Experimental Radial Profiles of Early Time (<4 mu s) Neutral and Ion Spectroscopic Signatures in Lightning-Like Discharges

This study presents experimental results for the radial and temporal variation of neutral and ion spectroscopic signatures emerging from the heated channel of lightning-like discharges diagnosed with a high speed (900,000 fps) imaging spectrograph. Light emissions emanate from three regions: an inner core (up to similar to 2 mm), an external sheath (up to similar to 4 mm) featuring a sudden temperature increase, and further optical emissions forming a dim glow from 4 mm up to 16 mm. The optical emissions are initially (<1.11 mu s) dominated by the N-2 first positive system at 660.8 nm and by the N II ion line at 661.05 nm. Between 1.11 and 3.33 mu s the optical emissions are dominated by H-alpha (656.3 nm) and O II ion (656.54 nm) lines. The N II ion line at 648.20 nm prevails in the outer dim glow region (9-12 mm) before 2.22 mu s. Spectroscopic signals were used to experimentally derive the time dynamics of the electron density and electron/gas temperature radial profiles, which allowed the estimation of the early time overpressure pulse, electrical conductivity and concentrations of key molecular species (N-2, NO, O-2, OH, H-2, N2O, NO2, HO2, O-3, and H2O) along the radial axis of the heated air plasma channel. These populations were calculated from the overpressure pulse, assuming that they were produced from humid (50%) air under thermal equilibrium conditions. OH is found to be the second most abundant molecular species (after NO) directly generated by heated lightning-like channels.