Self-consistent time dependent vibrational and free electron kinetics for CO₂dissociation and ionization in cold plasmas

Abstract

A self-consistent time dependent model, based on the coupling between the Boltzmann equation for free electrons, the non equilibrium vibrational kinetics for the asymmetric mode of CO 2 and simplified global models for the dissociation and ionization plasma chemistry, has been applied to conditions which can be met under pulsed microwave (MW), dielectric barrier discharge (DBD) and nanosecond pulsed discharges (NPD). Under MW discharge type conditions, the selected pulse duration generates large concentration of vibrational excited states, which affects the electron energy distribution function (eedf) through the superelastic vibrational collisions. Moreover, in discharge conditions, plateaux appear in the vibrational distribution function (vdf) through the vibrational-vibrational up pumping mechanism, persisting also in the post discharge. In post discharge conditions, also the eedf is characterized by plateaux due to the superelastic collisions between cold electrons and the CO 2 electronic state at 10.5 eV. The plateau in vdf increases the dissociation of pure vibrational mechanism (PVM), which can become competitive with the dissociation mechanism induced by electron molecule collisions. The PVM rates increase with the decrease of gas temperature, generating a non-Arrhenius behaviour. The situation completely changes under DBD and NPD type conditions characterized by shorter pulse duration and higher applied E/N values. Under discharge conditions, both vdf and eedf plateaux disappear, reappering in the afterglow.

Numerical model for the determination of the reduced electric field in a CO2 microwave plasma derived by the principle of impedance matching

“…Better results in narrowing down the γ range, and estimating T e may be obtained by exploring the usage of more complex models, e.g. [26][27][28]. It is clear that the method described in this paper is not very easy to apply, and it would therefore be both interesting and helpful to apply this method to plasma systems that are more standard, like a surface wave dis-charge, see for example [29].…”

“…It is known that more complete models exist, taking into account superelastic vibrational and electronical collisions, and the influence on plasma composition, e.g. see reference [26]. A more careful study using this approach takes into account the influence on the EEDF, not only of CO 2 [27], but also CO [28], which has a substantial molar fraction at these high temperatures.…”

“…The rates of electron impact dissociation and dissociative recombination at the temperatures and pressures in this paper are comparable to rates for thermal dissociation, mainly due to the large cross section of dissociation of CO. Also, the degrees of ionisation of both the diffuse (10 −5 ) and the contracted (10 −4 ) plasma are relatively high, and therefore letting the plasma (electron) rates compete with the thermal dissociation rates and changing the composition [26]. Therefore the narrowing down of the γ-range and the estimate of T e can be improved using the more complex models mentioned in [26][27][28]. Since the superelastic collisions have the effect of enlarging the EEDF in the tail (and be smoothened by electron-electron collisions) [27], the BOLSIG+ EEDFs are possibly underestimated.…”

Dissociative Electron Attachment From Vibrationally Excited Molecules in Nanosecond Repetitively Pulsed CO Discharges and Afterglows

“…A perspective of this work will be the insertion of the CO reacting kinetics developed in the present work, as well as in Pietanza et al (2018a,b) and Pietanza et al (2017a,b), in a complex model for the activation of CO 2 under non-equilibrium plasmas (Kozak and Bogaerts, 2014, 2015; Pietanza et al, 2016a,b, 2017c; Belov et al, 2017; Bogaerts et al, 2017a,b; Capitelli et al, 2017; Klarenaar et al, 2017; Silva et al, 2018). At the moment, the existing data of the dissociative electron attachment of CO 2 include only the global process in the cold gas approximation (i.e., the different vibrational ladders are in the ground state).…”

“…A third example, largely investigated in the present days, is the activation of CO 2 in cold plasmas, where the dissociation process involves either the electron impact dissociation process of asymmetric mode of CO 2 or the heavy particle dissociation processes assisted by vibrational excitation (Capitelli et al, 2017). In addition, the reacting CO 2 plasma forms CO and O 2 molecules that in turn undergo a complicated non-equilibrium vibrational kinetics coupled to the Boltzmann equation for eedf.…”