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

Abstract

Three dimensional electromagnetic modelling of a free-standing CO 2 microwave plasma has been performed, by describing the plasma as a dielectric medium. The relative permittivity and conductivity of the medium are parametrised. The waveguide geometry from experiment, including the tuner, is put into the model, knowing that this corresponds to maximum power transfer of the microwave generator to the plasma under plasma impedance matching conditions. Two CO 2 plasma discharge regimes, differing mainly in pressure, input power and temperature, have been studied. The model's validity has been checked through study of materials of known conductivity. From measurements of the neutral gas temperature and the plasma electron density profile, the reduced electric field is determined. From the parametrisation of the dielectric properties, a range for the effective electron-neutral collision frequency for momentum transfer is estimated. The results for the reduced electric field and the range of the electron neutral collision frequency obtained, are consistent as verified by simulations using BOLSIG+. In addition, from this comparison it is possible to narrow down the range of the collision frequencies, and to estimate the electron temperature. The reduced electric field lies between 80 and 180 Td for the relatively low pressure, low input power, the so-called 'diffuse' regime. For the relatively high pressure, high input power ('contracted') regime it lies between 10 and 60 Td. The normalised collision frequency lies between 1.6 and 2.3 for the diffuse regime, while for the contracted regime it lies between 2 and 3. The electron temperature ranges from 2 to 3 eV for the diffuse regime, and from 0.5 to 1 eV for the contracted regime.

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

“…It is an important measure to distinguish the operating conditions of different plasma types. A DBD exhibits a reduced electric field around 200 Td and above, whereas MW and GA plasmas (and some other plasma types, such as APGD) typically operate around 50-100 Td (Bongers et al, 2017;Pietanza et al, 2020;van den Bekerom et al, 2020), although a recent model for determining the reduced electric field in CO 2 MW plasma derived by the principle of impedance matching (Groen et al, 2019) predicted somewhat higher E/n values, i.e., 10-60 Td for the so-called contracted regime (higher pressure and power), but 80-180 Td for the so-called diffuse regime (lower pressure and power). As depicted in Figure 5, for reduced electric fields around 200 Td and above, the largest part of the electron energy (70-80%) goes to electronic excitation, while limited amounts are spent for dissociation and ionization (ca.…”

Advances in non-equilibrium $$\hbox {CO}_2$$ plasma kinetics: a theoretical and experimental review

“…With the Phelps cross section, instead, with the increase of T gas , the overall kinetics passes to a regime in which the DEM mechanism starts prevailing, with a more thermal behavior of the discharge, and the CO 2 conversion rates globally increases with T gas . Higher gas temperature MW discharges, with T gas in the range 3500-5500 K, were also investigated by Pietanza et al [271] to compare the self-consistent model results to recent experiments performed by Groen et al [324] in diffuse and contracted regimes. The kinetic values for the electron density, reduced electric field and electron temperature calculated by the selfconsistent model were compared to the same quantities measured estimated by Groen et finding a good qualitative agreement.…”

Resolving discharge parameters from atomic oxygen emission

“…However, here the electric field magnitude presents a rather flat profile in the whole plasma region up to r = 10 mm. We should notice that for higher radial positions, where n e (r) 7 × 10 16 m −3 and no electron-impact ionisation takes place, the simulation result of E/n g is assumed to have a minimum value of 1 Td, which also affects the calculation of T e and E. Conversely, in the 3D EM simulation results in Groen et al (2019), addressing a low-confinement discharge in the same MW reactor and assuming equation ( 8) as input profile, E/n g and E have rather flat profiles only in the first 4 mm near the axis, and then a k exc is the coefficient corresponding to excitation from ground-state only. k eff is an effective coefficient, corresponding to excitation from ground-state O(2p 4 3 P), O(3s 5 S 0 ) and O(3s 3 S 0 ).…”