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Soft x-ray spectra (10-70 ˚ A) from carbon and aluminium plasmas produced by high- intensity laser pulses have been studied as a function of laser pulse duration (130 fs-1.3 ps) and intensity (10 14 -5 10 17 Wc m 2 ) for two different laser wavelengths (248 and 800 nm). The x-ray energy emitted from the plasma into specific lines and into the continuum has been measured by means of an absolutely calibrated x-ray spectrograph and thus allows the water window x-ray yield from plasmas produced by long and short wavelength laser radiation to be compared directly. Comparison was made between the dependence of the x-ray energy and width of the x-ray lines on the laser pulse duration, wavelength and intensity. The experimental results have been related to hydrocode simulations.

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International audience; The interaction of 150 fs laser pulses with very thin plastic targets at an intensity of 5×1017 W/cm2 was investigated experimentally. Second-harmonic (SH) radiation was found to be emitted only in the specular direction. Both SH intensity and hard x-ray yield were found to be strongly dependent upon the laser polarization. The main features of SH emission are in agreement with a theoretical model which assumes resonance absorption as the source mechanism of electron plasma waves. Measurements suggest that, in conditions of maximum energy absorption, wave breaking of resonantly excited electron plasma waves takes place.

A diagnostic currently being used for ultrashort pulse laser matter interactions at the Lawrence Livemore National Laboratory is presented. Two cylindrical curved crystals are used as collection optics and diffractive elements in the von Hamos geometry. The dispersed x rays are focused onto the slit of an ultrafast x-ray streak camera. The small nature of the streak camera allows the diagnostic to be positioned close to the target, maximizing the solid angle. Using the geometric layout, the spectral resolution is estimated at E/ΔE∼1000.

Isochoric heating of solid-density matter up to a few tens of eV is of interest for investigating astrophysical or inertial fusion scenarios. Such ultra-fast heating can be achieved via the energy deposition of short-pulse laser generated electrons. Here, we report on experimental measurements of this process by means of time-and space-resolved optical interferometry. Our results are found in reasonable agreement with a simple numerical model of fast electron-induced heating. (C) 2013 AIP Publishing LLC.

Retrieving the characteristics of hot electrons produced in the interaction between solid targets and ultra-intense (I?>?1018?W?cm?2) laser pulses is essential for achieving?progress in our understanding of the interaction physics, which is?key for optimizing numerous downstream applications. Until now, various methods have been used, direct or indirect, but no correlation and no assessment of their respective merits were performed. Here we compare results retrieved from four different diagnostics, direct or indirect, as well as local or non-local, i.e. spectrometry of electrons, spectrometry of the protons accelerated by the electrons and optical probing of these beams expanding into vacuum from the targets. We show that measurements obtained locally at the target rear surface are consistent with those far away from the target and that one can use the diagnostics of the co-moving proton beams to retrieve information about electrons.

We demonstrate for the first time attosecond time scale control of collective electron motion in overdense plasmas driven by intense waveform-controlled few-cycle laser pulses

On presente une etude de l'ionisation optique de l'Argon a une pression voisine de l'atmosphere, par une impulsion laser femtoseconde avec un eclairement de 2-3 l O ^ W / c m 2 . L'effet de defocalisation, observe dans l'Argon statique a une pression de 1 bar, est notablement reduit par la mise en oeuvre d'un jet puise, d'une densite de 1-3 1 0 1 9 atomes/cirA L' emission Ka et le taux d'ionisation de l'Argon presentent un maximum tres localise pour une focalisation de l'impulsion laser en bord de jet. L'etat d'ionisation atteint dans ces conditions par l'Argon est de 5 a 6, en bon accord avec les predictions d'un modele

The authors present experimental results and numerical simulations of K-shell emission from Be-like, Li-like and He-like ions from a 100 fs laser-produced aluminium plasma at intensities in excess of 1016 W cm-2. The use of thin film targets of thickness comparable to the laser skin depth greatly facilitates the interpretation of time-integrated spectra and permits the study of quasi-homogeneous temperature plasmas with limited plasma line broadening effects. Time-dependent modelling shows that the X-ray pulse duration of the Be-like and Li-like emissions is much shorter than the laser pulse. Time-integrated model spectra are in good agreement with experiment.

