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We detail a laser plasma experiment aimed at enhancing laser to MeV electron energy coupling and then the x-ray dose produced when a short pulse laser propagates through a long preformed plasma. This study can be of interest not only for radiography of high areal mass objects requiring large doses but also for radiation safety of large scale laser facilities such as LMJ or NIF able to produce long preformed plasmas through which a short pulse laser can propagate. A low-intensity (∼1014 W/cm2) ns beam explodes a thin foil deposited on a high-Z solid target to generate an underdense plasma. An intense (>1018 W/cm2) and short (

This paper shows how electromagnetic pulses mitigation is performed on the LMJ-PETAL laser facility. In particular, the authors show mitigation suppression for joint shots with appropriate time delay between the LMJ nanosecond beams and the PETAL picosecond beam.

International audience; During the interaction of an intense picosecond laser pulse with a plasma created by a plastic foil ablated by a nanosecond laser pulse, relativistic electrons are produced. A metal solid target placed behind the foil allows converting these high-energy electrons into hard X-rays. The use of an ablated CH foil allows maximizing the conversion efficiency and thus the X-ray emission. In this study, the photon energy spectrum and dose are measured for different thicknesses of various metal targets such as tantalum. Numerical simulations including hydro-dynamical radiative, particle-in-cell, and Monte Carlo codes are made to give comparison with the experimental data. These are also compared with that of a bremsstrahlung emission and photon dose model in which the energy loss by Ohmic heating arising from the return current driven by the background electrons of the conductive target is taken into account [A. Compant La Fontaine, Phys. Plasmas 25, 043301 (2018)]. The results obtained allow for benchmarks to test the accuracy of this model and to check that the dose is maximized for high-Z solid targets and thickness in the mm range in the relativistic interaction regime at ultrahigh laser intensity (>10 18 W/cm 2). Published under license by AIP Publishing. https://doi.

The Task 1 of the ANR TRANSLIT project consisted on a transversal analysis of scientific, professional and institutional fields of the three information literacies: documentary information, computer and media. By the analysis and observation of texts issued by these three fields, called info-doc, info-data and info-media, the aim was to try to identify the convergences and discrepancies, the circulation of notions and to understand knowledge production dynamics at work in these fields. To do this, we analysed the corpus using textual analysis methods with a well-known tool: Alceste, a speech analysis software developed by Max Reinert and the CNRS (The National Centre for Scientific Research). The aim was to provide a deeper understanding of the orientation of texts and to be able to answer several questions: which are the terms, notions, authors and devices associated to information in the three fields? Which spannings and overlaps emerge from the comparison? Which aspects remain irreducible to a single discipline? Basing on which focal points can we define a transliteracy?; La Tâche 1 du projet ANR TRANSLIT a consisté en une analyse transversale des champs scientifiques, professionnels et institutionnels des trois cultures de l’information : info-documentaire, informatique et médiatique. Par l’analyse et l‘observation de textes issus de ces trois domaines, surnommés info-doc, info-data et info-media, il s’agissait de tenter de repérer les convergences et divergences, la circulation des notions et de comprendre les dynamiques de production de savoirs, à l’oeuvre dans ces domaines. Pour ce faire, nous avons utilisé les méthodes de l’analyse textuelle de corpus, avec un outil reconnu, Alceste, logiciel d’aide à l’analyse des discours, développé par Max Reinert et le CNRS. Il s’agissait d’apporter une connaissance sur ce que sont les orientations des textes et de pouvoir offrir des éléments de réponse à plusieurs questions : quels sont les termes, notions, auteurs et dispositifs associés à l’information dans les trois domaines ? Quels enjambements et recouvrements émergent de la comparaison ? Quels éléments restent irréductibles à une seule discipline ? A partir de quels points de convergence définir une translittératie ?

