap. When the discharge gasp excesses 6 mm, adding a shielding conical quartz also has an obvious enhanced effect on the production of •OH and H2O2 in compared with no shielding case. While adding a shielding straight tube with small diameter has a little effect on H2O2 production, even a negative effect on •OH production.
... Some studies, like the work of Marjanović et al., 26 are performed in highly controlled conditions that allow for detailed comparison with modeling; nonetheless a lot of studies focus on more complex electrode geometries motivated by applications. Marjanović et al. 26 report on different discharge modes in four different alcohol vapors covering Townsend, normal glow, and abnormal glow regimes with a focus on distinct transitions between the normal and abnormal glow regimes, while Yuan et al. 27 reported three discharge modes (streamer mode, glow-like mode, and abnormal glow/arc mode) in an oxygen discharge with liquid electrode plasma. Gershman and Belkind 28 extended this work to a study of the plasma properties of discharges generated in gas bubbles in hydrogels, which allowed them to assess the impact of dielectric constant and conductivity on the discharge. ...
In the ‘Plasma–Liquid Interactions’ Special Topic Collection, researchers explore the richness and full breadth of plasma– liquid interactions and their applications. The collection contains two perspective articles that discuss key questions related to fundamental processes at the plasma–liquid interface. Researchers introduce the concept of plasma-driven solution electro chemistry (PDSE) inspired by comparing plasma–liquid interactions with conventional electrolysis and develop ten questions that should be answered to enable researchers to make transformational advances in plasma–liquid interactions for a variety of applications. The Perspective by Vanraes and Bogaert focuses on that same plasma–liquid interface but emphasizes the gas phase processes rather than liquid-phase processes.
... Some studies, like the work of Marjanović et al., 26 are performed in highly controlled conditions that allow for detailed comparison with modeling; nonetheless a lot of studies focus on more complex electrode geometries motivated by applications. Marjanović et al. 26 report on different discharge modes in four different alcohol vapors covering Townsend, normal glow, and abnormal glow regimes with a focus on distinct transitions between the normal and abnormal glow regimes, while Yuan et al. 27 reported three discharge modes (streamer mode, glow-like mode, and abnormal glow/arc mode) in an oxygen discharge with liquid electrode plasma. Gershman and Belkind 28 extended this work to a study of the plasma properties of discharges generated in gas bubbles in hydrogels, which allowed them to assess the impact of dielectric constant and conductivity on the discharge. ...
Multi-color terahertz (THz) detector has attracted much attention in various applications because of the ability to obtain more comprehensive information simultaneously. THz quantum well photodetectors (QWPs) have great advantages in realizing multi-color detection because of high speed, sensitivity, and mature technology. In this work, QWPs based on antenna coupled microcavity (AM-QWP) and etched antenna coupled microcavity (EAM-QWP) structures are proposed to realize multi-color THz detection. Thanks to the combination of the microcavity resonance and surface plasmon polariton mode, AM-QWP achieves a coupling efficiency of one order of magnitude higher than that of the conventional 45° edge facet coupler (45°-QWP) in multiple bands. The EAM-QWP only retains the active region where the effective photocurrent is generated so that the coupling light is highly localized in a small area, improving the optical coupling efficiency by two orders higher compared with 45°-QWP. It is theoretically estimated that the responsivity of AM-QWP and EAM-QWP at the temperature of 4 K is 9.6–24.0 A/W and 78.4–196.0 A/W while their noise equivalent power (NEP) is 5.4 × 10⁻⁴–1.1 × 10⁻³ pW/Hz1/2 and 1.7 × 10⁻⁵–3.5 × 10⁻⁵ pW/Hz1/2, and the specific detectivity is 4.4 × 10¹²–8.9 × 10¹² and 6.9 × 10¹³–1.4 × 10¹⁴ cm Hz1/2/W, respectively. This work provides a guideline for the experimental realization of high-performance multi-color THz QWPs.
... The distance between the detector of the spectrometer and the center of the stainless steel bar was 100 mm. The excitation temperature (T exc ) of discharge was estimated as follows [17]: ...
Plasma contacting with liquid provides many charged particles and reactive species into the liquid. The difficulty in controlling or selecting each specific species has significantly limited its applications in industry. Here, we present a study on using voltage polarity to regulate the type of charged particles absorbing from plasma into liquid. Detailed understanding of the processes at the plasma-liquid interface, electrolysis due to switching in voltage polarity was investigated via a visual pH observation, measuring the concentration of H2O2 and solvated electrons. The results indicated that changing in voltage polarity strongly affects the plasma properties, chemical properties, and electrolysis process in liquid, and further in the types of reducing species for gold nanoparticle synthesis. The results also showed using a suitable frequency could improve the efficiency of absorption of H2O2from plasma into the bulk liquid and the yield in the production of gold nanoparticles. The results provide a way to select desired species from plasma into the liquid for a distinct purpose and accompanying other properties in the system of plasma contacting with liquid.
Herein, we studied the combined effects of the magnetic field and the alternating current driven air surface dielectric barrier discharge on ozone production, and found that a 0.13 T perpendicular magnetic field introduced into the discharge area significantly enhanced the ozone generation performance with a 36-108% increase in ozone number density and 24-80% increase in ozone yield depending on discharge voltage and frequency differences. To reveal the micro physico-chemical mechanism of the influence of a magnetic field and discharge parameters of discharge voltage and frequency on ozone generation, a plasma chemical reaction network involving electron collision-chain reactions was considered. The results show that these parameters jointly influence ozone generation by affecting electron collision reactions and chain chemical reactions by changing the mean electron energy and plasma gas temperature. In this study, both the experimental results and mechanism analysis suggested that an optimal discharge parameter for ozone generation is the magnetic field assisted, low frequency, high voltage (6.5 kHz, 6.5 kV) surface dielectric barrier discharge. These insights provide guidance for optimizing the discharge parameters of the magnetic field assisted discharge to increase ozone production and reduce energy consumption.
Cobalt oxide nanoparticles are widely used owing to their distinct properties such as their larger surface area, enhanced reactivity, and their superior optical, electronic, and magnetic properties when compared to their bulk counterpart. The nanoparticles are preferably synthesized using a bottom-up approach in liquid as it allows the particle size to be more precisely controlled. In this study, we employed microplasma to synthesize Co3O4 nanoparticles because it eliminates harmful reducing agents and is efficient and cost-effective. Microplasma reactors are equipped with copper wire electrodes to generate plasma and are simple to configure. The product was characterized using UV-Vis spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The experimental parameters that were varied for the synthesis were: with or without stirring, with or without indirect ultrasonication, and with or without capping agents (urea and sucrose). The results showed that the microplasma enabled Co3O4 nanoparticles to be successfully synthesized, with particle sizes of 10.9-17.7 nm, depending on the synthesis conditions.
