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 T e with sub-cycle time-resolution. However, it was not capable to give similar profiles for n e 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 n e 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 T e estimation and had to be removed. Values for electron temperature from the remaining levels remained at a similar value as for pure argon.

Disalignment rate coefficient of argon $\mathrm{2p_8}$ due to nitrogen collision

“…A significant drop of an order of magnitude in 1s state densities was observed when nitrogen concentration was increased up to 50 sccm. Similar behavior was observed in previous work [14] based purely on optical emission measurements and collisionalradiative modeling. Precise modeling of 2p 8 state requires the description of one more transition branch 2p 8 → 1s 2 at 978.45 nm.…”

“…The quenching rate is given as product of species density n and deactivation rate constant k q , reported in [11] for k q,Ar , [12] for k q,N 2 and [13] for k q,e . The electron density for argon admixture with nitrogen was expected to be in order of few 10 16 m −3 comparable to measured values in [14] for unmagnetized plasma and similar conditions. If the laser frequency matches the transition frequency 1s 4,m j → 2p 8,m i , the sub-level is pumped and its density can be obtained by…”

Optical properties of magnetized transient low-pressure plasma

“…Phase dependent development of plasma produced in same configuration and similar conditions has been investigated in previous works [12,24] where strong variations in plasma parameters as well as in 1s state densities were observed. Plasma dynamics under the conditions investigated here illustrated by phase resolved emission of 842.47 nm argon line is shown in figure 4.…”

Diagnostics of a high-pressure DC magnetron argon discharge with an aluminium cathode

“…Emission spectra of an argon low temperature plasma have been successfully analysed by modelling collisional and radiative processes responsible for the population of excited states [43,44]. In the complete model for argon [44,45] the populations of the first 14 excited states (Ar(1s) and Ar(2p) branches) were modelled including electron impact excitation from ground state, electron coupling between and within the Ar(1s) and Ar(2p) levels, and electron quenching and radiative processes including light trapping. It was shown that in the case of low temperature plasmas with moderate electron densities and temperatures, the collisional-radiative model (CRM) can be simplified to an extended corona model including excitation from ground and 1s states and radiation transfer processes [44,46].…”