Space resolved density measurements of argon and helium metastable atoms in radio-frequency generated He-Ar micro-plasmas

Space resolved concentrations of helium He* (³S₁) and argon Ar* (³P₂) metastable atoms in an atmospheric pressure radio frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. Even small absorptions down to 10^-4 could be measured using lock-in technique. The absolute density of metastable atoms densities at different rf-power, flow rate and gas mixture was deduced from measured absorption rates. Metastable concentrations range from 10⁹ to 10¹¹ cm^-3. Analysis of spectral profiles provided the pressure broadening coefficients of both metastable atoms by helium. The spatial distribution of metastable atoms in the plasma volume was obtained for various discharge conditions. Density profiles between the electrodes reveal the sheath structure and reflect the plasma excitation distributions in the discharge volume. It reveals the dominance of the α-mode discharge.     

Radiation Efficiency and Kinetic Calculations for the Positive Column in Hg/Ar Mixtures II. Impact of Tube Radius and Current Density

In a preceding part of the paper, based on experimental methods and on a corresponding kinetic model, especially the impact of the effective life times of the 6³P₁ and 6¹P₁ Hg resonance levels on the main properties of the low pressure Hg-–Ar positive column has been studied. In the presented second part these investigations, which have been performed under the aspect of their application in fluorescent lamps, will be continued. They are related to the effects of tube radius and discharge current density. New fluorescent lamps have reduced tube radii from R = 18 to 13 mm and compact lamps down to R = 5 mm. Thus it is of interest to study the electron distribution function and main macroscopic properties of such low pressure discharge plasmas, where the latter properties are obtained by adequate energy space averaging. At first, results are reported and discussed which are related to the dependence of main plasma parameters on the Hg partial pressure when reducing the discharge tube radius for constant discharge current i and argon pressure pAr. Starting from the electron distribution function the particle and energy budget will be studied in detail, especially of course the change of the ultraviolet radiation output from both Hg resonance levels 6³P₁ and 6¹P₁ and, in addition, of visible radiation. Then, to understand the effect of the discharge current density, as a representative example two cases with different values of R, i and PAr but with equal current density have been investigated and discussed. Altogether the investigations made will shed light upon the complex relations in the mentioned mixture plasma and give hints to select suitable parameter values which can be useful to improve light sources.     

Effect of atmospheric pressure pulsed discharge spatial inhomogeneity on Ar I and H I lines: Electron number density study

The spatial inhomogeneity of pulsed atmospheric pressure discharge in argon is investigated using the electron number density N_e diagnostics procedure applied to asymmetrically broadened Ar I lines. A dedicated fitting procedure is used for describing Ar I 703.0 nm line shape recorded from argon gas discharge and H I (at 486.13 and 656.28 nm) lines recorded from Ar-H₂ gas mixture discharge. The results revealed the change in N_e in both axial and radial directions. The additional Ar I lines at 614.5, 710.7, 731.2, and 731.6 nm, recorded from integral spatial radiation, are analysed as well to confirm the results from the plasma column region. The possibility of using AlO (B²∑⁺–X²∑⁺) and CN (B²∑⁺–X²∑⁺) molecular bands for gas temperature T_g measurements in this type of gas discharge source is demonstrated and T_g used as an input parameter for the N_e diagnostics procedure. For the proper identification of molecular band spectral lines, the Fortrat parabolas are constructed. The results obtained from Ar I 703.0 nm line indicate three different N_e values, with N_e1 ≈ 0.6 × 10¹⁶ cm⁻³, N_e2 ≈ 3.6 × 10¹⁶ cm⁻³, and N_e3 ≈ 19 × 10¹⁶ cm⁻³ measured from the plasma column. These N_e values increase in the cathode and anode region.     

Radiative Transitions and Temperature Time Dependences in Pulse Excited Microwave Discharges – (N2, Ar + N2)

Quantitative spectral and microwave measurements of vibrational temperatures and electron densities were performed for 2400 MHz non-isothermic pulse excited discharges in flowing nitrogen and argon at pressures (60–2700) Pa. A detailed analysis of the N₂ vibrational states population for the N₂ C³Π_u, X¹Σ_g⁺ electronic states has been carried out. The basic difficulties encountered when comparing the spectroscopically determined values of vibrational temperatures with corresponding quantities of the ground electronics state are mentioned and the time resolved dependences of the translational gas temperature in N₂ during the microwave pulse is evaluated. The steady state in the nitrogen pulse excited microwave plasma is reached within 3 · 10^?4 s, but generally, this time depends on the gas pressure in the discharge tube. In the Ar + N₂ mixtures the excitation conditions are complicated by the metastable argon atoms (3P_2,0) creating the nonequilibrium populations of electronic, vibrational and rotational N₂ states.     

