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International audience; Elemental analyses of thin films and optical surfaces with complex compositions are challenging as the standard analytical techniques based on measurement calibration are difficult to apply. In this chapter, we show that calibration-free laser-induced breakdown spectroscopy presents a powerful solution, enabling rapid quantitative analyses of multi-elemental thin films or tiny surface volumes of optical materials with high analytical performances. Examples are given for classically polished optical glass and a nickel-chromium-molybdenum alloy film of 150 nm thickness. Thus, broadband spectra, recorded under experimental conditions that enable simple and accurate modeling of plasma emission, are processed via a calibration-free approach based on the calculation of the spectral radiance of a plasma in local thermodynamic equilibrium to deduce the elemental composition. The validation via energy-dispersive X-ray spectroscopy, inductively coupled plasma atomic emission spectroscopy, and Rutherford backscattering spectrometry illustrates that calibration-free LIBS is not only a promising tool, but already a reliable analytical technique.

International audience; The purpose of this chapter is to give an introductive overview on the so-called calibration-free laser-induced breakdown spectroscopy that enables straightforward compositional analysis of materials via modeling of the plasma emission spectrum, without any need of measurement calibration with standard samples. The proposed physical models, their validity conditions, and the experimental requirements are discussed. Based on results reported in literature, a critical review of the analytical performance is given. Finally, the remaining challenges are outlined, and a tentative picture of the perspectives in terms of further improvements and potential applications is drawn.

International audience; The breakdown voltage (Vb) of gases can be reasonably estimated using Paschen's law, which is based on assumptions such as a perfectly uniform electric field, and infinite electrode size. This analytical approach has demonstrated remarkable capability in redicting Vb in numerous cases, by scaling it by the product of pressure and nter-electrode distance (p·d). This description is quite efficient and convenient over wide ranges of pressure and gap distance. However, the robustness of Paschen's law in predicting consistent values of Vb is severely compromised at gap distances shorter than typically 1 micrometer at atmospheric pressure. Several studies have provided experimental, theoretical, and simulation evidences highlighting these limitations. This raised significant concerns, both theoretical and practical, about the insulation properties of gases at short distances. This study focuses on experimentally investigating Vb of air in short electrode gaps, ranging from 100 nm to 6 μm. The electrode system consists of a silicon wafer coated with gold, connected to ground. The anode connected to high voltage is a sharp CuBe needle coated with gold, with 20 μm tip radius of curvature. The gap distance is controlled by a piezoelectric actuator. Ambient air is used at atmospheric pressure, allowing for comparisons with data from existing literature. Complementary experiments are also performed in a closed chamber allowing to vary the pressure from 10-4 mbar to 3.0 bar, and control the gas purity and water content.

The solution of Poisson-like equations defined on complex geometry is required for gyrokinetic simulations, which are important for the modelling of plasma turbulence in nuclear fusion devices such as the ITER tokamak. In this paper, we compare three solvers capable of solving this problem, in terms of the accuracy of the solution, and their computational efficiency. The first, the Spline FEM solver, uses C 1 polar splines to construct a finite elements method which solves the equation on curvilinear coordinates. The resulting linear system is solved using a conjugate gradient method. The second, the GmgPolar solver, uses a symmetric finite differences method to discretise the differential equation. The resulting linear system is solved using a tailored geometric multigrid scheme, with a combination of zebra circle and radial line smoothers, together with an implicit extrapolation scheme. The third, the Embedded Boundary solver, uses a finite volumes method on Cartesian coordinates with an embedded boundary scheme. The resulting linear system is solved using a multigrid scheme. The Spline FEM solver is shown to be the most accurate. The GmgPolar solver is shown to use the least memory. The Embedded Boundary solver is shown to be the fastest in most cases. All three solvers are shown to be capable of solving the equation on a realistic non-analytical geometry. The Embedded Boundary solver is additionally used to attempt to solve an X-point geometry, highlighting the problems with concave boundaries.

The present paper is concerned with the derivation, via asymptotic studies, of a reduced hybrid model describing the anisotropic fusion plasma dynamics in tokamaks. The parallel dynamics is governed by a kinetic equation, whereas the perpendicular dynamics is described by a Maxwellian distribution function, whose temperature T ⊥ satisfies an evolution equation, exchanging information with the parallel direction via some coupling terms. The reduced model is obtained from the underlying fully kinetic model, under the assumption of a strong magnetic field and strong collisionality in the perpendicular direction. From a numerical point of view, reduced models are very advantageous, permitting significant savings in computational times and memory. To improve the precision of the reduced description, we propose in this paper also first order correction terms with respect to the parameter describing the anisotropy, and discuss these terms from a physical point of view. This first order truncated model is new to our knowledge, meets the desired requirements of precision and efficiency, and its derivation is clearly exposed in this work, based on formal asymptotic studies.

The effect of divertor closure and nitrogen on the detachment process has been studied by performing 2D numerical simulations of TCV H-mode divertor scenarios with SOLEDGE3X-EIRENE edge plasma transport code. The outcomes reveal that: In the cases with only deuterium gas fuelling, there exists a divertor neutral pressure threshold about 0.76 Pa for detachment, despite the difference in divertor closure. Nitrogen can cool the target temperature with little effect on the upstream density and momentum loss but drops the upstream pressure, leading to a decrease in the target particle flux and divertor neutral pressure. Furthermore, when the radiation front starts to move up from the outer target, the peak parallel heat flux level at the outer target remains approximately the same (about 2.3 MW/m²), despite the difference in nitrogen seeding and divertor closure. An empirical partial detachment qualifier calibrated on AUG experimental data was compared with the TCV simulations. The results show good agreement in detachment state prediction, indicating the potential of this detachment qualifier to be applied in devices of different sizes, emphasizing the combined influence of input power entering the divertor, neutral pressure, and the concentration and species of impurity in achieving divertor detachment.

International audience; Coherent time-domain spectroscopy (TDS) using terahertz radiation is valuable for fundamental science, security, and medical applications. This study investigates the performance of air-biased coherent detection terahertz spectroscopy (ABCD-THz) when an extended plasma filament is created in the air over long distances. We report on the latest results obtained within the follow-up of the ALTESSE project (Bergé L. et al ., EPL , 126 (2019) 24001) whose objective is to measure a set of spectral signatures characterizing suspicious materials over meter-long distances. As one of the most critical steps towards routinely applying this technique, we verified the feasibility of a remote THz time-domain spectroscopy by loosely focusing two-color ultrashort laser pulses at more than 3 meters from the laser source. The absorption spectra of amino acids and explosives analyzed in such a filamentation geometry are compared with those obtained using a standard ABCD scheme where the plasma is generated at much shorter distances of ∼ 30 cm.

Event: SPIE Optics + Optoelectronics, 2023, Prague, Czech Republic; High-harmonic generation (HHG) in gaseous media is a workhorse tool in attosecond science. Its description origins from the microscopic scale of a single atom or molecule interacting with a driving IR-laser pulse. However, its practical realization usually employs a macroscopic volume of the gas. The macroscopic aspect aggregates all the microscopic emitters and brings novel physical mechanisms that drive the generation. One of the key mechanisms within the macroscopic scale is the shaping of the driving pulse due to the non-linear response of the medium. We present a comprehensive numerical model describing and coupling the physics on both scales. The model consists of different modules that provide different levels of approximation to choose an optimal trade-off between accuracy and computational cost. We then use it to address two generation schemes, where we provide a detailed picture together with experimental realizations. The first scheme uses a long medium homogeneously pre-ionized by an electrical discharge to optimize the phase-matching of the harmonic signal. This scheme allows, in particular, for optimizing HHG in long media where the control is difficult because the driving pulse undergoes strong reshaping and defocusing due to the non-linear response after the entrance to the medium. The second scheme introduces a mechanism to control the divergence of the harmonic beam in thin targets. The divergence is driven by shaping the wavefront of the driving pulse. This allows for spectrally selective focusing of the harmonic beams without the use of optics, which leads to inevitable losses in the XUV region.

In the present work we present a general framework which guarantees the existence of optimal domains for isoperimetric problems within the class of $C^{1,1}$-regular domains satisfying a uniform ball condition as long as the desired objective function satisfies certain properties. We then verify that the helicity isoperimetric problem studied by Cantarella, DeTurck, Gluck and Teytel in \cite{CDGT002} satisfies the conditions of our framework and hence establish the existence of optimal domains within the given class of domains. We additionally use the same framework to prove the existence of optimal domains among uniform $C^{1,1}$-domains for a first curl eigenvalue problem which has been studied recently for other classes of domains in \cite{EGPS23}., 11 pages, comments welcome!

This paper explores a semi-supervised object detection approach to detect hot spots on the internal wall of Tokamaks. A huge amount of data is produced during an experimental campaign by the infrared (IR) viewing systems used to monitor the inner thermal shields during machine operation. The amount of data to be processed and analysed is such that protecting the first wall is an overwhelming job. Automatizing this job with artificial intelligence (AI) is an attractive solution, but AI requires large labelled databases which are not readily available for Tokamak walls. Semi-supervised learning (SSL) is a possible solution to being able to train deep learning models with a small amount of labelled data and a large amount of unlabelled data. SSL is explored as a possible tool to rapidly adapt a model trained on an experimental campaign A of Tokamak WEST to a new experimental campaign B by using labelled data from campaign A, a little labelled data from campaign B and a lot of unlabelled data from campaign B. Model performances are evaluated on two labelled datasets and two methods including semi-supervised learning. Semi-supervised learning increased the mAP metric by over six percentage points on the first smaller scale database and over four percentage points on the second larger scale dataset depending on the employed method.

International audience; The quality of electron bunches accelerated by laser wakefields is highly dependant on the temporal and spatial features of the laser driver. Analysis of experiments performed at APOLLON PW-class laser facility shows that spatial instabilities of the focal spot, such as shot-to-shot pointing fluctuations or asymmetry of the transverse fluence, lead to charge and energy degradation of the accelerated electron bunch. It is shown that PIC simulations can reproduce experimental results with a significantly higher accuracy when the measured laser asymmetries are included in the simulated laser's transverse profile, compared to simulations with ideal, symmetric laser profile. A method based on a modified Gerchberg-Saxton iterative algorithm is used to retrieve the laser electric field from fluence measurements in vacuum in the focal volume, and accurately reproduce experimental results using PIC simulations, leading to simulated electron spectra in close agreement with experimental results, for the accelerated charge, energy distribution and pointing of the electron beam at the exit of the plasma.