In the application of atmospheric pressure plasma jet, due to the limitations of AC power supply with the frequency in the ranging of kHz, the study of the influence of power supply electrical parameters on the discharge basically only explore the change rules of plasma jet characteristics varying with a single driving electrical parameter (for example, voltage and frequency) changes. However, the discharge power usually changes in the process of a single electrical parameter change, which undoubtedly also affect the discharge performances including the plasma physical parameters and generated reactive species, resulting that the effect of the single driving parameters on the discharge cannot be reflected. In this study, an atmospheric pressure argon plasma jet is driven by a self-developed AC power supply with adjustable pulse modulated duty cycle. And combined with the diagnosis of the optical emission spectrum and the optical absorption spectrum, the influences of the voltage, frequency and pulse modulated duty cycle parameters on the gas temperature Tg, electron excitation temperature Texc, electron density ne, and OH radical particle number density of the plasma jet are studied under a fixation discharge power of 2 W. The results show that at the fixation power, the electron density ne does not change with the variation of electrical parameters as the linkage change of electrical parameters will offset the influence of a single parameter on the electron density, while the gas temperature Tg, electron excitation temperature Texc, and OH radical particle density are the most affected by the pulse modulated duty cycle, followed by driving voltage, and frequency has the least effect. Under the fixation power, when the frequency is reduced, the voltage will increase, and the gas temperature Tg, electron excitation temperature Texc, and OH radical particle number density will increase. On the contrary, although the voltage also increases when the pulse modulated duty cycle is reduced, it will lead a decrease in the gas temperature Tg, electron excitation temperature Texc, and OH radical particle number density. In addition, the results indicate that reducing the duty cycle of AC power can make the atmospheric pressure plasma jet produce more OH radical at lower gas temperature. This study provides a new insight into the influence of electrical parameters on the characteristics of atmospheric pressure plasma jets under a fixation power, and provides guidance for the selection of power parameters of plasma jets with low gas temperature and high density of reactive species, which is conducive to the development of atmospheric pressure plasma jets in biomedicine and other fields.
Plasma-enhanced decomposition is a promising method to achieve reliable ignition and better thruster performance with an ADN-based propellant. High-speed imaging and optical emission spectroscopy are used to characterize the discharge plasma produced from ADN-based propellant vapor and argon carrier gas within parallel planes. Discharge regimes are independently controlled by varying working parameters. The transition from abnormal glow discharge (AGD) to filamentary discharge (FD) is identified from voltage-current characteristics and sudden changes in excitation electron temperature (Te-exc) and electron density (Ne) under a pressure of 0.2–10 kPa. Instabilities develop because there are more freedom degrees and higher collision probabilities after ADN-based propellant vapor is added. The product of Te-exc and Ne shows a higher energy transfer efficiency from input power to vapor in the FD regime. Water molecules increase the net dissociative attachment rate and effectively quench Ne. Although preheated vapor (as the discharge medium) has a much lower Ne, more radicals appear such as OH, NH, CH, CN, N2, N2⁺, and C2, which promote chemical interactions in an electric field and increase the probability of successful ignition. This can be attributed to thermalization due to an increase in the translational kinetic energy and rotational excitation, which has an important impact on chemical reaction kinetics. Therefore, preheating is verified as critical for improving the ignition and performance of a plasma-assisted ADN-based thruster.
A simultaneous atomic absorption and emission spectrometer (AAS and AES) was developed using dielectric barrier discharge (DBD) microplasma for atomization and excitation. The feasibility of the simultaneous measurement scheme was evaluated through the detection of AAS and AES signals of Hg with hydride generation (HG) and photochemical vapor generation (PCVG) for the sample introduction into the microplasma. The potential application for the diagnostics of atomization and excitation processes in the microplasma was investigated, through which the effect of doped gases on atomization/excitation was also studied. The doped gas mainly affects the atomization and excitation processes by varied physical parameters and radical types and contents of the plasma. Simultaneous detection of AES and AAS signals is an ideal technique for plasma diagnostics, since the ground-state population could be monitored through AAS, while the excited-state population could be monitored by AES.
Plasma bubbles are regarded as a promising means of interacting plasma discharges and liquids due to their high efficiency in the generation of reactive species. The discharge mode and characteristics are significant factors that should be considered. In this study, the plasmas are initially generated in the gas phase and then driven by the gas flow to diffuse into the solution through the two holes at the lower part of the quartz tube to form plasma bubbles. The discharge modes, characteristics, and plasma–liquid interactions in two different configurations, i.e., bare electrode and dielectric-coated electrode, are investigated. It is found that the discharge mode induced for the two structures is different, with a hybrid-mode operating in the bare electrode design and a filamentary mode operating in the dielectric design. When the applied voltage is increased, a filamentary-to-spark transition occurs in the bare structure, while the discharge remains relatively stable in the dielectric design. Direct and intense contact between the discharge and the solution in the bare structure greatly promotes the physio-chemical reactions and results in obvious changes in H 2 O 2 concentration, solution pH, conductivity, and temperature. This study provides insights into hybrid gas–liquid discharges and reactor design for plasma bubble generation.
Solution pH is a significant parameter that affects the electrical characteristics of gas–liquid discharges and thus potentially produces different plasma chemistries for different plasma-engineered applications. In this study, the discharge characteristics and long-lived aqueous reactive species under different initial pH conditions were investigated. It was found that the discharge contained three phases in one pulse cycle, which occurred at the pulse rising edge, the falling edge, and between the rising and falling edges. The discharge intensity and average power at an initial solution pH of 7.0 are much lower than those obtained at an initial solution pH of 5.0 and 9.0. In contrast, the density ratio of N2 (C³Πu, v = 1)/N2 (C³Πu, v = 0) is much higher under neutral solution conditions, indicating that the relatively high vibrational energy was obtained in the gas–liquid discharge plasma. Concentrations of aqueous species H2O2, NO2–, and NO3– are higher at the initial pH of 9.0, indicating that different plasma intensities and chemical compositions (H⁺ and OH–) are involved in their generation and consumption processes. This study provides insights into understanding and controlling the characteristics of the gas–liquid discharge under different solution conditions.
Atmospheric pressure electrolyte cathode discharge plasma presents a great potential on detecting heavy metal pollutants. In this paper, a novel microsecond pulsed electrolyte cathode discharge was generated and coupled with temporal resolved atomic emission spectra to analyze Cu element, as one of the representative heavy metals. The waveforms of voltage and current, as well as optical emission spectra of discharge were investigated, and the experimental conditions were optimized. The spatiotemporal resolved spectra of Cu I (324.8 nm), OH (A²Σ–X²Π) and N2 (C³Πu–B³Πg) were diagnosed. The spatiotemporal distributions of gas temperature and electron density were calculated by the rotational temperature of OH (A) and the Stark broadening of Hβ (486.1 nm), respectively. It was found that the optimal detection sensitivity of Cu are obtained under the conditions of HNO3 solution with 1.0 pH value, 3.0 mL/min solution flow rate, 8.5 kV pulse peak voltage, and 100 μs pulse width. The interferences of spectral line of Cu I (324.8 nm) are mainly sourced by the spectral bands of N2 (C − B) and OH (A − X), which could be reduced by the temporal resolved spectra. The limit of detection of Cu was improved from 0.217 mg/L to 0.092 mg/L by acquiring spectra only in 25–100 μs, under the optimal conditions. Furthermore, the gas temperature and electron density play important roles in the spatiotemporal evolution of discharge and the improvement of detection sensitivity for elemental analysis.