We present modelling results to interpret Li-like satellite spectra recorded from solid Al targets irradiated with ultra-short (80 fs), high intensity 1016-1017 W cm-2 laser pulses. A fully time-dependent, non-local thermodynamic equilibrium (NLTE) atomic kinetics model must be used to account for the observed satellite peak-intensity distribution; a stationary NLTE model is not sufficient. Hot electron effects on the doubly excited states population kinetics are also considered. Detailed Stark broadened line profiles are employed to compute the synthetic spectra. Modelling results indicate that the satellite emission is emitted in a 100 fs pulse.

High-harmonic generation (HHG) from solid targets driven by ultra-high-intensity lasers is predicted to be a source of attosecond XUV pulses with shorter wavelengths and higher energies than those currently generated in gases [1,2]. A much less explored regime consists in focusing few-cycle pulses down to wavelength-limited spot sizes in order to reach the ultrahigh intensities using mJ pulse energies. This enables HHG from solid targets using waveform-controlled few-cycle pulses readily obtainable from compact kHz laser systems. Here, we show the first experimental demonstration of HHG from solid targets using intense CEP-controlled two-cycle laser pulses.

Electrostatic (E) fields associated with the interaction of a well-controlled, high-power, nanosecond laser pulse with an underdense plasma are diagnosed by proton radiography. Using a current 3D wave propagation code equipped with nonlinear and nonlocal hydrodynamics, we can model the measured E-fields that are driven by the laser ponderomotive force in the region where the laser undergoes filamentation. However, strong fields of up to 110 MV/m measured in the first millimeter of propagation cannot be reproduced in the simulations. This could point to the presence of unexpected strong thermal electron pressure gradients possibly linked to ion acoustic turbulence, thus emphasizing the need for the development of full kinetic collisional simulations in order to properly model laser-plasma interaction in these strongly nonlinear conditions., 12 pages, 4 figures, submitted to Physics of Plasmas

We demonstrate for the first time kHz-driven high-harmonic generation from solid targets with fully carrier-envelope phase-controlled few-cycle pulses.

International audience; We report the first demonstration of high-harmonic generation from plasma mirrors at a 1 kHz repetition rate. Harmonics up to nineteenth order are generated at peak intensities close to 10$^{18}$ W=cm$^2$ by focusing 1 mJ, 25 fs laser pulses down to 1:7 $\mu$m FWHM spot size without any prior wavefront correction onto a moving target. We minimize target surface motion with respect to the laser focus using online interferometry to ensure reproducible interaction conditions for every shot and record data at 1 kHz with unprecedented statistics. This allows us to unambiguously identify coherent wake emission as the main generation mechanism

Cet article traite du dimensionnement des diagnostics imageurs X a haute resolution spatiale (≤ 10μ m) concus et developpes pour le Laser MegaJoule. On introduira ceux-ci en abordant les objectifs physiques des experiences mettant en jeu ces imageurs ainsi que l’environnement hostile dans lequel ils seront susceptibles de travailler. On etudiera les concepts geometriques lies au caractere haute resolution des imageurs. Enfin, on s’attachera a l’aspect particulier de la conception de diagnostics imageant des rayons X avec une bande passante allant jusqu’a 20 keV.

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.

International audience; We report on a set of experiments in which solid targets of different atomic numbers (Z) were irradiated with laser pulses of time durations ranging from 300 fs to 33 ps, and energies up to 26 J. The time-resolved X-ray emission in the 7.6-8.1 Å spectral range was measured using an ultra-fast X-ray streak camera coupled with a conical Bragg crystal. In this way we were able to follow the dramatic modification of the spectral features as a function of the laser duration. The features evolve from a "ns-type" emission, characterized by narrow and well-defined spectral lines, to very broad spectral features, due not only to the Stark broadening but also to the proliferation of satellites lines. The measured spectra also show strong time dependence, which allows us to follow the time evolution of the hydrodynamic parameters. We then compare the derived parameter with the CHIVAS hydro-radiative simulations. The experimental results are also compared with the AVERROES/TRANSPEC collisional-radiative code, and with precise spectral line shape calculations (PPP and PrismSPECT). The results seem to indicate regimes of interaction where hot electrons play an important role on spectral line formation.

