Electron Excitation Temperature Research Articles

A large-scale filament-free planar plume generated by an argon plasma jet in a gas-confined barrier discharge geometry

Large-scale plumes in a plasma jet are desirable for fast processing of materials with large surface, which are normally composed of discharge filaments. A filamentary plume may cause nonuniform treatment or even damage to vulnerable samples. In this Letter, an argon plasma jet in a gas-confined barrier discharge geometry is proposed to generate a large-scale filament-free plume. Results indicate that the filament-free plume can only be sustained in a relatively low voltage amplitude (Vp), which transits to the filamentary plume with increasing Vp. There is only one negative discharge per voltage cycle for the filament-free plume, while both positive and negative discharges for the filamentary plume. Fast photography reveals that the negative discharge is diffuse, originating from the propagation of a negative streamer. On the contrary, the positive discharges are filamentary. Optical emission spectroscopy indicates that the filament-free plume has lower electron density, electron excitation temperature, and molecular vibrational temperature compared to the filamentary plume, while gas temperature keeps at a low value with varying Vp.

Characterization of cascaded arc He plasma in a compact linear plasma device using voltammetry and optical emission spectroscopy

Cascaded arc plasma has been widely applied in linear plasma devices (LPDs) to produce high flux plasma for the study of plasma-material interaction. In this work, cascaded arc He plasma produced in an LPD with a compact arrangement is investigated by voltammetry and optical emission spectroscopy (OES). The results show that the cathode potential increases with the discharge current while it firstly decreases and then increases as increasing the gas flow rate. A local reverse electric field is observed at low gas flow rates between two cascaded plates (i.e. floating electrodes) near the cathode. The OES’ results reveal that as the gas flow rate increases, the intensity of He I lines increases and the electron excitation temperature (T exc ) decreases. As increasing the discharge current, the intensity of He lines exhibits various trends at different gas flow rates, showing a monotonic decline at 1.94 slm and a first increase followed by a reduction at 3.52 slm. The T exc increases with the discharge current. These findings could preliminarily shed light on the properties of cascaded arc of He plasma in the compact LPD and aid in the optimization of the device to generate the high-flux divertor-relevant plasma.

Spatio-temporal dynamics of a microsecond pulsed Grimm-type glow discharge

Spatio-temporal dynamics of a PGD, in Ar6.0 at 6 mbar for some of Ar I, Ar II, Cu I, Cu II lines. Analysis of a high current peak during ignition of discharge, excitation and deexcitation processes, development of electron excitation temperature of Ar.

Effects of Surface-Wave-Sustained Argon Plasma Torch Interaction with Liquids

In this paper, an investigation of the interaction of a surface-wave-sustained argon plasma torch with liquids is presented. The plasma is produced by an electromagnetic wave traveling along the plasma–dielectric interface, and at the same time, the plasma is a part of this waveguide structure. Because the interaction of the plasma torch with water (liquid) results in modifications of the properties of both the treated water and the plasma itself, a detailed study of the effects in both media is required. The results of the experimental investigation of a surface-wave-sustained argon plasma torch interaction with liquids show significant changes in the plasma parameters, such as the electron excitation temperature Te and the average rotation temperature Trot. In addition, mechanical waves are produced both in the meniscus surface and in the plasma torch by the interaction between the plasma torch (ionized gas with charged particles and electric field) and the liquid surface, which is different from the effects produced by a neutral gas jet on a liquid surface. As a result of the plasma–water interaction, the water’s chemical and physical characteristics, such as the water conductivity, pH, and H2O2 concentration, are modified. As a possible application for water purification, the performed SWD treatment of model wastewater shows a significant variation in nitrate, ammonium, phosphate, and COD (chemical oxygen demand) concentration as a result of the treatment.

Ammonia Nitrogen Removal by Gas–Liquid Discharge Plasma: Investigating the Voltage Effect and Plasma Action Mechanisms

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.

