Excited Argon Research Articles

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.

Infrared Study of Room Temperature Atomic Layer Deposition of SnO2 Using Sn(CH3)4 and Plasma Excited Humidified Argon

Tin oxide (SnO2) is an oxide semiconductor material with an excellent optical transparency, where it is used as transparent conducting films. It is also used as a channel material for gas sensors. Recently atomic layer deposition (ALD) of SnO2 has been studied intensively since ALD allows the conformal deposition of the oxide film with an atomic level precision. However, the high process temperature in excess of 100 °C in the conventional SnO2 ALD is a major problem for its application for flexible electronics. We developed a room temperature deposition technique with a precursor of tetramethyl tin (Sn(CH3)4, TMT) and plasma excited humidified argon. The surface reaction of the precursor adsorption and its oxidization were monitored by infrared absorption spectroscopy (IRAS) with a multiple-internal-reflection (MIR) geometry.A p-type Si(100) substrate with a resistivity of 8-12 Ωcm was used as a sample. Prior to the experiment, the sample was cleaned with buffered HF acid and sulfuric acid-hydrogen peroxide mixture solutions. The Sn precursor of TMT was introduced with a variable leak valve. A plasma excited humidified argon source was installed to the ALD chamber as shown in Fig. 1. The source gas for OH-radical is a mixture of H2O vapor and Ar made by a water bubbler. The plasma was generated in a glass tube with an induction coil with a frequency of 13.56MHz and a RF power of 30 W. Before the ALD process, the substrate surface was treated with the plasma excited humidified argon for 5 min to be terminated with Si-OH. For the SnO2 deposition, we repeated the cycle of TMT saturation and plasma excited humidified argon at RT. The condition of TMT saturation and oxidization were determined by the MIR-IRAS experiment. In this experiment, the TMT saturation exposure and oxidization time were 2×105 Langmuir and 10 min, respectively. The oxidation state of the SnO2 film was evaluated by X-ray photoelectron spectroscopy (XPS). The thickness of the grown film was measured by spectroscopic ellipsometry.In Fig. 2, we show Sn 3d XPS spectra obtained from the RT grown SnO2 film, where we confirm the evolution of full oxidized state peak according to the increase in the ALD cycle number. We calculated the growth per cycle as 0.02 nm/cycle. In the conference, we present the results of RT ALD of SnO2 and discuss the surface-reaction mechanism based on the IRAS observation. Acknowledgement; This work was partly supported by JST-CREST, JSPS KAKENHI Grant Numbers 15H03536 and 15K13299. Figure 1

Time-Resolved Electron Temperature in an Argon Theta Pinch by Line Ratio Methods

Pulsed inductive argon plasma in an 80-J, 15-kV, 490-kHz theta pinch is interrogated using spectroscopic methods. Time-resolved electron temperature is obtained by coupling line intensity ratios with steady-state corona and collisional-radiative (CR) models. Neutral excited state argon transitions from the 2p to 1s subshells were utilized. The backfill pressures of 50 and 100 mtorr are of primary focus. Time-resolved electron temperature estimates are presented ranging from 1 to 11.1 eV for the corona model up to an excess of 80 eV for the CR model. Near-IR spectral emission is seen to increase rapidly near the first zero crossing of the oscillating discharge current while electron temperature increase lags by roughly one full discharge cycle later near the third zero crossing. An analysis of long exposures provides an account of substantial second-order diffracted spectra. Weak spectral signal quality for the short exposures of 0.25-μs yielded time-resolved spectral intensity trend lines of a low accuracy with an average percent difference of 69% between raw data and trend lines.

Novel focal point multipass cell for absorption spectroscopy on small sized atmospheric pressure plasmas.

A novel focal point multipass cell (FPMPC) was developed, in which all laser beams propagate through a common focal point. It is exclusively constructed from standard optical elements. Main functional elements are two 90(∘) off-axis parabolic mirrors and two retroreflectors. Up to 17 laser passes are demonstrated with a near-infrared laser beam. The number of laser passes is precisely adjustable by changing the retroreflector distance. At the focal point beams are constricted to fit through an aperture of 0.8 mm. This is shown for 11 beam passes. Moreover, the fast temporal response of the cell permits investigation of transient processes with frequencies up to 10 MHz. In order to demonstrate the applicability of the FPMPC for atmospheric pressure plasma jets, laser absorption spectroscopy on the lowest excited argon state (1s5) was performed on a 1 MHz argon atmospheric pressure plasma jet. From the obtained optical depth profiles, the signal-to-noise ratio was deduced. It is shown that an elevation of the laser pass number results in an proportional increase of the signal-to-noise ratio making the FPMPC an appropriate tool for absorption spectroscopy on plasmas of small dimensions.

