Study of Ar and Ar-CO2 microwave surfaguide discharges by optical spectroscopy

Abstract

A surfaguide microwave discharge operating at 2.45 GHz in Ar and Ar-CO2 mixtures is studied using diagnostics methods based on optical emission spectroscopy. The population densities of Ar metastable and resonant states of the lowest group of excited levels (1sx) are investigated for several experimental conditions using the self-absorption technique. It is found that the densities of these levels, ranging from 1017 to 1016 m−3 for the pure Ar case, are dependent on the discharge pressure and applied power. The electron temperature and electron density are calculated via the balances of creation/loss mechanisms of radiative and metastable levels. In the range of the studied experimental conditions (50–300 W of applied power and 0.5–6 Torr of gas pressure), the results have shown that lower values of electron temperature correspond to higher values of power and pressure in the discharge. Adding CO2 to the argon plasma results in a considerable decrease (about 3 orders of magnitude) of the Ar metastable atom density. The feasibility of using the ratio of two Ar emission line intensities to measure the electron temperature in CO2 discharges with small Ar admixtures is studied.

Understanding Microwave Surface‐Wave Sustained Plasmas at Intermediate Pressure by 2D Modeling and Experiments

“…The results from the models are extended with measurements of the plasma characteristics of the modeled set‐up. Using a self‐absorption method associated to optical emission spectroscopy (OES), the metastable argon density and electron temperature, and the electron density are evaluated in the range 80–170 Pa . The intensity of the excited Ar atom emission decreases whit increasing pressure similarly to the reported measurements in literature .…”

“…This allows building a system of equations by combining line ratios between the intensities of two emission lines produced by de‐excitation to two 1 s levels from two higher 2 p excited levels (see Figure b for a typical case). If one repeats this method for another pair of levels and pair of lines, all 1s populations can be determined . Further details on evaluating the excited state densities, and the electron density and temperature based on the emission spectra are given in ref …”

Understanding CO2 decomposition in microwave plasma by means of optical diagnostics

“…The increase in conversion with the decrease of the pulse repetition rate might be related to the characteristic time of dissociative recombination, which as a rough approximation, is given by . Under the assumptions of T e ∼ 1 eV, T gas ∼ 700 K and n e ∼ 10 11 cm −3 (typical values obtained in MSGDs), we obtain τ rec ∼ 350 μs. This value is higher than the off‐time duration at f > 1.4 kHz, meaning that the dissociative recombination is promoted at low pulse repetition rates.…”

“…Therefore, this must lead to an increase of Ar met densities and CO 2 decomposition. Adding more CO 2 (Ar < 90%) results in increasing of molecular quenching, which dramatically reduces the Ar met density (see, e.g., Silva et al) together with CO production (see Figure ). Finally, at even higher CO 2 contents (Ar < 40% in our case) the system becomes non‐sensitive to the Ar atom density variation due to the complete suppression of Ar met states through molecular (CO­ 2 ) quenching.…”

“…Let us note that a negligible conversion of N 2 was assumed, which is supported by (i) negligible emission of atomic nitrogen under CO 2 ‐5%N 2 mixtures and (ii) CO 2 ‐N 2 discharge modeling data showing insignificant N 2 dissociation at low N 2 admixtures . The rate coefficients relative to the excitation of CO and N 2 were calculated through a Maxwellian distribution (using electron temperature T e = 1 eV as measured by Silva et al) and the available excitation cross sections of CO and N 2…”