Density Of Metastable Atoms Research Articles

Investigations of argon and neon abnormal glow discharges in the presence of metastable atom density with fluid model

In this manuscript an investigation of a DC argon and neon abnormal glow discharges with metastable atom density is presented. The values of pressure lie between 133.32 and 330 Pa, and the voltage ranges from 250 to 400 V in the case of argon gas. In the case of neon gas, the pressure has a value of 399.92 Pa (3 Torr) and the voltage ranges from 300 to 500 V. In this framework, an analysis of abnormal glow discharge characteristics is carried out in the case of input data taken from the Boltzmann equation in the multi-term approximation, and in the case of input data obtained from BOLISG+ code. With these differences of input data in the same gas, the output results are different; it appears in the cathodic region. The spatio-temporal distributions of electron and ion densities, the potential and electric field, the mean electron energy and the metastable atom density are shown. A 1D fluid model is used to solve self-consistently the first three moments of the Boltzmann’s equation coupled with the Poisson’s equation. The role of the presence of metastable atom density in the fluid model is clearly because of the domination of ionization processes by both stepwise and penning ionizations. Our results are validated with those obtained by both recent paper and experimental results.

Space-time resolved density of helium metastable atoms in a nanosecond pulsed plasma jet: influence of high voltage and pulse frequency

Using tunable diode laser absorption spectroscopy, the spatio-temporal distributions of the helium He(23S1) metastable atoms’ density were measured in a plasma jet propagating in ambient air. The plasma jet was produced by applying short duration high voltage pulses on the electrodes of a DBD-like structure, at a repetition rate in the range 1–30 kHz. In addition to the metastable density, the spatial distribution of helium 587 nm emission intensity was also investigated to give insight into the excitation mechanisms of the He(33D) excited state inside the dielectric tube, in which no laser measurement can be performed. It is demonstrated that the shape of the radial distribution of helium He(23S1) metastable atoms strongly depends on the polarity of the applied voltage and on the repetition frequency. For positive applied voltages, a dramatic constriction of the excited species production is observed whenever the pulse repetition frequency is higher than 6 kHz, and the voltage higher than 5 kV. This shrinking of the jet structure induces an increase by one order of magnitude of the metastable atoms’ density in the jet centre which reaches values as high as 1014 cm−3. Beyond a critical distance, associated to a transition between a positive streamer and a negative one, the distribution of the excited atoms gets back to an annular structure. For the negative polarity, no shrinking effect correlated to the pulse repetition frequency was observed. The on-axis constriction of the excited species for the high repetition rate and positive polarity is attributed to a memory effect induced by the negative ions, having a lifetime of hundreds of microseconds, left between successive pulses at the periphery of the helium gas flow.

Diagnostics of atmospheric-pressure pulsed-dc discharge with metal and liquid anodes by multiple laser-aided methods

The density and temperature of electrons and key heavy particles were measured in an atmospheric-pressure pulsed-dc helium discharge plasma with a nitrogen molecular impurity generated using system with a liquid or metal anode and a metal cathode. To obtain these parameters, we conducted experiments using several laser-aided methods: Thomson scattering spectroscopy to obtain the spatial profiles of electron density and temperature, Raman scattering spectroscopy to obtain the neutral molecular nitrogen rotational temperature, phase-modulated dispersion interferometry to determine the temporal variation of the electron density, and time-resolved laser absorption spectroscopy to analyze the temporal variation of the helium metastable atom density. The electron density and temperature measured by Thomson scattering varied from 2.4 × 1014 cm−3 and 1.8 eV at the center of the discharge to 0.8 × 1014 cm−3 and 1.5 eV near the outer edge of the plasma in the case of the metal anode, respectively. The electron density obtained with the liquid anode was approximately 20% smaller than that obtained with the metal anode, while the electron temperature was not significantly affected by the anode material. The molecular nitrogen rotational temperatures were 1200 K with the metal anode and 1650 K with the liquid anode at the outer edge of the plasma column. The density of helium metastable atoms decreased by a factor of two when using the liquid anode.

