Low-Pressure DC Air Plasmas. Investigation of Neutral and Ion Chemistry

Abstract

The neutral and charged species present in a direct current (dc) hollow cathode, gas flow, air reactor are experimentally studied by quadrupole mass spectrometry. The degree of ionization of the plasma and the electron mean temperature with decreasing air pressure, for constant discharge current, are measured with a double Langmuir probe. The chemical composition of the plasma changes appreciably over the 3 x 10(-3) to 5 x 10(-2) mbar range investigated: at the lowest pressures studied, O2 dissociation is up to 60% and the concentration of NO is half that of N2; concerning ions, NO+ and N2+ are dominant for the whole pressure range. A kinetic model of the plasma including electrons, neutrals, and positive ions is developed to account for the experimental observations; it is consistent with energy balance and predicts that heterogeneous processes are the main source of NO and that the contribution of ions to the global chemistry of neutrals is of minor significance even for the lowest pressures.

Atom and Ion Chemistry in Low Pressure Hydrogen DC Plasmas

“…The experimental plasma reactor, shown in Figure 1, is basically the same as in our previous works on air plasmas 34 and H 2 + CH 4 + N 2 plasmas. 35,36 It consists of a grounded cylindrical stainless steel vessel (10 cm diameter, 34 cm length) and a central anode.…”

“…The reactor was electrically fed by a 0.2 A, 2000V, dc source through a 100 Ω ballast resistor. At the lowest operating pressures, an electron gun built in the laboratory 34 was used for ignition of the plasma. Steady-state plasma currents, I p ≈ 150 mA in the abnormal glow discharge mode, with output source voltages ∼400-460 V, were maintained during the experiments.…”

“…34 Thus, the ion lifetimes τ i are given by the expressions where N i and m i are the ion concentration and ion mass, which runs from m 1 ) 1 (H + ) to m 3 ) 3 (H 3 + ), respectively. The processes considered in the numerator of eq 5 are only those that contribute to the creation of a net electrical charge in the plasma.…”

Chemistry in glow discharges of H2/O2mixtures: diagnostics and modelling

“…The DC plasma reactor used for the experiments has been described in previous works [33,34]. It consists of a stainless steel grounded hollow cathode reactor (34 cm length, 10 cm diameter) with a central anode.…”

“…In the steady state, a small fraction of these hot atoms survives and can be detected either in the line profiles of Balmer emission lines of H atoms [81] or indirectly in the energy resolved mass spectra of H + and O + ions [81][82][83]. However, previous works carried out on this reactor for different gas mixtures at comparable pressures [33,34,[36][37][38][39] have shown that the assumption of a Maxwellian distribution is not a bad approximation for the global kinetics, and we have adopted it here for lack of better information on the actual eedf. The limitations of this approximation should be taken into account in the interpretation of the results.…”