T. Hall

Progress in the study of Warm Dense Matter

In the last few years, high power lasers have demonstrated the possibility to explore a new state of matter, the so-called warm dense matter. Among the possible techniques utilized to generate this state, we present the dynamic compression technique using high power lasers. Applications to planetary cores material (iron) will be discussed. Finally new diagnostics such as proton and hard-x-ray radiography of a shock propagating in a solid target will be presented.

Absolute equation of state measurements of iron using laser driven shocks

First absolute equation of state measurements obtained for iron with laser driven shock waves are presented. The shock velocity and the free surface velocity of compressed iron have been simultaneously measured by using a VISAR diagnostic, and step targets. The pressure range 1–8 Mbar has been investigated, which is directly relevant to planetary physics. The experiments have been performed at the Laboratoire pour l’Utilisation des Lasers Intenses of the Ecole Polytechnique.

A laser experiment for studying radiative shocks in astrophysics

In this article, we present a laboratory astrophysics experiment on radiative shocks and its interpretation using simple modelization.The experiment is performed with a 100-J laser ~pulse duration of about 0.5 ns! which irradiates a 1-mm 3 xenon gas-filled cell. Descriptions of both the experiment and the associated diagnostics are given. The apparition of a radiationprecursorintheunshockedmaterialisevidencedfrominterferometrydiagrams.Amodelincludingself-similar solutions and numerical ones is derived and fairly good agreements are obtained between the theoretical and the experimental results.

Preheating study by reflectivity measurements in laser-driven shocks

A study on preheating effects in laser-driven shock waves is presented. Two different diagnostics were used: the color temperature measurement deduced by recording the target rear side emissivity in two spectral bands, and the rear surface reflectivity measurement by using a probe beam. In order to test the response of the two diagnostics to the preheating, three types of targets characterized by different radiative properties were used. The greater sensitivity of the second diagnostic compared with the first was demonstrated. A model which calculates the reflectivity using a one-dimensional hydrodynamic code data was developed. In this model, the wave propagation equations in the expanding plasma using an appropriate model for the electron–ion collision frequency applicable to the cold solid-hot plasma transition were solved. The comparison between the calculated and measured reflectivities allows us to estimate the preheating process.

Subfemtosecond, coherent, relativistic, and ballistic electron bunches generated at ω0 and 2ω0 in high intensity laser-matter interaction

Harmonics of the laser light have been observed from the rear side of solid targets irradiated by a laser beam at relativistic intensities. This emission evidences the acceleration of subfemtosecond electron bunches by the laser pulse in front of the target. These bunches emit coherent transition radiation (CTR) when passing through the back surface of the target. The spectral features of the signal recorded for targets of thicknesses up to several hundred microns are consistent with the electrons being accelerated by both the laser electric field—via vacuum heating and/or resonance absorption,—and the v×B component of the Lorentz force. The spatial study of the radiation shows that the relativistic electrons causing the CTR radiation are coherent and propagate ballistically through the target, originating from a source with a size of the order of the laser focal spot.

Equation of state data experiments for plastic foams using smoothed laser beams

The importance of foams in laser produced plasmas has been recently pointed out in both Inertial Confinement Fusion (ICF) and astrophysics laboratory dedicated experiments. In this paper, Equation of State (EOS) data measurements of plastic porous materials have been experimentally determined using ns laser pulses smoothed with Phase Zone Plates (PZP). Foams of density in the range 20–400 mg/cm3 and thickness about 20 μm were used. A new original scheme for the targets has been designed which allowed, for the first time with laser, EOS data to be obtained for pressures ranging from 0.1 to 2.5 Mbars. Results are discussed and compared with available models.

Brominated plastic equation of state measurements using laser driven shocks

In order for brominated plastic (CHBr) to be used in future large lasers, such as the National Ignition Facility, capsule design, and equation of state (EOS) data are needed to address uncertainties in modeling. We have performed CHBr EOS measurements using the impedance matching technique. Laser beams spatially smoothed, and giving a spot size of 400 μm and intensities ⩽5×1013 W/cm2, produced high-quality shock waves allowing the simultaneous measurements of the shock velocities in two materials, one used as reference. Results are compared to other experiments and to EOS calculations. We obtained very good agreement with the theoretical curve for pressures ranging from 1 to 3 Mbar.

Conical crystal spectrograph for high brightness x-ray Kα spectroscopy in subpicosecond laser–solid interaction

A high brightness crystal spectrograph was designed and successfully used to study the x-ray Kα spectrum of aluminum as a diagnostic for target heating due to suprathermal electrons in subpicosecond laser–solid interaction experiments. Conical geometry was chosen in order to enhance spatial focusing, since an extremely low signal-to-noise ratio was expected for the photon flux, and to have a reasonable spectral range while occupying only a small solid angle within the target chamber. Very high image brightness is obtained through strong spatial focusing, as well as good spectral resolution. A simple analytical model and three-dimensional numerical simulation are presented to describe the crystal characteristics. The performance of the spectrograph was tested both on an optical bench and with a ray-tracing code. The experimental spectra allowed us to estimate the target temperature and characterize the fast electron transport. The spectrograph is considered to be particularly useful, in the configuration d...

High pressures generated by laser driven shocks: applications to planetary physics

High power lasers are a tool that can be used to determine important parameters in the context of Warm Dense Matter, i.e. at the convergence of low-temperature plasma physics and finite-temperature condensed matter physics. Recent results concerning planet inner core materials such as water and iron are presented. We determined the equation of state, temperature and index of refraction of water for pressures up to 7 Mbar. The release state of iron in a LiF window allowed us to investigate the melting temperature near the inner core boundary conditions. Finally, the first application of proton radiography to the study of shocked material is also discussed.

Optical properties of highly compressed polystyrene using laser-driven shockwaves

Plastic is a common material used for laboratory astrophysics and inertial confinement fusion. Experimental measurements of the optical properties of shock compressed polystyrene in the strongly coupled and correlated regime (0.7 Mbar<P<1.7 Mbar; 0.5<T<1.3 eV) are reported. A VISAR (Velocity Interferometer System for Any Reflector) system was used in addition to self-emission recording on a streak camera. This allowed a simultaneous access to reflectivity, instantaneous shock velocity and temperature. Mean ionization and conductivity of the material have then been deduced through the Drude model. The values found are more than an order of magnitude greater than published ones in this temperature domain.