Diffusion in Electronegative Discharges with Magnetic Fields.

Electronegative plasmas are important in a variety of electric discharge applications, such as plasma reactors, negative ion sources, and even electropositive discharges when contaminated with an electronegative impurity. The need for an understanding of the processes and phenomena associated with these electronegative discharges has spurred the development of numerical simulations and models. While many of the devices incorporate various configurations of external magnetic fields, specific attention to the influence of the magnetic field on the discharge operating point, structure, and stability is lacking. To address this deficiency, a collisional model for diffusion in three -component plasmas with an applied magnetic field is developed; it is an extension of Schottky theory allowing for negative ions and a magnetic field. This study analyzes the effect of magnetic fields on diffusion in three-component plasmas; provides an analytic solution for the collisional model in a magnetic field; evaluates the validity of effective diffusion coefficients through an analysis of the afterglow; and provides a regime of validity for the model in terms of magnetic field strength by analytically establishing the critical magnetic field for the onset of "anomalous" diffusion. When a discharge operating at constant current is subjected to an increase in axial magnetic field, the ionization frequency decreases more rapidly in an electronegative discharge than an electropositive discharge, due to negative ions reducing the ambipolar electric field. The model is compared qualitatively to a hydrogen discharge and quantitatively to a sulfur hexafluoride discharge with good agreement. The time-dependent model shows that, for a hydrogen discharge, one can obtain an enhancement of the negative ion density in the afterglow. Electronegative discharges are more unstable with respect to the helical mode instability than electropositive discharges, due to the larger axial electric field and weaker ambipolar electric field in an electronegative discharge.