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During the interaction of a short-pulse high-intensity laser with the preplasma produced by the pulse's pedestal in front of a high-Z metal solid target, high-energy electrons are produced, which in turn create an X-ray source by interacting with the atoms of the converter target. The current brought by the hot electrons is almost completely neutralized by a return current j→ driven by the background electrons of the conductive target, and the force exerted on the hot electrons by the electric field E→ which induces Ohmic heating j→.E→, produced by the background electrons, reduces the energy of the hot electrons and thus lowers the X-ray emission and photon dose. This effect is analyzed here by means of a simple 1-D temperature model which contains the most significant terms of the relativistic Fokker-Planck equation with electron multiple scattering, and the energy equations of ions, hot, and cold electrons are then solved numerically. This Ohmic heating energy loss fraction τOh is introduced as a corrective term in an improved photon dose model. For instance, for a ps laser pulse with 10 μm spot size, the dose obtained with a tantalum target is reduced by less than about 10% to 40% by the Ohmic heating, depending upon the plasma scale length, target thickness, laser parameters, and in particular its spot size. The laser and plasma parameters may be optimized to limit the effect of Ohmic heating, for instance at a small plasma scale length or small laser spot size. Conversely, others regimes not suitable for dose production are identified. For instance, the resistive heating is enhanced in a foam target or at a long plasma scale length and high laser spot size and intensity, as the mean emission angle θ0 of the incident hot electron bunch given by the ponderomotive force is small; thus, the dose produced by a laser interacting in a gas jet may be inhibited under these circumstances. The resistive heating may also be maximized in order to reduce the X-ray emission to lower the radiation level for instance in a safety radiological goal.During the interaction of a short-pulse high-intensity laser with the preplasma produced by the pulse's pedestal in front of a high-Z metal solid target, high-energy electrons are produced, which in turn create an X-ray source by interacting with the atoms of the converter target. The current brought by the hot electrons is almost completely neutralized by a return current j→ driven by the background electrons of the conductive target, and the force exerted on the hot electrons by the electric field E→ which induces Ohmic heating j→.E→, produced by the background electrons, reduces the energy of the hot electrons and thus lowers the X-ray emission and photon dose. This effect is analyzed here by means of a simple 1-D temperature model which contains the most significant terms of the relativistic Fokker-Planck equation with electron multiple scattering, and the energy equations of ions, hot, and cold electrons are then solved numerically. This Ohmic heating energy loss fraction τOh is introduced as a co...

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On an AIRIX facility, a high-intensity electron beam is used to generate an x-ray pulse for radiographic purposes. The measured electron beam spot size is found to be much larger than its computed value. It is shown that this discrepancy is removed considering ion emission at the target, under electron beam interaction. A model has been developed to simulate the plasma produced during the interaction of the electron beam with the target. The predictions have been compared with ion identification results developed on the PIVAIR prototype accelerator as well as with the x-ray spot size and dose measurements obtained on AIRIX. The simulations show that the ion effect can be reduced by placing a thin foil (100 µm thickness) at an appropriate distance from the target to catch the ions emitted and to stabilize the electron beam spot size. The X-spot size is then reduced, as found experimentally on AIRIX. Moreover, for a smaller thickness barrier foil (5 µm), the ion effect can even be suppressed, producing an X-spot size close to that obtained without ion emission.

We present the experimental set-up developed on an undulator beam line of Super-ACO to measure on an absolute scale the photoionisation cross section of multiply-charged ions. Recent results obtained for Ba and Sm ions are presented.

A time-of-flight diagnostic made of a double sheet sensor has been designed for the search of ions emitted by an electron–photon conversion target under the impact of an intense relativistic electron beam. This device was tested on the PIVAIR prototype accelerator. Signals attributed to ions emitted from the target were observed for different target elements. The ions C+ and H+ were identified by time-of-flight measurements, and their mean velocities were found to be equal to about 1.6×108 and 5×108 cm/s for C and Ta targets, respectively. The appearance, rise time and saturation values of the ionic current are close to those obtained from preliminary calculations based on an ionic emission model (VAPOR) which has been coupled to a Maxwell–Vlasov PIC code (M2V). The ionic current signal is characterised by a plateau, the level of which corresponds to the Child–Langmuir current (25 A for H+ ions). Furthermore, the fall-off of this signal is synchronised with the electron beam one. This behaviour represents the de-confinement of the ion beam under the effect of its own space charge.