THe WEST Team: http://west.cea.fr/WESTteam; International audience; Plasma facing components temperature measurement is required to ensure safe high power for long pulse tokamak operation and for physics studies. During the WEST phase 1, a set of 20 thermocouples (TC) and 4 fibers Bragg grating (FBG) have been exposed to five experimental campaigns with 3889 plasmas for a total plasma duration of about 7h15 and 21.2 GJ of cumulated injected energy. This paper presents the performances in terms of response time and high temperature sustaining assess with high heat flux tests performed in the HADES facility. The response time of the TC remains at the same level as before their installation in the divertor. However, the FBG have shown clear increase of their response time depending on the location due to a degradation of the thermal contact of the graphite adhesive and the FBG or component. Finally, one regenerated FBG has been exposed to high temperature, showing that absolute temperature measurement is found reliable up to 1100 °C with the start of some spectrum degradations. Full erasure of the grating is obtained after 10 s at 1200 °C well above the stable temperature of 900 °C of the regenerated FBG.

International audience; A. In the present paper, an efficient General Purpose Graphical Processing Unit (GPGPU)-based implementation of sparse grid Particle-In-Cell (PIC) methods is proposed. The parallelization, implementing novel strategies specific to Sparse-PIC methods and tailored to GPU architectures, provides speed-ups* as large as 100 on a single Tesla V100 GPU, with respect to sequential Computing Processing unit (CPU) execution; and a four order of magnitude reduction of the computational time in comparison with a standard PIC sequential CPU simulation. In addition, the simple implementation of the parallelization with the OpenACC framework offers portability to a large class of accelerators.

International audience; The main objective of WEST is to study the behavior of the ITER-like divertor monobloc components, test their resistance and ageing under ITER relevant heat loads.For the upcoming phase 2 of WEST, the lower divertor is now fully equipped with the ITER-like actively cooled bulk tungsten Plasma Facing Units (PFU), designed to withstand 10MW/m2 in steady state. This divertor is also equipped with an enlarged set of ThermoCouples (TCs x 16) and Fiber Bragg Gratings (FBGs) probes (x5) with 14 measurement spots each to evaluate the heat loading during the experiments. This paper presents the new TC and FBG diagnostics as well as the associated inverse method used to estimate the surface heat flux from the embedded temperature measurements in the divertor. The performance of each diagnostic will be evaluated with synthetic measurements generated with realistic heat flux intensity and distribution. The results of a parametrical analysis shows that each diagnostic is able to estimate the spatial distribution of the expected heat fluxes in WEST

WEST database analysis shows a correlation of the recycled neutral source around the separatrix with core performances. This observation questions the causality chain between particle source and turbulent transport up to the core in L-mode, high recycling plasmas, an unavoidable phase of all scenarios. The best core performances correlate with the lowest values of the density at the separatrix, n sep , similarly to ASDEX Upgrade (AUG) tokamak and Joint European Torus (JET) tokamak in H-mode (Verdoolaege et al 2021 Nucl. Fusion 61 076006). Reflectometry in the midplane provides n sep , while the temperature at the separatrix, T sep is inferred by the ‘two-point model’ using Langmuir probe data on divertor targets. Lower separatrix resistivity does not correlate with better core performances, unlike H-mode observations (Eich et al 2020 Nucl. Fusion 60 056016). As expected in the presence of an efficient neutral source due to recycling fluxes, n sep correlates with the D recycled particle flux at the divertor measured by visible spectroscopy. Coherently, at a given controlled central line integrated density n ˉ , lower n sep correlates with a larger density gradient around the separatrix as well as a larger global density peaking, n ˉ / ⟨ n ⟩ , measured by interferometry. The latter correlates as well with lower collisionality in the core, similarly to JET and AUG H-modes (Angioni et al 2007 Nucl. Fusion 47 1326). The correlations reported allow phrasing the subsequent causality question: what is the interplay chain between low neutral recycling at the divertor plates, low density at the separatrix, high density peaking at the separatrix, high global density peaking, higher central temperature and better core energy confinement quality? Understanding the causality chain is essential to prepare ITER operation and design DEMO scenarios where the ratio of the divertor leg to the ionization length will be larger and where the pumped flux with respect to the plasma volume will be lower than presently operating tokamaks.

A new method named B-TIS (Bourgeois & Davoine, J. Comput. Phys., vol. 413, 2020, 109426) has recently been proposed for suppressing the influence of numerical Cherenkov radiation that appears in particle-in-cell (PIC) simulation of laser wakefield acceleration (LWFA). However, while this method provides good results when applied to the already accelerated electrons, we show here that it cannot model correctly most of the plasma electron bulk interacting with the laser field. We thus investigate in this paper the origins of this limitation and propose an improved method for which this limitation is removed. This new method, named B-TIS3, can now be applied to a much broader variety of problems and improve the performance in comparison with the standard PIC algorithm. We show that, for an electron interacting directly with a laser pulse, this new technique offers greater accuracy in terms of momentum and motion than the conventional scheme used in many PIC codes. These improvements translate into more faithful energy spectrum and electric charge for the accelerated beam in simulations of vacuum laser acceleration (VLA) or LWFA involving direct laser acceleration (DLA) at low plasma density. This new method, easy to implement and not computationally demanding, should then prove useful to study in depth and help develop novel VLA, DLA and LWFA techniques.

A JOREK 3D non-linear MHD simulation of a disruption triggered by an argon massive gas injection in JET, which quantitatively reproduces the plasma current ( I p ) spike (Nardon et al 2021 Plasma Phys. Control. Fusion 63 115006), is analyzed in order to investigate the origin of the I p spike and its relation with magnetic stochasticity. The I p spike is associated to a current density (j φ ) profile relaxation which appears to result from Shear Alfvén Wave (SAW) propagation along stochastic field lines, as proposed by Boozer (2019 Plasma Phys. Control. Fusion 61 024002; 2020 Phys. Plasmas 27 102305), possibly complemented by a macroscopic E×B flow structure. Using axisymmetric JOREK simulations involving a mean field Ohm’s law, we verify that the level of hyper-resistivity associated to SAWs is consistent with the prediction made in (Boozer 2019 Plasma Phys. Control. Fusion 61 024002; Boozer 2020 Phys. Plasmas 27 102305), which connects the I p spike with the level of stochasticity. The relaxation comprises two main phases, the first one corresponding to a fast (0.1 ms) and almost complete j φ flattening in the q j φ flattening. During the first phase, strong E×B flows develop that play a key role in mixing impurities into the core.

In the next step magnetic fusion devices like DEMO, a large fraction of power, more than 95%, needs to be dissipated before the plasma impacts the dedicated regions on the wall, the so-called divertor targets. The divertor detachment is one of the solutions envisaged to protect target tiles by keeping the radiation front detached from the target, thus radiating most of the heating power on a larger surface, keeping the heat flux deposit on the target at a manageable level, and reducing the sputtering and accumulation of impurities. The present work is devoted to investigating the strategies to control detachment in the tokamak single null divertor configuration, which is planned for the International Thermonuclear Experimental Reactor (ITER) experiment presently in construction at Cadarache. The detachment must be feedback controlled in real-time to guarantee the continuous production of energy by the reactor and a long life of the plasma-facing components. To develop reliable feedback control methods, the first step is to use good control variables and actuators. With the help of edge numerical simulations and reduced models, this thesis aims to find the more appropriate control variable that will be used in a feedback scheme to adjust the actuators available in fusion reactors. 2D numerical simulations are obtained using SOLEDGE3X-EIRENE transport code with input parameters specially defined to reproduce the experimental scenarios in tokamaks, and more precisely in the TCV tokamak located in Switzerland and the WEST tokamak located in Cadarache, France. Profile feedback control has been developed as an interpretative tool to extract edge plasma transport coefficients from experimental data. Simulation results are compared with available experimental measurements and show good agreement. Modeling density scan is performed via adjusting deuterium gas puffing or impurity seeding to achieve different levels of detachment.The impacts of wall geometry (like divertor closure), impurities, and input power on detachment have been discussed. Simulation results show that the case with a more closed divertor has better performance in trapping the neutral particles and has higher neutral pressure near the target. Increasing the neutral pressure or impurity concentration in the divertor leads to greater power dissipation, thus facilitating the access to the detached regime. Both considering more closed divertor configurations and increasing nitrogen seeding will induce the divertor detachment at lower upstream separatrix density, up to 24% in the cases we have considered. The threshold values of variables like target heat flux, and radiation front height in the divertor for detachment are found to be not significantly affected by the change of divertor closure and impurities concentration. Higher input power can significantly increase the detachment threshold in upstream plasma density, divertor neutral pressure, target particle flux, etc. The dependence of numerical results from few physical quantities is also analyzed with the help of reduced models to find the observables that mostly affect the detachment process. Finally, the relative advantage of the different observables regarding detachment controllability is evaluated. A new criterion for detachment is proposed, which provides a good indicator of the optimal detachment condition in all the analyzed plasma scenarios (the situations with different levels of divertor closure, impurities concentration, and input power). Several strategies for controlling the divertor detachment have been designed based on good control variables and compared with existing control methods. The robustness of a control strategy for WEST tokamak has been investigated by numerical simulations, and it will be tested in future experiments.; Dans les machines de fusion magnétique de prochaine génération, comme DEMO, une grande partie de la puissance injectée et produite par les réactions de fusion doit être dissipée avant que le plasma n'entre en contact avec la paroi au niveau de composants appelés « cibles » dans le divertor. Le détachement du plasma est l'une des solutions envisagées pour protéger les tuiles des cibles en maintenant le front de rayonnement à une certaine distance de la cible. Ceci permet d’évacuer sous forme de rayonnement la majeure partie de la puissance de chauffage sur une plus grande surface et de maintenir le flux de chaleur à des niveaux compatibles avec leur design. Ce travail de thèse se concentre sur l'étude des caractéristiques du plasma détaché dans différentes configurations divertor de deux tokamaks : « TCV », situé en Suisse, et « WEST » situé à Cadarache, en France. Le détachement doit être contrôlé en temps réel pour garantir à la fois une production d’énergie continue par le réacteur, et une durée de vie longue pour composants face au plasma. Pour développer des méthodes de contrôle de rétroaction fiables, la première étape consiste à sélectionner les bonnes variables de contrôle et les bons actionneurs (injection de gaz, d’impureté, etc.). A l’aide de simulations numériques de bord et de modèles réduits, cette thèse vise à trouver la variable de contrôle la plus appropriée qui sera utilisée dans un système de rétroaction pour contrôler les actionneurs disponibles dans les réacteurs de fusion. Des simulations numériques 2D sont obtenues en utilisant le code de transport SOLEDGE3X-EIRENE avec des paramètres d'entrée spécialement définis pour reproduire les scénarios expérimentaux. Pour obtenir ces paramètres, une méthode de contrôle par rétroaction sur le profil du plasma a été développée comme un outil permettant de déterminer les coefficients de transport radial du plasma de bord à partir des données expérimentales. Les résultats des simulations sont comparés aux mesures expérimentales disponibles et montrent un bon accord. Les impacts de la forme géométrique de la paroi (comme le degré de fermeture du divertor), de la quantité d’impuretés injectées, et de la puissance d'entrée sont discutés. Les résultats de la simulation montrent que le cas avec un divertor plus fermé montre de meilleures performances pour piéger les particules neutres, avec une pression neutre plus élevée près de la cible. L’augmentation à la fois de la pression de neutres ou de la concentration en impuretés dans le divertor entraîne une plus grande dissipation de puissance, et facilite l'accès au régime détaché. Un divertor plus fermé, ainsi que l’injection d’azote peuvent permettre au plasma de se détacher avec une densité plasma dans la région centrale plus faible (jusqu'à 24%) dans les cas que nous avons examinés. Les valeurs seuils à partir desquelles le plasma détache pour les paramètres tels que le flux de chaleur sur la paroi et la hauteur du front de rayonnement ne sont pas significativement affectés par la modification de la fermeture du baffle ou la concentration d'impuretés. Une puissance d'entrée plus élevée peut augmenter de manière significative le seuil de détachement en termes de densité du plasma en amont, de la pression neutre du divertor, du flux de particules cibles, etc. La dépendance des résultats numériques pour quelques quantités physiques est également analysée à l’aide de modèles réduits pour trouver les observables qui affectent principalement le processus de détachement.Enfin, les avantages relatifs des différentes observables au regard la contrôlabilité du détachement sont analysés. Un nouveau critère pour le détachement est proposé, fournissant un bon indicateur de l’état de détachement optimal dans tous les scénarios de plasma analysés (les situations avec différents degré de fermeture du divertor, concentration d’impuretés, et puissance d’entrée). Plusieurs stratégies de contrôle du détachement du divertor sont conçues en fonction des variables de contrôle choisies et comparées aux méthodes de contrôle existantes. La robustesse d’une stratégie de contrôle du tokamak WEST est étudiée par des simulations numériques, et sera testée dans de futures expériences.

