Investigation of ion collision effect on electrostatic plasma-sheath transition in weakly ionized and weakly collisional discharge

Effects of ion collisions on electrostatic sheath formation are investigated experimentally under low-density and low-pressure Ar plasma conditions. Measurement results of space potential and ion velocity distribution using emissive probe and laser induced fluorescence with high spatial resolutions, respectively, show that the ion collision properties dominant in the presheath are maintained in the plasma-sheath transition region. The space potential measurement results show good agreement with Riemann's[1] kinetic solution of presheath-sheath transition, indicating that the thermal properties of ions and neutral gases contribute to space potential structure by the charge exchange and ionization properties near the sheath edge as well as the elastic collisions of ions-neutrals. It is found that the transition region is an area where the kinetic Bohm criterion is satisfied by the varying ion velocity distribution and the electron energy distriubtion in space[2]. The methods of determination of the plasma, sheath and transition region in this study can be extended to determining the boundary conditions for the behavior of ions, determining the spatial distribution of sheath edge, and studying the effect of negative ions on sheath formation in electronegative discharge.