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International audience; In the design of laser plasma electron injectors for multi-stage laser driven wakefield accelerators, the control of plasma density is a key element to stabilize the acceleration process. A cell with variable parameters is used to confine the gas and tailor the density profile. The gas filling process was characterized both experimentally and by fluid simulations. Results show a good agreement between experiments and simulations. Simulations were also used to study the effect of each of the gas cell parameters on the density distribution and show the possibility to finely control the density profile.

In this paper, the interaction of 10-TW laser pulses, focused at moderately relativistic intensity, with a supersonic he-lium gas-jet has been investigated by varying gas density and jet nozzle. We have successfully tested several advanced diagnostic devices to characterize the plasma and the accelerated electron bunches. Plasma densities have been measured by means of a fem-tosecond high-resolution interferometer, while the electron beams were analyzed with a stack of radiochromic films, a beam-profile monitor, a magnetic spectrometer, and a nuclear activation setup based on gamma-ray generation via electron bremsstrahlung. We present the results as well as the basic features and relevant details of such diagnostics whose performances can fit a large class of experiments.

X-ray spectroscopic diagnostics of laser-cluster interaction at the stage of nonadiabatic scattering of clusters and formation of a spatially uniform plasma channel has been performed. The experimental investigations have been carried out on a Ti:Sa laser setup with a pulse duration of about 65 fs and an energy up to 600 mJ. It has been shown that, within 10 ps from the beginning of a laser femtosecond pulse, the laser-cluster interaction forms a uniform plasma channel with a length of 0.4 to 1 mm with the parameters N e ∼ 1019−1020 cm−3 and T e ∼ 100 eV.

The objective of the Apollon 10 PW project is the generation of 10 PW peak power pulses of 15 fs at $1~\text{shot}~\text{min}^{-1}$ . In this paper a brief update on the current status of the Apollon project is presented, followed by a more detailed presentation of our experimental and theoretical investigations of the temporal characteristics of the laser. More specifically the design considerations as well as the technological and physical limitations to achieve the intended pulse duration and contrast are discussed.

International audience; We report on a study on controlled trapping of electrons, by field ionization of nitrogen ions, in laser wakefield accelerators in variable length gas cells. In addition to ionization-induced trapping in the density plateau inside the cells, which results in wide, but stable, electron energy spectra, a regime of ionization-induced trapping localized in the density down-ramp at the exit of the gas cells, is found. The resulting electron energy spectra are peaked, with 10% shot-to-shot fluctuations in peak energy. Ionization-induced trapping of electrons in the density down-ramp is a way to trap and accelerate a large number of electrons, thus improving the efficiency of the laser-driven wakefield acceleration.

In this work, we demonstrate that the contrast of high intensity laser pulses is a key issue for many problems dealing with high-field interaction with solid-density matter. As an example, we will discuss the interaction of an Ultra-High-Contrast (UHC), Ultra-Intense, 65 fs laser pulse with an overdense plasma through the proton acceleration from ultra-thin foils (from 0.08 μm to 105 μm). The symmetric feature of the ion beams emitted from both faces of the target will be demonstrated as well as the dominant role of the p component of the electric field rather than the ponderomotive force. Simulations performed with a 2D particle-in-cell code are in close agreement with all experimental data. UHC experiments prove to be a good as benchmarks for theories and models.

We investigate the interaction of a high-intensity (~5.1018 W/cm2) and short (~65 fs) laser pulse with thin foils (from 0.08 to 105 mum) in a regime of ultrahigh contrast (> 1010). This paper shows that for thicknesses less than about 10 mum, proton acceleration from both sides of the target presents quite symmetric features. Proton bunches emitted from each side show similar maximum energies and spatial characteristics. Moreover, we show that for ultrahigh-contrast pulses, the efficient acceleration mechanism is related to the Brunei effect and not to the ponderomotive force. Simulations performed with a 2-D particle-in-cell code are in close agreement with all experimental data.