We construct an unconventional divergence free discretization of updated Lagrangian ideal MHD over simplicial grids. The cell-centered FV method employed to discretize the conservation laws of volume, momentum and total energy is rigorously the same than the one developed to simulate hyperelasticity equations. By construction this moving mesh method ensures the compatibility between the mesh displacement and the approximation of the volume flux by means of the nodal velocity and the attached unit corner normal vector which is nothing but the partial derivative of the cell volume with respect to the node coordinate under consideration. This is precisely the definition of the compatibility with the Geometrical Conservation Law which is the cornerstone of any proper multi-dimensional moving mesh FV discretization. The momentum and the total energy fluxes are approximated utilizing the partition of cell faces into sub-faces and the concept of sub-face force which is the traction force attached to each sub-face impinging at a node. We observe that the time evolution of the magnetic field might be simply expressed in terms of the deformation gradient which characterizes the Lagrange-to-Euler mapping. In this framework the divergence of the magnetic field is conserved with respect to time thanks to Piola An unconventional divergence free FV discretization of Lagrangian MHD formula. Therefore, we solve the fully compatible updated Lagrangian discretization of the deformation gradient tensor for updating in a simple manner the cell-centered value of the magnetic field. Finally, the sub-face traction force is expressed in terms of the nodal velocity to ensure a semi-discrete entropy inequality within each cell. The conservation of momentum and total energy is recovered prescribing the balance of all the sub-face forces attached to the sub-faces impinging at a given node. This balance corresponds to a vectorial system satisfied by the nodal velocity. It always admits a unique solution which provides the nodal velocity. The robustness and the accuracy of this unconventional FV scheme has been demonstrated employing various representative test cases. Finally, it is worth emphasizing that once you have an updated Lagrangian code for solving hyperelasticity you also get an almost free updated Lagrangian code for solving ideal MHD ensuring an exact divergence free constraint of the magnetic field at the discrete level.

In the wake of steady progress in laser technology opening up to an ever wider range of physical configurations, a consistent research community developed around “ultra-high intensity” physics. During the last two decades, these works have uncovered a stable set of physical phenomena and applications, all essentially described by the classical mechanics of fields and particles at relativistic energies. The current situation is however likely to be a turning point, for radically different physical phenomena are expected in last-generation facilities, either planned or under construction, revealing the quantum aspect of particles and fields.Relying on a scheme explicitly designed to foster the ongoing transition, the present thesis fully takes ground in such context. By generating pulses exceeding forecast laser intensities by two to six orders of magnitude, relativistic plasma mirrors could allow entering the quantum-dominated regime of strong-field Quantum Electrodynamics, with fundamental and instrumental implications. Our research was devoted to try and predict some of them explicitly.Remarkably, in the most favorable case the combination of wavelength shortening and pulse amplification in the « harmonics beams » produced by relativistic plasma mirrors results in fields close to the Schwinger critical value, a landmark for the onset of coherent field decay into electron-positron pairs. This entails that quantum processes can occur in harmonics beams both in the presence of matter, or in a vacuum. In this thesis, we will be presenting the expectations derived from a closer study of these different scenarios. The self-interaction of light in a vacuum is studied first, assessing the potential of harmonics beams for a first empirical detection of this long-predicted effect. We will then display the transition from light self-interaction when no matter is there, to prolific pair creation as soon as one particle comes in. This process of pair creation occurs for different types of seed, a Schwinger pair, a dense target, copropagating electron bunches. All three cases were studied and their evolution towards a final state numerically simulated, reveling the possibility to create relativistic quantum electron-positron plasmas with exotic properties, or enter the Ritus-Narozhny regime for the first time, allowing to empirically test strong-field Quantum Electrodynamics even beyond its established theoretical framework.; Dans le sillage de progrès constants de la technologie laser ouvrant à une variété toujours croissante de configurations physiques, une communauté de recherche autonome s'est développée autour de la physique des ultra-hautes intensités. Au cours des deux dernières décennies, ces travaux ont couvert un ensemble stable de phénomènes physiques et d'applications associées, toutes essentiellement décrites par la mécanique classique relativiste des particules et des champs. La situation actuelle se trouve cependant certainement être un moment seuil, puisque des phénomènes radicalement nouveaux sont attendus dans les infrastructures de dernière génération, prévues ou en construction, révélant l'aspect quantique des particules et des champs.Reposant sur une méthode explicitement vouée à accélérer cette transition en cours, la présente thèse s'inscrit pleinement dans un tel contexte. En générant des impulsions excédant les plus hautes intensités conventionnellement envisagées de deux à six ordres de grandeur, les miroirs plasma pourraient permettre d'accéder au régime quantique profond de l'Électrodynamique Quantique en champs forts, avec des implication fondamentales comme instrumentales. Notre travail de recherche a consisté à prédire certaines d'entre elles explicitement.De manière remarquable, dans les cas les plus favorables la conjonction de la compression de la longueur d'onde et de l'amplification de l'impulsion des « faisceaux harmoniques » générés par les miroirs plasma relativistes produit des champs proches de la valeur critique de Schwinger, seuil de la conversion spontanée d'un champ cohérent en paire électron-positron. Cela implique que des processus quantiques peuvent survenir au sein de ces faisceaux harmoniques en présence de matière, ou dans le vide. Dans cette thèse, nous présentons les prédictions issues d'une étude poussée de ces différents scénarios. L'auto-interaction de la lumière dans le vide est exposée la première, de manière à établir le potentiel des faisceaux harmoniques pour une première détection expérimentale de cet effet prédit de longue date. La transition de la diffusion de la lumière par la lumière en l'absence de matière, à la création de paires prolifique dès qu'une particule intervient est détaillée ensuite. Ce processus de création de paires se produit différemment selon la particule initiale, une paire issue de l'effet Schwinger, une cible de matière dense, un faisceau d'électrons. Ces trois cas ont été étudiés, et leur évolution vers un état final simulée numériquement, révélant la possibilité de créer un plasma d'électrons-positrons relativiste et quantique aux propriétés exotiques, ou d'atteindre le régime de Ritus-Narozhny pour la première fois, permettant une observation empirique l'électrodynamique quantique des champs forts au-delà même des limites de son cadre théorique établi.

Free-electron lasers generate high-brilliance coherent radiation at wavelengths spanning from the infrared to the X-ray domains. The recent development of short-wavelength seeded free-electron lasers now allows for unprecedented levels of control on longitudinal coherence[1], opening new scientific avenues as ultra-fast dynamics on complex systems and X-ray nonlinear optics. While those devices rely on state-of-the-art large-scale accelerators, advancements on laser-plasma accelerators, which harness giga-volt-per-centimeter accelerating fields, showcase a promising technology as compact drivers for free-electron lasers. Using such miniaturized accelerators, exponential amplification of a shot-noise type of radiation in a self-amplified spontaneous emission configuration was recently achieved [2]. However, employing this compact approach for the delivery of temporally coherent pulses in a controlled manner remained a major challenge. Here, we present the experimental demonstration of a laser-plasma accelerator driven free-electron laser in a seeded configuration, where control over the radiation wavelength is accomplished. Furthermore, the appearance of interference fringes, resulting from the interaction between the phase-locked emitted radiation and the seed, confirms longitudinal coherence. Building on our scientific achievements, we anticipate a straightforward scaling to extreme-ultraviolet wavelengths, paving the way towards university-scale free-electron lasers, unique tools for a multitude of applications. [1] Meyer, M. FELs of europe: Whitebook on science with free electron lasers 8–19 (2016). [2] Wang, W. et al. Free-electron lasing at 27 nanometres based on a laser wakefield accelerator.

The terahertz (THz) frequency domain is of increasing interest for many applications, including medical imaging, molecular spectroscopy or modifications of condensed matter states. This thesis is devoted to the generation of such radiations by laser-plasma interactions, in which a laser-ionized gas acts as a converter from laser (optical) frequency to lower frequencies. We start by studying photoionization, from which a low-frequency current emitting in the THz frequency range is produced by introducing a second harmonic (or color) in the laser pulse ionizing the gas. Using a simple model, we describe qualitatively and quantitatively the gain in THz energy gain reported when the laser polarization is circular rather than linear. These analytical results are successfully compared with experimental data performed during the thesis. We then study analytically and numerically the Cherenkov wake radiation, through which the laser ponderomotive force excites an electron plasma wave, capable of propagating outside the plasma channel by coupling with a magnetic field orthogonal to the laser propagation axis. In the ultra-relativistic regime with strong magnetization (that is with cyclotron frequency higher than the plasma frequency), unexplored in the past, we achieve record THz amplitudes (> 100 GV/m) and high laser-to-THz energy conversion efficiencies (> 0.1%) by means of particle-in-cell, kinetic numerical simulations. Finally, when the magnetic field is directed parallel to the laser propagation axis, we highlight a novel efficient THz source (conversion efficiency > 0.1%) generating an azimuthally polarized THz field with high amplitude (>30 GV/m).; Le domaine des fréquences térahertz (THz) connaît un intérêt croissant pour ses applications nombreuses, incluant l'imagerie médicale, la spectroscopie moléculaire ou encore la modification d'états de la matière condensée. Cette thèse est consacrée à la génération de tels rayonnements par des interactions laser-plasma, dans lesquelles un gaz ionisé par une impulsion laser femtoseconde joue le rôle de convertisseur depuis la fréquence laser (optique) vers les basses fréquences. Nous commençons par étudier la photoionisation, à travers laquelle un courant basse-fréquence émettant dans la gamme THz est produit grâce à l'introduction d'une seconde harmonique (couleur) dans l'impulsion laser ionisant le gaz. A l'aide d'un modèle simple, nous décrivons qualitativement et quantitativement le gain en énergie THz constaté lorsque la polarisation laser est circulaire plutôt que linéaire. Ces résultats analytiques sont comparés avec succès à des données expérimentales obtenues pendant la thèse. Nous examinons ensuite analytiquement et numériquement le rayonnement de sillage Cherenkov, dans lequel la force pondéromotrice laser excite une onde à la fréquence plasma, pouvant être rayonnée hors du plasma par l'introduction d'un champ magnétique orthogonal à l'axe de propagation laser. En régime ultra-relativiste et de forte magnétisation (c'est-à-dire de fréquence cyclotron supérieure à la fréquence plasma électronique), inexploré jusqu'à présent, nous rapportons numériquement à l'aide de simulations particulaires et cinétiques des amplitudes THz record (> 100 GV/m) et de forts taux de conversion en énergie laser-THz (> 0.1%). Enfin, lorsque le champ magnétique est orienté parallèlement à l'axe de propagation laser, nous mettons en évidence une source de rayonnement nouvelle, produisant efficacement (rendement énergétique > 0.1%) un champ THz polarisé azimutalement et de forte amplitude (>30 GV/m).

