Spectroscopic diagnostics and modelling of a N2–Ar mixture discharge created by an RF helical coupling device: I. Kinetics of N2(B 3Πg) and N2(C 3Πu) states

Abstract

Optical emission spectroscopy, ranging from visible to near infrared, is used to determine densities and rotational temperatures of N2(B 3Πg) and N2(C 3Πu) states in a nitrogen–argon (0–95% Ar) discharge, under moderate pressures (200–400 Pa). The plasma is sustained by a helical cavity with an excitation frequency of 27 MHz and power fixed to 28 W. Firstly, in the case of a pure N2 discharge, the two states turn out to have a similar rotational temperature, which approximates the gas temperature reasonably well. With a gradual increase in the Ar concentration up to 95%, the rotational temperature of N2(C 3Πu) roughly doubles while that of N2(B 3Πg) stays unchanged at 430 ± 50 K regardless of the gas composition. Secondly, as observed, the densities of the N2(B 3Πg) and N2(C 3Πu) states increase with increasing Ar percentage in the gas mixture. The increase in the emission intensity values is less marked for positions corresponding to both ends of the cavity. In fact, the difference in the emission level between the power input and helix middle positions is reduced, revealing that the total discharge is more uniform along the cavity for large argon concentrations. The experimental results show a strong dependence of temperatures and densities on the Ar amount in the gas mixture. A kinetic model is developed to explain this phenomenon, which is then used in modelling density evolutions versus relative abundance of Ar and versus the position along the cavity axis. The model indicates the importance of the role of electron and metastable species in the above-described discharge.