Denis Gravelle

Study of the structure and deviation from equilibrium in direct current supersonic plasma jets

Mathematical modelling and optical emission spectroscopy are applied to study the effect of the chamber pressure on the structure and properties of supersonic plasma jets formed by a direct current arc. In this installation the plasma is created inside the nozzle where the flow is accelerated. As a result some deviation from thermal and ionization equilibrium can be found, even at the working chamber inlet. In this paper, by means of a two-temperature model, we study the argon jet flow using the data of the emission spectroscopy measurements to make realistic assumptions about the inlet boundary conditions. The results show that, when the chamber pressure is low, a strongly underexpanded jet with a Mach disc is formed. For the higher ambient pressure values, the core region of the jet changes to a mildly underexpanded structure with alternating oblique expansion and compression zones. The predicted shock zone positions are in a very good agreement with measurement. The general analysis shows that the deviation from local thermodynamic equilibrium in the jet is inversely related to the chamber pressure. Along the jet core the deviation from thermal equilibrium is less in the shock regions than in the expansion zones, where the electrons are heated by three-particle recombination. Downstream of the jet core the velocity drops, but the ionization and thermal equilibria are not attained because of the correlation between the characteristic recombination and the hydrodynamic times. Both the modelling and the emission spectroscopy show that the axial electron number density is much closer to its frozen value than to equilibrium value. The results obtained are helpful for different applications such as plasma processing, rocket propulsion systems and the simulation of re-entry conditions.

An analysis of LTE effects in inductively coupled RF plasmas

Spectroscopic measurements are carried out on inductively coupled radio-frequency (RF) argon plasma over the pressure range 120 to 760 Torr. The electron number density is obtained by two methods: Hbeta profile and the absolute intensity of the argon continuum. Number densities of excited argon levels are deduced from absolute line intensities. The electron density measured in the hottest zone of the plasma decreases from 1016 to 6*1015 cm-3 with the decrease of pressure from 760 to 120 Torr. Comparison between the different number densities shows that the atmospheric plasma is in local thermodynamic equilibrium (LTE) whereas weak departures from equilibrium are observed at pressures below 300 Torr.

Spectroscopic validation of the supersonic plasma jet model

Optical emission spectroscopy is applied to validate numerical simulations of supersonic plasma flow generated by induction torch with a convergent-divergent nozzle. The plasmas exhausting from the discharge tube with the pressure 0.4-1.4 atm. through two nozzle configurations (the outlet Mach number equals 1.5 and 3) into low-pressure (1.8 kPa) chamber are compared. Both modelling and experiments show that the effect of the nozzle geometry on physical properties of plasma jet is significant. The profiles of electron number density obtained from modeling and spectroscopy agree well and show the deviations from local thermodynamic equilibrium. Analysis of intercoupling between different sorts of nonequilibrium processes is performed. The results reveal that the ion recombination is more essential in the nozzle with the higher outlet number than in the nozzle with the lower outlet number. It is demonstrated that in the jets the axial electron temperature is quite low (3000-8000 K). For spectroscopic data interpretation we propose a method based on the definition of two excitation temperatures. We suppose that in mildly under expanded argon jets with frozen ion recombination the electron temperature can be defined by the electronic transitions from level 5p (the energy E = 14.5 eV) to level 4p (E = 13.116 eV). The obtained results are useful for the optimization of plasma reactors for plasma chemistry and plasma processing applications.

Spectroscopic study of a supersonic plasma jet generated by an ICP torch with a convergent-divergent nozzle

A supersonic argon and argon-hydrogen (3-5{%}) plasma jet generated by an induction plasma torch is studied by means of the methods of optical emission spectroscopy. The torch was operated at the input power of 20 kW and near atmospheric pressure. The supersonic jet with a periodic structure of expansion and compression zones is created by expanding the plasma through the Laval nozzle into a chamber maintained at the pressure around 1.8 kPa. Atomic argon lines with the upper level energies ranging from 13.3 to 15.5 eV, continuum emission and Hβ line profile are used to evaluate plasma parameters such as temperature and electron number density. Analysis based on the Boltzmann diagram, line-to-continuum ratio, population of continuum extrapolated level and Stark broadening reveals various stages of departure from thermodynamic equilibrium in the plasma flow. It is shown, among others, that the temperature derived from Boltzmann diagram does not follow the jet structure and reliable determination of electron temperature is questionable. An addition of several percent of hydrogen results in a significant quenching of populations of atomic states and nonequilibrium behaviour of continuum radiation.