Plasma parameters and electric field fluctuations in a cold micro-plasma jet interacting with a substrate

Interaction of a cold atmospheric pressure micro-plasma jet with different types of substrates having a range of electrical permittivity (εr), such as conductor (copper), semiconductor (p-type silicon), insulator (Teflon and quartz), and biological (goat skin), is carried out experimentally. The electric field fluctuations, cross-phase analysis, plasma parameters [electron excitation temperature (Texc) and electron density (ne)], average propagation velocity of the plasma bullet, gas temperature (Tg), power dissipated on the substrates, and substrate temperature are investigated during the interaction. Cross phase analysis and plasma bullet velocity support the return stroke phenomenon for high εr samples, such as copper and silicon, and a surface ionization wave is generated in the case of low εr samples, such as Teflon, quartz, and biological tissue. The highest substrate current (IS) is observed for copper due to its high conductivity. Tg and ne are affected by the interaction; however, Texc is observed to change only slightly. ne is comparatively higher for high εr samples, and for all samples, it increases initially and, thereafter, decreases as we get closer to the sample's surface. Tg is comparatively higher for low εr samples and increases axially downward from the orifice of the jet, it is lower and almost constant for copper and silicon. The electric field fluctuation (EZ and Eϕ components) frequencies lie up to ∼8 kHz with a peak amplitude at ∼1 kHz, which is found to be comparatively higher for low εr samples.

Enhancement of NOx production in water by combining an air bubble plasma jet and an external magnetic field

Production of NOx (NO2− + NO3−) in water with an air bubble discharge plasma jet under the influence of an external axial steady magnetic field was investigated experimentally. The gas phase plasma parameters, rotational (Tr), vibrational (Tv) and electronic excitation (Tx) temperatures, and electron density (ne), as well as the liquid phase pH and the concentrations of nitrite (NO2−) and nitrate (NO3−), were measured as a function of treatment time and magnetic field strength. It was found that Tr, Tv, Tx, and ne slightly increased as a function of magnetic field strength in the gas phase plasma. The pH decreased both with treatment time and magnetic field strength. In the maximum field strength of 290 mT, the concentrations of NO2− and NO3− were ∼82% and ∼74%, respectively, greater than with B=0. With B=290 mT, the energy cost for producing NOx was ∼78% lower than with B=0. The energy cost may likely be reduced due to decreasing radial diffusion loss of charged species in the discharge with increasing magnetic field strength.

Temporal evolution characteristics of the excited species in a pulsed needle-water discharge: effect of voltage and frequency

Transient spark discharge has broad application prospects due to its excellent performance in generating reactive oxygen species, but there is currently limited research on transient excited species, which are important precursors for the formation of these species. In this paper, we obtained a transient spark discharge with a needle-water electrode configuration, and then developed an optical detection system based on a photomultiplier to measure the temporal evolution of four excited species. It is observed that N2 (C) is produced mainly in the streamer phase, while H (n = 3), O (3p), and OH (A) are produced mainly in the spark phase, which results from the difference in threshold energy and pathway for generating different species. To investigate the variation mechanism of peak time and peak intensity during the evolution process, the gas temperature T g, electron density n e, and electron excitation temperature T exc were calculated by using the optical emission spectroscopy under different pulse voltage and pulse repetition frequency conditions. The results indicate that the discharge conditions influence the physical parameters of T g, n e, and T exc as well as the output voltage waveform, and then they jointly determine the evolution characteristics of excited species. These insights contribute to understanding the characteristics of excited species which affect the formation of reactive species by influencing the chemical chain reaction in the transient spark discharge.

“One stone three birds” plasma engraving strategy to boost the hydrogen evolution reaction activity of layered double hydroxides

Possessing large specific surface areas and rich metal redox sites, layered double hydroxides (LDHs) are potentially suitable oxygen evolution reaction catalysts. It is a pity that they usually show poor hydrogen evolution reaction (HER) activity on account of the limited conductivity and deficient active sites. Taking NiFe LDH nanosheets as an example, we develop a “one stone three birds” plasma engraving strategy to enhance the HER activity of NiFe LDH. The “three birds,” including the reduction of Ni2+ to Ni nanoparticles (Ni NPs), generation of more oxygen vacancies (Ov), and exfoliation of nanosheets into much thinner ones, can obviously improve the conductivity and active sites of NiFe LDH. The plasma processing can also enhance water adsorption and accelerate the Volmer step during HER. As expected, the plasma-engraved NiFe LDH (PEH) exhibits enhanced HER activity with a low overpotential of 22 mV at 10 mA cm−2 and a small Tafel slope of 38 mV dec−1 in 1 M KOH, much better than NiFe LDH (202 mV, 145 mV dec−1). By combining optical emission spectroscopy diagnosis and structural/electrochemical characterizations, the relationship among the electron excitation temperature (Texc) in plasma, the amount of Ni NPs and Ov in PEH, and the HER activity of PEH is established. Excitingly, the PEH also displays splendid HER activity in both alkaline real seawater and overall water splitting.