Radiation Efficiency and Kinetic Calculations for the Positive Column in Hg/Ar Mixtures. I. Thg Variation

Properties of the positive column of low pressure Hg/Ar discharge have been investigated from the point of view of its application in fluorescent lamps. The results obtained are based on experimental methods to determine both UV resonance radiation fluxes of the 6³ P₁ and 6¹P₁ Hg levels and to measure the electron distribution function by probes; in addition a kinetic model is applied for the theoretical interpretation of the experimental results and to give a deeper insight into the particle and energy budget. In the present first part of the paper the effects of recently determined τ_eff-values on radiation, net collision rates, power transfer and the electron energy distribution for extended parameter values are investigated especially in dependence on mercury partial pressure.     

Intensity of Ar spectral lines in an Ar-O2 glow discharge

Measurements of relative intensities of spectral lines and of plasma parameters have been carried out in an Ar-O₂ d.c. glow discharge under the following conditions: gas pressures of 1, 2 and 3 torr and partial pressures for O₂ of 0, 0.01, 0.02, 0.05 and 0.1 torr. Many of the Ar line intensities decreased when O₂ was added, while some were enhanced. Enhancement of the 6677A and 7504A lines has been most notable. The number density of metastable Ar atoms is decreased when O₂ is added. Upon addition of O₂, the electron temperature is decreased and the electron density increased, except at a gas pressure of 1 torr; the electric field strength in the positive column is also increased.     

Repetitive low-pressure volume discharge induced by attachment instability in an Ar/Cl<Subscript>2</Subscript> mixture

Ignition conditions and the characteristics of a repetitive volume discharge with a spherical anode and plane cathode are investigated. The discharge was ignited in Ar/Cl₂ mixtures (P≤2.0 kPa) used in excimer halogen lamps operating on the ArCl (B-X) 175-nm, Cl₂(D′-A′) 257-nm, and Cl ₂ ^** 195-to 200-nm molecular bands. At an interelectrode distance of 3 cm and a dc anode voltage of U _ch ≤1 kV, a stable repetitive pulsed discharge with a repetition rate of 1–50 kHz was ignited in chlorine or (0.1–2.0)/(0.04–0.12)-kPa Ar/Cl₂ mixtures. The development of attachment instability in the discharge plasma, in which the processes of the formation, decay, and diffusion of the Cl ₂ ⁻ and Cl⁻ negative ions play an important role, leads to the formation of a solitary pearlike plasma domain with an average diameter of 0.2–3.5 cm.     

Längsfeldstärke,Elektronentemperatur, mittlere Neutralgasdichte,Ionenstromdichte auf die Wand und mittlere Entladungsstromdichte in der Freifallsäule bei hohem Ionisationsgrad

On the basis of a foregoing paper new theoretical results for the positive column at low pressure and strong ionization, especially for discharges in noble gas ion lasers, are given. The mean velocity v_n0 of the neutral atoms reemitted from the wall is taken into account. The electric conductivity is calculated for an argon plasma. The formulas connecting the electron temperature, the mean neutral gas density, and the electric field strength are derived. The electron temperature, the axial electric field intensity, the degree of ionization, the axial electron drift velocity, the ion flux to the wall, and the force density causing the main part of gas pumping along the column are calculated as functions of the product of the mean current density and the tube radius, and of v_n0 for argon. The axial drift velocity of the electrons is still smaller than the mean thermal electron velocity for high discharge currents, except at very low gas pressures. In general, the ion flux to the wall is not directly proportional to the discharge current. The factor for the determination of the charged particle density by means of probe measurements at the wall is discussed. The self-magnetic field affects the discharge only at high electron temperature, high degree of ionization, and relatively large tube radius, i.e. at high current density and low gas pressure in not too narrow discharge channels.     