The characteristics of a sputtering plasma source using electron cyclotron resonance, operating on the ERIC facility dedicated to isotope separation by the plasma process, are described. A simple model based upon the particle conservation equation allows us to derive the ionization probability and the reflux fraction that govern the source operating conditions, both with and without carrier gas. Other data such as sputtering yields of Ar+ and Kr+ ions on Ni, Cu, Pd and Gd targets as well as their self-sputtering yields have been also determined for the first time in the case of gadolinium. The axial deposit profile of metal atoms on the lateral screen of the source is in accordance with the Yamamura semi-empirical differential sputtering yield, and the ionization coefficient of GdI is deduced.

The heating at ion-cyclotron resonance of various species of pure metallic plasmas (Mg, Ca, Zn, Ba and Yb) is measured experimentally with various diagnostics (using an energy analyser, Fabry-P?rot interferometry, laser absorption and induced fluorescence) and compared with the theory. The selectivity of the heating is found to be very sensitive to the characteristics of the plasma source. At each ion-cyclotron resonance, the refractive index of the fast wave exhibits an increase proportional to the square root of the abundance of the corresponding isotope, leading to a decrease of the exciting electric field and of the heating that is checked experimentally. The experiments and the theory show that the selective heating is proportional to the square of the antenna current. Also the increase of the heating measured with the static axial magnetic field is due to the increase of the fast-wave component as and to the decrease of the slow-wave component of the electric field by the space-charge effect. The decrease of the ionic heating with the plasma radius is due to the slow wave radial profile whose characteristic absorption length (3 cm) is about half the plasma radius.

Interaction of high-intensity laser pulses with solid targets results in generation of large quantities of energetic electrons that are the origin of various effects such as intense x-ray emission, ion acceleration, and so on. Some of these electrons are escaping the target, leaving behind a significant positive electric charge and creating a strong electromagnetic pulse long after the end of the laser pulse. We propose here a detailed model of the target electric polarization induced by a short and intense laser pulse and an escaping electron bunch. A specially designed experiment provides direct measurements of the target polarization and the discharge current in the function of the laser energy, pulse duration, and target size. Large-scale numerical simulations describe the energetic electron generation and their emission from the target. The model, experiment, and numerical simulations demonstrate that the hot-electron ejection may continue long after the laser pulse ends, enhancing significantly the polarization charge.

The first experiments on the National Ignition Facility (NIF) in the US started and will be followed by the Laser MegaJoule (LMJ) in France. Such facilities will provide unique tools for inertial confinement fusion (ICF) physics & for basic science. A petawatt short pulse laser (ps) is being added to the ns pulse beams of the LMJ. This is PETAL (PETawatt Aquitaine Laser), under construction on the LMJ site near Bordeaux (France). The Petal+ project is aiming at the design and construction of diagnostics dedicated to experiments with PETAL and LMJ laser beams. Within Petal+, three types of diagnostics are under study: a proton spectrometer, an electron spectrometer and a large-band X-ray spectrometer. The first goal of these diagnostics will be to characterize the secondary radiation and particle sources produced with PETAL. They will also be used for experiments using both ns and ps beams. In the present paper emphasis is put on the charged-particle diagnostics.

Plusieurs concepts etudies pour la realisation des sources X du domaine multi-keV/ns au domaine multi-MeV/ps sont abordes. Ces sources de rayonnement sont developpees dans le but de radiographier les cibles sur le Laser Megajoule (LMJ). Pour valider ces concepts et pour predimensionner ces sources de radiographie, de nombreuses campagnes experimentales ont ete menees sur des installations comme ALISE, GEKKO XII, OMEGA, OMEGA-EP et LIL.