International audience; Elemental analyses of thin films and optical surfaces with complex compositions are challenging as the standard analytical techniques based on measurement calibration are difficult to apply. In this chapter, we show that calibration-free laser-induced breakdown spectroscopy presents a powerful solution, enabling rapid quantitative analyses of multi-elemental thin films or tiny surface volumes of optical materials with high analytical performances. Examples are given for classically polished optical glass and a nickel-chromium-molybdenum alloy film of 150 nm thickness. Thus, broadband spectra, recorded under experimental conditions that enable simple and accurate modeling of plasma emission, are processed via a calibration-free approach based on the calculation of the spectral radiance of a plasma in local thermodynamic equilibrium to deduce the elemental composition. The validation via energy-dispersive X-ray spectroscopy, inductively coupled plasma atomic emission spectroscopy, and Rutherford backscattering spectrometry illustrates that calibration-free LIBS is not only a promising tool, but already a reliable analytical technique.

Since the invention of the Chirped Pulse Amplification technique, achievable laser intensities never stopped increasing. The upcoming facilities are expected to deliver several PW peak power on a focal spot of a few microns diameter. When particles interact with those extreme electromagnetic fields, nonlinear effects of a quantum nature can be dominant, described by the so-called strong-field quantum electrodynamics (SFQED). Among the prediction of SFQED two phenomena in particular attracted the interest of the community over the last decade. The first one, the nonlinear Compton scattering process, is the emission of a high energy gamma photon (from a hundred of MeV to tens of GeV) by an electron or a positron interacting with a strong background field. The second one is the nonlinear Breit-Wheeler (NBW) process, that consists in the creation of an electron-positron pair from the conversion of a high energy gamma photon interacting with the strong electromagnetic field. Thanks to this processes, multi-PW laser facilities open the way to prolific electron-positron pair production in the laboratory. The ultimate goal, although not yet achievable in current facilities, is the creation of an electron-positron pair plasma in the laboratory. This kind of plasma is of interest from a fundamental point of view for its exotic properties, but also because it is present in the astrophysical environment. The common thread in this thesis is the optimisation of pair creation in ultra intense lasers, for which two promising physical configurations achievable in the future were studied. The first one, in the so-called shower-regime, involves the head-on collision of a high intensity laser pulse with a flash of high energy gamma photons. A semi-analytical model was developed for this configuration that takes fully into account the laser pulse spatio-temporal distribution and allows to predict quickly the amount of produced pairs. The model prediction was in excellent agreement with three dimensional Particle-in-cell (PIC) simulations performed with the code SMILEI. A systematic study for different laser pulse configurations was performed, by considering a fixed total pulse energy, to match experimental conditions. In this study Laguerre-Gauss (LG) beams, or vortex beams, were also considered because of their interesting properties, such as a ring shape transverse intensity distribution, and the fact that they carry orbital angular momentum. The study allowed to identify the optimal focalisation in order to maximise pair production that, depending on the total energy available, is not necessarily the tightest focus. General guidelines for upcoming experiments were also provided. The second configuration studied involves two counter streaming laser beams, with seeding electrons in the focal plane. With this set-up an exponential growth (avalanche) of the pairs’ number can be obtained, as charges are constantly reaccelerated and emit gamma photons, further converted into pairs. The present work re-examined the conditions for the inset of the avalanche in a realistic 3D geometry for the laser pulse, and extended the study to LG beams. We studied semi-analytically the short time (less than a laser period) particle dynamics in the counter streaming lasers and showed that there are large qualitative differences in the onset of the cascade depending on the field configuration. The growth rate of pair production was derived from 3D PIC simulations and confirmed the qualitative differences. Moreover we showed that -at the same peak intensity- LG beams can have larger growth rates than Gaussian beams. The analysis of the short time particle dynamics allowed to propose a new model to predict the growth rate of the avalanche for an idealised configuration, with the perspective of an extension to a more realistic 3D counter-streaming beams configuration.; Depuis l’invention de la technique de Chirped Pulse Amplification, les intensités lasers atteignables n’ont cessé d’augmenter. Les futures installations devraient produire des puissances de plusieurs PW sur des tâches focales de quelques microns. Quand des particules interagissent avec ces champs extrêmes, des effets non-linéaires de nature quantique, décrits par l’électrodynamique quantique en champs forts (SFQED), peuvent devenir dominants. Parmi les prédictions de cette théorie, deux phénomènes on particulièrement attiré l’attention de la communauté ces dernières années. Le premier, la diffusion Compton non-linéaire, est l’émission d’un photon gamma de haute énergie (d’une centaine de MeV à des dizaines de GeV) par une charge interagissant avec le champ fort. Le second est le processus Breit-Wheeler non-linéaire (NBW), qui est la conversion d’un photon gamma de haute énergie en une paire électron positrons, lors de l’interaction avec un électromagnétique champ fort.Grâces à ces processus, les installations multi-PW ouvrent la voie vers une production de paires électrons-positrons abondante en laboratoires. L’objectif ultime, bien qu’inaccessible dans les installations actuelles, est la création d’un plasma de paires électron-positron. Ce genre de plasma est intéressant fondamentalement pour ces propriétés exotiques, mais aussi car il est présent dans des environnements astrophysiques. Le thème de cette thèse est l’optimisation de la production de paires dans les lasers ultra intenses. Pour cela deux configurations physiques prometteuses ont été étudiées. La première, appelée shower, est la collision frontale d’une impulsion laser haute intensité avec un flash de photons gamma de haute énergie. Un modèle semi-analytique a été développé pour cette configuration, prenant en compte la structure spatio-temporelle de l’impulsion laser, et permettant de prédire rapidement le nombre de paires produites. Ce modèle est en excellent accord avec des simulations Particle-in-cell (PIC) en trois dimensions réalisées avec le code SMILEI. Une étude systématique avec différentes configurations d’impulsions a été réalisée, en fixant l’énergie totale de l’impulsion pour se rapprocher des conditions expérimentales. Dans cette analyse, des faisceaux Laguerre-Gauss (LG), ou faisceaux à vortex, ont été étudiés en raison de leurs propriétés intéressantes, comme leur distribution d’identité en forme d’anneau ou le fait qu’ils transportent un moment angulaire orbital. Cette étude a permis d’identifier la focalisation optimale pour la création de paires, qui selon l’énergie totale disponible n’est pas nécessairement la focalisation maximale. Ce travail dispense aussi des conseils généraux pour les futures expériences. La seconde configuration étudiée est formée de deux faisceaux laser contre-propagatifs, avec des électrons placés initialement au plan focal. Dans ce cas une croissance exponentielle du nombre de paires (avalanche) peut-être obtenue, car les charges sont constamment réaccélérées et rayonnent des photons gamma, par la suite convertis en paires. Le présent travail réévalue les conditions de démarrage de la cascade dans une géométrie 3D réaliste pour les impulsions lasers, et étend l’étude aux faisceaux LG. Il est présenté une analyse semi-analytique de la dynamiques des particules sur les temps courts (petits comparés à la période laser) dans cette configurations de faisceaux contre-propagatifs. Il est montré qu’il y a de grandes différences qualitatives dans le déclenchement de l’avalanche suivant la configuration des champs. Le taux de croissance des paires obtenu de simulations PIC 3D confirme ces différences. De plus il est montré qu’à amplitudes maximum égales, les LG on un plus haut taux de croissances que les faisceaux gaussiens. L’analyse de la dynamique aux temps courts permet de proposer un nouveau modèle pour prédire le taux de croissance dans un cas idéal, avec le potentiel d’être étendu à des cas plus réalistes.