In this paper, a transient spark discharge is presented driven by a nanosecond pulse power with a needle-water electrode configuration in atmospheric nitrogen. The transient spark discharge concludes three phases, described as the streamer phase, the streamer-to-spark transition phase, and the spark phase. The amplitude of pulse voltage has a significant influence on the characteristics of the transient spark discharge. The streamer-to-spark transition time shortens with the increase of pulse voltage, and the spark current value increases with the increase of pulse voltage. Though the streamer-to-spark transition is not completely prevented, the gas temperature is still in a lower value (∼400 K), due to the short duration of spark current (200-400 ns). The electron density in the transient spark, calculated by the Stark broadening of the Hα line at 656 nm, is about 1.3 × 10¹⁷ cm⁻³, which is 2-3 orders of magnitude higher than that in other forms of gas-liquid discharge. The results indicate that the transient spark discharge plasma is in a highly non-equilibrium state and the results also present its other unique features of high electron density and abundant excited species.
An underwater Ar bubble discharge is excited by nanosecond pulsed power, and the effects of adding N2/O2 in Ar discharge on products are investigated. This work focus on regulating species concentration in plasma activated water (PAW), and then seeking specific concentrations for future application. The optical emission spectra (OES) and colorimetric chemical probes are used to diagnose and measure active species in plasma region or liquid phase. The H2O2 energy yield with Ar bubble can be up to 3.04 g kWh−1. The emission intensity characteristics of Ar (4p‐4s), OH (A‐X), Hα, and O (3p‐3s) in plasma region, and the concentrations of H2O2, , and in liquid are investigated with adding varying N2/O2 proportion in Ar bubble discharge. Non‐thermal plasma has prospect in green agriculture, and specific species concentrations are required for specific target. Here, Plasma activated water containing high concentration of species is produced by nanosecond argon bubble discharge. Meaningfully, species concentration can be regulated by adding N2/O2 in discharge, and a specific concentration may be sought for different application.
The plasma kinetics of Ar-H2O and H2O at atmospheric pressure are of interest for applications in biotechnology where rare-gas plasma jets treat liquid surfaces and in water treatment where discharges are generated in bubbles or directly in liquid water. Due to evaporation resulting from heat transfer to the liquid, for many conditions the mole fraction of water in the plasma can be large - approaching nearly pure water. In this paper, results are discussed from a combined experimental and computational investigation of the chemical kinetics in a high electron density plasma filament sustained in Ar-H2O at atmospheric pressure. The chemical kinetics were simulated using a 0D global model, validated by measurements of the absolute OH and H densities by laser induced fluorescence (LIF) and two-photon absorption LIF. The primary sources of H and OH during the discharge pulse are dissociative excitation transfer from metastable Ar atoms and Ar dimer excimers at low water concentration and electron impact dissociation of H2O at high water concentration. In spite of their similar sources, the density of OH was measured to be two orders of magnitude smaller than that of H at power densities on the order of 10⁵ Jm⁻³. This disparity is due to electron impact dissociation of OH during the discharge pulse and rapid reactions of OH in the presence of high H and O densities in the afterglow. It is often assumed that OH is the dominant non-selective reactive species in water-containing plasmas. These results reinforce the importance of atomic species such as H and O in water containing high energy density plasmas. A numerical parametric study revealed that the lowest energy cost for H2O2 production is achieved at low energy densities in pure water. The high concentration of atomic radicals, which rapidly recombine, results in an overall lower energy efficiency of reactive species production. In particular, the selectivity of H2O2 production decreases with increasing power density which instead favors H2 and O2 production.
Gas phase non-equilibrium plasmas jets containing water vapor are of growing interest for many applications. In this manuscript, we report a detailed study of an atmospheric pressure nanosecond pulsed Ar + 0.26% H2O plasma jet.
The plasma jet operates in an atmospheric pressure air surrounding but is shielded with a coaxial argon flow to limit the air diffusion into the jet effluent core. The jet impinges on a metal plate electrode and produces a stable plasma filament (transient spark) between the needle electrode in the jet and the metal plate. The stable plasma filament is characterized by spatially and time resolved electrical and optical diagnostics. This includes Rayleigh scattering, Stark broadening of the hydrogen Balmer lines and two-photon absorption laser induced fluorescence (TaLIF) to obtain the gas temperature, the electron density and the atomic hydrogen density respectively. Electron densities and atomic hydrogen densities up to 5×1022 m−3 and 2×1022 m−3 have been measured. This shows that atomic hydrogen is one of the main species in high density Ar-H2O plasmas. The gas temperature does not exceed 550 K in the core of the plasma.
To enable in situ calibration of the H TaLIF at atmospheric pressure a previously published O density calibration scheme is extended to include a correction for the line profiles by including overlap integrals as required by H TaLIF. The line width of H TaLIF, due to collision broadening has the same trend as the neutral density obtained by Rayleigh scattering. This suggests the possibility to use this technique to in situ probe neutral gas densities.
In this paper, atmospheric pressure N2/O2 plasma jets with homogeneous shielding gas excited by nanosecond pulse are obtained to generate simplex reactive nitrogen species (RNS) and reactive oxygen species (ROS), respectively, for the purpose of studying the simplex RNS and ROS to induce the myeloma cell apoptosis with the same discharge power. The results reveal that the cell death rate by the N2 plasma jet with N2 shielding gas is about two times that of the O2 plasma jet with O2 shielding gas for the equivalent treatment time. By diagnosing the reactive species of ONOO⁻, H2O2, OH and in medium, our findings suggest the cell death rate after plasma jets treatment has a positive correlation with the concentration of ONOO⁻. Therefore, the ONOO⁻ in medium is thought to play an important role in the process of inducing myeloma cell apoptosis.
A plasma-activated medium (PAM), which means a cell-culture medium irradiated with cold atmospheric plasmas or non-equilibrium atmospheric pressure plasma (NEAPP), has shown strong antitumor effects on various kinds of cells such as gastric cancer cells, human lung adenocarcinoma cells, human breast cancer cells and so on. In order to clarify the mechanism, it is extremely important to investigate the behaviors of stable and unstable reactive oxygen nitrogen species in culture medium irradiated by NEAPP. The roles of hydroxyl radicals (•OH) and nitric oxide (•NO) were studied to understand the dominant synthetic pathways of H2O2 and NO−2 in culture medium irradiated with NEAPP. In the PAM, •OH in the aqueous
phase was generated predominantly by photo-dissociation. However, most of the H2O2 nor NO−2 generated in the PAM did not originate from aqueous •OH and •NO. Pathways for the generation of H2O2 and NO−2 are suggested based on the high concentrations of intermediates generated at the gas/aqueous-phase interface following NEAPP irradiation. On the basis of
these results, the reaction model of chemical species in the culture medium is proposed.
