Solid material damaging induced by an intense and short electromagnetic pulse is accompanied by structural modifications, such as solid/solid phase transition, solid/liquid phase transition or ablation. In such an interaction, the energy is mainly absorbed by electrons, and then transferred to the lattice over a 1 − 10 ps time scale. Such out-of-equilibrium physics is the subject of intense experimental and theoretical work, rising fundamental questions about the thermal or non-thermal nature of phase transitions, the softening or hardening of chemical bonds, and the competition between thermal ablation and coulomb explosion. Here, an experimental technique based on pump-probe interfero-polarimetry in reflection, is presented. It allows us to measure the reflectivity and phase shift of an optical probe reflecting on the sample, in both P and S polarization directions, with a sub-100 fs time resolution. The accuracies on phase shift and on reflectivity are 10 mrad and 1%, respectively. These quantities depend on both the sample optical properties (dielectric function) and the heated sample hydrodynamics. Careful comparison of signals in P and S polarizations allows us to distinguish between optical properties and hydrodynamics contributions. Optical properties give information about the dynamics of the electron properties which drive the damage formation, while the hydrodynamic contribution includes sample surface motion and modofication of the electron density profile, at the nanometer scale. This interfero-polarimetry technique was employed to study damage on aluminum induced by an infrared ultrashort laser pulse (800 nm, 30 fs, 1 J:cm-2)
A numerical study of the interaction of short (15 fs), intense (1016 − 2 × 1017W/cm2), soft X-ray pulses with vanadium targets has been performed by means of a 1D hydrodynamics code. The code which considers the non-equilibrium between electrons and ions, includes a proper treatment of the atomic processes involved in the absorption of x-ray photons and in the evolution of the subsequent heated material. By associating the ablation depth to the spatial profile of the shock wave into the material, numerical values of this ablation depth have been obtained.
Interaction of femtosecond (30 fs-5 ps) and intense (up to 1017 W/cm2) laser pulses with Ar clusters (180 to 350 Å radius) has been studied. The laser absorption and the cluster heating have been measured using different diagnostics,
demonstrating the production of very hot and dense plasmas, in the keV range. A special attention has been devoted to the K-shell x-ray emission spectra (2.9-3.3 keV). X-ray emission has been observed from ions in very high charge states (Ar16+). Time-resolved measurements have been performed, giving evidence, for the first time, of extremely short x-ray pulses down to the sub-picosecond. Simulations based on both an hydrodynamic model and collisional-radiative atomic physics reveal an extremely brief x-ray emission burst consistent with measurements.
Rare gas cluster jets are an intermediate medium between solid and gas targets. Laser-cluster jets interaction may generate a great number of energetic particles such as X-rays, UV, high harmonics, ions, electrons and neutrons. To understand all the mechanisms involved in this interaction we need to make a complete study of individual cluster response to an ultra-short laser pulse. We studied the laser interaction with our Argon cluster gas jet, which is well characterized in cluster size and density, to enlarge the knowledge of this interaction. We measured absorption, heating and X-ray emission spectra versus laser parameters and clusters size (~15-30 nm). We show that there is a strong refraction effect on laser propagation due to the residual gas density. This effect was confirmed by laser propagation simulation with a cylindrical 2D particle code WAKE. The role played by refraction was to limit maximum laser intensity on the focal spot and to increase interaction volume. By this way, X-ray emission was observed with laser intensity not so far from the ionization threshold (few 1014 W.cm-2). We also studied plasma expansion both at cluster scale and focal volume scale and deduced the deposited energy distribution as a function of time. Thanks to a simple hydrodynamic model, we used these results to study cluster expansion. X-ray emission is then simulated by TRANSPEC code in order to reproduce X-ray spectra and duration. Those results revealed an extremely brief X-ray emission consistent with a preliminary measure by streak camera (~ps).
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.
Recent advances in femtosecond laser plasma x-rays sources have resulted in several experiments to explore the dynamics of physical and chemical processes on the femtosecond time scale. We present our most recent progresses on the development of an intense broadband x-ray source in the multi-keV range, for application to time-resolved EXAFFS experiments. Experiments have been realized with two different CPA laser systems having different pulse durations and characteristics. X-ray emissions in the 5KeV range generated form solid targets with the INRS Nd:Glass laser and the UCSD Ti:Sapphire laser have been characterized through high resolution and time resolved x-ray spectroscopy. The application of this source to time resolved EXAFS measurements with a sub-picosecond time resolution will also be discussed.
Hot dense lanthanum plasmas have been produced by irradiating a solid Lanthanum target with a high contrast 400fs laser pulses at 3 X 1018 W/cm2. The M-shell emission has been recorded in the wavelength range 10A-14A using a high-resolution crystal spectrometer. The emission has been time resolved using our PX1 x-ray streak camera with a temporal resolution of 350fs. A superconfiguration model for non-LTE plasmas coupled to hydrodynamics calculations is used to discuss the measurements. Our work indicates that a correct understanding of intermediate and high Z dense plasmas, created with femtosecond clean laser pluses, requires time resolved spectroscopy with femtosecond resolution and NLTE atomic physics coupled to plasma hydrodynamics.
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.
The analysis of the fine structure observed in spectra of hydro genic aluminum emitted from a constrained-flow plasma indicates the presence of the laser-induced satellites. The measured profiles of the Al Lyman line (beta) exhibit peaks in the wings consistent with theoretical predications of the spectral line modification by strong single-frequency electric fields. The experimental identification of laser satellites opens up significant applications in the diagnosis of transient plasmas submitted to external oscillating fields.
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.
We present recent results of our effort to develop an efficient, user-friendly, table-top ultrafast X-ray source. The factors affecting the duration and the intensity of the X-ray emission in the keV range are studied. Time-dependent calculation of the atomic physics coupled to a Fokker- Planck code is used for a quantitative analysis of the experimental results.
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