Potential formation near negative charge collecting substrate in negative ion plasma

Potential formation near a negative charge collecting substrate in a negati ve ion plasma was studied experimentally. We present the first observation of a potential double layer formed in front of a positively biased substrat e in pure SF6 discharge plasma. With the increase of SF6 gas in Ar gas, the plasma potential reduced to negative potential, which is lower than the wa ll potential. The energy distribution function of the negative charges in S F6 plasma predicted the presence of negative ions near the sheath edge and its disappearance at a position close to the substrate. This result implies that negative ions are kept inside the bulk plasma by the sheath electric field while few of them can reach the surface of the substrate.