Phenomena of non-thermal electrons from the X-ray imaging crystal spectrometer on J-TEXT tokamak

A high spectra resolution X-ray imaging crystal spectrometer has been implemented on J-TEXT Tokamak for the measurements of Kα spectra of helium-like argon and its satellite lines. The wavelength range of Kα spectra of helium-like argon is from 3.9494Å to 3.9944Å that includes the resonance line w, intercombination lines x and y, forbidden line z and numerous satellite lines, referenced using standard Gabriel notation. In low-density discharge, the intensity of q, r satellite lines and z lines can be significantly enhanced by non-thermal electrons. Non-thermal electrons are produced due to the low plasma density. The high hard X-ray flux from NaI detector and significant downshift electron cyclotron emissions from energetic runaway electrons also indicated that there is a large population of runaway electrons in the low-density discharge. The non-thermal part of electrons can affect the excitation/transition equilibrium or ionization/recombination equilibrium. The q line is mainly produced by inner-shell excitation of lithium-like argon, and the r line is partially produced by inner-shell excitation of lithium-like argon and dielectronic recombination of helium-like argon.

Energy efficiency analysis of ArF excimer laser system

The reliable functioning and continual optimizing of ArF excimer laser system is of importance when it comes to productization into the market from a laboratory test machine. The analysis of dynamic characteristics of the system is vital to understanding its operating mechanism and optimizing the design theoretically. In this article, one-dimensional fluid model is used to analyze the excimer laser discharge mechanism, and the content ratio of fluorine gas, argon gas, and neon gas, which constitute a gas mixture, is studied in a simulated ArF excimer laser system. Particles are treated as a fluid, which significantly reduces the computing cost in fluid model, and therefore is suitable for high-pressure situation. Four equations are included in one-dimensional fluid model, i.e., Boltzmann equation that describes electron energy distribution, ion continue equation that illustrates ion number density, Poisson's equation that shows the distribution of electric field, and photon rate equation that demonstrates laser outputting process. By combining these four equations, high pressure plasma discharge process and particles stimulated radiation process are studied, and calculation continues from one time step to another until the end of discharging process. The result of the calculation presents energy transfer process from three aspects:energy deposition efficiency, ArF* formation, and laser outputting. In the energy deposition process, the energy deposition efficiency is sensitive to the change of fluorine gas ratio while the variation of the content ratio of other two gases has a less influence on this process. In addition, there exists an optimal fluorine gas ratio that causes the highest energy deposition efficiency. In the ArF* formation process, the reaction between excited argon ions and fluorine gas is the main channel that generates ArF*. The proper increasing of fluorine gas ratio helps form ArF*. In the laser outputting process, photon loss is mainly because of the reaction between fluorine negative ions and photons. Therefore superfluous fluorine gas in the mixture leads to less photons, which eventually results in low energy efficiency of laser. By summarizing the three aspects of energy transfer process, the fluorine gas ratio in the gas mixture plays a significant role in determining the energy efficiency of ArF excimer laser system. This theory is verified by experiments, showing that the deviation of the optimized fluorine gas ratio severely reduces energy efficiency. This conclusion can guide us in optimizing the design and steady reliable function of ArF excimer laser system.

Kinetic modeling and sensitivity analysis of plasma-assisted oxidation in a H2/O2/Ar mixture

