Measurement of short-range correlations in shock-compressed plastic by short-pulse x-ray scattering.

We have performed short-pulse x-ray scattering measurements on laser-driven shock-compressed plastic samples in the warm dense matter regime, providing instantaneous snapshots of the system evolution. Time-resolved and angularly resolved scattered spectra sensitive to the correlation effects in the plasma show the appearance of short-range order within a few interionic separations. Comparison with radiation-hydrodynamic simulations indicates that the shocked plastic is compressed with a temperature of a few electron volts. These results are important for the understanding of the thermodynamic behavior of strongly correlated matter for conditions relevant to both laboratory astrophysics and inertial confinement fusion research.

Density measurement of low- Z shocked material from monochromatic x-ray two-dimensional images.

An experiment on LULI 2000 laser devoted to density determination of shocked plastic from a two-dimensional monochromatic x-ray radiography is presented. A spherical quartz crystal was set to select the He-alpha line of vanadium at 2.382 A and perform the image of the main target. Rear side diagnostics were implemented to validate the new diagnostic. The density experimental results given by radiography are in good agreement with rear side diagnostics data and hydrodynamical simulations. The pressure regime into the plastic is 2-3 Mbar, corresponding to a compression between 2.7-2.9.

Direct observation of strong ion coupling in laser-driven shock-compressed targets.

In this Letter we report on a near collective x-ray scattering experiment on shock-compressed targets. A highly coupled Al plasma was generated and probed by spectrally resolving an x-ray source forward scattered by the sample. A significant reduction in the intensity of the elastic scatter was observed, which we attribute to the formation of an incipient long-range order. This speculation is confirmed by x-ray scattering calculations accounting for both electron degeneracy and strong coupling effects. Measurements from rear side visible diagnostics are consistent with the plasma parameters inferred from x-ray scattering data. These results give the experimental evidence of the strongly coupled ionic dynamics in dense plasmas.

Transient development of backward stimulated Raman and Brillouin scattering on a picosecond time scale measured by subpicosecond thomson diagnostic.

The excitation and the relaxation of the plasma waves and ion acoustic waves (IAW), respectively, driven by stimulated Raman (SRS) and Brillouin (SBS) backscatterings have been experimentally investigated with short-pulse lasers. The spectra have been obtained with a 0.3 ps time resolution. It is shown that SRS develops before SBS and suddenly decays around the peak of the pump, as the IAW reaches saturation. On this short time scale, electron kinetic effects play a major role for SRS saturation, contrary to ion dynamics. These results are supported by particle-in-cell simulations.

Fast electron heating of a solid target in ultrahigh-intensity laser pulse interaction.

We report one of the first measurements of induced heating due to the transport of a fast electron beam generated by an ultrashort pulse laser interaction with solid targets. Rear-side optical reflectivity and emissivity have been used as diagnostics for the size and temperature of the heated zone. A narrow spot has been observed of the order of the laser focus size. Values up to approximately 10 eV at the target back surface were inferred from the experimental data and compared with the predictions of a hybrid collisional-electromagnetic transport simulation.

K(alpha) fluorescence measurement of relativistic electron transport in the context of fast ignition.

Electron transport within solid targets, irradiated by a high-intensity short-pulse laser, has been measured by imaging K(alpha) radiation from high- Z layers (Cu, Ti) buried in low- Z (CH, Al) foils. Although the laser spot is approximately 10 microm [full width at half maximum (FWHM)], the electron beam spreads to > or =70 microm FWHM within <20 microm of penetration into an Al target then, at depths >100 microm, diverges with a 40 degree spreading angle. Monte Carlo and analytic models are compared to our data. We find that a Monte Carlo model with a heuristic model for the electron injection gives a reasonable fit with our data.

Evidence of ultrashort electron bunches in laser-plasma interactions at relativistic intensities.

The second harmonic of the laser light (2omega(0)) is observed on the rear side of thick solid targets irradiated by a laser beam at relativistic intensities. This emission is explained by the acceleration by the laser pulse in front of the target of short bunches of electrons separated by the period (or half the period) of the laser light. When reaching the rear side of the target, these electron bunches emit coherent transition radiation at 2omega(0). The observations indicate that, in our conditions, the minimum fraction of the laser energy transferred to these electron bunches is of the order of 1%.

Fast electron transport in ultraintense laser pulse interaction with solid targets by rear-side self-radiation diagnostics.

We report on rear-side optical self-emission results from ultraintense laser pulse interactions with solid targets. A prompt emission associated with a narrow electron jet has been observed up to aluminum target thicknesses of 400 microm with a typical spreading half-angle of 17 degrees. The quantitative results on the emitted energy are consistent with models where the optical emission is due to transition radiation of electrons reaching the back surface of the target or due to a synchrotron-type radiation of electrons pulled back to the target. These models associated with transport simulation results give an indication of a temperature of a few hundred keV for the fast-electron population.

X-ray emission of a xenon gas jet plasma diagnosed with Thomson scattering.

We present the results of a benchmark experiment aimed at validating recent calculation techniques for the emission properties of medium and high-Z multicharged ions in hot plasmas. We use space- and time-resolved M-shell x-ray spectroscopy of a laser-produced gas jet xenon plasma as a primary diagnostic of the ionization balance dynamics. We perform measurements of the electron temperature, electron density, and average charge state by recording simultaneous spectra of ion acoustic and electron plasma wave Thomson scattering. A comparison of the experimental x-ray spectra with calculations performed ab initio with a non-local-thermodynamic-equilibrium collisional-radiative model based on the superconfiguration formalism, using the measured plasma parameters, is presented and discussed.