Experimental studies of ion flow near the sheath edge in multiple ion species plasma including argon, xenon and neon

The Bohm sheath criterion was studied with laser-induced fluorescence (LIF) in three ion species plasmas using two tunable diode lasers. It was found in the first LIF studies of three ion species plasma (Yip et al 2016 Phys. Plasmas 23 050703) in which krypton was added to a mixture of argon and xenon plasma confined in a multidipole, dc hot filament discharge, that the addition of krypton served to turn off instability enhanced collisional friction (IEF) found in two ion species plasma (Yip et al 2010 Phys. Plasmas ). In this study, neon, a less massive atomic gas than argon was added. Argon and xenon ion velocity distribution functions (IVDFs) were measured at the sheath-presheath boundary near a negatively biased boundary plate, and the Ne⁺ density was systematically increased. We found in both cases that once the added ion density significantly exceeded the density of the other two ions, IVDF measurements consistent with the absence of the instability were obtained, and the measured ion sheath edge speeds tended toward their individual Bohm velocities. For all other relative concentrations, the ions reached the sheath edge neither at their Bohm speeds nor the ion sound speed of the system, consistent, qualitatively, with the action of the IEF. (paper)