Coherent ultrashort X-ray pulses provide new ways to probe matter and its ultrafast dynamics1–3. One of the promising paths to generate these pulses consists of using a nonlinear interaction with a systemto strongly and periodically distort the waveform of intense laser fields, and thus produce high-order harmonics. Such distortions have so far been induced by using the nonlinear polarizability of atoms, leading to the production of attosecond light bursts4, short enough to study the dynamics of electrons in matter3. Shorter and more intense attosecond pulses, together with higher harmonic orders, are expected5,6 by reflecting ultraintense laser pulses on a plasma mirror—a dense (1023 electrons cm−3) plasma with a steep interface. However, short-wavelength-light sources produced by such plasmas are known to generally be incoherent7. In contrast, we demonstrate that like in usual low-intensity reflection, the coherence of the light wave is preserved during harmonic generation on plasma mirrors. We then exploit this coherence for interferometric measurements and thus carry out a first study of the laser-driven coherent dynamics of the plasma electrons.

The chirped pulse amplification (CPA) technique has opened new perspectives in the radiation-matter interaction studies using ultra-short laser pulses at ultra-relativistic intensities. In particular the original idea, proposed by Tajima and Dawson, of accelerating electrons by the huge electric fields of plasma waves which develop in the wake of a laser pulse propagating in a plasma, become feasible. Some laboratories all over the world have produced by such a technique collimated electron busts of hundreds of MeV along acceleration lengths of a few hundreds of microns. In other experiments, using thin solid targets, intense bursts of energetic protons have been at the same time detected. The proton acceleration mechanism is essentially based on the Coulomb force appearing at the thin solid target surface as a consequence of the previous escape of the energetic electrons from the target. In the paper some experimental results will be presented as well as the opportunities the INFN PLASMONX project will offer in this research field at LNF.

The first results of commissioning for the French Synchrotron Radiation Facility SOLEIL at 2.75 GeV are presented. Perspectives for the fourth generation light source based on the ARC-EN-CIEL project are described.

We present a high-contrast 10-fs Front-End for Ti:sapphire PW-lasers within the Apollon-10PW project. This injector uses OPCPA pumped at 100Hz by Yb-based CPA chain. Combination of OPCPA and XPW permits a >1012 contrast ratio.

The interaction of femtosecond laser pulses with SiO2 aerogel targets has been analyzed by x-ray spectroscopic methods. The use of an aerogel target with transparent grains makes it possible to considerably reduce the requirements on laser-pulse contrasts for which heating occurs without the formation of a preplasma. A nanoplasma with a density sevenfold higher than the solid-state density has been detected.

At the initiative of the CEA (Commissariat a l’Energie Atomique), discussions were conducted in France on fourth generation light sources. A new independent accelerator based radiation facility ARC-EN-CIEL (Accelerator Radiation Complex for ENhanced Coherent Intense Extended Light) is proposed, aiming at providing coherent femtosecond light pulses in the UV- to X-ray range for scientific applications. The project is based on a 700 MeV superconducting LINAC, providing low emittance 200 fs RMS electron bunches. They can be injected in undulators used in the Self Amplified Spontaneous Emission mode (SASE), or in the High Gain Harmonic Generation (HGHG), seeded with high harmonics in gases at 20 nm.The SASE source covers the 100–5 nm spectral range, the HGHG goes down to 0.8 nm.Two optional loops, for Energy Recovery or energy enhancement (1.4 GeV), will accommodate fs synchrotron radiation sources in the IR-, VUV- and X-ray ranges, together with a FEL oscillator providing radiation down to 10 nm, taking advantage of the optical development for lithography. The facility also proposes to test plasma acceleration and to provide a Thomson radiation source. Characteristics of the light source will be described.

We demonstrate the influence of the prepulses and ASE of ultrashort pulses interacting with a solid target by addressing the direct comparison of the harmonic spectra generated by reflection onto a solid target with and without the introduction of a plasma mirror system. Harmonics up to the 20th of the fundamental of the Ti-Sa laser are clearly visible in a situation free from any plasma expansion.

