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Measurements of high-temperature air emission spectra between 150 and 250 nm (VUV/UV) are presented. These measurements are calibrated in absolute intensity. The high-temperature air was produced using an atmospheric pressure plasma torch facility. The centerline temperature of the plasma jet is approximately 6700 K. A VUV emission spectroscopy system was adapted to the plasma torch facility to acquire spectra from 150 to 250 nm. Absolute intensity spectra were obtained in this wavelength range. They were compared with numerical predictions of the line-by-line spectroscopy code SPECAIR. The overall agreement between the SPECAIR predictions and measurements is good, particularly above 180 nm. Several modifications to the SPECAIR radiation code were carried out in order to improve agreement with experiments. These modifications significantly improved agreement though, at lower wavelengths, the data indicate that a source of emission remains unaccounted for by SPECAIR. Several possibilities for this missing source of emission are discussed, including photodissociation of the nitric oxide molecule.

Reentry radiation is one of the key phenomena to take into account while designing an Entry Descent and Landing (EDL) phase for sample return and some gas/ice giants’ missions. In the case of an ablative thermal protective system (TPS), the physicochemical processes involved produce a large amount of dust and phenolic gases that can obstruct the optical path. The QubeSat for Atmospheric Research and Measurement on Ablation mission (5.2 kg CubeSat launched in December 2019) gave a low-cost opportunity to develop a dedicated payload named “Imbedded Nano-platform-size Emission Spectrometer” (INES) to study the radiation of a reentry plasma in the presence of an ablative heatshield (Cork P50 from Amorim™). The payload is able to measure both radiation and thickness evolution of the TPS at the same location. The paper presents a passive method to prevent any contamination of the optical path from the ablative TPS to be able to measure radiation for a wide range of mission profiles and heatshield locations.

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.

Quantifying the thermal response of a heat shield is a key step in the design of a spacecraft to ensure its survivability during atmospheric entry. Recession and swelling of the thermal protection material have a drastic effect on both the heat transfer within the vehicle and aerothermodynamic transients. Postflight analysis of reentry can be achieved after recovery on Earth, but it is more difficult for entries on other bodies of the solar system. A dedicated instrumentation is necessary to understand the evolution of the thermal protection material thickness during flight. This paper investigates the current limitations of the available measurement techniques. A low-mass passive solution is proposed to measure with high accuracy the phenomena of recession and swelling. The QubeSat for Aerothermodynamic Research and Measurements on Ablation (QARMAN) CubeSat mission provides a flight opportunity to develop a dedicated payload to quantify the recession and swelling of an ablative heat shield made of P50 cork from AmorimTM. In addition, this payload allows us to study the radiation of a reentry plasma in the presence of ablation products through the same optical path. The performance of the measurement technique and the integration of the instrument are discussed for the QARMAN platform, demonstrating its applicability to a space mission.

Plasma, the fourth and most pervasive state of matter in the visible universe, is a fascinating medium that is connected to the beginning of our universe itself. Man-made plasmas are at the core of many technological advances that include the fabrication of semiconductor devices, which enabled the modern computer and communication revolutions. The introduction of low temperature, atmospheric pressure plasmas to the biomedical field has ushered a new revolution in the healthcare arena that promises to introduce plasma-based therapies to combat some thorny and long-standing medical challenges. This article presents an overview of where research is at today and discusses innovative concepts and approaches to overcome present challenges and take the field to the next level. It is written by a team of experts who took an in-depth look at the various applications of plasma in hygiene, decontamination, and medicine, made critical analysis, and proposed ideas and concepts that should help the research community focus their efforts on clear and practical steps necessary to keep the field advancing for decades to come.

International audience; This work aims to provide a better understanding of the cumulative effect of successive nanosecond repetitively pulsed (NRP) discharges on the ignition process. Fast chemiluminescence imaging of both the post-discharge and the following flame was used to analyze the ignition of a lean propane/air mixture (ϕ = 0.7) by a train of 82 NRP discharges at a pulse repetition frequency (PRF) of 30 kHz, in comparison with the traditional spark ignition. The transition from the non-equilibrium plasma to the ignition kernel has been imaged. It has been found that, differently from traditional spark ignition and despite similar experimental conditions, each NRP ignition develops in a unique way. NRP discharges generate both highly reactive species and thermal instabilities in the gap: the multi-pulse strategy produce a gas motion resulting in a jetting phenomenon, reported here for the first time. An algorithm within ImageJ software was used to quantitatively describe the observed jetting phenomenon. The results presented in this work suggest that jetting is the driver of the ignition initiated by NRP discharges, at least in the experimental conditions investigated here.

