Influence of negative-ion processes on steady-state properties and striations in molecular gas discharges

Abstract

Theoretical and experimental studies of negative-ion processes in weakly ionized glow discharges have been conducted. Emphasis in these investigations has been directed towards analysis of gas mixtures in which negative ions are produced by dissociative electron attachment of C${\mathrm{O}}_2$, CO, or ${\mathrm{O}}_2$. It is shown that attachment, detachment, and clustering reactions normally occurring in discharges containing these species can significantly affect both the steady-state and transient characteristics of the plasma, even when an external source of ionization is provided. The magnitude and electron temperature dependence of the electron-molecule attachment coefficient is found to be particularly important. Specifically, analysis shows that when the electron attachment coefficient has a strong positive dependence on electron temperature, and a magnitude exceeding that of the ionization coefficient, an ionization instability can occur. This instability will occur under these circumstances when the negative-ion concentration is comparable to the electron density. Numerical evaluation of the conditions required for this electron-attachment—induced mode of ionization instability results in good agreement with experimentally determined conditions for the onset of striations in gas mixtures for which the charged-particle kinetics can be calculated in detail. In other cases, observed differences between theory and experiment have been related to uncertainties associated with the loss mechanisms of clustered negative ions in the discharge.