Study of RF Helical Cavity Discharge in Nitrogen, Argon and in Mixture of Nitrogen–Argon by Optical Emission Spectroscopy

This paper presents the spatial characterization of Ar-N2 plasma induced by a helical cavity excited at 27 MHz and moderated pressure (400 Pa). The axial profiles of densities and gas temperature are determined through the emissions of the first positive (1) and second positive (2) systems of N2. Additionally, in case of pure argon plasma, the OH(AΣ→XΠ) emission system coming from impurities is investigated. Rotational and vibrational temperatures profiles of the OH(AΣ) species are deduced by comparison of simulated and measured spectra. Investigation of 2 and OH(AΣ→XΠ) emission systems shows the effect of excitation transfer between the impurities and Ar metastable atoms. The gas temperature values determined from 1 emissions led to flat profile, which contrasts with hollow density profiles of the corresponding emitters, whatever the gas mixture considered here. Introduction Many reactions occur in electrical gas discharge. Dynamics of such processes are investigated practically for their understanding and primarily exploitation. These processes are used in plasmachemistry reactors on volume synthesis of gases or on creation of excited, ionized and dissociated particles. Flow of these particles is subsequently utilized in their interaction with surface of various materials. These reactions are implemented in chemical lasers, too. Kinetic temperature of working gas, noted Tg, is one of parameters, which describes environment of the plasma and determines degrees of excitation of molecules. This parameter is directly affected by processes of diffusion (index of diffusion is generally proportional to Tg), velocity of the chemical reactions, efficiency of devices (e.g. ozonizer, plasma etching). Molecules of nitrogen are normally used with the aim to measure Tg in plasma by optical emission spectroscopy. Indeed, N2 molecules exchange rotational and translational energy faster with gas atom than electrons and, as a consequence, rotational distribution quickly achieves thermodynamic equilibrium with gas [Jolly, 1995]. In mixtures without nitrogen, it is possible with admixture of a little amount of nitrogen (e.g. 1 %) to measure temperature [Sahli et al., 1993]. The present work follows publication [Kilianova et al., 2007], where we presented the spatial characterization of gas temperature of nitrogen-argon plasma created by a helical coupling device excited at 13.56 MHz and low pressure (15 Pa). Here, we have continued our investigations on this device at higher pressure (400 Pa) and with a plasma excitation frequency about 27 MHz. The profiles along the cavity of relative densities of most emitting species are also presented. The gas temperature is determined through the rotational temperatures of first positive (1) and second positive (2) systems, denoted Tr(1) and Tr(2), respectively. Additionally, in the case of a pure argon discharge, we have monitored along the cavity the OH(AΣ→XΠ) emission system coming from impurities we have deduced rotational and vibrational temperatures of the OH(AΣ) species. Scheme of experimental set-up is given in Figure 1. The flowing plasma is maintained in a fused silica tube (20 mm inner diameter, length 80 cm). The discharge is generated by a helical cavity excited at 27 MHz. The gas is pumped by an oil rotary pump. We studied nitrogen-argon plasma with 0, 25, 50, 75 and 100 % of argon in the mixture. In all cases, the flow rate is fixed at 3.4 sccm for total gas pressure, measured by a capacitive gauge (MKS Baratron), about 400 Pa. The resulting gas velocity in absence of plasma is about 0.5 m.s. 62 WDS'08 Proceedings of Contributed Papers, Part II, 62–67, 2008. ISBN 978-80-7378-066-1 © MATFYZPRESS