The Chemical Origins of Plasma Contraction and Thermalization in CO₂ Microwave Discharges

Abstract

Thermalization of electron and gas temperature in CO2 microwave plasma is unveiled with first Thomson scattering measurements. The results contradict the prevalent picture of an increasing electron temperature that causes discharge contraction. It is known that as pressure increases, the radial extension of the plasma reduces from ~7 mm diameter at 100 mbar to ~2 mm at 400 mbar. We find that, simultaneously, the initial non-equilibrium between ~2 eV electron and ~0.5 eV gas temperature reduces until thermalization occurs at 0.6 eV. 1D fluid modelling, with excellent agreement with measurements, demonstrates that associative ionization of radicals, a mechanism previously proposed for air plasma, causes the thermalization. In effect, heavy particle and heat transport and thermal chemistry govern electron dynamics, a conclusion that provides a basis for ab initio prediction of power concentration in plasma reactors.

Avoiding solid carbon deposition in plasma-based dry reforming of methane

“…This approach provides quantitative measurements of the rotational (gas) temperature (assumed in equilibrium with T g ), as well as electron density and temperature, respectively. 29 We couple this laser scattering to a microwave plasma setup as schematically illustrated in Fig. 2.…”

“…More details regarding the laser scattering setup, along with a discussion of the improvements introduced to the diagnostics, are available elsewhere. 23,29…”

“…More details regarding the laser scattering setup, along with a discussion of the improvements introduced to the diagnostics, are available elsewhere. 23,29 Optical emission imaging Optical plasma emission is very suitable to reconstruct the plasma shape and estimate the discharge volume. 30,31 In this work, the plasma emission images are obtained using a CCD camera, as depicted in Fig.…”

Plasma activation mechanisms governed by specific energy input: Potential and perspectives

“…Moreover, electron kinetics were found to be affected by T gas . [ 99 ] Hence, the probability that colliding particles have sufficient energy for plasma activation (by electrons and among excited particles) can exceed α 0 ∙ E pl / E th , yielding an enhanced efficiency (>36.8%). However, the number of CO 2 molecules to reach the threshold for dissociation remains low, thus limiting conversion (<36.8%).…”

Insights into the limitations to vibrational excitation of CO2: validation of a kinetic model with pulsed glow discharge experiments

“…In fact, the former are low-excitation and non-equilibrium discharges (at least when operated at low pressure), whereas the latter can be described as 'warm' plasmas, where the gas (T g ) and vibrational (T v ) temperature are almost in equilibrium with each other, and are typically not much lower than the electron temperature (T e ) (i.e. T g = T v T e ) [12,15,16]. For 'warm' plasmas, with T g = 3000-4000 K (or more in case of contraction [15]), the conversion is mostly thermally-driven [17].…”