Probe methods used in diagnostics of plasma with two negative particles

Many errors can be introduced in calculus of plasma parameters by selection of plasma potential. In this paper a combination of two methods is proposed to find out the plasma potential. The study is made on Ar- SF6 plasma which contains two population of negative particles, electrons and negative ions. A combination of the direct fit method and test function method is proposed for better determination of plasma parameters. Our study is conceived to improve the probe measurements in plasma produced in electronegative gases, where the negative plasma particles consist of a mixture of thermal electrons and negative ions, respectively. In all cases plasma parameters calculated from probe characteristics depend on the value selected for plasma potential. The results on the comparative analysis of the influence of the plasma potential on the plasma parameters calculated from an experimental probe characteristics using the methods listed above are presented. The nature of probe surface can introduce errors in plasma potential determination. A good agreement, between the test function method and the direct fit method, is established for Vp=-2.64 V, value more closed by Vp1. The parameters of Ar-SF6 plasma were calculated for this value of plasma potential. (authors)