Simple Collisional-radiative Model Research Articles

Zeeman-resolved TDLAS using metastable levels of Ar in the weakly magnetized plasma of the linear plasma device PSI-2

Tunable diode laser absorption spectroscopy was applied at the linear plasma device PSI-2 to measure the magnetic field, temperature of argon and density of metastable species in a low density gas discharge. The measurements on the two metastable levels of Ar were performed by scanning the plasma column of PSI-2 at different radii. The obtained magnetic field using the lines at 763 and 772 nm (Ar) was found to be systematically lower (by 5% to 17%) than the calculated vacuum field. Part of the deviation arises from the line integration of the absorption signal. The radial gradient of the magnetic field strength combined with the radial metastable density determines the magnitude of this contribution (2%–3%). The temperature of the neutral gas was found to be essentially constant within the discharge chamber. The gas temperature rises with increasing cathode current and magnetic field due to an increase in the plasma density and, consequently, an increase in the energy transferred to the neutral gas by collisions with the charged particles. The density of the 4 s metastable level with J = 2 was found to be 8–9 times higher than that of the level with J = 0 similarly to observations by others in non-magnetized plasmas. To understand this trend a simple collisional-radiative model for the metastable argon 4s J = 2 level was developed. Depending on the treatment of the 4p levels it predicts a lower and an upper limit of the metastable density. The experimental values are within the limits predicted by the model indicating that the complex kinetics of the excitation and deexcitation collisional-radiative processes lead to this deviation from the statistical equilibrium.

Diagnostic of Ar-CO2 mixture plasma using a fine-structure resolved collisional radiative model

In the present work, we develop a collisional radiative (CR) model for the Ar-CO2 mixture plasma. The model is applied to the diagnostics of a recently reported low-pressure DC generated Ar-CO2 plasma of Rodríguez et al. [Phys. Plasmas 25, 053512 (2018)] utilizing their spectroscopic measurements who reported the plasma parameters at different concentrations of CO2 in Ar at various pressures and powers. Our CR model includes various population transfer mechanisms between the fine structure levels, viz., electron-impact excitation, radiative decay, and ionization, as well as their reverse processes, e.g., electron-impact de-excitation as well as two- and three-body recombination. The effects of radiation trapping and diffusion are also taken into account. The plasma parameters viz. electron density (ne) and electron temperature (Te) are obtained as a function of different pressures (0.2, 0.3, and 0.6 mbars) and discharge powers at 25 and 50% concentrations of CO2 in argon. These results are determined using measured intensities of seven intense emission lines out of 3p54p (2pi) → 3p54s (1si) fine-structure transitions. It is observed that both the electron density and electron temperature increase with the increase of CO2 concentration, which is in confirmation with experimental predictions. The populations obtained for 1si and 2pi levels from our CR model are also reported and compared respectively with the corresponding available values from the simple CR model and experiment of Rodríguez et al. [Phys. Plasmas 25, 053512 (2018)]. A significant difference in the populations is observed from the two models.

Optical Diagnostics of Shock-induced Argon Plasmas Based on a Simple Collisional-Radiative Model

Optical Diagnostics of Shock-induced Argon Plasmas Based on a Simple Collisional-Radiative Model

Determination of Electron Density and Attenuation of Electromagnetic Waves in Ar DBD Plasmas

Prediction of electron density is crucial to numerically evaluate the attenuation of electromagnetic waves in plasmas and to optimize the parameters. In this paper, a simple collisional-radiative model (CRM) to determine the electron density of Ar dielectric barrier discharge (DBD) plasmas for pressures ranging from 0.2 to 1 atm is introduced, and then, the attenuation of electromagnetic waves is analyzed. First, the radial average electron density of Ar DBD plasma is estimated by combining the measured current and a 1-D self-consist fluid numeric model. Second, based on a simple CRM, an optical emission spectroscopy method is used to determine the radial electron density distribution. Finally, according to the radial electron density distribution, the attenuation of electromagnetic waves is calculated at a frequency range from 10 MHz to 10 GHz. The result indicates that the electromagnetic waves with a relatively low frequency can be effectively attenuated by the plasma and the edge distribution of the plasma clearly affects the attenuation.

Investigation of transitions between metastable levels of the first excited configuration of palladium-like tungsten

Spectroscopy in the wavelength region of 330 nm to 400 nm for highly charged tungsten was performed using the High Temperature Super Conducting Electron Beam Ion Trap (SH-HtscEBIT) at Fudan University. Three lines from palladium-like tungsten (W28+) were identified as transitions between metastable levels in the first excited configuration ([Kr]4d94f). A second-order relativistic many-body perturbation theory approach and a simple collisional radiative model were used to theoretically study the fine structure levels of the 4d94f configuration. The calculated results show qualitative agreement with experiment. We conclude that some levels in the 4d94f excited configuration have extremely long lifetimes and may exhibit extraordinarily high populations, possibly leading to indirect ionization in, for example, fusion plasmas.

