On the response of the electric field in oxygen DC discharges to negative current pulses - the effect on N2 admixtures

Studies of the transient behavior of macroscopic plasma parameters are important with respect to the problem of stability of the plasma as well as to obtaining information about the constants of dominant elementary processes. The purpose of this work is to investigate the time behavior of the electric field strength in the electronegative plasma of the positive column of oxygen dc discharges when the current decreases suddenly. The experiments were carried out in a glass tube 5.0 cm in diameter with aluminum hollow electrodes separated by 50 cm. The measurements of the axial potential gradient have been performed using two Langmuir probes at a distance of 5.0 cm at the axis of the tube. These probes were connected with a difference amplifier and an oscilloscop. A pentode within the circuit of the discharge allowed to disturb the current by applying of a square-wave voltage on the grid of the pentode. Fig. 1 shows an example of the record of the probe voltage when the current decreases abruptly. If the current jumps from Io to the low value I the gradient decreases at the same time. However, after that it doesn't increase exponentially with the time as in the case of electropositive gases but the discharge is maintained at a small electric field during the time t1. Fig. 2 demonstrates the dependence of t1 on the ratio I/Io for pure oxygen at different pressure conditions as well as for oxygen with 5% N2 admixture