Inertial confinement fusion experiments on large laser facilities such as the LMJ in Bordeaux or the NIF in the United States, involve the propagation of lasers through large plasmas (several millimeters). A large number of instabilities called wave coupling are likely to appear, and scatter the light in a different direction from the incident electromagnetic wave. In particular, Raman and Brillouin backscattering, as well as energy exchange between laser beams result from these wave couplings. Techniques called optical smoothing are used in large facilities to try to reduce these phenomena. The beams, once smoothed, are composed of many micrometric hot spots called speckles. During these experiments, the beams, focused around a target, are going to cross each other. The coupling of two coherent electromagnetic waves in a non-linear medium (the plasma), can induce an energy exchange between the two beams. The crossing of the lasers creates an interference grating, where the ponderomotive force expels the electrons. The ions follow the electrons due to the electrostatic spring force, generating a density modulation, or acoustic wave, which diffracts the electromagnetic waves from one beam to the other. This exchange, called Cross-beam Energy Transfer (CBET) takes place if the lasers have slightly different frequencies, or if the latter are equal but the plasma is moving in the direction of the acoustic wave. We have shown that, although these two situations are often considered equivalent in hydrodynamic models, they are in fact different. This is due to the fact that the exchange is commonly calculated by considering the laser beams as plane waves, i.e. neglecting the laser smoothing. In order to demonstrate this non-equivalence, we first studied an academic situation, considering the crossing of two laser beams each constituted of 4 Gaussian speckles. Different simulations have been performed with a particle-in-cell kinetic code, solving the Vlasov and Maxwell equations. The simulations showed that when the interaction is induced by a moving plasma, plane wave models are able to predict the exchange between the Gaussian beams. By contrast, when the exchange is induced by different laser frequencies, plane wave models overestimate the energy transfer. Moreover, we were able to distinguish two different configurations for the case where the laser frequencies are different. In the first case, the acoustic waves from different speckle crossings are in phase, and a constructive interference results from their interaction. In the other case, the waves are out of phase resulting in a destructive interference. It has been shown that although the exchange is greater in the in-phase case, the exchange remains lower than in the plasma flow case. The phase shift is therefore not the only source of difference between the two situations in which CBET appears. These results were obtained by considering a weakly Landau-damped plasma, i.e. a situation where the acoustic wave propagates and may encounter several speckle crossings before being damped. In a second step, a more realistic modeling of the smoothed beams has been adopted. For this purpose, the fields of a smoothed laser beam have been computed in an exact way, allowing to perform more accurate simulations and to build a model taking into account the real structure of the speckles. The previous results concerning the non-equivalence between the plasma flow and wavelength shift cases have been confirmed, even in the case of a strongly Landau-damped plasma. We were then able to show that the resonance conditions allowing the energy transfer to take place are also affected by the laser smoothing. In particular, the resonance width is broadened by the spatial smoothing.; Les expériences de fusion par confinement inertiel sur les grandes installations laser, telles que le LMJ à Bordeaux ou le NIF aux États-Unis, nécessitent la propagation des lasers à travers des plasmas de grandes tailles (plusieurs millimètres). Un grand nombre d'instabilités dites de couplages d'ondes peuvent apparaître, et diffusent la lumière dans une direction différente de l'onde électromagnétique incidente. En particulier, les diffusions arrières Raman et Brillouin, ainsi que l'échange d'énergie entre faisceaux laser en résultent. Des techniques dites de lissage optique sont utilisées pour tenter de réduire ces phénomènes. Les faisceaux, une fois lissés, sont constitués de nombreux points chauds micrométriques nommés speckles. Lors de ces expériences, les faisceaux, focalisés autour d'une cible, vont être amenés à se croiser.Le couplage de deux ondes électromagnétiques cohérentes dans un milieu non-linéaire (le plasma), peut induire un échange d'énergie entre les faisceaux. Le croisement des lasers crée un réseau d'interférences, où la force pondéromotrice expulse les électrons. Les ions suivent les électrons à cause de la force de rappel électrostatique. Cela crée une modulation de densité, ou onde acoustique, qui diffracte les ondes électromagnétiques d'un faisceau vers l'autre. Cet échange, nommé Cross-beam Energy Transfer (CBET) a lieu si les lasers ont des fréquences différentes, ou si ces dernières sont égales mais que le plasma est en mouvement dans la direction de l'onde acoustique. Nous avons montré que, bien que ces deux situations soient souvent considérées comme équivalentes dans les modèles hydrodynamiques, elles sont en réalité différentes. Ceci est dû au fait que l'échange est communément calculé en considérant les faisceaux laser comme des ondes planes, c'est-à-dire en négligeant le lissage laser. Afin de démontrer cette non-équivalence, nous avons, dans un premier temps, étudié une situation académique, en considérant le croisement de deux faisceaux lasers constitués chacun de 4 speckles Gaussiens. Différentes simulations ont été effectuées grâce à un code cinétique "particle-in-cell", résolvant les équations de Vlasov et de Maxwell. Les simulations ont montré que lorsque l'interaction est induite par un plasma en mouvement, les modèles du type onde plane sont en mesure de prédire l'échange entre les faisceaux Gaussiens. Au contraire, lorsque l'échange est induit par des fréquences laser différentes, ces modèles surestiment le transfert d'énergie. Nous avons aussi mis en évidence deux configurations distinctes dans le cas où les fréquences laser sont différentes. Dans la première situation, les ondes acoustiques issues de différents croisements de speckles sont en phase, et une interférence constructive résulte de leur interaction. Dans l'autre situation, les ondes sont déphasées ce qui donne une interférence destructive. Nous avons montré que, bien que l'échange soit plus important dans la situation en phase, il demeure inférieur au cas flot de plasma. Le déphasage n'est donc pas la seule cause de la différence entre une interaction avec ou sans flot. Nos résultats ont été obtenus en considérant un plasma faiblement amorti, où l'onde acoustique se propage et peut rencontrer plusieurs croisements de speckles avant que son amplitude n'ait significativement décru. Dans un second temps, nous avons considéré une situation plus proche de celle d'un croisement de faisceaux lissés. Nous avons calculé exactement le champ d'un faisceau laser lissé, ce qui a permis d'effectuer des simulations plus précises et de construire un modèle tenant compte de la structure réelle des speckles. Les résultats précédents ont été confirmés, même dans le cas d'un plasma fortement amorti. Nous avons également pu montrer que les conditions de résonance permettant au transfert d'énergie d'avoir lieu sont elles aussi affectées par le lissage laser. En particulier, le lissage spatial augmente la largeur de résonance.

Electromagnetic radiation within the terahertz (THz) frequency range is of great interest for applications in remote sensing and time-domain spectroscopy. The laser-induced plasmas are promising mediums for generating THz radiation. It has been recently reported that focusing femtosecond Bessel pulses inside dielectrics induces a high aspect ratio over-critical plasmas. Here we show that the intense resonantly driven electrostatic fields at the so-called critical surface lead to THz radiation emission. Through three-dimensional particle-in-cell simulation and analytical derivation, we have investigated the emission of THz radiation. We show that the THz radiation is associated with a hot population of electrons trapped in ambipolar electric fields of the double layers., Comment: 10 pages, 4 figures, accepted for Physics of Plasmas

International audience; Active ingredients containing fluorinated organic compounds such asFluoroalkylamines or pyrimidine-based molecules are promising in the field of agriculture (pesticides and herbicides) and pharmacology (antibiotics). The massive use of these molecules will result in a massive increase in waste containing this type of molecule. Developed countries have restrictive waste management policies, which is not the case in developing countries. In the latter, we are witnessing a proliferation of storage areas and open-air disposal of waste, sometimes from developed countries. These practices have enormous consequences on the environment such as air, soil and water pollution and therefore on human health. One of the solutions already proven on solid waste would be the use of plasma torches. These torches can reach high temperatures (5,000 K to 20,000 K). However, the use of these means of treatment is not without danger since toxic or lethal molecules could be produced. In order to understand these difficulties, we propose to study the influence of air on the chemical composition of a plasma based on fluoroalkylamines (trifluoroethylamine: C2H4F3N, nonafluoropentylamine: C5H4F9N, etc.), at atmospheric pressure and at atmospheric pressure. local thermodynamic equilibrium (E.T.L), in a temperature range from 500 K to 20,000 K. In order to obtain the chemical composition of the plasma, we use the Gibbs free energy minimization method. The results obtained show that dangerous and toxic gaseous chemical species such as CF2, CO, HCN and HF appear at low temperatures with high concentration.; Les principes actifs contenant des composés organiques fluorés comme les fluoroalkylamines ou les molécules à base de pyrimidine sont prometteurs dans le domaine de l’agriculture (pesticides et herbicides) et de la pharmacologie (antibiotiques). L’utilisation massive de ces molécules aura pour conséquence une augmentation massive de déchets contenant ce type de molécules. Les pays développés ont des politiques contraignantes en matière de gestion de déchets ce qui n’est pas le cas dans les pays en voie de développement. Dans ces derniers, nous assistons à une prolifération des aires de stockage et des éliminations à l’air libre des déchets parfois issus des pays développés. Ces pratiques ont d’énormes conséquences sur l’environnement comme la pollution de l’air, des sols et de l’eau et par conséquent sur la santé humaine. L’une des solutions déjà éprouvé sur des déchets solides serait l’utilisation de torches à plasma. Ces torches peuvent atteindre des températures élevées (5.000 K à 20.000 K). Cependant, l’utilisation de ces moyens de traitement n’est pas sans danger puisque des molécules toxiques ou létales pourraient être produites. Afin d’appréhender ces difficultés, nous proposons d’étudier l’influence de l’air sur la composition chimique d’un plasma à base de fluoroalkylamines (trifluoroéthylamine : C2H4F3N, nonafluoropentylamine : C5H4F9N, …), à la pression atmosphérique et à l’équilibre thermodynamique local (E.T.L), dans une gamme de températures allant de 500 K à 20.000 K. Afin d’obtenir la composition chimique du plasma, nous utilisons la méthode de minimisation de l’énergie libre de Gibbs. Les résultats obtenus montrent que des espèces chimiques gazeuses dangereuses et toxiques comme CF2, CO, HCN et HF apparaissent aux basses températures avec de forte concentration.

International audience; This chapter is dedicated to the presentation of basic concepts of plasma diagnostics via spectroscopy techniques, including optical emission spectroscopy and absorption spectroscopy. The measurements of temperature and electron density are illustrated via several examples of transient atmospheric plasmas generated by electrical and optical discharges, for which the validity conditions and the measurements limitations are discussed. Finally, the influence of self-absorption on plasma emission is discussed, and the appropriate modeling using the radiation transfer equation is introduced. This chapter is addressed to beginners in optical spectroscopy to help them discovering this very powerful technique. Readers wishing to use this technique to diagnose their plasmas are advised to consult more specialized books.

The study is focused on the impact of different magnetic field configurations of a high-power impulse magnetron sputtering (HiPIMS) in a nonreactive mode on the film precursors. Ionized flux fraction and total flux deposited onto the substrate were measured with the magnetic quartz crystal microbalance probe placed in front of the target racetrack. Particularly, we investigated the degree of magnetron balancing and the geometry of the magnetic field above the Ti target surface (4 in. diameter), as crucial factors influencing the thin film deposition, for different HiPIMS pulse lengths. Three unbalanced (II type) magnetron configurations have been chosen for this study: two symmetric geometries—with a regular magnetic field (B parallel to the target about 80 and 35 mT) and one asymmetric (highly unbalanced) magnetron configuration with an intermediate magnetic field (B parallel to the target about 48 mT). The HiPIMS was operated keeping constant the peak current at 43 A for C0-E0 and C10-E0 B-field configurations and a lower value, 33 A, when operating in C10-E10 configuration. In addition to the peak current, the pulse frequency was kept constant at 100 Hz but the pulse length (power on-time, Ton) was varied from 50 up to 100 μs. Obviously, the pulse power and the average power continuously increase with the length of the pulse. The results reveal a significant difference in the trends of the deposition rate and ionized flux fraction reaching the substrate with respect to the degree of balancing of the magnetron. It was found that the ionized fraction of metal arriving at the substrate reaches its maximum for the pulse length between Ton ≈ 70–80 μs in both symmetric cases, with strong and weak magnetic fields. The ionized fraction of Ti atoms in the asymmetric configuration increased in all measured range with the pulse length and the growth rate has a smooth increase.

A method for the optimisation and advanced studies of a laser-plasma electron injector is presented, based on a truncated ionisation injection scheme for high quality beam production. The SMILEI code is used with laser envelope approximation and a low number of particles per cell to reach computation time performances enabling the production of a large number of accelerator configurations. The developed and tested workflow is a possible approach for the production of large dataset for laser-plasma accelerator optimisation. A selection of functions of merit used to grade generated electron beams is discussed. Among the significant number of configurations, two specific working points are presented in details. All data generated are left open to the scientific community for further study and optimisation.