Zur Radialstruktur der diffusionsbestimmten positiven Säule der Sauerstoff-Entladung bei niederen Gasdrucken

The radial distribution of the electrons, negative and positive ions in the cylindrical positive column of the direct current discharge in 0₂ has been calculated. The negative ions are formed by dissociative attachment (0₂ + e →^? + 0), the destruction of negative ions is effected by the reverse process (0^? + 0 0₂ + e). The pressure is assumed to be low enough that the radial variation of 0-concentration can be neglected. By use of the condition of the quasineutrality and the ambipolarity for the radial current from the balance equations of the electrons and the negative ions two differential equations of second order are derived, which has been solved numerically for different values of the ratio z_j/z_a and β[0]. Numerical results show that the radial distributions of the charge carriers are not-proportional. If z_j ≈? z_a the radial distribution of the electron density is enlarged, compared to a Bessel distribution. The radial distribution of the negative ions is also enlarged in the centre of the column, but near the wall of the tube a lack in the density exists. The effective diffusion coefficient of the negative ions is negative, i.e., they are moving from the wall of the tube to the centre of the discharge. The enlargement of the radial distributions of the charge carriers, which results in an improvement of the economy of the balance of electrons, increases if z_j/z_a and β/[0] are decreased.     

On Electron Concentration Distribution along the Radius of the Positive Column of a Glow Discharge in Electronegative Gases

It is shown in this paper that the form of the electron concentration distribution along the radius of the positive column in an oxygen discharge depends on the relative concentration of negative ions α = n_-/n_e in the plasma. It is found that in discharges in electronegative gases everywhere along the radius of the positive column, where α > 10, the electron concentration is equal to its value on the tube axis (plane electron distribution along the radius of the positive column). It is shown that an increase of the discharge current leads to the decrease of α and to the change of the distribution of profile n_e from plane to parabolic. However, weak laminar oxygen pumping along the positive column leads to more plane an electron concentration distribution because an increase of the negative ions concentration takes place. It is induced by the decrease of oxygen atoms density on account of their carrying out by the flow.     

Impact broadening and shift rates for the 6p2 3PJ′ → 7s 3P J o transitions of lead induced by collisions with argon and helium

The collisional broadening and shift rate coefficients of the 283.39 nm (6p ^{2 3}P₀ → 7s ³P₁ ^o ), 364.06 nm (6p ^{2 3}P₁ → 7s ³P₁ ^o ), 368.45 nm (6p ^{2 3}P₁ → 7s ³P₀ ^o ) and 405.90 nm (6p ^{2 3}P₂ → 7s ³P₁ ^o ) Pb lines by He and Ar have been measured by fitting the experimental absorption line shapes to theoretical Voigt profiles. The absorption measurements were performed in a resistively heated, Pb loaded oven with an integrated dc noble gas discharge to produce also Pb atoms in the 6p ^{2 3}P¹ and 6p ^{2 3}P² metastable states. The diffusion of the metastable atoms out of the discharge zone into the neutral noble-gas atmosphere enabled the line-shape and shift measurement of the lines involving the metastable states without the influence of the discharge plasma.     

Zur Mikrowellendiagnostik stromstarker Entladungen mit Skineffekt. Teil II: Bestimmung der Plasmaparameter eines angeströmten Stickstoffbogens bei Atmosphärendruck

Microwave diagnostics of an inhomogeneous, dense, cylindrical plasma column may be used to determine the complex impedance and a mean noise temperature in the range of strong skin effect (theory see part I). This case is realized if the specific d.c. impedance E/I falls below a critical value, e.g. 20 Ω/cm for ω = 10¹⁰. It is shown, that for any radial conductivity profile the complex impedance is a function of E/I only. A formula is derived for calculating the noise temperature of the column as a weighted mean value due to the local plasma temperature and conductivity. In the case of strong skin effect this mean value equals the electron temperature of plasma regions near the boundary of the conducting diameter. These results in connection with spectroscopical diagnostics are use to determine some plasma parameters of an arc discharge in a streaming nitrogen atmosphere at 760 Torr. The temperature profil was obtained from the intensity of the 3371 Å molecular band and then the conductivity profile was calculated using an energy balance equation. The results show that even in the 10 A-case the arc plasma is not in thermal equilibrium, the difference T_e—T_g being about 1,500 ºK. This large difference is due to the flow of neutral gas streaming downward the discharge chamber. The length of the discharge (1·3 cm) is not sufficient for the neutral atoms to reach their equilibrium temperature.     