International audience; The formation of thermal sparks generated by nanosecond pulses is studied experimentally and numerically. The increase of the electron number density, up to 10 19 cm-3 , is measured with sub-nanosecond resolution using Stark broadening of N + , Stark broadening of Hα, continuum emission of the electron, and the intensity of N + lines. Our experimental results are then compared to 0-D kinetic simulations including the electron-impact ionization of excited electronic states of N and O. The electron temperature, in equilibrium with the gas temperature, reaches 42,000K, which agrees with our experimental findings and the literature on the thermal spark.

International audience; An electronic-specific collisional-radiative (CR) model of Nitric Oxide (NO) is presented and applied to the study of a recombining air plasma. This plasma is produced at local thermodynamic equilibrium at 8000 K and 1 atm by a 50-kW Inductively Coupled Plasma torch and passes at high-velocity through a water-cooled tube that forces rapid cooling and recombination. The electronic-specific model takes into account NO(C 2 Π) predissociation, spontaneous emission from the C 2 Π and D 2 Σ + states towards the A 2 Σ + state, and quenching of the excited states by various colliders. This model, alongside two other electronic-specific CR models developed for NO at NASA, is compared to measurements of NO excited states and electron densities along the tube length. Better agreement is obtained with the model presented in this work, especially because of the NO(C 2 Π) predissociation and NO C 2 Π and D 2 Σ + spontaneaous emission toward NO(A 2 Σ +). The electron overpopulation is also explained by the partial equilibrium between N and O atoms and electrons, and the slow depletion of N and O atoms by the NO recombination reactions.

International audience; Non-equilibrium plasmas are known to improve ignition and extend flammability limits. In this study, NRP discharges of 2 mJ are applied at 20 kHz in the recirculation zone of a premixed methane-air bluff-body burner with a lean equivalence ratio of 0.8. The operating point is close to the lean blow out limit and NRP discharges efficiently enhance the combustion process. Discharge effects are investigated and the role of N2(C) is quantified with spectroscopic measurements.

International audience; The formation and decay of the thermal spark generated by a single nanosecond high-voltage pulse between pin electrodes are characterized in this study. The influence of air pressure in the range 50-1000 mbar is investigated at 300 K. By performing short-gate imaging and Optical Emission Spectroscopy (OES), we find that the thermal sparks exhibit an intense emission from excited electronic states of N+, in contrast with non-thermal sparks for which the emission is dominated by electronic transitions of N2. Spark thermalization consists of the following steps: (i) partial ionization of the plasma channel accompanied by N2 emission, (ii) creation of a fully ionized filament at the cathode characterized by N+ emission, (iii) formation of a fully ionized filament at the anode, (iv) propagation of these filaments toward the middle of the interelectrode gap, and (v) merging of the filaments. The formation of the filaments, steps (ii) and (iii), occurs at sub-nanosecond timescales. The propagation speed of the filaments is on the order of 104 m/s during step (iv). For the 1-bar condition, the electron number densities are measured from the Stark broadening of N+ and Hα lines, with spatial and temporal resolution. The electron temperature is also determined, from the relative emission intensity of N+ excited states, attaining a peak value of 48,000 K. In the post-discharge, the electron number density decays from 4x1019 to 2x1018 cm-3 in 100 ns due to an isentropic expansion. During this decay phase, the plasma is found to be in chemical equilibrium. Comparisons are given with previous experiments from the literature. Expressions for the Van der Waals and resonant broadenings of Hα, Hβ, and several lines of O, O+, N and, N+ are derived in the appendix

International audience; We present absolute intensity measurements of CO 4th positive emission from a CO 2 /Ar plasma jet produced using an atmospheric pressure plasma torch facility. The centerline temperature of the plasma jet is approximately 6300 K. A VUV emission spectroscopy system was adapted to the plasma torch facility to measure spectrally resolved emission down to 140 nm. The emission from the plasma in the VUV, UV and near IR is consistent with thermochemical equilibrium. Carbon lines in the UV and VUV are found to be optically thick and their amplitude is therefore substantially affected by the line broadening parameters used for the calculation. For the CO 4th positive emission, these measurements test the performance of various models for the electronic transition moment function (ETMF). These ETMFs are used to calculate sets of Einstein coefficients that are used by SPECAIR to calculate the spectrally resolved emission of the plasma for comparison with experiments. We find that the EMTF of Kirby and Cooper and Spielfiedel accurately reproduce the measured emission from 150-215 nm.

