Temporal-spatial distribution of [formula omitted] metastable molecules in the pulsed positive streamer in the needle-to-plate gap in sub-atmospheric pressure nitrogen

Abstract

The morphology of pulsed positive streamer corona discharge, such as the channel shape and branching, depends on the operating gas and its pressure. The streamer morphology in the gases containing nitrogen is usually visualized by the optical imaging based on the optical emission spectroscopy (OES) of N2(C3Πu) and N2(B3Πg) states (2nd positive and 1st positive N2 system, respectively). The OES can also be employed for determining the concentrations of N2 molecules in those short-lived radiative states. However, the conventional OES is of the restricted use for the diagnostics of N2 molecules in long-lived metastable states, e.g. for monitoring the N2(A3∑u+) metastable molecules, which play an important role in the collisional-radiative processes in electric discharges in the nitrogen containing atmosphere. Since the knowledge of temporal-spatial concentration and distribution of N2(A3∑u+) metastable species is important for describing the behavior of the pulsed positive streamer corona discharge, therefore, in this experiment we employed also a laser-induced fluorescence (LIF) technique to investigate behavior of the N2(A3∑u+) molecules formed by the streamer. Both diagnostics, OES and LIF used in this experiment for investigating the pulsed positive streamer corona discharge were performed under various pressures of nitrogen. Owing to the use of the 2-stage MPC ICCD camera in our experiment, the investigation provided the first LIF imaging of the temporal-spatial distribution of N2(A3∑u+) metastable molecules in the pulsed positive corona streamer in the needle-to-plate electrode gap in nitrogen at sub-atmospheric pressures. The time-dependent LIF images enabled estimating the N2(A3∑u+) metastable number density (3 × 1014 cm−3) in the pulsed positive streamer discharge at nitrogen pressure of 30 kPa.