A portable (200 � 100 � 100 mm 3 ), high-luminosity, spherically bent crystal spectrometer was designed to measure very low emissivity X-ray spectra of different elements with spatial resolution in a wide spectral range (1.2� 19.6 ( A). A large (50 � 15 mm 2 ) open aperture mica spherically bent crystal with R ¼ 150 mm was used as dispersive and focusing element. This spectrometer was associated with a large sensitive area (f ¼ 40 mm) micro-channel plates assembly. This apparatus provides simultaneously high spectral (l=dlB1800) and spatial (100–200mm) resolutions. Its large tunability allowed, without any adjustment of the spectrometer set-up, to record spectra in the 1.38–17.5 ( A wavelength range. We used the X-ray emission of femtosecond laser-produced plasmas from different materials ((CF2)n, CaF2, Cu, Al) to test the spectrometer. Thanks to the high sensitivity (high collection efficiency) of the system, high quality space-resolved X-ray spectra of Fluorine and Aluminum plasmas were obtained on a single laser shot. r 2001 Elsevier Science B.V. All rights reserved.

By means of spatially resolved high-resolution X-ray spectroscopy, we have investigated the generation of fast ions at various laser installations with different flux densities and laser wavelengths. It is demonstrated that the fast ion generation in laser-produced plasma can be achieved for a very low level of the averaged laser intensity on the target. The time-of-flight mass spectrometry ion diagnostics and X-ray spectrographs give very close results for the energy distribution of the thermal ion component. For higher energies, however, we found significant differences: the spatially resolved high-resolution spectrographs expose the presence of suprathermal ions, while the time-of-flight method does not. Suprathermal ion energies Eion plotted as a function of the qλ2 parameter show a large scatter far above the experimental errors. The cause of these large scatters is attributed to a strong nonuniformity of the laser intensity distribution in the focal spot. The analysis by means of hydrodynamics and spectral simulations show that the X-ray emission spectrum is a complex convolution from different parts of the plasma with strongly different electron density and temperature. It is shown that the highly resolved Li-like satellite spectrum near Heαcontains significant distortions even for very low hot electron fractions. Non-Maxwellian spectroscopy allows determination of both the hot electron fraction and the bulk electron temperature.

International audience; Applications using multi-PW lasers necessitate high temporal pulse quality with a tremendous contrast ratio (CR). The first crucial prerequisite to achieve multi-PW peak power is the generation of ultrashort pulses with good spectral phase quality. Second, to avoid any deleterious pre-ionization effect on targets, nanosecond contrast better than 10$^{12}$ is also targeted. In the framework of the Apollon 10 PW French laser program, we present a high-contrast 10 fs front-end design study to inject highly energetic Ti:sapphire PW lasers. The CR has been measured and analyzed in different time ranges highlighting the different major contributions for each scale.

International audience; The stability of beams of laser wakefield accelerated electrons in dielectric capillary tubes is experimentally investigated. These beams are found to be more stable in charge and pointing than the corresponding beams of electrons accelerated in a gas jet. Electron beams with an average charge of 43 pC and a standard deviation of 14% are generated. The fluctuations in charge are partly correlated to fluctuations in laser pulse energy. The pointing scatter of the electron beams is measured to be as low as 0.8 mrad (rms). High laser beam pointing stability improved the stability of the electron beams.

Atomic argon clusters were created using various types of nozzles and gas jet backing pressures. High temperature plasmas were created by irradiating the clusters with high intensity 35 fs laser pulses. The data obtained from these experiments were compared with each other as well as an experiment using a longer pulse ( 60 fs ) higher flux density laser. Detailed spectroscopic analysis of high resolution X-ray data near the Heα,Heβ, and Lyα lines of Ar was consistent with a two-temperature collisional-radiative model incorporating the effects of highly energetic electrons. Each variation has an effect on the rate of cluster expansion; therefore, results of the analysis provide useful insight into the behavior of such plasmas over short time scales. It was demonstrated for the first time, that by using sufficiently large clusters, hydrogen-like ions of Ar with an ionization potential of approximately 4120 eV could be obtained with a 35 fs laser pulse with a relatively low flux density of 1×10 17 W/cm 2 .