International audience; Optical emission measurements of CO 4th positive radiation, calibrated in absolute intensity, are compared with SPECAIR calculations done using the electronic transition moment function (ETMF) of Spielfiedel and Kirby and Cooper. The emission measurements are spectrally resolved and span the VUV and UV spectral regions, with a minimum reported wavelength of approximately 140 nm. The source for the CO 4th positive emission is an atmospheric plasma produced using an inductively coupled plasma torch facility. A mixture of CO2 and Ar is injected and heated via the induction fields to produce the hot CO responsible for the observed 4th positive emission. The comparison between experimental measurements and SPECAIR calculations shows that the ETMF used reproduces well the observed emission.

Two collisional-radiative (CR) models including electronic states of atomic nitrogen are compared with recent measurements performed in a nitrogen/argon recombining plasma at atmospheric pressure. The collisional-radiative models are in good agreement with one another but overpredict the experimental atomic nitrogen densities by one order of magnitude.

International audience; Measurements of ground state atomic nitrogen inside of a nanosecond repetitively pulsed (NRP) discharge operating at pressures between 0.1-5 bar are performed using a femtosecond two-photon absorption laser induced fluorescence (fs-TALIF) technique. The main goal of this work is to develop a quench-free diagnostic technique which would allow measurements at elevated pressures with high spatial and temporal resolution. Quantitative information is extracted from the TALIF signal via a novel calibration technique based on direct absorption measurements performed in a low-pressure DC discharge. The VUV measurements were done at the Soleil synchrotron facility using their unique high-resolution Fourier-transform spectrometer (⁄ =). During this preliminary work, fs-TALIF measurements of N(4 S) are demonstrated in the post-discharge of the NRP between 1-500 µs after the nanosecond pulse. A maximum number density of N-atoms of × was measured at 1 µs after the pulse when the discharge was operated at 1 bar in pure nitrogen. Importantly, the limit of detection of the fs-TALIF technique was determined to be ()~. This is approximately two orders of magnitude lower than previously reported by ns-TALIF.

International audience; In this paper, the mechanism of air ionization by a single nanosecond discharge under atmospheric conditions is studied by numerical simulations. The plasma kinetics is solved with ZDPlasKin and the electron energy distribution function is calculated with BOLSIG+. The originality of the model is to consider not only the excited electronic states of N2, but also the excited electronic states of O and N. These states are shown to have a primary importance in the ionization of the plasma for ne > 10 17 cm-3. It is shown that a non-equilibrium plasma (Te > Tgas) at ne = 10 17 cm-3 can reach full ionization and thermalization (Te = Tgas ≈ 3 eV, ne ≈ 10 19 cm-3) in less than half a nanosecond under a field usually encountered in nanosecond discharges.

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.

International audience; Nonequilibrium infrared emission spectra representative of a Mars atmospheric entry mission are modeled using RADIS, a spectral code for CO and CO 2 based on the HITEMP-2010 and CDSD-4000 databases. The modeled experimental data for the free-flow and the forebody radiation were obtained in the JAXA expansion tube facility. In the expanding flow, good agreement with the experimental data can be obtained using a slightly nonequilibrium distribution and a homogeneous flow. The model suggests that all three CO 2 vibration modes share a same vibrational temperature slightly higher than the gas temperature. In the shock layer, the forebody radiation includes both CO 2 and CO emission features. It can be correctly predicted by assuming thermal and chemical equilibrium at 4000 ± 500 K. Nomenclature p = pressure [bar] T r ot = rotational temperature of the gas [K] T tr ans = translational temperature of the gas [K] T vib = vibrational temperature of the gas [K] T vib 1 , T vib 2 , T vib 3 = for CO 2 , temperature of each vibration mode: symmetric, bending and asymmetric vibration, respectively [K] x CO , x CO 2

International audience; We present optical emission spectroscopy measurements in recombining nitrogen plasma flows at atmospheric pressure. An inductively coupled plasma torch is used to create an equilibrium plasma, which 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 2 (3), 2 (3) and 2 + (2 +) is studied to measure the nonequilibrium vibrational density distributions within these electronic states. These densities are found to be highly overpopulated in comparison with their corresponding equilibrium values, which is consistent with previous results in recombining flows. The measured densities are also compared with the predictions of a 2-temperature model. This 2-temperature model underpredicts the measured densities but is closer to the measured distributions than the equilibrium densities. The total measured radiation is approximately 100,000 times stronger than the corresponding equilibrium radiation. The 2-T model estimate of this radiation is much closer yet still underestimates by a factor of 10 the measured radiation. These data are intended as a new dataset to test the recombining plasma models used to simulate afterbody flows during atmospheric reentry.

