Striations in a plasma column

Abstract

An investigation into wave interactions between self-excited ionization waves - striations - in the positive column of a direct current argon discharge is described. Since such self-excited ionization waves are determined by the ionization processes in the discharge, they are inherently nonlinear and a variety of nonlinear effects may be observed.After reviewing the theory and experimental observations for small amplitude linear ionization waves in gas discharges, experimental data obtained from an argon discharge, 5 cm in diameter, 110 cms in length, run at gas pressures between 0.1- 1.5 torr and discharge currents of 20- 250mA, are presented. Linear interactions between large amplitude self-excited waves of the same frequency, and nonlinear mode coupling between waves of different frequencies and wave numbers are reported and the characteristics of each wave type are described. In order to interpret some of the phenomena observed for the self-excited waves, experiments where an external alternating voltage was applied across the discharge were performed. Following a general discussion of wave modulation the wave-wave mode coupling is interpreted as due to nonlinear interactions which may arise during the initial nonlinear growth of ionization waves. A physical model for the nonlinear behaviour is described. By including a nonlinear term in the basic theory of striations expressions are derived which account for the mode-coupling and support the physical interpretation.- 2 -Theoretical techniques previously used to describe the nonlinear behaviour of water waves are outlined, and then applied to ionization waves to predict further nonlinear effects which could not be obtained from the modified linear theory. Finally, a general overall view of two-wave interactions between ionization waves in a discharge is presented using, as an example, the results from a neon discharge. The experimental observations are related to the linear and nonlinear growth of the waves.