In this work, we present a contribution on the applicability of the recursive least squares method used for the parametric identification of a corona discharge phenomenon at small distances. Furthermore, we show the influence of the choice of the forgetting factor for a better performance of the identification operation and the quality of estimation of the identified parameters. The identification process is based on experimental input/output measurements. The validation of the parameter results is done by a physical analysis of the behaviors of these parameters and by comparing the output calculated according to these parameters, with the real output obtained experimentally. The results show that with a constant forgetting factor close to 1 ( $\lambda $ $=$ 0.99), parameter quality improves but output accuracy may vary. In contrast, a variable forgetting factor enhances both parameter quality and model output consistently. A good agreement observed between the real and calculated outputs confirms both the good choice of the forgetting factor and the precision of the estimated parameters as well as the validity of the identified model in general.

The presented research work is devoted to the measurements of the absolute densities of ground state OH radicals in an atmospheric pressure helium plasma jet launched in free air onto three different targets. The OH density is measured by using laser induced fluorescence (LIF). The plasma is generated inside a glass tube by a dielectric barrier discharge powered by a mono-polar pulsed high voltage. Estimation of the absolute density is based on a quantitative analysis of the spectral and temporal component of the LIF signal and does not require any assumption on the densities of colliding species. Based on this analysis, the spatially-resolved density distributions of OH radicals are determined in front of three different targets (metallic, dielectric, and ultra-pure water) placed in the open air at 2 cm from the exit of the glass tube. In order to also investigate the temporal behaviour of the OH densities, and the influence of the gas flow rate and of the amplitude of the voltage-pulse, planar LIF has been performed investigating the fluorescence signal (i) during the intervals of the discharge pulses, (ii) for various gas flow rates, and (iii) as a function of the amplitude of the voltage pulse. In the free jet configuration without a target, the OH densities were determined to be in the order of 1019m−3 , whereas they increased up to a factor of 10 in contact with a target. The spatial distributions were also influenced by the dielectric properties of the target: in the case of the metallic target, the maximum of the OH density was found to be in the center between the exit of the glass tube and the target, whereas in the case of the dielectric target, the maximum was found to be close to the surface of the target. In the case of ultra-pure water, the densities were equally distributed between the exit of the glass tube and the target. With this work, the influence of a target on the production of OH radicals was demonstrated, which enables the optimisation of the plasma parameters for an efficient production of OH radicals in front of a target with various dielectric properties.

The generation of arcs in air at atmospheric pressure induced by static voltage and triggered by x-ray pulses is analyzed. A static voltage is set between a cathode and an anode, and the arcing process is triggered by irradiating the air gap of a pin-to-plane switch with an x-ray photon pulse in the MeV range. This x-ray pulse produces a weakly ionized non-equilibrium air plasma (called the pre-plasma), which reduces the breakdown threshold. The influence of this pulsed x-ray on the arcing process is analyzed. The reduction of the breakdown voltage has been quantified, and for low static voltages, the creation of a sheath that is responsible for a delay in the arcing process is highlighted. Electrical measurements (currents and voltages) and optical emission spectroscopy have been performed to characterize the arcs in terms of electron density, temperature, and electric conductivity. A good agreement between measurements and 3D Maxwell calculations is achieved, which allows us to determine the electric conductivity of the arc in both self-triggered and x-ray-triggered regimes.

A low-temperature plasmas jet is generated by a dielectric barrier discharge powered by a pulsed high voltage in helium flow (3 L min−1) at atmospheric pressure in the presence of different targets (a glass slide, ultra-pure water and a grounded metal plate) positioned perpendicular to the plasma propagation axis. Experimental electrical characterizations such as discharge current, voltage and power, and optical characterizations such as intensified camera ICCD, Schlieren imaging and emission spectroscopy to follow specific excited species have been achieved. The transition from laminar to turbulent regime was observed during the discharge ignition with a larger spreading of the plasma on the surface target with lower dielectric permittivity. The generation of two discharges during each voltage pulse is highlighted during the propagation of the ionization wave which has a variable speed along the plasma axis not depending on the target kind. The evolution of some active species (such as OH, O and excited nitrogen and helium) are investigated using time resolved mapping of the emissions of radiative excited species propagating in ambient air between the plasma jet output and the target. For a low relative permittivity target (glass), the volume ionization wave at its arrival on the target spreads on its surface thus behaving as a surface ionization wave. For the highest relative permittivity (metal), a conductive channel appears between the target surface and the plasma jet during the first discharge, followed by a diffuse plasma plume from the target surface towards the plasma jet after the impact of the ionization wave on the target. A hybrid behavior is highlighted for the ultra-pure water which leads to a short spreading of the ionization wave on the target surface, the formation of a conductive channel in ambient air between tube output and target and the formation of a plasma plume on the target surface.