Metastable helium atom density in a single electrode atmospheric plasma jet during sample treatment

The metastable He atoms play an important role in atmospheric pressure plasma jet (APPJ) chemistry processes and in the plasma generation. This work presents cavity ring-down spectroscopy (CRDS) investigation of metastable helium atom (2) densities in a single electrode APPJ during sample treatment. A spatially resolved density distribution of a free jet (without sample) was measured at a He flow rate of 2 slm. The maximum measured density of a free jet was around cm−3. With the insertion of a sample the densities increased up to 10 times. Helium metastable atoms, in a single electrode helium APPJ (2 slm, ≈2.5 kV, pulsed DC, 10 kHz repetition rate), decayed exponentially with a mean lifetime of μs. Eight different samples of the same sizes but different conductivities were used to investigate the influence of a sample material on the He metastable densities. The correlation between sample conductivities and metastable He densities above the sample surface was found. Metastable He density can also be further increased with decreasing sample distance, increasing conductive sample surface area and by increasing He flow.

Study of Ar and Ar-CO2 microwave surfaguide discharges by optical spectroscopy

A surfaguide microwave discharge operating at 2.45 GHz in Ar and Ar-CO2 mixtures is studied using diagnostics methods based on optical emission spectroscopy. The population densities of Ar metastable and resonant states of the lowest group of excited levels (1sx) are investigated for several experimental conditions using the self-absorption technique. It is found that the densities of these levels, ranging from 1017 to 1016 m−3 for the pure Ar case, are dependent on the discharge pressure and applied power. The electron temperature and electron density are calculated via the balances of creation/loss mechanisms of radiative and metastable levels. In the range of the studied experimental conditions (50–300 W of applied power and 0.5–6 Torr of gas pressure), the results have shown that lower values of electron temperature correspond to higher values of power and pressure in the discharge. Adding CO2 to the argon plasma results in a considerable decrease (about 3 orders of magnitude) of the Ar metastable atom density. The feasibility of using the ratio of two Ar emission line intensities to measure the electron temperature in CO2 discharges with small Ar admixtures is studied.

Electrical Characteristics of an Argon Glow Discharge in the Presence of Metastable Atom Density

The paper deals with numerical modelling studies of DC glow discharge in argon with metastable atom density. The values of pressure is between 133.32 Pa and 330 Pa, and voltage ranges from 250 to 400 V in the case of input data taken from the Boltzmann equation in multi term approximation. In the frameworks, an analysis of glow discharge characteristics is carried out in the case of input data obtained from BOLISG+ code. As conclusion of these different input data in the same gas the output results are different and they appear in the cathode region. The spatio-temporal distributions of electron and ion densities, the potential and electric field, the mean electron energy and the metastable atom density are shown. A 1D fluid model is used to solve self-consistently the first three moments of the Boltzmann equation coupled with the Poisson's equation. The results are validated with those obtained by recent papers.

Measurement of the helium 23S metastable atom density by observation of the change in the 23S–23P emission line shape due to radiation reabsorption

In helium discharge plasmas, the relative emission intensities of the fine-structure transitions belonging to the HeI 23S–23P transition can be affected by radiation reabsorption. Since the magnitude of the reabsorption depends on the density and temperature of the 23S metastable atoms, their density can be determined by measuring the 23S–23P emission line shape using a high wavelength-resolution spectrometer. In this study, the applicable conditions of the method in terms of the opacity and line broadening are revealed, and possible causes of errors in the measurement, i.e. spatial distributions of the density and temperature and the effects of external magnetic and electric fields, are investigated. The effect of reabsorption under an external magnetic field is experimentally confirmed using a glow discharge plasma installed in a superconducting magnet.

Detection of fast electrons in pulsed argon inductively-coupled plasmas using the 420.1–419.8 nm emission line pair

Pulsed rf plasmas exhibit many differences as compared to continuous wave plasmas with the same average power levels, including large temporal variations in the electron temperature, with a sharp spike when the power is applied and falling dramatically in the afterglow. We present a comparison of time-resolved measurements of the effective electron temperature in pulsed inductively-coupled plasmas by means of (i) optical emission spectroscopy (OES) using different sets of argon emission lines and (ii) Langmuir probe measurements. One OES diagnostic used six strong Ar() emission lines in the 700–800 nm wavelength range, the second used only the Ar 420.1–419.8 nm line pair. For pulsed plasmas with long afterglow periods, the line pair method reveals the presence of a significant number of hot electrons ( eV) at the start of the pulse. Under these conditions, the metastable atom density is very low, and the diagnostic using the Ar() emission lines is ineffective for determining the electron temperature. For later parts of the pulse and pulsed plasmas with short periods (i.e. 10 μs), the metastable density is high and the two OES methods yield similar results which are also in agreement with probe measurements.