The generation of fast particles (both ions and electrons) in a femtosecond laser-produced plasma has been studied experimentally and the possibility of applying particle-in-cell (PIC) codes to the description of the experimental results has been investigated. The energy distribution function of fast ions has been measured directly by means of X-ray spectroscopy. It is shown that this diagnostic can also be used as an indirect method to measure fast electrons inside the plasma. The hot electron distribution in the 0:1–2 MeV energy range was measured directly using a standard electron magnetic spectrometer. The comparison of the experimental results with the numerical simulations show: (1) the electromagnetic PIC-FC code can be successfully applied for simulating plasmas created by the interaction of femtosecond-laser pulses with solid targets and (2) the existence of a transient population of hot electrons with a non-Maxwellian distribution typical of interaction experiments with short-pulse lasers. ? 2001 Elsevier Science Ltd. All rights reserved.

A study on the interaction of a 10 TW, 60 fs, Ti–Sapphire laser with a high-density long-scale pulsed nitrogen gas jet is reported. Experimental data on the laser propagation are analyzed with the help of a ray-tracing model. The plasma dynamics is investigated by means of time-resolved shadowgraphy and time-integrated high-resolution x-ray spectroscopy. Shadowgrams show that the plasma does not expand during the first 55 ps, while x-ray spectra exhibit an unusual continuum-like structure attributed to hollow atoms produced by charge exchange process between bare nuclei expelled from the plasma and molecules of the surrounding gas. The interpretation of the results is supported by particle-in-cell simulations. The question of x-ray lasing is also examined using a hydrodynamic code to simulate the long lasting regime of recombination.

We recall the main characteristics of high harmonic generation (HHG) by intense laser pulses in gas, which proceed from the fact that HHG is a laser-driven non linear process. We particularly outline the properties of intrinsic, spatial and temporal coherence which are investigated experimentally. We show that they can be controlled through various phase-matching parameters, on account of the particular form of the atomic dipole phase involved in HHG. Finally, we present two feasibility experiments demonstrating that this XUV source can be effectively used in time-resolved interferometry measurements, in particular in the diagnostic of laser-produced ultra-short dense plasmas.

High order harmonic generation in gases is now a widely studied non linear process. The research in this domain shows two main directions. On the one hand, researchers are working on the different fundamental aspects involved in high order generation. On the other hand, they have developed several applications of this unique XUV source. High order harmonics (HOH) present a good beam quality, which allows to focus the harmonic radiation to very tight spots1–3, a high brightness and a high repetition rate (up to 1 kHz)4 –6. Since high order harmonic generation is a field driven process, the properties of the laser that is used to generate the harmonics are partially transferred to the XUV radiation. As a result, the harmonic pulse presents a short duration, equal to or shorter than the generating laser pulse duration 7 ,8 depending on the harmonic order. The harmonic radiation is also naturally synchronized with the fundamental driving IR laser. These two last properties are very important for all experiments of the pump probe type. Up to now high order harmonics have already been used in atomic and molecular spectroscopy9,10. This type of experiments can also be applied in solid state physics for instance to probe the relaxation time of electrons that have been excited in the conduction band of a dielectric material11. Another important property of the harmonic source is the good temporal12 and spatial13–15 coherence that can be obtained in some generation conditions. Moreover, a very unique property in this XUV range is the possibility to produce two mutually coherent sources either separated in space or in time. In the following, we discuss the generation of these phase-locked harmonic sources and their applications to plasma diagnostic.

The interaction of a 60-fs laser pulse with argon clusters was experimentally studied. It is shown that cluster heating by an intense femtosecond pulse preceded by a picosecond prepulse can produce plasma whose ionization state is determined by the prepulse properties, while the mean energy of hot electrons is determined by the main pulse intensity. A simple model of cluster plasma evolution is suggested, allowing an adequate description of its X-ray emission.

An x-ray laser scheme based on the recombination of a fully stripped nitrogen plasma is presented. Plasma is assumed to be created by the optical-field ionization of a nitrogen gas jet of 10(19) cm-3 atomic density by an ultrashort (60 fs), high-intensity (3 x 10(19) W/cm2) Ti:sapphire laser. Results of two-dimensional particle-in-cell simulations, modeling laser-plasma interaction, parametric heating, and ponderomotive effects are presented. Hydrodynamic and kinetics calculations are performed and predict important local gain for H-like nitrogen transitions at 25 and 134 A, following fast collisional recombination for specific plasma conditions.