We report experiments demonstrating enhanced coupling efficiencies of high-contrast laser irradiation to nanofabricated conical targets. Peak temperatures near 200 eV are observed with modest laser energy (10 J), revealing similar hot-electron localization and material heating to reduced mass targets (RMTs), despite having a significantly larger mass. Collisional particle-in-cell simulations attribute the enhancement to self-generated resistive (approximately 10 MG) magnetic fields forming within the curvature of the cone wall, which confine energetic electrons to heat a reduced volume at the tip. This represents a different electron confinement mechanism (magnetic, as opposed to electrostatic sheath confinement in RMTs) controllable by target shape.

We will present the ELI Project consisting in a 200PW laser based high intensity physics users infrastructure. A French single beamline laser prototype called ILE/APOLLON is presently under construction. Associated technological bottlenecks will be described.

International audience

International audience; no abstract

Nickel nanowires present >90% absorption in an effective skin-depth ~1 mum, making efficient X-ray converters. We present new theoretical and experimental results for intensities from small-signal up to very clean relativistic pulses.

We will discuss the two mechanisms involved in high-order harmonic generation from plasma mirrors, and show that they can be clearly identified experimentally. The phase and coherence properties of these harmonics will be analyzed.

Fast adiabatic plasma heating of a thin solid target irradiated by a high intensity laser has been observed by an optical fast interferometry diagnostic. It is driven by the hot electron current induced by the laser plasma interaction at the front side of the target. Radial and longitudinal temperature profiles are calculated to reproduce the observed rear-side plasma expansion. The main parameters of the suprathermal electrons (number, temperature, and divergence) have been deduced from these observations.

The postacceleration of laser-generated protons through conventional drift-tube linear accelerators (linacs) (DTLs) has been simulated with the particle-code PARMELA. The proton source is generated on the rear surface of a target irradiated by an high-intensity short-pulse (350 fs) laser and focused by a microlens that allows selecting collimated protons at 7 0.1 MeV with root-mean-square unnormalized transverse emittance of 0.180 mm mrad. The simulations show that protons can be accelerated by one DTL tank to more than 14 MeV with un normalized transverse-emittance growth of 8 and 22.6 in different transverse directions when considering a total proton charge of 0.112 mA. This first numerical study shows that coupling between laser-plasma accelerators with traditional accelerators is possible, allowing a luminosity gain for the final beam.

We have compared the acceleration of high-energy ions from the rear surface of thin foils for various prepulse contrast conditions of the ultraintense laser pulse irradiating the targets. The experiments were performed using the Laboratoire pour I'Utilisation des Lasers Intenses 100-TW laser facility with 15-20 J energy pulses of >1018 W/cm2 intensity irradiating aluminum targets of variable thicknesses. The laser pulse contrast ratio ahead of the main pulse (prepulse) was varied using either a fast Pockels cell or a single or double plasma mirror (PM). The latter was installed at an intermediate field position, in between the focusing optics and the target. This way, the effect of reducing the laser prepulse by the PM was optimized. With both methods improving the laser pulse contrast, we have observed that one can significantly reduce the thickness of the target used for proton acceleration and, at the same time, increase 1) the maximum energy of the accelerated protons and 2) the energy conversion efficiency of the process. This offers interesting perspectives from high-energy (5-10 MeV) ion acceleration using sub-petawatt laser facilities. A full characterization of contrast enhancement due to the PM, as well as a detailed analysis of the accelerated proton beams, is presented.

