Plasma Electronegativity Measurement using Electron and Ion Saturation Current Ratio

Saturation current ratio method is considered to be one of the simplest methods for negative ion diagnostics. In this method, a Langmuir probe is used to measure the current-voltage characteristic, wherein the ratio of electron saturation to the positive ion saturation current is referred to as the saturation current ratio (SCR). As the negative ions' mobility is relatively lower than the electrons' mobility, negative ions generally get stagnated in the bulk plasma owing to the electrons' confining field. Consequently, this significantly alters the positive ion flux at the sheath edge [1]. Therefore, SCR reduces with the increase in negative ion density. However, this reduction is also observed for pure electropositive argon plasma. This anomaly is perhaps attributed to the expanding ionic sheath with negative bias, ground sheath resistance, and an inaccurate determination of plasma potential. The edge-to-centre positive ion density also becomes an important factor while modelling the SCR method for a range of negative ion densities. Therefore, this work aims to address the various limitations associated with the SCR method and investigate the whole range of applicability of this method. This study proposes a hairpin probe-assisted SCR method for better perceiving the negative ion determination [2].

References:

[1] Singh P, Swati and Karkari S K 2022 Equilibrium properties of inhomogeneous partially-magnetized plasma containing negative ions J. Phys. D. Appl. Phys. 55 235201

[2] Singh P, Dahiya S, Pandey A K and Karkari S 2023 Hairpin probe assisted saturation current ratio method to determine plasma electronegativity Plasma Sources Sci. Technol. 32 045013