We investigate the harmonics generation from a pure dielectric target when submitted to laser intensities in the 1018W/cm2. We demonstrate the negative influence of the prepulses and ASE by addressing the direct comparison of the harmonic spectra with and without the introduction of a perfectly controlled plasma mirror system. Harmonics up to the 20th of the fundamental of the Ti-Sa laser are clearly visible in a situation free of any plasma expansion.
Multi-keV X-ray source from intense laser-cluster interaction was experimentally studied. A special effort was first made in order to characterize the cluster target. K-shell emission of Argon clusters (around 3 keV) was then studied when irradiated by kHz, 30 fs, 1017 W.cm-2 laser pulses. High-resolution spectra are presented, in this spectral range, as a function of laser duration and average cluster size. Signature of very highly charged ions (Ar16+) was observed with relatively low intensity laser pulses (few 1015 W.cm-2). This feature is not yet clearly understood nor reproduced by simulations. Optimal laser pulse duration was observed for X-ray production, depending on the cluster size. For the first time to our knowledge, the duration of K-shell X-ray bursts was measured with a home-made streak camera to be on the picosecond scale.
X-ray spectra of a few picosecond duration were emitted by aluminum, selenium and samarium thin foils irradiated with a 100 TW, 300 fs laser at 0.53 μm wavelength. They were measured in the 1600 eV range with high temporal and spectral resolution, using a high-speed streak camera coupled to a conical Bragg crystal. Gradients were limited by using thin foils (300 to 800 Å) deposited on a 50 μm gold pinhole. Frequency Domain Interferometry was set to measure the velocity of the critical density at the rear of the target and deduce the electron temperature. A few picosecond duration X-ray spectra have been measured. Sm spectra showed no spectral features in the measured wavelength range, providing a spectrally homogeneous backlighter for absorption spectroscopy. The duration of the emission was shorter when observed through a pinhole. 1-D hydrodynamic simulations coupled to an atomic collisional-radiative code have been used to simulate the X-ray emission of aluminum. The main features of the experimental time resolved spectra, obtained for the pinhole target have been well reproduced, for an initial temperature of 700 ± 100 eV.
This paper reviews the highlights of the high intensity laser-plasma experiments achieved with the six-beam and the 100 TW LULI laser facilities, as well as the progress of the LULI 2000 project. This covers fields of laser fusion, equations of state, hgih energy particle emission, atomic physics, X-ray production and laser developments.
We have studied the influence of the target properties on laser-accelerated proton and ion beams generated by the LULI multi-terawatt laser. A strong dependence of the ion emission on the surface conditions, conductivity, shape and material of the thin foil targets were observed. We have performed a full characterization of the ion beam using magnetic spectrometers, Thompson parabolas, radiochromic film and nuclear activation techniques. The strong dependence of the ion beam acceleration on the conditions on the target back surface was found in agreement with theoretical predictions based on the target normal sheath acceleration (TNSA) mechanism. Proton kinetic energies up to 25 MeV have been observed.
Recent high temporal resolution Ni-like x-ray laser experiments have yielded important insights into the output characteristics of picosecond pumped x-ray lasers. However, current experimental observations do not fully explain the plasma dynamics which are critical to the gain generation within the x-ray laser medium. A theoretical study of the Ni-like Silver x-ray laser has therefore been undertaken to compliment our experimental results, in an attempt to further our understanding of the processes at play in yielding the observed x-ray laser output. Preliminary findings are presented within this paper.
Ultra-short x-ray sources are generated by focusing sub- picosecond lasers on massive targets. The emission duration of a samarium x-ray source produced with a 100 TW sub- picosecond laser was measured using an ultra-fast X-ray streak camera. The spectral range was limited around 7.5-8.5 angstrom, the range in which samarium can be used as a backlighter for K(alpha) aluminum absorption experiments. The spectral time-evolution and the duration of samarium emission were measured. Preliminary calculations performed with non-local-thermodynamic equilibrium atomic physics show the plasma cooling which occurs with a characteristic time longer than predicted by radiative hydrocode simulations.
We have performed studies of keV x-ray production from (formula available in paper) rare gas clusters submitted to intense IR laser pulses. Up to (formula available in paper) per pulse at a moderate atomic density have been observed. High resolution spectroscopy studies in the case of (formula available in paper) clusters have also been performed, gibing unambiguous evidence of highly charge ions with K vacancies production. We have determined the photon energies and the absolute photon emission yields as a function of several physical parameters governing the interaction: size and atomic number of the clusters, peak intensity of the laser. Unexpectedly low laser intensity thresholds have been measured. The result obtained indicate nevertheless that x-rays may be emitted before cluster explosion on a subpicosecond time scale, and that several mechanisms must be involved in the first stage of the production of the hot nanoplasma induced from each cluster.
We have characterized the ultrafast solid-liquid transition of InSb and CdTe semiconductors by time resolved x-ray diffraction in the femtosecond timescale. Visible spectroscopic data were obtained together with x-ray measurements to characterize the dense electron-ho9le plasma at the origin of the phase transition following the IR excitation.