New types of space resolved X-ray spectra produced in light matter experiments with high intensity lasers have been investigated experimentally and theoretically. This type of spectra is characterised by the disappearance of distinct resonance line emission and the appearance of very broad emission structures due to the dielectronic satellite transitions associated to the resonance lines. Atomic data calculations have shown, that rather exotic states with K-shell vacancies are involved. For quantitative spectra interpretation we developed a model for dielectronic satellite accumulation (DSA-model) in cold dense optically thick plasmas which are tested by rigorous comparison with space resolved spectra from ns-lasers. In experiments with laser intensities up to 10 19 W/cm 2 focused into nitrogen gas targets, hollow ion configurations are observed by means of soft X-ray spectroscopy. It is shown that transitions in hollow ions can be used for plasma diagnostic. The determination of the electron temperature in the long lasting recombining regime is demonstrated. In Light-matter interaction experiments with extremely high contrast (up to 10 10 ) short pulse (400 fs) lasers electron densities of n e ≈3×10 23 cm −3 at temperatures between kT e =200–300 eV have been determined by means of spectral simulations developed previously for ns-laser produced plasmas. Expansion velocities are determined analysing asymmetric optically thick line emission. Further, the results are checked by observing the spectral windows involving the region about the He α -line and the region from the He β -line to the He-like continuum. Finally, plasmas of solid density are characteristic in experiments with heavy ion beams heating massive targets. We report the first spectroscopic investigations in plasmas of this type with results on solid neon heated by Ar-ions. A spectroscopic method for the determination of the electron temperature in extreme optically thick plasmas is developed.

We demonstrate that frequency-domain interferometry can be performed in the extreme ultraviolet range using high-order harmonics. We first show that two phase-locked harmonic sources delayed in time can be generated in the same medium despite ionization. This gives insight into the dynamics of the generation and ionization processes. We then apply the technique to the study of the temporal evolution of an ultrashort laser-produced plasma at the femtosecond time scale.

We have observed evidence of the emission of energetic He-and H-like ions of fluorine more than 1 MeV produced via the optical field ionization (OFI) from a solid target irradiated by an intense I=(2-4)x10(18) W/cm(2) (60 fs, lambda=800 nm), obliquely incident p-polarized pulse laser. The measured blue wing of He(alpha), He(beta), and Ly(alpha) lines of fluorine shows a feature of the Doppler-shifted spectrum due to the self-similar ion expansion dominated by superthermal electrons with the temperature T(h) approximately 100 keV. Using a collisional particle-in-cell simulation, which incorporates the nonlocal-thermodynamic-equilibrium ionization including OFI, we have obtained the plasma temperature, line shape, and maximal energy of accelerated ions, which agree well with those determined from the experimental spectra. The red wing of ion spectra gives the temperature of bulk plasma electrons.

We present the characteristics of a high-density pulsed gas jet developed to study the interaction of a short-pulse, high-intensity laser with a large scale underdense plasma. The jet was characterized by laser interferometry. Measurements of density profile were performed for a 5 mm diam cylindrical nozzle and for a 20×1 mm slit. In this latter case, a systematic study of the output density as a function of the backing pressure and of the valve opening duration is presented. At 1 mm from its outlet, the cylindrical nozzle provides densities up to 6.9×1019 cm−3 for an 8 bar backing pressure with a 3 mm full width at half maximum Gaussian-shaped profile. Under the same conditions, the slit produces a 20 mm long flat-top profile with a maximum density of 2.1×1019 cm−3.

For the first time registration of high-resolution soft x-ray emission and atomic data calculations of hollow-atom dielectronic satellite spectra of highly charged nitrogen have been performed. Double-electron charge-exchange processes from excited states are proposed for the formation of autoionizing levels in high-intensity laser-produced plasmas, when field-ionized ions penetrate into the residual gas. Good agreement is found between theory and experiment. Plasma spectroscopy with hollow ions is proposed and a temperature diagnostic for laser-produced plasmas in the long-lasting recombining regime is developed.

A hydrodynamic model of the interaction of a short, intense laser pulse with moderate-density gases is constructed. The model systematically takes into account both tunneling-ionization processes and the relativistic motion of the electrons that are produced by the ionization of the gas in the relativistically strong field of the pulse. X-ray spectroscopy data agree well with the proposed theory.