The post‐acceleration of laser‐generated protons through conventional drift tube linear accelerators has been simulated with the particle code Parmela. The proton source is generated on the rear surface of a target irradiated by an high‐intensity (1019 W⋅cm−2) short‐pulse (350 fs) laser and focused by a microlens that allows selecting collimated protons at 7±0.1 MeV with rms unnormalized emittance of 0.180 mm.mrad. The simulations show that protons can be accelerated by one drift tube linac tank to more than 14 MeV with unnormalized emittance growth of 8 in x and 22.6 in y directions when considering a total proton charge of 0.112 mA. This result shows for the first time that coupling between laser‐plasma accelerators with traditional accelerators is possible, allowing a luminosity gain for the final beam.

Nanostructured metal targets exhibit low-intensity linear optical absorption > 95%. Is such absorption also possible for ultra-intense femtosecond laser pulses? Analytic theory (low intensities) and particle-in-cell simulations (high intensities) show similarities and remarkable differences.

Nickel nanowires present >90% absorption in an absorption depth ~1 µm, making efficient x-ray converters at high energy-densities. We present new theoretical and experimental results for intensities from small-signal up to relativistic ultrafast pulses.

High-energy petawatt lasers require large diffraction gratings for pulse compression. As an alternative to meter-sized gratings, we demonstrate the capability of a tiled-grating system to compress multiterawatt subpicosecond laser pulses. Using a 100 TW-class Nd:glass chirped-pulse amplification laser facility, we report on the performance of a two-grating mosaic to compress high-energy pulses to 2.5 J, 450 fs (5.5 TW) in air with a beam size of 50 mm and energy transmission of 63%. Stability of the grating mosaic alignment was realized by use of an accurate nanopositioning optomechanical system. The output Gaussian spectrum was preserved from grating-gap spectral clipping and was free of modulation.

Specular reflection is one of the most fundamental processes of optics. At moderate light intensities generated by conventional light sources this process is well understood. But at those capable of being produced by modern ultrahigh-intensity lasers, many new and potentially useful phenomena arise. When a pulse from such a laser hits an optically polished surface, it generates a dense plasma that itself acts as a mirror, known as a plasma mirror (PM). PMs do not just reflect the remainder of the incident beam, but can act as active optical elements. Using a set of three consecutive PMs in different regimes, we significantly improve the temporal contrast of femtosecond pulses, and demonstrate that high-order harmonics of the laser frequency can be generated through two distinct mechanisms. A better understanding of these processes should aid the development of laser-driven attosecond sources for use in fields from materials science to molecular biology.

We present and characterize a very efficient optical device that employs the plasma mirror technique to increase the contrast of high-power laser systems. Contrast improvements higher than 10(4) with 50% transmission are shown to be routinely achieved on a typical 10 TW laser system when the pulse is reflected on two consecutive plasma mirrors. Used at the end of the laser system, this double plasma mirror preserves the spatial profile of the initial beam, is unaffected by shot-to-shot fluctuations, and is suitable for most high peak power laser systems. We use the generation of high-order harmonics as an effective test for the contrast improvement produced by the double plasma mirrors.

The acceleration of high-energy ion beams (up to several tens of MeV per nucleon) following the interaction of short ( t < 1ps ) and intense (Iλ2 > 1018W cm-2μm2) laser pulses with solid targets has been one of the most active areas of research in the last few years. The exceptional properties of these beams (high brightness and high spectral cutoff, high directionality and laminarity, short burst duration) distinguish them from those of the lower energy ions accelerated in earlier experiments at moderate laser intensities. In view of these properties, laser-driven ion beams can be employed in a number of groundbreaking applications in the scientific, technological and medical areas. This paper reviews the current state-of-the-art, highlights recent developments and indicate future directions of this research area.

