Joining active plasma and sheath when the plasma contains two species of positive ions

The complete plasma–sheath is examined in the situation where there is an active plasma generated by electron impact ionization in a mixture of gases, with a view to determining the structure. It is already well known that at low pressures and for a collisionless sheath, a composite Bohm criterion applies. The way in which this relationship breaks down at higher pressures is examined. However, specifically for the situation with two positive ions, we find no evidence of structure in the plasma, but rather that the two ion species behave similarly in the outer regions adjacent to the wall.