High‐Speed Spectroscopy of Lightning‐Like Discharges: Evidence of Molecular Optical Emissions

Abstract

High speed spectra (between ∼380 nm and ∼800 nm) of meter‐long lightning‐like discharges recorded at 672,000 fps and 1,400,000 fps (with 1.488 and 0.714 μs time resolutions and 160 ns exposure time) show optical emissions of neutral hydrogen, singly ionized nitrogen, oxygen, and doubly ionized nitrogen which are similar to those found in natural lightning optical emissions. The spectra recorded in the near ultraviolet‐blue range (380–450 nm) and visible‐near infrared (475–793 nm) exhibited features of optical emissions corresponding to several molecular species (and emission bands) like cyanide radical (CN) (Violet bands), N2 (Second Positive System), N2+ (first negative system), C2 (Swan band) and CO (Quintet and Ångström bands). Molecular species can be formed at regions of the lightning‐like channel where the gas temperature would be milder and/or in the corona sheath surrounding the heated channel. We have quantified and compared electron densities and temperatures derived from different sets of neutral and ion line emissions and have found different sensitivities depending on the lines used. Temperatures derived from ion emissions are higher and change faster than those derived from neutral emissions.

Optical Signatures Associated With Streamers and Leaders of Laboratory Discharges

“…Additionally, we image the discharges at the moment of strong emissions obtained by the photometers with short‐time exposure pictures. The optical emissions associated with the discharges within the response range of our filters can be attributed mainly to the radiation of the second positive system of nitrogen molecules (Gallimberti et al., 1974) in the blue (337 nm) and to the radiation of the neutral O I line in the red (777 nm), which is in reality a triplet with sublines at 777.19, 777.42, and 777.54 nm (Kieu et al., 2021).…”

“…A broad review on lightning spectroscopy and the high‐speed spectra of meter‐long laboratory discharges in a wide range (between 380 and 800 nm) were recently explored by Kieu et al. (2021). During the breakdown, they have found several optical emissions similar to those found in natural lightning.…”

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

“…Then we calculated the FWHA of the Lorentzian curve obtained from the H α fitting parameters. We only took into account those rows where the peak value of H α was three times higher than the standard deviation of the signal for that row (values over 3‐sigma) to ensure that we had real values of electron density instead of noise (Kieu et al., 2021).…”

“…We obtained the variation in time of the radial profile of the overpressure in the heated plasma channel with respect to ambient pressure ( δ p = 1) by following the method described in Kieu et al. (2021). For this we use the electron density and temperature radial profiles obtained, and compute the overpressure as $$${\delta }_{p}=\frac{{N}_{e}^{\mathrm{exp}\,}}{{N}_{e}^{LTE\left({T}_{g}\right)}},$$$ where $${N}_{e}^{\mathrm{exp}\,}$$ is the electron density obtained experimentally and $${N}_{e}^{LTE\left({T}_{g}\right)}$$ is the electron density obtained when equilibrium is assumed at a certain gas temperature.…”

“…The analysis of the spectrum of distant plasmas allows to infer physical properties as the gas temperature or the electron density. So far, the method to obtain a spectral curve from 2D spatial-spectral images, consists of integrating the entire image in the spatial dimension, so the signal-to-noise ratio (SNR) is enhanced in the spectral dimension (Kieu et al, 2020(Kieu et al, , 2021. However, this method usually ignores the spatial information that could reveal the radial distribution of electron density and/or temperature from the core of the lightning-like plasmas to its surface.…”