Abstract
In this paper, we studied the electropositive to electronegative sheath transition in a weakly ionized plasma and the mathematical condition that describes it. This condition is derived from the Bohm criterion in a two-scale formalism. We show that this transition is characterized by the formation of a triple layer structure in the sheath. Finally, we analyze the sheath thickness as a function of the temperature and density of the negative ions, i.e. for different values of plasma electronegativity. We prove that this sheath thickness has a universal dependence on the ionization rate, due to the universality of the Bohm criterion. When the plasma goes from electropositive to electronegative behavior, it continuously varies even though the sheath edge and its corresponding electric potential displays a discontinuity.
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