Using global models, micro hollow cathode discharges (MHCDs) are compared to radiofrequency atmospheric pressure plasma jets (APPJs) in terms of reactive oxygen species (ROS) production. Ar/O-2 gas mixtures are investigated, typically with a small percentage of oxygen in argon. The same chemical reaction set, involving 17 species and 128 chemical reactions in the gas phase, is used for both devices, operated in the typical geometries previously published; the APPJ is driven by a radiofrequency voltage across a 1 mm gap, at atmospheric pressure, while the MHCD is driven by a DC voltage source, at 100 Torr and in a 400 mu m hole. The MHCD may be operated either in the self-pulsing or in the normal (stationary) regime, depending on the driving voltage. The comparison shows that in both regimes, the MHCD produces larger amounts of O-2*, while the APPJ produces predominantly reactive oxygen ground state species, O and O-3. These large differences in ROS composition are mostly due to the higher plasma density produced in the MHCD. The difference in operating pressure is a second order effect.
This paper presents an AC-excited argon discharge generated using a gas-liquid (two-phase) hybrid plasma reactor, which mainly consists of a powered needle electrode enclosed in a conical quartz tube and grounded deionized water electrode. The discharges in the gas-phase, as well as in the two-phase, exhibit two discharge modes, i.e., the low current glow-like diffuse mode and the high current streamer-like constrict mode, with a mode transition, which exhibits a negative resistance of the discharges. The optical emission spectral analysis shows that the stronger diffusion of the water vapor into the discharge region in the two-phase discharges boosts up the generation of OH (A-X) radicals, and consequently, leads to a higher rotational temperature in the water-phase plasma plume than that of the gas-phase discharges. Both the increase of the power input and the decrease of the argon flow rate result in the increase of the rotational temperature in the plasma plume of the water-phase discharge. The stable two-phase discharges with a long plasma plume in the water-phase under a low power input and gas flow rate may show a promising prospect for the degradation of organic pollutants, e.g., printing and dyeing wastewater, in the field of environmental protection.
Electron density is one of the key parameters in the physics of a gas discharge. In this contribution the application of the Stark broadening method to determine the electron density in low temperature atmospheric pressure plasma jets is discussed. An overview of the available theoretical Stark broadening calculations of hydrogenated and non-hydrogenated atomic lines is presented. The difficulty in the evaluation of the fine structure splitting of lines, which is important at low electron density, is analysed and recommendations on the applicability of the method for low ionization degree plasmas are given. Different emission line broadening mechanisms under atmospheric pressure conditions are discussed and an experimental line profile fitting procedure for the determination of the Stark broadening contribution is suggested. Available experimental data is carefully analysed for the Stark broadening of lines in plasma jets excited over a wide range of frequencies from dc to MW and pulsed mode. Finally, recommendations are given concerning the application of the Stark broadening technique for the estimation of the electron density under typical conditions of plasma jets.
An atmospheric-pressure N2–Ar plasma is investigated by means of optical emission spectroscopic diagnosis concerning the variation of its fundamental parameters, electron density and plasma temperature, and concentrations of ionized molecular nitrogen, atomic nitrogen, and excited argon with the tuning variables, such as the input power and the ratio of N2 in N2–Ar mixture gas, in the discharge region of the plasma torch. Moreover, qualitative discussions are delivered with respect to the mechanisms for nitrogen dissociation and influence of the Ar component on the N2 plasma discharge at atmospheric pressure.
Miniaturized atmospheric pressure glow discharges (APGDs) were generated in contact with small sized flowing liquid cathode systems. As anodes a solid pin electrode or a miniature flow Ar microjet were applied. Both discharge systems were operated in the open to air atmosphere. Hydrogen peroxide (H2O2) as well as ammonium (NH4+), nitrate (NO3−), and nitrite (NO2−) ions were quantified in solutions treated by studied discharge systems. Additionally, an increase in the acidification of these solutions was noted in each case. Emission spectra of the near cathode zone of both systems were measured in order to elucidate mechanisms that lead to the formation of active species in gas and liquid phases of the discharge. Additionally, the concentration of active species in the liquid phase (H2O2, NH4+, NO3− and NO2−) was monitored as a function of the solution uptake rate and the flow rate of Ar. The suitability of investigated discharge systems in the water treatment was tested on artificial wastewaters containing an organic dye (methyl red), hardly removable by classical methods non-ionic surfactants (light Triton x-45 and heavy Triton x-405) and very toxic Cr(VI) ions. Preliminary results presented here indicate that both investigated flow-through APGD systems may successfully be applied for the efficient and fast on-line continuous flow chemical degradation of toxic and hazardous organic and inorganic species in wastewater solutions.
The results of a study of the nanosecond discharge in H2 gas at pressures of (1–3) × 105 Pa using fast-framing photography and space- and time-resolved spectroscopy are presented. The discharge is initiated by the application of a high-voltage pulse with an amplitude of ~100 kV and duration of ~5 ns to a blade cathode placed at a distance of 20 mm from the anode. The results show the dynamics of the discharge formation and the build-up of the plasma electron density in the discharge channels close to and at a distance from the edge of the cathode. The results obtained are compared to those obtained in recent studies of similar discharges in air and He gas. It was shown that the time and space evolution of the plasma light emission in the H2 gas discharge is very similar to that in air. Namely, the generation of the plasma is mainly confined to the plasma channels initiated at the top and bottom edges of the cathode electrode and that there are no new plasma channels formed from the explosive emission centres along the blade as it was obtained in earlier experiments with He gas. Spectroscopic measurements showed that the plasma density reaches 2 × 10^17 cm−3 and 1.6 × 10^16 cm−3 in the vicinity of the cathode and the middle of the anode–cathode gap, respectively, for a plasma electron temperature of <1.5 eV. The values of plasma electron density and the previously presented results of electric field measurements allow calculation of the resistance of the plasma channels.
Plasmas in bubbles in water are being investigated for their ability to produce chemically reactive species for water purification and medical treatment. The gas forming the bubble is potentially a design parameter for water purification as the type and rate of production of active species may be controllable by the type of gas in the bubble. In this paper, we report on a computational investigation of the dynamics of plasmas in bubbles in water sustained in different gases. Images, optical spectra and plasma properties are discussed for plasmas in bubbles of N2, Ar and He in water, and compared to experiments. The differences in plasma dynamics and spatial distribution of the plasma (e.g., volume discharge or surface hugging) when using different gases depend in large part on the electron energy relaxation length, and the rate of diffusion of water vapour into the interior of the bubble. Electron impact dissociative excitation of water vapour, electron impact excitation of dissociation products and excitation transfer from the plasma excited injected bubble gases to water vapour all contribute to plasma emission. Variations in the contributions of these processes are responsible for differences in the observed optical spectra and differences in radical production.
In this Letter, we report that the air gas-liquid diffuse discharge plasma excited by bipolar nanosecond pulse in quartz container with different bottom structures at atmospheric pressure. Optical diagnostic measurements show that bountiful chemically and biologically active species, which are beneficial for effective sterilization in some areas, are produced. Such diffuse plasmas are then used to treat drinking water containing the common microorganisms (Candida albicans and Escherichia coli). It is found that these plasmas can sterilize the microorganisms efficiently.