International audience; The mechanisms controlling the hydrodynamic effects induced by nanosecond pulsed discharges applied between two pin electrodes in air at atmospheric conditions are investigated experimentally and numerically. A cylindrical plasma kernel is formed between the electrodes during the discharge. After the discharge, schlieren images show that the cylindrical kernel evolves in different ways depending on the gap distance: (1) for short gap distances (< 4), the cylindrical kernel collapses to form a torus-like structure. In this case the flow field presents a significant source of vorticity; (2) for larger gaps (≥ 4), the kernel retains its initial cylindrical shape and cools down primarily through heat diffusion. The objective of this work is to understand the mechanisms leading to the formation of these two hydrodynamic regimes. To this end, simulations were performed and compared with the experimental schlieren images. It is shown that the main source of vorticity is the baroclinic torque caused by the cylindrical blast wave that follows the fast energy addition during the discharge. Finally, a criterion is given to predict the occurrence of the two hydrodynamic regimes. This criterion is then validated against experimental results from the literature.

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.

International audience; Shock layer radiation from atomic lines can represent the main source of radiation in the afterbody region of a space capsule during Earth entry. Studying the kinetics of excited states of atomic nitrogen is therefore important to correctly predict its corresponding radiation. The CentraleSupelec plasma torch facility provides a stable equilibrium plasma and can be used to study plasma kinetics and radiation in a controlled environment. Previous experiments were performed to supply measurements of excited states of molecular nitrogen in a nonequilibrium recombining flow. We extend these measurements to atomic lines to get measurement of excited states of atomic nitrogen. We performed optical emission spectroscopy from VUV to NIR and obtain measurements of atomic nitrogen states above 10eV. The VUV region allow access to low energy states of nitrogen. A departure from Boltzmann distribution is observed for the high excited states and their densities tend to be equal to the Saha-Boltzmann distribution. This was previously predicted and is experimentally observed in this paper. These results may be used to validate kinetic codes.

A vibrational state-to-state kinetic model is developed and compared with recent measurements performed in a nitrogen/argon recombining plasma at atmospheric pressure. It is shown that the flow experiences high vibrational non-equilibrium, which slows down the recombination process of nitrogen atoms. The present state-to-state model overpredicts the experimental atomic nitrogen densities.

International audience; Nanosecond repetitively pulsed discharges can be classified into three types: the corona, the glow and the spark. The experimental parameters triggering the transition between these regimes are well understood. Corona and glow are non-equilibrium plasmas. The spark plasma can be in non-equilibrium or in equilibrium, the equilibrium regime being called hereafter the arc regime. The transition from the non-equilibrium spark to the arc regime for nanosecond discharges is not entirely understood. This is accompanied by a sharp rise of the gas temperature and electron number density that cannot be explained by conventional mechanisms. In this paper, spatially and temporally resolved emission was performed before, during and after the arc transition, which was found on a time scale below 1 ns. We compare the measurements of the electron number density and the electronic temperature before and after the transition. According to the present results, the electron number density rises from typical values of 10 15 to 10 19 cm-3 and the gas temperature increases from 1,000 K to 40,000 K. The presented measurements are intended to help explain the transition mechanism.

International audience; Presented herein is a feasibility study for absolute measurements of ground state atomic nitrogen inside of a nanosecond repetitive pulse (NRP) discharge using a femtosecond two-photon absorption laser induced fluorescence (TALIF) technique. A two-photon absorption scheme (λ = ×206.6 nm) is used to probe the ground state population of atomic nitrogen (2p 3 4 S) inside the discharge. The scheme involves the excitation of the 3p 4 S3/2 level with the subsequent fluorescence collection taking place from the radiative decay to the 3s 4 P triplet state around 745 nm. The main challenge is to demonstrate the ability to perform the measurements at above atmospheric conditions where the typical ns-TALIF diagnostics is unable to provide information due to the high rate of collisional quenching. As such, the current investigation focuses on high-pressure discharges in pure nitrogen. We demonstrate here the ability to measure fs-TALIF signals from atomic nitrogen in NRP discharges operated at pressures between 0.1-5 bar. Additionally, we perform fs-TALIF measurements in Krypton and we identify the main fluorescence channels using the λ = ×204.13 nm excitation scheme. We report that significant saturation effects was observed in both mixtures due to the presence of photoionization which resulted in a departure from the I 2 dependency of the TALIF intensity.

