Conversion of CO2 into CO and O is studied in a flowing gas surfaguide pulsed microwave discharge operating with CO2 and CO2 + N2 gas mixtures under different conditions. Optical emission spectroscopy, including actinometry (using N2), vibrational (N2 molecule) and rotational (CO and N2 molecules) analysis are utilized. Both time- and space-resolved measurements are performed. The results show the essential changes of the CO2 conversion rate, its energetic efficiency, and the gas and vibrational temperatures along the gas flow direction in the discharge. The spatial distribution of the power absorbed in the plasma is analyzed. It is also confirmed that the vibrational excitation is a key factor in the CO2 dissociation process in this type of plasma. It is suggested that the obtained dissociation rates can be further optimized by varying the gas composition, as well as the power applied to the discharge.

https://iopscience.iop.org/article/10.1088/0963-0252/23/2/025009/pdf

Optical characterization of a microwave pulsed discharge used for dissociation of CO2

The 'two-term' approximation (representation of the electron distribution by the first two terms of an expansion in spherical harmonics in velocity space) continues to occupy a central role in the low-temperature plasma physics literature, in spite of the mass of evidence illustrating its inadequacy in the swarm (free diffusion) limit for many molecular gases. Part of the problem lies in the failure of many authors to specify quantitatively what they mean when they say that the two-term approximation is 'acceptable'. Thus for example, an error of 10% in transport coefficients may well be acceptable in many plasma applications, but for analysis of highly accurate swarm experiments to compare with ab initio and beam-derived cross-sections, 0.1% or less is required, making 'multi-term' analysis mandatory. While reconciliation of the swarm and plasma literature along the lines of two different accuracy regimes may thus be possible, we dispute claims that the two-term approximation is generally satisfactory for inversion of swarm experiment data to obtain electron impact cross-sections. The unsatisfactory nature of other assumptions implicit in much of the modern plasma kinetic theory literature is also discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/36/24/006/pdf

Is the classical two-term approximation of electron kinetic theory satisfactory for swarms and plasmas?

/pdf/foundations-of-modelling-of-nonequilibrium-low-temperature-1aohvkqwlj.pdf

Foundations of modelling of nonequilibrium low-temperature plasmas

This work presents the LisbOn KInetics Boltzmann solver (LoKI-B), a simulation tool to model non-equilibrium low-temperature plasmas, produced from different gas mixtures for a wide range of working conditions. LoKI-B (to become open-source) provides the solution to the homogeneous and stationary two-term electron Boltzmann equation including: first and second-kind collisions, electron-electron collisions and spatial or temporal electron density growth models to account for the production of secondary electrons born in ionisation events. On output, it yields the electron energy distribution function and different electron macroscopic parameters. The tool can handle simulations for any gas mixture, accounting for the electronic, vibrational and rotational internal degrees of freedom of the atomic / molecular excited states present in the plasma.

https://iopscience.iop.org/article/10.1088/1361-6595/ab0537/pdf

The LisbOn KInetics Boltzmann solver

Travelling-wave-sustained discharges

In the first part of this paper we briefly review some basic concepts of kinetic theory. The concept of the velocity distribution function is first introduced and its meaning is discussed. Then, the Boltzmann equation is presented on physical grounds and it is shown that the fluid equations are its moments. In the second part, the Boltzmann equation for free electrons in a low-temperature plasma is analysed. It is shown how this equation can approximately be solved for electrons under a HF field of frequency ω (including the particular case of a dc field, which corresponds to the limit ω = 0) by using a first-order double expansion in spherical harmonics in velocity space and a Fourier series in time. The electron transport parameters, particle balance and energy balance are analysed from a general point of view. Finally, an application to argon and nitrogen is given. The effects of changes in the field frequency on the electron energy distribution function, transport parameters and power balance are discussed. The importance of the coupling between the electron and the vibrational kinetics in N2 is emphasized.

https://iopscience.iop.org/article/10.1088/0963-0252/9/4/308/pdf

C M Ferreira

Papers

Electron kinetics in atomic and molecular plasmas