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We experimentally investigate the fast ( < 1 ps ) isochoric heating of multi-layer metallic foils and subsequent high-pressure hydrodynamics induced by energetic electrons driven by high-intensity, high-contrast laser pulses. The early-time temperature profile inside the target is measured from the streaked optical pyrometry of the target rear side. This is further characterized from benchmarked simulations of the laser-target interaction and the fast electron transport. Despite a modest laser energy ( < 1 J ), the early-time high pressures and associated gradients launch inwards a strong compression wave developing over 10 ps into a 140 Mbar blast wave, according to hydrodynamic simulations, consistent with our measurements. These experimental and numerical findings pave the way to a short-pulse-laser-based platform dedicated to high-energy-density physics studies.

The rapidly growing ultrafast science with X-ray lasers unveils atomic scale processes with unprecedented time resolution bringing the so called “molecular movie” within reach. X-ray absorption spectroscopy is one of the most powerful x-ray techniques providing both local atomic order and electronic structure when coupled with ad-hoc theory. Collecting absorption spectra within few x-ray pulses is possible only in a dispersive setup. We demonstrate ultrafast time-resolved measurements of the LIII-edge x-ray absorption near-edge spectra of irreversibly laser excited Molybdenum using an average of only few x-ray pulses with a signal to noise ratio limited only by the saturation level of the detector. The simplicity of the experimental set-up makes this technique versatile and applicable for a wide range of pump-probe experiments, particularly in the case of non-reversible processes.

Fast-electron beam stopping mechanisms in media ranging from solid to warm dense matter have been investigated experimentally and numerically. Laser-driven fast electrons have been transported through solid Al targets and shock-compressed Al and plastic foam targets. Their propagation has been diagnosed via rear-side optical self-emission and Kα X-rays from tracer layers. Comparison between measurements and simulations shows that the transition from collision-dominated to resistive field-dominated energy loss occurs for a fast-electron current density ~5 × 1011 A cm−2. The respective increases in the stopping power with target density and resistivity have been detected in each regime. Self-guided propagation over 200μm has been observed in radially compressed targets due to ~1kT magnetic fields generated by resistivity gradients at the converging shock front.

The role of self generated magnetic fields in the transport of a heat wave following a nanosecond laser irradiation of a solid target is investigated. Magnetic fields are expected to localize the electron carrying the heat flux but at the same time are affected in their evolution by the heat flux itself. We performed simultaneous measurements of heat wave propagation velocity within the target and magnetic fields developing on the target surface. These were compared to results obtained by numerical magneto-hydrodynamic modeling, including self-generated B fields. The comparison shows that longitudinal heat flow is overestimated in the simulations. Similarly, but most notably, the radial expansion of the magnetic fields is underestimated by the modeling. The two are likely linked, the more pronounced radial drift of B-fields induces a rotation of heat flux in the radial direction, and corresponding longitudinal heat flux inhibition. This suggests the need for improving present modeling of self-generated magnetic fields evolution in high power laser-matter interaction.

We present results from an experimental characterization of fast electron transport in high density plasmas created by 1D shock wave compression. The Kα fluorescence from a Cu layer embedded in Al or CH foil targets is measured. We use long laser pulses (LP) with 180 J, 1.5 ns, 0.53μm to compress the foils by shock wave propagation to 2-3 times their solid density and heat them to ∼ 4eV (close to the Fermi temperature). A counter-propagating high-intensity short laser pulse (SP), with 40 J, 1 ps, 57×1019 Wcm−2, generates intense currents of fast electrons which propagate through the deep regions of the target just before shock breakthrough. The results are compared to the uncompressed, solid density case (without the LP beam). The complete set of measurements is compared to numerical results, including a 2D hydrodynamic description of the compression and pre-pulse effects, 2D PIC simulations of the SP beam interaction and both hybrid and PIC simulations of the electron transport in the target depth and sheaths. In the case of the non-compressed targets we need to take fast electron refluxing into account to reproduce the experimental results. By exploring the domain of warm temperatures, we identify a regime for the incident fast electron current density, 1010 < jh < 1012 Acm−2, for which the collective mechanisms of electron transport differs appreciably between solid density and compressed matter.

X-ray polarization spectroscopy is a useful diagnostic tool for measuring the velocity distribution of electrons inside plasma. A new polarization measurement was performed at relativistic laser intensity using a laser pulse (10 J in ∼1 ps) from Alisé facility at CEA/CESTA. Chlorinated triple-layer targets were irradiated, and polarization degrees of C1-Heα line were measured. Obtained polarization degrees are negative in shallow region from the target surface and positive in deep region. This result is qualitatively consistent with the experimental results at non-relativistic intensity. Moreover, de-polarizations due to isotropic excitation and elastic collision, which were predicted by a model calculation and a time-dependent atomic kinetic code, were observed.

Study on x-ray polarization spectroscopy was performed for intense-laser-produced plasma under laser-oblique incidence. The laser pulse was focused onto a polyvinylchloride target at angles of 67 degrees and 7 degrees to the target normal at average intensities of 1017-18 W/cm2. There were differences in the spectra for shifted Kα lines of C1 atoms between the oblique and nearly normal incidence, indicating the low laser-energy absorption in the oblique incidence. A difference in polarization degrees of C1 Heα lines was also observed, and this means the polarization of incident laser pulse affect velocity distribution functions of fast electrons in intense-laser-produced plasma.

We report on X-ray diagnostics results from an experiment on fast electrons propagation in cylindrically compressed targets. It was performed on the VULCAN TAW laser facility at RAL (UK) using four long pulses (1ns, 70 J each at 2ω) to compress a cylindrical polyimide target filled with CH foam at 3 different initial densities. The cylindrical geometry allows us to reach temperatures and densities higher than those obtained in planar geometry compression. 2D hydrodynamic simulations predicted a core density range from 4 to 8 g/cm3 and a core temperature from 30 eV up to 175 eV at maximum compression. An additional short laser pulse (10 ps, 160 J at ω) was focused on a Ni foil at one of the cylinder edges in order to generate a fast electrons current propagating along the compressed target. A X-ray radiography diagnostic was implemented in order to estimate the core plasma conditions of the compressed cylinder. Moreover two Bragg X-ray spectrometers collected the Kα fluorescence from the target so as to determine the variations of fast electrons population during the compression.

X-ray polarization spectroscopy was studied to derive directly the velocity distribution function (VDF) of hot electrons propagating in plasma created with a high intensity laser pulse. Polarization measurement was made at around 1018 W/cm2 using a laser pulse (∼10 J in ∼1 ps) from Alisé facility at CEA/CESTA. Chlorinated triple-layer targets were irradiated, and C1 Heα line was observed with an x-ray polarization spectrometer. Polarization degrees were measured as a function of the target overcoat thickness. It is found that the polarization is weakly negative for thin coating, but becomes positive, and finally zero for thick coating. This result is consistent with predictions made with a time-dependent atomic kinetics code developed to gain an insight into the generation of polarized C1 Heα radiation. The de-polarization on the surface is attributed to excessive bulk electron temperature and that in the deep region to elastic-scattering processes by the isotropic bulk electrons in dense region.

We performed an experiment with cone targets in planar geometry devoted to the study of fast electron generation, propagation, and target heating. This was done at LULI with the 100 TW laser at intensities up to 1019 W/cm2. Fast electrons penetration, with and without cones, was studied with different diagnostics (Kα imaging, Kα spectroscopy, visible emission) for ω or 2ω irradiation. At ω, the pre-plasma generated by the laser pedestal fills the cone and prevents the beam from reaching the tip.