Spatio-temporal evolution of a pulsed chlorine discharge

Abstract

A one-dimensional fluid model of a pulsed (square-wave power modulated) chlorine discharge was developed in order to study the spatiotemporal evolution of species densities and electron temperatures for various pressures, powers, pulsing frequencies and duty ratios. Simulation results show spontaneous separation of the plasma into an ion-ion core and an electron-ion edge during the power 'on' (active glow) fraction of the cycle. A transition from an electron-dominated plasma to an ion-ion plasma occurs during the power 'off' (afterglow) fraction of the cycle, under the conditions examined. The formation of an ion-ion plasma is favoured at lower power levels, higher pressures, and lower duty ratios. A minimum afterglow time is required for an ion-ion plasma to form and the negative ions to reach the walls. Increasing the afterglow period increases the fraction of time an ion-ion plasma is sustained in the reactor. The evolution of negative ion density profiles is especially complex due to the formation of self-sharpening fronts during power 'on' and subsequent back-propagation of the fronts during the power 'off' stage of the pulse. When possible, simulation results are compared to reported experimental data. In general, good agreement is obtained, except that the measured dependence of electron density on pulse period and duty ratio is more complex than predicted.

Negative ion density fronts

“…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…”

“…When power is turned off in the afterglow, electrons disappear rapidly because of diffusion to the walls and attachment to gas molecules. Late in the afterglow, the electron density and temperature are too low for any significant electrostatic fields to exist, and a transition occurs from an electron-dominated plasma to a positive ion -negative ion (ion-ion) plasma [8,9,10]. After that time, it is possible to extract negative ions from the plasma.…”

Fundamentals and applications of ion–ion plasmas

“…Fig. 2 shows the evolution of species densities and electron temperature at the center of a parallel plate pulsed chlorine discharge (20 mtorr, 1 W/cm 3 power density, no applied electrode bias), once a periodic steady-state is achieved [13]. The time window 0-50 ms represents the time power is ''ON'' (activeglow) while the time window 50-100 ms represents the time power is ''OFF'' (afterglow) during a pulse.…”

“…In a collisional electronegative plasma containing electrons, one kind of positive ions and one kind of negative ions, the electrostatic field in the bulk is given by [13] …”

“…In practice, some electrons may still be present. Ion-ion plasmas may form in the (temporal) afterglow of pulsed discharges in electronegative gases [13,14]. The plasma grows during power ON (activeglow) and decays during power OFF (afterglow).…”