Study of the evolution of pulsed plasma under an external longitudinal magnetic field

An experimental study on the role of an external longitudinal magnetic field on the characteristics changes of pulsed plasma stream, produced in argon medium, is carried out at different time spans of its evolution. The spectroscopic observations are time integrated and are carried out at different times of plasma formation for pulsed discharge. This study gives insights into the recombination and diffusion phase of the plasma species in the presence of the magnetic field. The transition of plasma species from a dominant recombination phase to a diffusional phase is well revealed by the density profile during the time evolution. Moreover, the decrease in electron temperature and the increase in electron excitation temperature explain the energy transfer to electrons due to metastable quenching, and the system gradually approaches equilibrium. The magnetic field also affects the transitions of the ionized argon population between different energy levels. It is found that faster decay occurs for transitions of different plasma species to non-metastable states, while the populations of metastable states exist for a longer time. In addition, the time-resolved morphology changes of the plasma stream are also observed by high speed imaging, which shows the flow structure of the plasma stream at different time frames. The imaging of the plasma stream evolution shows the initial ejection of the plasma sheet from the electrode assembly, its detachment, the steady flow, and gradually its nature of dying out.

Optical and electrical investigation of plasma generated by high-energy self-stabilized spark ignition system

Spark discharge plasma is commonly used for ignition in internal combustion engines. The environmental performance of internal combustion engines with forced ignition is improved when operating under lean mixture conditions. High-energy ignition systems are needed to ensure reliable ignition of lean mixtures. The ignition of a combustible mixture is influenced by several plasma parameters, such as the temperature of its various components, the size of the plasma, and the deposited energy. It is, therefore, beneficial to know these parameters. Here, we present optical and electrical investigation of plasma generated in ambient air by a novel high-energy self-stabilized spark ignition system. The electrical investigation showed two high current pulses, with the current amplitude of ∼40 and ∼150 A. The energy is deposited to the spark gap mainly during the second current pulse, and it is increasing from 213 to 541 mJ with the increasing gap size from 3 to 13 mm. The energy efficiency increases with the gap as well, from around 23% to 58%. Time-resolved emission spectra enabled us to estimate the evolution of the gas temperature, electron excitation temperature, and electron density in the generated plasma. It was found that the highest electron density, 3–4 × 1017 cm−3, correlates with the maximum of the second pulse current. We observed a specific plasma evolution between the two current pulses, with an increase in temperature from 4500 to 7500 K and a contraction of the plasma channel diameter from 3.3 to 0.5 mm.

A double‐mode planar argon plume produced by varying the distance from an atmospheric pressure plasma jet

AbstractAtmospheric pressure planar plumes are desirable for the applications of low temperature plasmas, such as rapid modification of large‐scale surfaces. Up to now, only single‐mode planar plumes with either a streamer mode or a filamentary mode have been reported in the literature. Distinctive from the single‐mode planar plumes, a double‐mode argon planar plume has been generated in this article, which operates in the streamer mode with a larger distance away from a plasma jet and transits to the filamentary mode with decreasing the distance. Discharge characteristics and plasma parameters are compared for the two modes. Results indicate that the streamer mode and the filamentary mode correspond to pulsed and humped discharges respectively. Fast photography reveals that the streamer‐mode plume is composed of stochastically branching streamers, while the filamentary‐mode plume results from a series of moving filaments similar to those in barrier discharge. In contrast to the streamer mode, the filamentary mode has lower excited electron temperature and vibrational temperature, whereas higher electron density and gas temperature. In addition, better hydrophilicity of polyethylene terephthalate surface is achieved in the filamentary mode.

Obtaining plasma parameters by Langmuir probes and optical emission spectroscopy in low-pressure DC plasma

The plasma environment in low-pressure systems allows coating different materials such as glasses, textiles, metals, etc. at the atomic level. Although vacuum coating is one of the best methods for thin film coating, the coating quality is closely affected by the spatial and temporal non-uniformities inside the reactor. Therefore it is quite important to determine the plasma properties within the chamber. The determination of plasma parameters such as electron temperature, electron excitation temperature, electron and ion densities can be realized using Langmuir probe systems, and by Optical Emission Spectroscopy (OES) by using Boltzmann plot method. Knowing such parameters can provide important information about specific reaction mechanisms, as well as it provides invaluable information that can increase reproducibility in various discharge process control in thin film coating. In this study, a homemade single Langmuir probe (s-LP) and a multiple Langmuir probe system (m-LP) were used to calculate electron temperature, electron density, saturation current, and plasma potential of the plasma at different pressures and at different voltages. One of the objectives of this work was to evaluate the electron temperature and ion density inside the plasma during a coating process. The second objective was to find the spatial change of electron temperature in the axial direction between cathode and anode by m-LP system including 8 independent tips. The last objective was to measure the axial plasma electron temperature distribution inside the vacuum chamber by using OES.

