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This experimental and numerical study is focused on the formation of fully ionized plasmas in ambient air by nanosecond pulsed discharges, namely the thermal spark. The first contribution of this article is the experimental characterization of the electron number density during the pulse. An increase of the electron number density up to 1019 cm−3 was measured with sub-nanosecond resolution via three techniques based on optical emission spectroscopy (OES): Stark broadening of H α , Stark broadening of N+/O+, and the continuum emission of electrons. The discharge diameter is measured with sub-nanosecond resolution using calibrated OES of the N+ and O+ lines. All measurements indicate a transition to a micrometric-size filament of fully ionized plasma in approximately 0.5 ns. The second main contribution of this work is the development of a 0D kinetic mechanism to explain this observation. The mechanism includes 100 reactions, 12 species, and 12 excited electronic states. Particular attention is paid to modeling the N2, N2 +, N, and O electronic state kinetics using the electronic states as additional pseudo-species. Our results show that including the electron-impact ionization of the excited electronic states of N and O, in addition to those of N2, is necessary to explain the experimental results, emphasizing the key role of excited state kinetics in the thermal spark formation.

This work presents a femtosecond two-photon absorption laser-induced fluorescence (fs-TALIF) diagnostic for measuring ground-state atomic nitrogen in nanosecond repetitively pulsed (NRP) discharges. Absolute atom density is obtained from the TALIF signal via a novel calibration technique based on one-photon direct absorption measurements performed in a low-pressure DC discharge. The Vacuum Ultraviolet (VUV) measurements were done at the Soleil synchrotron facility using the high-resolution Fourier-transform spectrometer (minimum linewidth Δ ν ~ = 0.08 cm−1). The main goal of this work was to develop a quench-free diagnostic technique, which would allow measurements at elevated pressures with high spatial and temporal resolution. Here fs-TALIF measurements of N(4S) are demonstrated in the NRP post-discharge between 1–500 μs after the nanosecond high-voltage pulse. A maximum number density of N-atoms of 5 × 1016 cm−3 was measured at 1 μs after the pulse when the discharge was operated at 1 bar in pure nitrogen. This corresponds to a dissociation fraction of ∼0.1%. The fs-TALIF technique at high laser intensity regime (>1 TW cm−2) calibrated using VUV absorption was compared with the fs-TALIF at low laser intensity regime (−2) calibrated via the well-established non-saturated TALIF technique using krypton as an etalon gas. It was found that the two measurements of N(4S) in the NRP post-discharge agree within a factor of 3. Importantly, the limit of detection of the fs-TALIF at high laser intensity regime was determined to be n N ( S 4 ) ∼ 1 0 12 c m − 3 . This is approximately one order of magnitude better than previously reported by ns-TALIF in low-pressure discharges.

Rotational temperature measurements of N2(B3Πg), N2(C3Πu) and N 2 + ( B 2 Σ u + ) obtained by optical emission spectroscopy (OES) are compared with previous gas temperature measurements obtained with Raman spectroscopy in a nonequilibrium recombining nitrogen plasma at atmospheric pressure. The plasma is produced in a state of thermal and chemical equilibrium at about 7000 K using an inductively coupled plasma torch. It is then forced out of chemical equilibrium by imposing rapid cooling through a water-cooled tube. The gas temperature measured by Raman spectroscopy is about 3200 K at the exit of the tube. The rotational temperatures of N2(B) and N2(C) measured with OES are found to be in good agreement with the gas temperature, but the rotational temperature of N2 +(B) is found to be significantly above the gas translational temperature. We propose that this higher rotational temperature is the result of N2 +(B) being formed by the charge exchange reaction between N2(A3Σu +) and N+. Kinetic and QSS simulations are performed to support this assumption and a rate constant for this reaction is proposed.

We present optical emission spectroscopy measurements from the vacuum ultraviolet to the near-infrared in a recombining nitrogen plasma flow. An inductively coupled plasma torch is used to create an equilibrium plasma at atmospheric pressure. This plasma is then forced to recombine by flowing through a water-cooled tube. For certain conditions, the plasma is forced out of chemical equilibrium. The emission of excited nitrogen atoms, essential for reentry radiation, is measured to study the nonequilibrium distributions of the electronic states. The densities of the low-lying energy states are found to follow a Boltzmann distribution while the high-lying states are closer to a Saha–Boltzmann distribution—consistent with previous predictions of recombining nonequilibrium plasmas. The measured densities are then compared with the predictions of a two-temperature model, which is found to underpredict the measured densities. The total measured radiation is around 5 orders of magnitude stronger than the equilibrium radiation. The 2-T model estimate of this radiation is much closer yet still underestimates the measured radiation. A model using a single electronic temperature is therefore not sufficient to predict the measurements. The measurements presented in this paper will be useful to test more detailed state-to-state recombination models.

