Experimental and numerical study on discharge characteristics in atmospheric pressure nanosecond pulsed CO2 plasmas

Atmospheric pressure nanosecond pulsed CO2 discharges have attracted interest because of their importance in plasma-assisted CO2 conversion and the transient physics of streamer discharges. In this work, the characteristics of atmospheric pressure nanosecond pulsed CO2 pin-to-pin discharge were investigated by electrical measurements, time-resolved optical emission spectroscopy (OES), intensified charge coupled device (ICCD) imaging, and numerical simulation. The results show that the discharge current rises rapidly after breakdown and then decays. The ICCD images demonstrate that the discharge originates near the high-voltage needle electrode and then expands radially and axially, rapidly bridging the discharge gap. The streamer propagation exhibits a stochastic branching tendency, which is consistent with the limited seed-electron supply and weak photoionization characteristics reported for CO2 streamers. The maximum streamer-head axial propagation velocity reaches up to 1.78 × 105 m/s. The OES measurements show that the discharge spectrum is dominated by atomic and ionic radiation, while no clear molecular bands suitable for gas temperature diagnosis are observed. Therefore, a representative gas–temperature range was estimated from independent physical constraints, and its influence on the Stark broadening-based electron density diagnosis was evaluated. Within this temperature range, the inferred electron density is weakly sensitive to the assumed gas temperature. Based on the Stark broadening of the Hα and O I lines, the temporal evolution of electron density was obtained, and its normalized temporal evolution showed a similar general trend to that obtained from the simulation. These results provide insight into transient discharge development in atmospheric pressure nanosecond pulsed CO2 plasmas and demonstrate the value of combining experiment with simulation for plasma parameters difficult to access by experiment.