A torch nozzle design to improve plasma spraying techniques

The effects of nozzle design on the plasma characteristics of supersonic flow conditions are investigated using a D.C. plasma jet under low-pressure plasma spraying conditions. Comparison is made between a 5 mm I.D. standard nozzle and a 13 mm E.D. (exit diameter) Mach 3 Laval nozzle. Emission spectroscopy is used to study the temperature and electron density distributions in the plasma jets produced by the different nozzle configurations. The effects of the observed modifications of the temperature and electron density fields on the properties of the plasma sprayed deposits are studied using Rene 80 powders which are sprayed using both types of nozzle under similar operating conditions. The results show that the M3 Laval nozzle provides a better spraying efficiency and spraying density than a standard anode nozzle under similar conditions.

Measurements of temperature and electron number density in a dc argon-nitrogen plasma torch: Supersonic operation

The diagnostics study on supersonic argon/nitrogen plasma jets expanded into a low-pressure test chamber is carried out by means of emission spectroscopy and enthalpy probe measurement techniques. The spatial distributions of electron density, temperatures, and associated shock structure effects in plasma jets are investigated in conjunction with their direct dependency upon the chamber pressure. The experimental results show the occurrence and the position of different zones; i.e., supersonic expansion, stationary shock waves and subsonic jet at pressures below 51 kPa. Flow fluctuations due to the oblique shock wave at 39 kPa background pressure are observed and discussed. The electron density profiles show variations along the plasma axis that coincide with the position of the shock waves. The experimental results show the transition from the moderately under-expanded to the strongly under-expanded jet structure induced by lowering of the chamber pressure.

Emission spectroscopic study of a low pressure supersonic Ar-H2 DC plasma jet

An emission spectroscopic study is reported of the temperature and the electron density distributions in a DC plasma jet at atmospheric and low pressure conditions. A standard DC plasma spraying torch is used with an Ar-H2 mixture as the plasma gas (17% H2 by volume) at a power level of 30 kW (current=600 A, voltage=50 V). Results were obtained for chamber pressures of 100 kPa, 52 kPa and 20 kPa. Electron density measurements were carried out by the Stark broadening of the Hbeta line and from the continuum recombination of the Ar+ and H+ ions. The temperature is obtained by using a Boltzmann plot of the neutral argon lines and the absolute intensity measurements of the argon lines. The results confirm that local thermodynamic equilibrium (LTE) conditions prevail throughout most of the temperature field at pressures of 100 and 53 kPa. Important deviations from LTE conditions are noted at a chamber pressure of 20 kPa. The results at the lowest pressure condition (20 kPa) show important axial variations of the electron density which coincide with the observed diamond shock wave in the plasma jet.

Thermal plasma reactor for the processing of gaseous hydrocarbons

Abstract An experimental thermal plasma reactor is developed to study the direct conversion of natural gas to liquid hydrocarbons. A preliminary theoretical study of the equilibrium composition and of the reaction kinetics underlined the need to maintain the reaction mixture at temperature in the range of 1300 to 1500 K for at least one second residence time in the reactor. While the relatively long residence time required is incompatible with standard plasma reactor technology, it was achieved by the combined use of a d.c. plasma torch for the rapid heating of the reaction mixture to the required temperature, and of resistive heating elements to extend the high temperature zone throughout the one-meter long tubular reactor.

Chemical non-equilibrium modelling of an argon–oxygen supersonic ICP

In this paper, a non-equilibrium mathematical model for an argon?oxygen inductively coupled plasma (ICP) torch with a supersonic nozzle is developed without making chemical equilibrium assumptions. Reaction rates of dissociation and recombination of diatomic gas and ionization are taken into account. Higher-order approximations of the Chapman?Enskog method are used to obtain better accuracy for transport properties, taking advantage of the most recent sets of collision integrals available in the literature.In order to validate the developed model, results are compared qualitatively and quantitatively with existing experimental data. The calculated results for the axial temperature profile for pure argon less than 10?mm above the substrate are in good agreement with spectroscopic measurements.

Spectroscopic study of a high-power transferred arc: ArII transition probability measurements

A spectroscopic investigation of an atmospheric pressure high-power transferred argon arc operated over a wide range of experimental conditions revealed the existence of interesting physical characteristics. It was shown that local thermodynamic equilibrium (LTE) was established in the column. However, in the immediate vicinity of the cathode tip, the absolute intensity of some atomic lines do not remain constant on a given isotherm. This phenomenon is not observed on ionized argon lines. Owing to the high temperatures occurring and due to column stability downstream of the cathode tip, the discharge was used to determine transition probabilities of ions.