Excited Argon Atoms Research Articles

Microwave atmospheric pressure plasma jets for wastewater treatment: Degradation of methylene blue as a model dye

The degradation of methylene blue in aqueous solution as a model dye using a non thermal microwave (2.45 GHz) plasma jet at atmospheric pressure has been investigated. Argon has been used as feed gas and aqueous solutions with different concentrations of the dye were treated using the effluent from plasma jet in a remote exposure. The removal efficiency increased as the dye concentration decreased from 250 to 5 ppm. Methylene blue degrades after different treatment times, depending on the experimental plasma conditions. Thus, kinetic constants up to 0.177 min−1 were obtained. The higher the Ar flow, the faster the degradation rate. Optical emission spectroscopy (OES) was used to gather information about the species present in the gas phase, specifically excited argon atoms. Argon excited species and hydrogen peroxide play an important role in the degradation of the dye. In fact, the conversion of methylene blue was directly related to the density of argon excited species in the gas phase and the concentration of hydrogen peroxide in the aqueous liquid phase. Values of energy yield at 50% dye conversion of 0.296 g/kWh were achieved. Also, the use of two plasma applicators in parallel has been proven to improve energy efficiency.

U-Shaped Ultrahigh Frequency Atmospheric Pressure Plasma Jet With Magnetic Loop Antenna

An atmospheric pressure argon plasma was generated using a magnetic loop antenna with a pulse modulated ultrahigh frequency signal of 850 MHz. Finite-element simulations were performed to study the patterns of gas flow and electromagnetic waves. Fast photography of the plasma jet was used to study temporal behavior of the plasma jet. Emission and mass spectrometry were used to identify active species in the plasma jet. The detected dominant species included excited argon atoms and singly charged ions of argon, nitrogen, oxygen, nitrogen oxide, and water. To study the effect of the plasma jet on biological samples, a thin onion epidermal membrane was treated with the plasma jet. The hydrophobic layer displayed hydrophilic characteristics after treatment for 1 min.

A comprehensive model of Penning energy transfers in Ar − CO2 mixtures

Ionizing energy transfer mechanisms due to the excited argon atoms, called Penning transfers, have been investigated for various Ar − CO2 mixtures at 0.4, 0.8, 1.2 and 1.8 atm gas pressures. The Penning energy transfer probabilities are extracted from the systematic gas gain measurements carried out in cylindrical proportional counters. In this report, contributions of the several transfer processes are identified by studying the pressure and mixing proportion dependence of the transfer rates with a model.

3-D Numerical Characterization of a Microwave Argon PECVD Plasma Reactor at Low Pressure

A 3-D model of a microwave plasma (mwp)-enhanced chemical vapor deposition (PECVD) reactor at 2.45 GHz in argon at low pressure describing self-consistently, the coupling of the microwave energy into the plasma is presented. The characteristics of the discharge are simulated using a fluid plasma model which solves the electron and ion continuity equations, electron energy balance equation, and the Poisson’s equation by finite element method, using COMSOL Multiphysics software. The physical behavior of the microwave PECVD discharge, such as plasma density, electron temperature, electric field and plasma potential, are simulated and analyzed. The chemical reactions considered in this paper are: elastic, superelastic, excitation, ionization, penning ionization and metastable quenching processes, involving electrons, ions (Ar+), neutral atoms (Ar), and excited metastable argon atoms (Ar*). The plasma characterization results are studied for a gas temperature of 300 K, a gas pressure of 100 mtorr and a microwave power of 600 W. The effect of varying gas pressure from 50 to 200 mTorr has been studied. The obtained results turn out to be in agreement with previous measurements and show that this kind of model can lead to a better understanding of the physical processes occurring in this kind of microwave reactor and thus allow optimization of this device.

Mid-infrared ultrafast laser pulses induced third harmonic generation in nitrogen molecules on an excited state.

We report on generation of third harmonic from nitrogen molecules on the excited state with a weak driver laser pulse at a mid-infrared wavelength. The excited nitrogen molecules are generated using a circularly polarized intense femtosecond pulse which produces energetic electrons by photoionization to realize collisional excitation of nitrogen molecules. Furthermore, since the third harmonic is generated using a pump-probe scheme, it enables investigation of the excited-state dynamics of nitrogen molecules produced under different conditions. We also perform a comparative investigation in excited argon atoms, revealing different decay dynamics of the molecules and atoms from the excited states in femtosecond laser induced filaments.

