Kinetic features of redistribution of gas phase components during spark discharge

Abstract

Plasma discharge in a gas phase favors multi com bination excitation and ionization of atoms, mole cules, and ions in the medium [1]. New stable mole cules and molecular ions formed in these processes [2] significantly change the internal parameters and gen eral character of development of the ionization pro cess. This fact is clearly illustrated by the instability of volume discharge in sealed off CO 2 lasers, where the gradual accumulation of plasma reaction products leads to transformation of the volume discharge into spark discharge [3].Electric discharge in a closed volume is widely used in practice, in particular, in detectors and control measurement instruments [4,5]. Despite the high practical significance, many aspects of the aforemen tioned phenomena still remain unclear. These include specific features in the kinetics of spark discharge in a multicomponent gas phase. The multistep character of plasmachemical reactions involved in discharge and the mechanisms of transformation of the initial com ponents into final products still have to be elucidated. The influence of exchange processes in the interelec trode space on the rate of plasma transformations is also insufficiently studied. The answers to these and many other questions can be obtained based on an analysis of the variation of pressure P in the gas phase, voltage U c applied to the storage capacitor, and spark discharge frequency f, which reflect all transforma tions in the system and the can be monitored during the entire operation of plasma discharge. To the best of the authors' knowledge, no detailed information on changes in these parameters during spark discharge has been reported until now. In order to fill this gap, the present work was aimed at gaining experimental data on the kinetic features of the redistribution of gas phase components during spark discharge, which pro vides answers to many questions.The experimental setup and methods used in this study have been described in detail previously [6]. We have only modified the method of renewal of the gas phase inside the discharge cell in the regime of elec tron conservation, which was achieved using a three way valve [cell-vacuum pump (~0.1 Pa)-ambient atmosphere]. This allowed us to strictly fix the primary discharge gap width H in a series of experiments with various U c values.A series of 5 h experiments have been carried out, in which only the applied voltage was varied within U c = 7.53-10.5 kV at an unchanged interelectrode gap width H = 1.51 mm. In order to fix the discharge char acteristics of a capacitor bank (referred to below as "capacitor"), which are determined by the relaxation time τ = RC, the RC chain in all experiments was the same with R = 9.7 MΩ and C = 0.075 μF. The pressure variations ΔP(t) = P 0 -P(t) were measured relative to the initial (atmospheric) pressure P 0 in the cell. Figure 1 presents the main results of our investiga tion. As can be seen from these data, the slowest con version of components takes place at U c = 7.53-7.87 kV (Fig. 1, curves 1 and 2). T...