International audience; An open-source nonequilibrium emission and absorption infrared line-by-line code is presented. RADIS is built around the HITRAN, HITEMP and CDSD databases for molecules in their electronic ground state. Energy levels are read from tabulated databases or calculated from Dunham expansions. Boltzmann, Treanor, and arbitrary vibrational distributions can be generated. In its present version, nonequilibrium spectra can be computed for CO2 and CO, and equilibrium spectra can be computed for all species of the HITRAN database. New species can be added without modification to the core code. Several optimization strategies are described that reduce computation times of high-temperature, high-resolution spectra with millions of lines down to less than a minute. Additionally, RADIS includes a line survey tool, a built-in look-up database to reduce convergence times when fitting experimental spectra, and a multi-slab module to simulate line-of-sight experiments. Validation cases against existing spectral codes and experimental results from various plasma sources are presented. Finally, RADIS is applied to improve the modeling of the radiation from a plasma torch and a nonequilibrium expansion-tube experiment. RADIS is distributed under LGPLv3 license

International audience; We present spectrally resolved measurements of CO 4th positive radiation down to 170 nm under equilibrium conditions. The emission is calibrated in absolute intensity by using an argon discharge calibrated to NIST standards. The measurements were undertaken to validate radiation models available in the literature. NASA EAST measurements had noted a discrepancy between radiation model predictions and their shock tube measurements. Our goal was to perform these measurements under equilibrium conditions and compare against model predictions. To that end, we selected several datasets for the electronic transition moment available in the literature. These were used to calculate state-specific Einstein coefficients for use with the radiation code SPECAIR. Three datasets were tested and two were found to give good agreement with the experimental results.

International audience; This work presents an experimental investigation of the hydrodynamic effects induced by nanosecond and conventional spark discharges. The energy deposited in sparks in the short breakdown time (~1 mJ/mm) induces hydrodynamic effects that redistribute the energy over a large volume (~1 cm 3) surrounding the initial plasma channel. This process influences the subsequent formation of the ignition kernel and the initiation of combustion. The experimental results presented in this paper were obtained with a set of synchronized diagnostics including Schlieren, OH Planar Laser Induced Fluorescence and electrical measurements of the energy deposited in the plasma. It is shown that the motion of the gas excited after the discharge breakdown depends not only on the total deposited energy but also on the dynamics of the energy input in the plasma. Finally, the effects of nanosecond sparks are compared with those of conventional sparks used for internal combustion engines. We show that, with 20 times less energy, the nanosecond spark produces a twice bigger excited gas volume than the conventional spark. This is because the energy deposited by the nanosecond spark during the breakdown stage is three times higher than for the conventional spark.

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.

International audience; This article presents a model to describe the effects of non-equilibrium plasma discharges on gas temperature and species concentration, in the set of equations governing the combustion phenomena. Based on the results reported in the literature, the model is constructed by analysing the channels through which the electric energy is deposited. The two main channels by which the electrons produced during the discharge impact the flow are considered: (1) the excitation and the subsequent relaxation of electronic states of nitrogen molecules which leads to an ultrafast increase of gas temperature and species dissociation within the discharge characteristic time; and (2) the excitation and relaxation of vibrational states of nitrogen molecules which causes a much slower gas heating. The model is fully coupled with multi-dimensional flow balance equations with detailed transport coefficients and detailed combustion chemical kinetic mechanisms. This high level of NRP discharge modelling allows computing high Reynolds flows by means of Direct Numerical Simulations and, therefore, a better understanding of plasma-assisted ignition phenomena in practical configurations. A sequence of discharge pulses in air and methane–air mixture in quiescent and turbulent flow configurations are studied with this model. The results show the minor impact of the vibrational energy on mixture ignition and how the increase of the turbulence spreads this vibrational energy and intermediate combustion species around the discharge zone, minimizing the cumulative effect of multiple pulses. In contrast, the production of O atoms during the discharge has a strong impact on the ignition delays and ignition energies (number of discharge pulses). The results also underscore the impact of the initial turbulent flow Reynolds number and the spatial distribution of turbulent eddies, relative to the discharge channel, on the number of pulses needed to ignite the mixture.

