Transition of the sheath structure in an electrostatic probe from electropositive to electronegative plasma

A simple model for the sheath structure which takes place in the surroundings of a metallic plane is used to analyze the Bohm criterion in electronegative plasmas. As it is well known, the Bohm criterion which determines the electric potential at the sheath edge can be multivalued in electronegative plasmas. In this article we analyze which value is the correct one. It is shown that when the positive ion current provided by choosing the highest potential at the sheath edge is equal to the one obtained by choosing the lowest, a transition in the sheath dynamics is produced. A new statement of the Bohm criterion which has great advantages is proposed.