Investigation of sheath properties in Ar/SF6 dc discharge plasma

Properties of plasma sheath boundary in multicomponent plasma with negative ions have been investigated experimentally. The modified Bohm sheath criterion derived for thermal electron-thermal negative ion-cold positive ion model predicts the decrease of positive ion drift speed toward the sheath when the negative ion concentration is gradually increased. This modified Bohm criterion has been discussed in this experimental investigation. The experimental parameters (e.g. discharge voltage, discharge current) set for argon plasma production are kept floating during the addition of SF6 gas. Two important effects caused by the presence of negative ions, e.g. decrease in Bohm speed of positive ions toward the sheath and reduction in plasma density have been observed. The reduction in the ion flux toward the sheath therefore causes the expansion of the sheath. The numerical solution of Poisson's equation, which includes the contribution of negative ions, is compared with the experimental findings.