The spatial and temporal evolution of the ozone density has been measured in a 1 cm electrode gap tip to plane positive pulsed corona discharge in air at atmospheric pressure with no gas flow. It was expected to observe a higher ozone density in the vicinity of the discharge gap but ozone has been found at a quasi uniform density level up to 10 cm from the electrode gap after only 30 s of ignition. Such a wide distribution of ozone in the reactor cell is attributed to strong turbulence mostly triggered by the ionic wind. The mapping of the ozone density associated to the power measurement led to the estimation of the ozone production yield in term of mass per electrical energy.

This work is devoted to experimental analyses of plasmas induced by electrostatic discharges in dielectric materials. Electrostatic discharges are produced in polytetrafluoroethylene (Teflon) and polyethylene samples irradiated by a 6-MeV electron beam generated by a linear accelerator facility. The time and space evolution of the conductivity of the plasma is determined by microwave transmission measurements across a cavity followed by comparison with 3D Maxwell calculations. Furthermore, plasma parameters such as average electron energies and densities of electrons and neutral background species are determined from a 0D collisional radiative model. This analysis infers a plasma expansion velocity of about 150 km/s and a maximum electric conductivity of about 40 S/m. The electron density is estimated to be about 1012 cm−3, and electron average energies are up to 60 eV, while neutral species densities do not exceed 1018 cm−3.

This work is devoted to the characterization of plasmas produced by an intense pulsed relativistic electron beam propagating in air at atmospheric pressure. A large range of a time integrated dose is investigated [0.75 to 7.4] kGy(air) inducing electron densities from 2 × 1012 up to 1.6 × 1014 cm−3. The air plasmas are analyzed by two complementary diagnostics: microwave absorption and optical emission spectroscopy, to deduce plasma parameters during the creation and relaxation phases, respectively. A reduced air kinetics model is used to calculate transmission measurements during the electron beam pulse and to infer electric conductivities, which are compared to the results of three-dimensional Maxwell calculations. Optical emission spectra are measured and compared to calculated ones, giving rotational and vibrational temperatures during the relaxation phase of the plasma. These plasma characterizations may be used to model non-equilibrium atmospheric air plasma chemistry in the framework of, for instance, streamer dynamics as well as corona or dielectric barrier discharges.

We elaborate a reduced kinetics model to study humid air plasmas at atmospheric pressure generated by X-ray irradiation. The originality of the present approach is to use the experimental results of the transmission measurements, in the case of a microwave signal by the X-ray-induced plasma filled waveguide, to fit the calculated time evolutions of some plasma parameters such as average electron energies and an effective loss coefficient. The reduced kinetics model used to restitute the transmission measurements is based on the solution of a one-dimensional transport of a guided microwave signal coupled to the calculation of the complex electric conductivity of the plasma. The conductivity is calculated using a simplified kinetics scheme based on three species (electrons, positive ions, and negative ions) and coupled to the electron energy balance equation. The input parameters of the model are the collision cross sections of the electrons impact with air molecules (N-2, O-2, and H2O) and the electron energy distribution functions pre-tabulated for a large set of average electron energies. The latter takes into account the main processes leading to the decrease of average electron energies. This model is more generally usable for the modelling of weakly ionized atmospheric air plasmas during, for instance, the streamer development in corona or dielectric barrier discharges.

Electron-caused electromagnetic pulse (ECEMP) is susceptible to occur into constitutive dielectrics of satellites during their mission due to natural space ionizing environment. This paper reports the numerical and experimental characterizations of the electromagnetic fields subsequently generated by the electrostatic discharges (ESDs) from the analysis of the resonant cavity modes and the replacement currents. ESDs are produced when the polytetrafluoroethylene (PTFE or Teflon) samples are irradiated by a 6-MeV electron beam generated by the linear accelerator (LINAC) facility. The measured electromagnetic fields and currents are reproduced by a parametric analysis based on the 3-D Maxwell solution. Such a reproduction of the measured currents and magnetic fields allowed us to quantify the temporal and spectral characteristics of the ESD. It is shown that ESD is produced by bulk currents in the dielectric composed by a fast contribution (rise time lower than 100 ps) and a slower one (of about 200-ns duration).