Study of a contracted glow in low-frequency plasma-jet discharges operating with argon

In this work, we present an experimental and theoretical study of a low frequency, atmospheric plasma-jet discharge in argon. The discharge has the characteristics of a contracted glow with a current channel of submillimeter diameter and a relatively high voltage cathode layer. In order to interpret the measurements, we consider the separate modeling of each region of the discharge: main channel and cathode layer, which must then be properly matched together. The main current channel was modeled, extending a previous work, as similar to an arc in which joule heating is balanced by lateral heat conduction, without thermal equilibrium between electrons and heavy species. The cathode layer model, on the other hand, includes the emission of secondary electrons by ion impact and by additional mechanisms, of which we considered emission due to collision of atoms excited at metastable levels, and field-enhanced thermionic emission (Schottky effect). The comparison of model and experiment indicates that the discharge can be effectively sustained in its contracted form by the secondary electrons emitted by collision of excited argon atoms, whereas thermionic emission is by far insufficient to provide the necessary electrons.

Coherent Forward Scattering Spectrometer as a Wavelength Tuning Indicator for Diode Laser Atomic Absorption Spectrometry

A coherent forward scattering spectrometer was used as a wavelength tuning indicator for diode laser atomic absorption spectrometry. Atomic absorption transition of excited argon atoms at 842.46 nm was investigated. Argon atoms were excited in a glow discharge. Coherent forward scattering signal attained maximum at 0.1 T of external magnetic field and at 12 mA discharge current in 266 Pa (2 Torr) of argon. When the lasing wavelength crossed an atomic absorption line, a spike-like signal was observed. When a continuous coherent forward scattering signal was observed, the lasing wavelength was confirmed to approach within about ±3 pm from the absorption transition of argon.

Electron confinement and heating in microwave-sustained argon microplasmas

We systematically measure and model the behavior of argon microplasmas sustained by a broad range of microwave frequencies. The plasma behavior exhibits two distinct regimes. Up to a transition frequency of approximately 4 GHz, the electron density, directly measured by Stark broadening, increases rapidly with rising frequency. Above the transition frequency, the density remains approximately constant near 5 × 1020 m–3. The electrode voltage falls with rising frequency across both regimes, reaching approximately 5 V at the highest tested frequency. A fluid model of the plasma indicates that the falling electrode voltage reduces the electron temperature and significantly improves particle confinement, which in turn increases the plasma density. Particles are primarily lost to the electrodes at lower frequencies, but dissociative recombination becomes dominant as particle confinement improves. Recombination events produce excited argon atoms which are efficiently re-ionized, resulting in relatively constant ionization rates despite the falling electron temperature. The fast rates of recombination are the result of high densities of electrons and molecular ions in argon microplasmas.

Measurement of Excited Argon Atom in Glow Discharge Plasma by Diode Laser Coherent Forward Scattering Spectrometry (DL-CFS)

Measurement of Excited Argon Atom in Glow Discharge Plasma by Diode Laser Coherent Forward Scattering Spectrometry (DL-CFS)

Numerical modelling of the nonequilibrium expansion process of argon plasma flow through a nozzle

A two-temperature thermal and chemical nonequilibrium model is developed and applied to investigate the expansion processes of an argon plasma flow through a Laval nozzle. This model describes in a self-consistent manner the gas flow and heat transfer, the coupling of the electric energy deposited into the plasma, and the reaction kinetics including the contribution of excited species. It is found that the plasma is far from thermodynamic equilibrium in the entire argon plasma flow expansion process through a nozzle. Significant temperature discrepancies between electrons and heavy species are found in the cooler outer region. The dominant chemical kinetic processes in different plasma gas expansion regions are presented and discussed. It is noted that although the number density of excited argon atoms (Ar*) is much lower than that of other species in the argon plasma, Ar* play important roles in the ionization and recombination processes, and in arc attachment to the anode.

Two-Temperature Chemical-Nonequilibrium Modelling of a High-Velocity Argon Plasma Flow in a Low-Power Arcjet Thruster

A numerical simulation has been performed of a high-velocity argon plasma arc flow in a low power arcjet including a finite-rate chemical kinetic model. Electrons, ions, molecular ions ( $$ {\text{Ar}}_{2}^{ + } $$ ), neutral atoms including the ground and excited argon atoms (Ar*) are treated as separate species in the plasma mixture. The chemical reactions considered are excitation, de-excitation, ionization and recombination processes, in which reactions involving excited argon atoms (Ar*) and molecular ions ( $$ {\text{Ar}}_{2}^{ + } $$ ) are taken into account. The relative importance of different production and loss processes in determining the densities of excited argon atoms and ions is calculated inside the constrictor and expansion portion of the nozzle. The roles of the excited argon atoms and molecular ions are investigated. It is found that excited argon atoms play an important role in the ionization of argon atoms in the core of plasma arc, while the molecular ions have a significant effect on the recombination process at the arc fringes inside the constrictor and in the arc attachment zone of the anode.

