Characterization of CO₂ microwave plasma based on the phenomenon of skin-depth-limited contraction

Abstract

The sub-atmospheric CO 2 microwave plasma is known to contract to a narrow filament with rising pressure as result of a mode transition. This changing state of contraction is investigated in relation to its dielectric properties, in order to directly relate the discharge parameters to the discharge radius. The electron density and gas temperature are measured, respectively, by 168 GHz microwave interferometry and Doppler broadening of the 777 nm oxygen emission lines. The plasma is operated in steady state with 1400 W at 2.45 GHz, between 100 mbar to 400 mbar. Electron density values in the central region range from 10 18 to 10 20 m −3 between the discharge modes, while the gas temperature increases from 3000 K to 6500 K, in good agreement with previously reported values. Based on the dielectric properties of the discharge in relation to the plasma radius, it is found that the discharge column constitutes a radius of a single skin depth. Implications of these insights on the conditions of previously reported CO 2 dissociation experiments are discussed.

Plasma Technology for CO2 Conversion: A Personal Perspective on Prospects and Gaps

“…It was indeed demonstrated, both experimentally (e.g., van Rooij et al, 2015;Bongers et al, 2017;den Harder et al, 2017;van den Bekerom et al, 2019;Wolf et al, 2019Wolf et al, , 2020 and computationally (e.g., Berthelot and Bogaerts, 2017;Heijkers and Bogaerts, 2017;Kotov and Koelman, 2019), that in practice, the vibrational non-equilibrium is not the leading mechanism in (sub)atmospheric MW or GA CO 2 plasmas, and that the CO 2 conversion is thermal, although Pietanza et al (2020) suggested to act with caution on this assumption, as their model still showed clear deviations from equilibrium, even at temperatures of 3,500-5,500 K.…”

“…In MW and GA plasmas, the electron temperature is still higher than the gas temperature (hence, they are not really thermal plasmas), but the vibrational and gas temperature are often close to each other (i.e., VT equilibrium), so they can also be considered as quasi-thermal plasmas. For this reason, the CO 2 conversion proceeds mainly by thermal reactions in MW and GA plasmas at practical operating conditions (van Rooij et al, 2015;Berthelot and Bogaerts, 2017;Bongers et al, 2017;den Harder et al, 2017;Heijkers and Bogaerts, 2017;Kotov and Koelman, 2019;van den Bekerom et al, 2019;Wolf et al, 2019Wolf et al, , 2020.…”

“…However, in recent years, there is growing insight that optimizing the vibration-induced dissociation pathway may not be the most realistic strategy. In fact, experiments indicate that thermal CO 2 conversion gives rise to quite high energy efficiencies (up to 40-50%) (van Rooij et al, 2015;Bongers et al, 2017;den Harder et al, 2017;van den Bekerom et al, 2019;Wolf et al, 2019Wolf et al, , 2020. Indeed, the rate coefficients of thermal reactions increase with gas temperature, thus enhancing the conversion.…”

“…The above suggestions to improve the energy efficiency by promoting the vibrational dissociation pathway are not straightforward to realize in practice. Indeed, recent insights based on both modeling and experiments have revealed that the CO 2 conversion in (sub)atmospheric pressure MW and GA plasmas proceeds by thermal reactions (van Rooij et al, 2015;Berthelot and Bogaerts, 2017;Bongers et al, 2017;den Harder et al, 2017;Heijkers and Bogaerts, 2017;Kotov and Koelman, 2019;van den Bekerom et al, 2019;Wolf et al, 2019Wolf et al, , 2020, and this can also give rise to quite high energy efficiencies (up to 40-50%). As explained above, the rate coefficients of the thermal reactions between various plasma species rise upon higher gas temperature, thus enhancing the conversion.…”

“…Note that other papers by different researchers also reported similar behavior. Typically, three different operating regimes could be distinguished for (sub)atmospheric MW plasmas (i.e., diffuse and two different constricted modes) depending on pressure (e.g., den Harder et al, 2017) and the constricted regimes were studied in detail by Wolf et al (2019Wolf et al ( , 2020, who also confirmed the dominance of thermal dissociation.…”