Spatiotemporal Optical Emission Measurement of Pulsed Streamer Discharges in Air by Time-Correlated Photon Counting

In this study, we performed spatiotemporally resolved optical emission spectroscopy of pulsed streamer discharges generated in atmospheric-pressure air by applying a DC voltage to a needle-to-sphere electrode configuration. Time-correlated single-photon counting was employed to achieve sub-nanosecond time resolution, enabling observation of the temporal evolution of weak emissions after discharge collapse and resolution of various emission species with high sensitivity. The electrodes consisted of a 0.5 mm diameter stainless-steel needle and a 15 mm diameter stainless-steel sphere, separated by a 4 mm gap. A positive voltage applied to the needle produced repetitive current pulses at ~20 kHz. Temporal measurements were conducted using a Time-to-Amplitude Converter (TAC) and a multichannel analyzer, with TAC converting the time difference between the current pulse and detected photon into a voltage signal. Spatial resolution was achieved by moving the electrodes with a Z-stage. The results clearly distinguished primary and secondary streamer phases. For the N₂ second positive system (SPS), the secondary streamer was more intense, while the first negative system (FNS) showed weaker emission in the secondary phase. Based on the SPS/FNS intensity ratio, the spatiotemporal development of the electric field and relative electron density were estimated. In addition, the influence of the working gas on streamer propagation was evaluated, highlighting the influence of minor componets.