This paper describes a study evidencing the creation of a corona discharge induced by natural high voltage found above the Earth’s surface due to the vertical potential gradient at an altitude of 125 meters. Experiments were made in fine weather in autumn and with little wind, on the Balloon Generali, a tethered helium balloon that takes tourists up for views above Paris. The measuring equipment was installed in the gondola suspended from the tethered balloon. A metal point electrode was also suspended from the balloon at the same level as the gondola at a lateral distance of 8 m. During the ascent, as soon as the altitude of the gondola exceeded about 65 m, a current coming from the metal point electrode was detected. The value of this current fluctuated but generally increased with height and reached an intensity of up to 320 nA at 125 m. When the electrode tip approached the gondola, which was electrically grounded via the pull cable, the current was lower; when the tip was replaced by a larger rounded electrode, the current was observed to disappear. The combination of these observations clearly demonstrated the presence of a corona discharge caused by the vertical potential gradient, and in turn, the existence of a corona discharge without high voltage power supply evidenced the obtaining of a natural high voltage.

In order to investigate the effects of low temperature plasmas on germination of Arabidopsis thaliana seeds, a dielectric barrier discharge device generating the plasma in ambient air was used. To highlight the different plasma effects on the seed surface, saline and osmotic stresses were considered in the case of reference Col-0 seeds and two further seed coat mutants gl2 and gpat5 to better analyse the seed surface changes and their consequences on germination. The GL2 gene encode a transcription factor controlling the balance between the biosynthesis of fatty acids in the embryo and the production of mucilage and flavonoid pigments in the seed coat. The GPAT5 gene encode for an acyltransferase necessary for the accumulation of suberin in the seed coat which is essential for the embryo protection. The testa and endosperm ruptures are identified to note the germination stage. An increasing of germination rate, possibly due to the modification of mantle layers structure, is observed in most of cases, even in presence of saline or osmotic stress, after plasma treatment. Furthermore, we demonstrated that the germination rate of the gl2 mutant seeds is increased by at most 47% after plasma treatment, contrariwise, the germination of gpat5 mutant being initially lower is inhibited by the same plasma treatment. The scanning electron microscopy pictures and confocal microscopy fluorescence both showed changes of the exterior aspects of the seeds after plasma treatment. Considering these results, we assumed that lipid compounds can be found on the surface. To validate this hypothesis, permeability tests were performed, and it was clearly shown that a permeability decrease is induced by the low temperature plasma treatment.

A complex OD collisional-radiative model (CRM) experimentally validated was used to describe in details the time evolution of the densities of all the species present in a non-equilibrium plasma generated in a background gas mixture composed by 64%H + 31%O + 5%C. This initial CRM involving 1050 reactions and 79 species is gradually reduced in term of number of reactions and species in order to be easily implemented in LSP particle-in-cell (PIC) simulations and to optimize the characteristics of an electron beam generated by a surface flashover on a velvet cathode. The reduction was done using comparisons between the electron densities calculated with the complete initial set of reactions and species. The reduction was successfully performed in three steps leading to a first reduced CRM (99 reactions and 26 species) followed by a second reduced CRM (nine reactions and eight species) and a third CRM (seven reactions and four species). The latter is obtained by lumping all the hydrogen excited states in a single fictitious sate, the spontaneous emission frequencies of these excited states in a single fictitious frequency and the three atoms of the gas mixture onto one single fictitious atom. The electron density evolution is well reproduced even with the most reduced CRM (third CRM) that leads to a mean relative error of about 3% without never exceeding 11%.

A high power electron diode is used to generate an intense high quality pulsed electron beam (2.6 kA, 4 MeV) in order to produce a bremsstrahlung radiation pulse for the first axis of the flash radiography facility named Epure. The plasma created by surface flashover of the velvet cathode provides a zero work function emitter used to produce the space-charge-limited electron beam. In order to optimize the design of diodes and to better understand the plasma dynamics, we have developed a 0D Collisional-Radiative Model (CRM) which describes the time-dependent evolution of the density of the plasma species in three plasma compositions: 100% H, 95% H + 5% C, and 64% H + 31% O + 5% C. The non-equilibrium electron energy distribution functions (EEDFs) are pre-calculated for a large range of electron mean energies for each mixture. EEDFs and cross sections of electron impact processes are then used to tabulate the electron rate coefficients needed to simulate the time-dependent plasma species densities. The evolution of the electron mean energy is estimated through the electron beam current density itself inferred from particle-in-cell calculations experimentally validated. Visible emission spectroscopy measurements were performed in order to investigate the plasma composition, to measure the Hα/Hβ intensity ratio, and to estimate the electron density. It was shown that the electron density in our setup is between 1014 and 1016 cm−3. Experimental results were compared to the 0D CRM. Results from the time-dependent CRM show that for the three studied plasma compositions, maximum electron densities range from 0.9 × 1014 cm−3 to 1.7 × 1014 cm−3 and that the electron energy averaged over the time interval of the electronic beam pulse is about 3.5 eV. An estimate of the electron energy inside the plasma during the relaxation provides values ranging from 0.10 eV to 0.12 eV based on comparison between the calculated and measured Hα/Hβ intensity ratio averaged over 5 μs. Moreover, main reactions involved in the evolution of the electron density were pointed out.