The capability to reproduce atmospheric entry phenomena in ground-based facilities is crucial for studying the entry of space missions and of any objects de-orbiting or entering the Earth’s atmosphere. During entry, these objects are exposed to intense mechanical and thermal stresses due to interactions with the high-temperature plasma that forms in front of them. Complex physical and chemical phenomena occur at hypersonic conditions. The characterization of plasma–meteoroid interactions requires the ability to understand the thermochemical state of the plasma flowfield, both in the free stream and in the boundary layer surrounding the meteoroid, as well as the determination of the evolution of the meteoroid under the effect of the plasma. The latter requires monitoring the temporal evolution of the shape and surface temperature of the material during melting and ablation processes and the ability to characterize the evolution of the structure and composition of the material. Several complementary ground-based facilities worldwide can be used for that purpose. A few of them are presented in this chapter. They include the SCIROCCO and GHIBLI facilities of CIRA in Italy, the HEAT facility at SITAEL (Italy) and the CP50 Plasma Torch facility at CentraleSupelec (CNRS, France). These ground-based facilities operate under a wide variety of conditions, with test times ranging from less than 1 s (HEAT) to continuous (SCIROCCO, GHIBLI, CP50), specific enthalpies up to 30 MJ kg−1 and heat fluxes up to 30 MW m−2 . The following sections present these various facilities and illustrate their measurement capabilities. Illustrative results from experiments on various materials, including meteorites, are presented. Additional experimental facilities and results may be found in.

The mechanism of air ionization by a single nanosecond discharge under atmospheric conditions is studied using numerical simulations. The plasma kinetics are solved with ZDPlasKin and the electron energy distribution function is calculated with BOLSIG+. The model includes the excited electronic states of O and N atoms, which are shown to play the main role in plasma ionization for n e > 1016 cm−3. For electric fields typical in nanosecond discharges, a non-equilibrium plasma (T e > T gas) is formed at ambient conditions and remains partially ionized for about 12 nanoseconds (n e < 1016 cm−3). Then, the discharge abruptly reaches full ionization (n e ≈ 1019 cm−3) and thermalization (T e = T gas ≈ 3 eV) in less than half a nanosecond, as also encountered in experimental studies. This fast ionization process is explained by the electron impact ionization of atomic excited states whereas the fast thermalization is induced by the elastic electron–ion collisions.

This article reports on experiments in a nonequilibrium plasma produced by nanosecond repetitively pulsed (NRP) spark discharges in water vapor at 450 K and atmospheric pressure. The objective is to determine the electron number density in the post-discharge, with spatial and temporal resolution, to gain a better understanding of the discharge development and chemical kinetics. Electron number densities were measured in water vapor from the broadenings and shifts of the H α and H β lines of the hydrogen Balmer series and of the atomic oxygen triplet at 777 nm. For an average reduced electric field of about 150 Td, high electron densities up to 3 × 1018 cm−3 are measured at the cathode, up to 5 × 1017 cm−3 at the anode, and up to 4 × 1016 cm−3 in the interelectrode gap. The high density near the electrodes is attributed to ionization enhancement and secondary electron emission due to the high electric field in the plasma sheath. In the middle of the inter-electrode gap, we show that the electron density mainly decays by electron attachment reactions. The dissociation fraction of water vapor is estimated to be around 2% in the middle of the gap.

Spatial profiles of N2+ concentration are measured in an atmospheric pressure nitrogen glow discharge by cavity ring-down spectroscopy. At discharge currents of ~100 mA, we measure centreline ion concentrations on the order of 1012 cm-3, and radial profiles with radial-half-maximum of ~1 mm. The measurements have sub-ppm concentration sensitivity, and sub-millimetre spatial resolution. Using a collisional-radiative model we relate the ion concentrations to electron number densities, and we find good agreement with spatially integrated electrical measurements. The measured concentrations indicate significant nonequilibrium in the discharges.

We have applied Cavity Ring-Down Spectroscopy (CRDS) to investigate Nanosecond Repetitively Pulsed Discharge (NRPD) in nitrogen and air at atmospheric pressure in a pin-to-pin electrode configuration. We have measured the concentration of N2(A) by probing absorption transitions in the near Infrared at 770 nm. We report on preliminary results about the measurement of the N2(A) concentration and its evolution under different discharge regimes. We present here a comparison for an air plasma between the time evolution of N2(A) measured by CRDS and the temporal evolution of atomic oxygen measured by two-photon absorption laser-induced fluorescence (TALIF). This study will allow us to compare our experimental results to those given by a kinetic model and help provide a better understanding of the plasma physicochemical processes.