The formation of transient species (OH, NO2, NO radicals) and long-lived chemical products (O3, H2O2,
,
) produced by a gas discharge plasma at the gas–liquid interface and directly in the liquid was measured in dependence on the gas atmosphere (20% oxygen mixtures with nitrogen or with argon) and pH of plasma-treated water (controlled by buffers at pH 3.3, 6.9 or 10.1). The aqueous-phase chemistry and specific contributions of these species to the chemical and biocidal effects of air discharge plasma in water were evaluated using phenol as a chemical probe and bacteria Escherichia coli. The nitrated and nitrosylated products of phenol (4-nitrophenol, 2-nitrophenol, 4-nitrocatechol, 4-nitrosophenol) in addition to the hydroxylated products (catechol, hydroquinone, 1,4-benzoquinone, hydroxy-1,4-benzoquinone) evidenced formation of NO2, NO and OH radicals and NO+ ions directly by the air plasma at the gas–liquid interface and through post-discharge processes in plasma-activated water (PAW) mediated by peroxynitrite (ONOOH). Kinetic study of post-discharge evolution of H2O2 and
in PAW has demonstrated excellent fit with the pseudo-second-order reaction between H2O2 and
. The third-order rate constant k = 1.1 × 103 M−2 s−1 for the reaction
was determined in PAW at pH 3.3 with the rate of ONOOH formation in the range 10−8–10−9 M s−1. Peroxynitrite chemistry was shown to significantly participate in the antibacterial properties of PAW. Ozone presence in PAW was proved indirectly by pH-dependent degradation of phenol and detection of cis,cis-muconic acid, but contribution of ozone to the inactivation of bacteria by the air plasma was negligible.
This paper is devoted to the experimental investigation of arc cutting of mild steel using plasmas generated in gas and liquid media. Due to different chemical compositions, the examined media have different thermophysical properties, which affect the properties of the generated plasma and cutting performance. The experiments are performed on 15 mm mild steel plates using commercial equipment at 60 A to approach real operation conditions in application areas. The studied gases are chosen according to recommendations of the world's leading manufacturers of arc cutting equipment for mild steel. Specific differences between plasma gases are discussed from the point of view of properties of the gas and the generated plasma, amount of removed material, kerf shape and overall energy balance of the cutting process. The paper describes the role of exothermic reaction of iron oxidation for oxygen cutting and explains its neglect for liquid cutting. This paper explains the potential of facilitating the cutting process by modification of the plasma gas chemical composition and flow rate.
We present a numerical model of a surface microdischarge (SMD) in humid air at atmospheric pressure. Our model includes over 50 species and 600 elementary reactions and consists of two, coupled well-mixed regions: a discharge layer with both charged and neutral species and an afterglow region consisting only of neutral species. Multiple time steps employed in our model enable capturing rapid dynamic behaviour in the discharge layer as well as the relatively slow diffusion and reaction in the afterglow. A short duration, high electric field is assumed to be excited at 10 kHz in the discharge region with power density maintained at 0.05 W cm−2. Among the predicted dominant species in the afterglow are O3, N2O5, N2O, HNO3, H2, NO3, H2O2, HNO2 and NO2. The results are in qualitative agreement with Fourier transform infrared absorption spectroscopy. Our simulation results show that density of those reactive species continues to evolve significantly in time, even after ~15 min of SMD exposure. This result suggests that SMD treatments on the order of minutes or less may involve significant neutral species concentration and flux transients, potentially affecting interpretation of results.
A novel spectroscopic method is proposed for the measurement of electron density and temperature in atmospheric pressure dielectric barrier discharges using nitrogen gas. Simplified collisional-radiative models for the electronic and the vibrational states yield two separate continuity equations as a function of the electron density and the temperature with the coefficients expressed in terms of rotational temperature, vibrational temperature, and emission intensity ratio between the first positive system and the second positive system of nitrogen molecules. The electron density and the temperature in nonequilibrium atmospheric pressure plasmas can be determined by solving the continuity equations with the coefficients estimated from the spectroscopic measurements. It was confirmed by applying to a high power dielectric barrier discharge, where the measured plasma parameters were in good agreement with the estimation by using the electron conductivity of the discharge.
We present an analysis of the procedure for plasma electron number density, Ne, diagnostics based on the
comparison of theoretical and experimental shape or width of hydrogen Balmer lines. Low Ne diagnostics,
requiring an extension of available theoretical Stark broadening data tables was examined first. The difficulties
encountered during experimental line profile analysis at low Ne are discussed and appropriate procedures
suggested. The widely adopted deconvolution of experimental profile by fitting with a Voigt function
is examined and it is shown that this deconvolution introduces large systematic error in Stark width determination.
This uncertainty can be decreased but never completely avoided by fixing the Gaussian part of
the Voigt function. Simple formulas of satisfactory accuracy for deconvolution at the half width of experimental
profile were investigated and their application recommended. The contribution of Van der Waals broadening
to the experimental profile is examined and the correction for its contribution discussed. Approximate
and reliable formulas for the evaluation of Ne from the Stark width were critically evaluated. To estimate the
applicability of different sets of theoretical data for Ne diagnostics, a comparison of theory versus experiments
was carried out and best data tables were recommended. For low Ne diagnostics the application of higher
members of Balmer series is advised whenever possible.
In this study, we investigated the inactivation characteristics of Geobacillus stearothermophilus spores under different plasma exposure conditions using low-pressure microwave plasma in nitrogen, oxygen and an air-simulated (N2:O2=4:1) gas mixture. The microwave-excited surface-wave plasma discharges were produced at low pressure by a large volume device. The directly plasma-exposed spores, up to 106 populations, were successfully inactivated within 15, 10 and 5 min of surface-wave plasma treatment using nitrogen, oxygen and an air-simulated gas mixture, respectively, as working gases within the temperature of 75 °C. The contribution of different inactivation factors was evaluated by placing different filters (e.g. a LiF plate, a quartz plate and a Tyvek® sheet) as indirect exposure of spores to the plasma. It was observed that optical emissions (including vacuum UV (VUV)/UV) play an important role in the inactivation process. To further evaluate the effect of VUV/UV photons, we placed an evacuated isolated chamber, inside which spores were set, into the main plasma chamber. The experimental results show that the inactivation time by VUV/UV photons alone, without working gas in the immediate vicinity of the spores, is longer than that with working gas. This suggests that the VUV/UV emission is responsible not only for direct UV inactivation of spores but also for generation of reactive neutral species by photoexcitation. The scanning electron microscopy images revealed significant changes in the morphology of directly plasma-exposed spores but no change in the spores irradiated by VUV/UV photons only.
In this paper it is shown that electronic quenching of OH(A) by water prevents thermalization of the rotational population distribution of OH(A). This means that the observed ro-vibrational OH(A–X) emission band is (at least partially) an image of the formation process and is determined not only by the gas temperature. The formation of negative ions and clusters for larger water concentrations can contribute to the non-equilibrium. The above is demonstrated in RF excited atmospheric pressure glow discharges in He–water mixtures in a parallel metal plate reactor by optical emission spectroscopy. For this particular case a significant overpopulation of high rotational states appears around 1000 ppm H2O in He. The smallest temperature parameter of a non-Boltzmann (two-temperature) distribution fitted to the experimental spectrum of OH(A–X) gives a good representation of the gas temperature. Only the rotational states with the smallest rotational numbers (J ≤ 7) are thermalized and representative for the gas temperature.