C-R Model for Ar plasmas using reliable excitation cross-sections

We have used reliable electron excitation cross sections in a simple Collisional-Radiative Model (CRM) to obtain the population densities of various excited levels for low electron temperature Ar discharges. Cross sections are obtained by using relativistic distorted wave theory for transitions from ground as well as excited states to the different higher lying levels of the Ar atom. We have compared our calculated population density results for 3p54s levels with recent exp erimental OES measurements.

Determination of metastable level densities in a low-pressure inductively coupled argon plasma by the line-ratio method of optical emission spectroscopy

The line-ratio method of optical emission spectroscopy (OES) is used for the diagnosis of plasma parameters. In this work, electrostatic probe-assisted OES is employed to measure metastable level densities from spectral lines and electron energy distribution functions (EEDFs) in a low-pressure inductively coupled argon plasma. Emission spectroscopy is based on plasma modelling through a simple collisional–radiative model. The line intensities of Ar(3p54p → 3p54s) are modified due to the plasma reabsorption at relatively high pressures where the plasma becomes optically thick. To consider this effect, a pressure dependence factor αij(P) is first derived from both the measured intensity and pressure-dependent cross-section for electron excitation. It is found that the obtained metastable densities range from 1.3 × 109 to 1.2 × 1010 cm−3 and their ratios are nearly constant by a factor of about 3–5 in the investigated pressure range (3–50 mTorr). The effect of non-Maxwellian EEDF on the metastable densities is also discussed. The results measured by the line-ratio method are consistent with that of the OES-branching fraction method taking into account the photon escape factor to treat the radiation trapping.

Diagnosis of a low pressure capacitively coupled argon plasma by using a simple collisional-radiative model

This paper proposes a simple collisional-radiative model to characterise capacitively coupled argon plasmas driven by conventional radio frequency in combination with optical emission spectroscopy and Langmuir probe measurements. Two major processes are considered in this model, electron-impact excitation and the spontaneous radiative decay. The diffusion loss term, which is found to be important for the two metastable states (4s[3/2]2, 4s′[1/2]0), is also taken into account. Behaviours of representative metastable and radiative states are discussed. Two emission lines (located at 696.5 nm and 750.4 nm) are selected and intensities are measured to obtain populated densities of the corresponding radiative states in the argon plasma. The calculated results agree well with that measured by Langmuir probe, indicating that the current model combined with optical emission spectroscopy is a candidate tool for electron density and temperature measurement in radio frequency capacitively coupled discharges.

A simple collisional–radiative model for low-temperature argon discharges with pressure ranging from 1 Pa to atmospheric pressure: kinetics of Paschen 1s and 2p levels

A simple collisional–radiative model for the Paschen 1s and 2p levels is proposed for low-temperature argon discharges. This model can predict the population distribution of 1s and 2p levels over a wide discharge pressure range 1–105 Pa and ionization ratio range 10−6–10−3. The modelling results are found to be in good agreement with observed optical emissions from several different types of argon discharges at 1, 100 and 105 Pa. By using the model, the dominant kinetic processes of 1s and 2p levels are investigated for an electron beam plasma, an inductively coupled plasma, a capacitively coupled plasma and a microwave microplasma. A kinetic diagram is given, which can be used to identify the kinetic state of 1s and 2p levels in many low-temperature argon discharges reported in the literature. This model is also useful for obtaining discharge parameters from optical emissions in low-temperature argon discharges.

Determining the electron temperature and the electron density by a simple collisional–radiative model of argon and xenon in low-pressure discharges

A simple collisional–radiative model for argon and xenon is used, in conjunction with optical emission spectroscopy (line-ratio technique), to determine the electron temperature and electron density in low-pressure discharges containing argon and xenon. Satisfactory agreement is obtained between this method and the Langmuir probe for an inductively coupled plasma containing neon, argon and xenon. This method is applied for a capacitive discharge containing fluorocarbon, argon and xenon. The electron temperatures and electron densities obtained under various discharge conditions are compared with those reported in the literature by other techniques.

Spectroscopic Measurement of the Electron Temperature and the Metastable Densities by Using a Simple Collisional-Radiative Model in a Low-Pressure Inductively-Coupled Argon Plasma

Spectroscopic Measurement of the Electron Temperature and the Metastable Densities by Using a Simple Collisional-Radiative Model in a Low-Pressure Inductively-Coupled Argon Plasma

A simple collisional–radiative model for low-pressure argon–oxygen mixture discharges

The simple collisional–radiative model is investigated in low-pressure discharges containing argon and oxygen. Using this model, one can obtain a simple relationship between the electron density and the intensity ratio of two argon emission lines without knowing electron temperature. It is found that the relationship is valid in these discharges regardless of oxygen concentration, if argon np levels with zero total angle momentum quantum numbers are selected. This is because that the kinetics of these levels is not significantly affected by the metastables under low-pressure conditions. The application and validity of this model in other kinds of low-pressure plasmas, such as discharges contain CF4, are also discussed.

A simple collisional–radiative model for low-pressure argon discharges

A simple collisional–radiative model for low-pressure argon discharges is proposed. Using this model and the measured cross section data from recent literature, the relationship between the electron density and some emission line intensities is investigated, resulting in a semi-empirical formula relating the electron density and the emission intensity ratio of argon lines (from levels 3p and 4p). The electron density determined from this model is compared with that from a Langmuir probe in an inductively coupled argon plasma. The application and validity of this model in other kinds of low-pressure argon plasmas, such as capacitively coupled and helicon plasmas, or in discharges containing both argon and other gases are discussed.

