Atomic and molecular processes in plasma decomposition method of hydrocarbon gas

In order to understand the decomposition process of hydrocarbons in a hydrogen (H) plasma, a Monte Carlo simulation of collisional transport of a methane (CH₄) molecule was developed. The model simulates collision reactions with plasma ions and electrons (including dissociation, excitation, ionization, and charge exchange) and elastic collisions with residual H₂ gas. The interaction with a surrounding wall was also considered (reflection from the wall, deposition on the wall, and reemission of carbon (C) and hydrocarbons (CH_x) by physical and chemical sputtering). In a low-temperature plasma, because the decomposition process was mainly dominated by charge exchange with plasma ions followed by dissociative recombination with electrons, many neutral C and CH_x species were obtained. At high temperature, the ionized C^y+ and CH_x⁺ species were the dominant ones because of the dissociative ionization and excitation by electrons. Comparable to our previous experiment, the calculated decomposition rate of CH₄ into neutral and ionized C atoms was ∼50% for a temperature of 15 eV and a density of 3.5 × 10¹⁷ m^-3. Nevertheless, the calculated distribution of C and CH_x deposits on the vessel wall were localized in the upstream of the plasma, which was different from the experimental setup. (author)