The plasma separation process by ion cyclotron resonance heating (ICRH) is studied both theoretically and experimentally on two devices: the first one called ERIC (Ion Cyclotron Resonance Experiment) at Saclay (France) [P. Louvet, Proceedings of the 2nd Workshop on Separation Phenomena in Liquids and Gases, Versailles, France, 1989, edited by P. Louvet, P. Noe, and Soubbaramayer (Centre d’Etudes Nucleaires de Saclay and Cite Scientifique Parcs et Technopoles, Ile de France Sud, France, 1989), Vol. 1, p. 5] and the other one named SIRENA at the Kurchatov Institute, Moscow, Russia [A. I. Karchevskii et al., Plasma Phys. Rep. 19, 214 (1993)]. The radio frequency (RF) transversal magnetic field is measured by a magnetic probe both in plasma and vacuum and its Fourier spectrum versus the axial wave number kz is obtained. These results are in agreement with the electromagnetic (EM) field calculation model based on resolution of Maxwell equations by a time‐harmonic scheme studied here. Various axial boundary con...

The paper describes the preparation of the short-pulse high-energy laser PETAL that will be coupled to the French megajoule laser (LMJ) of CEA. The LMJ/PETAL facility will be opened to academic access for the international research community. In parallel diagnostics are being developed within the PETAL project and many physical problems are being addressed ranging from the study of the problems of radiation generation and activation issues to the problem of generation of large amplitude electromagnetic pulses.

We have analyzed the coupling of ultraintense lasers (at {approx}2x10{sup 19} W/cm{sup 2}) with solid foils of limited transverse extent ({approx}10 s of {mu}m) by monitoring the electrons and ions emitted from the target. We observe that reducing the target surface area allows electrons at the target surface to be reflected from the target edges during or shortly after the laser pulse. This transverse refluxing can maintain a hotter, denser and more homogeneous electron sheath around the target for a longer time. Consequently, when transverse refluxing takes places within the acceleration time of associated ions, we observe increased maximum proton energies (up to threefold), increased laser-to-ion conversion efficiency (up to a factor 30), and reduced divergence which bodes well for a number of applications.

Results of an experimental study of multi-MeV bremsstrahlung x-ray sources created by picosecond laser pulses are presented. The x-ray source is created by focusing the short pulse in an expanding plasma obtained by heating a solid target with a time-delayed nanosecond laser beam. The high-energy part of the x-ray spectrum and emission lobe are inferred from photonuclear activation techniques. The x-ray dose is measured with silicon diodes. Two-dimensional images of the source are reconstructed from a penumbral imaging technique. These results indicate the creation of a relatively small source, below 200 µm diameter, delivering doses up to 12 mrad in air at 1 m with x-ray temperature up to 2.8 MeV. The diagnostics used give access to a whole set of coherent experimental results on the x-ray source properties which are compared to extensive numerical simulations. X-ray intensity and temperature are found to increase with the size of the preplasma. ©2009 American Institute of Physics

International audience; Un des axes principaux de la formation disciplinaire des PLC2 Documentation à l'IUFM de Basse-Normandie privilégie la construction des concepts informationnels par les stagiaires, dans un premier temps, afin de favoriser, dans un second temps, au travers d'un travail de transposition didactique et de conception de situations d'enseignement, l'appropriation de ces connaissances par les élèves eux-mêmes. L'élaboration de cartes conceptuelles, qui permet la modélisation des différentes catégories de connaissances convoquées dans une situation informationnelle complexe, est apparue aux formatrices comme un outil intellectuel pertinent de conceptualisation. L'analyse de cartes réalisées par les stagiaires montre, au-delà d'un certain nombre de difficultés, les avantages de cette méthode et l'enrichissement qu'elle apporte à la maîtrise des connaissances informationnelles ainsi qu'à la réflexion didactique et à la conception des situations d'enseignement-apprentissage.