The modelling of streamer-induced emission in atmospheric pressure, pulsed positive corona discharge: N2 second positive and NO-γ systems

The potential of employing nitric oxide (NO) as a spectrometric probe to monitor the evolution of free electrons is explored in this article for conditions occurring during the propagation of a positive streamer in atmospheric pressure nitrogen. An extension of the spectroscopic approach, based on a comparison of the relative emission intensity of electronic states with different excitation thresholds is proposed, considering typical space and timescales of streamers. Basic characteristics of electron-impact excitation processes are evaluated for N2(X 1Σg+), N2(C 3Πu) and NO(A 2Σ+) electronic states utilizing recently published cross section data. Emission of the N2 second positive (2.PG) and NO-γ systems is inspected by means of a simple collisional-radiative model considering energetic Maxwellian electrons as the primary excitation source of involved electronic states. A simple tool for evaluating `spectroscopic Maxwellian electron temperature' from relative intensities of the N2 2.PG and NO-γ systems emissions is provided.

Intensity ratio between Lyman-α1and -α2lines of hydrogenlike titanium observed in an electron-beam ion trap

Lyman-$\ensuremath{\alpha}$ lines of hydrogenlike ${\mathrm{Ti}}^{21+}$ have been observed with an electron-beam ion trap. The intensity ratio between Lyman-${\ensuremath{\alpha}}_{1}$ and -${\ensuremath{\alpha}}_{2}$ was measured at electron energies of 10.6, 24.7, and 49.6 keV (2.12, 4.96, and 9.96 in threshold units). The linear polarization of Lyman-${\ensuremath{\alpha}}_{1}$ was obtained by comparing the experimental intensity ratio at an observation angle of $90\ifmmode^\circ\else\textdegree\fi{}$ with the total emission cross section ratio calculated by a simple collisional radiative model. The present results are compared with existing theoretical results.

Dissociation of nitrogen in flowing DC glow plasmas

Results of a mass spectrometric investigation of dissociation of nitrogen in DC glow plasmas in a flowing gas system in the pressure range 1-3 Torr, electron density range (1-8)*1010 cm-3 electron temperature range 2.0-2.7 eV and linear flow rate range 1.1-5.8 m s-1 are presented. The extent of dissociation is of the order of approximately 1%. A simple collisional-radiative model is presented which incorporates several molecular electronic excited states, nitrogen-containing ions, and N atoms. The N atom production is presented as a function of electron density; the predictions of the model are in agreement with the experimental results within an order of magnitude.

Spectroscopic characterization of a reactive low-pressure RF plasma: investigation of the electronic distribution functional tail

With the aid of spectroscopic and microwave measurements, the authors developed a simple collisional radiative model (CRM) for helium that they applied in a low-pressure range (<20 mTorr) where atom-atom inelastic collisions are negligible. Exploiting the two main populating channels of the n=3 helium levels, and assuming that they involve two electron groups, they deduced the gas temperature and two electronic temperatures characterizing these two electron groups. The electron energy distribution function tail temperature, higher than the core one, varies between 4.0 and 7.5 eV according to gas composition (Ar-He-F2), total pressure and coupled RF power values.

Dissociation of nitrogen in dc glow plasmas

Dissociation of nitrogen in dc glow plasmas in flowing-gas systems has been studied quantitatively in the pressure range 1–3 Torr, n e range 3–8 × 10 10 cm −3, and kT e range 2–3 eV. The extent of dissociation, which is of the order of 1% and varies with experimental conditions, has been for the first time expressed fully as a function of electron density and electron temperature. A simple collisional-radiative model predicts well the trends of dissociation as a function of plasma parameters, and predicts absolute values of per cent dissociation, in the worst case, within an order of magnitude.

Charge transfer between Ar and Mg in an inductively coupled plasma

Relative level population densities for several excited states of magnesium atom and ion have been measured at several RF powers and spatial positions. These were used to construct Saha-Boltzmann plots that show that the 2 S and 2 D levels of Mg + are overpopulated relative to the results obtained from a simple collisional-radiative model. The degree of overpopulation has been rationalized using a simple model previously developed for Mg-Ar charge transfer.

Lokale Diagnostik einer Argon‐Niederdruckentladung mit Hilfe der resonanten Laserstreuung

AbstractWe report on the experimental investigations on a laser‐like low pressure argon gas discharge. The discharge parameters varied in the range from 0,05 to 1 Torr for the gas pressure and from 10 to 30 A for the discharge current.Local electron densities and temperatures have been measured with an adjustable spherical probe.Spatially resolved measurements of the temperatures and drift velocities of the Ar+‐ions in the radial and axial direction of the positive column have been carried out by the resonant laser light scattering (fluorescence technique).The radial density profiles of the excited Ar+‐ions have been determined in two different ways and have been compared with the calculations based on a simple collisional radiative model.