Effect of surface produced negative ions on near wall sheath

The role of surface produced negative ions in a negative ion source on the sheath structure, which takes place in the surroundings of a metallic plane immersed in a plasma, is analysed using a simple model. It is well known that the Bohm criterion which determines the electric potential at the sheath edge can be multi-valued in electronegative plasmas. It is observed in this study that a multi-valued sheath potential equally holds well in the case of a negative ion source where surface production mechanism of negative ions also exists. Due to the presence of surface produced negative ions, the positive ion current flux towards the metallic plane increases and the sheath potential with respect to the metallic plane increases.