In this work we have performed experimental research into the influence of ion dynamics on the profiles of the Hα and Hβ lines of the hydrogen Balmer series. In order to understand this influence the electron density of a microwave plasma column at atmospheric pressure is measured from the Stark broadening of both lines. However, in this case Kepple–Griem's theory is not used, as usual, but a new computational model based on the μ-ion model that includes the effect of the ion dynamics on the profiles. The results obtained show that the difference between the electron density values from both the Hα and Hβ lines is about 3%. So, it is possible to use the Hα line for the diagnosis of the electron density in those cases in which it is not possible to use the Hβ line; for example, when the Hβ line is not intense enough.
Spectral line shapes and shifts of non-hydrogenic spectral lines of neutral atoms are considered for diagnostics of low electron density high pressure plasmas. Difficulties in application and limitations of this spectroscopic diagnostic technique are discussed in detail. The simultaneous presence of comparable Stark, Van der Waals and sometimes resonance broadening is discussed and a procedure to deduce the electron number density from the measured line width and/or shift is described. The importance and requirement of high accuracy theoretical and experimental data for plasma diagnostics are also discussed.
Results of the treatment of multi-crystalline silicon with low-pressure inductive plasma are presented. Plasma treatment was found to increase silicon electrical conductivity to a greater value relative to its initial state; the time to reach this maximum was found to depend strongly on temperature between 120 and 400 °C. An effect of plasma parameters on collection efficiency and diffusion length was observed by EBIC measurements. An actinometry method based on optical emission spectroscopy measurements was used to determine the molar fraction of monatomic hydrogen produced in plasma in the range between 5% and 8% during the treatment. The excitation temperature calculated by Boltzmann's method ranged between 4500 and 8000 K depending on the plasma gas composition, the pressure and the applied power. A model was developed using the CHEMKIN III® software to compute the role of operational parameters in hydrogen–silicon interactions. The aim of this work is to elucidate the relation between the plasma characteristics and the efficiency of hydrogen passivation on multi-crystalline silicon.
We use a global (volume averaged) model to study the dissociation processes and the presence of negative ions and metastable species in a low pressure high density O2/Ar discharge in the pressure range 1–100 mTorr. The electron density and the fractional dissociation of the oxygen molecule increases with increased argon content in the discharge. We relate this increase in fractional dissociation to an increase in the reaction rate for electron impact dissociation of the oxygen molecule which is due to the increased electron temperature with increased argon content in the discharge. The electron temperature increases due to higher ionization potential of argon than for molecular and atomic oxygen. We find the contribution of dissociation by quenching of the argon metastable Arm by molecular oxygen (Penning dissociation) to the creation of atomic oxygen to be negligible. The negative oxygen ion O− is found to be the dominant negative ion in the discharge. Dissociative attachment of the oxygen molecule in the ground state and in particular the metastable oxygen molecule O2(a 1Δg) are the dominating channels for creation of the negative oxygen ion O−.
A laser initiation and radio frequency (rf) sustainment technique has been developed and improved from our previous work to create and sustain large-volume, high-pressure air and nitrogen plasmas. This technique utilizes a laser-initiated, 15 mTorr partial pressure tetrakis (dimethylamino) ethylene seed plasma with a 75 Torr background gas pressure to achieve high-pressure air/nitrogen plasma breakdown and reduce the rf power requirement needed to sustain the plasma. Upon the laser plasma initiation, the chamber pressure is raised to 760 Torr in 0.5 s through a pulsed gas valve, and the end of the chamber is subsequently opened to the ambient air. The atmospheric-pressure plasma is then maintained with the 13.56 MHz rf power. Using this technique, large-volume (1000 cm <sup>3</sup>) , high electron density (on the order of 10<sup>11–12</sup> cm <sup>-3</sup> ), 760 Torr air and nitrogen plasmas have been created while rf power reflection is minimized during the entire plasma pulse utilizing a dynamic matching method. This plasma can project far away from the antenna region (30 cm), and the rf power budget is 5 W / cm <sup>3</sup> . Temporal evolution of the plasma electron density and total electron-neutral collision frequency during the pulsed plasma is diagnosed using millimeter wave interferometry. Optical emission spectroscopy (OES) aided by SPECAIR, a special OES simulation program for air-constituent plasmas, is used to analyze the radiating species and thermodynamic characteristics of the plasma. Rotational and vibrational temperatures of 4400–4600±100 K are obtained from the emission spectra from the N <sub>2</sub>(2+) and N <sub>2</sub><sup>+</sup>(1-) transitions by matching the experimental spectr-
um results with the SPECAIR simulation results. Based on the relation between the electron collision frequency and the neutral density, utilizing millimeter wave interferometry, the electron temperature of the 760 Torr nitrogen plasma is found to be 8700±100 K (0.75±0.1 eV ) . Therefore, the plasma deviates significantly from local thermal equilibrium.
This introductory review on plasma health care is intended to provide the interested reader with a summary of the current status of this emerging field, its scope, and its broad interdisciplinary approach, ranging from plasma physics, chemistry and technology, to microbiology, biochemistry, biophysics, medicine and hygiene. Apart from the basic plasma processes and the restrictions and requirements set by international health standards, the review focuses on plasma interaction with prokaryotic cells (bacteria), eukaryotic cells (mammalian cells), cell membranes, DNA etc. In so doing, some of the unfamiliar terminology-an unavoidable by-product of interdisciplinary research-is covered and explained. Plasma health care may provide a fast and efficient new path for effective hospital (and other public buildings) hygiene-helping to prevent and contain diseases that are continuously gaining ground as resistance of pathogens to antibiotics grows. The delivery of medically active 'substances' at the molecular or ionic level is another exciting topic of research through effects on cell walls (permeabilization), cell excitation (paracrine action) and the introduction of reactive species into cell cytoplasm. Electric fields, charging of surfaces, current flows etc can also affect tissue in a controlled way. The field is young and hopes are high. It is fitting to cover the beginnings in New Journal of Physics, since it is the physics (and non-equilibrium chemistry) of room temperature atmospheric pressure plasmas that have made this development of plasma health care possible.
Atmospheric pressure air plasmas are often thought to be in local thermodynamic equilibrium owing to fast interspecies collisional exchange at high pressure. This assumption cannot be relied upon, particularly with respect to optical diagnostics. Velocity gradients in flowing plasmas and/or elevated electron temperatures created by electrical discharges can result in large departures from chemical and thermal equilibrium. This paper reviews diagnostic techniques based on optical emission spectroscopy and cavity ring-down spectroscopy that we have found useful for making temperature and concentration measurements in atmospheric pressure plasmas under conditions ranging from thermal and chemical equilibrium to thermochemical nonequilibrium.
