Micron-scale mapping of megagauss magnetic fields using optical polarimetry to probe hot electron transport in petawatt-class laser-solid interactions

The transport of hot, relativistic electrons produced by the interaction of an intense petawatt laser pulse with a solid has garnered interest due to its potential application in the development of innovative x-ray sources and ion-acceleration schemes. We report on spatially and temporally resolved measurements of megagauss magnetic fields at the rear of a 50-μm thick plastic target, irradiated by a multi-picosecond petawatt laser pulse at an incident intensity of ~1020 W/cm2. The pump-probe polarimetric measurements with micron-scale spatial resolution reveal the dynamics of the magnetic fields generated by the hot electron distribution at the target rear. An annular magnetic field profile was observed ~5 ps after the interaction, indicating a relatively smooth hot electron distribution at the rear-side of the plastic target. This is contrary to previous time-integrated measurements, which infer that such targets will produce highly structured hot electron transport. We measured large-scale filamentation of the hot electron distribution at the target rear only at later time-scales of ~10 ps, resulting in a commensurate large-scale filamentation of the magnetic field profile. Three-dimensional hybrid simulations corroborate our experimental observations and demonstrate a beam-like hot electron transport at initial time-scales that may be attributed to the local resistivity profile at the target rear

Proton beam generation by ultra-high intensity laser-solid interaction

International audienceWe report on some recent experimental results on proton production from ultra-intense laser pulse interaction with thin aluminium and plastic foil targets. These results were obtained at Laboratoire d'Optique Appliqueacutee with the 100 TW 'salle jaune' laser system, delivering 35 fs laser pulses at 0.8 μm, reaching a maximum intensity on target of a few 10^19 W/cm^2. In such extreme interaction conditions, an intense and collimated relativistic electron current is injected from the plasma created on the laser focal spot into the cold interior of the target. Its transport through dense matter, ruled by both collisions and self-induced (electro-magnetic) field effects, is the driving mechanism for proton acceleration from the rear side of thin foils: when reaching and leaving the foil rear-side, the fast electrons create a large charge separation and a huge electrostatic field with a maximum value of few TV/m, capable of accelerating protons. A parametric study as a function of the laser driver and target parameters indicates an optimal value for target thickness, which strongly depends on the laser prepulse duration. In our experiments, we did irradiate targets of various materials (CH, Al, Au) changing the prepulse duration by using fast Pockels cells in the laser chain. CR-39 nuclear track detectors with Al filters of different thickness and a Thomson parabola were used to detect proton generation. The best results were obtained for 2 μm Al targets, leading to the generation of proton energies with energies up to 12 MeV

Early Bronze Age pebble installations from Tell es-Safi/Gath, Israel: evidence for their function and utilization

Pebble stone installations are commonly found at various Early Bronze Age sites in the southern Levant. However, their function is often assumed or unknown. Thirteen circular pebble installations were found scattered throughout a residential neighbourhood dating to the Early Bronze Age III at Tell es-Safi/Gath. Five such installations were recently studied by implementing an integrated micro-archaeological approach by which all micro- and macro-artefacts were analysed using various analytical techniques. Based on the analysis of ash-micro remains identified in the sediments, associated plant remains, flint and pottery, we suggest that these installations were used for food-processing, cooking and/or other domestic low-heat tasks. The installations first appear at Tell es-Safi/Gath during the Early Bronze Age III, and seem to disappear during later periods. The functional roles of these installations are discussed in comparison to finds from other Early Bronze Age sites, and of other food preparation traditions known from other periods and cultures

Production of energetic proton beams with lasers

International audienceA new method for accelerating proton beams, based on the use of intense and short laser pulses, is presented. It is shown that by focusing an ultrashort laser pulse onto a thin foil target, a proton beam with energy up to tens of MeV is produced. Due to the original properties of these beams unexplored fields in science will be discovered. In addition, this compact and low cost approach would probably be of great interest for medical applications

LASER-driven fast electron dynamics in gaseous media under the influence of large electric fields

We present the results of experiments performed at the LULI laboratory, using the 100 TW laser facility, on the study of the propagation of fast electrons in gas targets. The implemented diagnostics included chirped shadowgraphy and proton imaging. Proton images showed the presence of very large fields in the gas (produced by charge separation). In turn, these imply a strong inhibition of propagation, and a slowing down of the fast electron cloud as it penetrates in the gas. Indeed chirped shadowgraphy images show a reduction in time of the velocity of the electron cloud from the initial value, of the order of a fraction of c, over a time scale of a few picoseconds. © 2009 American Institute of Physics

Ultraintense Laser-Produced Fast-Electron Propagation in Gas Jets

International audienceWe study the propagation of fast electrons in a gas at different densities. A large relativistic electron current is produced by focusing a short-pulse ultrahigh-intensity laser on a metallic target. It then propagates in a gas jet placed behind the foil. Shadowgraphy in the gas shows an electron cloud moving at sub-relativistic average velocities. The experiment shows (i) the essential role of the density of background material for allowing propagation of fast electrons, (ii) the importance of the ionization phase which produces free electrons available for the return current, and (iii) the effect of electrostatic fields on fast-electron propagation

Energetic electrons generation in high intensity and ultra-short laser pulse interactions with thin foil or low density gas jet targets

Energy and angular distributions of the fast outgoing electron beam induced by the interaction of 1-2 J, 30 fs, 3-20x1018 W/cm2 laser with a thin foil or a gas jet target are characterized by using both an electron spectrometer and Bremsstrahlung induced photo-nuclear reactions. The supra-thermic electron beams production was investigated for a solid target versus its thickness and its Z number, and for a gas jet target versus its pressure. Using a polyethylene target and a supersonic Helium gas jet target, we measured, respectively, up to 4x108 and 3x109 electrons produced per laser pulse, with energies up to, respectively, 60 MeV and 160 MeV. The associated Boltzmann temperature of these electrons is colder for thin foils (9 MeV) than for gas jet (18 MeV). About, respectively 0.06% and 1% of the laser energy has been converted to outgoing electrons with energies above 5 MeV. Such electrons leave the plasma in the laser direction within a cone with an opening angle of, respectively, 2.5° and 8.5°. We discuss the physical processes of electron acceleration. Numerical calculations show a good agreement with the experiments