Presheaths in weakly collisional plasmas with two ion species

Summary form only given, as follows. In plasmas with two or more ion species, it is usually assumed that ions leave plasma at their individual Bohm velocities. In collisionless plasmas, this result can be achieved if ions fall through a presheath with a plasma potential drop of T/sub e//2e. If ionization is ignored, such a potential drop cannot exist. If ionization is included with the same profile for each species, a solution can exist, but the presheath length will equal the chamber length and the potential drop must exceed T/sub e//2e. With weakly collisional plasmas, mobility limited flow can be assumed for weak electric fields but not for velocities close to the individual species Bohm velocities. Experimental data from emissive probes, Mach probes and LIF in an argon/helium plasma suggest another solution to the problem. It appears that the plasma potential drop exhibits two consecutive presheaths and that neither ion leaves at their individual Bohm velocity.