Correlating metastable-atom density, reduced electric field, and electron energy distribution in the post-transient stage of a 1-Torr argon discharge

Temporal measurement of electron density, metastable-atom density, and reduced electric field are used to infer the dynamic behavior of the excitation rates describing electron-atom collision-induced excitation in the positive column of a 1 Torr argon plasma by invoking plausible assumptions regarding the shape of the electron energy distribution function performed in Adams et al (2012 Phys. Plasmas 19 023510). These inferred rates are used to predict the 420.1 nm to 419.8 nm argon emission ratio, which agree with experimental results when the assumptions are applicable. Thus the observed emission ratio is demonstrated to be dependent on the metastable-atom density, electron density, and reduced electric field. The established confidence in the validity of this emission-line-ratio model allows us to predict metastable argon-atom density during the post-transient phase of the pulse as suggested by De Joseph et al (2005 Phys. Rev. E 72 036410). Similar inferences of electron density and reduced electric field based on readily available diagnostic signatures may also be afforded by this model.

Computational study of RF micro-hollow cathode discharge

Radiofrequency discharges can generate non-equilibrium and stable micro-plasmas without a streamer in micro-hollow cathode reactors. In this work, we present the results of a fluid model describing the mechanisms occurring in a micro-reactor in argon plasma, generated by an excitation of 13.56 MHz at high pressures (100 Torr) with and without a secondary emission of electrons. The results of the simulation improve the understanding of the effect of micro-hollow cathodes and of the influence of excited atoms in ionization and sustainment of argon discharge at low voltage (150 V). Simulation results showed that the maximum of penning ionization and stepwise ionization rates are respectively about 25% and 11% of the maximum of direct ionization rate. The metastable atom density reaches a maximum value of 1020/m3 inside the hole and have also two humps near to oppsite cathodes. Accordingly the excitation rate is significant in all the space.

An investigation of electrophysical and thermophysical characteristics of low-frequency low-pressure inductive discharge of transformation type

The electrophysical and thermophysical characteristics of the low-frequency transformation-type induction discharge in argon in a wide parameter range (the discharge current density varies from 0.1 to 0.8 A/cm2 and the argon pressure is 15–6000 Pa) are studied. A self-consistent radial model of plasma of the low-frequency low-pressure transformation-type induction discharge in argon, which is based on the simultaneous solution of balance equations for the electron density and metastable argon atom density and balance equations for electron energy and gas temperature, is designed. The dependencies of the electric field strength on the argon pressure and discharge current are calculated. It is shown that the numerical results are in good agreement with the measurement results and describe their basic features, namely, the falling current-voltage discharge characteristic and the availability of the local minimum in the dependence of the electric field strength on the pressure. The radial distributions of the plasma parameters of the low-frequency induction transformation-type discharge (electron density, gas temperature, and electron temperature) are obtained. It is shown that the gas heating-up accounting leads to a substantial change in the plasma parameters of the inductively coupled discharge.

Determination of the de-excitation probability of argon metastable (1s5 and 1s3) atoms on aluminum, stainless steel, silicon, quartz and Pyrex surfaces

The de-excitation probability of argon 1s5 and 1s3 metastable atoms on aluminum (alloy 2024), stainless steel (304), silicon (100), quartz (JGS1) and Pyrex surfaces is measured with a low pressure (1–5 mTorr) pulsed capacitively coupled plasma. The de-excitation probability is deduced from the loss rate of the metastable atoms in the late afterglow, which is obtained from the temporal evolution of the metastable atom density measured by laser absorption. Under the low pressure condition, the loss of the metastable atoms is mainly due to volume diffusion and de-excitation on the surface. The measured de-excitation probability for both Ar*(1s5) and Ar*(1s3) atoms ranges from 0.74 to 0.88 on these five materials, with an uncertainty of ±0.11. The measured probabilities on the Pyrex surface are in good agreement with those reported in previous work, and those on aluminum, stainless steel, silicon and quartz surfaces are reported here for the first time.