Effect of external longitudinal magnetic field on the dynamics of pulsed plasma stream

A spectroscopic study is carried out to investigate pulsed plasma stream produced in a coaxial plasma accelerator under the influence of an external longitudinal magnetic field. The dynamics of Argon plasma stream that includes the excitations and emissions of plasma species and its different parameter variations on the application of an external longitudinal magnetic field are reported in this work. The behavior of the ionized and neutral species of argon plasma is studied from their intensity distribution profiles which indicate the occurrence of faster recombination as compared to the field-free case. The variation of pressure reveals a significant influence on the recombination and diffusion process of the plasma species. Moreover, the variation of cathode voltage leads to variation in input energy to the system which influences the ionization and excitation processes of the plasma species. A plasma density (Ne) jump is achieved from 1021–1022 m−3 at a relatively higher pressure ∼8 bar plenum pressure in presence of the magnetic field. Saturation of the emissions and excitations of plasma species are revealed from the intensity profile and electron excitation temperature (Texc) profile which occur at different pressure and voltage both for field and field-free cases. The electron temperature (Te) is estimated using the FLYCHK code by comparing the simulated charge state distributions with the experimental findings. Using known parameters in the code, the different estimated Te shows a linear and an exponential decrement respectively in the absence and presence of the magnetic field. Maximum Te of 2.2 eV is reported which decreases to 1.51 eV at a significant increase of plasma density of 1022 m−3.

Spectral diagnosis of atmospheric pressure AC argon plasma jet at constant power

In the application of atmospheric pressure plasma jet, because the frequency of AC power supply is limited in the kHz range, the research on the influence of power supply electrical parameters on discharge is basically aimed at the variation of plasma jet characteristics with a single driving electrical parameter ( such as voltage and frequency). However, the discharge power usually changes with a single electrical parameter changing, which can undoubtedly affect the discharge performances including the plasma physical parameters and generated reactive species, resulting in the failure to reflect the influence of the single driving parameter on the discharge. In this study, an atmospheric pressure argon plasma jet is driven by a home-made AC power supply with adjustable pulse modulated duty cycle. And combining 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 <i>T</i><sub>g</sub>, electron excitation temperature <i>T</i><sub>exc</sub>, electron density <i>n</i><sub>e</sub>, and OH radical particle number density of the plasma jet are studied under a constant discharge power of 2 W. The results show that at the constant power, the electron density n<sub>e</sub> 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 <i>T</i><sub>g</sub>, electron excitation temperature <i>T</i><sub>exc</sub>, and OH radical particle density are most affected by the pulse modulated duty cycle, followed by driving voltage, and the frequency effect is the smallest. Under the constant power, as the frequency decreases, the voltage will increase, and also the gas temperature <i>T</i><sub>g</sub>, electron excitation temperature <i>T</i><sub>exc</sub>, and OH radical particle number density will increase. On the contrary, although the voltage also increases as the pulse modulated duty cycle decreases, the gas temperature <i>T</i><sub>g</sub>, electron excitation temperature <i>T</i><sub>exc</sub>, and OH radical particle number density are all reduced. In addition, the results indicate that reducing the duty cycle of AC power can make the atmospheric pressure plasma jet produce more OH radicals 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 constant power, and also presents a guidance for choosing 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.

Investigation on plasma enhanced decomposition of ammonium dinitramide (ADN) based propellant with optical diagnosis

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.

