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Accueil > A propos du LPP > Communication > Actualités archivées > 2021 > Georgy Pokrovskiy defended his PhD "Dissociation of carbon dioxide in pulsed plasma at high electric fields : role of energy exchange with electronically excited species"

Georgy Pokrovskiy defended his PhD "Dissociation of carbon dioxide in pulsed plasma at high electric fields : role of energy exchange with electronically excited species"

On December 7, 2021, Georgy Pokrovskiy defended his PhD "Dissociation of carbon dioxide in pulsed plasma at high electric fields : role of energy exchange with electronically excited species", supervised by Svetlana Starikovskaia.

Abstract
The Thesis is devoted to study of dissociation of carbon dioxide at moderate pressure in nanosecond capillary pulsed discharges at high levels of reduced electric field and specific deposited energy. The main purpose of the research was to focus on such a discharge regime that facilitates domination of excitation of electronic degrees of freedom of active species over vibrational ones.
The experimental studies have been done in two different types of capillaries – the so called ’thin’ and FTIR’ capillaries. It has been shown that the discharge develops as a stable fast ionization wave (FIW) in CO2 in both types of capillaries. Any portion of gas was being treated by train of three high voltage pulses separated by 250 ns in each experiment. The main discharge characteristics such as electric current, reduced electric field and specific deposited energy have been measured in all three pulses. The electron density as a function of time has been measured in the thin capillary. The peak electron density was found to be 2×1015 cm−3 in the first high voltage pulse. This value corresponds to the ionization degree of 0.5% at the pressure of 15.5 mbar.
Since it was technically possible to obtain time-resolved profile of the longitudinal reduced electric field E/n in the thin capillary, measurements of E/n as a function of time have been done in it. The values of the E/n in the thin capillary were about 1000 Td in the FIW and 300 Td behind the front of the FIW. Specific deposited energy was found to be about 1.5eV/particle. As for the FTIR capillary, it was only possible to estimate E/n and specific deposited energy. It can be still concluded that both values are two times higher than in the thin capillary.
Measurements of steady-state values of the dissociation fraction α and the energy efficiency of the dissociation η have been done in the FTIR capillary in effluent gas. The FTIR has been used for the measurements. The values of α and η were around 20 % at low frequency pulse regime. When the pulse frequency increased, the αvalue tended to a saturation threshold of 92 %, η of the process was around 8 %.
Optical emission spectroscopy (OES) in both thin and FTIR capillaries hasn’t re- vealed any qualitative difference between the acquired spectra. The emission spectra demonstrate an abundant presence of electronically excited states of CO2+ ion and C atom lying between 18 and 25 eV with respect to the ground state of CO2. It has been shown that excitation of electronic degrees of freedom does dominate over vibrational ones and supplies high values of dissociation fraction of CO2 during the discharge. Presence of dissociation of CO onto C and O has been noticed.
Measurements of radial profile of the electron density by ICCD imaging in the mid- dle of the discharge have been done. The profile has been shown to have a maximum in the axis of the capillary and to monotonously decrease with respect to direction of walls. The behaviour of the profile didn’t change within the high voltage pulses and between them.
Gas temperature has been measured by OES of second positive system of nitrogen which has been added to CO2 as a small admixture. The relevance of rotational temperature of nitrogen and gas temperature of the mixture has been justified. The measurements of temperature have been done both in both types of capillar- ies. The temperature in FTIR capillary was equal to 2000 K after the third pulse. As for the thin capillary, it has been heated up to 1100 K after the third pulse. The phenomenon of fast heating of CO2 in a nanosecond discharge has been therefore confirmed.
Zero-dimensional numerical modeling of the discharge has been also done. It has shown a good agreement between experimental and calculated results. It has been shown that temporal dynamics of CO2+ , O2+ and C2O4+ ions as well as electronic states CO(a3Π), O(1D, 1S) defines the most important kinetic processes and heating of gas during and between the pulses.

Jury
- Erik JOHNSON Directeur de Recherche CNRS, Ecole Polytechnique, LPICM, France Examinateur
- Vasco GUERRA Professeur, Instituto Superior Tecnico, Portugal Rapporteur
- Nicolas NAUDE Maître des conférences, Université de Toulouse, France Rapporteur
- Olivier GUAITELLA Ingénieur de recherche, Ecole Polytechnique, LPP, France Examinateur
- Gianpiero COLONNA Senior Researcher CNR, University of Bari, Italie Examinateur
- Giorgio DILECCE Senior Researcher CNR, Trento University, Italie Examinateur
- Svetlana STARIKOVSKAIA Directrice de recherche, Ecole Polytechnique, LPP, France Directrice de thèse

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