International audience; Previous experimental and numerical results have shown that in multi-MeV Bremsstrahlung x-ray sources created by picosecond laser pulses, the delivered on-axis dose increases if the laser interacts with under-critical plasmas. The effect of the laser-plasma interaction as a function of the plasma density gradient is thus first studied here, by focusing the short pulse of the Alisé laser on a solid tantalum target coated with plastic. Interaction conditions are modified by irradiating the target front side with a secondary nanosecond heating beam, prior to the main pulse. The length of the expanding plasma is modified by adjusting the delay between the interaction and heating beams. Various diagnostics give access to a whole set of consistent experimental results on the x-ray source properties which are compared successfully to self-consistent numerical simulations obtained with coupled PIC and Monte Carlo codes. All source parameters increase as the plasma length increases, in reason of the increasing number of high-energy electrons near the axis. In a second part of this talk, we use our simulation tools to predict the x-ray and neutron (produced by photonuclear interactions in the target) flash characteristics that can be expected on future PW lasers. Favourable scaling laws for the production of an intense, directional and short duration x-ray source versus intensity are observed. However, these findings also stress that radioprotection will become an important issue for future petawatt lasers operating at the kJ level.

The dynamics of femtosecond light pulses ionizing noble gases from single or multiple ionization schemes are numerically compared, with the emphasis on two-color pulses. We discuss the influence of multiply-charged ions on the excitation of harmonics in focused propagation and on the supercontinuum and clamping intensity of laser filaments. Depending on the atom’s first binding energy, multiple ionization based on Ammosov–Delone–Krainov (ADK) tunneling rates can increase the peak intensities, the number of harmonics or the spectral blueshifts, compared with quasistatic hydrogen-like rates. With two-color pulses, noticeable differences occur in the frequency domain corresponding to broadband terahertz pulses.

We present absolute measurements for photoionisation of multiply-charged ions N2+ and O3+ in the ground and metastable states. Multiconfiguration Dirac Fock calculations were performed for N2+ and are tested here by comparison with experimental results. Our experimental and theoretical results are also compared to the theoretical data available in the Opacity Project data base.

When a high-intensity laser pulse hits a solid target, its pedestal creates a preplasma. The interaction of the main laser pulse, linearly polarized, with this preplasma produces relativistic electrons. These electrons subsequently penetrate inside the target, with high atomic number, and produce bremsstrahlung emission, which constitutes an x-ray source that may be used in various applications such as radiography of high area density objects, photonuclear studies or positron production. This x-ray source is mainly defined by its photon dose, which depends upon the laser, preplasma and target characteristics. In new facilities the radioprotection layout design can be obtained by numerical simulations, which are somewhat tedious. A simple model giving the photon dose per laser energy unit is obtained by using the mean bremsstrahlung cross section of electrons interacting with the atoms of the conversion target. It is expressed versus the fraction ηel of the laser energy absorbed into the forward hot electrons, their mean kinetic energy E, the photon lobe emission mean angular aperture and the target characteristics, i.e. thickness, element, atomic mass and atomic number. The parameters ηel, E and are analysed by applying the energy and momentum flux conservation laws during the laser–plasma interaction in the relativistic regime in an underdense and overdense plasma, including the hole-boring effect. In addition, these quantities are parametrized versus the normalized laser vector potential a0 and the preplasma scale length Lp by using a full set of numerical simulations, in the laser intensity domain 1018–1021 W cm−2 and preplasma scale length range 0.03–400µm. These simulations are done in two- and three-dimensional geometry with the CALDER particle-in-cell code, which computes the laser–plasma interaction, and with the MCNP Monte Carlo code, which calculates the bremsstrahlung emission. The present model is compared with the simulations and with experimental results.

