Temporal evolution of electron density and temperature in low pressure transient Ar/N2 plasmas estimated by optical emission spectroscopy

Abstract

A recently published method for the analysis of phase-resolved optical emission spectra was extended in order to permit estimation of time-resolved electron density profiles. The previously presented method combined collisional-radiative modelling with a self-absorption method to estimate the evolution of Te with sub-cycle time-resolution. However, it was not capable to give similar profiles for ne as the model was insensitive to its variations. The extensions proposed in this work describe a way to also estimate the electron density with sub-cycle time resolution from the changing rates of the argon Paschen 1s states. The method was applied to a low-pressure DBD-jet operated with argon and several argon–nitrogen mixtures with up to 4% N2. Good agreement among evaluation of ne from changing rates of individual 1s states was observed during the collisional phase and the full-cycle temporal profile could be calculated from relative changes in light emission. Electron densities exhibited a drop for larger admixtures of nitrogen and ranged from 1017 m−3 to 1018 m−3. As assumed in a previous work, the electron temperature model worked without explicit consideration of additional processes even when N2 affected the plasma. However, presumably due to collisional quenching by nitrogen, two argon Paschen 2p levels were found to be inappropriate for Te estimation and had to be removed. Values for electron temperature from the remaining levels remained at a similar value as for pure argon.