Transitions between the streamer and spark modes affect the stability of gas–liquid discharges, limiting their practical applications, while the nature of such transitions is poorly understood. Here we clarify the often neglected effect of a dielectric on the gas–liquid discharge stability, using direct and indirect grounding configurations. Discharge modes and plasma characteristics in these two configurations are investigated. The discharge appears in a transient spark mode in the direct grounding while in a streamer mode in the indirect case. It is shown that the dielectric significantly improves the discharge stability. The gas temperature and electron density in the transient spark are 380 K and 1017/cm3 with a 30 kV pulse voltage, respectively, which are higher than those in the streamer mode. Gas–liquid discharge is one of the hottest research topics in the area of nonequilibrium plasmas, in which stability is a key factor. This paper aims to present the effect of a dielectric on the stability of gas–liquid discharge, by comparing the discharge modes and plasma characteristics of nanosecond pulse gas–liquid discharge in atmospheric nitrogen with two different grounding configurations.
In this paper, a pulsed electrolyte cathode discharge is generated for the purpose of detecting metal elements by atomic emission spectrometry in atmospheric air. The discharge image, and the waveforms of voltage and current are obtained for studying the discharge mode. To understand the mechanisms of metal atomic excitation, the plasma temperature and the electron density of discharge are obtained by the spectra of N2 (C-B, Δν = −2) and Hβ (486.1 nm), respectively. Also, the effects of the solution pH, solution flow rate, discharge gap, and discharge voltage on the emission intensities of Cu and Fe are discussed to acquire the optimal experimental conditions. It is found that the pulsed electrolyte cathode discharge is a kind of atmospheric pressure glow discharge, and it can analyze metal elements accurately and sensitively. The gas temperature and electron density play important roles in the improvement of emission intensities of metal elements.
In this paper, a linear shape nanosecond pulsed dielectric barrier discharge is generated at atmospheric pressure for improving the hydrophilic property of aramid fibers. The discharge images, waveforms of voltage and current, and optical emission spectra of discharge are obtained to investigate plasma characteristics, and the water contact angles, scanning electron microscopy, and X-ray photoelectron spectroscopy are employed to estimate the modifying effects of plasma and investigate modification mechanisms. It is found that 75 s is an optimal treatment time in air under 2 mm discharge gap, 28 kV pulse peak voltage, and 100 Hz pulse repetition rate, and the energy density of discharge is about 2.1 J/cm². The improvement of aramid fiber hydrophilicity is due to the increasing of surface roughness and the formation of polar functional groups such as CN and OCO. Besides, the content of O2 has an obvious influence on plasma treatment, and CC is more easily oxidized under a high O2 concentration owing to the role of O atoms.
The decomposition of methylene blue (MB) via a novel double-chamber dielectric barrier discharge (DBD) reactor in different carrier gases (air and oxygen) was investigated. The results showed that the degradation efficiency of MB was 99.98% using O2 plasma for 20 min, while it was only 85.3% using air plasma for 100 min. In addition, the concentrations of nitrite, nitrate, ozone and hydrogen peroxide in aqueous phase and the oxidizing ability of the oxidants were measured to explore the various results obtained in different carrier gases. The formation of nitrogenous species was considered to be the main reason for the low degradation efficiency of the air plasma. The accumulation of oxidants enhanced the degradation efficiency of the MB in the O2 plasma. Both the combined effects of ozonation and plasma with oxygen bubbling and the reaction poisoning with air bubbling were enhanced in the double- chamber DBD reactor. The decomposition routes of MB and byproducts formation were also proposed.
Production rates of hydroxyl radical (•OH) formation in a gas-liquid plasma discharge reactor with pure water and argon were quantified using gas (carbon monoxide: CO) and liquid (ethanol) radical scavengers. The major oxidation products were acetaldehyde from ethanol and carbon dioxide (CO2) from CO. Total •OH production rates were estimated from the •OH required to form these products and hydrogen peroxide (H2O2). Ethanol completely depleted H2O2 suggesting that it is formed in or near the plasma-liquid interface. •OH production rates were higher with CO, and CO had a smaller effect on H2O2 suggesting additional •OH is available for other reactions at the plasma-gas interface. The best case total •OH production was approximately 25% of the thermodynamic limit for water dissociation based upon measured CO2 from CO.
In this work, a novel direct current (DC) atmospheric pressure rotating gliding arc (RGA) plasma reactor has been developed for plasma-assisted chemical reactions. The influence of the gas composition and the gas flow rate on the arc dynamic behaviour and the formation of reactive species in the N2 and air gliding arc plasmas has been investigated by means of electrical signals, high speed photography, and optical emission spectroscopic diagnostics. Compared to conventional gliding arc reactors with knife-shaped electrodes which generally require a high flow rate (e.g., 10-20 L/min) to maintain a long arc length and reasonable plasma discharge zone, in this RGA system, a lower gas flow rate (e.g., 2 L/min) can also generate a larger effective plasma reaction zone with a longer arc length for chemical reactions. Two different motion patterns can be clearly observed in the N2 and air RGA plasmas. The time-resolved arc voltage signals show that three different arc dynamic modes, the arc restrike mode, takeover mode, and combined modes, can be clearly identified in the RGA plasmas. The occurrence of different motion and arc dynamic modes is strongly dependent on the composition of the working gas and gas flow rate.
Non-equilibrium atmospheric-pressure plasmas have recently become a topical area of research owing to their diverse applications in health care and medicine, environmental remediation and pollution control, materials processing, electrochemistry, nanotechnology and other fields. This review focuses on the reactive electrons and ionic, atomic, molecular, and radical species that are produced in these plasmas and then transported from the point of generation to the point of interaction with the material, medium, living cells or tissues being processed. The most important mechanisms of generation and transport of the key species in the plasmas of atmospheric-pressure plasma jets and other non-equilibrium atmospheric-pressure plasmas are introduced and examined from the viewpoint of their applications in plasma hygiene and medicine and other relevant fields. Sophisticated high-precision, time-resolved plasma diagnostics approaches and techniques are presented and their applications to monitor the reactive species and plasma dynamics in the plasma jets and other discharges, both in the gas phase and during the plasma interaction with liquid media, are critically reviewed. The large amount of experimental data is supported by the theoretical models of reactive species generation and transport in the plasmas, surrounding gaseous environments, and plasma interaction with liquid media. These models are presented and their limitations are discussed. Special attention is paid to biological effects of the plasma-generated reactive oxygen and nitrogen (and some other) species in basic biological processes such as cell metabolism, proliferation, survival, etc. as well as plasma applications in bacterial inactivation, wound healing, cancer treatment and some others. Challenges and opportunities for theoretical and experimental research are discussed and the authors' vision for the emerging convergence trends across several disciplines and application domains is presented to stimulate critical discussions and collaborations in the future.
During the last two decades atmospheric (or high) pressure non-thermal plasmas in and in contact with liquids have received a lot of attention in view of their considerable environmental and medical applications. The simultaneous generation of intense UV radiation, shock waves and active radicals makes these discharges particularly suitable for decontamination, sterilization and purification purposes. This paper reviews the current status of research on atmospheric pressure non-thermal discharges in and in contact with liquids. The emphasis is on their generation mechanisms and their physical characteristics.
