M. Rabec Le Gloahec

Electron and photon production from relativistic laser-plasma interactions

The interaction of short and intense laser pulses with plasmas is a very efficient source of relativistic electrons with tunable properties. In low-density plasmas, we observed bunches of electrons up to 200 MeV, accelerated in the wakefield of the laser pulse. Less energetic electrons (tens of megaelectronvolt) have been obtained, albeit with a higher efficiency, during the interaction with a pre-exploded foil or a solid target. When these relativistic electrons slow down in a thick tungsten target, they emit very energetic Bremsstrahlung photons which have been diagnosed directly with photoconductors, and indirectly through photonuclear activation measurements. Dose, photoactivation, and photofission measurements are reported. These results are in reasonable agreement, over three orders of magnitude, with a model built on laser–plasma interaction and electron transport numerical simulations.

Electronic conduction in shock-compressed water

The optical reflectance of a strong shock front in water increases continuously with pressure above 100 GPa and saturates at ∼45% reflectance above 250 GPa. This is the first evidence of electronic conduction in high pressure water. In addition, the water Hugoniot equation of state up to 790 GPa (7.9 Mbar) is determined from shock velocity measurements made by detecting the Doppler shift of reflected light. From a fit to the reflectance data we find that an electronic mobility gap ∼2.5 eV controls thermal activation of electronic carriers at pressures in the range of 100–150 GPa. This suggests that electronic conduction contributes significantly to the total conductivity along the Neptune isentrope above 150 GPa.

Time-resolved nanosecond imaging of the propagation of a corona-like plasma discharge in water at positive applied voltage polarity

This paper is an experimental study of a pulsed filamentary plasma discharge inside liquid water in pin-to-plane electrode configuration. Time-resolved electrical and imaging diagnostics have been performed. The initiation and the propagation of the discharge have been studied for several experimental parameters. The propagation is continuous and is followed by reilluminations at low water conductivity. The measured propagation velocity of the plasma discharge is 30?km?s?1 for the secondary positive mode. This velocity was found to be surprisingly constant whatever the experimental parameters and especially as a function of the water conductivity.

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.

X-ray source studies for radiography of dense matter

Studies of short-pulse laser-generated hard x-ray (18–60 keV) sources, suitable for radiographs of large samples of dense matter, are presented. The spatial and dynamic resolutions for different target types and laser parameters have been investigated. A high quality radiograph with good spatial resolution in two dimensions was demonstrated by irradiating freestanding thin W wires. The influence of the geometry for the quality of the radiograph, which is crucial for the design of experiments probing laser-compressed matter, is reported.

Strong absorption, intense forward-Raman scattering and relativistic electrons driven by a short, high intensity laser pulse through moderately underdense plasmas

The propagation of a short and intense laser pulse (1.057 μm, 350 fs, 1017 W/cm2–2×1019 W/cm2) through preformed undercritical plasmas (≈5%–40% of nc) has been experimentally investigated on the 100-TW laser facility at the Laboratoire pour l’Utilisation des Lasers Intenses. The transmission and reflection of the 1 μm laser pulse, the forward- and backward-Raman (respectively, F-SRS and B-SRS) scattered light and the emission of fast electrons are reported. Significant absorption occurs in these plasmas, which is found to increase with the laser intensity. B-SRS is strongly driven at 1017 W/cm2 and gradually decreases at higher intensities. It is shown that the transmission is low and only weakly dependent on the laser intensity. In contrast, the forward Raman scattering continuously increases with the laser intensity, up to 7% of the incident energy at 2×1019 W/cm2 in the lowest density case. The relativistic electrons accelerated in the forward direction appear to be correlated with the F-SRS. The exper...

Laser-driven shock waves for the study of extreme matter states

During the last ten years, the ability of high power lasers to generate high energy density shocks has made them a reliable tool to study extreme states of matter. These states of matter are relevant in many important physics areas such as astrophysics, planetology and ICF physics. Here, we present some representative studies performed by using a driven laser shock: melting of iron at pressures relevant for geophysics, developments of new techniques to measure the density of highly compressed matter and a study of a radiative shock.

Astrophysical jet experiments

We present an experimental characterization of jet propagation in an ambient medium. An intense laser (LULI2000) was used to generate the plasma jet using foam filled cone target. We observed, with several diagnostics, a perturbation in the interaction region between the jet and the ambient medium. The effect of the ambient medium on the jet velocity is also presented.

Fast electron transport and heating in solid-density matter

Two experiments have been performed to investigate heating by high-intensity laser-generated electrons, in the context of studies of the fast ignitor approach to inertial confinement fusion (ICF). A new spectrometer and layered targets have been used to detect K α emission from aluminum heated by a fast electron beam. Results show that a temperature of about 40 eV is reached in solid density aluminum up to a depth of about 100 μm.

X-ray scattering from dense plasmas

We review the potential of x-ray scattering as a dense plasma diagnostic and present data taken from experiments in which x-ray scattering from dense plasmas is developed as a diagnostic tool. In one type of experiment the scattered photons are detected as a function of angle using direct detection onto a CCD chip. Such experiments are designed primarily to observe the static ion–ion structure factor, which is expected to dominate the scattering for moderate to high Z plasmas at a few electronvolts temperature. In a second type of experiment we have used a curved crystal to observe spectrally resolved x-ray scattering at a fixed angle. This experiment was designed to observe the dynamical structure factor of the plasma.