International audience; The principal objective of Apollon-10P project is the generation of 10 PW peak power pulses of 15 fs duration at 1 shot/minute. The technological challenges and the current status of the Apollon laser system are discussed in detail.

This paper reports on experimental investigations on relativistic self-focusing and self-channeling of a terawatt laser pulse (0.7 TW/spl les/P/spl les/15 TW) in an underdense plasma. We present results obtained with picosecond (/spl tau/=1 ps) and subpicosecond (/spl tau/=0.4 ps) pulses. In the "long pulse" regime, modifications in the laser propagation are observed for P

We present an experimental investigation of the propagation of a 1-ps-terawatt laser pulse in a hydrogen gas jet. A study of the output beam profile shows that beam divergence is significantly reduced when laser power is close to---but lower than---the critical power for relativistic self-focusing. It is shown that this effect depends dramatically on the gas used.

We review the main results concerning high-order generation processes from the point of view of a potential user of this new source of XUV radiation. The perspectives for optimizing the source, both in efficiency and in spectral range, its characteristics and in particular, its coherence properties, are discussed. Finally, we describe two experiments, which demonstrate the usefulness of the harmonics as a short-pulse, coherent source in the XUV domain.

The propagation of intense, picosecond laser pulses in tenuous plasmas is studied for conditions close to those required for relativistic self‐focusing. Solutions of the steady‐state wave‐envelope equations are used to obtain experimentally measurable quantities such as the Thomson‐scattered light at 90°, and the far‐field radiation pattern. It is demonstrated that in the presence of electron cavitation, the Thomson signal has a longitudinal structure which is generally the converse of the intensity pattern. Furthermore, beam collimation in the far‐field depends on where the laser is focused in the plasma, and may actually be degraded if self‐channeling occurs.

Reported is on the propagation and on the focusing of a 1064 nm 1 ps multiterawatt laser pulse in a H 2 gas jet. The focusing is diagnosed by viewing the second harmonic emission. It is shown that high intensities can be maintained in the presence of a high density plasma.

Cross polarized wave generation (XPW) is a well established technique for contrast enhancement of ultrashort pulses in high energy laser systems [1]. Efficient conversion with XPW only occurs at high intensities. Weaker, unconverted pre and post pulses are thus rejected by the second polarizer, thereby improving the temporal contrast of the pulse. XPW has several drawbacks especially in terms of simultaneously achieving high conversion efficiencies and output energies. Limitations in seeding the nonlinear filter with high energies arise from the upper intensity limit of white light generation while high conversion efficiencies require excellent beam quality.

We present a compact and energy-scalable crossed polarized wave generation setup with waveguide filtering. A 1.4 mJ, 14.5 fs FTL, 10−4 contrast ratio enhanced XPW pulse is produced with 11 mJ, 40 fs input pulse.

We present a compact and energy-scalable ultrashort laser setup based on waveguide filtering and cross polarized wave generation. A 650 µJ, 15.5 fs, 10-10 contrast ratio XPW pulse is produced with 3.3 mJ, 25 fs input pulses

We report a new experimental situation where the main contribution to the energy of electrons submitted to an ultrastrong laser field is given by the ponderomotive force. Electrons released from a low-density gas medium with energies up to 5 keV have been detected, which is far above the energy acquired during the ionization process. Numerical simulations of the electron trajectories are in good agreement with the experimental results

Cette contribution donne une rapide description de quelques phenomenes physiques lies a l'ionisation multiphotonique d'atomes en champs laser relativement intenses, de 10 12 a 10 15 W/cm 2 . Cette ionisation conduit a la production d'ions multicharges (etude de la dynamique de formation des ions), ionisation au-dessus du seuil (etude de l'energie des electrons crees) et a la generation d'harmoniques d'ordre tres eleve. Un atome place dans un champs laser intense, va emettre des radiations qui sont les harmoniques impaires du champs laser excitateur, avec une distribution en amplitude presque constante