Photoionization of multiply charged ions of the Ba isonuclear sequence up to Ba 6 + has been studied in a beam-beam experiment. A very strong increase in the resonance structures was observed when moving from Ba 2 + to Ba 6 + . Absolute values of the photoionization cross sections were measured for Ba 2 + and Ba 3 + ions. The interpretation of the results is provided using theoretical multiconfiguration Dirac-Fock and relativistic random phase approximation calculations, showing that the collapse of the nf orbitals occurs for Ba 4 + .

A beam diagnostic made of a double sheet sensor is designed measure ions emitted by an electron-photon conversion target under the impact of an intense relativistic electron beam. This device is tested on the PIVAIR prototype accelerator. Signals attributed to ions emitted from the target are observed for different target elements: Ta, C, Ti, Au and light species are identified by their time of flight. The mean velocities of C/sup +/ and H/sup +/ ions are found to be respectively equal to 1.2 /spl times/10/sup 8/ cm/s and 5 /spl times/10/sup 8/ cm/s for C and Ta targets. The ionic appearance and rise time signals are closed to the expected values. Besides, the current signals are characterized by a plateau to which level corresponds to the Child-Langmuir current (25 A for H/sup +/ ions). Furthermore, the fall-off of this signal is found to be in synchronism with the electron beam signal. This behaviour represents the deconfinement of the ion beam under the effect of its own space charge. These experimental results confirm the preliminary calculations obtained with the M2V Maxwell-Vlasov PIC code which has been coupled with an ionic emission model.

We present new results obtained on photoionization processes in Xe3+ to Xe7+ ions, using ion spectrometry. The experiment combines for the first time a beam of multiply-charged ions produced in an electron cyclotron resonance (ECR) ion source and a beam of synchrotron radiation emitted from an undulator. These results provide a complete picture of the behavior of resonant and continuum photoionization processes along the Xe isonuclear sequence, following the complete stripping, electron by electron, of the 5p subshell.We present new results obtained on photoionization processes in Xe3+ to Xe7+ ions, using ion spectrometry. The experiment combines for the first time a beam of multiply-charged ions produced in an electron cyclotron resonance (ECR) ion source and a beam of synchrotron radiation emitted from an undulator. These results provide a complete picture of the behavior of resonant and continuum photoionization processes along the Xe isonuclear sequence, following the complete stripping, electron by electron, of the 5p subshell.

The interaction of a high intensity laser pulse on the preplasma of a high-Z solid target produced by the pulse's pedestal generates high-energy electrons. These electrons subsequently penetrate inside the solid target and produce bremsstrahlung photons, generating an x-ray source which can be used for photonuclear studies or to radiograph high area density objects. The source characteristics are compared for targets with thin (20 μm) and thick (100 μm) Au foils on the Omega EP laser at Laboratory for Laser Energetics. Simulations using the particle-in-cell code CALDER show that for a 20 μm thickness Au target, electrons perform multiple round-trips in the target under the effect of the laser ponderomotive potential and the target electrostatic potential. These relativistic electrons have random transverse displacements, with respect to the target normal, attributed to electrostatic fluctuation fields. As a result, the x-ray spot size is increased by a factor 2 for thin target compared to thick targets, in agreement with experimental results. In addition, the computed doses agree with the measured ones provided that electron recirculation in the thin target is taken into account. A dose increase by a factor 1.7 is then computed by allowing for recirculation. In the 100 μm target case, on the other hand, this effect is found to be negligible.

Results of an experiment to characterise a MeV Bremsstrahlung x-ray emission created by a short (

In order to achieve effective isotopic separation by the Plasma Separation Process (PSP) near the ion cyclotron resonance, experiments are performed on the ERIC device,1,2 and in parallel theory is developed. Some recent results dealing with the basic properties of the electromagnetic wave induced in the plasma are presented. Also the conditions for which the heating is the most selective are given. In addition experimental measurements of ionic temperatures of resonant isotopes are compared with theoretical results.