F. M. Dias

On the self-consistent modeling of a traveling wave sustained nitrogen discharge

We present a self-consistent formulation to study low-pressure traveling wave (azimuthally symmetric surface transverse magnetic mode) driven discharges in nitrogen. The theoretical model is based on a self-consistent treatment of the electron and heavy particle kinetics, wave electrodynamics, gas thermal balance, and plasma–wall interactions. The solution provides the axial variation (as a result of nonlinear wave power dissipation along the wave path) of all discharge quantities and properties of interest, such as the electron energy distribution function and its moments, population densities of all relevant excited and charged species [N2(X 1Σg+,ν),N2(A 3Σu+,a′ 1Σu−,B 3Πg,C 3Πu,a 1Πg,w 1Δu), N2+, N4+, e], gas temperature, degree of dissociation [N(4S)]/N, mean absorbed power per electron, and wave attenuation. A detailed analysis of the energy exchange channels among the degrees of freedom of the heavy particles is presented. Particular attention is paid to the axial variation of the gas and wall tempe...

On the axial structure of a nitrogen surface wave sustained discharge: Theory and experiment

A model for a surface wave sustained nitrogen discharge accounting in a self-consistent way for electron and heavy particles kinetics and discharge electrodynamics has been developed. The system under analysis is a plasma column produced by a traveling, azimuthally symmetric (m=0 mode) surface wave. The model is based on a set of coupled equations consisting of the electron Boltzmann equation and the rate balance equations for the most important excited species—vibrationally, N2(X 1Σg+, ν), and electronically excited states, N2(A 3Σu+, a′ Σu−, B 3Πg, C 3Πu, a 1Πg)—and charged particles (e, N2+, N4−) in the discharge. Electron collisions with nitrogen molecules of the first and the second kind and electron–electron collisions are accounted for in the Boltzmann equation. The field strength necessary for steady-state operation of the discharge is obtained from the balance between the total rates of ionization (including direct, stepwise, and associative ionization) and of electronic losses (due to diffusion ...

Wave driven N2–Ar discharge. II. Experiment and comparison with theory

Discharges in N2–Ar mixtures are experimentally investigated by means of optical emission and absorption spectroscopy, probe diagnostic techniques, and radiophysic methods. The experimental results provide insight into the mechanisms of wave-to-plasma power transfer, N2 dissociation, creation of N2+ ions, and excitation of metastable states [N2(A 3Σu+),Ar(3P2)]. These results are analyzed in the framework of the theoretical predictions of a model developed in a companion article.

Spatially resolved experimental investigation of a surface wave sustained discharge in nitrogen

A spatially resolved experimental investigation of the electron energy distribution function (EEDF) and wave propagation characteristics in a high frequency surface wave (SW) discharge in nitrogen has been performed. The measurements reveal the specific changes of the EEDF and its integrals which occur as a result of a different coupling between the electrons and the inhomogeneous SW electric field as a function of gas pressure. At a pressure of 0.5 Torr it was found that the EEDF and corresponding integrals depend on the spatial position, which means that local plasma response occurs. As a result, a strong radial inhomogeneity of the discharge is observed which relates to the radial variation of the wave field intensity. On the contrary, at 0.05 Torr nonlocal plasma response is observed. In a molecular plasma, electron energy relaxation occurs faster than in inert gases, this being why the transition from the local to the nonlocal regime takes place here at a lower gas pressure. A turning back of the axi...

Microwave air plasma source at atmospheric pressure: Experiment and theory

An experimental and theoretical investigation of the axial structure of a surface wave (2.45 GHz) driven atmospheric plasma source in air with a small admixture (1%) of water vapor has been performed. Measurements of the gas temperature and of the intensities of the O(777.4 nm), O(844.6 nm), and O(630 nm) atomic lines and the NO(γ) molecular band versus input power and axial position were carried out. Amplitude and phase sensitive measurements have also been performed to derive the surface wave dispersion characteristics. The experimental results are analyzed in terms of a one-dimensional theoretical model based on a self-consistent treatment of particle kinetics, gas dynamics, and wave electrodynamics. The predicted gas temperature and emission line intensities variations with power and axial position are shown to compare well with experiment. “Hot” excited O atoms (with kinetic energy ∼2 eV) have been detected.