Effect of helium/argon gas ratio in a He-Ar-Cu<Superscript>+</Superscript> IR hollow-cathode discharge laser: modeling study and comparison with experiments

The He-Ar-Cu⁺ IR laser operates in a hollow-cathode discharge, typically in a mixture of helium with a few-% Ar. The population inversion of the Cu⁺ ion levels, responsible for laser action, is attributed to asymmetric charge transfer between He⁺ ions and sputtered Cu atoms. The Ar gas is added to promote sputtering of the Cu cathode. In this paper, a hybrid modeling network consisting of several different models for the various plasma species present in a He-Ar-Cu hollow-cathode discharge is applied to investigate the effect of Ar concentration in the gas mixture on the discharge behavior, and to find the optimum He/Ar gas ratio for laser operation. It is found that the densities of electrons, Ar⁺ ions, Ar_m ^* metastable atoms, sputtered Cu atoms and Cu⁺ ions increase upon the addition of more Ar gas, whereas the densities of He⁺ ions, He₂ ⁺ ions and He_m ^* metastable atoms drop considerably. The product of the calculated Cu atom and He⁺ ion densities, which determines the production rate of the upper laser levels, and hence probably also the laser output power, is found to reach a maximum around 1–5 % Ar addition. This calculation result is compared to experimental measurements, and reasonable agreement has been reached. Received: 14 October 2002 / Revised version: 28 November 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +32-3/820-23-76, E-mail: annemie.bogaerts@ua.ac.be     

Etude spectroscopique de l'effet d'anode

Spectral lines of LiI emitted by an anodic plasma, produced during electrolysis of a KCl-LiCl mixture, have been observed. This paper deals with diagnostic studies of this plasma and is based on a comparison of observed line profiles for LiI(2P-3D), (2P-4S) and (2P-4D, 4F) with calculated values derived from the Griem-Baranger impact theory. We have found, from the ensemble of the results, that the electron density in the plasma center is N_e(0) ? 1·8×10¹⁷ cm^-3, the electron temperature is T_e(0) ? 9×10³ °K, and the plasma thickness is l ? 200 μ.     

Numerical study of the effect of gas flow in low pressure inductively coupled Ar/N<Subscript>2</Subscript> plasmas

The effect of gas flow in low pressure inductively coupled Ar/N₂ plasmas operating at the rf frequency of 13.56 MHz and the total gas pressure of 20 mTorr is studied at the gas flows of 5–700 sccm by coupling the plasma simulation with the calculation of flow dynamics. The gas temperature is 300 K and input power is 300 W. The Ar fractions are varied from 0% to 95%. The species taken into account include electrons, Ar atoms and their excited levels, N₂ molecules and their seven different excited levels, N atoms, and Ar⁺, N⁺, N₂ ⁺, N₄ ⁺ ions. 51 chemical reactions are considered. It is found that the electron densities increase and electron temperatures decrease with a rise in gas flow rate for the different Ar fractions. The densities of all the plasma species for the different Ar fractions and gas flow rates are obtained. The collisional power losses in plasma discharges are presented and the effect of gas flow is investigated.     

Optical characterization of atmospheric‐pressure plasma needle

The atmospheric‐pressure plasma needle is a promising source that can be used efficiently for different industrial applications. A radio frequency (RF) (13.56 MHz) generator was used to generate a He–O₂/Ar mixture plasma. The ground‐state oxygen atomic density [O] was calculated as a function of discharge parameters by “actinometry”. The Ar‐I (2p₁ → 1s₂) line at 750 nm and the O‐I (³P → ³S) line at 844 nm were used to estimate the [O] atomic density. The rotational temperature T _R of He–O₂/Ar mixture was measured from the rotational levels of the “first negative system” (FNS) by using the “Boltzmann plot”. The effect of discharge parameters on the atomic oxygen density [O] and the gas temperature was monitored. These results show that [O] density increases with RF power and O₂ concentration, but decreases with the gas flow rate. Whereas the gas temperature increases with increase in the input RF power, it decreases with increase in the gas flow rate and O₂ concentration in the mixture. Since the [O] atomic density contributes to plasma‐based biomedical applications, the proposed optimum conditions for plasma‐based decontamination of heat‐sensitive materials in the present study are 0.6% oxygen, 500 sccm flow rate, and 26 W RF power.     

