Study of the ion flow in plasma presheaths

The validation of the generalized Bohm criterion for two-ion species plasma and mechanisms of single-ion and two-ion species flow in presheaths were described in this dissertation. A new technique to determine the edge of an electron-free sheath with an emissive probe was presented. The transition region between the sheath and presheath was studied and it was found that the length of the transition region was independent of the ion-neutral collision length and was much longer than one Debye length. A fluid dispersion relation for single and multiple ion species presheaths was derived. A modified mobility-limited flow model for single ion species presheath nu = muE 1-n2 c2s was developed, where nu is the ion drift velocity, mu is the ion mobility, E is the electric field and cs is the ion sound velocity. Comparing to ion acoustic wave measurements, the modified mobility-limited flow model was verified to work throughout the presheath but the mobility-limited flow model does not work near the sheath-presheath boundary. Two-ion species flow in the presheath was studied in an He (3.0mTorr) + Ar (0.2mTorr) plasma by Laser-induced fluorescence (LIF) for Ar ion velocities and ion acoustic wave measurements. It was found that the velocity of each ion species approaches the other near the sheath-presheath boundary and the common velocity is the ion sound velocity of the system. It was also found that both ion species flow in the presheath are mobility limited by comparing calculations from nu = muE with the LIF and wave measurements. Hala's observations of the ion-ion two-stream instability varying with ion concentration and position from the plasma boundary were reproduced in this work. The theoretical analysis from both the fluid and kinetic equations were made and compared to the observations. The variation of the instability with total pressure with a constant ratio of partial pressure was measured for the first time. Frequency spikes of approximately a few hundred kHz were observed in both He + Ar plasmas and pure Ar plasmas and were identified as a two-stream instability involving secondary electrons generated at the chamber wall and the colder bulk species electrons.