The study of presheath instabilities in two ion species plasmas

Summary form only given. The presheath is a region of weak electric field that accelerates ions into the sheath at the plasma boundary. When the plasma consists of two ion species of different masses, ion-ion two-stream instability can be excited by two ion streams with different drift velocities that are created in the presheath. The experiments were performed in multi-dipole DC plasmas with He-Ar gas mixtures (P/sub total//spl ges/1.0 mTorr, n/sub e//spl ges/1/spl times/10/sup 9/ cm/sup -3/, T/sub e//spl les/2 eV). The ion concentration of each species 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 and a spectrum analyzer. Since charge exchange dominated in our presheath, the ion temperature increased approaching the sheath-presheath boundary. A fluid and a kinetic dispersion relation taking the ion temperature into account are applied and compared to describe these instabilities.