Cette contribution donne une rapide description de quelques phenomenes physiques lies a l'ionisation multiphotonique d'atomes en champs laser relativement intenses, de 10 12 a 10 15 W/cm 2 . Cette ionisation conduit a la production d'ions multicharges (etude de la dynamique de formation des ions), ionisation au-dessus du seuil (etude de l'energie des electrons crees) et a la generation d'harmoniques d'ordre tres eleve. Un atome place dans un champs laser intense, va emettre des radiations qui sont les harmoniques impaires du champs laser excitateur, avec une distribution en amplitude presque constante

In a recent experiment [1] a high efficiency regime of stable electron acceleration to kinetic energies ranging from 10 to 40 MeV has been achieved. The main parameters of the electron bunches are comparable with those of bunches provided by commercial Radio‐Frequency based Linacs currently used in Hospitals for Intra‐Operative Radiation Therapy (IORT). IORT is an emerging technique applied in operating theaters during the surgical treatment of tumors. Performances and structure of a potential laser‐driven Hospital accelerator are compared in detail with the ones of several commercial devices. A number of possible advantages of the laser based technique are also discussed.

High-energy electrons can be produced in interactions of intense, ultra-short laser pulses with plasmas. Experiments conducted in the regime of moderate laser power (a few terawatts [TW]) are attracting increasing attention for their possibility of optimizing the acceleration process. Here we report the successful production of several-MeV electron bunches in interactions of femtosecond laser pulses from a 10 TW tabletop laser with supersonic gas-jets. The laser–plasma interaction and the obtained electron bunches have been characterized in detail, and conditions for stable and reproducible acceleration have been found. The accelerated electron bunches have been characterized by means of the measurement of the induced photo-activation of a gold sample via bremsstrahlung-generation of photons with suitable energy. The obtained result opens up a wide range of possible applications of the compact electron source for the concerns of nuclear physics studies. Some of them are briefly considered in this paper.

Multiply charged ion production in He, Ne, Ar and Xe has been investigated using a 1 ps Nd-glass laser pulse at lambda =1053 nm with intensities in the 1013-1018 W cm-2 range. All the outer-shell electrons are removed except in Ne gas in which ion charge states up to Ne7+ are observed. The influence of the polarization of the laser radiation is also investigated. Experimental ion production rates are compared with those predicted by tunnelling and barrier-suppression ionization models.

International audience; We report on an innovative two-dimensional imaging extreme ultraviolet (XUV) interferometer operating at 32 nm based on the mutual coherence of two laser high order harmonics (HOH) sources, separately generated in gas. We give the first evidence that the two mutually coherent HOH sources can be produced in two independent spatially separated gas jets, allowing for probing centimeter-sized objects. A magnification factor of 10 leads to a micron resolution associated with a subpicosecond temporal resolution. Single shot interferograms with a fringe visibility better than 30% are routinely produced. As a test of the XUV interferometer, we measure a maximum electronic density of 3×10^20 cm^−3 1.1 ns after the creation of a plasma on aluminum target.

Emission coherente de sillage et dynamique plasma C. Thaury1, F. Quere1, H. George1, R.A. Loch2, J.-P. Geindre3, G. Bonnaud1, P. Monot1 et Ph. Martin1 1Service des Photons, Atomes et Molecules, IRAMIS/DSM, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France 2Laser Physics and Nonlinear Optics Group, Faculty of Science and Technology, MESA+ Institute for Nanotechnology, University of Twente, The Netherlands 3Laboratoire pour l’Utilisation des Lasers Intenses, CNRS, Ecole Polytechnique, 91128 Palaiseau, France

We use optical interferometry to study the propagation of femtosecond laser pulses in gases. We show the measurements of propagation in a nitrogen gas jet and we compare the results with propagation in He under the same irradiation conditions. We find that in the case of nitrogen, the detailed temporal structure of the laser pulse can be tracked and visualized by measuring the phase and the resulting electron-density map. A dramatically different behavior occurs in He gas jets, where no details of the temporal structure of the laser pulse are visible. These observations are explained in terms of the ionization dynamics of nitrogen compared to helium. These circumstances make ${\text{N}}_{2}$ gas sensitive to variations in the electric field and, therefore, allow the laser-pulse temporal and spatial structures to be visualized in detail.

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.