Characterization of helium surface-wave plasmas at intermediate pressures (5–50 Torr): temperatures and density of metastable atoms in the 23s level

A stationary surface wave discharge is studied in helium gas at intermediate pressure. By optical emission spectroscopy, rotational temperatures derived from impurities molecular bands have been studied, as well as the excitation temperature from the Boltzmann plot method of helium lines, as a function of gas pressure, absorbed power and axial position. The rotational temperature of OH molecules is supposed to be in thermal equilibrium with the plasma gas and is used for determining its temperature. The influence of the tube diameter has also been studied; it was observed that the tubes with smaller diameter lead to higher gas temperature. By optical absorption spectroscopy, the density of metastable atoms (in 23s level) has been studied as a function of the parameters. An important dependence on the gas pressure has been observed. Indeed, a one order of magnitude reduction is obtained when the pressure increases from 5 to 50 Torr. The density of metastable atoms remains constant as a function of power and almost all along the plasma column. Finally, a collisional-radiative model was used to compare the experimental to the theoretical results of the density of metastable atoms providing at the same time estimations of the density and the temperature of the electrons as well as the density of helium (atomic and molecular) ions. Results showed a good agreement when the tube diameter is large, but for smaller tubes, the agreement was obtained only for the higher gas pressures.

Combined optical emission and resonant absorption diagnostics of an Ar-O2-Ce-reactive magnetron sputtering discharge

We report the results on combined optical characterization of Ar-O2-Ce magnetron sputtering discharges by optical emission and resonant absorption spectroscopy. In this study, a DC magnetron sputtering system equipped with a movable planar magnetron source with a Ce target is used. The intensities of Ar, O, and Ce emission lines, as well as the absolute densities of Ar metastable and Ce ground state atoms are analyzed as a function of the distance from the magnetron target, applied DC power, O2 content, etc. The absolute number density of the Arm is found to decrease exponentially as a function of the target-to-substrate distance. The rate of this decrease is dependent on the sputtering regime, which should be due to the different collisional quenching rates of Arm by O2 molecules at different oxygen contents. Quantitatively, the absolute number density of Arm is found to be equal to ≈3×108cm−3 in the metallic, and ≈5×107cm−3 in the oxidized regime of sputtering, whereas Ce ground state densities at the similar conditions are found to be few times lower. The absolute densities of species are consistent with the corresponding deposition rates, which decrease sharply during the transition from metallic to poisoned sputtering regime.

Absolute number densities of helium metastable atoms determined by atomic absorption spectroscopy in helium plasma-based discharges used as ambient desorption/ionization sources for mass spectrometry

The absolute number densities of helium atoms in the 2s 3S1 metastable state were determined in four plasma-based ambient desorption/ionization sources by atomic absorption spectroscopy. The plasmas included a high-frequency dielectric barrier discharge (HF-DBD), a low temperature plasma (LTP), and two atmospheric-pressure glow discharges, one with AC excitation and the other with DC excitation. Peak densities in the luminous plumes downstream from the discharge capillaries of the HF-DBD and the LTP were 1.39×1012cm−3 and 0.011×1012cm−3, respectively. Neither glow discharge produced a visible afterglow, and no metastable atoms were detected downstream from the capillary exits. However, densities of 0.58×1012cm−3 and 0.97×1012cm−3 were measured in the interelectrode regions of the AC and DC glow discharges, respectively. Time-resolved measurements of metastable atom densities revealed significant random variations in the timing of pulsed absorption signals with respect to the voltage waveforms applied to the discharges.