Investigation on spheroidization of refractory tungsten powders by laminar DC plasma torch

The spherical tungsten powders were prepared by a low-power DC laminar plasma torch. At a discharge current of 50 A, the tungsten powder fed downstream cannot be spheroidized by using Ar discharge, while a small number of spherical particles can be observed with upstream feeding. When N2 is added to the working gas, the input power is increased to maintain the same discharge current, which leads to the increase of plasma jet length, thereby expanding the effective high temperature region. Optical emission spectroscopy measurements show that the electron excitation temperature of N2-Ar mixed discharge is higher than that of argon discharge. In addition, compared with argon plasma, N2-Ar mixed gas plasma has larger thermal conductivity due to the molecular dissociation. Therefore, the heat is more efficiently transferred from the plasma to the tungsten particles. The improvement of plasma jet and heat transfer characteristics by introducing nitrogen into the discharge contributes to the increase of spheroidization rate. EDS analysis shows that the oxygen content in the tungsten particles decreases after spheroidization, which is caused by the evaporation of tungsten oxide during plasma treatment.

Discharge Characteristics of Atmospheric Pressure Ar/N₂/O₂ a Plasma Jet Under a Square Wave Pulse

Plasma jets have broad application in the fields of sterilization and material processing. In recent years, studies have shown that adding a small amount of active gas into a single inert gas can change the distribution of active particles in the plasma. This not only produces the desired active species but also changes the length of the jet to make it more suitable for plasma applications. In this article, the characteristics of plasma jets were studied under an argon atmosphere with a small amount of nitrogen and oxygen. By measuring the voltage and current waveforms, luminescence images, and emission spectra under the three mixed gases, the main parameters such as discharge power, the three main particle emission spectrum intensities (the OH spectral line, the second positive band of nitrogen molecules and its diffraction band, argon atomic line), electron excitation temperature, molecular vibration temperature, and molecular rotation temperature are calculated. The results show two discharges in one cycle under the square wave pulse voltage. The addition of nitrogen and oxygen reduces the peak current of the Ar plasma jet but increases the power. The addition of N2 increases and stabilizes the length of the jet, while oxygen decreases the length. The spectral line intensities of OH and Ar atoms decrease with the increase in N2 doping concentration, while the spectral line intensity of nitrogen gas increases. The addition of oxygen reduces the intensity of all the three emission spectra. The electron excitation temperature is the highest in the mixed gas (Ar/N2/O2) and the lowest in pure argon. In the mixed gas (Ar/N2), the vibration temperature and rotation temperature are basically unchanged. After the addition of oxygen, the vibration temperature remained unchanged, but the rotation temperature increased slightly.

Vibrational Emission Study of the CN and C2 in Nylon and ZnO/Nylon Polymer Using Laser-Induced Breakdown Spectroscopy (LIBS).

The laser-induced breakdown spectroscopy (LIBS) technique was performed on polymers to study the neutral and ionic emission lines along with the CN violet system (B2Σ+ to X2Σ+) and the C2 Swan system (d3 Пg–a3 Пu). For the laser-based emission analyses, the plasma was produced by focusing the laser beam of a Q-switched Nd: YAG laser (2ω) at an optical wavelength of 532 nm, 5 ns pulse width, and a repetition frequency of 10 Hz. The integration time of the detection system was fixed at 1–10 ms while the target sample was positioned in air ambiance. Two organic polymers were investigated in this work: nylon and nylon doped with ZnO. The molecular optical emission study of nylon and doped nylon polymer sample reveals CN and C2 molecular structures present in the polymer. The vibrational emission analysis of CN and C2 bands gives information about the molecular structure of polymers and dynamics influencing the excitation structures of the molecules. Besides, it was further investigated that the intensity of the molecular optical emission structure strongly depends on the electron number density (cm−3), excitation temperature (eV), and laser irradiance (W/cm2). These results suggest that LIBS is a reliable diagnostic technique for the study of polymers regarding their molecular structure, identification, and compositional analysis.

Varying magnetic field strength as an effective approach to boost up the plasma signal in laser-induced breakdown spectroscopy

Externally variable magnetic field was incorporated with the combination of laser induced breakdown spectroscopy (LIBS) to enhance the emission characteristics of aluminum (Al) plasma. Significant emission enhancement of laser induced plasma (LIP) was obtained at different magnetic field strengths, for instance, enhancement factors of about 1.2, 1.3 and 1.4 times were observed at field-strength of 0.4, 0.5 and 0.6 T, respectively. The electron-impact excitation rates and recombination process were increased at higher field-strengths, which led to the higher emission signal due the stronger plasma confinement by the field. The electron number density and electron temperature were measured using the spectroscopic techniques at several delay times. At higher field strengths, both electron density and electron excitation temperature showed an increased trend as compared to the case when No-field was applied. Hence, the research has significance for enhancing the plasma signal which led to improve the LIBS sensitivity.