The study of presheath instabilities in two ion species plasmas

Summary form only given, as follows. The presheath is a region of weak electric field that accelerates ions into the sheath at the plasma boundary. In two ion species plasmas, instabilities can be excited by the two ion streams with different drifting velocities that are created in presheath. The experiments were performed in multi-dipole DC plasmas with He-Ar gas mixtures. The concentration of ion species in the two ion species plasmas was determined by measuring ion acoustic wave phase velocity and electron temperature in the bulk region. Instabilities changing with different partial pressure, positions and discharge current were observed by using a cylindrical probe biased to collect either ion saturation current or electron saturation current and a spectrum analyzer. Correlations of instabilities between two positions without time delay were studied by placing two identical cylindrical probes both parallel to the boundary plate.