On the Plasma Chemistry of an RF Discharge Containing Aluminium Tri‐Isopropoxide Studied by FTIR Spectroscopy

In Ar and Ar/N₂ radio frequency (RF) discharges with admixtures of aluminium tri‐isopropoxide (ATI) the fragmentation of this metal‐organic precursor was studied by means of Fourier Transform Infrared (FTIR) spectroscopy using an optical long path cell providing an optical length of l = 17.2 m. The experiments were performed in an asymmetric capacitively coupled process plasma at a frequency of f = 13.56 MHz and at pres‐sure values in the range of p = 1–10.5 Pa. The discharge power was chosen between P = 10–100 W. Using FTIR spectroscopy the evolution of the concentrations of ATI and of six stable molecules, CH₄, C₂H₂, C₂H₄, C₂H₆, CO and HCN, was monitored under flowing conditions at gas flows of Φ_total = 0.5–14.5 sccm in the discharge. The concentrations of the reaction products were measured tobe between 2 x 10¹² molecules cm^–3 as e.g. found for C₂H₄and C₂H₆, and 5 x 10¹³ molecules cm^–3, as e.g. in the case of CO. In the plasma a complete dissociation of the precursor ATI was found at a power value of about P = 80 W independent on the admixture of Ar or N₂. The fragmentation efficiency (F_E) of the reaction products which originate from the ATI molecules ranges between 0.2 and 4 x 10¹⁶ molecules J^–1 while the fragmentation rate (F_R) reached up to 2.5 x 10¹⁸ molecules s^–1. The multi component detection ability of the spectrometer served to analyse the carbon balance of the by‐product formation. For all experiments, the carbon balance never exceeded 25%. Therefore, in the plasmas the majority of the provided carbon is most likely deposited at the reactor walls or forms dust particles or higher molecular C_xH_y. The conversion efficiencies (C_E) of the produced molecular species ranges between 0.1 x 10¹⁵ molecules J^–1 for C₂H₄ and 5 x 10¹⁵ molecules J^–1 for C₂H₆ depending on the discharge conditions of the RF plasma. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of the dependence of the radiative properties of high-frequency electrodeless lamps in an Hg-Ar mixture on the pressure of mercury vapor

The radiative characteristics of high-frequency electrodeless lamps in a mixture of mercury and argon have been studied theoretically and experimentally as functions of the cold spot temperature (the pressure of mercury vapor). The intensity of the mercury lines at 404.7, 435.8, and 546.1 nm, corresponding to the triplet transition (7³ S ₁-6³ P _0,1,2) in the visible spectral region, as well as the intensity of the UV resonance line (6³ P ₁-6¹ S ₀) at 253.7 nm, has been measured. A model describing the physical processes in the discharge plasma and including the kinetics of excited states of mercury and argon atoms has been suggested. The parameters of the discharge plasma and the electromagnetic field have been calculated self-consistently through the numerical solution of the system of equations of electron density and energy balance and population balance of excited levels of argon and mercury atoms, as well as the Maxwell equations. The model developed has allowed us to calculate the intensities of the mercury emission lines at 253.7, 404.7, 435.8, and 546.1 nm and to compare the results with experimental data. The relative intensities of the mercury spectral lines corresponding to the triplet transition 7³ S ₁-6³ P _0,1,2 have been calculated for the first time on the basis of a self-consistent model of the discharge.     

Modelling of the Positive Column in an Argon-Hexamethyl-disiloxane dc Glow Discharge

A model is presented of the positive column of a dc glow discharge in argon with small admixtures of hexamethyldisiloxane (HMDS). The axial electric field, the ion production rates for direct-, stepwise-, pair-, and Penning ionization, the densities of metastable Ar atoms and of electrons, and the wall current of HMDS ions are calculated in dependence on HMDS admixture and discharge current density. For the calculations particle balance equations were used for a diffusion determined plasma in a mixture of two gaseous components. The reaction rates for the electron collision processes were determined applying the electron distribution function calculated for pure argon. Taking into account PENNING ionization of HMDS molecules by metastable argon atoms the decrease of electric field for increasing HMDS admixtures is according to the experimen-tally measured values. Also ion wall currents and electron densities are compared with experimen-tal values for thin film formation rate and results of probe measurements.     

悬浮型微波共振探针在电负性容性耦合等离子体中电子密度的测量

本文首先利用悬浮型微波共振探针测量了Ar等离子体的电子密度,并与朗缪尔双探针的测量结果进行了比较,表明了微波共振探针在低密度等离子体测量的可行性.对40.68 MHz单射频容性耦合Ar/SF₆和SF₆/O₂等离子体的测量结果表明:电负性气体SF₆掺入Ar等离子体显著降低了等离子体电子密度,但随着增加SF₆的流量,电子密度表现为缓慢下降;而O₂掺入SF₆等离子体中,电子密度则随着O₂流量的增加表现为持续的下降.另外,40.68 MHz/13.56 MHz双频激发的SF₆/O₂容性耦合离子体的电子密度并不随低频功率的变化而变化.本文对上述的实验现象进行了初步的解释.