Since its creation, LULI has given an important contribution to laser plasma physics and Inertial Confinement Fusion studies. We will review some major results obtained these last two years with a 600 ps laser chain and with the recent 100-TW, 300fs ultra-intense laser chain. These result cover a wide spectrum of laser plasma physics research such as laser plasma interaction, laser plasma acceleration, fast ignitor, atomic processes in dens plasmas, shock waves generation and x-rays laser.
The emission from plasmas created with fs-lasers provides sub-picosecond x-ray pulses in the keV-range. Intense emission of K(alpha) lines as well as quasi continuum x-rays can be used for time-resolved diffraction and spectroscopy, i.e. to study lattice or atomic dynamics with sub-picosecond resolution by using a laser pump x-ray probe technique. The x-ray yield and x-ray pulse duration of the laser plasma source depend on the laser parameters and the target design, such as intensity, laser wavelength, pulse duration and prepulse level. To accumulate as many photons as possible of the isotropic source an efficient large aperture optic has to be used to select an x-ray line or a wavelength range and focus the radiation onto the sample. It is shown that the use of toroidally bent crystals provides the possibility to refocus 10-4 of the photons emitted in the whole solid angel to spot size of around 80 micrometers with a temporal broadening below 100 fs. Combinations of bent focusing crystals with a flat sample crystal for fast x-ray diffraction application are discussed. Experiments showing the temporal response of laser heated crystals are presented and compared with theoretical simulations based on Takagi-Taupin theory.
Optical pump, x-ray diffraction probe experiments have been used to study the lattice dynamics of organic materials using a laser-produced plasma x-ray source. The x-ray source is generated from a 10 Hz, 26 mJ, 120 fs laser beam focused on a silicon wafer target. The emitted K(alpha ) x-ray radiation is used to probe a cadmium arachidate Langmuir-Blodgett film and a TlAP crystal optically perturbed at laser fluences from 1.8 J/cm2 to 27 J/cm2. Ultrafast disordering inside the lattice -- within a time scale below 600 fs to few tens of picoseconds -- is clearly observed and produce a drop of the probe x-ray diffracted signal.
Short-pulse laser-produced plasmas look very promising for the generation of sub-picosecond X-rays. By combining several experimental techniques, we have significantly progressed towards a better understanding of ultrafast laser-matter interaction. The X-ray yield is a sensitive function of the electron density gradient scale length of the target plasma. In this work, the scale length has been changed by varying the temporal separation between the main laser pulse and a lower intensity prepulse. X-ray spectroscopic diagnostics of the plasma parameters have been used from the analysis of resonance and dielectronic satellite lines. The angular and energy distribution of suprathermal electrons emitted during the ultrafast laser- plasma interaction have been measured as a function of laser polarization and prepulse delay. Frequency-domain interferometry and optical measurements of the reflected probe pulse have been used to study the velocity and the gradient scale length of the expanding plasma. The Kα emission yield peaks for a scale length where resonant absorption is optimized. Hydrodynamic simulations have been performed to investigate the plasma dynamics and the basic processes which control the X-ray emission duration and intensity. Applications of ultrashort Kα X-rays to the diagnostic of solid plasma conditions and as a source for time-resolved diffraction and spectroscopy of transient chemical, biological or physical phenomena are underway.
The collision of laser-produced plasmas has been diagnosed by x-ray spectroscopy and imaging. The two colliding plasmas are produced on Al thin foils at a distance of 200 to 900 micrometers irradiated at (lambda) equals 0.53 micrometers with laser intensities of 3 X 1013 to 6 X 1013 W/cm2. Interpretation of the plasmas was visualized by replacing one of the foils material by magnesium. The main diagnostics were x-ray crystal optics based on flat, cylindrical, and toroidal crystals viewing the inter-target space. A multifluid eulerian monodimensional hydrodynamic code coupled with a radiative-atomic package provided simulations of the experiments. Hydrodynamic 2D simulations calculating the lateral expansion of the plasma enabled a reliable treatment of reabsorption along the line of sight of the spectrographs. The size and the time duration of the collision, the plasma parameters in the collision region (Te, Ti, and ne) and interpenetration were measured. The hydrocode simulations give a good understanding of the behavior of the collision in function of intertarget distance and laser intensity.
Jean-Claude Gauthier, S. Bastiani, Patrick Audebert, Jean-Paul Geindre, K. Neuman, Tom Donnelly, M. Hoffer, Roger Falcone, Ronnie Shepherd, Dwight Price, William White
We have studied theoretically and experimentally the x-ray production above 1 keV from femtosecond laser plasmas generated on periodically modulated surface targets. Laser energy coupling to plasma surface waves has been modeled using a numerical differential method. Almost total absorption of incident laser radiation is predicted for optimized interaction conditions. Silicon gratings have been irradiated by a 120 fs Ti: sapphire laser at irradiances in excess of 1016W/cm2. X-ray intensities above 1.5 keV (K-shell lines) have been measured as a function of the incidence angle. Results show a distinct x-ray emission maximum for the first order diffraction angle and are in good qualitative agreement with our theoretical predictions.
