J Amorim

The role of rotational mechanisms in electron swarm parameters at low reduced electric field in N2, O2 and H2

The homogeneous Boltzmann equation for electrons in N2, O2 and H2 is solved under the classical two-term approximation, for reduced electric fields in the interval 10 − 4 −10 Td where the electron-neutral encounters are limited to elastic, rotational and vibrational collisions. Rotational excitations/de-excitations are described using the following three different approaches: the discrete inelastic/superelastic collisional operator, written for a number of rotational levels that depends on the molecular gas and the specific rotational cross sections considered; the continuous approximation for rotations; a modified version of the continuous approximation for rotations, including a Chapman–Cowling corrective term proportional to the gas temperature. The expression of the rotational collision operator for this latter approach is deduced here and the results show that it bridges the gap between the discrete and the continuous descriptions at low/intermediate reduced electric fields. The calculations are compared with the measurements for the available swarm parameters to assess the validity of the different approaches and cross sections adopted to describe the rotational mechanisms.

Experimental study of unconfined surface wave discharges at atmospheric pressure by optical emission spectroscopy

A surface wave discharge (SWD) in argon at atmospheric pressure generated by a surfatron device was studied by optical emission spectroscopy (OES). Two distinct situations were investigated; (i) a discharge plasma in open air and (ii) a discharge plasma totally confined in a quartz tube. The electron density ne, electron temperature Te and gas temperature Tg were investigated as a function of applied power and gas flow rate. The self-absorbing method was used to estimate the population of the metastable state Ar(1s5). These physical quantities were determined through optical measurements along the plasma axis of symmetry. The profile of the electron density presented a maximum value under certain conditions, in contrast with typical electron density profiles of SWDs which are usually monotonically decreasing. A correlation between the electron density and the metastable state Ar(1s5) was found in one of these cases, suggesting that stepwise ionization from metastable states and non-local kinetics play an important role on the unexpected increase in ionization degree along the discharge.

Optical and electrical diagnostics of a spark-plug discharge in air

Optical and electrical diagnostics were used to investigate a spark discharge using a commercial spark-plug operating in the glow phase regime. Voltage and current were measured in order to characterize the discharge. The gas temperature was estimated as a function of time and duty cycle using ro-vibrational spectra of the second positive system of nitrogen by comparison between experimental and simulated spectra. It was found that 1600 K ≤ Tg ≤ 2800 K. The reduced electric field varied between 1 and 1000 Td after the end of the current pulse. Using the line intensity method between one line from the ionic argon and the other from the neutral atom, the electronic temperature was measured and found to be between 17 000 and 20 000 K. The electron density was determined from the broadening of Hα line and was found to be 4.0 × 1014 cm−3 ≤ ne ≤ 1.7 × 1015 cm−3.

Electric and spectroscopic properties of argon-hydrogen RF microplasma jets at atmospheric pressure

Microplasma jets of argon–hydrogen (Ar–H2) gas mixture were generated by 144.0 MHz radio-frequency (RF) waves at powers of 5 W, 10 W, 20 W and 50 W. The experimental setup employed creates stable microplasmas at atmospheric pressure from 5.0 mm up to 20.0 mm visual glow lengths. We have determined the rms voltages, the rms electric currents and the power absorptions of these microplasma jets. By making use of optical spectroscopy, the emission spectra of Ar–H2 microplasma jets were recorded in the range 3060–8200 A, in order to estimate the axial distribution profiles of electron density, rotational temperature, excitation temperature and hydrogen atomic temperature.

Experimental and theoretical study of atmospheric-pressure argon microplasma jets

Surface-wave discharges in argon at atmospheric pressure were experimentally studied by optical emission spectroscopy (OES) and mass spectrometry (MS). OES was employed to determine the rotational temperature using the ultraviolet OH band, Q1 branch and found to be between 450 and 970u2009K. The electron density (5u2009u2009u2009×u2009u2009u20091013u2009cm−3u2009u2009≤u2009u2009neu2009u2009≤u2009u20097u2009u2009u2009×u2009u2009u20091014u2009cm−3) was estimated using the Hβ line profile, and produced by dissociation of the water present as an impurity in the Ar gas. The electron temperature (0.63u2009eVu2009u2009≤u2009u2009Teu2009u2009≤u2009u20091.3u2009eV) was estimated using a collisional–radiative (CR) model that takes the input measured intensities of four emission lines originating from 2p states including 2p2, 2p4, 2p6, and 2p10. The density of the metastable state Ar(1s5) (2.0u2009u2009u2009×u2009u2009u20091011u2009cm−3u2009u2009≤u2009u2009Ar(1s5)u2009u2009≤u2009u20094.2u2009u2009u2009×u2009u2009u20091012u2009cm−3) was estimated by means of OES using the self-absorbing method. Positive and negative ions were probed along the plasma column using MS. A theoretical model based on the solution of the homogeneous electron Boltzmann equation, considering inelastic and superelastic collisions with the Ar(1s) states and electron–electron collisions, coupled with a system of rate balance equations describing the creation and destruction of the most important heavy particles, is proposed. The experimental results are compared with theoretical ones obtained from a self-consistent model of these discharges, providing physical insight into the basic mechanisms and phenomena ruling the discharges.

Simulating nitrogen molecular plasma spectra using artificial neural networks

We introduce a method based on artificial neural networks (ANNs) to simulate molecular spectra under experimental conditions. The spectra of a nitrogen discharge were measured under variable pressure ranging from 0.3 to 2.0 Torr and discharge current varying from 5.0 to 50.0 mA and they have been used as a training set. Our procedure uses a system of two ANNs. The first one extracts the physical characteristics of the training set. The second one uses the information collected from the first to simulate the spectra under conditions that have not been presented in the training sample. The parameters used to assure the quality of the simulated spectra were the temperatures. Our simulated spectra have agreed with those measured experimentally. These results have indicated the robustness of the ANN in simulating molecular spectra. These facts stimulated us to investigate the ability of our system in generating spectra under different conditions of experimental resolution. The results show that our method could be used to improve the accuracy of experimental temperature whenever the experimental spectrum has a poor resolution.

Causes of plasma column contraction in surface-wave-driven discharges in argon at atmospheric pressure

In this work we compute the main features of a surface-wave-driven plasma in argon at atmospheric pressure in view of a better understanding of the contraction phenomenon. We include the detailed chemical kinetics dynamics of Ar and solve the mass conservation equations of the relevant neutral excited and charged species. The gas temperature radial profile is calculated by means of the thermal diffusion equation. The electric field radial profile is calculated directly from the numerical solution of the Maxwell equations assuming the surface wave to be propagating in the TM_{00} mode. The problem is considered to be radially symmetrical, the axial variations are neglected, and the equations are solved in a self-consistent fashion. We probe the model results considering three scenarios: (i) the electron energy distribution function (EEDF) is calculated by means of the Boltzmann equation; (ii) the EEDF is considered to be Maxwellian; (iii) the dissociative recombination is excluded from the chemical kinetics dynamics, but the nonequilibrium EEDF is preserved. From this analysis, the dissociative recombination is shown to be the leading mechanism in the constriction of surface-wave plasmas. The results are compared with mass spectrometry measurements of the radial density profile of the ions Ar^{+} and Ar_{2}^{+}. An explanation is proposed for the trends seen by Thomson scattering diagnostics that shows a substantial increase of electron temperature towards the plasma borders where the electron density is small.