Modeling electronegative discharges at low pressure

Abstract

Summary form only given, as follows. In a previous study, a macroscopic analytic model was developed for a plasma discharge with a three component (electronegative) core and an electropositive edge region. Both regions were treated in the high pressure approximation of constant mobility for the positive ions. We extend the treatment to low pressures for which the ion thermal velocity within the electropositive region is much less than the ion flow velocity by using a variable mobility model with constant mean free path for the positive ions. The density at the interface between the electropositive region and the sheath is determined by generalizing the low pressure electropositive solutions to a finite flow boundary condition at the interface with the electronegative plasma. The results are also extended to a wider parameter range in which the electropositive region disappears and a more general Bohm loss condition holds at the plasma-sheath interface. It is also possible to have an additional transition region in which the flow at the edge of the electronegative region is required to be sonic. The resulting algebraic equations are solved numerically over all parameter ranges and compared to analytical approximations. It is shown that the formalism can be extended to two positive ion species (four plasma components), and explicit equations for oxygen are presented.