Optical diagnostics of low-pressure RF-DBD Ar/CH₄ plasma: mapping electron temperature and density versus power, pressure, and gas flow

This paper presents an experimental study of an RF-DBD plasma, establishing how variations in operational parameters affect its characteristics and discharge behavior. The study identified the optimal parameters for plasma ignition – including pressure, applied power, and gas flow rate – and quantified their influence on key plasma characteristics, namely electron temperature, electron density, and Hα emission intensity as an optical indicator of hydrogen-containing species. Emission spectroscopy was employed to characterize the argon and argon-methane plasmas, with particular attention paid to the hydrogen Hα line (656.2 nm) in the mixture. For argon plasma, the electron temperature increased from ~ 0.98 eV to ~ 1.08 eV as the pressure was raised from 0.5 to 1.0 Torr at a constant discharge power of 4 W. Under the same pressure and power conditions, the electron temperature for an argon-methane plasma was approximately ~ 1.2 eV. A further increase in both power and gas flow rate leads to a decrease in electron temperature. Concurrently, the electron density was found to be on the order of 1014– 1015 m− 3. Material properties were examined using scanning electron microscopy (SEM) for morphology and Raman spectroscopy for structural analysis and defect characterization. The deposited carbon-based materials – films on the RF electrode and particles on the reactor wall – exhibited an amorphous structure. These findings elucidate the fundamental relationship between plasma parameters and gas conversion efficiency, providing a valuable framework for the controlled hydrogen production for energy applications.