Langmuir probe, optical, and mass characterization of a DC CO2–H2 plasma

Abstract

We present a detailed Langmuir probe, optical emission spectroscopy (OES), and quadrupole mass spectrometry (QMS) characterization of a DC CO2–H2 plasma mixture, complemented by calculations of the electron energy distribution functions (EEDFs) and ionization rates of CO2 plasma with varying H2 ratios using Boltzmann equation (BE) solver BOLSIG+, assuming a bi-Maxwellian distribution. Both the measured and calculated EEDFs as a function of the H2 concentration agreed well and showed a bi-Maxwellian distribution. The measured and calculated electron temperatures Te as a function of the increment in the H2 concentration (0–100%) increased in the range of 2.5–3.1 eV. The measured and calculated electron densities (Ne) as a function of H2 concentration exhibited the same increasing behavior (approximately 1010 cm−3), which confirms that the mixture composition directly influences the plasma-related parameters and results in a large fraction of H atoms by reaction e + H2 → H + H. An ascending Te would result in higher ionization rates (explaining the observed increase in electron densities), which agrees with the ionization rate behavior obtained by the BE calculation. Both OES and QMS techniques detected the species H (through the lines Hα, Hβ, and Hγ), CO, CO2, CO2+, O2, OH, O, C2, CO, and CO+. An analysis of the CO/CO2 and O2/CO2 ratios would clarify that OH is formed from O2 + H → OH + O rather than other reactions involving CO species. At a 100% CO2 concentration, CO and O2 formations proceed in accordance with the stoichiometry of 2CO2 → 2CO + O2.