In most applications helium-based plasma jets operate in an open-air environment. The presence of humid air in the plasma jet will influence the plasma chemistry and can lead to the production of a broader range of reactive species. We explore the influence of humid air on the reactive species in radio frequency (rf)-driven atmospheric-pressure helium–oxygen mixture plasmas (He–O2, helium with 5000 ppm admixture of oxygen) for wide air impurity levels of 0–500 ppm with relative humidities of from 0% to 100% using a zero-dimensional, time-dependent global model. Comparisons are made with experimental measurements in an rf-driven micro-scale atmospheric pressure plasma jet and with one-dimensional semi-kinetic simulations of the same plasma jet. These suggest that the plausible air impurity level is not more than hundreds of ppm in such systems. The evolution of species concentration is described for reactive oxygen species, metastable species, radical species and positively and negatively charged ions (and their clusters). Effects of the air impurity containing water humidity on electronegativity and overall plasma reactivity are clarified with particular emphasis on reactive oxygen species.
This study focuses on the generation and loss of reactive oxygen species (ROS) in low-temperature atmospheric-pressure RF (13.56 MHz) He + O2 + H2O plasmas, which are of interest for many biomedical applications. These plasmas create cocktails of ROS containing ozone, singlet oxygen, atomic oxygen, hydroxyl radicals, hydrogen peroxide and hydroperoxyl radicals, i.e. ROS of great significance as recognized by the free-radical biology community. By means of one-dimensional fluid simulations (61 species, 878 reactions), the key ROS and their generation and loss mechanisms are identified as a function of the oxygen and water content in the feed gas. Identification of the main chemical pathways can guide the optimization of He + O2 + H2O plasmas for the production of particular ROS. It is found that for a given oxygen concentration, the presence of water in the feed gas decreases the net production of oxygen-derived ROS, while for a given water concentration, the presence of oxygen enhances the net production of water-derived ROS. Although most ROS can be generated in a wide range of oxygen and water admixtures, the chemical pathways leading to their generation change significantly as a function of the feed gas composition. Therefore, care must be taken when selecting reduced chemical sets to study these plasmas.
A kinetic scheme for non-equilibrium discharge in nitrogen-oxygen mixtures is developed, which almost wholly describes chemical transformations of particles in the cold (200 K<or=T<or=500 K) vibrationally unexcited gas. The kinetic scheme includes processes of excitation of electronic states, destruction and ionization of heavy particles by electron impact, associative ionization, electron attachment and detachment, electron-ion and ion-ion recombination, chemical transformations of neutral particles (in ground and excited electronic states) and ion conversion. On the basis of kinetic modelling in the framework of the kinetic scheme proposed, the influence of the electronic excitations of nitrogen molecules and atoms on air composition dynamics is analysed.
This monograph presents a comprehensive description of the theoretical
foundations and experimental applications of spectroscopic methods in
plasma physics research. The first three chapters introduce the
classical and quantum theory of radiation, with detailed descriptions of
line strengths and high density effects. The next chapter describes
theoretical and experimental aspects of spectral line broadening. The
following five chapters are concerned with continuous spectra, level
kinetics and cross sections, thermodynamic equilibrium relations,
radiative energy transfer, and radiative energy losses. The book
concludes with three chapters covering the basics of various
applications of plasma spectroscopy to density and temperature
measurements and to the determination of some other plasma properties.
Over one thousand references not only guide the reader to original
research covered in the chapters, but also to experimental details and
instrumentation. This will be an important text and reference for all
those working on plasmas in physics, optics, nuclear engineering, and
chemistry, as well as astronomy, astrophysics and space physics.
Atmospheric-pressure, non-equilibrium plasmas are susceptible to instabilities and, in particular, to arcing (glow-to-arc transition). Spatially confining the plasma to dimensions of 1 mm or less is a promising approach to the generation and maintenance of stable, glow discharges at atmospheric-pressure. Often referred to as microdischarges or microplasmas, these weakly-ionized discharges represent a new and fascinating realm of plasma science, where issues such as the possible breakdown of 'pd scaling' and the role of boundary-dominated phenomena come to the fore. Microplasmas are generated under conditions that promote the efficient production of transient molecular species such as the rare gas excimers, which generally are formed by three-body collisions. Pulsed excitation on a sub-microsecond time scale results in microplasmas with significant shifts in both the temperatures and energy distribution functions associated with the ions and electrons. This allows for the selective production of chemically reactive species and opens the door to a wide range of new applications of microplasmas. The implementation of semiconductor and microelectronics and MEMs microfabrication techniques has resulted in the realization of microplasma arrays as large as 250,000 devices. Fabricated in silicon or ceramics with characteristic device dimensions as small as 10 µm and at packing densities up to 104 cm−2, these arrays offer optical and electrical characteristics well suited for applications in medical diagnostics, displays and environmental sensing. Several microplasma device structures, including their fundamental properties and selected applications, will be discussed.
Electrical and optical emission properties of a burning plasma between a liquid cathode and a metal anode are presented in this paper. The plasma has constricted contact points at the liquid cathode and is clearly filamentary in nature near the water surface.The cathode voltage drop depends on conductivity rather than pH and is significantly different for distilled water and electrolyte solutions. An acidification of the liquid due to the plasma is always observed.The rotational temperature of OH and N2 in the bulk of the plasma is, respectively, in the range 3200–3750 K and 2500–2750 K. The rotational temperature of nitrogen near the metal anode is typically two times smaller. Electron densities near the cathode measured by Stark broadening of Hβ are in the range (5.5–8.0) × 1014 cm−3, the atomic excitation temperatures in the range 5750–7250 K. Differences in electrical and optical emission properties between the cases when distilled water and electrolyte solutions are used as cathode are discussed in detail.
In this paper the rotational temperature of OH(A–X) and rotational population distribution of OH(A) are investigated for streamer discharges in bubbles and glow discharges with liquid electrodes, both at atmospheric pressure. The influence of the filling gas is investigated in detail and the non‐Boltzmann nature of the rotational population distributions is discussed. It is shown that the rotational population distribution of OH(A) is even at atmospheric pressure an image of the formation process or is at least influenced by it. As a consequence the rotational temperature is in this case not a good estimate of the gas temperature as the rotational population distribution is not an image of a kinetic temperature. In some cases rotational states with small rotational numbers offer a possibility to obtain the gas temperature. The influence of these results on the determination of gas temperatures in the field of liquid plasmas is discussed.
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The decomposition of three β-lactam antibiotics (amoxicillin, oxacillin and ampicillin) in aqueous solution was investigated using a dielectric barrier discharge (DBD) in coaxial configuration. Solutions of concentration 100 mg/L were made to flow as a film over the surface of the inner electrode of the plasma reactor, so the discharge was generated at the gas-liquid interface. The electrical discharge was operated in pulsed regime, at room temperature and atmospheric pressure, in oxygen. Amoxicillin was degraded after 10 min plasma treatment, while the other two antibiotics required about 30 min for decomposition. The evolution of the degradation process was continuously followed using liquid chromatography-mass spectrometry (LC-MS), total organic carbon (TOC) and chemical oxygen demand (COD) analyses.