Point‐projection K‐shell absorption spectroscopy has been used to measure absorption spectra of transient plasma created by an ultra‐short laser pulse. The 1s‐2p and 1s‐3p absorption lines of weakly ionized aluminum and the 2p‐3d absorption lines of bromine were measured over an extended range of densities in a low‐temperature regime. Independent plasma characterization was obtained using frequency domain interferometry diagnostic (FDI) that allows the interpretation of the absorption spectra in terms of spectral opacities. Assuming local thermodynamic equilibrium, spectral opacity calculations have been performed using the density and temperature inferred from the FDI diagnostic to compare to the measured absorption spectra. A good agreement is obtained when non‐equilibrium effects due to non‐stationary atomic physics are negligible at the x‐ray probe time.

Low temperature poly-silicon thin film transistors (LTPS TFT) technology is very suitable for low cost fingerprint sensor. In this paper, a new LTPS TFT capacitive sensor is presented. The proposed sensing scheme is based on charge sensitive amplifiers using correlated double sampling techniques (CDS) for offset voltage cancellation. Simulation results exhibit that the random offset voltage is successfully compensated.

We present a general expression for the maximum ion energy observed in experiments with thin foils irradiated by high-intensity laser pulses. The analytical model is based on a radially confined surface charge set up by laser accelerated electrons on the target rear side. The only input parameters are the properties of the laser pulse and the target thickness. The predicted maximum ion energy and the optimal laser pulse duration are supported by dedicated experiments for a broad range of different ions.

We present a new mechanism for high-order harmonic generation by reflection of a laser beam from an overdense plasma, efficient even at moderate laser intensities (down to $I{\ensuremath{\lambda}}^{2}\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{15}\text{ }\text{ }\mathrm{W}\text{ }{\mathrm{cm}}^{\ensuremath{-}2}\text{ }\ensuremath{\mu}{\mathrm{m}}^{2}$). In this mechanism, a transient phase matching between the electromagnetic field and plasma oscillations within a density gradient leads to the emission of harmonics up to the plasma frequency. These plasma oscillations are periodically excited in the wake of attosecond electron bunches which sweep across the density gradient. This process leads to a train of unevenly spaced chirped attosecond pulses and, hence, to broadened and chirped harmonics. This last effect is confirmed experimentally.

We demonstrate that in ultraintense ultrafast laser-matter interaction, the interplay of laser-induced oscillating space-charge fields with laser E and B fields can strongly affect whether the interaction is relativistic or not: stronger laser fields may not in fact produce more relativistic plasma interactions. We show that there exists a regime of interaction, in the relation of laser intensity and incident angle, for which the Brunel effect of electron acceleration is strongly suppressed by AC gyromagnetic fields, at a frequency different from the laser field. Analytically and with 1.5D particle-in-cell modeling, we show that from gyromagnetic effects, even in the absence of usual J x B second-harmonic contributions, there are strong effects on the harmonic emission and on the generation of attosecond pulses.

We report on first measurements of the transverse characteristics of laser-produced energetic ion beams in direct comparison to results for laser accelerated proton beams. The experiments show the same low emittance for ion beams as already found for protons. Additionally, we demonstrate that the divergence is influenced by the charge over mass ratio of the accelerated species. From these observations we deduced scaling laws for the divergence of ions as well as the temporal evolution of the ion source size.

High harmonic generation from ultra-intense laser-matter interaction can be generated by both linear and relativistic means. In experiments with intensity up to a few 10 19 Wcm -2 , we show the distinctions and means to control each.

In the last few years, intense research has been conducted on laser‐accelerated ion sources and their applications. These sources have exceptional properties, i.e. high brightness and high spectral cut‐oft high directionality and laminarity, short burst duration. These proton sources open new opportunities for ion beam generation and control, and could stimulate development of compact ion accelerators for many applications. We have studied the variations of the proton acceleration characteristic time with target and laser parameters. We used these variations to correct one of the model recently developed to predict maximum energies of laser‐accelerated protons for low energy, short duration laser pulses. We have also developed an ultra‐fast laser‐triggered micro‐lens that allows tunable control of the beam divergence as well as energy selection, therefore solving two of the major problems that these proton beams were facing. We used PIC simulations to explain the focusing and energy selection mechanisms, ...