Spatial profiles of electron and metastable atom densities in positive polarity fast ionization waves sustained in helium

Fast ionization waves (FIWs), often generated with high voltage pulses over nanosecond timescales, are able to produce large volumes of ions and excited states at moderate pressures. The mechanisms of FIW propagation were experimentally and computationally investigated to provide insights into the manner in which these large volumes are excited. The two-dimensional structure of electron and metastable densities produced by short-pulse FIWs sustained in helium were measured using laser-induced fluorescence and laser collision-induced fluorescence diagnostics for times of 100–120 ns after the pulse, as the pressure was varied from 1 to 20 Torr. A trend of center-peaked to volume-filling to wall-peaked electron density profiles was observed as the pressure was increased. Instantaneous FIW velocities, obtained from plasma-induced emission, ranged from 0.1 to 3 × 109 cm s−1, depending on distance from the high voltage electrode and pressure. Predictions from two-dimensional modeling of the propagation of a single FIW correlated well with the experimental trends in electron density profiles and wave velocity. Results from the model show that the maximum ionization rate occurs in the wavefront, and the discharge continues to propagate forward after the removal of high voltage from the powered electrode due to the potential energy stored in the space charge. As the pressure is varied, the radial distribution of the ionization rate is shaped by changes in the electron mean free path, and subsequent localized electric field enhancement at the walls or on the centerline of the discharge.

Measurements of helium 23S metastable atom density in low-pressure glow discharge plasmas by self-absorption spectroscopy of HeI 23S–23P transition

The helium 23S metastable atom densities are experimentally evaluated by self-absorption spectroscopy of the HeI 23S–23P transition spectra in two kinds of cylindrical glow discharge plasmas, which have different radii and are operated under different pressures of 300 and 20 Pa. The spectra are measured by using an interference spectroscopy system with a wavelength resolution of about 60 pm, and the relative intensities of the fine structure transitions are analyzed. It is found that the method is in principle applicable to plasmas with the pressure up to about the atmospheric pressure and electron density on the order of up to 1022 m−3. For a plasma with an absorption length of 10 mm and a spatially uniform temperature of 300 K, the method is sensitive to the metastable atom density roughly from 1016 to 1019 m−3.

Modeling of Direct Current Atmospheric Pressure Argon Discharge in Two-Dimensional

The finite element computational package COMSOL multiphysics were used to simulate a bar plate dc discharge in argon at atmospheric pressure. The basic plasma properties such as electron density, ion density, metastable atom density, electron temperature, electric voltage and electric field were studied. The current-voltage (I-V) characteristic of numerical model is in good agreement well with experimental data. This model is simple and insightful as a theoretical tool for argon atmospheric pressure discharges.

Ignition and extinction phenomena in helium micro hollow cathode discharges

Micro hollow cathode discharges (MHCD) were produced using 250 μm thick dielectric layer of alumina sandwiched between two nickel electrodes of 8 μm thickness. A through cavity at the center of the chip was formed by laser drilling technique. MHCD with a diameter of few hundreds of micrometers allowed us to generate direct current discharges in helium at up to atmospheric pressure. A slowly varying ramped voltage generator was used to study the ignition and the extinction periods of the microdischarges. The analysis was performed by using electrical characterisation of the V-I behaviour and the measurement of He*(3S1) metastable atoms density by tunable diode laser spectroscopy. At the ignition of the microdischarges, 2 μs long current peak as high as 24 mA was observed, sometimes followed by low amplitude damped oscillations. At helium pressure above 400 Torr, an oscillatory behaviour of the discharge current was observed just before the extinction of the microdischarges. The same type of instability in the extinction period at high pressure also appeared on the density of He*(3S1) metastable atoms, but delayed by a few μs relative to the current oscillations. Metastable atoms thus cannot be at the origin of the generation of the observed instabilities.

Selective and non-selective excitation/ionization processes in analytical glow discharges: excitation of the ionic spectra in argon/helium mixed plasmas

The increasing use of mixed gases, including helium mixtures, in glow discharge mass spectrometry has led to a need to understand the resulting signal enhancements due to changes in excitation and ionization processes. To understand more fully these processes in glow discharge, we have carried out complementary optical emission spectrometry experiments. The presence of added gases in the plasma gas can cause major changes in the number density of plasma gas ions and metastable atoms and so affects the intensities of spectral lines involving selective and non-selective excitation & ionization processes. We report the results of an experimental investigation on the effect of Ar/He mixed plasmas on the relative emission intensities of various analyte materials including copper, iron and titanium. Selective and non-selective excitation processes which are mainly dependent on the nature of the plasma gas and analyte material are observed and discussed.