Ns Pulse and Hybrid Discharges for Plasma Chemistry and Plasma Catalysis Applications

Electric field in a capacitively coupled, non-self-sustained “hybrid” RF discharge in nitrogen, with external ionization generated by high-voltage ns pulses, has been measured by ps Electric Field Induced Second Harmonic generation. The combined ns pulse / sub-breakdown RF waveform generates a diffuse plasma between two plane dielectric-covered electrodes. The measurements are made in the bulk of the plasma and in the sheaths. Reduced RF electric field in the sheaths is significantly higher compared to that in the plasma, exceeding 100 Td, due to the displacement of the electrons by the drift oscillations. However, kinetic modeling indicates that the electron impact ionization in the sheaths is largely ineffective, due to the low electron density. Peak reduced RF electric field in the plasma ranges from 10 to 40 Td, indicating the efficient vibrational excitation of N₂ by electron impact. Hybrid ns pulse / RF plasmas can be used for the selective vibrational excitation of molecular species, such as N₂, CO₂, CO, and H₂. In the present work, this approach is used to study the CO₂ dissociation kinetics in CO₂-N₂, and the plasma-catalytic ammonia generation in N₂-H₂. Time-resolved vibrational populations of CO₂ and CO, translational-rotational temperature, and CO number density are measured by mid-IR, tunable Quantum Cascade Laser Absorption Spectroscopy. Vibrational populations of N₂ are measured by CARS. The NH₃ number density is measured by FTIR absorption spectroscopy. The results identify the effect of the targeted vibrational excitation on the process yield.