This paper reviews the electron- and heavy-particle impact processes governing the population/depletion of the states of N, O, N2, and N2+, which can be significantly affected by nonequilibrium conditions while strongly radiating in lunar-return shock layers. The collisional-radiative model developed in this work includes electron-impact excitation, ionization, heavy-particle impact excitation, vibration–electron, vibration–translation, dissociation and bound–bound radiative mechanisms. The available experimental and theoretical reaction rates governing these processes were critically reviewed to produce a reliable set of rate constants. The collisional-radiative model was then applied to typical nonequilibrium thermodynamic conditions encountered during lunar-return Earth reentry. The effect of heavy-particle impact processes is discussed.

International audience; We present an experimental study of lean mixture ignition by nanosecond repetitively pulsed (NRP) discharges. The plasma is created in a lean propane/air mixture at pressure up to 10 bar and equivalence ratio 0.7, premixed in a constant volume vessel. We characterize the initial spark radius, the ignition kernel development and the flame propagation as a function of pressure (up to 10 bar) and the pulse energy (1–6 mJ per pulse). Comparisons with a conventional igniter show that better results are obtained with NRP discharges in terms of flame propagation speed, in particular at high pressure, due to the increased wrinkling of the flame front that is induced by NRP discharges.

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.

We present the results of an experimental determination of the quenching rate of the metastable 2P state of atomic nitrogen by nitrogen molecules. 2D density profiles of N(2P) and N(4S) atoms were measured by absolute optical emission spectroscopy inside a tube placed downstream of an atmospheric pressure pulsed discharge in nitrogen. The results are analyzed by means of a detailed kinetic model taking into account the major processes of production and depletion of N(2P). The quenching rate constant of N(2P) by N2 molecules determined with this method at atmospheric pressure and T = 300 K is k = (3 ± 1) × 10−17 cm3 s−1.

International audience; We present a radiative heat flux probe capable of directly accessing a radiative boundary layer around an ablative sample. Unlike previous approaches that have suffered from pollution of the optical system by ablative species, our probe utilizes a system for preventing contamination of the optical system. We are therefore able to make measurements over a long period of time during testing in a plasma torch facility. The probe operates on a battery-powered system and can therefore be installed in facilities without optical access. It is designed for use under a wide variety of conditions within ground test facilities. Prototype tests of the probe were performed in our 50 kW plasma torch facility to study an equilibrium air flow around an ablative CBCF turning wedge and flat surface samples. Further tests of the probe were performed in the much larger plasmatron facility at VKI to demonstrate probe functionality.

International audience; Previous experiments performed at Stanford University studied the properties of a recombining non-equilibrium nitrogen plasma at atmospheric pressure. They used an inductively coupled plasma torch to create an equilibrium plasma and then imposed rapid cooling within a water-cooled tube in order to force rapid recombination and generate non-equilibrium distributions of ground and excited species. Emission spectroscopy was used to measure the plasma properties. Comparisons with numerical simulations showed that CFD codes did not correctly predict the temperature drop and the radiative flux leaving the plasma. This paper discusses a re-analysis of these experiments using new temperature measurements performed using Raman spectroscopy. A key result is that the measured temperature drop due to the imposed cooling is even higher than that measured by prior emission spectroscopy measurements. In addition to the Raman measurements, optical emission measurements were also performed and used to measure the atomic nitrogen density. From these measurements, we show that a large portion of the plasma enthalpy is stored in non-recombined nitrogen atoms. This is an important consideration for CFD codes looking to model atmospheric reentry dynamics.

International audience; We report on Raman scattering and optical emission spectroscopy (OES) measurements in recombining atmospheric pressure plasmas of air and nitrogen. An inductively coupled plasma torch is used to create an equilibrium plasma, which is then forced to rapidly recombine by flowing through a water-cooled tube. For all conditions, temperature measurements are performed using OES and Raman scattering at the exit of tubes of varying lengths. The density of atomic nitrogen is also determined. Evidence of strong chemical nonequilibrium is found in a number of cases. For these cases, we observe that the rotational temperatures measured with OES differ from those measured with Raman scattering, and that the atomic nitrogen density is elevated with respect to equilibrium. A power balance analysis confirms that a large fraction of gas enthalpy is stored in the non-recombined nitrogen atoms. For cases where the plasma remains in equilibrium, we perform numerical simulations using the Eilmer3 computational fluid dynamics (CFD) code. Eilmer3 does not predict the observed drop in gas temperature measured using Raman scattering and OES. Prior efforts by the CFD community have also failed to correctly predict this temperature drop. The results presented in this paper are therefore intended as validation test cases for CFD simulations.