A numerical scheme describing the oxidation process induced by single and multiple dielectric barrier discharges in a H2/O2/Ar mixture is developed. Path flux and sensitivity analyses are performed in order to identify important reaction paths. The analyses are conducted for an initial mixture of 2000 ppm H2 and 3000 ppm O2 balanced in Ar at a constant pressure of 1 atm and temperatures ranging from 474 K to 1008 K, covering both below and above the second explosion limit. Results with only one discharge as well as with repetitive discharges at rates between 1 and 5 kHz are discussed. Three sets of reaction schemes leading to H2O formation are identified: two short-time-scale schemes involving negative ions and O(1D), respectively, and a long-time-scale scheme involving ground state radicals, which is responsible for the majority of the H2O formation and driven by plasma dissociation of H2 and O2. At temperatures below the second explosion limit, the last scheme drives the straight chain propagation mechanism without causing exponential growth of radicals, where the reactions of HO2 play important roles on the overall fuel oxidation. At temperatures above the second explosion limit, H and O atoms produced from the dissociation of reactants by deactivation of excited argon subsequently trigger ignition significantly reducing the ignition delay. The consumption of H2 and O2 which follows is governed by conventional high temperature chain branching kinetics and occurs at a rate equivalent to that of the purely thermal reaction. Increasing the pulse repetition rate of the discharge at low temperatures results in a shift of the rate-limiting steps from reactions of H atoms to those of the OH radical. Furthermore, the kinetics of metastable Ar*m, HO2, O2(a1Δg), and O(1D) are analyzed and their implications to other systems are discussed.

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.

Modification of TiO2 powder via atmospheric dielectric barrier discharge treatment for high performance lithium-ion battery anodes

The main objective of this study is to improve the electrochemical performances of TiO2 Li-ion anode material by introducing plasma treatment on TiO2 powder. A specially designed atmospheric dielectric barrier discharge plasma generator feasible to modify powders is proposed. The rate capacity of 20min plasma-treated TiO2 anode revealed nearly 20% increment as compared to that of pristine TiO2 at the rates of 0.5, 1, 2, 5, 10C. As-treated TiO2 was first analyzed by the X-ray diffractometer and high resolution transmission electron microscope confirmed that there was no noticeable surface morphology and microstructure change from plasma treatment. In addition, plasma-treated TiO2 was reduced with increasing treatment duration. Significant amount of excited argon (Ar∗) and excitation of a nitrogen second positive system (N2∗) were discovered using optical emission spectroscopy (OES). It was believed that Ar∗ and N2∗ contributed to TiO2 surface reduction as companied by formation of oxygen vacancy. A higher amount of oxygen vacancy increases the chance of allowing excited nitrogen to dope onto surface of TiO2 particle. Electrochemical properties of TiO2 were raised due to the production of oxygen vacancy and nitrogen doping. These findings enhance the understanding of the atmospheric plasma treatment on the potential application of TiO2 anode material in Li-ion battery.

Effects of gas composition, focus ring and blocking capacitor on capacitively coupled RF Ar/H2 plasmas

The capacitively coupled discharges in Ar/H2 mixtures operating at a RF frequency of 13.56 MHz and the total gas pressure of 100 Pa are investigated by means of a two-dimensional fluid model, in which a blocking capacitor is serially connected to the power electrode. The gas mixtures of Ar with 0–10% H2 and the blocking capacitors of 1 and 100 nF are taken into account. The applied voltage is 200 V and the gas temperature is considered to be 300 K. The density profiles of all the plasma species in the discharges are obtained. Results show that the plasma discharge is concentrated over the focus ring, not being in the center of the reactor. A large potential difference and a high capacitive power deposition appear at the surface area between the substrate and focus ring. With the addition of small amount of H2 to Ar, the electron density markedly decreases. It is found that the reductions of the metastable excited argon density and the metastable pooling ionization rate due to the addition of H2 are the main reason that causes the electron density decrease. The self DC-bias generated by the blocking capacitor is examined.

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.

Electrical and optical properties of gallium-doped zinc oxide thin films prepared by Ion-Beam-Assisted Deposition

Gallium doped zinc oxide films were prepared by Ion-Beam-Assisted Deposition. The effects of argon and oxygen bombardment flux on electrical and optical properties were studied according to the existence of excited species. The prepared films were polycrystalline in nature with c-axis perpendicular to the substrate. The resistivity of the film decreased to 1.35 × 10−3 Ω-cm, and the optical transparency increased up to 80%, as the discharge current in ion gun was increased. The variation of discharge current mostly affected the grain size and crystallinity, which in turn affected the carrier mobility. Too much argon bombardment may disrupt grain growth, which resulted in higher resistivity. An optical emission spectrometer (OES) was used to examine the optical emission spectra of the ion beam, mainly for the excited oxygen and argon species. It was found the O*/Ar* ratio peaked with the increase of discharge current set in ion gun, then, decreased. This trend was similar to that of the electrical properties.