This article describes the first experiment of our groups to combine monochromatic x-ray imaging with a time-resolving detector i.e., a streak camera and a 120 ps gated framing camera. The aim of setting such a time-resolved diagnostic is to image the x-ray emission from colliding plasmas with high spatial resolution in a very narrow spectral window. Both camera types were tested and the adjustment procedure for the crystal was tested with film as a detector. The obtained spectral and spatial resolution of the x-ray microscope was measured.
To simulate the interaction of high laser intensity with solid targets, we have used the 1D code FILM in which the collisional plasma absorption is calculated by solving the linear electromagnetic field for p and s polarization. For p-polarized light the collision frequency is adjusted so that the field in the critical region of the plasma never exceeds the maximum field allowed by the wave breaking limit. Energy transport by thermal conduction is described with the help of the delocalized heat flux theory. The ponderomotive force resulting from the huge filed is taken into account. The calculated temperatures and ion densities are used as an input to a time-dependent atomic physics code. Non-stationary ionization dynamics is demonstrated.
We present preliminary results of L-shell absorption spectroscopy in an expanding germanium laser-produced plasma. The experiments were performed on the Octal laser at the Centre d'Etudes de Limeil-Valenton. The ion state distribution was inferred from the absorption spectrum. The radiation used to probe the plasma is produced by interaction of an auxiliary laser beam with a 80-micrometers diameter praseodymium wire, allowing a spatial resolution of the absorption spectrum. Temporal resolution is obtained by adjusting the delay between the driving beams and the probe beam. Modeling with a radiative hydrocode and an atomic kinetics code gives qualitative agreement with the experiments.
This study deals with the mechanic impulse transmitted to a solid target by soft x rays. We have measured the impulse transmitted to thin aluminum layers by x-ray radiation around 1.2 keV produced by a laser-irradiated copper target. The results are compared to radiative hydrocode simulations with the code XRAD.
Daniel Schirmann, Alain Mens, Richard Sauneuf, Roger Verrecchia, Patrick Audebert, Jean-Claude Gauthier, Jean-Paul Geindre, Andre Antonetti, Jean-Paul Chambaret, G. Hamoniaux, Andre Mysyrowicz
To simultaneously improve the spatial and temporal performances of streak tubes, we have developed bilamellar x-ray streak tubes with the help of Philips Components (now Philips Photonics). We describe the structure of one of them, the P850X, and the prototype of the camera which uses it, the C850X. We used 80 fs pulse dye lasers developed at LOA (ENSTA) to characterize this instrument in UV and x-ray range. The results show that its time resolution is less than 2 ps even with 1.5 keV energy photons and its spatial resolution much better than 16 lp/mm. This instrument was used for studying the time evolution of x-ray emission from 80 fs and 1 ps laser created plasmas at very high intensities.
Alain Mens, Dominique Gontier, J.-C. Huilizen, Richard Sauneuf, Daniel Schirmann, Roger Verrecchia, Patrick Audebert, Jean-Claude Gauthier, Jean-Paul Geindre, Andre Antonetti, Jean-Paul Chambaret, G. Hamoniaux, Andre Mysyrowicz
High resolution x-ray streak cameras using tubes with bilamellar electron optics have been developed for a long time in CEL-V, in collaboration with Philips Components (now Philips Photonics) for the tubes. The last improvements on their electronics, implemented on the commercial cameras developed with ARP, have allowed us to get the best results from the tubes, especially for the quality of their focusing. We have already been able to verify that the performance of the C850X camera in the UV range is as good as expected from calculations. New experiments made in the x-ray range show that it is also true in the x-ray range. We describe the results of these experiments and we also show preliminary results obtained with the C750X camera (which is identical to C850X except that its deflection structure is a meander travelling wave line instead of conventional plates) which show that the sweep speed reached gives us a 1 ps temporal resolution in the x-ray range.
We present preliminary experiments and calculations performed to optimize the photoionization of an He-like aluminum plasma. The high emissivity of the 3d-4f M-band of tantalum or tungsten is used as the pump. The plasmas are produced by two 500 ps duration beams of a frequency doubled Nd-glass laser. The pump beam is delayed by 1 ns with respect to the main beam. The Al plasma ionization state has been measured with K-alpha absorption measurements. X-ray diodes and space-resolving crystal spectrographs have been used to measure the intensity of the pump source in the desired spectral range. Optimization of the pumping scheme is analyzed with a numerical description of the photopumping process by a collisional-radiative modeling of the Al plasma including the X-ray pump.
Solid targets have been irradiated by 80 fs laser pulses at 1017 W/cm2 laser irradiance. Spectra obtained in the 0.7 - 0.9 nm wavelength range show that x ray emission occurs at near-solid electron densities (Ne $OM 1023 cm-3). The analysis of the K(alpha) emission line points out the role of hot, fast electrons in the interaction physics. A time-resolved Schlieren imaging technique shows that the ultrashort laser pulses strikes a preplasma created by the laser-amplified spontaneous emission (ASE) with an electron density gradient scale length less than 1 micrometers.
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