Molecular dissociation in N2–H2 microwave discharges

A microwave N2?H2 discharge driven by a travelling surface wave is investigated as a source of ground state N(4S) and H(1s) atoms. Experimental investigations have been carried out in a plasma source operating at 2.45?GHz at low-pressure conditions (p = 0.5?2?Torr). By means of optical emission spectroscopy and probe diagnostic techniques, the population densities of ground state atoms have been detected. The dissociation kinetics is discussed in the framework of a theoretical model based on a self-consistent treatment of the main discharge balances, wave electrodynamics and plasma?wall interactions. Electron?ion surface recombination processes involving and ions are the most important sources of N(4S) gas phase atoms for the conditions considered. The relative number of N(4S) atoms in respect to the total neutral density remains approximately constant for percentages of H2 between 10% and 50% at nearly constant electron density. The competitive interplay of two important source channels of H(1s) atoms, namely electron impact dissociation of H2 and H2 dissociation via the quenching of nitrogen and metastables, determines a smooth decrease of hydrogen dissociation when the amount of hydrogen increases up to 50% in the mixture.

A travelling wave sustained hydrogen discharge: modelling and experiment

A model has been developed for a surface wave sustained hydrogen discharge which, in a self-consistent way, accounts for the main plasma balances governing the discharge production, including bulk and surface processes. The approach used self-consistently describes the axial discharge structure, i.e. the axial distribution of charged particle concentrations, population densities of excited species and neutrals, taking into account inhomogeneous gas heating along the plasma column as well as plasma-wall interactions. A spatially resolved experimental investigation into the distribution of electron density, atomic line intensity and gas temperature confirms the main trends of the model predictions.

Effect of gas heating on the spatial structure of a traveling wave sustained Ar discharge

In this work we report a theoretical and experimental study of the influence of gas heating on the spatial structure of a microwave Ar discharge sustained by a traveling surface wave. The theoretical analysis is based on a discharge model which couples in a self-consistent way electron and heavy particle kinetics, discharge electrodynamics, and gas thermal balance. The set of coupled equations used includes the electron Boltzmann equation, the rate balance equations for the most important excited species and charged particles, the gas thermal balance equation, and the equations describing wave propagation and power dissipation. The principal collisional and radiative processes which determine the populations in the Ar(3p54s) and Ar(3p54p) levels are accounted for. The field strength necessary for steady-state discharge operation is obtained from the balance between total rates of ionization (including direct and step-wise ionization and energy pooling reactions) and of electron loss due to the diffusion t...

Microwave N2–Ar plasma torch. II. Experiment and comparison with theory

Spatially resolved emission spectroscopy techniques have been used to determine the gas temperature, the electron, and N2+ ion densities and the relative emission intensities of radiative species in a microwave (2.45 GHz) plasma torch driven by a surface wave. The experimental results have been analyzed in terms of a two-dimensional theoretical model based on a self-consistent treatment of particles kinetics, gas dynamics, and wave electrodynamics. The measured spatial variations in the various quantities agree well with the model predictions. The radially averaged gas temperature is around 3000 K and varies only slowly along the discharge zone of the source but it drops sharply down to about 400 K in the postdischarge. The experimental wave dispersion characteristics nearly follow the theoretical ones, thus confirming that this plasma source is driven by a surface wave.

Spatial structure of a slot-antenna excited microwave N2–Ar plasma source

The spatial structure of a large-scale, slot-antenna excited (2.45GHz) surface wave plasma source operating in N2–Ar mixtures is investigated. A self-consistent theoretical model is developed in the local approximation to investigate the entire spatial structure of the system, including the discharge zone sustained by the field of the TM140 surface mode and the remote plasma zone. Maxwell’s equations and the rate balance equations for the most important excited species—vibrationally and electronically excited states, ions, and N(S4) atoms—and the electron Boltzmann are consistently solved. The pumping of the higher νth levels of N2(XΣg+1,ν) molecules is shown to be very effective and to strongly influence the remote plasma kinetics. Collisions of N2(XΣg+1,ν) molecules with N(S4) atoms are responsible for the increase in the number densities of electrons and electronically excited states N2(AΣu+3,BΠg3,CΠu3,a′Σu−1) in the “far” remote plasma zone.