Génération et propagation de champs électromagnétiques dans des gaz et diélectriques irradiés

Les travaux de recherche restitués dans ce document ont été effectués sur la période [2010 – 2020] au CEA-Gramat, et traitent des effets des rayonnements ionisants sur les systèmes électroniques. Plus précisément, des études ont été menées sur la génération et la propagation de champs électromagnétiques induits par l’interaction de flux de rayonnements ionisants intenses avec des milieux gazeux et diélectriques. Les applications concernées par ces études portent sur la vulnérabilité de systèmes électroniques (militaires et civils) soumis à des flux de rayonnements ionisants pouvant être d’origine naturelle (comme l’environnement spatial) ou nucléaire. Les effets produits par ces contraintes sur les systèmes électroniques sont quantifiés par simulations numériques hautes performances au moyen des codes de calculs Monte-Carlo et Maxwell-Vlasov Particle-In-Cell fonctionnant sur les supercalculateurs en service au CEA/DAM. Les résultats de ces simulations numériques sont comparés à des mesures de grandeurs radiatives et électromagnétiques obtenues à l’issue de la mise en oeuvre de grands instruments de physique expérimentale, plus précisément des générateurs de Marx et des accélérateurs linéaires. Une part importante de ces travaux a donc consisté à quantifier les termes sources produits par ces générateurs d’une part, ainsi qu’à développer les diagnostics permettant l’analyse des phénomènes d’interaction champs / milieux ionisés d’autre part. Des modèles ont été développés pour prendre en compte les phénomènes non-linéaires induits par l’irradiation à haut flux de gaz et de diélectriques. Les perspectives associées à ces travaux portent sur le couplage de ces modèles avec les codes de transport Particle-In-Cell hautes performances du CEA/DAM

Spectroscopies d'émission et d'absorption appliquées à l'analyse de plasmas produits par laser

This work has been performed in the purpose of probing solid surfaces by using LIPS (laser induced plasma) technique. The behavior of the produced plasma is very difficult to simulate owing to the shortness of the involved time and space scale. Besides classical emission spectroscopy diagnostics, we have put into operation an original absorption experiment which provides more reliable and numerous informations about the plasma temperature and number densities. This study, which couples experiment and theory through the resolution of the radiative transfer equation, is based on the comparison between the experimental and the simulated profiles of the 308.21 and 396.15 nm Al(I) resonant lines. Special attention has been turned to the ion and electron Stark broadening and self-absorption effects. An important departure from equilibrium is put into evidence throughout the plasma recombination. The measurement of the aluminum fundamental state which is of prime importance in the validation of any collisional-radiative model dedicated to this complex situation has been carried out in a large pressure range (5 to 1E5 Pa). The measurement of the relaxation times highlights the major role played by the diffusion in the relaxation of the plasma species.Ce travail a été réalisé dans le but de sonder des surfaces solides en utilisant la technique « LIPS » (Spectroscopie de plasmas induits par laser). Le comportement de ce type de plasmas est difficile à simuler étant donné la petitesse des échelles de temps et d'espace considérées. En plus du diagnostic classique de spectroscopie d'émission, nous avons mis en oeuvre une expérience originale d'absorption qui produit des valeurs fiables et de nombreuses informations sur la température et les densités constitutives du plasma. Cette étude, qui couple expérience et théorie dans le cadre de la résolution de l'équation de transfert radiatif, est basée sur la comparaison entre des spectres calculés et expérimentaux des raies à 308.21 et 396.15 nm de l'aluminium. Une attention spéciale a été portée à l'écriture de l'élargissement électronique et ionique par effet Stark et aux effets d'auto-absorption. Un écart important à l'équilibre a été mis en évidence tout au long de la relaxation du plasma. La mesure de la densité de l'état fondamental de l'aluminium, qui est de première importance pour valider un code collisionnel - radiatif consacré à cette situation complexe, a été menée à bien dans une large gamme de pression (5 à 1E5 Pa). La mesure des constantes de temps de relaxation met en évidence le rôle majeur joué par la diffusion dans la relaxation des espèces du plasma

Influence of electron beam spectra on internal electrostatic discharge in Dielectrics

International audienc

Characterization of Electrostatic Discharges induced Plasmas in dielectrics irradiated by multi-MeV electron beam

International audienceThis 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

Improving sample preheating capabilities for dynamic loading on high-pulsed power drivers

