MODELING ELECTRONEGATIVE DISCHARGES AT LOW-PRESSURE

In a previous study, a macroscopic analytical model was developed for a plasma discharge with a three-component (electronegative) core and a n electropositive edge region. Both regions were treated in the high-p ressure approximation of constant mobility for the positive ions. We e xtend the treatment to low pressures, for which the ion thermal veloci ty within the electropositive region is much less than the ion flow ve locity, by using a variable mobility model with constant mean free pat h for the positive ions. The density at the interface between the elec tropositive region and the sheath is determined by generalizing a low- pressure electropositive solution to a finite-flow boundary condition at the interface with the electronegative plasma. The results are also extended to include an additional transition region in which the flow within the electronegative region is not allowed to exceed the local ion sound speed, which can result in an abrupt decrease in negative io n density. The approximation of a parabolic negative ion profile resul ts in algebraic equations which are solved numerically over a range of parameters. Typical cases are compared with particle-in-cell (PIC) Mo nte Carlo simulations.