This paper concerns the 3D simulation of corona discharge using high performance computing (HPC) managed with the message passing interface (MPI) library. In the field of finite volume methods applied on non-adaptive mesh grids and in the case of a specific 3D dynamic benchmark test devoted to streamer studies, the great efficiency of the iterative R&B SOR and BiCGSTAB methods versus the direct MUMPS method was clearly demonstrated in solving the Poisson equation using HPC resources. The optimization of the parallelization and the resulting scalability was undertaken as a function of the HPC architecture for a number of mesh cells ranging from 8 to 512 million and a number of cores ranging from 20 to 1600. The R&B SOR method remains at least about four times faster than the BiCGSTAB method and requires significantly less memory for all tested situations. The R&B SOR method was then implemented in a 3D MPI parallelized code that solves the classical first order model of an atmospheric pressure corona discharge in air. The 3D code capabilities were tested by following the development of one, two and four coplanar streamers generated by initial plasma spots for 6 ns. The preliminary results obtained allowed us to follow in detail the formation of the tree structure of a corona discharge and the effects of the mutual interactions between the streamers in terms of streamer velocity, trajectory and diameter. The computing time for 64 million of mesh cells distributed over 1000 cores using the MPI procedures is about 30 min ns(-1), regardless of the number of streamers.

Two plasma devices at atmospheric pressure (air dielectric barrier discharge and helium plasma jet) have been used to study the early germination of Arabidopsis thaliana seeds during the first days. Then, plasma activated waters are used during the later stage of plant development and growth until 42 days. The effects on both testa and endospserm ruptures during the germination stage are significant in the case of air plasma due to its higher energy and efficiency of producing reactive oxygen species than the case of helium plasma. The latter has shown distinct effects only for testa rupture. Analysis of germination stimulations are based on specific stainings for reactive oxygen species production, peroxidase activity and also membrane permeability tests. Furthermore, scanning electron microscopy (SEM) has shown a smoother seed surface for air plasma treated seeds that can explain the plasma induced-germination. During the growth stage, plants were watered using 4 kinds of water (tap and deionized waters activated or not by the low temperature plasma jet). With regards to other water kinds, the characterization of the tap water has shown a larger conductivity, acidity and concentration of reactive nitrogen and oxygen species. Only the tap water activated by the plasma jet has shown a significant effect on the plant growth. This effect could be correlated to reactive nitrogen species such as nitrite/nitrate species present in plasma activated tap water.

We present and compare six simulation codes for positive streamer discharges from six different research groups. Four groups use a fully self-implemented code and two make use of COMSOL Multiphysics®. Three test cases are considered, in which axisymmetric positive streamers are simulated in dry air at 1 bar and 300 K in an undervolted gap. All groups use the same fluid model with the same transport coefficients. The first test case includes a relatively high background density of electrons and ions without photoionization. When each group uses their standard grid resolution, results show considerable variation, particularly in the prediction of streamer velocities and maximal electric fields. However, for sufficiently fine grids good agreement is reached between several codes. The second test includes a lower background ionization density, and oscillations in the streamer properties, branching and numerical instabilities are observed. By using a finer grid spacing some groups were able to reach reasonable agreement in their results, without oscillations. The third test case includes photoionization, using both Luque's and Bourdon's Helmholtz approximation. The results agree reasonably well, and the numerical differences appear to be more significant than the type of Helmholtz approximation. Computing times, used hardware and numerical parameters are described for each code and test case. We provide detailed output in the supplementary data, so that other streamer codes can be compared to the results presented here.

We investigated a new cathode design and beam transport trough the EPURE axis 1 injector in order to increase the beam intensity from the nominal 2.0 kA to 2.6 kA by using a larger size velvet cathode. For this purpose we developed a time-dependent model based on Particle-In-Cell method in order to simulate the inj ector. A qualitative comparison of experimental and calculated results is made. The study provides a detailed understanding of the beam dynamics inside this type of high current, high energy inj ector [1].