Experimental studies of the two-ion species flow in the plasma presheath

The ion flow created in the presheath of weakly ionized He+Ar plasma is studied experimentally. Ion acoustic wave measurements combined with previous LIF measurements [Phys. Rev. Lett. 90, 145001 (2003)] suggest that the Ar ion and He ion drift velocities approach each other near the sheath-presheath boundary. Their common velocity is the ion sound velocity of the system. The Ar ion and He ion drift velocities are also calculated assuming mobility-limited flow, including symmetric and asymmetric ion-neutral collisions, combined with the measured plasma potential profile. Comparing these calculations to the wave and LIF data shows that both ion species drift in the presheath with mobility-limited flow until very close to the sheath edge.