Spectroscopic diagnosis of an atmospheric-pressure waveguide-based microwave N2–Ar plasma torch

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.

Effect of the electron energy distribution on total energy loss with argon in inductively coupled plasmas

The total energy lost per electron-ion pair lost εT is investigated with the electron energy distribution function (EEDF). The EEDFs are measured at various argon powers in RF inductively coupled plasma, and the EEDFs show a depleted distribution (a discontinuity occurring at the minimum argon excitation threshold energy level) with the bulk temperature and the tail temperature. The total energy loss per electron-ion pair lost εT is calculated from a power balance model with the Maxwellian EEDFs and the depleted EEDFs and then compared with the measured εT from the floating probe. It is concluded that the small population of the depleted high energy electrons dramatically increases the collisional energy loss, and the calculated εT from the depleted EEDFs has a value that is similar to the measured εT.

Non-local thermodynamic equilibrium effects on isentropic coefficient in argon and helium thermal plasmas

In the present work, two cases of thermal plasma have been considered; the ground state plasma in which all the atoms and ions are assumed to be in the ground state and the excited state plasma in which atoms and ions are distributed over various possible excited states. The variation of Zγ, frozen isentropic coefficient and the isentropic coefficient with degree of ionization and non-equilibrium parameter θ(= Te/Th) has been investigated for the ground and excited state helium and argon plasmas at pressures 1 atm, 10 atm, and 100 atm in the temperature range from 6000 K to 60 000 K. For a given value of non-equilibrium parameter, the relationship of Zγ with degree of ionization does not show any dependence on electronically excited states in helium plasma whereas in case of argon plasma this dependence is not appreciable till degree of ionization approaches 2. The minima of frozen isentropic coefficient shifts toward lower temperature with increase of non-equilibrium parameter for both the helium and argon plasmas. The lowering of non-equilibrium parameter decreases the frozen isentropic coefficient more emphatically in helium plasma at high pressures in comparison to argon plasma. The increase of pressure slightly reduces the ionization range over which isentropic coefficient almost remains constant and it does not affect appreciably the dependence of isentropic coefficient on non-equilibrium parameter.

Electron-impact excitation of argon at intermediate energies

Large-scale $R$-matrix-with-pseudostates calculations for electron collisions with argon atoms, using a recently developed parallel version of our B-spline $R$-matrix code, are reported. The calculations were carried out in the semirelativistic $\mathit{jK}$-coupling scheme. They are intended to provide converged (with respect to the number of coupled states) results for electron-impact excitation of individual target states with dominant configurations $3{p}^{5}4s$, $3{p}^{5}4p$, $3{p}^{5}3d$, and $3{p}^{5}5s$ for incident electron energies from threshold to 300 eV. The close-coupling expansion includes 500 target states, with the lowest 78 states representing the bound spectrum and the remaining 422 the ionization continuum. The results reveal dramatic reductions of the predicted excitation cross sections at intermediate energies due to a strong influence of coupling to the target continuum and the higher-lying Rydberg states. Comparison with available experimental data for excitation raises questions about the absolute normalization in the measurements.

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.

Ion-beam excitation of liquid argon

The scintillation light of liquid argon has been recorded wavelength and time resolved with very good statistics in a wavelength interval ranging from 118 nm through 970 nm. Three different ion beams, protons, sulfur ions and gold ions, were used to excite liquid argon. Only minor differences were observed in the wavelength-spectra obtained with the different incident particles. Light emission in the wavelength range of the third excimer continuum was found to be strongly suppressed in the liquid phase. In time-resolved measurements, the time structure of the scintillation light can be directly attributed to wavelength in our studies, as no wavelength shifter has been used. These measurements confirm that the singlet-to-triplet intensity ratio in the second excimer continuum range is a useful parameter for particle discrimination, which can also be employed in wavelength-integrated measurements as long as the sensitivity of the detector system does not rise steeply for wavelengths longer than 190 nm. Using our values for the singlet-to-triplet ratio down to low energies deposited a discrimination threshold between incident protons and sulfur ions as low as ∼2.5 keV seems possible, which represents the principle limit for the discrimination of these two species in liquid argon.