Submicrosecond Spectroscopy of Lightning‐Like Discharges: Exploring New Time Regimes

Abstract

Submicrosecond (0.476 μs per frame with an exposure time of 160 ns) high‐resolution (0.38 nm) time‐resolved spectra of laboratory‐produced lightning‐like electrical discharges have been recorded for the first time within the visible spectral range (645–665 nm). The spectra were recorded with the GrAnada LIghtning Ultrafast Spectrograph (GALIUS), a high‐speed imaging spectrograph recently developed for lightning research in the IAA‐CSIC. Unprecedented spectral time dynamics are explored for meter long laboratory electrical discharges produced with a 2.0 MV Marx generator. The maximum electron density and gas temperature measured in a timescale of ≤0.50 μs (160 ns) were, respectively, ≃1018 cm−3 and ≃32,000 K. Overpressure in the lightning‐like plasma channel, black‐body dynamics, and self‐absorption in spectral lines were investigated.

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

“…Even though the measured peak currents of these lightning‐like discharges were quite humble (∼3 kA) compared to peak currents in real lightning, the estimation of the temperature and electron density gave similar results: measured electron densities reached values of up to ∼10 18 cm −3 and the temperatures were as high as ∼32,000 K. The analyses carried out in Kieu et al. (2020) also indicated a possible disruption of equilibrium behind the shock front in the very early times.…”

“…Details of these calculations can be found in Kieu et al. (2020). Finally, for spectra recorded with grism R4 in the near‐infrared the temperature was calculated from the ratio of the line intensities of the triplets 777 and 795 nm.…”

“…This generator can produce sparks up to 900 kV in Switching Impulse (SI) and Lightning Impulse (LI) modes. The experimental setup used here can be found elsewhere (Kieu et al, 2020). In the SI mode, the voltage slowly rises up to its maximum peak in 100 µs and then it decays in ∼ 0.5 µs.…”

“…The emergence of the 795 nm line is weaker than the 777 nm one because, though they have very similar Einstein coefficients, the excitation energy of the 795 nm triplet is higher (∼ 14.04 eV) than that of the 777 nm triplet (∼ 10.73 eV). It is interesting to note that, as it is seen from high speed photometric recordings from space, lightning 777 nm optical radiation inside thunderclouds has a duration of several milliseconds due to scattering by water and/or ice particles before it is finally absorbed (Luque et al, 2020;Soler et al, 2020). Lightning near-infrared optical emissions inside thunderclouds are more absorbed than near-ultraviolet and blue radiation (Luque et al, 2020).…”

“…For visible-near infrared spectra recorded with grism R2 we used the pair of lines at 648 nm and 661 nm associated with singly ionized nitrogen, and the pair of O I neutral lines 715 nm and 777 nm. Details of these calculations can be found in Kieu et al (2020). Finally, for spectra recorded with grism R4 in the near-infrared the temperature was calculated from the ratio of the line intensities of the triplets 777 nm and 795 nm.…”