Laser‐accelerated ion sources open new opportunities for ion beam generation and control, and could stimulate development of compact ion accelerators for many applications. The mechanisms of proton acceleration with solid targets have been intensively studied over the past years, and new target or laser setups are now needed to obtain even higher maximum proton energies. PIC simulations have shown that using ultra thin targets, the maximum proton energy can be greatly increased. The laser can pass through the target and heat target electrons more efficiently. Experiments were conducted to test the feasibility of ultra thin targets laser interaction. PIC simulations were performed and successfully compared to the experimental results. Recently, experiments have shown that a gaseous target can produce proton beams with characteristics comparable to those obtained with solid targets. PIC simulations were also used to study proton acceleration with an underdense target. The optimum thickness obtained corresponds to the thickness where the laser absorption and transmission are equal, and depends greatly on laser and target parameters. The plasma hot electron temperature has also been found to depend on both laser and target parameters. We developed a simple model for the scaling of the optimum thickness for proton acceleration on target and laser parameters.

The extension of time-resolved X-ray diffraction to the subpicosecond domain is an important challenge, as the nature of chemical reactions and phase transitions is determined by atomic motions on these timescales. An ultimate goal is to study the structure of transient states with a time resolution shorter than the typical period of vibration along a reaction coordinate (around 100 fs). Biological processes that can be initiated optically have been studied extensively by ultrafast infrared, visible and ultraviolet spectroscopy1. But these techniques probe only electronic states, whereas time-resolved crystallography should be able to directly monitor atomic positions. Here we show that changes in the X-ray diffraction pattern from an organic film heated by a laser pulse can be monitored on a timescale of less than a picosecond. We have studied the response of a Langmuir–Blodgett multilayer film of cadmium arachidate to laser heating by observing changes in the intensity of one Bragg peak for different delays between the perturbing optical pulse and the X-ray probe pulse. A strong decrease in intensity is seen within a picosecond of heating, resulting from disorder introduced to the layers of cadmium atoms before thermal expansion of the film (which ultimately leads to its destruction) has time to occur.

The acceleration of multi-MeV protons from the rear surface of thin solid foils irradiated by an intense ($\ensuremath{\sim}{10}^{18}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$) and short ($\ensuremath{\sim}1.5\text{ }\text{ }\mathrm{ps}$) laser pulse has been investigated using transverse proton probing. The structure of the electric field driving the expansion of the proton beam has been resolved with high spatial and temporal resolution. The main features of the experimental observations, namely, an initial intense sheath field and a late time field peaking at the beam front, are consistent with the results from particle-in-cell and fluid simulations of thin plasma expansion into a vacuum.

Since their discovery, laser accelerated ion beams have been the subject of great interest. The ion beam peak power and beam emittance is unmatched by any conventionally accelerated ion beam. Due to the unique quality, a wealth of applications has been proposed, and the first experiments confirmed their prospects. Laser ion acceleration is strongly linked to the generation and transport of hot electrons by the interaction of ultra-intense laser light with matter. Comparing ion acceleration experiments at laser systems with different beam parameters and using targets of varying thickness, material and temperature, some insight on the underlying physics can be obtained. The paper will present experimental results obtained at different laser systems, first beam quality measurement on laser accelerated heavy ions, and ion beam source size measurements at different laser parameters. Using structured targets, we compare information obtained from micro patterned ion beams about the accelerating electron sheath, and the influence of magnetic fields on the electron transport inside conducting targets.

We have used point-projection K-shell absorption spectroscopy to infer the ionization and recombination dynamics of transient aluminum plasmas. Two femtosecond beams of the 100 TW laser at the LULI facility were used to produce an aluminum plasma on a thin aluminum foil (83 or 50 nm), and a picosecond x-ray backlighter source. The short-pulse backlighter probed the aluminum plasma at different times by adjusting the delay between the two femtosecond driving beams. Absorption x-ray spectra at early times are characteristic of a dense and rather homogeneous plasma. Collisional-radiative atomic physics coupled with hydrodynamic simulations reproduce fairly well the measured average ionization as a function of time.