Analysis of microdischarges in asymmetric dielectric barrier discharges in argon

Theoretical and experimental studies of two different discharge modes in asymmetric dielectric barrier discharges in argon at atmospheric pressure have been performed. The first mode appears to be the well-known filamentary microdischarge with non-striated positive column whereas the second mode is characterized by discharge instabilities and the appearance of striations. Both experiment and model calculations predict a transition from the first mode to the second mode when the applied voltage amplitude is increased above approximately 2 kV. The reliability of the employed fluid model is confirmed by comparison of the current–voltage characteristics obtained by model calculations and measurements for different applied voltage amplitudes. The results of the model calculations point out that in the second discharge mode the ionization of excited argon atoms prevents the total recombination of charge carriers between two subsequent discharge events. This leads to the occurrence of the memory from one discharge to the following one, which plays an important role in mode transition.

A dielectric-barrier discharge enhanced plasma brush array at atmospheric pressure

This study developed a large volume cold atmospheric plasma brush array, which was enhanced by a dielectric barrier discharge by integrating a pair of DC glow discharge in parallel. A platinum sheet electrode was placed in the middle of the discharge chamber, which effectively reduced the breakdown voltage and working voltage. Emission spectroscopy diagnosis indicated that many excited argon atoms were distributed almost symmetrically in the lateral direction of the plasma. The concentration variations of reactive species relative to the gas flow rate and discharge current were also examined.

Time-resolved tunable diode laser absorption spectroscopy of excited argon and ground-state titanium atoms in pulsed magnetron discharges

Time-resolved investigations of excited argon atom density and temperature and ground-state titanium atom density during high-power impulse magnetron sputtering (HiPIMS, repetition frequency 100 Hz) and direct current pulsed magnetron (repetition frequency 2.5 kHz) discharges (PMDs) in argon employing a titanium target were performed. Atom density and temperature were measured with the help of tunable diode laser absorption spectroscopy. Excited argon atoms form during the discharge pulse and again by three-body electron ion recombination in the afterglow. Similarly, the temperature of excited (metastable) argon atoms rises during the plasma on phase and again during the afterglow. The observed temporal evolution of the temperature is faster than expected from thermal conductivity considerations, which is taken as an indication that metastable and ground-state argon atoms are not in thermal equilibrium. The time dependence of titanium atoms can be explained by recombination and diffusion. The results provide new insights into the physics of PMDs.

Development of a stable dielectric-barrier discharge enhanced laminar plasma jet generated at atmospheric pressure

A stable nonthermal laminar atmospheric-pressure plasma source equipped with dielectric-barrier discharge was developed to realize more efficient plasma generation, with the total energy consumption reduced to nearly 25% of the original. Temperature and emission spectra monitoring indicates that this plasma is uniform in the lateral direction of the jet core region. It is also found that this plasma contains not only abundant excited argon atoms but also sufficient excited N2 and OH. This is mainly resulted from the escape of abundant electrons from the exit, due to the sharp decrease of sustaining voltage and the coupling between ions and electrons.

Characterization of stable brush-shaped large-volume plasma generated at ambient air

A brush-shaped, large-volume plasma was generated at ambient pressure with a dc power supply and flowing argon gas, as well as a narrow outlet slit. Based on the V–I curve and emission profiles obtained in our experiment, the plasma shows some typical glow discharge characteristics. The electron density in the positive column close to the anode is about 1.4×1014cm-3 high, which is desirable for generating abundant amounts of reactive species in the plasma. Emission spectroscopy diagnosis indicates that many reactive species, such as excited argon atoms, excited oxygen atoms, excited nitrogen molecules, OH and C2 radicals, etc., generated within the plasma are distributed symmetrically and uniformly, which is preferable to some chemical reactions in practical applications. Spectral measurement also shows that the concentration of some excited argon atoms increases with the argon flow rate when the applied voltage is unvaried, while that of these excited argon atoms declines with the discharge current in the normal/subnormal glow discharge mode with the argon flow rate fixed. The plasma size is about 15 mm × 1 mm × 19 mm (L, W, H), when 38-W of discharge power is used. Such a laminar brush-shaped large-volume plasma device ensures not only efficient utilization of the plasma gas, but also effective processing of objects with large volume and complicated structure that are susceptible to high temperatures.

