Implementation of a single-shot LIF technique for 2-D imaging of metastable nitrogen molecules in a discharge afterglow at sub-atmospheric pressures

The nonequilibrium kinetics of nitrogen metastable N 2 ( A 3 Σ u + ) manifold is of a high interest for the non-thermal plasma diagnostics. A review of laser induced fluorescence (LIF) diagnostic techniques of N 2 ( A 3 Σ u + ) metastable molecules in nitrogen discharges indicates the need to introduce single-shot LIF measurements. The main obstacle in introducing the single-shot technique is the extremely weak fluorescence of N 2 ( A 3 Σ u + ) species, which monitoring requires a very sensitive and fast detector. In this paper, 2-D imaging technique of metastable nitrogen molecules in a nitrogen discharge via single-shot LIF is presented. The experiments involved a sub-atmospheric pressure nitrogen afterglow of a positive-pulsed streamer discharge that was generated in a modified wire-to-plate electrode system. Using 2-D single-shot LIF technique, instantaneous images of the spatial and temporal distributions of the metastable N 2 ( A 3 Σ u +, v”=2) species and streamer optical emission in the nitrogen afterglow were captured for the first time. The LIF from N 2 ( A 3 Σ u +, v”=2) metastable molecules and the streamer emission from the excited nitrogen molecules had similar spatial distribution. The key factors for successful single-shot LIF measurements were the intensive streamers produced by the modified wire-to-plate electrode system with a short gap distance, and the use of an intensified charge-coupled detector (ICCD) camera equipped with a two-stage microchannel plate (MCP). The single-shot LIF measurement has the advantage s of offering the instantaneous distribution of N 2 ( A 3 Σ u + ) species in the discharge, short measuring time, and better spatial resolution. All of this enable a deeper insight into the nonequilibrium kinetics of nitrogen discharges, making the single-shot LIF technique a powerful diagnostic tool, especially when the LIF signal is weak in stochastic discharges.