Observation of a significant drop of electron density in cascaded arc argon plasma doped with oxygen gas using laser Thomson scattering

In this work, the electron density (n(e)) and electron temperature (T-e) of cascaded arc argon plasma regulated by adding electronegative oxygen gas have been investigated using laser Thomson scattering diagnostic technique. The results indicate that the addition of O-2 gas causes a significant decrease of n(e), which drops from 10(20) m(-3) to 10(17) m(-3). This is mainly attributed to the dissociative recombination reaction between electrons and O-2(+) molecular ions. Meanwhile, the formation of negative ions, O-2(-) and O-, consumes electrons and further makes n(e) decrease. But, T-e remains nearly unchanged with the increase of O-2 ratio from 0% to 10%. This is probably due to that the electron energy loss by the electron collisions with O-2 molecules in the ground state balances the electron heating induced by the super-elastic collisions with the highly vibrational excited O-2 molecules.