Probe and Emission Spectrometry Diagnostics in Hollow Cathode Magnetron

This paper deals with the characterization of an ionized physical vapor deposition (IPVD) by means of hollow cathode magnetron. Langmuir probe, optical emission spectroscopy measurements were used to study a mechanism for the production of excited argon and copper atoms and ions. The kinetic processes of excitation were considered and the main processes were determined using results of measurements. The pressure range is 0.5 - 10 mTorr with 1- 5 kW discharge power. Plasma parameters such as electron densities and temperatures, electron energy distribution function, plasma space and floating potentials as a function of the position, pressure and power in the growth chamber were measured. The plasma density is up to 1012 cm?3 at 20 cm from the magnetron for 10 mTorr.

Influence of Additive Gas on Electrical and Optical Characteristics of Non-equilibrium Atmospheric Pressure Argon Plasma Jet

Electrical and optical properties of an argon plasma jet were characterized. In particular, effects of an additive gas, namely nitrogen or oxygen, on these properties were studied in detail. The plasma jet was found to be of a glow-like discharge, which scarcely changed upon the injection of an additive gas, either directly or through a glass capillary. Optical emission spectroscopy characterization revealed that excited argon atoms were the predominant active species in this plasma jet. Metastable argon atoms were highly quenched, and N2(C3Πu) became the main energy carrier following nitrogen injection. When oxygen was added to the afterglow zone through a glass capillary, no significant quenching effect was observed and the number of oxygen atoms decreased with the increase in oxygen concentration. Finally, to demonstrate an application of this plasma jet, a high-density polyethylene surface was treated with argon, argon/nitrogen, and argon/oxygen plasmas.

Nitrogen grafting onto polycarprolactone by a simple surface modification with atmospheric pressure glow discharge (Ar-APGD) and promoted neonatal human fibroblast growth

Plasma surface modifications of polymer scaffolds using biomolecules such as oxygen, nitrogen, and other active grafting molecules have been studied to enhance biological responses such as cell attachment, spreading, and proliferation. According to the reports, nitrogen grafting requires corrosive or mixture gas environment, or post treatment. This study aimed to evaluate a simple atmospheric pressure plasma surface modification in order to graft nitrogen derivatives and to promote biological responses. In this study, a polycarprolactone (PCL) film was modified within 10 min by argon atmospheric pressure discharge (Ar-APGD). Excited argon atoms, nitrogen atoms, oxygen atoms, and hydroxyl functional groups were observed from the optical emission spectra of the discharge. Decreased carbonyl functional groups and ether functional groups were observed; notably, immobilized nitrogen was observed on the PCL surface after the Ar-APGD treatment. Promoted neonatal Human Dermal Fibroblast (nHDF) growth patterns were observed on the Ar-APGD-treated surface.

On plasma parameters of a self-organized plasma jet at atmospheric pressure

Electron temperature and electron concentration in the active zone of a miniaturized radio frequency (RF) non-thermal atmospheric pressure plasma jet in argon have been determined using two independent approaches: the spectroscopic measurement of the broadening of Balmer H $_\beta$ and H $_\gamma$ lines and a time-dependent, spatially two-dimensional fluid model of a single discharge filament. The plasma source has been configured as a capacitively coupled RF jet (27.12 MHz, 8 W generator output power) with two outer ring electrodes around a quartz capillary with diameter of 4.0 mm between which Ar flows at typical rates of 0.3 slm. The discharge has been operated in a self-organized mode, where equidistant, stationary filaments rotate regularly with a constant frequency at the inner wall of the outer capillary. For the purpose of calculating the spectral line broadening different models applicable at higher electron concentration have been evaluated. Resulting electron concentrations are between 2.2 and 3.3 × 1014 cm-3. The calculation according to the line broadening model provides electron temperatures between 20 000 and 30 000 K which is in agreement with the results of the fluid model calculations. Here, a broad radial profile with a maximal value of about 22 000 K in the centre of the column and an electron concentration of about 7 × 1013 cm-3 have been obtained. Moreover, the results of the model calculations reveal a structural change of the filament from the dielectric surface through the sheath to the column. The axially inhomogeneous region has an extension of about 0.5 mm. In the column a concentration of about 1013 cm-3 has been found for the excited argon atoms, whose collisions with electrons represent the most important ionization channel there.