The size of a hard Kα x-ray source ($${\mathrm{E}}_{{\rm{K}}_{\rm{\alpha }}}$$ E K α = 17.48 keV) produced by a high intensity femtosecond laser interacting with a solid molybdenum target is experimentally investigated for a wide range of laser intensity (I ~ 1017–2.8 × 1019 W/cm2) and for four values of the temporal contrast ratio (6.7 × 107 10). Results point out the size enlargement of the x-ray source with the increase of laser intensity and with the deterioration of temporal contrast. It amounts up to sixteen times the laser spot size at the highest laser intensity and for the lowest temporal contrast ratio. Using hydrodynamic simulations, we evaluate the density scale length of the pre-plasma L/λ just before the main pulse peak. This allows us to show that a direct correlation with the laser absorption mechanisms is not relevant to explain the large size broadening. By varying the thickness of the molybdenum target down to 4 µm, the impact of hot electron scattering inside the solid is also proved irrelevant to explain the evolution of both the x-ray source size and the Kα photon number. We deduce that the most probable mechanism yielding to the broadening of the source size is linked to the creation of surface electromagnetic fields which confine the hot electrons at the solid surface. This assumption is supported by dedicated experiments where the evolution of the size enlargement of the x-ray source is carefully studied as a function of the laser focal spot size for the highest contrast ratio.

Due to their high cost of acquisition and operation, there are still a limited number of high-yield, high-flux neutron source facilities worldwide. In this context, laser-driven neutron sources offer a promising, cheaper alternative to those based on large-scale accelerators, with, in addition, the potential of generating compact neutron beams of high brightness and ultra-short duration. In particular, the predicted capability of next-generation petawatt (PW)-class lasers to accelerate protons beyond the 100 MeV range should unlock efficient neutron generation through spallation reactions. In this paper, this scenario is investigated numerically through particle-in-cell and Monte Carlo simulations, modeling, respectively, the laser acceleration of protons from thin-foil targets and their subsequent conversion into neutrons in secondary heavy-ion targets. Laser parameters relevant to the 1 PW LMJ-PETAL and 1-10 PW Apollon systems are considered. Under such conditions, neutron fluxes exceeding $10^{23}\,\rm n\,cm^{-2}\,s^{-1}$ are predicted, opening up attractive fundamental and applicative prospects., Comment: 16 pages, 13 figures

International audience; Two-color air plasma based broadband terahertz (THz) generation and subsequent air biased coherent detection (ABCD) of the THz field are presented. Both source and detection system are characterized experimentally and numerically, yielding excellent agreement of measured and simulated signals. We reveal that it is crucial to model the whole optical setup and include the various pump distortions, like temporal and spatial walk-off as well as ellipticity of polarization, in order to interpret the experimental measurements correctly. Moreover, it turns out that geometrical effects in the ABCD scheme shape the recorded THz spectra and need to be taken into account. We confirm that THz electric fields with peak amplitude in the MV/cm range and large spectral bandwidth are produced, allowing us to demonstrate THz spectroscopy with molecular samples.

For an energy mix, the mastery of nuclear fusion offers the opportunity for a abundant and renewable source of energy. The tokamaks showed the best performance for confining a fusion plasma using magnetic fields and were chosen as the next generation of machines for research in fusion (ITER). The evacuation of heat flows and the increase in the time of containment are the two main challenges for tokamaks. They are directly in connection with the perpendicular, essentially turbulent transport towards the wall. For improve confinement, mode H is sought. It creates a pedestal on the pressure in the edge plasma, reducing the perpendicular transport but is associated with relaxations, the ELMs, responsible for a transitory influx of heat on the walls beyond tolerable limits. To remove or mitigate ELMs, disturbances 3D Magnetics (MPs) have been added through external coils. The impacts MPs on the ELMs, the L-H transition, their screening or their resonance on the magnetic field have been studied. Few data exist on their impact on the turbulence due to the difficulty of experimental measurements and the complexity of the tools numbers needed.In this thesis, we address the issue by modeling the edge plasma, from the outside of the core to the SOL. 2 complementary codes are used: a fluid code Turbulent electrostatic 3D, TOKAM3X and a medium field, SOLEDGE3X_HDG.; Pour un mix énergétique, la maîtrise de la fusion nucléaire offre l’opportunité d’une source d’énergie abondante et renouvelable. Les tokamaks ont montré lesmeilleurs performances pour confiner un plasma de fusion grâce à des champs magnétiques et ont été choisis comme la prochaine génération de machines pour la recherche en fusion (ITER). L’évacuation des flux de chaleur et l’augmentation du temps de confinement sont les deux principaux défis pour les tokamaks. Ils sont directement en lien avec le transport perpendiculaire, essentiellement turbulent, vers le mur. Pour améliorer le confinement, lemode H est recherché. Il crée un piedestal sur la pression dans le plasma de bord, réduisant le transport perpendiculaire mais est associé à des relaxations, les ELMs, responsables d’un afflux de chaleur transitoire sur les murs au-delà des limites tolérables. Pour supprimer oumitiger les ELMs, des pertubations magnétiques 3D (MPs) ont été ajoutées grâce à des bobines externes. Les impacts des MPs sur les ELMs, la transition L-H, leur écrantage ou leur résonance sur le champ magnétique ont été étudiés. Peu de données existent sur leur impact sur la turbulence dû à la difficulté de mesures expérimentales et à la complexité sur les outils numériques nécessaires. Dans cette thèse, nous abordons la question par la modélisation du plasma de bord, de l’extérieur du coeur à la SOL. 2 codes complémentaires sont utilisés : un code fluide 3D électrostatique turbulent, TOKAM3X et un à champs moyens, SOLEDGE3X_HDG.

In the era of information, the reliability of telecommunication by radio wave and/or via satellites hadbecome an important challenge. In this context, it is crucial to quantify the lost of signal intensity provoked by irregularities in the ionosphere. Among them, we have Equatorial Plasma Bubble (EPB) and striations, which results from interchange instabilities, similar to the classical Rayleigh-Taylor Instability (RTI). This is starting from this ascertain-ment that we try to adapt already known non-linear models used for hydrodynamical RTI to ionospheric plasma counterpart, namely the generalized Rayleigh-Taylor Instability (GRTI). The GRTI takes into account for the fact that ionospheric plasma is only weakly ionized, so that the friction drag with the neutral atmosphere cannot be neglected. As a consequence, we extended theses non-linear models and put into evidence two regimes: the inertial regime, where we retrieve previous result of RTI and the collisional regime, where the collision between neutrals and ions is predominant. It has also been confronted to numerical simulations performed with two codes developed in CEA-DAM, CLOVIS, working with a ideal-MHD model and ERINNA, working with a electrostatic model. Thus, we show, with new analaytical models and simulations, that in the single mode case the structure grows as √gλ⁄6π in the inertial regime and as g⁄∨in the collisional regime, For multi-mode case the bubble front grows as αbgt² in the inertial, while its transverse scale follows the same tendency, and as g⁄∨in in the collisional case, while maintaining a rather constant size of bubbles. In conclusion, we present new analytical models that describe the non-linear growth of ionospheric instabilities, like EPBs and striations, as well as the non-linear interaction between multiple bubbles. We hope that one day they could be coupled with electromagnetic wavepropagation codes to quantify the loss in telecommunication.; À l’ère de l’information, la fiabilité des télécommunications par ondes radio et/ou par satellites est devenue un défi important. Dans ce contexte, il devient crucial de quantifier la perte d’intensité du signal provoquée par des irrégularités dans l’ionosphère. Parmi celles-ci, nous avons l’"Equatorial Plasma Bubble" (EPB) et les striations, qui résultent d’instabilités d’interchange, similaires à l’instabilité de Rayleigh-Taylor (RTI). C’est à partir de ce constat que nous essayons d’adapter les modèles non linéaires déjà connus et utilisés pour la RTI hydrodynamique à son pendant pour les plasmas ionosphériques, à savoir l’Instabilité Rayleigh-Taylor généralisée (GRTI). La GRTI tient compte du fait que le plasma ionosphérique n’est que faiblement ionisé, de sorte que la force de friction avec l’atmosphère neutre ne peut être négligée. En conséquence, nous avons étendu ces modèles non linéaires et mis en évidence deux régimes : le régime inertiel, où nous retrouvons les résultats précédents de la RTI et le régime collisionnel, où la collision entre les neutres et les ions est prédominante. Elles ont également été confrontées à des simulations numériques réalisées avec deux codes développés au CEA-DAM, CLOVIS, travaillant avec un modèle de MHD idéale et ERINNA, travaillant avec un modèle électrostatique. Ainsi, nous montrons avec des nouveaux modèles analytiques et des simulations que dans le cas d’un mono-mode, la croissance de la structure suit √gλ⁄6π dans le régime inertiel et g⁄∨in dans le régime collisionnel. Dans le cas d’un multi-mode, la croissance du front de bulles suit αbgt² dans le régime inertiel, tandis que sa taille transverse suit la même tendance, et g⁄∨in dans le régime collisionnel, tout en maintenant une taille plutôt constante de bulles. En conclusion, nous présentons de nouveaux modèles analytiques qui décrivent la croissance non linéaire des instabilités ionosphériques, comme les EPB et les striations, ainsi que l’interaction non linéaire entre plusieurs bulles. Nous espérons qu’un jour ils pourront être couplés avec des codes de propagation d’ondes électromagnétiques pour quantifier les pertes en télécommunication.

