Negative Ion Density Fronts during Ignition and Extinction of Plasmas in Electronegative Gases

Abstract

Negative ion density fronts have been shown to occur in electronegative steady-state plasmas with hot electrons. In this Letter, we report theoretical and numerical results on the spatiotemporal evolution of negative ion density fronts during plasma ignition and extinction (afterglow). During plasma ignition, the negative ion fronts are analogous to hydrodynamic shocks. This is not the case during plasma extinction where, although negative ions diffuse freely in the plasma core, the negative ion front propagates towards the chamber walls with a nearly constant velocity.

Negative ion density fronts

“…In this paper, we have demonstrated that weakly damped perturbations propagate in collisional multi-species weakly ionized plasmas with a speed , where u is the 21 negative ion drift velocity. Due to the nonlinear dependence of the signal speed on the ion density, discontinuities in ion density (ion fronts) may appear due to steepening of ion density profiles, analogous to gasdynamic shocks.…”

“…We have studied many different cases for few EN gases [8,21,32,36]. The negative ion fronts are clearly seen during the active (power "on") glow if: 1) the plasma electronegativity is not…”

“…In each of these regions different physical processes dominate. The proposed methodology is based on a fundamental phenomenon, which appears in multi-component plasma that the plasma stratifies into regions with different ion composition [17][18][19][20][21]. For example electronegative plasmas with hot electrons often separate into core region, containing the negative ions (electronegative region), and an electron-ion edge region, near the walls, which contains essentially no negative ions (electropositive region).…”

“…The idea was applied to gas discharges [15,16,28] and semiconductor plasmas [17,29]. In papers [20,21] it was shown, that similar narrow shock-like structures-fronts, which separate the plasma regions with smooth 7 profiles and different ion composition, naturally arise also in currentless collisional non-isothermal plasma.…”

“…5 and 6, respectively. The spatio-temporal evolution of such fronts has been studied theoretically and by a numerical "experiment" in an argon/oxygen pulsed discharge [21,32]. In pulsed discharges, power to the discharge is modulated (e.g., square wave modulation) with a specific frequency and duty ratio.…”