Observation of two-ion-stream instability in sheath-presheath transition region by LIF measurement

Abstract

The each drift velocities of ion species at the sheath edge in a multi-ion-species plasma has been researched over 20 years. In 1991, Riemann developed a criterion for the ion drift velocity at the sheath edge, however, the criterion could not determine the specific value of the drift velocity. Recently Baalrud et al.[1]suggested a model, determining the drift velocities of each ion species at the sheath edge, satisfying the generalized Bohm criterion for two-ion-species plasmas. He considered the two-stream-instability enhanced collisional friction between two ion species inside the presheath region. Later, the model was experimentally demonstrated by Hershkowitz and Yip et al.[2]. They measured the ion drift velocities at the sheath edge using the LIF Doppler velocimetry. Although the ion drift velocity at the sheath edge had been identified, some properties of the presheath region, in which the ion drift velocity are formed, require further research, especially when the two-stream-instability is excited. In this study, we observed two properties of the presheath region, which arethe spatial variation of the space potential and the ion drift velocities. They are both important because they determine the ion flux toward a plasma-facing surface. Experiments were conducted in a weakly-ionized and weakly-collisional Ar/Xe mixture multi-dipole plasma source. The spatial profiles of the ion drift velocity and space potential were measured by using the LIF velocimetry and the emissive probe, respectively. From the result, we observed the region where a relative velocity of the two ion species hold constant even though the space potential in the region continuously decreased. Especially, the relative velocity was identical to the estimation of Baalrud's model. The potential difference in that region increased when the instability is strongly excited. It might be because the strong momentum transfer between two ion species of different masses consumes more energy. The results will be presented at the conference with some analyses.