Since the 1960s, the most used gas for electric space propulsion has been xenon, a heavy atom that is easy to ionize. However, although it is a product of air distillation, which is a priori inexhaustible, the very low concentration of xenon in the atmosphere limits its annual production and the growing demand makes it necessary to find a sustainable alternative with comparable performance. Iodine, an element next to xenon in the periodic table, is a viable candidate to succeed it despite its molecular and electronegative nature. In this work, we try to understand how and why iodine, in its plasma state, can serenely consider the battle with its competitors, and in particular krypton, in the space race. The study of low-pressure iodine plasmas was conducted in two complementary directions. On the one hand, two experimental iodine plasma set-ups (a gridded-ion-thruster and a quartz cell) were set up and developed, within which numerous electrical and optical diagnostics (Langmuir probe, photodetachment of negative I- ions, absorption on excited atomic states, TALIF on the fundamental atomic level) were developed and successfully applied, some of them for the first time. These experimental works allowed measuring the absolute values of different species of the plasma (electrons, I- negative ions, positive ions, molecules, population difference between the first two atomic states), to highlight the non-negligible presence of an excited atomic state at 0.94 eV, to shift all the excited atomic levels downwards by 0.167 cm-1 with respect to the reference spectroscopic table, or to measure the degree of molecular dissociation, approaching, for sufficient power, more than 95 %. Also, the temperature of the atoms was measured and heating mechanisms much more prevalent than in noble gas plasmas were brought to light and discussed. On the other hand, a rigorous bibliography on collisional data (census and questioning of the effective sections and reaction rates measured or calculated) of atomic and molecular iodine, allowed the development of numerical simulation tools aiming at reproducing the functioning of iodine plasmas. Firstly, a global model, averaged in volume and allowing the description of the essential characteristics of the plasma, was largely updated with the new set of cross-sections and enriched to take into account the heating of neutral species. Secondly, a one-dimensional multi-fluid model of the three major neutral species in an iodine plasma (atom in its ground state, first excited atomic state, and molecular ground state) has been developed to simulate their densities, velocities, and temperatures along the thrust axis of the thruster. The inertia terms and heat fluxes of each species were taken into account.Comparisons between experiments and models proved to be relevant and led to several conclusions. At low injection flux or pressure (around 1 mTorr for our thruster), the plasma is completely dissociated and behaves like a noble gas plasma, composed almost entirely of atoms, I+ atomic positive ions, and electrons. In this operating regime, where dissociation is energetically inexpensive, iodine can revolutionize the field of electric propulsion by being more efficient than its noble competitors (argon, krypton, and xenon) in producing ions, and thus generating better thrust. At higher rates, however, the I2 molecules are no longer negligible and easily produce negative I- and molecular I2+ ions. Under these conditions, iodine is no longer competitive for propulsion, with krypton offering even better performance.; Depuis les années 1960, le gaz privilégié dans le domaine de la propulsion spatiale électrique est le xénon, un atome lourd et facile à ioniser. Néanmoins, bien que produit de la distillation de l’air, a priori inépuisable, la très faible concentration de xénon dans l’atmosphère limite sa production annuelle et la demande grandissante obligent à trouver une alternative pérenne, aux performances comparables. L’iode, élément voisin du xénon dans le tableau périodique, est un candidat viable pour lui succéder malgré sa nature moléculaire et électronégative. Dans ce travail, nous tâchons de comprendre comment et pourquoi l’iode, à l’état plasma, peut envisager sereinement la bataille avec ses concurrents, et notamment le krypton, dans la course à l’espace. L’étude des plasmas d’iode, à basse pression, a été menée dans deux directions complémentaires. D’une part, ont été mis en place et développés, deux montages expérimentaux de plasmas d’iode (un propulseur à grille et une cellule en quartz) au sein desquels de nombreux diagnostiques électriques et optiques (sonde de Langmuir, photodétachement des ions négatifs I-, absorption sur des états atomiques excités, TALIF sur le niveau atomique fondamental) ont été développés et appliqués avec succès, dont certains pour la première fois. Ces travaux expérimentaux ont permis de mesurer les valeurs absolues de différentes espèces du plasmas (électrons, ions négatifs I-, ions positifs, molécules, différence de population entre les deux premiers états atomiques), de mettre en évidence la présence non négligeable d’un état atomique excité à 0.94 eV, de décaler tous les niveaux excités atomiques de 0.167 cm-1 vers le bas par rapport à la table spectroscopique de référence ou encore de mesurer le degré de dissociation moléculaire, avoisinant pour une puissance suffisante, plus de 95 %. Egalement, la température des atomes a été mesurée et des mécanismes de chauffage bien plus prégnants que dans des plasmas de gaz nobles ont été mis en lumière et discutés. D’autre part, un travail rigoureux de bibliographie sur les données collisionnelles (recensement et questionnement des sections efficaces et taux de réaction mesurés ou calculés) de l’iode atomique et moléculaire, a permis de développer des outils de simulation numérique visant à reproduire le fonctionnement des plasmas d’iode. Premièrement, un modèle global, moyenné en volume et permettant de décrire les caractéristiques essentielles du plasma, a été largement mis à jour avec le nouveau jeu de sections efficaces et enrichi pour prendre en compte le chauffage des espèces neutres. Deuxièmement, un modèle multi-fluide unidimensionnel des trois espèces neutres majoritaires dans un plasma d’iode (atome dans son niveau fondamental, premier état atomique excité et molécule dans son état fondamental) a été développé afin de simuler leurs densités, vitesses et températures le long de l’axe de poussée du propulseur. Les termes d’inertie et les flux de chaleur de chaque espèce ont été pris en compte.Les comparaisons entre les expériences et les modèles se sont révélées pertinentes et ont permis de dresser plusieurs conclusions. A faible flux d’injection ou faible pression (autour de 1 mTorr pour notre propulseur), le plasma est complètement dissocié et se comporte comme un plasma de gaz noble, composé quasi uniquement d’atomes, d’ions atomiques positifs I+ et d’électrons. C’est dans ce régime de fonctionnement, où la dissociation est peu coûteuse énergétiquement, que l’iode peut révolutionner le domaine de la propulsion électrique en étant plus efficace que ses concurrents nobles (argon, krypton et xénon) pour produire des ions, et donc engendrer une meilleure poussée. A plus haut débit, en revanche, les molécules I2 ne sont plus négligeables et produisent facilement des ions négatifs I- et moléculaires I2+. Dans ces conditions là, l’iode n’est plus du tout compétitif pour la propulsion, le krypton offrant même de meilleures performances.

The results derived in this thesis are part of a long-term research activity on the kinetic description of nonlinear electron electron plasma waves (EPW) growing at a rate much smaller than the plasma frequency. EPW's resulting from stimulated Raman scattering (SRS) in laser fusion experiments are such slowly growing waves, and laser-plasma interaction in such experiments are one of the main motivation of the work presented in this dissertation. Thanks to the results derived in this thesis, the EPW's nonlinear properties may be now described from the moment when theses waves grow from thermal noise up to the time when they break. Indeed, we provided a self-consistent derivation of the nonlinear electron distribution function, the nonlinear frequency shift of the wave, and of its scalar and vector potentials. From the knowledge of the latter quantity we could derive the magnetic generated by the nonlinear EPW, which is therefore not electrostatic. Moreover, we showed that no solution to the nonlinear dispersion relation could exist beyond a maximum amplitude, which allowed us to derive an upper for the wave breaking limit. This result, together with that on the nonlinear frequency shift, were used in a code based on a nonlinear ray tracing to simulate SRS. This allowed us to derive very accurately, and about 10⁷ times faster than with a Particle In Cell code, the EPW transverse spectrum, when this wave resulted from the SRS of laser wave focused inside the plasma. Finally, we addressed the stability of the wave we described theoretically. We first solved, for the first time, the exact theory for the trapped particles instability. For a smooth enough distribution function, we showed that the spectrum of unstable modes derived theoretically was in very good agreement with that derived from kinetic simulations using a Vlasov code. Moreover, we pointed out the difficulty to address, both theoretically and numerically, the stability of a very slowly growing wave, because of the abrupt variations of the distribution function close to the separatrix. This let conclude on the future research activity to pursue in order to overcome this difficulty.; Les travaux présentés dans cette thèse s'inscrivent dans un effort à long terme sur la description cinétique des ondes plasma électroniques (EPW), croissant à un taux beaucoup plus petit que la fréquence plasma. C'est le cas des ondes créées par diffusion Raman stimulée (SRS) dans les expériences de fusion par confinement inertiel, l'étude de l'interaction laser-plasma dans de telles expériences étant une des motivations premières de la thèse. Grâce aux résultats que nous avons obtenus, couplés à ceux de précédents travaux, nous pouvons désormais décrire complètement, et de manière très précise, les propriétés d'une onde plasma électronique créée par SRS, depuis le moment où elle croît à partir du bruit thermique jusqu'au moment où elle se brise. Nous avons ainsi calculé de manière auto-cohérente la fonction de distribution électronique, le décalage non linéaire en fréquence de l'onde plasma, ainsi que ses potentiel scalaire et vecteur. La connaissance de cette dernière quantité nous a permis de déduire le champ magnétique créé par une onde plasma non linéaire, qui n'est donc pas purement électrostatique. De plus, en montrant que la relation de dispersion non linéaire n'admettait pas de solution au-delà d'une certaine amplitude, nous avons pu donner une borne supérieure à la limite de wave breaking pour une onde plasma électronique. Les résultats obtenus sur le décalage en fréquence non linéaire et sur la limite de wave breaking ont été utilisés pour simuler la SRS à l'aide d'un code basé sur un tracé de rayons non linéaire. Cela nous a permis de calculer très précisément, et environ 10⁷ fois plus vite qu'avec un code Particle In Cell, le spectre des modes transverses de l'EPW, dans le cas où elle résultait de la SRS d'une onde laser focalisée dans le plasma. Enfin, dans un dernier temps, nous avons étudié la stabilité des ondes plasma que nous avons décrites. Nous avons tout d'abord résolu, pour la première fois, la théorie exacte de l'instabilité des particules piégées. Nous avons montré que, dans le cas d'une fonction de distribution lisse, le spectre des modes instables calculé théoriquement était en excellent accord avec celui déduit de simulations cinétiques de type Vlasov. De plus, nous avons mis en évidence la difficulté de traiter, à la fois théoriquement et numériquement, le cas d'ondes croissant très lentement, à cause de la variation abrupte de la fonction de distribution électronique près de la séparatrice. Nous indiquons alors les pistes de recherche à poursuivre pour surmonter cette difficulté.

International audience; Resonant filament-assisted mode conversion (FAMC) scattering of high harmonic fast waves (HHFW) by cylindrical field-aligned density inhomogeneities can efficiently redirect a fraction of the launched HHFW power flux into the parallel direction. Within a simplified analytic approach, this contribution compares the parallel propagation, reflection and dissipation of nearly resonant FAMC modes for three magnetic field line geometries in the scrape-off layer, in the presence of radio-frequency (RF) sheaths at field line extremities and phenomenological wave damping in the plasma volume. When a FAMC mode, excited at the HHFW antenna parallel location and guided along the open magnetic field lines, impinges onto a boundary at normal incidence, we show that it can excite sheath RF oscillations, even toroidally far away from the HHFW launcher. The RF sheaths then dissipate part of the power flux carried by the incident mode, while another part reflects into the FAMC mode with the opposite wave vector parallel to the magnetic field. The reflected FAMC mode in turn propagates and can possibly interact with the sheath at the opposite field line boundary. The two counter-propagating modes then form in the bounded magnetic flux tube a lossy cavity excited by the HHFW scattering. We investigate how the presence of field line boundaries affects the total HHFW power redirected into the filament, and its splitting between sheath and volume losses, as a function of relevant parameters in the model.

