CO₂ conversion by NRP discharges: spotlight on two parallel CO production mechanisms.

The conversion of CO₂ by Nanosecond Repetitively Pulsed (NRP) discharges at atmospheric pressure is investigated using Optical Emission Spectroscopy (OES) and nanosecond optical imaging. These measurements allow us to distinguish two coexisting regions with radically different thermophysical conditions. The first region is bright, mostly atomic, and near full ionization. The gas temperature in this region thermalizes with the electron temperature at approximately 30,000 K. This is the first identification of the thermal-spark NRP regime in CO₂. The second region revealed by our measurements is faint, mostly molecular, and its ionization degree is about 0.1%. The gas temperature is about 1,000 K while the electron temperature is about 20,000 K: this region is of the non-thermal spark type. While the thermal region dominates the emission, energy measurement and calculations show that the non-thermal region consumes most of the electric energy deposited and accounts for more than 90% of CO production.