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

In this work we will show the first experimental radial profiles of early time (<4 μs) neutral and ion spectroscopic signatures of laboratory lightning-like discharges, recorded with the GrAnada Lightning Ultrafast Spectrograph (GALIUS), which is a portable, ground-based slit spectrograph designed and developed at the Instituto de Astrofísica de Andalucía, in Granada, Spain. It provides 2D spatial-spectral images, being each row the spectrum of the plasma channel at a different radial position. Hence, we produced 20 lightning-like discharges of 30 mm length and 8 ± 2 mm mean width and we recorded their 2D spatial-spectral images at 900 kfps with 0.79 µs exposure time, spectral resolution better than 0.38 nm and spectral dispersion of 0.58 mm/px, from 645.0 nm to 663.0 nm. We generated the lightning-like discharges with an automated Wimshurst machine, being their mean peak voltage and current 32.70 kV and 149.58 A. Under these settings we could experimentally quantify the profiles of both electron density and electron/gas temperature along the radial dimension of the lightning-like plasma channels and their temporal dynamics. To do so, we analyzed each row of the 2D spatial-spectral images of the heated channel of every lightning-like discharge at different times, to follow the radial and temporal variation of neutral (atoms and molecules) and ion spectroscopic signatures. These measurements allowed us to estimate the evolution of the radial profiles of electrical conductivity, overpressure and populations of key chemical species (N2, NO, O2, OH, H2, N2O, NO2, HO2, O3 and H2O) produced along the radius of the plasma channel. Our results are consistent with those obtained from 1D cylindrical radiation-hydrodynamic simulations using an exact (the discrete-ordinates method (DOM-S12)) and approximate (the P1 model approach) solutions of the radiation transfer equations.