Contemporary ultraintense, short-pulse laser systems provide extremely compact setups for the production of high-flux neutron beams, such as those required for nondestructive probing of dense matter, research on neutron-induced damage in fusion devices or laboratory astrophysics studies. Here, by coupling particle-in-cell and Monte Carlo numerical simulations, we examine possible strategies to optimise neutron sources from ion-induced nuclear reactions using 1-PW, 20-fs-class laser systems. To improve the ion acceleration, the laser-irradiated targets are chosen to be ultrathin solid foils, either standing alone or preceded by a plasma layer of near-critical density to enhance the laser focusing. We compare the performance of these single- and double-layer targets, and determine their optimum parameters in terms of energy and angular spectra of the accelerated ions. These are then sent into a converter to generate neutrons via nuclear reactions on beryllium and lead nuclei. Overall, we identify configurations that result in neutron yields as high as $\sim 10^{10}\,\rm n\,sr^{-1}$ in $\sim 1$-cm-thick converters or instantaneous neutron fluxes above $10^{23}\,\rm n\,cm^{-2}\,s^{-1}$ at the backside of $\lesssim 100$-$\mu$m-thick converters. Considering a realistic repetition rate of one laser shot per minute, the corresponding time-averaged neutron yields are predicted to reach values ($\gtrsim 10^7\,\rm n \,sr^{-1}\,s^{-1}$) well above the current experimental record, and this even with a mere thin foil as a primary target. A further increase in the time-averaged yield up to above $10^8\,\rm sr^{-1}\,s^{-1}$ is foreseen using double-layer targets., Comment: 16 pages, 8 figures, 2 tables

Sander Bekeschus and Sylvain Iséni equally contributed to this work as first authors.; International audience; Parameter studies of plasma treatment are informative about the optimal use of this technology in biomedical applications such as the argon radio-frequency plasma jet kINPen. However, the interdependence of the plasma-dissipated power in relation to input current and feed gas modulation on the resulting biological consequences has not been studied so far. To this end, a parameter study is presented, and the effect on human immune cell viability was investigated across different input current power and argon with oxygen/nitrogen feed gas admixture settings. It was found that with both nitrogen and oxygen admixtures, a concentration-dependent change in plasma-dissipated power emerged, which converged at 27.5 mA and 26.5 mA, respectively. The extent of cytotoxicity in immune cells confirmed the relevance of these findings, which were in congruency with the plasma dissipated powers identified. These findings underline the critical role and input parameter-dependent action of plasma sources for biomedical application.Dr Sander Bekeschus and Dr Sylvain Iséni equally contributed to this work as first authors.

Non-thermal plasma is widely considered as an effective technology for applications in agriculture. Particularly, numerous reports studies have highlighted the role of plasma-activated water (PAW) for seeds germination, plant growth, stress tolerance, and antibacterial ability. The present study investigated the effects of PAW on lettuce (Lactuca Sativa L.) seed germination and seedling growth. PAW was achieved by using an atmospheric pressure dielectric barrier discharge in Ar (50%) –N2 (40%)–O2 (10%) gas mixture for treatment time ranging from 5 to 30 mn. The physicochemical properties of PAW (temperature, pH, electrical conductivity, concentrations of nitrate, nitrite, and hydrogen peroxide) were evaluated. Results show that water-activated during moderate time, 10 to 20 mn, contains reactive oxygen and nitrogen species at relevant concentration levels to have active impacts on seed germination and seedling growth. Germination potential significantly increased by about 117%, 56%, and 77% after 15 mn of treatment, for the first 3 days, respectively, compared to control. For long time PAW (25–30 mn), the germination rate is either constant or decreases. Positive effects of PAW treatments were registered on the growth parameters of seedling including stem and root length, leaf weight, leaf area and chlorophyll content, and the vigor of seedlings. Chlorophyll content significantly increased by 220% for PAW-treated 15 mn and by about 165% for PAW-treated 20 mn, respectively, compared with control. Additionally, PAW induce morphological changes on lettuce seeds which are associated with oxidizing species leading to better water and nutrients uptake.

International audience; Image plates (IPs) are a popular detector in the field of laser driven ion acceleration, owing to their high dynamic range and reusability. An absolute calibration of these detectors to laser-driven protons in the routinely produced tens of MeV energy range is, therefore, essential. In this paper, the response of Fujifilm BAS-TR IPs to 1–40 MeV protons is calibrated by employing the detectors in high resolution Thomson parabola spectrometers in conjunction with a CR-39 nuclear track detector to determine absolute proton numbers. While CR-39 was placed in front of the image plate for lower energy protons, it was placed behind the image plate for energies above 10 MeV using suitable metal filters sandwiched between the image plate and CR-39 to select specific energies. The measured response agrees well with previously reported calibrations as well as standard models of IP response, providing, for the first time, an absolute calibration over a large range of proton energies of relevance to current experiments.

International audience; This paper presents the performance portable implementation of a kinetic plasma simulation code with C++ parallel algorithm to run across multiple CPUs and GPUs. Relying on the language standard parallelism stdpar and proposed language standard multi-dimensional array support mdspan, we demonstrate that a performance portable implementation is possible without harming the readability and productivity. We obtain a good overall performance for a mini-application in the range of 20 % to the Kokkos version on Intel Icelake, NVIDIA V100, and A100 GPUs. Our conclusion is that stdpar can be a good candidate to develop a performance portable and productive code targeting the Exascale era platform, assuming this approach will be available on AMD and/or Intel GPUs in the future.

The ITER NBI is based on negative hydrogen ions extracted from caesiated ion sources. The 3D particle-in-cell Monte Carlo collision code Orsay negative ion extraction (ONIX) models the beamlet formation of negative ions in such sources where surface production plays an important role. A coupling scheme between ONIX and the ion-optics code ion beam simulator (IBSimu) has been developed and compared to other particle simulation approaches. This extends the computational domain such that the complete grid system can be included while only marginally increasing the computational cost. The properties of the accelerated ONIX beamlet are studied and compared to standalone IBSimu calculations, which are based on a simplified plasma model. The comparison provides insight about the effect of approximations made in ion-optics codes, which were also used to design the ITER NBI grid systems. ONIX volume and surface produced negative ions have a different angular distribution in the accelerated beamlet. The ONIX volume produced particles have a similar core divergence compared to standalone IBSimu calculations, but there is more halo in the IBSimu angular distribution. In the ONIX simulations, a Debye sheath is formed between the plasma and the grid, which repels negatively charged particles. The sheath decreases the extracted current density at the edge of the aperture for volume produced ions. Contrarily, surface produced particles are directly extracted near the edge of the aperture. Particles extracted near the edge of the aperture are highly divergent at the end of the grid system, independent of their initial angle. To summarize, the presence of the plasma sheath around the apertures in the plasma grid as calculated by ONIX decreases the halo from volume produced particles compared to standalone IBSimu.

International audience

International audience; Ray-based cross-beam energy transfer (CBET) models have become a common feature of the radiation-hydrodynamic codes used to simulate inertial confinement fusion experiments. These models are necessary for achieving better agreement with experimental measurements, but their detailed implementation can vary widely between the codes and often rely on artificial multipliers. To address this, a series of 2D and 3D test cases has been developed with validated solutions from wave-based calculations. Comparisons of various ray-based CBET models to the wave-based calculations highlight the essential physics that is required for accurate ray-based CBET modeling. Quantitative comparison metrics and/or field data from the wave-based calculations have been made available for use in the validation of other ray-based CBET codes.

International audience; We describe two numerical investigations performed using a 3D plasma Monte-Carlo code, developed to study hot-electron transport in the context of inertial confinement fusion. The code simulates the propagation of hot electrons in ionized targets, using appropriate scattering differential cross sections with free plasma electrons and ionized or partially ionized atoms. In this paper, we show that a target in the plasma state stops and diffuses electrons more effectively than a cold target (i.e., a target under standard conditions in which ionization is absent). This is related to the fact that in a plasma, the nuclear potential of plasma nuclei has a greater range than in the cold case, where the screening distance is determined by the electronic structure of atoms. However, in the ablation zone created by laser interaction, electrons undergo less severe scattering, counterbalancing the enhanced diffusion that occurs in the bulk. We also show that hard collisions, i.e., collisions with large polar scattering angle, play a primary role in electron beam diffusion and should not be neglected. An application of the plasma Monte-Carlo model to typical shock ignition implosions suggests that hot electrons will not give rise to any preheating concerns if their Maxwellian temperature is lower than 25–30 keV, although the presence of populations at higher temperatures must be suppressed. This result does not depend strongly on the initial angular divergence of the electron beam set in the simulations.

International audience; We describe the development of a 3D Monte-Carlo model to study hot-electron transport in ionized or partially ionized targets, considering regimes typical of inertial confinement fusion. Electron collisions are modeled using a mixed simulation algorithm that considers both soft and hard scattering phenomena. Soft collisions are modeled according to multiple-scattering theories, i.e., considering the global effects of the scattering centers on the primary particle. Hard collisions are simulated by considering a two-body interaction between an electron and a plasma particle. Appropriate differential cross sections are adopted to correctly model scattering in ionized or partially ionized targets. In particular, an analytical form of the differential cross section that describes a collision between an electron and the nucleus of a partially ionized atom in a plasma is proposed. The loss of energy is treated according to the continuous slowing down approximation in a plasma stopping power theory. Validation against Geant4 is presented. The code will be implemented as a module in 3D hydrodynamic codes, providing a basis for the development of robust shock ignition schemes and allowing more precise interpretations of current experiments in planar or spherical geometries.

Magnetic reconnection can occur when two plasmas, having anti-parallel components of the magnetic field, encounter each other. In the reconnection plane, the anti-parallel component of the field is annihilated and its energy released in the plasma. Here, we investigate through laboratory experiments the reconnection between two flux tubes that are not strictly anti-parallel. Compression of the anti-parallel component of the magnetic field is observed, as well as a decrease of the reconnection efficiency. Concomitantly, we observe delayed plasma heating and enhanced particle acceleration. Three-dimensional hybrid simulations support these observations and highlight the plasma heating inhibition and reconnection efficiency reduction for these obliquely oriented flux tubes.

International audience; The High-order harmonic generation (HHG) is a workhorse tool of the whole attosecond science. Our intention is to have a comprehensive computational picture of HHG in a gas phase. Such a picture requires to couple two scales: 1) The macroscopic model of the driving pulse strongly affected by a non-linear propagation. 2) A single microscopic system – an atom or a molecule interacting with the pulse – described naturally by quantum physics.The primary goal is to provide an ab-initio fully-numerical solver of the process. The microscopic aspect is covered by solving the Time-dependent Schrödinger equation (TDSE) and the resulting harmonic field is computed by the means of the diffraction integral based on the Hankel transform. Finally, the non-linear propagation of the driving pulse is computed by the unidirectional solver [1].Coupling these solvers already provided a detailed insight in generating mechanisms. We used the multi-scale approach to model the control of the focusing properties of the generated field in thin targets [2] without a need of an XUV optics. Next, we modelled the optimisation of HHG in long media controlling the initial degree of the ionisation of the generating medium [3]. Both these schemes were investigated in close collaboration of experimentalists and theorists. The numerical model proved its indispensable role toextract key mechanisms and to sort complex interplays of microscopic and macroscopic effects within the process. The multi-scale approach reached a good quantitative agreement with experiments.Finally, the fully numerical model is computationally expensive. Our palette is then enriched by simpler tools (modelling both IR- and harmonic beams by Gaussian optics, using simplified models of the microscopic response, etc.). A large parametric space may be searched by these simple models and promising configuration pivot refinements and detailed studies by the full model.