Insights into nanosecond repetitively pulsed CO2 discharges by time-resolved optical emission spectroscopy

Nanosecond repetitively pulsed discharges are seen as a promising way to achieve plasma (- catalytic) conversion of CO2 into value added compounds in a commercially interesting, industrially viable manner; the non-equilibrium conditions unlock thermodynamically unfavourable reaction pathways. It has been demonstrated that a further enhancement of conversion is possible by utilizing a burst pulse pattern rather than a continuous one, under the hypothesis that this further augments the nonequilibrium features of the plasma and provides suitable conditions for vibrational ladder climbing to occur [1]. The short duration nature of these discharges --as well as the fast equilibration times at atmospheric pressure-- necessitate fast diagnostics that are capable of probing the time evolution of discharge conditions in a non-invasive manner. While running in low frequency continuous- or burst-mode, each pulse in the discharge is uncorrelated to the preceding one, i.e. it does not encounter a memory effect and will forms its own streamer. Decreasing the pulse separation however means that a pulse will encounter a progressively stronger memory effect from the preceding pulse, altering discharge conditions and even traversing the electrode gap along the same discharge channel [1, 2]. As such, the electrical characteristics of the discharge as well as the emission produced by it change considerably. Time-resolved spectroscopic analysis of specific spectral lines has allowed both the study of transient conditions in the discharge as well as identifying underlying mechanisms or reaction pathways for the emission....