International audienceThe CEA operates several High-Pulsed Power (HPP) drivers for dynamic loading experiments. The aim of these experiments is to provide quantitative information about the response of various materials of interest, mainly under quasi-isentropic compression. In order to improve our ability to explore these materials’ behavior over a wide range of thermodynamic paths and starting from various non-ambient conditions, we developed a device capable of pre-heating both metallic and nonmetallic samples up to several hundred degrees prior to loading. This device is based on conductive heating and on a configuration that allows homogeneous heating with unprecedented temperature stability on our HPP platforms. Moreover, it is designed to allow efficient sample heating, within extremely severe electromagnetic environments associated with such platforms. The main features of this preheating device, whose design was guided by extensive thermal simulations, are presented, along with various technical solutions that enabled its insertion in a reliable experimental configuration on our HPP drivers. The results obtained from preliminary experiments on a composite material (carbon fibers embedded in epoxy resin) and on a high purity copper sample preheated to 323 K and 573 K, respectively, are presented. The performance and robustness of this heating device are potentially valuable for extending the range of studies in dynamic loading experiments for various materials under ramp compression using HPP drivers

Femtosecond coherent pulses in the keV range from inner-shell transitions pumped by a betatron source

International audienceWe present a new method to generate ultra-short X-ray laser pulses by using the recently demonstrated laser-driven betatron source to photo-pump inner-shell transitions. The proposed compact set-up will then open the route to a wide range of applications. The betatron spectrum and ion-population kinetics are modeled and the temporal evolution of the gain coefficient for the K-α transitions is assessed. Using measured values of divergence, duration, and number of photons per pulse of the betatron source as input parameters, local gain values close to 60 cm^−1 are calculated for nitrogen at 3.2 nm. Significant gain values are also numerically obtained at shorter wavelengths (for neon at 1.5 nm) when the betatron energy distribution is optimized as suggested by recent laser wakefield electron acceleration experiments

Pump requirements for betatron-generated femtosecond X-ray laser at saturation from inner-shell transitions

International audienceWe study pump requirements to produce femtosecond X-ray laser pulses at saturation from inner-shell transitions in the amplified spontaneous emission regime. Since laser-based betatron radiation is considered as the pumping source, we first study the impact of the driving laser power on its intensity. Then we investigate the amplification behavior of the K-a transition of nitrogen at 3.2 nm (395 eV) from radiative transfer calculations coupled with kinetics modeling of the ion population densities. We show that the saturation regime may be experimentally achieved by using PW-class laser-accelerated electron bunches. Finally, we show that this X-ray laser scheme can be extended to heavier atoms and we calculate pump requirements to reach saturation at 1.5 nm (849 eV) from the K-a transition of neon. © Springer-Verlag 2012

An optimized kHz two-colour high harmonic source for seeding free-electron lasers and plasma-based soft x-ray lasers

International audienceFree-electron lasers (FEL) and plasma-based soft x-ray lasers (PSXL) have been recently evolving very fast from the vacuum ultraviolet to the soft x-ray region. Once seeded with high harmonics, these schemes are considered as the next generation soft x-ray light sources delivering ultrashort pulses with high temporal and spatial coherence. Here, we present a detailed experimental study of a kHz two-colour high harmonic generation performed in various gases and investigate its potential as a suitable evolution of the actual seeding sources. It turns out that this double harmonic content source is highly tuneable, controllable and delivers intense radiation (measured here with a calibrated photodiode) with only one order of magnitude difference in the photon yield from 65 to 13 nm. Then, first and foremost, injections could be achieved at wavelengths shorter than what was previously accessible in FEL and PSXL and/or additional energy could be extracted. Also, such a strong and handy seed could allow the saturation range of FEL devices to be greatly extended to shorter wavelengths and would bring higher spectral as well as intensity stabilities in this spectral zone

Fourier-limited seeded soft x-ray laser pulse

International audienceWe present what we believe to be the first measurement of the spectral properties of a soft x-ray laser seeded by a high-order harmonic beam. Using an interferometric method, the spectral profile of a seeded Ni-like krypton soft x-ray laser (32.8 nm) generated by optical field ionization has been experimentally determined, and the shortest possible pulse duration has been deduced. The source exhibits a Voigt spectral profile with an FWHM of 3.1±0.3 mÅ, leading to a Fourier-transform pulse duration of 4.7 ps. This value is comparable with the upper limit of the soft x-ray pulse duration determined by experimentally investigating the gain dynamics, from which we conclude that the source has reached the Fourier limit. The measured bandwidth is in good agreement with the predictions of a radiative transfer code, including gain line narrowing and saturation rebroadening

Aberration-free high-harmonic source generated with a two-colour field

International audienceImaging experiments of ultrafast phenomena of matter at nanometre-scale require intense, short pulse duration and diffraction-limited soft–X-ray beams, nowadays almost only provided by free-electron lasers. Here, we focused on a table-top soft–X-ray source, which fulfils these fundamental criteria and in addition presents high temporal coherence, the high harmonics generated with a two-colour field (ω+2ω). These harmonics revealed to be free from aberration just by slightly spatially filtering the laser used for generation (ω). Indeed, the measured wavefront distortions, equal to λ/17 RMS at 44 nm, correspond to a diffraction-limited beam. This behaviour is explained by an additional spatial nonlinear filtering effect of the driving laser wavefront, induced here in our particular but simple geometrical configuration of generation by the 2ω component