D Pagnon

Kinetic model of a low-pressure N/sub 2/-O/sub 2/ flowing glow discharge

A self-consistent kinetic model is developed to study dc flowing glow discharges in N/sub 2//O/sub 2/ mixtures. This model includes the calculation of electron energy distribution functions and electron rate coefficients coupled with detailed vibrational kinetics of N/sub 2/ molecules, chemical kinetics taking into account a large set of neutral, excited and charged species, interaction of N and O atoms at the discharge tube wall, and the thermal balance of the discharge. The results of this model agree reasonably well with the measurements of the electronic density, the gas temperature, the reduced electric field, the vibrational temperature of N/sub 2/ and the concentration of O, N atoms, NO molecules, N/sub 2/(C), N/sub 2//sup +/(B), and NO(/spl gamma/) excited states. The comparison was performed in a N/sub 2/-O/sub 2/ discharge at pressure p=2 Torr, for discharge currents I=15, 30, and 80 mA, a flow rate Q=100 sccm, and O/sub 2/ percentages ranging from 0 up to 100%,. >

On the use of actinometry to measure the dissociation in O2 DC glow discharges: determination of the wall recombination probability

A study has been performed to re-investigate the actinometric technique used to determine the absolute concentration of O atoms in DC O2 flowing glow discharges for pressures ranging from 0.36 to 2 Torr and discharge currents ranging from 5 to 80 mA in Pyrex tubes of three different diameters (16, 7 and 4 mm). Actinometric measurements using O(3P-3S) 844 nm, O(5P-5S) 777 nm and Ar(2p1-1S2) 750 nm transitions are compared to VUV absorption spectrometry. The choice of the excitation cross sections for the calculations of atomic excitation rates as a function of the reduced electric field using a Boltzmann code and the contribution of the quenching processes of the excited states are discussed. The dissociation ratio (O)/(O2) can be determined from the ratio of intensities I844/I750 by the relation (O)/(O2)=C3P2p1 I844/I750. We have found that C3P2p1 remains constant (C3P2p1=2.6*10-3) throughout the range of experimental conditions investigated. The recombination probability gamma of the O atoms at the wall is calculated and correlated to the wall temperature of the Pyrex tubes. The variation of the recombination probability as a function of the wall temperature is fitted by the relation gamma =0.94exp(-1780/Twall) for 300

Experimental and theoretical investigation of a N2-O2 DC flowing glow discharge

A study has been performed to investigate the kinetics of production of N atoms and NO molecules in DC N2-O2 flowing glow discharges at pressure rho =2.0 Torr for discharge currents I=15, 30, 80 mA. Measurements of the plasma parameters, reduced electric field E/N, electronic density ne, as well as of the gas temperature Tg, the vibrational temperature Tnu of N2 molecules and the concentration of N atoms and NO molecules have been carried out over the whole range of oxygen percentages (0-100%). This set of measurements is compared to the results of a one-dimensional model including a detailed vibrational kinetics, a large number of chemical reactions and the thermal balance of the discharge.

Transient phenomena in closed electron drift plasma thrusters : insights obtained in a french cooperative program

This paper presents some aspects of the research developed in the frame of a coordinated program launched in France in 1996 and devoted to plasma thrusters for space technologies. Relevant results of physical studies have been selected from the literature with the addition of recent original results. The thrusters within the scope of this research are diagnostic equipped versions of industrial realizations, in a thrust level range of 0.1 N and electrical power 1.5 kW. The optical and electrical diagnostics concern studies of the thruster plasma and of the thruster plume. Transient phenomena in these two regions, related to discharge current fluctuations or oscillations on a typical time scale of 40 µs, have been space-time characterized. This has been achieved by developing a large panel of diagnostics including RFEA, Langmuir probes, OES, fast camera imaging and electron drift Hall current probe. They lead to a coherent representation of these phenomena , in rather good qualitative agreement with 1D modelling. But they emphasize also the importance of 2D effects. Insights obtained through combined LIF (on Xe+ ions) and OES diagnostics are also presented. They concern the ionization-acceleration region in the thruster plasma, where intrusive diagnostics are disturbing in nature, and open a new step for a significant improvement of the detailed understanding of these thrusters. Such improvements are required when looking at the final goal of a predicable modelling simulation able to help the design of optimized structures at various thrust levels, in spite of the important work devoted to these devices in the former USSR and by Russian teams in Moscow at the MIREA, MAI-RIAME and KOURCHATOV Institutes.

Experimental study of a pre-ionized high power pulsed magnetron discharge

This paper is focused on experimental studies of a high power pulsed magnetron discharge stabilized by low current pre-ionization. Time resolved studies were performed for a Cu target by optical emission spectroscopy and electrical measurements for different pressures of Ar buffer gas. Due to the elimination of the statistical delay time and a fast discharge current rise the quasi-stationary state was reached in 6 µs. The quasi-stationary state is characterized by an extremely high and pressure independent discharge current density of ~10 A cm−2 and stable Cu+ and Cu++ emissions. Such fast discharge dynamics permits the magnetron cathode current to be driven with a pulse of duration of the order of a few µs, significantly shorter than in other devices. During this short time, the plasma does not have time to undergo the transition from the glow to the arc discharge even at the extremely high cathode loads met in our case. Different stages of the fast discharge development are identified and the composition of the magnetized plasma as a function of the pressure is discussed in detail.

Fundamental aspects in non-reactive and reactive magnetron discharges

In this paper, which deals with physical processes and problems that are involved in magnetron discharges used for the deposition of thin films and material coatings, we emphasize the aspects connected to discharge physics: energy deposition, behaviour of the discharge in reactive gases in connection with plasma–surface interactions. We also present recent works on ionized physical vapour deposition (IPVD) in which the usual PVD magnetron sputtering is assisted by an additional discharge in order to ionize the sputtered neutral vapour and to achieve a better control and quality of the deposited material in industrial applications. We restrict ourselves to planar magnetrons.

Spectroscopy and Kinetics of an Oxygen Glow Discharge

For several years, low pressure oxygen discharges have been extensively used for various applications such as surface treatment, oxidation, polymer etching, etc. The investigation of such plasma is difficult as it is a complex medium composed of electrons, positive and negative ions, excited molecules and atoms. This work deals with the kinetics of these species in the positive column of a low pressure glow discharge. This type of discharge is particularly well suited for kinetic studies as it is a homogeneous medium where the electric field, the gas temperature and the concentration of all the species are uniform along the plasma column. Different diagnostic techniques have been used to determine the concentrations of the active species. They have been reviewed by Wayne 1. Different models have also been proposed including the calculation of the electron energy distribution function, and the resolution of the kinetic equations governing the concentrations of the active species 3-8. However it is difficult to comparethe experimental results and the model predictions because the experimental conditions are different from those used in the model. The interaction between the walls and the oxygen atoms 2,9,10 and gas purity are of particular importance. For this reason, we found it necessary to measure the plasma parameters: electric field E, electron density ne and the concentrations of the main excited species: